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Shore overloads during shoring removal
Engineering Structures 32 (2010) 3629–3638
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Shore overloads during shoring removal M. Azkune, I. Puente ∗ , A. Santilli Department of Mechanical Engineering, Institute of Civil Engineering, Tecnun (University of Navarra), Manuel de Lardizabal 13, 20018 San Sebastian, Spain
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Article history: Received 2 September 2009 Received in revised form 24 May 2010 Accepted 9 August 2010 Available online 9 September 2010 Keywords: Shore removal Multistory buildings Flat concrete slabs Overloads Falsework Concrete construction Shoring Reshoring
abstract The considerable overloads originating during the shore removal process can affect the structural safety of a multistory concrete building under construction. With an incorrect shoring removal reversal of stresses may occur which can cause concrete cracking and excessive deflections. Moreover, excessive overloads will damage the falsework, producing important economical losses due to its elevated cost. A measured program has been conducted during the shore stripping at different building levels. Results show that in general the Refined Method is adequate in a conservative form for the shore removal procedure. Then, the influence of different parameters such as concrete strength or steel reinforcement in shore removal overloads was studied. The use of the actual concrete strength and modeling the steel reinforcement produces a more accurate theoretical result. Nevertheless, these considerations do not bring major changes (less than 4%). Finally, shore overloads originating in five different shore removal procedures in a typical structure have been compared, establishing some criteria for a safe shore stripping sequence. For example the best stripping procedure found consists of removing shores by rows. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction For economic reasons, there is ever more pressure to construct multistory concrete buildings at a faster pace. Thus, the strength of a slab usually is not enough to support an upper floor when it is cast. Consequently, self-weight of a newly poured slab is distributed between lower partially hardened slabs interconnected by shores and/or reshores. Moreover, an excessive number of shored floors are not advisable. To keep the structure shored to the foundation would increase construction costs considerably, due to the fact that the excessive number of shores and falsework needed would be too high. This procedure could also result in exceeding ultimate shore loads on the lower floors. The temporary supporting structure is removed when the slab is sufficiently resistant which allows the liberated shores to be used for the construction of upper floors. This reduces the number of shores needed and allows work by other trades to proceed on the lower floors. Structural safety, however, is also of paramount importance. Load carrying capacities of shores and slabs must not be exceeded. A significant percentage of structural collapses during construction is caused by excessive loads on both the shoring system and
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Corresponding author. Tel.: +34 943219877; fax: +34 943311442. E-mail address: [email protected] (I. Puente).
0141-0296/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2010.08.007
interconnected slabs. The accidents that occurred in Cocoa Beach, Florida in 1981 reported by Lew et al. [1] and in Fairfax County, Virginia in 1973 reported by Carino et al. [2], where a large number of workers lost their lives, are well known examples. It is recognized that a detailed knowledge of shore-slab interaction may well have prevented some of these accidents. Nevertheless, due to factors such as the variable nature of concrete properties, determining load values and distribution between shores and slabs is complex. Several research studies have been conducted to accurately determine the strength properties of a concrete building during construction. Concrete has been studied by authors such as Price [3], Klieger [4], Gardner and Poon [5] or Carino et al. [6], who have analyzed the influence of factors such as curing temperature in concrete strength evolution. The first known research paper on the subject of load estimation was published in 1952 when Nielsen [7] presented a method which was too complex to be used in practice. In 1963 Grundy and Kabaila [8] developed the Simplified Method. This pioneering work was based on the following assumptions, which make the method clear and easy to apply: 1. Relative to the bending stiffness of slabs, the axial stiffness of shores and reshores is assumed to be infinite. 2. Despite the fact that concrete properties vary with age, all slabs are assumed to possess equal flexural stiffness. 3. The lowest level of shores or reshores is assumed to be supported on a completely rigid foundation.
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This Simplified Method is the most widely used method in the construction industry, and is also suggested in the shoring/ reshoring guide published by ACI Committee 347, 2R-05 [9]. Other theoretical models have been proposed in recent years. In general, these models include modifications that try to improve the results achieved by the Simplified Method, as well as to verify the accuracy of this method. In 1985 Liu et al. [10] developed the Refined Method. This finite element based computational method proposes a more realistic model of the actual structure. However, this method is cumbersome for practical use. Liu et al. [11] compared on-site measurements with results obtained by both the Simplified and the Refined Methods, and concluded that the Refined one predicts more accurate values. Nevertheless, they pointed out that the Simplified Method can be used if the results for the maximum shore load and slab moments are corrected by a modification coefficient that varies from 1.05 to 1.10. Other models have been developed by Gardner [12], Stivaros and Halvorsen [13], Chen and Mossallam [14], Mossallam and Chen [15], El-Shahhat and Chen [16], Duan and Chen [17], Fang et al. [18] and Miranda de Almeida et al. [19]. But the significant amount of theoretical research published contrasts with the absence of detailed experimental data related to the subject area. The first on-site measurements consisted of measuring shore loads and comparing experimental data with the Simplified Method. For example, Agarwal and Gardner [20], Lasisi and Ng [21] and Moragues et al. [22,23] carried out these types of measurements during the construction of high-rise concrete buildings. Data collected in Agarwal and Gardner [20], Lasisi and Ng [21] referred only to shores located at intermediate levels of the building, and the authors concluded that the Simplified Method acceptably predicts maximum shore and slab loads during construction. On the other hand, Moragues et al. [22,23] measured shore loads starting from the lowest level. They pointed out that the Simplified Method overestimates the maximum shore and slab loads in 77.5% and 36.4% respectively. A recent extensive experimental work was carried out by Puente et al. [24]. One hundred and two shores, distributed between three floors, were instrumented with strain gages. The authors compared theoretical results proposed by different methods with field measurements. It was concluded that the Refined Method proposed by Liu et al. [10] is the most accurate theoretical method, and that Duan and Chen’s [17] Improved Simplified Method is a quite accurate method (with deviations lower than 15% between theoretical and experimental values) which does not require structural analysis software. These experimental data consisted of measuring shore loads after a construction step has been determined. Therefore, measurements do not show the evolution of loads during an operation or during the time period between two consecutive operations. Several measurements have been conducted to study the load redistribution between two consecutive constructional steps. Rosowsky et al. [25] and Fang et al. [26] obtained continuous registers of shore loads during the curing process of concrete slabs. In both research projects, it was concluded that load variations during this period are mainly related to the continuous increase in slab and beam stiffness. Azkune et al. [27] also registered continuous measurements of shore loads during the curing process of the slab. Nevertheless, with respect to the previous authors, they pointed out that the redistribution of loads in this phase, at least in time periods no longer than a week, are mainly determined by ambient temperature variations. Therefore, shore loads fluctuate according to the temperature changes registered on-site. Finally, Azkune et al. [27] proposed modified models which predict adequately the load variations between consecutive steps. With respect to measurements of dynamic loads, Rosowsky et al. [25] and Azkune and Puente [28] measured shore loads during
the casting of the floor. In both cases loads were measured on shores located underneath the slab which was being poured. The registered peak loads were compared with the construction live loads proposed by the ACI Committee 347 [29] and the European UNE-EN 12812 [30]. It was concluded that both standards are adequate and safe. Rosowsky et al. [25] have collected experimental data during the shore stripping process. The measurements consisted of registering the redistribution of loads due to the removal of several shores located on the same floor. They concluded that shore removal originates considerable overloads on the remaining shores. During the shore stripping process of a slab, the load of a removed shore is redistributed between the concrete structure and the remaining shores. This redistribution may lead to abrupt overloads that can cause damage to the shores, which would imply important economical losses. In the present work, on-site measurements have been carried out during several shore stripping processes of slabs. The objective of this work is to determine the magnitude of the shore overloads during the shore removal process and to determine the factors that affect the load’s redistribution. Several shore removal procedures are investigated and their effect on the shore overloads is evaluated. The results of this study will help contractors develop cost effective and safe shore removal procedures. 2. Field measurements Field measurements were concentrated on shore load variations during the shore removal. Special attention was paid to shores supported on the ground because experimental works such as Moragues et al. [22] and Puente et al. [24] have shown that maximum shore loads take place at the bottom floor. Therefore, shore loads were measured during the shore removal at the two lowest levels of the studied building. 2.1. Construction site description The measurements were conducted during the construction of the Playa Gaztetape Building in Getaria, a city located on the Basque coast in the north of Spain. The structure is a seven-story flat slab type residential apartment building with four similar levels underground parking. Floor to ceiling height is 2.65 m for the garage levels, and 2.90 m for the residential floors. The total area is 1800 m2 per floor. Each parking floor was poured in 6 sections of 250 m2 plus a ramp zone. The residential floors were poured in 8 sections of similar area. The thickness of the slab is 25 cm for all levels with a design concrete strength of 25 MPa. The shores used were adjustable steel shores with an allowable shore load of 18.5 kN. The planned shoring scheme consisted of three levels of shores with no reshores, with a construction cycle of one floor per eleven days. The instrumented shores were arranged in two different sections of the parking floors in both cases, these shores were placed in a five-column module, where concrete columns are 40×40 cm. The plain views of the two measurement sections are shown in Figs. 1 and 2. The arrangement of 34 instrumented shores is also included in each figure. Shores on the far left and right sides of Row 1 were not instrumented since the supporting shoring system at these areas consisted of more than one shore root. 2.2. Measurement system set up Thirty four shores with strain gages were placed on the selected floor. Four strain gages connected in a full Wheatstone bridge
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Fig. 1. Plain view of the instrumented shores arrangement in section B.
