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The past 20 years of telecommunication structures in Portugal
Engineering Structures 48 (2013) 472–485
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Engineering Structures journal homepage: www.elsevier.com/locate/engstruct
The past 20 years of telecommunication structures in Portugal Rui Travanca ⇑, Humberto Varum, Paulo Vila Real Civil Engineering Department, University of Aveiro, Portugal
a r t i c l e
i n f o
Article history: Received 24 April 2012 Revised 16 October 2012 Accepted 18 October 2012 Available online 27 November 2012 Keywords: Towers Guyed masts Telecommunications Wind action Standards Failures
a b s t r a c t This paper reviews the analysis and design of telecommunication structures and presents the main problems observed for various types of structures. The nation of Portugal is selected for a case study, and more specifically, the evolution of Portuguese structural design standards for telecommunications systems is summarised using comparative analyses that cover a subset of the most relevant topics for design, including wind profiles, drag coefficients, dynamic effects and reliability classes. These analyses focus on characterisation of the effects of wind action, which plays a fundamental role in the behaviour and design of these structures. Following the comparative analyses of standards, the more common problems observed over the past 20 years in guyed masts and towers located in Portugal are presented and discussed. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
2. Standards evolution in design for wind action
Masts and towers represent the most important physical supports for the installation of the radio equipment used in transmission of the electromagnetic waves that provide such services of unquestionable importance as radio, television and/or mobile communications. Over the last few decades, the demand for these types of structures has increased due to the requirements for their use in the telecommunication sector. With the advent of mobile communications, this demand has become even greater. Unfortunately, the number of failures observed in telecommunications structures is high compared with other structures of equal economic and social importance. A great number of the failures observed are due to poor design, which results in unsafe structures that can suffer from full collapse [1–5]. The analysis and design of masts and towers requires specific knowledge and expertise, particularly with respect to guyed masts. Towers are lightweight structures with high slenderness and high flexibility. The newly adopted structural forms, the increasing strength of the materials used in construction and the consequent changes in structural stiffness, mass distribution and damping properties should lead to a new design approach, particularly for wind demands. The dynamic response becomes important when these structures exhibit a first natural frequency below 1 Hz. Therefore, dynamic analysis is necessary to determine whether the resonance response could be significant compared with the background response.
2.1. Generalities
⇑ Corresponding author. Tel.: +351 234370049; fax: +351 234370094. E-mail address: [email protected] (R. Travanca). 0141-0296/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engstruct.2012.10.012
Up until the 1970s, specific design procedures were not available for the analysis and design of notably tall and slender masts and towers. This lacuna was recognised by the industry in the early 1960s, when a rapid growth of these structures took place to accommodate the large expansion in radio and television broadcasting. As a consequence, a Working Group of the IASS (namely the WG4) was set up to study the behaviour of these structures and to produce the Recommendations for analysis and design. These Recommendations [6] were published in 1981 and formed the basis for the development of many additional national and international standards on this subject. At that time, the Recommendations contained such innovative ideas in structural design as structural reliability, the adoption of dynamic response procedures and the consideration of uncertainties associated with wind action. The innovative characteristics of such procedures were recognised by their adaption in the design standards for other types of structures and loadings [3–5]. The RSA [7], the Portuguese standard for accountable actions in the design of buildings and bridges, was published in 1983. Despite the special emphasis on buildings and bridges, other types of structures were also covered, including towers. It is important to note that the RSA was the only standard for action definition in Portugal. Even so, and as presented in this research, specific rules for the design of masts and towers were not adequately covered, which had both positive and negative effects with respect to safety. For example, ignoring the
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Nomenclature Latin upper-case letters A1 effective area total projected area when viewed normal to the face A2 (outer contour) Ac total projected area when viewed normal to the face of the circular-section members in the face in subcritical regimes Ac.sup total projected area when viewed normal to the face of the circular-section members in the face in supercritical regimes Ac1 effective area corresponding to subcritical circular-section members Ac2 effective area corresponding to supercritical circularsection members Aref reference area B2 background factor F global force actuation Fm,W mean wind load on the tower in the direction of the wind FT,W equivalent gust wind load on the tower in the direction of the wind Iv turbulence intensity L turbulent length scale R2 resonance response factor Latin lower-case letters cf force coefficient cf,0,c force coefficient for sections composed of subcritical circular-section members
cf,0,c,sup cf,S cf,S,0,j
cr cscd co fL h kp qp v vm w Z
force coefficient for sections composed of supercritical circular-section members wind force coefficient of the bare structure section force coefficients for sections composed of flat-sided, subcritical circular and supercritical circular-section members roughness factor structural factor Orography factor non-dimensional frequency height above the ground peak factor peak velocity pressure wind speed mean wind velocity wind dynamic pressure reference height
Greek upper-case letters W solidity ratio Greek lower-case letters dfc1 force coefficient for sections composed of subcritical circular-section members dfc2 force coefficient for sections composed of supercritical circular-section members k solidity ratio
2.2.1. Wind action Early codes treated wind action as a static wind pressure and often incorporated a factor to account for the drag of the structure. The dynamic effects were ignored in this procedure. Representative plots of extreme wind speeds are defined in the RSA [7], normalised to the design 50-year return period and specified as having
a 5% probability of not being exceeded in the structure lifetime. Table 1 shows the Portuguese territory zoning for the definition of wind action in both standards, i.e., the RSA [7] and the Portuguese National Annex for the Eurocode 1 (EC1) [8]. Similar to other standards of the 1970s and 1980s, power laws were applied to predict the distribution of the extremes of the wind loads. This approach has the advantage of simplicity and represents the observed profiles rather well [3]. However, a more accurate log-law procedure is now available and has been adopted in recent design standards such as the Eurocode 1 [8]. The wind speed profiles apply to all types of terrain, from smooth to rough, although areas of transition from one type to another require careful consideration. The RSA [7] only specifies two terrain categories (see Table 2). The Portuguese National Annex for the Eurocode 1 [8] contains a large differentiation that prescribes four categories for terrain (see Table 3). Additionally, the RSA does not account for the effect of terrain roughness and height in the turbulent component of the wind speed, prescribing instead a fixed value of 14 m/s for the two defined terrain categories. This value is also independent of the considered height. However, it is well known that the height and terrain roughness affects the mean wind speed as well as the turbulent component, i.e., the mean wind speed reaches a maximum value for smooth terrain, and the turbulence is minimal compared with that of rough terrain. Fig. 1 presents the wind pressure
Table 1 Portuguese territory zones (RSA [7] and Eurocode 1 [8]).
Table 2 Terrain roughness (RSA [7]).
dynamic effects of wind loading and an inadequate distribution of wind action over the height of the structure often resulted in an underestimation of the structural response. However, in many cases, this oversight was clearly offset by the use of conservative design criteria based on general standards for the design of steel structures that did not properly account for the behaviour of light slender lattice frames [3]. Nevertheless, the Annex I of the RSA specified that the variability of wind speed resulting from its turbulent nature can be assessed using spectral and spatial correlations of wind velocity, and this information should be available through specialised literature. More importantly, dynamic response identification procedures and consideration of ice loading are other examples of aspects that were not covered by the RSA [7]. With the implementation of the Eurocodes [8,9] all of these topics are finally covered by the national standards. 2.2. Comparative analysis between the RSA and Eurocodes
Zone A Zone B
The generalisation of Portuguese territory except for the regions belonging to zone B Azores and Madeira and mainland regions located not further than 5 km from the coast or at altitudes above 600 m
Type I Type II
Roughness of sites located within urban areas where medium and large buildings predominate Roughness of other locations, particularly rural areas and the outskirts of urban areas
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Table 3 Terrain categories (Eurocode 1 [8]). Cat. I Cat. II Cat. III
Cat. IV
Coastal area exposed to the open sea Areas with low vegetation, such as grass and isolated obstacles (trees, buildings) with separations of at least 20 obstacle heights Areas with regular cover of vegetation or buildings or containing isolated obstacles with separations of no more than 20 obstacle heights (i.e., villages, suburban terrain, permanent forest) Areas in which at least 15% of the surface is covered with buildings with an average height that exceeds 15 m
profiles for both codes. Despite the different premises of these two standards, it can be observed from Fig. 1 that the profile type I (RSA) fits well with the profile Cat. IV (EC1). Additionally, the profile type II (RSA) has similar equivalence with categories II and III (EC1), producing a substantial distinction in the effect of the ground roughness. However, category I (EC1) does not match any comparable profile in the RSA (see also Tables 2 and 3). 2.2.2. Topography effects The major topographic features (i.e., escarpments, hills and ridges) have strong effects on the wind pressure distribution
(a) Zone A
(b) Zone B Fig. 1. Wind pressure profiles (RSA [7] and Eurocode 1 [8]).
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(a) RSA [7]
475
(b) Eurocode 1 [8] Fig. 2. Topography effects.
because the wind that flows over these features is modified by their presence. The topographic features act as clear obstructions to the boundary layers and accelerate the wind near the ground, leading to increased wind pressure on obstacles located in such regions. Because of its specified purpose, telecommunication structures are frequently located at points of higher elevation. Many standards include procedures for estimating the so-called ‘speed-up effect’, and most use different methods to account for the increase in the design wind speed or pressure. Differences in topography as well as terrain height effects will lead to different design wind pressures from each of these standards. Recently updated standards, e.g., the Eurocode 1 [8], provide detailed procedures to determine wind loads that account for the speed-up effect due to topographic features [3,5]. Thus, the procedures included in the RSA [7] are out of date when compared with current state of knowledge; the RSA procedures do not consider the speed-up effect, are based on corrections at the height of the base of the structure and take into account the position and the slope of the terrain only. Fig. 2 illustrates the different approaches considered by the RSA [7] and the Eurocode 1 [8].
