Development of composite glass beams – A review

Engineering Structures 101 (2015) 1–15

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Review article

Development of composite glass beams – A review K. Martens ⇑, R. Caspeele, J. Belis Ghent University, Department of Structural Engineering, Ghent, Belgium

a r t i c l e

i n f o

Article history: Received 12 January 2015 Revised 1 July 2015 Accepted 2 July 2015

Keywords: Composite glass beams Experimental bending test Post-breakage performance Robustness

a b s t r a c t In architecture, there is a growing trend to include more transparency in structures. To increase the transparency, structural elements such as columns, beams, floors and roofs can be built in glass. However, glass is a brittle material and weak in tension, making it less evident to function as a key structural element. To provide robust and safe elements, researchers have tried to develop hybrid glass elements in which glass is combined with other materials. For the case of structural beams, composite glass beams were developed. These beams have typically T- or I-sections, in which the web is a glass laminate and the flanges are composed of another material. Both entities are then put together by using an adhesive or a bolted connection system. Also other concepts exist which have a layered section or in which the materials are combined to make trusses. In this paper, all developments and experimental investigations of this kind of glass beams are summarised using a classification based on the combined material. The concepts are evaluated considering load–displacement diagrams from bending tests. Most of the concepts described illustrate beneficial failure behaviour when subjected to bending, meaning significant post-breakage strength and ductility. Several concepts even possess a post-breakage strength that is larger than the initial glass failure load, while demonstrating extensive deformation capacity. With respect to practical applications, the latter concepts can be considered as very suitable as they generally provide significant robustness. Ó 2015 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Summary of developments and research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.1. Combination of glass and timber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2. Combination of glass and reinforced concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.3. Combination of glass and GFRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.4. Combination of glass and metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.5. Combination of glass and plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Proposals for further research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1. Introduction To increase transparency, the application of structural glass elements in buildings is a trend in today’s world. However, the ⇑ Corresponding author at: Technologiepark Zwijnaarde 904, 9052 Zwijnaarde, Ghent, Belgium. E-mail address: [email protected] (K. Martens). http://dx.doi.org/10.1016/j.engstruct.2015.07.006 0141-0296/Ó 2015 Elsevier Ltd. All rights reserved.

application of glass for structural use is not that evident. The material has important shortcomings such as its brittle behaviour, much lower tensile strength than compressive strength, the risk of fracture due to defects such as material imperfections or inclusions and the decrease of strength in time due to surface damage. As a consequence, the safety of ‘traditional’ elements, such as glass beams and façade fins (typically consisting of laminated glass) is mainly provided through over-dimensioning techniques such as

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sacrificial glass panes and unnecessary large glass thicknesses. However, these techniques are certainly not economical and when such elements fail, it is still in a quasi-brittle way as these measures do not increase their ductility. Moreover, the interlayer is not able to provide sufficient ductility during failure. To overcome this undesired failure behaviour, efforts have been made to introduce ductility by including other materials in the element’s section, creating hybrid glass beams. Such hybrids possess post-breakage (i.e. after glass breakage) capacity and ductility, which results in an increased structural safety. In general, hybrids can be subdivided according to the amount of material used in conjunction with glass and according to the shape of the resulting beam section in either ‘composite glass beams’ or ‘reinforced and post-tensioned glass beams’ (see Fig. 1). For the case of ‘composite glass beams’, the other material has a relatively high share in the beam’s section (typically higher than 25%) and is actively contributing to the load-carrying behaviour of the element from the beginning of loading. ‘Composite glass beams’ are typically Tand I-section beams in which the web consists of (laminated) glass, whereas the flanges are made of another material. Both entities are connected by using an adhesive or a bolted connection system. Also other concepts were developed which have layered sections, alternating between glass and the complementary material. In contrast, in the ‘reinforced and post-tensioned glass beams’ subdivision, relatively small amounts of other material (less than 10%) are added to the beam’s section which only become active in the post-breakage phase (reinforced glass beams) and/or change the load-carrying behaviour of the glass section in the initial linear elastic phase (post-tensioned glass beams can posses a higher initial glass failure load). Additionally, the added material generally does not alter the shape of the beam section (i.e. a rectangular glass section will stay rectangular after the reinforcement has been added). The latter concepts correspond well to the concepts of reinforced and post-tensioned concrete beams. A lot of work has been performed in this field [1–11]. This information is not included in this paper, however some important references on this topic are included at the end of this work. Also a zone of overlap exists between both subdivisions, as certain members possess characteristics of both categories. For instance, concepts have been developed in which triangular glass panes are used as load bearing diaphragms in steel trusses. This type of beams was called TVT-beam or Trabes Vitreae Tensegrity [12,13]. Satisfying the definition above, this concept is considered a composite glass beam. However, the steel truss members were post-tensioned. As a result, this beam could also be categorised as a post-tensioned glass beam. Furthermore, the section of a layered beam satisfies the requirements for a composite glass beam, however, the relative percentage of other material is smaller than 25% (but higher than 10%). Therefore, a zone of overlap is included