Fig. 2. Plain view of the instrumented shores arrangement in section E.
Fig. 3. Shore instrumentation and protection.
configuration were used: two active gages were placed on opposite arms to eliminate the influence of the bending strain of the shore,
and two passive gages were placed to compensate for temperature. A picture of some instrumented shores is shown in Fig. 3(a).
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Fig. 4. Removal process of instrumented shores.
All gages were protected against mechanical shocks and humidity. A picture of some shores protection is shown in Fig. 3(b). A four-path connector was also attached to the bridge installed at each shore. This connector allowed an easy monitoring of shore loads by a portable data logger. A picture of some instrumented shores in place at the construction site is shown in Fig. 4. Before placement on the construction site, the ratio between the transmitted load and microstrain for each instrumented shore was measured in a laboratory. Since steel shores were used, it was assumed that a linear relation existed between load and microstrain. During the calibration procedure, 8 points of the load versus microstrain curve were measured. The mean square error between actual values and the linear approximation was lower than 0.2% in all cases. Thermal compensation was also evaluated for each shore, and shores with a compensation error higher than 30 N/°C were discarded. Finally, the precision of the measurement system was evaluated on site using load cells placed beneath four instrumented shores. The difference between results provided by load cells and strain gages was less than 8% in all cases. 3. On-site measurement results Fig. 5. Three different measured shore removal procedures.
Experimental data was collected during the shore removal at three different slabs: the two lowest floors of one section (B) and the lowest floor of another section (E). The formwork was removed at 3 days after pouring. Shores were measured during the stripping operations. Different shore removal procedures were carried out in each case. Studied cases are listed below:
• Case 1: Shore removal at level −4, section B. • Case 2: Shore removal at level −3, section B. • Case 3: Shore removal at level −4, section E. Loads were measured after shores were partially stripped. In Case 1, loads were measured for the remaining shores after the removal of the four central shores of each row. This situation has been illustrated in Fig. 4. The opposite procedure was carried out in Case 2, measuring the loads of the three central shores of each row after the removal of the outer shores. Finally, loads on shores located at the central row were measured after the removal of two other rows. The procedures carried out in each case are shown schematically in Fig. 5. Considerable overloads were observed on some shores during its stripping process. In the three cases studied, maximum overloads up to 3 kN were measured; approximately 15% of the total
load in most critical cases. Maximum and average values of measured overloads are included in Table 1. From Table 1 it can be seen that important overloads have appeared in all cases. The most critical cases are 1 and 3. In fact, the maximum loads during the construction of every building occurred when shores were supported on the ground, before the removal. Consequently, attention must be paid to the most loaded shores of a floor, especially in the case of the lowest level. In these situations, a 3 kN overload may cause some permanent damage to the shores. Therefore, the shore stripping process must be properly performed in order to avoid excessive overloads at the critical shores. 4. Theoretical model for the shore removal operation It was observed in the previous section that some shores can be subjected to considerable overloads during the shore removal procedure. Therefore, it would be advantageous to obtain an optimum stripping sequence to minimize these overloads. In this respect, the ACI Committee 347 [29] Standard suggests shore/reshore stripping methods that will not damage the concrete structure.
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Fig. 6. Shore load variations during shore removal for Case 1. Table 1 Measured maximum and average overloads during shore removal. Case
Row
1 2 3
1 1
2 2 2
3 3
Maximum overload (kN)
Average overload (kN)
Maximum absolute load (kN)
Average absolute load (kN)
2.94 3.14
1.87 3.04
21.78 9.50
16.45 9.09
2.40 2.08 2.66
2.00 2.13
1.48 1.43 1.86
Nevertheless, existing codes or standards do not propose a theoretical method to model the shore removal operation. An accurate model will provide necessary data concerning the overloads that shores are subjected to, which will help determine the optimum shore removal procedure. A 3D model is necessary for a correct modeling of the shore stripping operation. With a 2D model load variations caused by the removal of shores located out of the modeled plane cannot be determined. Considering the work of Puente et al. [24], the 3D refined method developed by Liu et al. [10] was selected for the comparison between theoretical and experimental results. In Figs. 6–8, theoretical overloads are compared with on-site measurements. Each graph illustrates a row of shores. Each shore stripping has been modeled adding two forces, one downwards for the upper slab and one upwards for the lower slab. Each one was of the same magnitude as the load in the removed shore. In general, the refined method presented greater maximum shore loads than the measured results. Therefore, the refined method is considered as conservative. Depending on which case was analyzed the accuracy of the theoretical values has changed. Nevertheless, the refined method correctly models the trends of measured shore overloads as can be seen in Case 3, where one can observe that in both the theoretical and experimental results the shore position has considerable influence. Therefore, the order in which the shore removal is carried out can affect considerably the magnitude of the overloads. In consequence, it could be concluded that the refined method is
1.05 1.45
21.86 3.68 18.67
20.59 5.37
16.09 2.89 14.83
12.42 3.67
adequate for the selection of an optimum shore removal sequence. Even though it is not accurate in all cases, the model does, nevertheless, predict adequately the trend of the redistribution of the loads transmitted by the removed shores. 4.1. Influence of slab stiffness In this section, the influence of slab stiffness value on load redistribution originated during shore removal is studied. The elastic modulus Ec is the key parameter in concrete slab stiffness and deflection. In this analysis, the elastic modulus was obtained from the theoretical concrete strength development over time. All slabs were poured with 25 MPa concrete (28-day design strength). The calculation of the elastic modulus was based on the expressions provided by the CEB-FIP 1990 [31] code. Different analyses were carried out in order to study the influence of concrete strength over the shore stripping overloads. Therefore, Case 3 was recalculated with concrete characteristic strengths of 20 and 30 MPa. It can be observed in Fig. 9 that the concrete strength does not have any significant influence on the shore overloads. Maximum shore load variations of 3.2% have been obtained with a 5 MPa concrete characteristic variation. It can be concluded that stiffer slabs lead to lower shore overloads, as the elastic modulus of concrete increases with its strength. Therefore, a stiffer slab will support a greater portion of the load that the removed shore was transmitting, causing lower overloads on the remaining shores.
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Fig. 7. Shore load variations during shore removal for Case 2.
Fig. 8. Shore load variations during shore removal for Case 3.