2.2.3. Drag coefficients Because the structural analysis and design of lattice towers are generally undertaken in the face-on and 45° directions only, the RSA [7] prescribes drag coefficients only for the wind acting on the face and at the corner of the structure, and this last category is applicable to towers with a square base. However, according to the procedures prescribed in Part 3–1 of the Eurocode 3 [9], wind action can be determined for any angle of incidence. As prescribed in the RSA, the drag coefficients applied for lattice towers are more conservative in general than those presented in the Eurocode 3 [9]. This observation can be confirmed in Fig. 3. As far as the authors are aware, no experimental results exist for solidity ratios greater than 0.6, but as the solidity ratio tends to 1, the drag coefficient should clearly tend to a solid bare section. It should be noted that Part 3–1 of the Eurocode 3 [9] includes methods that account for one of the most difficult topics to standardise, i.e., the wind resistance of such non-structural elements as ladders, feeders, platforms or antennas. Additional research is needed on this topic. The non-structural elements mounted on these structures can take on a wide variety of forms in terms of position and shapes. As reported by several authors, it has become common to conduct a separate analysis of the structural and non-structural elements without correctly considering the interference between these different elements [3,5,10].
2.2.4. Dynamic effects The dynamic analysis of towers may be performed in the frequency domain based on the characteristics that depend on the frequencies of both of the wind action and the structural properties of the structure. In this approach, a wind gust is characterised on a probabilistic basis, i.e., by statistical descriptions of relevant properties such as frequency content and spatial distribution. However, simplified procedures may be used under certain conditions. According to the Eurocodes [8,9], a quasi-static analysis may be adopted using the appropriate gust response factors. These factors depend on several parameters, including the fundamental frequency, the damping of the structure and the wind action properties. Thus, the peak pressure is amplified by the allocation of a gust response factor and is subsequently treated as a static action. The RSA [7] provides a static analysis that only applies if the fundamental frequency exceeds 0.5 Hz. Therefore, the fundamental frequency is used only to verify and validate the applicability of this simplified procedure. The dynamic effects are included in the Eurocodes [8,9]. For guyed masts, such procedures may not be appropriate because the dynamic modes are not well separated. Additionally, it is difficult to perform dynamic analysis using conventional methods. In response to the gusty winds, the complex interactions between the mast and the guy ropes typically result in a large number of active vibration modes. Additionally, aerodynamic damping forces acting on the guys provide an effective means for suppressing resonance in the mast. The dynamic analysis is further complicated by the random nature of wind loads, which vary both in time and location on the mast [3]. Although dynamic analysis methods are available and are able to address all of these factors, these methods are complex and require specialised computer software. Consequently, procedures have been developed that simulate a full dynamic response analysis using static analysis through patch loading techniques. Such patch wind models were first introduced in the IASS Recommendations [6], and currently, this model has been refined and adopted in selected National Codes as well in Part 3–1 of the Eurocode 3 [9]. The RSA [7] does not cover guyed masts. Consequently, it is not possible to directly compare the dynamic effects. Nonetheless, in the case study presented in this paper, the influence of this parameter on the overall results is appraised when applicable to a self-supported lattice tower. 2.2.5. Towers with inclined legs The wind speed at any instant can be described as the sum of an average speed and a fluctuating part. The fluctuating part represents the turbulent wind component and is consequently variable both in time and location. Therefore, a cautious approach is required to obtain the resultant forces on the brace members for
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(a) Triangular structures
(b) Square structures (wind on face) Fig. 3. Drag coefficients (RSA [7] and Eurocode 3 [9]).
towers that include inclined legs, also referred to as ‘‘Eiffelated’’ towers, because the application of the gust wind to all of the height can lead to small forces on the bracing members [3,5]. Traditionally, static procedures (such as that prescribed in the RSA [7]) that use a uniform gust wind profile could indicate lower forces in such bracings, which can lead to unsafe structural design. Current procedures (such as the one prescribed in the
Eurocode 3 [9]) avoid this situation with a patch loading approach. 2.2.6. Reliability classes The principle of three reliability classes for masts and towers was established in 1981 by the IASS Recommendations [6] and was followed by all of the other relevant standards [3,5]. For
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(c) Square structures (wind on corner) Fig. 3. (continued)
Table 4 Reliability differentiation for masts and towers [9]. Class 1
Class 2 Class 3
Masts and towers built on unmanned sites in open countryside; masts and towers in which the failure would not be likely to cause injury to people All masts and towers that cannot be defined as class 1 or 3 Masts and towers erected in urban locations or where failure is likely to cause injury to people
example, Part 3–1 of the Eurocode 3 [9] introduces three classes of reliability (Table 4) corresponding to different partial safety factors (Table 5) and applies them to permanent and variable actions. The RSA [7] makes no such distinctions and applies the safety factors used for current structures, e.g., 1.5 for variable actions if unfavourable. As stated previously, this practice undoubtedly leads to conservative design criteria that fortunately offset the underestimation of the structural response. In the case study presented in this paper, this counterbalance is discussed in additional detail. 2.3. Case study In the case study presented, a self-supported lattice tower with a triangular base and a height of 40.5 m is analysed (Fig. 4). This analysis focuses only on the wind action and does not consider any exposed areas related to such non-structural elements as ladders, feeders, platforms or antennas. The main differences found in the results obtained when using different standards (i.e., RSA [7] and Eurocodes [8,9]) will be presented, compared and discussed. In this analysis, it was assumed that the lattice tower is located in Portugal in a rural region far from the coast at an altitude of 200 m. For quantification of the wind action, we assumed that the structure is located in zone A (according to RSA [7] and the Portuguese National Annex for the Eurocode 1 [8]) with a soil roughness of type II (according to RSA [7]) and a terrain category of type
Table 5 Partial factors for actions [9]. Type of effect
Reliability class
Permanent actions
Variable actions
Unfavourable
1 2 3
1.0 1.1 1.2
1.2 1.4 1.6
1.0 1.0
0.0 1.0
Favourable All classes Accidental situations
II (according to the Portuguese National Annex for Eurocode 1 [8]). To simplify the case study, the effects of topography are not taken into account. The elastic forces and natural frequencies were obtained using SAP2000 software from Computers and Structures, Inc. [11]. The value obtained for the fundamental frequency was 1.90 Hz. The Appendix A of the paper presents the determination of the acting forces due to the wind action applied only to the bare structure using the RSA [7] and the Eurocodes [8,9]. Table 6 compares the results obtained with these two distinct standards in terms of the following: (i) shear force at the base (F), (ii) axial force in the legs located at the base (N1), (iii) axial force in the bracing members located at the base (N2), (iv) design force in the legs located at the base (NSd,1) and (v) design force in the bracing members located at the base (NSd,2). Although the difference in shear force at the base presents an increase of 13.5% over the value determined by Eurocodes [8,9] in relation to the RSA [7], it can be observed that the difference in axial force for the legs located at the base is higher (16.9%). This increase is also justified by the difference found in the wind pressure distribution for the height and, as a result, in the static forces applied. However, this disparity is even larger for bracing members with a difference of 29.6%. This result is justified not only by the wind pressure distribution at the height but also by the patch loading approach prescribed in Eurocodes [8,9]. All of these differences are attenuated in the design forces considered in the combination of actions. For reliability
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(a) Photograph
(b) Draft
Fig. 4. Self-supported lattice tower with a triangular base.
Table 6 Overall comparison of the results. F (kN)
N1 (kN)
N2 (kN)
NSd,1 (kN) Class 1
RSA [7] EC [8,9] Dif. (%)
29.79 33.81 13.5
239.35 279.74 16.9
5.77 7.48 29.6
359.03 335.69 6.5
NSd,2 (kN) Class 2
Class 1
Class 2
391.64 9.1
8.66 8.98 3.7
10.47 21.0
class 1, the bracing members present only a slight difference (3.7%), and there is an inversion of the axial force for the legs located at the base ( 6.5%). For reliability class 2, the difference of the design forces in the legs located at the base is 9.1% and 21.0% for the bracing members. In this study, the reliability class 1 would be appropriate, and thus, the differences obtained and presented could be considered as negligible. 3. Structure typologies used in the telecommunication sector Telecommunication structures typologies vary widely across countries according to their uses, location and more commonly used materials. For example, in Portugal and Spain, where masts and towers are often installed in locations 500 m above the ground, the height of the structures will reach 60 m. In contrast, in Denmark and Holland, where the elevation of the land is practically zero, masts and towers with heights of over than 250 m are common [3]. The weight of a self-supported tower and thus its cost will vary approximately with the square of the height. For a guyed mast, the cost will vary proportionally with the
expression H1.5, where H is the height of the tower [3]. Self-supported towers up to 50 m high are commonly used throughout Europe for supporting antennas and are normally more suitable in terms of the overall costs compared with guyed masts, despite the higher quantity of steel used. The construction, excavation, land cost and subsequent maintenance leads to a more economical solution due to unnecessary replacement of guys, a simple operation that nonetheless involves additional time and therefore additional costs. However, along with the elimination of guys and lowland occupancy, another great advantage of self-supported lattice towers is their high torsional stiffness, an important aspect in the use of directional antennas (links) that impose a loading eccentricity due to wind action [3,5]. In this study, and for the simplicity of presentation of the results and analysis, the structures were grouped into six distinct typologies as described in Table 7 and illustrated in Fig. 5. Table 7 General description of structures typologies. Typology 1 Typology 2 Typology 3
Typology 4 Typology 5 Typology 6
Self-supported lattice tower with a square base, generally with angle profiles and bolted connections Self-supported lattice tower with a triangular base, generally with tubular profiles and with bolted connections Monopole with octagonal, dodecagonal or hexadecagonal cross-sections, with several modules generally linked by forced fit Monopole of tubular cross-section, with several modules linked by bolted connections Lattice guyed mast with triangular or square sections This typology groups all other types of towers not included in the other typologies
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Typology 1
Typology 2
Typology 3
Typology 4
Typology 5
Typology 6
Fig. 5. Structures typologies.
Table 9 Description of levels of structural integrity.
Table 8 Distribution of structures by typologies.
Level 1: comply
Typology
Qty.
%
1 2 3 4 5 6
42 96 136 80 18 13
10.9 24.9 35.3 20.8 4.7 3.4
The data collected, presented and discussed in this research are not referenced for reasons of professional confidentiality. However, it is possible to indicate that the maximum height of the structures analysed in this study is 100 m, and the average height stands at 35 m. Additionally, all of the research work is based on design projects, inspection reports and/or structural assessment studies of all structures. Table 8 presents the distribution of the 385 structures used in this research according to typology and can be extrapolated to all of the structures used in the telecommunication sector in Portugal. As an example, a clear observation from Table 8 is the preponderance of monopoles (typologies 3 and 4), representing 56.1% of all structures studied. It is also possible to verify that self-supported lattice towers with square or triangular bases (typologies 1 and 2) represent 35.8% of all structures. The higher propensity for the use of monopoles can be justified by their visual impact, an aspect that will not be discussed in this paper.