Fig. 1. Subdivision of hybrid glass beams.

in Fig. 1. This paper presents an overview of the developments in the category of ‘composite glass beams’, followed by an evaluation of all these concepts. The paper focusses merely on experimental research. Experimental work is extremely important in structural glass design of this kind. Also for the sake of validating numerical and analytical models, experimental work is essential. However, it is stated that a significant amount of numerical and analytical investigations on these concepts is also available for several of the treated concepts. For more information in this regard, the authors refer to the proper papers used in the text. Up to now, five different materials have been used in combination with glass to produce composite glass beams. More specifically, these are timber, concrete, Glass Fibre Reinforced Polymer (GFRP), steel and plastics. A classification is proposed according to these types of combining material. 2. Summary of developments and research Composites are typically built up out of a combination of several materials that are more or less equally present and which make specific contributions to the characteristics of the element. The goal of producing composites is to create synergy (a new ’material’ with characteristics that are a combination of the beneficial material properties of the constituents). For the case of composite glass beams, the goal is to increase the post-breakage strength and ductility by combining the high compressive strength and transparency of glass with the tensile resistance and/or ductility of the other material. In the following subsections, an overview of developments and research with respect to composite glass beams performed up to now is presented. Subdivisions are based on the different combined materials. 2.1. Combination of glass and timber The first concept uses timber as the material to be combined with glass. In a first application, the beams typically exhibit I-sections that consist of a glass web and timber flanges. They are connected to each other using an adhesive (see Fig. 2(a)–(f)) [14–18]. The performance of these beams was tested through four-point bending tests. First, Hamm tested the concept by building 4000 mm long test specimens [14]. The section was made out of 10 mm thick annealed float glass and varying timber flange sections glued to the glass with a polyurethane adhesive (see Fig. 2(a)). Good post-breakage behaviour was reported from all tests as can be seen in Fig. 3(a) (p. 8). Also Kreher performed an experimental test series in which he varied the type of glass, glass thickness and the timber flange sections [15] (see Fig. 2(b)). The beam specimens had a span of 2000 mm. The tests demonstrated a ductile failure behaviour and an increased residual strength up to 300%. The concept investigated by Kreher was applied in the construction of the Palafitte hotel in Switzerland where 6000 mm long beams were applied [16] (see Fig. 2(c)). Next, Cruz & Pequeno investigated this concept using laminated glass [17]. The web of the beams had a height of 500 mm and consisted of two laminated 6 mm annealed float glass panes. The rectangular timber flanges had a cross section of 70 mm  100 mm (see Fig. 2(d)). 15 composite beams having three different adhesives (polymer, silicone and polyurethane) and varying spans between 650 and 3200 mm, were examined. Cruz & Pequeno also investigated a rectangular section in which the timber flanges were placed between two laminated glass webs (see Fig. 2(e)). Fig. 3(b) (p. 8) illustrates the load–displacement curves from four-point bending tests on both beam sections in case of a 3200 mm long span, using silicone adhesive.

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Fig. 2. Timber-glass composite beam sections according to: (a) Hamm [14], (b) Kreher [15], (c) section for the Palafitte hotel [16], (d) Cruz & Pequeno [17], (e) rectangular section [17], (f) Premrov et al. [18], (g) Blyberg [19–21], (h) Hulimka & Kozlowski [22] and (i) Kozlowski et al. [23]. (Dimensions in mm.)