Therefore, a variation of 5 MPa in concrete strength does not significantly affect the theoretical overloads. Furthermore, it is assumed that strength values calculated with the CEB-FIP 1990 [31] code, are close to actual values. Another factor that affects the slab’s flexural stiffness is steel reinforcement. The elastic modulus of steel is much greater than that of concrete, so a steel reinforced slab is slightly stiffer than those previously considered. The analysis was repeated taking into account the presence of the steel reinforcement in the slab. An equivalent slab thickness was defined from the steel amount and the ratio between the elastic modulus of steel and concrete. Therefore, in this new analysis each slab has a different thickness depending on its age and steel reinforcement. The new analysis was carried out for Case 3. The theoretical overloads obtained are represented in Fig. 10. Table 2 shows the values of Ec and the slab thickness taken into account in this study, considering as ‘‘age’’ the difference in the time between
Fig. 9. Influence of concrete compressive strength variations on shore removal overloads.
the concreting of each level and the shoring removal of the last level. A 25 MPa concrete characteristic strength was assumed for this analysis. It can be observed that the results of the new analysis matched the on-site measurements better. Nevertheless, the introduction of steel reinforcement in the model produces small variations, lower than 4%, on the theoretical overloads. Moreover, the model which neglects the steel reinforcement is also on the safe side. In conclusion, the introduction into the model of actual concrete strength values and steel reinforcement helps to improve its accuracy. Variations registered in results, however, are not excessive. Consequently, acceptable values are obtained using the characteristic strength of the concrete and neglecting the steel reinforcement. Furthermore, these assumptions are both on the safe side.
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Fig. 10. Influence of steel reinforcement on shore removal overloads. Table 2 Elastic modulus and equivalent slab thickness of each level in Case 3. Level
Age (days)
Ec (GPa)
he (cm)
−4 −3 −2
29 18 6
32.079 31.035 27.688
25.55 25.57 25.65
Fig. 12. Theoretical loads before removing the lowest level of shores.
41 MPa and its modulus of elasticity is 35 GPa. The elasticity modulus of wooden shores is 7.75 GPa, and the compression strength of wood is 5.6 MPa. For the present study, it is assumed that there are two levels of shores and one level of reshores, with a construction rate of one floor per week. 5.2. Refined structure model
Fig. 11. Plan view of the analyzed module (shore numbered).
5. Optimum shore removal sequence From on-site measurements it has been observed that shore overloads are highly influenced by the shore’s position in the module under consideration. Therefore, greater or lower shore overloads will result depending on the order in which shores are removed. The objective, in this section, is to establish general criteria for an optimum shore removal sequence. The optimum procedure should lead to greater overloads at less loaded shores, with no significant overloads on the shores that bear a greater load. That is, the objective in a stripping process in which the maximum shore load is not increased. 5.1. Description of the studied structure Different shore stripping procedures have been applied over the typical structure studied by Liu et al. [10]. The plain view of the analyzed structure module is shown in Fig. 11. Main dimensions of the four columns module are also included. All floors are similar. Story height is 2.80 m, with 18 cm slab thickness. The cross-section of the shores is 50 × 100 mm. Regarding material parameters: 28-day cylinder strength of concrete is
The Refined Method was selected for the comparison between different shore removal procedures. In the previous section, it was demonstrated that the Refined Method adequately predicts the shore overloads originated during this operation. Different shore stripping procedures were analyzed for the lowest level, the most critical with respect to the falsework. In the model, the elastic deformation of columns was neglected. That is, vertices A, B, C and D in Fig. 11 of the studied slab were considered as fixed supports. It is assumed that slab edges are free, i.e. the effects of slab continuity are ignored. Theoretically obtained shore load distribution is shown in Fig. 12, before the beginning of shore removal on the lowest level. In this situation, the two poured slabs are shored to the foundation. From this figure, it can be seen that, on the lowest level, the most loaded shores are located in the central zone of the module. Therefore, special attention must be paid to the removal of these shores. 5.3. Shore removal procedures Different, commonly employed, on-site shore stripping sequences have been applied throughout the studied structure. Overloads on all shores were calculated in each case.
• Procedure 1: a typical zig-zag shore removal. It consists of removing the shores in one direction from one of the shore rows located at one slab edge, and then continuing with the adjacent row in the opposite direction and so on until the opposite row is reached. This operation is carried out in two possible directions: procedure1A: stripping according to larger shore distance rows (X direction in Fig. 11) as shown in Fig. 13(a), and procedure 1B: stripping according to shorter shore distance rows (Y direction in Fig. 10) as shown in Fig. 13(b). • Procedure 2: shore removal in spiral. The shore located at the module center is removed first, and the operation is completed removing shores in a spiral. The shore removal order is shown in Fig. 13(c). • Procedure 3: stripping alternating rows. The most loaded row of shores, the central row in direction X , is removed first.
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Fig. 13. Shore removal order by the different procedures.
Next, adjacent rows are removed, and so on until rows located between columns are reached. Shores located in the same row are removed in order, starting from a slab edge to the opposite one. The shore removal sequence is shown in Fig. 13(d). • Procedure 4: similar to Procedure 3, but also alternating the removal of the shores located within a row. First, the central shore of the row should be removed, then both adjacent shores, and so on until the removal of the shores located at both edges of the row. ACI Committee 347 [29] also recommends beginning with the removal of shores placed in the middle of a bay, since a more adequate slab load distribution is obtained. The shore removal order is shown in Fig. 13(e). • Procedure 5: removal of the most loaded shore. Apparently, this is the optimum procedure. It consists of removing the most loaded shore at every moment. This procedure could not be used on-site, since the worker cannot know which is the most loaded shore at a given time. This analysis, however, will allow the evaluation of the differences with other more common shore removal procedures described previously. Shore removal order is shown in Fig. 13(f). A new analysis was carried out for each procedure when a shore was removed, calculating the overloads originated on the remaining shores at each stage. 5.4. Analysis results The maximum overloads (max ol) and the maximum absolute loads (max absl) that were transmitted by shores during the
stripping procedures are included in Table 3. In this table shore represents the shore numbered according with Fig. 11. It can be concluded that different shore overloads were obtained depending on the stripping procedure. The most critical situation was observed in Procedure 1A, since an absolute load of 15.35 kN was reached at shore 20, after a 5.28 kN overload. This is perhaps one of the most common on-site procedures, where workers start removing shores from a slab edge and continue by rows until the opposite slab edge is reached. Applying the same criterion for the other direction, maximum absolute shore load is reduced by 9% (13.97 kN), with a 3.90 kN overload (26% reduction). Therefore, overloads are considerably reduced by taking the precaution of stripping by rows in the direction where the distance between columns is less. Spiral shore removal did not reduce maximum overloads. Late removal of shores located at the central row edges (shores 2 and 20) led to considerable overloads on both shores. Maximum overload originated on shore 2 was 5.16 kN, with a maximum absolute shore load of 15.23 kN. Procedure 3 involved a significant improvement with respect to previous procedures. Since the most loaded row was removed first, maximum overloads were generated on the less loaded shores. Consequently, maximum absolute shore loads decreased. Maximum shore overload was 4.05 kN, causing a maximum absolute shore load of 12.97 kN at shore 1. Therefore, with respect to the initial case 1A, maximum overload value was reduced by 23% and maximum absolute load by 16%. Procedure 4 did not lead to significant improvements when compared to Procedure 3. Albeit the overloads on shores placed