Level 2: comply/ limited
Level 3: not comply
Level 4: not comply/ reinforcement
Level 5: not comply/ replacement
The structure meets all requirements of structural safety for the considered loads and the applicable national standards The structure meets all requirements of structural safety for the considered loads and the applicable national standards but is already at its maximum capacity such that any increases in loading must be preceded by structural strengthening The structure does not meet all requirements of structural safety for the considered loads and the applicable national standards; no instructions are given for whether the structure should be strengthened or replaced The structure does not meet all requirements of structural safety for the considered loads the applicable national standard; explicit instructions are given that the structure should be strengthened The structure does not meet all the requirements of structural safety for the considered loads and the applicable national standards; explicit instructions are given that the structure should be replaced
4. Main problems observed in Portugal From the analysis of the problems recently observed in structures used in the telecommunication sector in Portugal, it is reasonable to conclude that design errors are the most frequent cause for collapse or early replacement events. In addition, and in relation to the aspects mentioned previously, market pressure demanding lighter and more economical solutions has generated
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R. Travanca et al. / Engineering Structures 48 (2013) 472–485 Table 10 Distribution of the structures by levels of structural integrity. Level
Qty.
%
1 2 3 4 5
307 42 6 21 9
79.7 10.9 1.6 5.5 2.3
Table 11 Distribution by levels of structural integrity of structures in typology 1. Level
Qty.
%
1 2 3 4 5
17 10 1 14 0
40.5 23.8 2.4 33.3 0.0
Table 12 Distribution by levels of structural integrity of structures in typology 2. Level
Qty.
%
1 2 3 4 5
80 12 1 3 0
83.3 12.5 1.1 3.1 0.0
Table 13 Distribution by levels of structural integrity of structures in typology 3. Level
Qty.
%
1 2 3 4 5
129 7 0 0 0
94.9 5.1 0.0 0.0 0.0
(a) Collapse of a type 3 monopole
Table 14 Distribution by levels of structural integrity of structures in typology 4. Level
Qty.
%
1 2 3 4 5
63 12 3 1 1
78.8 15.0 3.8 1.2 1.2
structures with limitations in terms of fatigue safety in certain cases. A definition of the basic requirements for structural design is a key issue. To address this need, proper communication between the designer and client is necessary to create an economical structure with adequate safety and performance. Unfortunately, as stated by Støttrup-Andersen [1], the combination of an inexperienced client and a designer who is not familiar with the special problems associated with the design of masts and towers is common. Over the years, this combination has created many structures that are not suitable for their intended purpose: they may be either too safe, or in the worst case, they may be unsafe. The latter case may lead to damage or even the total collapse of the structures. This paper highlights the observed features related to structural integrity and safety. For data analysis and discussion, five levels of structural integrity were defined (Table 9). The reference to the national standards only covers the RSA [7] for action definitions because the Eurocodes are still optional at present. In fact, in all structural design projects and/or studies covered in this research work, the Eurocode 1 [8] was not used for action definitions. One conclusion arising from this research that applies to all typologies is that, despite the limitations of the RSA already presented and discussed in this paper, in many cases, the RSA itself was not applied properly for the wind action definition. Thus, certain existing structures may not contain adequate safety measures. Hereafter, this issue will be defined as the ‘‘improper definition of wind action’’. In addition, we note that level 2 represents the scenario in which a compromise was reached between the structural designer and the operator for the definition of the solution for that specific structure. In many cases, this solution involves a reduction of exposed areas with the elimination of platforms and the replacement of antennas or other non-structural elements to reduce the exposed area. The global analysis for the 385 structures is shown in Table 10. The largest group of 307 structures (79.7%) is classified at level 1.
(b) Detail at the base
Fig. 6. Collapse of a monopole of typology 3.
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(a) General view of the type 4 monopole
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(b) Detail of rupture
Fig. 7. Collapse of a monopole with a tubular cross-section.
Table 15 Distribution by levels of structural integrity of structures in typology 5. Level
Qty.
%
1 2 3 4 5
12 0 1 2 3
66.7 0.0 5.5 11.1 16.7
The remaining 78 structures (20.3%) are classified at levels 2, 3, 4 or 5, and are therefore limited in use, indicated for strengthening or subject to explicit instructions for replacement. For a deeper analysis of the results, the next sections present the results per typology as well as the most probable associated causes. 4.1. Main problems observed in structures of typology 1 Table 11 shows the distribution by level of structural integrity for structures in typology 1, i.e., self-supported steel lattice towers with a square base. As shown in Table 11, strengthening was required for 14 towers (33.3%); this reinforcement can be achieved by the introduction of angles in the legs and with duplication of the bracing members. Ten towers (23.8%) are classified at level 2 or are limited in use, but for these cases, it is always possible to add structural strengthening. The main reasons for the observed problems that lead to the need for strengthening are the following:
(a) Guyed mast 15 m high
(i) errors in structural design mainly due to the improper definition of wind action, (ii) additions to the height of the tower without strengthening of the existing structure and (iii) loading beyond the capacity of the structure. 4.2. Main problems observed in structures of typology 2 Table 12 presents the distribution of structural integrity for structures classified in typology 2, i.e., self-supported lattice towers with a triangular base. The results indicate that for this typology, 80 towers (83.3%) are classified in level 1, 12 towers (12.5%) are limited in use, and only four towers (4.2%) present severe problems. Despite the overall result obtained for this typology, which is clearly positive compared with other typologies with a small number of serious problems, it is possible to indicate the following main causes that lead to the need for structural strengthening: (i) errors in design particularly due to improper definition of wind action, (ii) excessive loading beyond the carrying capacity of the structure and (iii) errors in assembly and elimination of structural elements that may compromise structural safety. 4.3. Main problems observed in structures of typology 3 Table 13 presents the distribution by levels of structural integrity of structures in typology 3, i.e., self-supported monopoles with
(b) Detail of crack in masonry wall
Fig. 8. Ineffective guy system.
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(a) Guyed mast 15 m high
(b) Detail of anchorage in a cantilever steel element
Fig. 9. Anchorages in flexible elements.
(a) Collapse of a 21 m guyed mast
(b) Detail of the mast base after collapse
Fig. 10. Problems in anchorages.
(a) Detail of concrete slab
(b) Detail of anchorage
Fig. 11. Pullout of the anchorages in concrete.
octagonal, dodecagonal or hexadecagonal cross-sections. From the analysis of the results, it can be concluded that this typology displays a higher success rate, with 129 monopoles (94.9%) in level 1 and only 7 monopoles (5.1%) in level 2, i.e., limited in usage. Therefore, there is no record of serious problems for this typology.
Although the scenario observed is quite positive, it is important to note that this typology in general exhibits a fundamental frequency close to that of the wind spectrum, and therefore, the dynamic response of the structure can be greatly amplified. This issue was not addressed adequately in the RSA [7]. To highlight this
R. Travanca et al. / Engineering Structures 48 (2013) 472–485 Table 16 Distribution by levels of structural integrity of structures in typology 6. Level
Qty.
%
1 2 3 4 5
6 1 0 1 5
46.1 7.7 0.0 7.7 38.5
assertion, measurements were taken on two self-supported monopoles of typologies 3 and 4 using FBG-based accelerometers as described in a previous paper [12]. The values obtained for the fundamental frequency in these two monopoles were 0.66 Hz and 0.41 Hz [12]. Fig. 6 shows the collapse of a monopole of typology 3 in which it is possible to observe the rupture at its base. 4.4. Main problems observed in structures of typology 4 Table 14 presents the distribution of structural integrity for structures in typology 4, i.e., monopoles with tubular crosssections. The overall results are considered positive, with 63 monopoles (78.8%) classified in level 1, 12 monopoles (15.0%) limited in use and 5 monopoles (6.2%) presenting serious problems. However, as mentioned for the monopoles in typology 3, many of the structures in this typology also display a frequency close to that of the wind spectrum, and therefore, the dynamic response can be largely amplified. It is also important to note that these structures, especially those with heights larger than 30 m, are usually composed of tubular steel profiles with a diameter/thickness ratio greater than 100. It is known that the resistance of these profiles is rather sensitive to imperfections (snap-through behaviour). In all documents reviewed for this study, the phenomena of local plate instability were not accounted for in estimating the resistance of thin-walled steel plates, and therefore, the resistance of the structure may have been overestimated. Another important point to underline through the analysis of the various documents used in this research, and that justifies the overly optimistic scenario, was the disregard for two important phenomena in the analysis, namely, (i) vortex-shedding and (ii) fatigue. If the frequency caused by vortex-shedding coincides with the fundamental frequency of the structure, resonant vibrations may occur. Because
(a) Lack of verticality of the monopole
483
this condition occurs for a critical wind speed that is often observed, fatigue also may become relevant depending on the number of cycles and the loading [3,5,13,14]. As an example of these two linked phenomena, the collapse of a monopole with a tubular cross-section, is shown in Fig. 7. 4.5. Main problems observed in structures of typology 5 Table 15 presents the distribution by levels of the structural integrity of the structures classified in typology 5, i.e., lattice guyed masts with triangular or square sections. Although the results presented do not highlight the fact significantly, with 12 masts (66.7%) in level 1 and 6 masts (33.3%) presenting severe problems, this typology certainly requires particular attention. The scenario presented may represent an optimistic scenario given the large number of collapses registered in recent years. Based on the samples studied, the value of 33.3% presented in this study may underestimate the actual problem. The authors of this research deem it relevant to underline this issue despite the results obtained, presented and discussed in this work. Guyed masts are structures that require complex analysis, and many experts have stated that ‘‘a guyed mast is one of the most complicated structural problems that an engineer may face’’ [1– 5]. This statement is unfortunately underlined by the fact that the number of collapses of these masts is far greater than that for other structures. As mentioned previously, the first error falls on the choice of the structure: for small structures, the choice of guyed masts does not seem to be suitable for the reasons presented in Section 3. In the cases presented in this study, which are all related to small guyed masts, the failures were caused by design errors and/or bad construction practices, e.g., ineffective guy systems and/or deficient anchorages, as shown in the four cases illustrated in Figs. 8–11. Fig. 8 illustrates a small guyed mast with a square section and a height of 15 m, with one guy level at 9 m composed of two guys with opening angles of 180°, i.e., with a guy system that is totally ineffective. Furthermore, other serious deficiencies have been detected, including inappropriate anchorages in masonry walls that present severe cracks and severe corrosion in steel members. Fig. 9 presents a small guyed mast with a triangular section and a height of 15 m with two levels of guys with opening angles of 120° and with two guys fixed to a cantilevered steel profile.