As can be seen, the I-section and rectangular section possess a post-initial crack strength of respectively 185% and 130%. Both beams demonstrated significant ductility. In the research of Premrov et al. [18], the web of the beam consisted of an 8 mm thick annealed float glass sheet. The C24 timber flanges were bonded to the glass web using a silicone adhesive (see Fig. 2(f)). 12 beams with a length of 4800 mm were produced. One beam was tested in four-point bending, using a span of 4320 mm. The load–displacement diagram is given in Fig. 3(c) (p. 8). One can observe a large post-initial crack strength (first crack at 3.2 kN and beam failure at 10.45 kN) and significant ductility for this beam. Also another concept of timber-glass composite beams was investigated. In this concept, the timber flanges of the I-section exist out of one piece as illustrated in Fig. 2(g), (h) and (i). Firstly, Blyberg et al. performed an experimental testing programme on twelve beam specimens in which an acrylate adhesive was used to realise the bond between a 10 mm float glass web and two types of Laminated Veneer Lumber (LVL) flanges, which differed in groove width [19–21] (see Fig. 2(g)). Two different edge finishings for the glass web were also tested. For comparative reasons, one beam was built using a silicone adhesive. The beam specimens had a span of 3500 mm. The load–displacement curves in Fig. 3(d) (p. 8) prove the beneficial failure behaviour of this concept. Later on, Hulimka & Kozlowski tested the concept by using a silicone adhesive to realise the bond [22]. For the glass web an 8 mm thick annealed float glass pane was used. The beams were 1800 mm long and had a test span of 1500 mm. Again, two different groove widths were used for the timber flanges (see Fig. 2(h)). The post-breakage behaviour can be seen in Fig. 3(e) (p. 8). Recently, Kozlowski et al. examined the concept for different types of glass and different types of adhesives [23]. The web consisted of an 8 mm annealed float glass pane (ANG) or an 8 mm heat

strengthened glass pane (HSG). A silicone (Sikasil SG-500; S), acrylate (SikaFast 5221; A) and an epoxy (3M DP490; E) were used to bond the timber flanges to the glass. The wooden flanges (60 mm wide) were finger-jointed pine studs (see Fig. 2(i)). The beam specimens were tested in four-point bending, with a span of 4320 mm. The resulting load–displacement diagram is depicted in Fig. 3(f) (p. 8). The results lead to the conclusion that heat-strengthened glass (HSG) beams establish higher loads at initial cracking than annealed float glass beams, but they fail in a brittle way (without post-breakage strength). Therefore, annealed float glass is recommended in glass-timber beams. Furthermore, the beams with stiff adhesives present a behaviour that is close to full composite action. The beams produced with the softer silicone exhibited a 25% lower stiffness. Both concepts demonstrate a safe failure behaviour (i.e. sufficient post-breakage strength and ductility). The timber flange at the tensile side of the beam acts as reinforcement. When a crack has developed, the timber forms a crack bridge through which the tensile stresses can be transferred, securing the beam’s carrying capacity. However, current research is still addressing several problems with this beam concept, e.g. the large difference in behaviour in the case of varying moisture, as wood has a hygroscopic nature [24]. The latter characteristic will introduce further demands on the adhesive used for bonding glass and wood in terms of flexibility. A possible solution could be a type of wood treatment that limits the moisture dependency [25]. However, this has not been tested yet for timber-glass composites. 2.2. Combination of glass and reinforced concrete A second example of composites is the combination of glass and reinforced concrete, investigated by Freytag [26]. As was the case

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Fig. 3. Load–displacement diagrams for timber-glass composite beams according to: (a) Hamm [14], (b) Cruz and Pequeno [17], (c) Premrov et al. [18], (d) Blyberg et al. (A = Acrylate, S = Silicone; L = Large groove width, S = Small groove width; N = No finish, P = Polished edges) [19], (e) Hulimka and Kozlowski [22] and (f) Kozlowski et al. (A = Acrylate, S = Silicone; E = Epoxy, AF = Annealed float glass; HS = Heat-strengthened glass) [23].

for timber, the beams have an I-shaped section. The web consisted of a glass laminate composed of three fully tempered 8 mm glass panes. The flanges were composed of ultra-high-performance reinforced concrete. Both entities were connected to each other by placing the web in a mould in which the concrete for the flanges was poured. To achieve a proper connection, the glass was pre-treated by roughening its contact surfaces (an enamelled coating containing fine corundum was brushed onto the glass and was

subsequently annealed during the thermal toughening process). The concrete was reinforced by steel rebars in the top flange and by either prestressing tendons or steel rebars in the bottom flange (see Fig. 4(a) and (b), respectively). The beams (7800 mm) were tested through four-point bending. The tests yielded a high load-bearing capacity for this type of beams. Also, the experiments illustrated a strong influence of prestressing on the load-bearing capacity. The prestressed beam

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Fig. 4. Reinforced concrete-glass composite beam according to Freytag [26]. (Dimensions in mm.)