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Table 3 Maximum shore overloads and maximum absolute loads during shore removal (kN). Shore
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Before removal
8.92 10.07 8.92 7.18 9.04 10.23 9.04 7.18 7.54 9.17 10.34 9.17 7.54 7.18 9.04 10.23 9.04 7.18 8.92 10.07 8.92
Procedure 1A
Proc 1B
Max ol
Max absl
Max ol
Max absl
Max ol
Proc 2 Max absl
Max ol
Max absl
Max ol
Max absl
Max ol
Max absl
– 0.49 0.76 0.48 2.21 2.63 1.50 0.19 0.62 2.08 3.35 2.61 1.00 1.11 2.79 3.52 2.12 0.79 2.98 5.28 4.38
8.92 10.56 9.68 7.66 11.26 12.86 10.54 7.37 8.15 11.24 13.69 11.77 8.54 8.29 11.84 13.75 11.16 7.98 11.90 15.35 13.30
1.26 1.43 4.10 – 1.04 2.82 3.63 1.03 0.24 1.18 3.21 3.70 1.57 0.36 1.01 3.19 3.52 1.47 0.20 3.90 2.52
10.18 11.50 13.01 7.18 10.08 13.05 12.67 8.21 7.77 10.34 13.55 12.87 9.10 7.55 10.05 13.42 12.56 8.65 9.11 13.97 11.44
4.10 5.16 2.95 1.03 1.32 2.04 1.17 1.11 1.57 1.98 – 1.23 1.30 1.47 2.08 0.69 0.80 0.90 2.44 4.58 3.77
13.01 15.23 11.87 8.21 10.37 12.27 10.21 8.29 9.10 11.14 10.34 10.40 8.84 8.65 11.12 10.92 9.85 8.09 11.36 14.64 12.69
4.05 1.92 1.71 1.02 3.59 1.27 2.66 1.47 1.57 3.67 1.28 2.82 1.57 1.46 3.48 1.06 2.74 1.03 2.48 – 3.14
12.97 11.99 10.62 8.20 12.63 11.51 11.70 8.65 9.10 12.83 11.62 11.99 9.10 8.64 12.52 11.29 11.79 8.21 11.40 10.06 12.05
3.09 1.80 3.98 1.46 2.61 1.04 3.43 1.46 1.29 1.85 – 2.60 1.29 1.32 2.35 0.69 3.16 1.32 3.14 1.92 4.05
12.01 11.87 12.90 8.64 11.65 11.27 12.48 8.64 8.83 11.02 10.34 11.76 8.83 8.51 11.40 10.92 12.20 8.51 12.05 11.99 12.97
3.14 1.92 4.05 1.46 2.74 1.27 3.59 1.46 1.29 1.85 – 2.60 1.29 1.32 2.35 0.69 3.16 1.32 2.95 1.59 3.83
12.05 11.99 12.97 8.64 11.79 11.51 12.63 8.64 8.83 11.02 10.34 11.76 8.83 8.51 11.40 10.92 12.20 8.51 11.87 11.65 12.75
in the center of the module were reduced, maximum loads were similar to those calculated in Procedure 3. Therefore, the removal of shores alternating the position within the row did not reduce maximum loads when compared with Procedure 3. Finally, maximum loads did not decrease when Procedure 5 was checked. In fact, the procedure of removing the most loaded shore is almost the same as Procedure 4. Therefore, from a practical point of view, Procedure 3 gets optimum results. Hence, the shore removal process should be carried out by rows: the most loaded row of shores should be removed first, then the adjacent rows and finally the less loaded rows. The use of this procedure will be especially important when maximum shore loads are present. During the construction of any multistory concrete building, removal of the lowest level of shores, which are supported on ground is the most critical, since maximum shore loads occur at this stage. For a typical 4 column module supported by shores with a similar tributary area, the most loaded row is located in the center according to the direction corresponding with the longest bay (X direction in the example studied here). 6. Conclusions The overloads originating during the shore removal process can cause shore failure in some cases. Damage to the falsework can produce important economic losses due to its elevated cost. Therefore, on-site measurements were conducted during the shore stripping at different levels. Based on these experimental measurements, the adequacy of the Refined Method for the shore removal operation modeling was evaluated. Finally, the shore overloads originated in different shore removal procedures were studied. From this study the following conclusions can be stated: 1. During the shore removal process, shores are subjected to considerable load increments. Overloads of up to 3 kN (15% of the total load, approximately) have been reached during onsite measurements, which represent approximately 10% of the shore capacity. 2. The Refined Method models adequately the shore removal procedure. The Refined 3D Method predicts correctly trends within the redistribution of loads. The theoretical model proposes overload values greater than real ones. Therefore, it is a model that proposes values which lean on the safe side, as shown in Figs. 6–8.
Proc 3
Proc 4
Proc 5
3. Overloads generated during shore removal are influenced by the stiffness of slabs and shores. Hence, use of actual concrete strength and modeling of steel reinforcement produces more accurate theoretical results. Nevertheless, relatively low strength variations and steel reinforcement consideration do not lead to important variations of theoretical values (less than 4%). Therefore, modeling the slab with its project characteristic strength and without its steel reinforcement leads to less accurate but acceptable results. 4. The overload supported by each shore is influenced by its relative position within the module. Therefore, different overloads are produced depending on the shores which are removed. Consequently, the stripping order affects the maximum shore loads considerably. 5. The best stripping procedure found consists of removing shores by rows. First, the most loaded row must be removed, then the adjacent rows and so on until the removal of less loaded rows. For a typical symmetric 4 column module, the most loaded row, in the most critical situation, is the central one in the longest bay direction. 6. Although an alternating removal of the shores located within a row does not produce a reduction on shore overloads, it could be beneficial for the concrete slab. By stripping the shores in the middle first, the slab will be loaded as designed. First, the central shore of the row should be removed, then both adjacent shores and so on until the removal of shores located at both edges of the row. Acknowledgements This research was sponsored by the Basque Government (Departamento de Educación, Universidades e Investigación and Departamento de Industria, Comercio y Turismo, Project number UE2005-1) and the Spanish Government (Ministerio de Fomento, Programa Nacional de Construcción, Project Ref. 80003/A04). Ulma Construcción also sponsored this study and supplied part of the material needed during the measurements. The authors also wish to acknowledge the collaboration of Construcciones Imaz in the collection of experimental data. The opinions expressed in this work are those of the writers and do not necessarily reflect the points of view of the sponsors.
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References [1] Lew HS, Carino NJ, Fattal SG. Cause of the condominium collapse in Cocoa Beach, Florida. Concr Int 1982;4(8):64–73. [2] Carino NJ, Leyendecker EV, Fattal SG. A review of the Skyline Plaza collapse. Concr Int 1983;5(7):35–42. [3] Price WH. Factors influencing concrete strength. ACI J Proc 1951;47(2): 417–32. [4] Klieger P. Effect of mixing and curing temperature on concrete strength. ACI J Proc 1958;54(6):1063–81. [5] Gardner NJ, Poon SM. Time and temperature effects on tensile, bond, and compressive strengths. ACI J Proc 1976;73(7):405–9. [6] Carino NJ, Hamlin MJ, Snell LM. Properties of concrete at early ages. Concr Int 1989;11:51–4. [7] Nielsen K. Loads on reinforced concrete floor slabs and their deformations during construction. Bulletin no.15, final report. Stockholm: Swedish Cement and Concrete Research Institute. Royal Institute of Technology; 1952. [8] Grundy P, Kabaila A. Construction loads on slabs with shored formwork in multistory buildings. ACI J Proc 1963;60(12):1729–38. [9] ACI Committee 347. Guide for shoring/reshoring of concrete multistory buildings (ACI 347.2R-05). Farmington Hills (MI): American Concrete Institute; 2005. p. 18. [10] Liu XL, Chen WF, Bowman MD. Construction load analysis for concrete structures. J Struct Eng ASCE 1985;111(5):1019–36. [11] Liu XL, Chen WF, Bowman MD. Construction loads on supporting floors. Concr Int: Des Construct 1985;7(12):21–6. [12] Gardner NJ. Shoring, reshoring, and safety. Concr Int: Des Construct 1985;7(4): 28–34. [13] Stivaros PC, Halvorsen GT. Shoring/reshoring operations for multistory buildings. ACI Struct J 1990;87(5):589–96. [14] Chen WF, Mossallam K. Concrete buildings analysis for safe construction. Boca Raton: CRC Press; 1991. [15] Mossallam KH, Chen WF. Determining shoring loads for reinforced concrete construction. ACI Struct J 1991;88(3):340–50. [16] El-Shahhat M, Chen WF. Improved analysis of shore-slab interaction. ACI Struct J 1992;89(5):528–37.