(b) Rupture of steel plates at corners
Fig. 12. Monopole with a square cross-section and a 40 m height.
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Fig. 10 shows a 21 m guyed mast with a square section that collapsed due to problems with the anchorages. Fig. 11 presents the details of a small guyed mast with a triangular section and a height of 12 m with two guy levels at 6 m and 12 m, composed of three guys per level; this structure collapsed after pullout of the concrete anchorages. The main causes for the problems observed in this typology are the following: (i) gross errors in design (e.g., a poor guy system, ineffective anchorage, incorrect assessment of the resistance and/ or stiffness of steel members, improper definition of wind action as prescribed in RSA) and (ii) gross errors in construction (e.g., anchorages placed in masonry walls or other weak or fragile elements, inadequate tensioning or lack of guy re-tensioning). 4.6. Main problems observed in structures of typology 6 Table 16 shows the distribution by levels of the structural integrity of the structures classified in typology 6, i.e., all types of structures not previously mentioned. Although the table presents a scenario that could be considered rather negative, with six structures (46.1%) in level 1, one structure (7.7%) in level 2 and six structures (46.2%) presenting severe problems, it must be noted that this typology only contains 13 structures, and the large number of problems observed are related to a single type of structure from a single manufacturer that contained serious design errors leading to the replacement option. Additionally, for this typology, the primary cause of the noted problems is related to design errors. Fig. 12 presents the case of a 40 m monopole formed by cold-form steel plates with a square cross-section and a width that varies with the height. This monopole is composed of eight different modules with a height of 5 m each linked together with bolted connections at the corners. The dimensions at the base of the monopole include a 1.2 m width and a 6 mm plate thickness, and at the top, a 0.54 m width and a 5 mm plate thickness. Several problems were detected, namely, the lack of verticality of the monopole, buckling of the steel plates and rupture of steel plates in various locations, as shown in Fig. 12. In the analysis of the de-
sign project, it was possible to identify several errors as follows: (i) improper definition of wind action and (ii) overestimation of the resistances of the cross-sections, i.e., an elastic analysis was performed without area reduction (effective cross-sectional area of the compression zone of the plate [9,15]) in sections classified as class 4, according to Part 1–1 of the Eurocode 3 [16]. 5. Conclusions This study presents the main differences observed between the former Portuguese national standard (RSA) and the Eurocodes 1 and 3; it also compares the relevant parameters and principles related to the design of telecommunication structures. One case study was presented for the comparison between these standards. This work also highlighted the considerable progress in development that occurred between these standards, which are separated by nearly three decades. Despite the obvious differences, which were presented and discussed throughout the paper, it was concluded that a self-supported lattice tower with a triangular base designed according to the RSA standard should present no major safety problems when assessed for safety according to the specifications in the Eurocodes. However, the same cannot be concluded if the reliability class is higher or for structures with lower fundamental frequencies for which the structural response would be greatly amplified. These aspects were not considered in the procedures included in the RSA. The main problems observed in telecommunication structures in Portugal are summarised. Based on the analysis of 385 structures located in Portugal, it was possible to identify the most common problems, and it was concluded that these problems are mainly a consequence of design errors. As observed in this research, the number of failures in this type of structures is high compared with other structures of similar importance. Appendix A Tables A.1– A.4.
Table A.1 Determination of wind forces (RSA [7]). Panel
h (m)
V (m/s)
W (kN/m2)
Ac1 (m2)
Ac2 (m2)
A1 (m2)
A2 (m2)
K
dfc1
dfc2
F (kN)
1 2 3 4 5 6 7
6.00 12.00 18.00 24.00 30.00 36.00 40.50
39.00 39.93 42.12 43.78 45.14 46.30 47.07
0.93 0.98 1.09 1.18 1.25 1.31 1.36
1.42 1.16 0.80 0.76 0.61 0.56 1.36
2.63 2.63 2.63 2.63 2.32 2.02 0.00
4.05 3.79 3.43 3.39 2.93 2.58 1.36
16.61 12.41 10.31 10.31 10.16 10.01 7.26
0.24 0.31 0.33 0.33 0.29 0.26 0.19
1.55 1.50 1.49 1.49 1.51 1.54 1.61
1.13 1.09 1.07 1.08 1.11 1.13 1.08
4.83 4.51 4.37 4.66 4.37 4.13 2.98
Table A.2 Determination of the structural parameters (Eurocode 1 [8]). vm (m/s)
Iv
L (m)
fL
SL
B2
gh
gb
Rh
Rb
R2
V (Hz)
kp
cs.cd
34.36
0.15
130.71
7.23
0.04
0.69
10.30
0.55
0.09
0.71
0.24
0.97
3.74
1.02
Table A.3 Determination of exposed areas and drag coefficients (Eurocode 3 [9]). Panel
Ac (m2)
Ac.sup (m2)
As (m2)
Ac (m2)
w
K1
K2
cf,0,c
cf,0,c,sup
cf,S,0,j
cf,S
cf
Aref (m2)
1 2 3 4 5 6 7
1.42 1.16 0.80 0.76 0.61 0.56 1.36
2.63 2.63 2.63 2.63 2.32 2.02 0.00
4.05 3.79 3.43 3.39 2.93 2.58 1.36
16.61 12.41 10.31 10.31 10.16 10.01 7.26
0.24 0.31 0.33 0.33 0.29 0.26 0.19
0.80 0.80 0.80 0.80 0.80 0.80 0.80
0.24 0.31 0.33 0.33 0.29 0.26 0.20
1.42 1.35 1.32 1.33 1.36 1.40 1.50
1.09 1.09 1.10 1.10 1.09 1.09 1.08
1.20 1.17 1.15 1.15 1.15 1.15 1.50
1.20 1.17 1.15 1.15 1.15 1.15 1.50
1.20 1.17 1.15 1.15 1.15 1.15 1.50
4.05 3.79 3.43 3.39 2.93 2.58 1.36
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R. Travanca et al. / Engineering Structures 48 (2013) 472–485 Table A.4 Determination of wind forces (Eurocode 1 and 3 [8,9]). Panel
z (m)
cr
co
vm (m/s)
Iv
qp (kN/m2)
cf
Aref (m2)
Fm,W (kN)
FT,W (kN)
1 2 3 4 5 6 7
6.00 12.00 18.00 24.00 30.00 36.00 40.50
0.91 1.04 1.12 1.17 1.22 1.25 1.27
1.00 1.00 1.00 1.00 1.00 1.00 1.00
24.56 28.12 30.20 31.67 32.82 33.75 34.36
0.21 0.18 0.17 0.16 0.16 0.15 0.15
0.93 1.13 1.25 1.34 1.41 1.47 1.51
1.20 1.17 1.15 1.15 1.15 1.15 1.50
4.05 3.79 3.43 3.39 2.93 2.58 1.36
1.83 2.19 2.25 2.44 2.26 2.12 1.50
4.62 5.14 5.13 5.51 5.11 4.83 3.46
References [1] Støttrup-Andersen U. Analysis and design of masts and towers. In: Proceedings of the international symposium on lightweight structures in civil engineering. Warsaw: Ed. Obrebski, JB; 2002. p. 137–66. [2] Støttrup-Andersen U. Mast and towers. In: Domingo A, Lazaro, C. editors, Proceedings of the International Association for Shell and Spatial Structures (IASS) symposium. Valencia; 2009. p. 127–38. [3] Smith B. Communication structures. London: Thomas Telford; 2007. [4] Smith B. 50 years in the design of towers and masts. From IASS recommendations to current procedures. In: Domingo A, Lazaro C. editors, Proceedings of the International Association for Shell and Spatial Structures (IASS) symposium. Valencia; 2009. p. 139–52. [5] Travanca R. Torres para radiocomunicações: Patologias e dimensionamento, master thesis. University of Aveiro; 2010 [in Portuguese]. [6] IASS Working Group 4. Recommendations for the design and analysis of guyed masts. In: International association of shell and spatial structures. Madrid; 1981. [7] RSA. Regulamento de Segurança e Acções para Estruturas de Edifícios e Pontes. Decreto-Lei n.° 235/83 de 31 de Maio; 1983 [in Portuguese]. [8] CEN. Comité Europeu de Normalização. NP EN 1991–1-4, Eurocódigo 1 – Acções em estruturas, Parte 1–4: Acções do vento; 2010 [in Portuguese].
[9] CEN. European Committee for Standardisation. EN 1993–3-1, Eurocode 3 – design of steel structures, Part 3–1: towers, masts and chimneys – towers and masts; 2006. [10] Carril Jr C, Isyumov N, Brasil R. Experimental study of the wind forces on rectangular latticed communication towers with antennas. J Wind Eng Ind Aerod 2003;91:1007–22. [11] CSI. SAP2000 version 9: Basic analysis reference manual. California: Computers and Structures Inc. Berkeley; 2004. [12] Antunes P, Travanca R, Varum H, André P. Dynamic monitoring and numerical modelling of communication towers with FBG based accelerometers. J Constr Steel Res 2012;74:58–62. [13] Repetto M, Solari G. Dynamic alongwind fatigue of slender vertical structures. J Eng Struct 2001;23:1622–33. [14] Jia J. Wind and structural modelling for an accurate fatigue life assessment of tubular structures. J Eng Struct 2011;33:477–91. [15] CEN. Comité Europeu de Normalização. NP EN 1993–1-5, Eurocódigo 3 – Projecto de estruturas de aço, Parte 1–5: Elementos estruturais constituídos por placas; 2012 [in Portuguese]. [16] CEN. Comité Europeu de Normalização. NP EN 1993–1-1, Eurocódigo 3 – Projecto de estruturas de aço, Parte 1–1: Regras gerais e regras para edifícios; 2010 [in Portuguese].