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cracks ran into the contact zone where the glass is covered by concrete. As a consequence, the static system for the transmission of shear forces between the components changed from a stiff shear panel (uncracked glass) to a series of small glass teeth between the cracks. These teeth are tremendously loaded when shear forces are transmitted. Due to this weaker system, the bond stress in the contact surfaces increased. However, no bond failure was observed. Upon further loading, the glass struts between the cracks got overloaded and failed by crushing or spalling. Each breakage of glass struts weakened the web. The beam finally collapsed when the central glass pane failed at one of the sides of the beam where the shear is transferred to the supports. In spite of its high load-bearing capacity, this type of beam has some drawbacks. Firstly, the manufacturing process is rather complicated and the glass surface needs special treatment. Secondly, fully tempered glass has no post-breakage stiffness (compared to annealed float glass). So, when the glass web breaks near the supports, shear forces can only be transferred to the supports through the bottom concrete flange. As the latter is not designed for this (and should not be), it will fail instantly. So, failure of the glass web near the supports means instant collapse of the beam. Finally, the quality of the glass and the interlayer could deteriorate in time due to the corrosive effects of the alkaline characteristics of concrete. 2.3. Combination of glass and GFRP

Fig. 5. Load–displacement diagram for the prestressed concrete-glass beam according to Freytag [26].

reached a failure load that is 2.5 times the failure load of the simply reinforced beam. The load–displacement curve for the prestressed beam is depicted in Fig. 5. As the reader may observe, the curve does not pass through the origin of the graph and starts at a negative displacement value. This can easily be explained by the prestressing process. Actually, the test existed out of two phases. Firstly, the prestress was applied which gave the beam an upward negative deflection. Subsequently, the beam was loaded in four-point bending (a load of about 75 kN was required to lower the beam back to zero displacement). In this figure, one can observe a very stiff and linear behaviour until cracking in the outer glass panes occurred. A lot of these

A third concept of composite beams is introduced by the combination of glass and Glass Fibre Reinforced Polymers (GFRP). Two example beam configurations were investigated. A first one is composed by alternating 8 mm thick float glass sheets with sheets of GFRP. This can be done horizontally, as well as vertically. The sheets were connected through a two-component epoxy resin, creating beam sections of 80 mm  170 mm (see Fig. 6(a) & (b)) [9,27,28]. The 2500 mm long beams, tested through four-point bending, illustrated a good post-breakage behaviour in which the final deformation is many times the initial cracking deformation (see Fig. 7(a) (p. 15)). The latter observation indicates ductile behaviour. For the GFRP-H beams (GFRP-glass composite beams with horizontally layered section), the ultimate failure load is larger than the load at first cracking. This is not the case for the GFRP-V beams (GFRP-glass composite beams with vertically layered section): after initial cracking of the glass sheets, the capacity of the beams is maintained as the tensile strength is provided by the GFRP sheets. Upon further loading, many cracks developed and grew until they reached the top of the beam (see the many load drops in the diagrams), resulting in final collapse. The second configuration is similar to the I-beam concept illustrated in the timber-glass and concrete-glass combinations described previously. This type consists of a 12 mm annealed float glass web and GFRP flanges, connected to the web through an

Fig. 6. GFRP-glass composite beams according to: (a) GFRP-H and (b) GFRP-V Speranzini et al. [28], (c) Correia et al. [29], (d) Valarinho et al. [30] and (e) the Glass & Façade Technology Research Group [31]. (Dimensions in mm.)

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Fig. 7. Load–displacement diagrams for GFRP-glass composite beams according to: (a) Speranzini et al. [27,28,9], (b) Correia et al. [29], (c and d) Valarinho et al. [30], (e) the Glass & Façade Technology Research Group [31].

adhesive as illustrated in Fig. 6(c) & (d). In a first investigation, four-point bending tests with a span of 1.50 m were performed on 1.80 m long beams. The beams were built with either a polyurethane or an epoxy adhesive, both having a thickness of 2 mm (see Fig. 6(c)) [29]. In Fig. 7(b) (p. 15), the load–displacement diagrams for both adhesives are illustrated. After initial glass breakage, the load could be increased until the full capacity of the composite beam was reached. In this phase, increased glass cracking lowered the bending stiffness of the beam, resulting in higher vertical displacements. It can be concluded that both beams demonstrated very good behaviour in terms of post-cracking strength and ductility. It is also concluded that the type of adhesive plays an important

role in the load-bearing behaviour. The polyurethane has a lower stiffness, which resulted in a good ductile behaviour, but with lower initial stiffness, post-cracking strength and ultimate load capacity of the composite beams. A post-breakage performance of about 153% was recorded. On the contrary, the beams with epoxy illustrated the reverse. A post-breakage performance of almost 200% was achieved. The choice of adhesive will depend on the requirements of the composite beam in its practical application. However, it is stated that the performance of the beams is better with the epoxy adhesive. Subsequently, a second investigation followed with a slightly different beam section, as illustrated in Fig. 6(d) [30]. Three kinds of adhesives were used in this test, i.e. two polyurethane adhesives