[17] Duan MZ, Chen WF. Improved simplified method for slab and shore load analysis during construction. Project report CE-STR-95-24. West Lafayette (IN): Purdue University; 1995. [18] Fang DP, Zhu HY, Geng CD, Liu XL. Floor load distribution in reinforced concrete buildings during construction. ACI Struct J 2001;98(2):149–56. [19] Miranda De Almeida JF, Silva MR, Ramalho MA. A new procedure for the analysis of construction loads in multistory reinforced concrete structures. Struct Des Tall Spec Build 2003;12:293–315. [20] Agarwall RK, Gardner NJ. Form and shore requirements for multistory flat slab type buildings. ACI J Proc 1974;71(11):559–69. [21] Lasisi MY, Ng SF. Construction loads imposed on high-rise floor slabs. Concr Int: Des Construct 1979;1(2):24–9. [22] Moragues JJ, Catalá J, Salort V, Sirvent PL. Transmisión de Cargas entre Forjados, durante el Proceso Constructivo: Medidas Realizadas en Obra. Hormigón y Acero 1991;179:37–47. [23] Moragues JJ, Catalá J, Pellicer E. An analysis of concrete framed structures during the construction process. Concr Int: Des Construct 1996;18(11):44–8. [24] Puente I, Azkune M, Insausti A. Shore–slab interaction in multistory reinforced concrete buildings during construction: an experimental approach. Eng Struct 2007;29(5):731–41. [25] Rosowsky DV, Philbrick Jr TW, Huston DR. Observations from shore load measurements during concrete construction. J Perform Construct Facil ASCE 1997;11(1):18–23. [26] Fang DP, Zhu HY, Geng CD, Liu XL. On-site measurements of load distribution in reinforced concrete buildings during construction. ACI Struct J 2001;98(2): 157–63. [27] Azkune M, Puente I, Insausti A. Effect of ambient temperature on the redistribution of loads during construction of multi-storey concrete structures. Eng Struct 2007;29(6):933–41. [28] Azkune M, Puente I. Evolución de la distribución de cargas entre puntales y forjados durante la construcción. Hormigón y Acero 2007;245:79–91. [29] ACI Committee 347. Guide to formwork for concrete. 1988. [30] UNE-EN 12812. Cimbras, Requisitos de comportamiento y diseño general. 2008. [31] CEB-FIP Model Code. Comité Euro-International du Béton—Federation Internationale de la Precontrainte, Lausanne. 1990.
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Shore overloads during shoring removal M. Azkune, I. Puente ∗ , A. Santilli Department of Mechanical Engineering, Institute of Civil Engineering, Tecnun (University of Navarra), Manuel de Lardizabal 13, 20018 San Sebastian, Spain
article
info
Article history: Received 2 September 2009 Received in revised form 24 May 2010 Accepted 9 August 2010 Available online 9 September 2010 Keywords: Shore removal Multistory buildings Flat concrete slabs Overloads Falsework Concrete construction Shoring Reshoring
abstract The considerable overloads originating during the shore removal process can affect the structural safety of a multistory concrete building under construction. With an incorrect shoring removal reversal of stresses may occur which can cause concrete cracking and excessive deflections. Moreover, excessive overloads will damage the falsework, producing important economical losses due to its elevated cost. A measured program has been conducted during the shore stripping at different building levels. Results show that in general the Refined Method is adequate in a conservative form for the shore removal procedure. Then, the influence of different parameters such as concrete strength or steel reinforcement in shore removal overloads was studied. The use of the actual concrete strength and modeling the steel reinforcement produces a more accurate theoretical result. Nevertheless, these considerations do not bring major changes (less than 4%). Finally, shore overloads originating in five different shore removal procedures in a typical structure have been compared, establishing some criteria for a safe shore stripping sequence. For example the best stripping procedure found consists of removing shores by rows. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction For economic reasons, there is ever more pressure to construct multistory concrete buildings at a faster pace. Thus, the strength of a slab usually is not enough to support an upper floor when it is cast. Consequently, self-weight of a newly poured slab is distributed between lower partially hardened slabs interconnected by shores and/or reshores. Moreover, an excessive number of shored floors are not advisable. To keep the structure shored to the foundation would increase construction costs considerably, due to the fact that the excessive number of shores and falsework needed would be too high. This procedure could also result in exceeding ultimate shore loads on the lower floors. The temporary supporting structure is removed when the slab is sufficiently resistant which allows the liberated shores to be used for the construction of upper floors. This reduces the number of shores needed and allows work by other trades to proceed on the lower floors. Structural safety, however, is also of paramount importance. Load carrying capacities of shores and slabs must not be exceeded. A significant percentage of structural collapses during construction is caused by excessive loads on both the shoring system and
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Corresponding author. Tel.: +34 943219877; fax: +34 943311442. E-mail address: [email protected] (I. Puente).
0141-0296/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2010.08.007
interconnected slabs. The accidents that occurred in Cocoa Beach, Florida in 1981 reported by Lew et al. [1] and in Fairfax County, Virginia in 1973 reported by Carino et al. [2], where a large number of workers lost their lives, are well known examples. It is recognized that a detailed knowledge of shore-slab interaction may well have prevented some of these accidents. Nevertheless, due to factors such as the variable nature of concrete properties, determining load values and distribution between shores and slabs is complex. Several research studies have been conducted to accurately determine the strength properties of a concrete building during construction. Concrete has been studied by authors such as Price [3], Klieger [4], Gardner and Poon [5] or Carino et al. [6], who have analyzed the influence of factors such as curing temperature in concrete strength evolution. The first known research paper on the subject of load estimation was published in 1952 when Nielsen [7] presented a method which was too complex to be used in practice. In 1963 Grundy and Kabaila [8] developed the Simplified Method. This pioneering work was based on the following assumptions, which make the method clear and easy to apply: 1. Relative to the bending stiffness of slabs, the axial stiffness of shores and reshores is assumed to be infinite. 2. Despite the fact that concrete properties vary with age, all slabs are assumed to possess equal flexural stiffness. 3. The lowest level of shores or reshores is assumed to be supported on a completely rigid foundation.