Contents lists available at SciVerse ScienceDirect
Engineering Structures journal homepage: www.elsevier.com/locate/engstruct
The past 20 years of telecommunication structures in Portugal Rui Travanca ⇑, Humberto Varum, Paulo Vila Real Civil Engineering Department, University of Aveiro, Portugal
a r t i c l e
i n f o
Article history: Received 24 April 2012 Revised 16 October 2012 Accepted 18 October 2012 Available online 27 November 2012 Keywords: Towers Guyed masts Telecommunications Wind action Standards Failures
a b s t r a c t This paper reviews the analysis and design of telecommunication structures and presents the main problems observed for various types of structures. The nation of Portugal is selected for a case study, and more specifically, the evolution of Portuguese structural design standards for telecommunications systems is summarised using comparative analyses that cover a subset of the most relevant topics for design, including wind profiles, drag coefficients, dynamic effects and reliability classes. These analyses focus on characterisation of the effects of wind action, which plays a fundamental role in the behaviour and design of these structures. Following the comparative analyses of standards, the more common problems observed over the past 20 years in guyed masts and towers located in Portugal are presented and discussed. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
2. Standards evolution in design for wind action
Masts and towers represent the most important physical supports for the installation of the radio equipment used in transmission of the electromagnetic waves that provide such services of unquestionable importance as radio, television and/or mobile communications. Over the last few decades, the demand for these types of structures has increased due to the requirements for their use in the telecommunication sector. With the advent of mobile communications, this demand has become even greater. Unfortunately, the number of failures observed in telecommunications structures is high compared with other structures of equal economic and social importance. A great number of the failures observed are due to poor design, which results in unsafe structures that can suffer from full collapse [1–5]. The analysis and design of masts and towers requires specific knowledge and expertise, particularly with respect to guyed masts. Towers are lightweight structures with high slenderness and high flexibility. The newly adopted structural forms, the increasing strength of the materials used in construction and the consequent changes in structural stiffness, mass distribution and damping properties should lead to a new design approach, particularly for wind demands. The dynamic response becomes important when these structures exhibit a first natural frequency below 1 Hz. Therefore, dynamic analysis is necessary to determine whether the resonance response could be significant compared with the background response.
2.1. Generalities
⇑ Corresponding author. Tel.: +351 234370049; fax: +351 234370094. E-mail address: [email protected] (R. Travanca). 0141-0296/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engstruct.2012.10.012
Up until the 1970s, specific design procedures were not available for the analysis and design of notably tall and slender masts and towers. This lacuna was recognised by the industry in the early 1960s, when a rapid growth of these structures took place to accommodate the large expansion in radio and television broadcasting. As a consequence, a Working Group of the IASS (namely the WG4) was set up to study the behaviour of these structures and to produce the Recommendations for analysis and design. These Recommendations [6] were published in 1981 and formed the basis for the development of many additional national and international standards on this subject. At that time, the Recommendations contained such innovative ideas in structural design as structural reliability, the adoption of dynamic response procedures and the consideration of uncertainties associated with wind action. The innovative characteristics of such procedures were recognised by their adaption in the design standards for other types of structures and loadings [3–5]. The RSA [7], the Portuguese standard for accountable actions in the design of buildings and bridges, was published in 1983. Despite the special emphasis on buildings and bridges, other types of structures were also covered, including towers. It is important to note that the RSA was the only standard for action definition in Portugal. Even so, and as presented in this research, specific rules for the design of masts and towers were not adequately covered, which had both positive and negative effects with respect to safety. For example, ignoring the
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Nomenclature Latin upper-case letters A1 effective area total projected area when viewed normal to the face A2 (outer contour) Ac total projected area when viewed normal to the face of the circular-section members in the face in subcritical regimes Ac.sup total projected area when viewed normal to the face of the circular-section members in the face in supercritical regimes Ac1 effective area corresponding to subcritical circular-section members Ac2 effective area corresponding to supercritical circularsection members Aref reference area B2 background factor F global force actuation Fm,W mean wind load on the tower in the direction of the wind FT,W equivalent gust wind load on the tower in the direction of the wind Iv turbulence intensity L turbulent length scale R2 resonance response factor Latin lower-case letters cf force coefficient cf,0,c force coefficient for sections composed of subcritical circular-section members
cf,0,c,sup cf,S cf,S,0,j
cr cscd co fL h kp qp v vm w Z
force coefficient for sections composed of supercritical circular-section members wind force coefficient of the bare structure section force coefficients for sections composed of flat-sided, subcritical circular and supercritical circular-section members roughness factor structural factor Orography factor non-dimensional frequency height above the ground peak factor peak velocity pressure wind speed mean wind velocity wind dynamic pressure reference height
Greek upper-case letters W solidity ratio Greek lower-case letters dfc1 force coefficient for sections composed of subcritical circular-section members dfc2 force coefficient for sections composed of supercritical circular-section members k solidity ratio
2.2.1. Wind action Early codes treated wind action as a static wind pressure and often incorporated a factor to account for the drag of the structure. The dynamic effects were ignored in this procedure. Representative plots of extreme wind speeds are defined in the RSA [7], normalised to the design 50-year return period and specified as having
a 5% probability of not being exceeded in the structure lifetime. Table 1 shows the Portuguese territory zoning for the definition of wind action in both standards, i.e., the RSA [7] and the Portuguese National Annex for the Eurocode 1 (EC1) [8]. Similar to other standards of the 1970s and 1980s, power laws were applied to predict the distribution of the extremes of the wind loads. This approach has the advantage of simplicity and represents the observed profiles rather well [3]. However, a more accurate log-law procedure is now available and has been adopted in recent design standards such as the Eurocode 1 [8]. The wind speed profiles apply to all types of terrain, from smooth to rough, although areas of transition from one type to another require careful consideration. The RSA [7] only specifies two terrain categories (see Table 2). The Portuguese National Annex for the Eurocode 1 [8] contains a large differentiation that prescribes four categories for terrain (see Table 3). Additionally, the RSA does not account for the effect of terrain roughness and height in the turbulent component of the wind speed, prescribing instead a fixed value of 14 m/s for the two defined terrain categories. This value is also independent of the considered height. However, it is well known that the height and terrain roughness affects the mean wind speed as well as the turbulent component, i.e., the mean wind speed reaches a maximum value for smooth terrain, and the turbulence is minimal compared with that of rough terrain. Fig. 1 presents the wind pressure
Table 1 Portuguese territory zones (RSA [7] and Eurocode 1 [8]).
Table 2 Terrain roughness (RSA [7]).
dynamic effects of wind loading and an inadequate distribution of wind action over the height of the structure often resulted in an underestimation of the structural response. However, in many cases, this oversight was clearly offset by the use of conservative design criteria based on general standards for the design of steel structures that did not properly account for the behaviour of light slender lattice frames [3]. Nevertheless, the Annex I of the RSA specified that the variability of wind speed resulting from its turbulent nature can be assessed using spectral and spatial correlations of wind velocity, and this information should be available through specialised literature. More importantly, dynamic response identification procedures and consideration of ice loading are other examples of aspects that were not covered by the RSA [7]. With the implementation of the Eurocodes [8,9] all of these topics are finally covered by the national standards. 2.2. Comparative analysis between the RSA and Eurocodes
Zone A Zone B
The generalisation of Portuguese territory except for the regions belonging to zone B Azores and Madeira and mainland regions located not further than 5 km from the coast or at altitudes above 600 m
Type I Type II
Roughness of sites located within urban areas where medium and large buildings predominate Roughness of other locations, particularly rural areas and the outskirts of urban areas
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Table 3 Terrain categories (Eurocode 1 [8]). Cat. I Cat. II Cat. III
Cat. IV
Coastal area exposed to the open sea Areas with low vegetation, such as grass and isolated obstacles (trees, buildings) with separations of at least 20 obstacle heights Areas with regular cover of vegetation or buildings or containing isolated obstacles with separations of no more than 20 obstacle heights (i.e., villages, suburban terrain, permanent forest) Areas in which at least 15% of the surface is covered with buildings with an average height that exceeds 15 m
profiles for both codes. Despite the different premises of these two standards, it can be observed from Fig. 1 that the profile type I (RSA) fits well with the profile Cat. IV (EC1). Additionally, the profile type II (RSA) has similar equivalence with categories II and III (EC1), producing a substantial distinction in the effect of the ground roughness. However, category I (EC1) does not match any comparable profile in the RSA (see also Tables 2 and 3). 2.2.2. Topography effects The major topographic features (i.e., escarpments, hills and ridges) have strong effects on the wind pressure distribution
(a) Zone A
(b) Zone B Fig. 1. Wind pressure profiles (RSA [7] and Eurocode 1 [8]).
R. Travanca et al. / Engineering Structures 48 (2013) 472–485
(a) RSA [7]
475
(b) Eurocode 1 [8] Fig. 2. Topography effects.
because the wind that flows over these features is modified by their presence. The topographic features act as clear obstructions to the boundary layers and accelerate the wind near the ground, leading to increased wind pressure on obstacles located in such regions. Because of its specified purpose, telecommunication structures are frequently located at points of higher elevation. Many standards include procedures for estimating the so-called ‘speed-up effect’, and most use different methods to account for the increase in the design wind speed or pressure. Differences in topography as well as terrain height effects will lead to different design wind pressures from each of these standards. Recently updated standards, e.g., the Eurocode 1 [8], provide detailed procedures to determine wind loads that account for the speed-up effect due to topographic features [3,5]. Thus, the procedures included in the RSA [7] are out of date when compared with current state of knowledge; the RSA procedures do not consider the speed-up effect, are based on corrections at the height of the base of the structure and take into account the position and the slope of the terrain only. Fig. 2 illustrates the different approaches considered by the RSA [7] and the Eurocode 1 [8].
2.2.3. Drag coefficients Because the structural analysis and design of lattice towers are generally undertaken in the face-on and 45° directions only, the RSA [7] prescribes drag coefficients only for the wind acting on the face and at the corner of the structure, and this last category is applicable to towers with a square base. However, according to the procedures prescribed in Part 3–1 of the Eurocode 3 [9], wind action can be determined for any angle of incidence. As prescribed in the RSA, the drag coefficients applied for lattice towers are more conservative in general than those presented in the Eurocode 3 [9]. This observation can be confirmed in Fig. 3. As far as the authors are aware, no experimental results exist for solidity ratios greater than 0.6, but as the solidity ratio tends to 1, the drag coefficient should clearly tend to a solid bare section. It should be noted that Part 3–1 of the Eurocode 3 [9] includes methods that account for one of the most difficult topics to standardise, i.e., the wind resistance of such non-structural elements as ladders, feeders, platforms or antennas. Additional research is needed on this topic. The non-structural elements mounted on these structures can take on a wide variety of forms in terms of position and shapes. As reported by several authors, it has become common to conduct a separate analysis of the structural and non-structural elements without correctly considering the interference between these different elements [3,5,10].
2.2.4. Dynamic effects The dynamic analysis of towers may be performed in the frequency domain based on the characteristics that depend on the frequencies of both of the wind action and the structural properties of the structure. In this approach, a wind gust is characterised on a probabilistic basis, i.e., by statistical descriptions of relevant properties such as frequency content and spatial distribution. However, simplified procedures may be used under certain conditions. According to the Eurocodes [8,9], a quasi-static analysis may be adopted using the appropriate gust response factors. These factors depend on several parameters, including the fundamental frequency, the damping of the structure and the wind action properties. Thus, the peak pressure is amplified by the allocation of a gust response factor and is subsequently treated as a static action. The RSA [7] provides a static analysis that only applies if the fundamental frequency exceeds 0.5 Hz. Therefore, the fundamental frequency is used only to verify and validate the applicability of this simplified procedure. The dynamic effects are included in the Eurocodes [8,9]. For guyed masts, such procedures may not be appropriate because the dynamic modes are not well separated. Additionally, it is difficult to perform dynamic analysis using conventional methods. In response to the gusty winds, the complex interactions between the mast and the guy ropes typically result in a large number of active vibration modes. Additionally, aerodynamic damping forces acting on the guys provide an effective means for suppressing resonance in the mast. The dynamic analysis is further complicated by the random nature of wind loads, which vary both in time and location on the mast [3]. Although dynamic analysis methods are available and are able to address all of these factors, these methods are complex and require specialised computer software. Consequently, procedures have been developed that simulate a full dynamic response analysis using static analysis through patch loading techniques. Such patch wind models were first introduced in the IASS Recommendations [6], and currently, this model has been refined and adopted in selected National Codes as well in Part 3–1 of the Eurocode 3 [9]. The RSA [7] does not cover guyed masts. Consequently, it is not possible to directly compare the dynamic effects. Nonetheless, in the case study presented in this paper, the influence of this parameter on the overall results is appraised when applicable to a self-supported lattice tower. 2.2.5. Towers with inclined legs The wind speed at any instant can be described as the sum of an average speed and a fluctuating part. The fluctuating part represents the turbulent wind component and is consequently variable both in time and location. Therefore, a cautious approach is required to obtain the resultant forces on the brace members for
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(a) Triangular structures
(b) Square structures (wind on face) Fig. 3. Drag coefficients (RSA [7] and Eurocode 3 [9]).
towers that include inclined legs, also referred to as ‘‘Eiffelated’’ towers, because the application of the gust wind to all of the height can lead to small forces on the bracing members [3,5]. Traditionally, static procedures (such as that prescribed in the RSA [7]) that use a uniform gust wind profile could indicate lower forces in such bracings, which can lead to unsafe structural design. Current procedures (such as the one prescribed in the
Eurocode 3 [9]) avoid this situation with a patch loading approach. 2.2.6. Reliability classes The principle of three reliability classes for masts and towers was established in 1981 by the IASS Recommendations [6] and was followed by all of the other relevant standards [3,5]. For
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(c) Square structures (wind on corner) Fig. 3. (continued)
Table 4 Reliability differentiation for masts and towers [9]. Class 1
Class 2 Class 3
Masts and towers built on unmanned sites in open countryside; masts and towers in which the failure would not be likely to cause injury to people All masts and towers that cannot be defined as class 1 or 3 Masts and towers erected in urban locations or where failure is likely to cause injury to people
example, Part 3–1 of the Eurocode 3 [9] introduces three classes of reliability (Table 4) corresponding to different partial safety factors (Table 5) and applies them to permanent and variable actions. The RSA [7] makes no such distinctions and applies the safety factors used for current structures, e.g., 1.5 for variable actions if unfavourable. As stated previously, this practice undoubtedly leads to conservative design criteria that fortunately offset the underestimation of the structural response. In the case study presented in this paper, this counterbalance is discussed in additional detail. 2.3. Case study In the case study presented, a self-supported lattice tower with a triangular base and a height of 40.5 m is analysed (Fig. 4). This analysis focuses only on the wind action and does not consider any exposed areas related to such non-structural elements as ladders, feeders, platforms or antennas. The main differences found in the results obtained when using different standards (i.e., RSA [7] and Eurocodes [8,9]) will be presented, compared and discussed. In this analysis, it was assumed that the lattice tower is located in Portugal in a rural region far from the coast at an altitude of 200 m. For quantification of the wind action, we assumed that the structure is located in zone A (according to RSA [7] and the Portuguese National Annex for the Eurocode 1 [8]) with a soil roughness of type II (according to RSA [7]) and a terrain category of type
Table 5 Partial factors for actions [9]. Type of effect
Reliability class
Permanent actions
Variable actions
Unfavourable
1 2 3
1.0 1.1 1.2
1.2 1.4 1.6
1.0 1.0
0.0 1.0
Favourable All classes Accidental situations
II (according to the Portuguese National Annex for Eurocode 1 [8]). To simplify the case study, the effects of topography are not taken into account. The elastic forces and natural frequencies were obtained using SAP2000 software from Computers and Structures, Inc. [11]. The value obtained for the fundamental frequency was 1.90 Hz. The Appendix A of the paper presents the determination of the acting forces due to the wind action applied only to the bare structure using the RSA [7] and the Eurocodes [8,9]. Table 6 compares the results obtained with these two distinct standards in terms of the following: (i) shear force at the base (F), (ii) axial force in the legs located at the base (N1), (iii) axial force in the bracing members located at the base (N2), (iv) design force in the legs located at the base (NSd,1) and (v) design force in the bracing members located at the base (NSd,2). Although the difference in shear force at the base presents an increase of 13.5% over the value determined by Eurocodes [8,9] in relation to the RSA [7], it can be observed that the difference in axial force for the legs located at the base is higher (16.9%). This increase is also justified by the difference found in the wind pressure distribution for the height and, as a result, in the static forces applied. However, this disparity is even larger for bracing members with a difference of 29.6%. This result is justified not only by the wind pressure distribution at the height but also by the patch loading approach prescribed in Eurocodes [8,9]. All of these differences are attenuated in the design forces considered in the combination of actions. For reliability
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(a) Photograph
(b) Draft
Fig. 4. Self-supported lattice tower with a triangular base.
Table 6 Overall comparison of the results. F (kN)
N1 (kN)
N2 (kN)
NSd,1 (kN) Class 1
RSA [7] EC [8,9] Dif. (%)
29.79 33.81 13.5
239.35 279.74 16.9
5.77 7.48 29.6
359.03 335.69 6.5
NSd,2 (kN) Class 2
Class 1
Class 2
391.64 9.1
8.66 8.98 3.7
10.47 21.0
class 1, the bracing members present only a slight difference (3.7%), and there is an inversion of the axial force for the legs located at the base ( 6.5%). For reliability class 2, the difference of the design forces in the legs located at the base is 9.1% and 21.0% for the bracing members. In this study, the reliability class 1 would be appropriate, and thus, the differences obtained and presented could be considered as negligible. 3. Structure typologies used in the telecommunication sector Telecommunication structures typologies vary widely across countries according to their uses, location and more commonly used materials. For example, in Portugal and Spain, where masts and towers are often installed in locations 500 m above the ground, the height of the structures will reach 60 m. In contrast, in Denmark and Holland, where the elevation of the land is practically zero, masts and towers with heights of over than 250 m are common [3]. The weight of a self-supported tower and thus its cost will vary approximately with the square of the height. For a guyed mast, the cost will vary proportionally with the
expression H1.5, where H is the height of the tower [3]. Self-supported towers up to 50 m high are commonly used throughout Europe for supporting antennas and are normally more suitable in terms of the overall costs compared with guyed masts, despite the higher quantity of steel used. The construction, excavation, land cost and subsequent maintenance leads to a more economical solution due to unnecessary replacement of guys, a simple operation that nonetheless involves additional time and therefore additional costs. However, along with the elimination of guys and lowland occupancy, another great advantage of self-supported lattice towers is their high torsional stiffness, an important aspect in the use of directional antennas (links) that impose a loading eccentricity due to wind action [3,5]. In this study, and for the simplicity of presentation of the results and analysis, the structures were grouped into six distinct typologies as described in Table 7 and illustrated in Fig. 5. Table 7 General description of structures typologies. Typology 1 Typology 2 Typology 3
Typology 4 Typology 5 Typology 6
Self-supported lattice tower with a square base, generally with angle profiles and bolted connections Self-supported lattice tower with a triangular base, generally with tubular profiles and with bolted connections Monopole with octagonal, dodecagonal or hexadecagonal cross-sections, with several modules generally linked by forced fit Monopole of tubular cross-section, with several modules linked by bolted connections Lattice guyed mast with triangular or square sections This typology groups all other types of towers not included in the other typologies
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Typology 1
Typology 2
Typology 3
Typology 4
Typology 5
Typology 6
Fig. 5. Structures typologies.
Table 9 Description of levels of structural integrity.
Table 8 Distribution of structures by typologies.
Level 1: comply
Typology
Qty.
%
1 2 3 4 5 6
42 96 136 80 18 13
10.9 24.9 35.3 20.8 4.7 3.4
The data collected, presented and discussed in this research are not referenced for reasons of professional confidentiality. However, it is possible to indicate that the maximum height of the structures analysed in this study is 100 m, and the average height stands at 35 m. Additionally, all of the research work is based on design projects, inspection reports and/or structural assessment studies of all structures. Table 8 presents the distribution of the 385 structures used in this research according to typology and can be extrapolated to all of the structures used in the telecommunication sector in Portugal. As an example, a clear observation from Table 8 is the preponderance of monopoles (typologies 3 and 4), representing 56.1% of all structures studied. It is also possible to verify that self-supported lattice towers with square or triangular bases (typologies 1 and 2) represent 35.8% of all structures. The higher propensity for the use of monopoles can be justified by their visual impact, an aspect that will not be discussed in this paper.