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This Simplified Method is the most widely used method in the construction industry, and is also suggested in the shoring/ reshoring guide published by ACI Committee 347, 2R-05 [9]. Other theoretical models have been proposed in recent years. In general, these models include modifications that try to improve the results achieved by the Simplified Method, as well as to verify the accuracy of this method. In 1985 Liu et al. [10] developed the Refined Method. This finite element based computational method proposes a more realistic model of the actual structure. However, this method is cumbersome for practical use. Liu et al. [11] compared on-site measurements with results obtained by both the Simplified and the Refined Methods, and concluded that the Refined one predicts more accurate values. Nevertheless, they pointed out that the Simplified Method can be used if the results for the maximum shore load and slab moments are corrected by a modification coefficient that varies from 1.05 to 1.10. Other models have been developed by Gardner [12], Stivaros and Halvorsen [13], Chen and Mossallam [14], Mossallam and Chen [15], El-Shahhat and Chen [16], Duan and Chen [17], Fang et al. [18] and Miranda de Almeida et al. [19]. But the significant amount of theoretical research published contrasts with the absence of detailed experimental data related to the subject area. The first on-site measurements consisted of measuring shore loads and comparing experimental data with the Simplified Method. For example, Agarwal and Gardner [20], Lasisi and Ng [21] and Moragues et al. [22,23] carried out these types of measurements during the construction of high-rise concrete buildings. Data collected in Agarwal and Gardner [20], Lasisi and Ng [21] referred only to shores located at intermediate levels of the building, and the authors concluded that the Simplified Method acceptably predicts maximum shore and slab loads during construction. On the other hand, Moragues et al. [22,23] measured shore loads starting from the lowest level. They pointed out that the Simplified Method overestimates the maximum shore and slab loads in 77.5% and 36.4% respectively. A recent extensive experimental work was carried out by Puente et al. [24]. One hundred and two shores, distributed between three floors, were instrumented with strain gages. The authors compared theoretical results proposed by different methods with field measurements. It was concluded that the Refined Method proposed by Liu et al. [10] is the most accurate theoretical method, and that Duan and Chen’s [17] Improved Simplified Method is a quite accurate method (with deviations lower than 15% between theoretical and experimental values) which does not require structural analysis software. These experimental data consisted of measuring shore loads after a construction step has been determined. Therefore, measurements do not show the evolution of loads during an operation or during the time period between two consecutive operations. Several measurements have been conducted to study the load redistribution between two consecutive constructional steps. Rosowsky et al. [25] and Fang et al. [26] obtained continuous registers of shore loads during the curing process of concrete slabs. In both research projects, it was concluded that load variations during this period are mainly related to the continuous increase in slab and beam stiffness. Azkune et al. [27] also registered continuous measurements of shore loads during the curing process of the slab. Nevertheless, with respect to the previous authors, they pointed out that the redistribution of loads in this phase, at least in time periods no longer than a week, are mainly determined by ambient temperature variations. Therefore, shore loads fluctuate according to the temperature changes registered on-site. Finally, Azkune et al. [27] proposed modified models which predict adequately the load variations between consecutive steps. With respect to measurements of dynamic loads, Rosowsky et al. [25] and Azkune and Puente [28] measured shore loads during
the casting of the floor. In both cases loads were measured on shores located underneath the slab which was being poured. The registered peak loads were compared with the construction live loads proposed by the ACI Committee 347 [29] and the European UNE-EN 12812 [30]. It was concluded that both standards are adequate and safe. Rosowsky et al. [25] have collected experimental data during the shore stripping process. The measurements consisted of registering the redistribution of loads due to the removal of several shores located on the same floor. They concluded that shore removal originates considerable overloads on the remaining shores. During the shore stripping process of a slab, the load of a removed shore is redistributed between the concrete structure and the remaining shores. This redistribution may lead to abrupt overloads that can cause damage to the shores, which would imply important economical losses. In the present work, on-site measurements have been carried out during several shore stripping processes of slabs. The objective of this work is to determine the magnitude of the shore overloads during the shore removal process and to determine the factors that affect the load’s redistribution. Several shore removal procedures are investigated and their effect on the shore overloads is evaluated. The results of this study will help contractors develop cost effective and safe shore removal procedures. 2. Field measurements Field measurements were concentrated on shore load variations during the shore removal. Special attention was paid to shores supported on the ground because experimental works such as Moragues et al. [22] and Puente et al. [24] have shown that maximum shore loads take place at the bottom floor. Therefore, shore loads were measured during the shore removal at the two lowest levels of the studied building. 2.1. Construction site description The measurements were conducted during the construction of the Playa Gaztetape Building in Getaria, a city located on the Basque coast in the north of Spain. The structure is a seven-story flat slab type residential apartment building with four similar levels underground parking. Floor to ceiling height is 2.65 m for the garage levels, and 2.90 m for the residential floors. The total area is 1800 m2 per floor. Each parking floor was poured in 6 sections of 250 m2 plus a ramp zone. The residential floors were poured in 8 sections of similar area. The thickness of the slab is 25 cm for all levels with a design concrete strength of 25 MPa. The shores used were adjustable steel shores with an allowable shore load of 18.5 kN. The planned shoring scheme consisted of three levels of shores with no reshores, with a construction cycle of one floor per eleven days. The instrumented shores were arranged in two different sections of the parking floors in both cases, these shores were placed in a five-column module, where concrete columns are 40×40 cm. The plain views of the two measurement sections are shown in Figs. 1 and 2. The arrangement of 34 instrumented shores is also included in each figure. Shores on the far left and right sides of Row 1 were not instrumented since the supporting shoring system at these areas consisted of more than one shore root. 2.2. Measurement system set up Thirty four shores with strain gages were placed on the selected floor. Four strain gages connected in a full Wheatstone bridge
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Fig. 1. Plain view of the instrumented shores arrangement in section B.
Fig. 2. Plain view of the instrumented shores arrangement in section E.
Fig. 3. Shore instrumentation and protection.
configuration were used: two active gages were placed on opposite arms to eliminate the influence of the bending strain of the shore,
and two passive gages were placed to compensate for temperature. A picture of some instrumented shores is shown in Fig. 3(a).
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Fig. 4. Removal process of instrumented shores.
All gages were protected against mechanical shocks and humidity. A picture of some shores protection is shown in Fig. 3(b). A four-path connector was also attached to the bridge installed at each shore. This connector allowed an easy monitoring of shore loads by a portable data logger. A picture of some instrumented shores in place at the construction site is shown in Fig. 4. Before placement on the construction site, the ratio between the transmitted load and microstrain for each instrumented shore was measured in a laboratory. Since steel shores were used, it was assumed that a linear relation existed between load and microstrain. During the calibration procedure, 8 points of the load versus microstrain curve were measured. The mean square error between actual values and the linear approximation was lower than 0.2% in all cases. Thermal compensation was also evaluated for each shore, and shores with a compensation error higher than 30 N/°C were discarded. Finally, the precision of the measurement system was evaluated on site using load cells placed beneath four instrumented shores. The difference between results provided by load cells and strain gages was less than 8% in all cases. 3. On-site measurement results Fig. 5. Three different measured shore removal procedures.
Experimental data was collected during the shore removal at three different slabs: the two lowest floors of one section (B) and the lowest floor of another section (E). The formwork was removed at 3 days after pouring. Shores were measured during the stripping operations. Different shore removal procedures were carried out in each case. Studied cases are listed below:
• Case 1: Shore removal at level −4, section B. • Case 2: Shore removal at level −3, section B. • Case 3: Shore removal at level −4, section E. Loads were measured after shores were partially stripped. In Case 1, loads were measured for the remaining shores after the removal of the four central shores of each row. This situation has been illustrated in Fig. 4. The opposite procedure was carried out in Case 2, measuring the loads of the three central shores of each row after the removal of the outer shores. Finally, loads on shores located at the central row were measured after the removal of two other rows. The procedures carried out in each case are shown schematically in Fig. 5. Considerable overloads were observed on some shores during its stripping process. In the three cases studied, maximum overloads up to 3 kN were measured; approximately 15% of the total
load in most critical cases. Maximum and average values of measured overloads are included in Table 1. From Table 1 it can be seen that important overloads have appeared in all cases. The most critical cases are 1 and 3. In fact, the maximum loads during the construction of every building occurred when shores were supported on the ground, before the removal. Consequently, attention must be paid to the most loaded shores of a floor, especially in the case of the lowest level. In these situations, a 3 kN overload may cause some permanent damage to the shores. Therefore, the shore stripping process must be properly performed in order to avoid excessive overloads at the critical shores. 4. Theoretical model for the shore removal operation It was observed in the previous section that some shores can be subjected to considerable overloads during the shore removal procedure. Therefore, it would be advantageous to obtain an optimum stripping sequence to minimize these overloads. In this respect, the ACI Committee 347 [29] Standard suggests shore/reshore stripping methods that will not damage the concrete structure.