Level 2: comply/ limited
Level 3: not comply
Level 4: not comply/ reinforcement
Level 5: not comply/ replacement
The structure meets all requirements of structural safety for the considered loads and the applicable national standards The structure meets all requirements of structural safety for the considered loads and the applicable national standards but is already at its maximum capacity such that any increases in loading must be preceded by structural strengthening The structure does not meet all requirements of structural safety for the considered loads and the applicable national standards; no instructions are given for whether the structure should be strengthened or replaced The structure does not meet all requirements of structural safety for the considered loads the applicable national standard; explicit instructions are given that the structure should be strengthened The structure does not meet all the requirements of structural safety for the considered loads and the applicable national standards; explicit instructions are given that the structure should be replaced
4. Main problems observed in Portugal From the analysis of the problems recently observed in structures used in the telecommunication sector in Portugal, it is reasonable to conclude that design errors are the most frequent cause for collapse or early replacement events. In addition, and in relation to the aspects mentioned previously, market pressure demanding lighter and more economical solutions has generated
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R. Travanca et al. / Engineering Structures 48 (2013) 472–485 Table 10 Distribution of the structures by levels of structural integrity. Level
Qty.
%
1 2 3 4 5
307 42 6 21 9
79.7 10.9 1.6 5.5 2.3
Table 11 Distribution by levels of structural integrity of structures in typology 1. Level
Qty.
%
1 2 3 4 5
17 10 1 14 0
40.5 23.8 2.4 33.3 0.0
Table 12 Distribution by levels of structural integrity of structures in typology 2. Level
Qty.
%
1 2 3 4 5
80 12 1 3 0
83.3 12.5 1.1 3.1 0.0
Table 13 Distribution by levels of structural integrity of structures in typology 3. Level
Qty.
%
1 2 3 4 5
129 7 0 0 0
94.9 5.1 0.0 0.0 0.0
(a) Collapse of a type 3 monopole
Table 14 Distribution by levels of structural integrity of structures in typology 4. Level
Qty.
%
1 2 3 4 5
63 12 3 1 1
78.8 15.0 3.8 1.2 1.2
structures with limitations in terms of fatigue safety in certain cases. A definition of the basic requirements for structural design is a key issue. To address this need, proper communication between the designer and client is necessary to create an economical structure with adequate safety and performance. Unfortunately, as stated by Støttrup-Andersen [1], the combination of an inexperienced client and a designer who is not familiar with the special problems associated with the design of masts and towers is common. Over the years, this combination has created many structures that are not suitable for their intended purpose: they may be either too safe, or in the worst case, they may be unsafe. The latter case may lead to damage or even the total collapse of the structures. This paper highlights the observed features related to structural integrity and safety. For data analysis and discussion, five levels of structural integrity were defined (Table 9). The reference to the national standards only covers the RSA [7] for action definitions because the Eurocodes are still optional at present. In fact, in all structural design projects and/or studies covered in this research work, the Eurocode 1 [8] was not used for action definitions. One conclusion arising from this research that applies to all typologies is that, despite the limitations of the RSA already presented and discussed in this paper, in many cases, the RSA itself was not applied properly for the wind action definition. Thus, certain existing structures may not contain adequate safety measures. Hereafter, this issue will be defined as the ‘‘improper definition of wind action’’. In addition, we note that level 2 represents the scenario in which a compromise was reached between the structural designer and the operator for the definition of the solution for that specific structure. In many cases, this solution involves a reduction of exposed areas with the elimination of platforms and the replacement of antennas or other non-structural elements to reduce the exposed area. The global analysis for the 385 structures is shown in Table 10. The largest group of 307 structures (79.7%) is classified at level 1.
(b) Detail at the base
Fig. 6. Collapse of a monopole of typology 3.
R. Travanca et al. / Engineering Structures 48 (2013) 472–485
(a) General view of the type 4 monopole
481
(b) Detail of rupture
Fig. 7. Collapse of a monopole with a tubular cross-section.
Table 15 Distribution by levels of structural integrity of structures in typology 5. Level
Qty.
%
1 2 3 4 5
12 0 1 2 3
66.7 0.0 5.5 11.1 16.7
The remaining 78 structures (20.3%) are classified at levels 2, 3, 4 or 5, and are therefore limited in use, indicated for strengthening or subject to explicit instructions for replacement. For a deeper analysis of the results, the next sections present the results per typology as well as the most probable associated causes. 4.1. Main problems observed in structures of typology 1 Table 11 shows the distribution by level of structural integrity for structures in typology 1, i.e., self-supported steel lattice towers with a square base. As shown in Table 11, strengthening was required for 14 towers (33.3%); this reinforcement can be achieved by the introduction of angles in the legs and with duplication of the bracing members. Ten towers (23.8%) are classified at level 2 or are limited in use, but for these cases, it is always possible to add structural strengthening. The main reasons for the observed problems that lead to the need for strengthening are the following:
(a) Guyed mast 15 m high
(i) errors in structural design mainly due to the improper definition of wind action, (ii) additions to the height of the tower without strengthening of the existing structure and (iii) loading beyond the capacity of the structure. 4.2. Main problems observed in structures of typology 2 Table 12 presents the distribution of structural integrity for structures classified in typology 2, i.e., self-supported lattice towers with a triangular base. The results indicate that for this typology, 80 towers (83.3%) are classified in level 1, 12 towers (12.5%) are limited in use, and only four towers (4.2%) present severe problems. Despite the overall result obtained for this typology, which is clearly positive compared with other typologies with a small number of serious problems, it is possible to indicate the following main causes that lead to the need for structural strengthening: (i) errors in design particularly due to improper definition of wind action, (ii) excessive loading beyond the carrying capacity of the structure and (iii) errors in assembly and elimination of structural elements that may compromise structural safety. 4.3. Main problems observed in structures of typology 3 Table 13 presents the distribution by levels of structural integrity of structures in typology 3, i.e., self-supported monopoles with
(b) Detail of crack in masonry wall
Fig. 8. Ineffective guy system.
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(a) Guyed mast 15 m high
(b) Detail of anchorage in a cantilever steel element
Fig. 9. Anchorages in flexible elements.
(a) Collapse of a 21 m guyed mast
(b) Detail of the mast base after collapse
Fig. 10. Problems in anchorages.
(a) Detail of concrete slab
(b) Detail of anchorage
Fig. 11. Pullout of the anchorages in concrete.
octagonal, dodecagonal or hexadecagonal cross-sections. From the analysis of the results, it can be concluded that this typology displays a higher success rate, with 129 monopoles (94.9%) in level 1 and only 7 monopoles (5.1%) in level 2, i.e., limited in usage. Therefore, there is no record of serious problems for this typology.
Although the scenario observed is quite positive, it is important to note that this typology in general exhibits a fundamental frequency close to that of the wind spectrum, and therefore, the dynamic response of the structure can be greatly amplified. This issue was not addressed adequately in the RSA [7]. To highlight this
R. Travanca et al. / Engineering Structures 48 (2013) 472–485 Table 16 Distribution by levels of structural integrity of structures in typology 6. Level
Qty.
%
1 2 3 4 5
6 1 0 1 5
46.1 7.7 0.0 7.7 38.5
assertion, measurements were taken on two self-supported monopoles of typologies 3 and 4 using FBG-based accelerometers as described in a previous paper [12]. The values obtained for the fundamental frequency in these two monopoles were 0.66 Hz and 0.41 Hz [12]. Fig. 6 shows the collapse of a monopole of typology 3 in which it is possible to observe the rupture at its base. 4.4. Main problems observed in structures of typology 4 Table 14 presents the distribution of structural integrity for structures in typology 4, i.e., monopoles with tubular crosssections. The overall results are considered positive, with 63 monopoles (78.8%) classified in level 1, 12 monopoles (15.0%) limited in use and 5 monopoles (6.2%) presenting serious problems. However, as mentioned for the monopoles in typology 3, many of the structures in this typology also display a frequency close to that of the wind spectrum, and therefore, the dynamic response can be largely amplified. It is also important to note that these structures, especially those with heights larger than 30 m, are usually composed of tubular steel profiles with a diameter/thickness ratio greater than 100. It is known that the resistance of these profiles is rather sensitive to imperfections (snap-through behaviour). In all documents reviewed for this study, the phenomena of local plate instability were not accounted for in estimating the resistance of thin-walled steel plates, and therefore, the resistance of the structure may have been overestimated. Another important point to underline through the analysis of the various documents used in this research, and that justifies the overly optimistic scenario, was the disregard for two important phenomena in the analysis, namely, (i) vortex-shedding and (ii) fatigue. If the frequency caused by vortex-shedding coincides with the fundamental frequency of the structure, resonant vibrations may occur. Because
(a) Lack of verticality of the monopole
483
this condition occurs for a critical wind speed that is often observed, fatigue also may become relevant depending on the number of cycles and the loading [3,5,13,14]. As an example of these two linked phenomena, the collapse of a monopole with a tubular cross-section, is shown in Fig. 7. 4.5. Main problems observed in structures of typology 5 Table 15 presents the distribution by levels of the structural integrity of the structures classified in typology 5, i.e., lattice guyed masts with triangular or square sections. Although the results presented do not highlight the fact significantly, with 12 masts (66.7%) in level 1 and 6 masts (33.3%) presenting severe problems, this typology certainly requires particular attention. The scenario presented may represent an optimistic scenario given the large number of collapses registered in recent years. Based on the samples studied, the value of 33.3% presented in this study may underestimate the actual problem. The authors of this research deem it relevant to underline this issue despite the results obtained, presented and discussed in this work. Guyed masts are structures that require complex analysis, and many experts have stated that ‘‘a guyed mast is one of the most complicated structural problems that an engineer may face’’ [1– 5]. This statement is unfortunately underlined by the fact that the number of collapses of these masts is far greater than that for other structures. As mentioned previously, the first error falls on the choice of the structure: for small structures, the choice of guyed masts does not seem to be suitable for the reasons presented in Section 3. In the cases presented in this study, which are all related to small guyed masts, the failures were caused by design errors and/or bad construction practices, e.g., ineffective guy systems and/or deficient anchorages, as shown in the four cases illustrated in Figs. 8–11. Fig. 8 illustrates a small guyed mast with a square section and a height of 15 m, with one guy level at 9 m composed of two guys with opening angles of 180°, i.e., with a guy system that is totally ineffective. Furthermore, other serious deficiencies have been detected, including inappropriate anchorages in masonry walls that present severe cracks and severe corrosion in steel members. Fig. 9 presents a small guyed mast with a triangular section and a height of 15 m with two levels of guys with opening angles of 120° and with two guys fixed to a cantilevered steel profile.