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Fig. 6. Shore load variations during shore removal for Case 1. Table 1 Measured maximum and average overloads during shore removal. Case
Row
1 2 3
1 1
2 2 2
3 3
Maximum overload (kN)
Average overload (kN)
Maximum absolute load (kN)
Average absolute load (kN)
2.94 3.14
1.87 3.04
21.78 9.50
16.45 9.09
2.40 2.08 2.66
2.00 2.13
1.48 1.43 1.86
Nevertheless, existing codes or standards do not propose a theoretical method to model the shore removal operation. An accurate model will provide necessary data concerning the overloads that shores are subjected to, which will help determine the optimum shore removal procedure. A 3D model is necessary for a correct modeling of the shore stripping operation. With a 2D model load variations caused by the removal of shores located out of the modeled plane cannot be determined. Considering the work of Puente et al. [24], the 3D refined method developed by Liu et al. [10] was selected for the comparison between theoretical and experimental results. In Figs. 6–8, theoretical overloads are compared with on-site measurements. Each graph illustrates a row of shores. Each shore stripping has been modeled adding two forces, one downwards for the upper slab and one upwards for the lower slab. Each one was of the same magnitude as the load in the removed shore. In general, the refined method presented greater maximum shore loads than the measured results. Therefore, the refined method is considered as conservative. Depending on which case was analyzed the accuracy of the theoretical values has changed. Nevertheless, the refined method correctly models the trends of measured shore overloads as can be seen in Case 3, where one can observe that in both the theoretical and experimental results the shore position has considerable influence. Therefore, the order in which the shore removal is carried out can affect considerably the magnitude of the overloads. In consequence, it could be concluded that the refined method is
1.05 1.45
21.86 3.68 18.67
20.59 5.37
16.09 2.89 14.83
12.42 3.67
adequate for the selection of an optimum shore removal sequence. Even though it is not accurate in all cases, the model does, nevertheless, predict adequately the trend of the redistribution of the loads transmitted by the removed shores. 4.1. Influence of slab stiffness In this section, the influence of slab stiffness value on load redistribution originated during shore removal is studied. The elastic modulus Ec is the key parameter in concrete slab stiffness and deflection. In this analysis, the elastic modulus was obtained from the theoretical concrete strength development over time. All slabs were poured with 25 MPa concrete (28-day design strength). The calculation of the elastic modulus was based on the expressions provided by the CEB-FIP 1990 [31] code. Different analyses were carried out in order to study the influence of concrete strength over the shore stripping overloads. Therefore, Case 3 was recalculated with concrete characteristic strengths of 20 and 30 MPa. It can be observed in Fig. 9 that the concrete strength does not have any significant influence on the shore overloads. Maximum shore load variations of 3.2% have been obtained with a 5 MPa concrete characteristic variation. It can be concluded that stiffer slabs lead to lower shore overloads, as the elastic modulus of concrete increases with its strength. Therefore, a stiffer slab will support a greater portion of the load that the removed shore was transmitting, causing lower overloads on the remaining shores.
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Fig. 7. Shore load variations during shore removal for Case 2.
Fig. 8. Shore load variations during shore removal for Case 3.
Therefore, a variation of 5 MPa in concrete strength does not significantly affect the theoretical overloads. Furthermore, it is assumed that strength values calculated with the CEB-FIP 1990 [31] code, are close to actual values. Another factor that affects the slab’s flexural stiffness is steel reinforcement. The elastic modulus of steel is much greater than that of concrete, so a steel reinforced slab is slightly stiffer than those previously considered. The analysis was repeated taking into account the presence of the steel reinforcement in the slab. An equivalent slab thickness was defined from the steel amount and the ratio between the elastic modulus of steel and concrete. Therefore, in this new analysis each slab has a different thickness depending on its age and steel reinforcement. The new analysis was carried out for Case 3. The theoretical overloads obtained are represented in Fig. 10. Table 2 shows the values of Ec and the slab thickness taken into account in this study, considering as ‘‘age’’ the difference in the time between
Fig. 9. Influence of concrete compressive strength variations on shore removal overloads.
the concreting of each level and the shoring removal of the last level. A 25 MPa concrete characteristic strength was assumed for this analysis. It can be observed that the results of the new analysis matched the on-site measurements better. Nevertheless, the introduction of steel reinforcement in the model produces small variations, lower than 4%, on the theoretical overloads. Moreover, the model which neglects the steel reinforcement is also on the safe side. In conclusion, the introduction into the model of actual concrete strength values and steel reinforcement helps to improve its accuracy. Variations registered in results, however, are not excessive. Consequently, acceptable values are obtained using the characteristic strength of the concrete and neglecting the steel reinforcement. Furthermore, these assumptions are both on the safe side.
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Fig. 10. Influence of steel reinforcement on shore removal overloads. Table 2 Elastic modulus and equivalent slab thickness of each level in Case 3. Level
Age (days)
Ec (GPa)
he (cm)
−4 −3 −2
29 18 6
32.079 31.035 27.688
25.55 25.57 25.65
Fig. 12. Theoretical loads before removing the lowest level of shores.
41 MPa and its modulus of elasticity is 35 GPa. The elasticity modulus of wooden shores is 7.75 GPa, and the compression strength of wood is 5.6 MPa. For the present study, it is assumed that there are two levels of shores and one level of reshores, with a construction rate of one floor per week. 5.2. Refined structure model
Fig. 11. Plan view of the analyzed module (shore numbered).
5. Optimum shore removal sequence From on-site measurements it has been observed that shore overloads are highly influenced by the shore’s position in the module under consideration. Therefore, greater or lower shore overloads will result depending on the order in which shores are removed. The objective, in this section, is to establish general criteria for an optimum shore removal sequence. The optimum procedure should lead to greater overloads at less loaded shores, with no significant overloads on the shores that bear a greater load. That is, the objective in a stripping process in which the maximum shore load is not increased. 5.1. Description of the studied structure Different shore stripping procedures have been applied over the typical structure studied by Liu et al. [10]. The plain view of the analyzed structure module is shown in Fig. 11. Main dimensions of the four columns module are also included. All floors are similar. Story height is 2.80 m, with 18 cm slab thickness. The cross-section of the shores is 50 × 100 mm. Regarding material parameters: 28-day cylinder strength of concrete is
The Refined Method was selected for the comparison between different shore removal procedures. In the previous section, it was demonstrated that the Refined Method adequately predicts the shore overloads originated during this operation. Different shore stripping procedures were analyzed for the lowest level, the most critical with respect to the falsework. In the model, the elastic deformation of columns was neglected. That is, vertices A, B, C and D in Fig. 11 of the studied slab were considered as fixed supports. It is assumed that slab edges are free, i.e. the effects of slab continuity are ignored. Theoretically obtained shore load distribution is shown in Fig. 12, before the beginning of shore removal on the lowest level. In this situation, the two poured slabs are shored to the foundation. From this figure, it can be seen that, on the lowest level, the most loaded shores are located in the central zone of the module. Therefore, special attention must be paid to the removal of these shores. 5.3. Shore removal procedures Different, commonly employed, on-site shore stripping sequences have been applied throughout the studied structure. Overloads on all shores were calculated in each case.
• Procedure 1: a typical zig-zag shore removal. It consists of removing the shores in one direction from one of the shore rows located at one slab edge, and then continuing with the adjacent row in the opposite direction and so on until the opposite row is reached. This operation is carried out in two possible directions: procedure1A: stripping according to larger shore distance rows (X direction in Fig. 11) as shown in Fig. 13(a), and procedure 1B: stripping according to shorter shore distance rows (Y direction in Fig. 10) as shown in Fig. 13(b). • Procedure 2: shore removal in spiral. The shore located at the module center is removed first, and the operation is completed removing shores in a spiral. The shore removal order is shown in Fig. 13(c). • Procedure 3: stripping alternating rows. The most loaded row of shores, the central row in direction X , is removed first.
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Fig. 13. Shore removal order by the different procedures.
Next, adjacent rows are removed, and so on until rows located between columns are reached. Shores located in the same row are removed in order, starting from a slab edge to the opposite one. The shore removal sequence is shown in Fig. 13(d). • Procedure 4: similar to Procedure 3, but also alternating the removal of the shores located within a row. First, the central shore of the row should be removed, then both adjacent shores, and so on until the removal of the shores located at both edges of the row. ACI Committee 347 [29] also recommends beginning with the removal of shores placed in the middle of a bay, since a more adequate slab load distribution is obtained. The shore removal order is shown in Fig. 13(e). • Procedure 5: removal of the most loaded shore. Apparently, this is the optimum procedure. It consists of removing the most loaded shore at every moment. This procedure could not be used on-site, since the worker cannot know which is the most loaded shore at a given time. This analysis, however, will allow the evaluation of the differences with other more common shore removal procedures described previously. Shore removal order is shown in Fig. 13(f). A new analysis was carried out for each procedure when a shore was removed, calculating the overloads originated on the remaining shores at each stage. 5.4. Analysis results The maximum overloads (max ol) and the maximum absolute loads (max absl) that were transmitted by shores during the
stripping procedures are included in Table 3. In this table shore represents the shore numbered according with Fig. 11. It can be concluded that different shore overloads were obtained depending on the stripping procedure. The most critical situation was observed in Procedure 1A, since an absolute load of 15.35 kN was reached at shore 20, after a 5.28 kN overload. This is perhaps one of the most common on-site procedures, where workers start removing shores from a slab edge and continue by rows until the opposite slab edge is reached. Applying the same criterion for the other direction, maximum absolute shore load is reduced by 9% (13.97 kN), with a 3.90 kN overload (26% reduction). Therefore, overloads are considerably reduced by taking the precaution of stripping by rows in the direction where the distance between columns is less. Spiral shore removal did not reduce maximum overloads. Late removal of shores located at the central row edges (shores 2 and 20) led to considerable overloads on both shores. Maximum overload originated on shore 2 was 5.16 kN, with a maximum absolute shore load of 15.23 kN. Procedure 3 involved a significant improvement with respect to previous procedures. Since the most loaded row was removed first, maximum overloads were generated on the less loaded shores. Consequently, maximum absolute shore loads decreased. Maximum shore overload was 4.05 kN, causing a maximum absolute shore load of 12.97 kN at shore 1. Therefore, with respect to the initial case 1A, maximum overload value was reduced by 23% and maximum absolute load by 16%. Procedure 4 did not lead to significant improvements when compared to Procedure 3. Albeit the overloads on shores placed
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Table 3 Maximum shore overloads and maximum absolute loads during shore removal (kN). Shore
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Before removal
8.92 10.07 8.92 7.18 9.04 10.23 9.04 7.18 7.54 9.17 10.34 9.17 7.54 7.18 9.04 10.23 9.04 7.18 8.92 10.07 8.92
Procedure 1A
Proc 1B
Max ol
Max absl
Max ol
Max absl
Max ol
Proc 2 Max absl
Max ol
Max absl
Max ol
Max absl
Max ol
Max absl
– 0.49 0.76 0.48 2.21 2.63 1.50 0.19 0.62 2.08 3.35 2.61 1.00 1.11 2.79 3.52 2.12 0.79 2.98 5.28 4.38
8.92 10.56 9.68 7.66 11.26 12.86 10.54 7.37 8.15 11.24 13.69 11.77 8.54 8.29 11.84 13.75 11.16 7.98 11.90 15.35 13.30
1.26 1.43 4.10 – 1.04 2.82 3.63 1.03 0.24 1.18 3.21 3.70 1.57 0.36 1.01 3.19 3.52 1.47 0.20 3.90 2.52
10.18 11.50 13.01 7.18 10.08 13.05 12.67 8.21 7.77 10.34 13.55 12.87 9.10 7.55 10.05 13.42 12.56 8.65 9.11 13.97 11.44
4.10 5.16 2.95 1.03 1.32 2.04 1.17 1.11 1.57 1.98 – 1.23 1.30 1.47 2.08 0.69 0.80 0.90 2.44 4.58 3.77
13.01 15.23 11.87 8.21 10.37 12.27 10.21 8.29 9.10 11.14 10.34 10.40 8.84 8.65 11.12 10.92 9.85 8.09 11.36 14.64 12.69
4.05 1.92 1.71 1.02 3.59 1.27 2.66 1.47 1.57 3.67 1.28 2.82 1.57 1.46 3.48 1.06 2.74 1.03 2.48 – 3.14
12.97 11.99 10.62 8.20 12.63 11.51 11.70 8.65 9.10 12.83 11.62 11.99 9.10 8.64 12.52 11.29 11.79 8.21 11.40 10.06 12.05
3.09 1.80 3.98 1.46 2.61 1.04 3.43 1.46 1.29 1.85 – 2.60 1.29 1.32 2.35 0.69 3.16 1.32 3.14 1.92 4.05
12.01 11.87 12.90 8.64 11.65 11.27 12.48 8.64 8.83 11.02 10.34 11.76 8.83 8.51 11.40 10.92 12.20 8.51 12.05 11.99 12.97
3.14 1.92 4.05 1.46 2.74 1.27 3.59 1.46 1.29 1.85 – 2.60 1.29 1.32 2.35 0.69 3.16 1.32 2.95 1.59 3.83
12.05 11.99 12.97 8.64 11.79 11.51 12.63 8.64 8.83 11.02 10.34 11.76 8.83 8.51 11.40 10.92 12.20 8.51 11.87 11.65 12.75
in the center of the module were reduced, maximum loads were similar to those calculated in Procedure 3. Therefore, the removal of shores alternating the position within the row did not reduce maximum loads when compared with Procedure 3. Finally, maximum loads did not decrease when Procedure 5 was checked. In fact, the procedure of removing the most loaded shore is almost the same as Procedure 4. Therefore, from a practical point of view, Procedure 3 gets optimum results. Hence, the shore removal process should be carried out by rows: the most loaded row of shores should be removed first, then the adjacent rows and finally the less loaded rows. The use of this procedure will be especially important when maximum shore loads are present. During the construction of any multistory concrete building, removal of the lowest level of shores, which are supported on ground is the most critical, since maximum shore loads occur at this stage. For a typical 4 column module supported by shores with a similar tributary area, the most loaded row is located in the center according to the direction corresponding with the longest bay (X direction in the example studied here). 6. Conclusions The overloads originating during the shore removal process can cause shore failure in some cases. Damage to the falsework can produce important economic losses due to its elevated cost. Therefore, on-site measurements were conducted during the shore stripping at different levels. Based on these experimental measurements, the adequacy of the Refined Method for the shore removal operation modeling was evaluated. Finally, the shore overloads originated in different shore removal procedures were studied. From this study the following conclusions can be stated: 1. During the shore removal process, shores are subjected to considerable load increments. Overloads of up to 3 kN (15% of the total load, approximately) have been reached during onsite measurements, which represent approximately 10% of the shore capacity. 2. The Refined Method models adequately the shore removal procedure. The Refined 3D Method predicts correctly trends within the redistribution of loads. The theoretical model proposes overload values greater than real ones. Therefore, it is a model that proposes values which lean on the safe side, as shown in Figs. 6–8.
Proc 3
Proc 4
Proc 5
3. Overloads generated during shore removal are influenced by the stiffness of slabs and shores. Hence, use of actual concrete strength and modeling of steel reinforcement produces more accurate theoretical results. Nevertheless, relatively low strength variations and steel reinforcement consideration do not lead to important variations of theoretical values (less than 4%). Therefore, modeling the slab with its project characteristic strength and without its steel reinforcement leads to less accurate but acceptable results. 4. The overload supported by each shore is influenced by its relative position within the module. Therefore, different overloads are produced depending on the shores which are removed. Consequently, the stripping order affects the maximum shore loads considerably. 5. The best stripping procedure found consists of removing shores by rows. First, the most loaded row must be removed, then the adjacent rows and so on until the removal of less loaded rows. For a typical symmetric 4 column module, the most loaded row, in the most critical situation, is the central one in the longest bay direction. 6. Although an alternating removal of the shores located within a row does not produce a reduction on shore overloads, it could be beneficial for the concrete slab. By stripping the shores in the middle first, the slab will be loaded as designed. First, the central shore of the row should be removed, then both adjacent shores and so on until the removal of shores located at both edges of the row. Acknowledgements This research was sponsored by the Basque Government (Departamento de Educación, Universidades e Investigación and Departamento de Industria, Comercio y Turismo, Project number UE2005-1) and the Spanish Government (Ministerio de Fomento, Programa Nacional de Construcción, Project Ref. 80003/A04). Ulma Construcción also sponsored this study and supplied part of the material needed during the measurements. The authors also wish to acknowledge the collaboration of Construcciones Imaz in the collection of experimental data. The opinions expressed in this work are those of the writers and do not necessarily reflect the points of view of the sponsors.
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