(b) Rupture of steel plates at corners
Fig. 12. Monopole with a square cross-section and a 40 m height.
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Fig. 10 shows a 21 m guyed mast with a square section that collapsed due to problems with the anchorages. Fig. 11 presents the details of a small guyed mast with a triangular section and a height of 12 m with two guy levels at 6 m and 12 m, composed of three guys per level; this structure collapsed after pullout of the concrete anchorages. The main causes for the problems observed in this typology are the following: (i) gross errors in design (e.g., a poor guy system, ineffective anchorage, incorrect assessment of the resistance and/ or stiffness of steel members, improper definition of wind action as prescribed in RSA) and (ii) gross errors in construction (e.g., anchorages placed in masonry walls or other weak or fragile elements, inadequate tensioning or lack of guy re-tensioning). 4.6. Main problems observed in structures of typology 6 Table 16 shows the distribution by levels of the structural integrity of the structures classified in typology 6, i.e., all types of structures not previously mentioned. Although the table presents a scenario that could be considered rather negative, with six structures (46.1%) in level 1, one structure (7.7%) in level 2 and six structures (46.2%) presenting severe problems, it must be noted that this typology only contains 13 structures, and the large number of problems observed are related to a single type of structure from a single manufacturer that contained serious design errors leading to the replacement option. Additionally, for this typology, the primary cause of the noted problems is related to design errors. Fig. 12 presents the case of a 40 m monopole formed by cold-form steel plates with a square cross-section and a width that varies with the height. This monopole is composed of eight different modules with a height of 5 m each linked together with bolted connections at the corners. The dimensions at the base of the monopole include a 1.2 m width and a 6 mm plate thickness, and at the top, a 0.54 m width and a 5 mm plate thickness. Several problems were detected, namely, the lack of verticality of the monopole, buckling of the steel plates and rupture of steel plates in various locations, as shown in Fig. 12. In the analysis of the de-
sign project, it was possible to identify several errors as follows: (i) improper definition of wind action and (ii) overestimation of the resistances of the cross-sections, i.e., an elastic analysis was performed without area reduction (effective cross-sectional area of the compression zone of the plate [9,15]) in sections classified as class 4, according to Part 1–1 of the Eurocode 3 [16]. 5. Conclusions This study presents the main differences observed between the former Portuguese national standard (RSA) and the Eurocodes 1 and 3; it also compares the relevant parameters and principles related to the design of telecommunication structures. One case study was presented for the comparison between these standards. This work also highlighted the considerable progress in development that occurred between these standards, which are separated by nearly three decades. Despite the obvious differences, which were presented and discussed throughout the paper, it was concluded that a self-supported lattice tower with a triangular base designed according to the RSA standard should present no major safety problems when assessed for safety according to the specifications in the Eurocodes. However, the same cannot be concluded if the reliability class is higher or for structures with lower fundamental frequencies for which the structural response would be greatly amplified. These aspects were not considered in the procedures included in the RSA. The main problems observed in telecommunication structures in Portugal are summarised. Based on the analysis of 385 structures located in Portugal, it was possible to identify the most common problems, and it was concluded that these problems are mainly a consequence of design errors. As observed in this research, the number of failures in this type of structures is high compared with other structures of similar importance. Appendix A Tables A.1– A.4.
Table A.1 Determination of wind forces (RSA [7]). Panel
h (m)
V (m/s)
W (kN/m2)
Ac1 (m2)
Ac2 (m2)
A1 (m2)
A2 (m2)
K
dfc1
dfc2
F (kN)
1 2 3 4 5 6 7
6.00 12.00 18.00 24.00 30.00 36.00 40.50
39.00 39.93 42.12 43.78 45.14 46.30 47.07
0.93 0.98 1.09 1.18 1.25 1.31 1.36
1.42 1.16 0.80 0.76 0.61 0.56 1.36
2.63 2.63 2.63 2.63 2.32 2.02 0.00
4.05 3.79 3.43 3.39 2.93 2.58 1.36
16.61 12.41 10.31 10.31 10.16 10.01 7.26
0.24 0.31 0.33 0.33 0.29 0.26 0.19
1.55 1.50 1.49 1.49 1.51 1.54 1.61
1.13 1.09 1.07 1.08 1.11 1.13 1.08
4.83 4.51 4.37 4.66 4.37 4.13 2.98
Table A.2 Determination of the structural parameters (Eurocode 1 [8]). vm (m/s)
Iv
L (m)
fL
SL
B2
gh
gb
Rh
Rb
R2
V (Hz)
kp
cs.cd
34.36
0.15
130.71
7.23
0.04
0.69
10.30
0.55
0.09
0.71
0.24
0.97
3.74
1.02
Table A.3 Determination of exposed areas and drag coefficients (Eurocode 3 [9]). Panel
Ac (m2)
Ac.sup (m2)
As (m2)
Ac (m2)
w
K1
K2
cf,0,c
cf,0,c,sup
cf,S,0,j
cf,S
cf
Aref (m2)
1 2 3 4 5 6 7
1.42 1.16 0.80 0.76 0.61 0.56 1.36
2.63 2.63 2.63 2.63 2.32 2.02 0.00
4.05 3.79 3.43 3.39 2.93 2.58 1.36
16.61 12.41 10.31 10.31 10.16 10.01 7.26
0.24 0.31 0.33 0.33 0.29 0.26 0.19
0.80 0.80 0.80 0.80 0.80 0.80 0.80
0.24 0.31 0.33 0.33 0.29 0.26 0.20
1.42 1.35 1.32 1.33 1.36 1.40 1.50
1.09 1.09 1.10 1.10 1.09 1.09 1.08
1.20 1.17 1.15 1.15 1.15 1.15 1.50
1.20 1.17 1.15 1.15 1.15 1.15 1.50
1.20 1.17 1.15 1.15 1.15 1.15 1.50
4.05 3.79 3.43 3.39 2.93 2.58 1.36
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R. Travanca et al. / Engineering Structures 48 (2013) 472–485 Table A.4 Determination of wind forces (Eurocode 1 and 3 [8,9]). Panel
z (m)
cr
co
vm (m/s)
Iv
qp (kN/m2)
cf
Aref (m2)
Fm,W (kN)
FT,W (kN)
1 2 3 4 5 6 7
6.00 12.00 18.00 24.00 30.00 36.00 40.50
0.91 1.04 1.12 1.17 1.22 1.25 1.27
1.00 1.00 1.00 1.00 1.00 1.00 1.00
24.56 28.12 30.20 31.67 32.82 33.75 34.36
0.21 0.18 0.17 0.16 0.16 0.15 0.15
0.93 1.13 1.25 1.34 1.41 1.47 1.51
1.20 1.17 1.15 1.15 1.15 1.15 1.50
4.05 3.79 3.43 3.39 2.93 2.58 1.36
1.83 2.19 2.25 2.44 2.26 2.12 1.50
4.62 5.14 5.13 5.51 5.11 4.83 3.46
References [1] Støttrup-Andersen U. Analysis and design of masts and towers. In: Proceedings of the international symposium on lightweight structures in civil engineering. Warsaw: Ed. Obrebski, JB; 2002. p. 137–66. [2] Støttrup-Andersen U. Mast and towers. In: Domingo A, Lazaro, C. editors, Proceedings of the International Association for Shell and Spatial Structures (IASS) symposium. Valencia; 2009. p. 127–38. [3] Smith B. Communication structures. London: Thomas Telford; 2007. [4] Smith B. 50 years in the design of towers and masts. From IASS recommendations to current procedures. In: Domingo A, Lazaro C. editors, Proceedings of the International Association for Shell and Spatial Structures (IASS) symposium. Valencia; 2009. p. 139–52. [5] Travanca R. Torres para radiocomunicações: Patologias e dimensionamento, master thesis. University of Aveiro; 2010 [in Portuguese]. [6] IASS Working Group 4. Recommendations for the design and analysis of guyed masts. In: International association of shell and spatial structures. Madrid; 1981. [7] RSA. Regulamento de Segurança e Acções para Estruturas de Edifícios e Pontes. Decreto-Lei n.° 235/83 de 31 de Maio; 1983 [in Portuguese]. [8] CEN. Comité Europeu de Normalização. NP EN 1991–1-4, Eurocódigo 1 – Acções em estruturas, Parte 1–4: Acções do vento; 2010 [in Portuguese].
[9] CEN. European Committee for Standardisation. EN 1993–3-1, Eurocode 3 – design of steel structures, Part 3–1: towers, masts and chimneys – towers and masts; 2006. [10] Carril Jr C, Isyumov N, Brasil R. Experimental study of the wind forces on rectangular latticed communication towers with antennas. J Wind Eng Ind Aerod 2003;91:1007–22. [11] CSI. SAP2000 version 9: Basic analysis reference manual. California: Computers and Structures Inc. Berkeley; 2004. [12] Antunes P, Travanca R, Varum H, André P. Dynamic monitoring and numerical modelling of communication towers with FBG based accelerometers. J Constr Steel Res 2012;74:58–62. [13] Repetto M, Solari G. Dynamic alongwind fatigue of slender vertical structures. J Eng Struct 2001;23:1622–33. [14] Jia J. Wind and structural modelling for an accurate fatigue life assessment of tubular structures. J Eng Struct 2011;33:477–91. [15] CEN. Comité Europeu de Normalização. NP EN 1993–1-5, Eurocódigo 3 – Projecto de estruturas de aço, Parte 1–5: Elementos estruturais constituídos por placas; 2012 [in Portuguese]. [16] CEN. Comité Europeu de Normalização. NP EN 1993–1-1, Eurocódigo 3 – Projecto de estruturas de aço, Parte 1–1: Regras gerais e regras para edifícios; 2010 [in Portuguese].