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(a gap-filling one and a structural one, called Sikaflex and Sikaforce respectively) and one epoxy adhesive (Sikadur). Because of the varying viscosity of the adhesives, their thickness was adapted ranging from 1 mm to 2 mm. Firstly, four-point bending tests with a span of 1.4 m were conducted on 1.5 m long beams. In a second stage, 3.0 m long beams were tested in a symmetrical 5-point bending test having two spans of 1.40 m. The aim of this research was to evaluate the feasibility of applying such beams in statically indeterminate systems. For the statically determinate case, Fig. 7(c) (p. 15) illustrates the load–displacement curves for all three adhesives. As was the case in the first investigation, it is stated that all beams exhibited a pseudo-ductile behaviour after initial glass cracking, which is highly dependent on the type of adhesive. Highest post-cracking strengths were found for the epoxy adhesive. For these beams an average post-breakage performance of 282% was reached. In contrast, the highest ductility was achieved for the polyurethane beams that exhibited a post-breakage performance of 165% (Sikaflex). The adhesive also influenced the cracking pattern of the beams. Few concentrated cracks were reported for the ductile gap-filling polyurethane-bonded beams, while a much more regular pattern was observed for the other types. Furthermore, the amount of composite action was determined using strain gauge measurements. It was concluded that almost full composite action was observed for the epoxy and structural polyurethane adhesives, while significant slip was encountered with the gap-filling polyurethane adhesive, resulting in a low interaction level. Subsequently, the statically indeterminate five-point bending tests were performed. The resulting load–displacement diagrams for the three kinds of adhesives are presented in Fig. 7(d) (p. 15). It is concluded that a similar behaviour as in the statically determinate case was observed, i.e. a linear elastic stage followed by a pseudo-ductile behaviour [30]. However, this ductile behaviour was different in both spans. Again, the type of adhesive played an important role in the behaviour of the beams. Regarding cracking load, cracking pattern, initial stiffness, deflection and composite action, the same conclusions as in the statically determinate case can be drawn. However, a different post-breakage performance was observed and now the beam with gap-filling polyurethane achieved the highest value (202%). The beams bonded with the other polyurethane and epoxy adhesive achieved a value of 130% and 180% respectively. This phenomenon is explained by the higher force redistribution capacity of the first beam, resulting from its high ductility. Also the other two types of beams illustrated force redistribution, although in a far less extent. It was concluded that a higher moment redistribution capacity is achieved when an adhesive with a lower stiffness is used. However, it is stated that in this case the redistribution is a consequence of stiffness reduction in several sections (i.e. damage) and not the result of the mechanical behaviour of the materials involved (e.g. steel, reinforced concrete). The beams bonded with the structural polyurethane and epoxy adhesive failed by sudden and explosive disintegration of the glass web. For the other beam (gap-filling polyurethane), failure was mainly due to lateral bending and crushing of the glass web below one of the loaded sections. It is concluded that the post-cracking behaviour observed in the simply supported beams can be reproduced in two-span beams, making use of the cross-sectional redundancy, the force redistribution and the statically indeterminate support conditions. For both cases, the mechanical properties of the adhesive are highly influencing the performance of the beams. Recently, the Glass & Façade Technology Research Group at the University of Cambridge completed a project in which the feasibility of composite components consisting of glass and GFRP was investigated [31]. As a part of this project, a series of GFRP-glass beams were tested through four-point bending. The 500 mm long

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beams had a span of 460 mm during the tests. The beam’s section is depicted in Fig. 6(e). The web consisted of a GFRP rectangular hollow section (RHS) and for the flanges, 10 mm thick monolithic toughened glass panes were used. The connection between web and flanges was executed with an epoxy or an acrylic adhesive or by applying a sliding joint using polytetrafluoroethylene (PTFE) plates instead. The load–displacement diagrams for the three types of beams are illustrated in Fig. 7(e) (p. 15). The first load drops for the adhesively bonded beams correspond to FRP shearing. Initial glass failure occurred at 7 mm for the beam using PTFE plates, at 8 mm for the epoxy-based beam and at about 11 mm for the acrylate-based beam. The residual load-carrying behaviour was entirely addressed to the GFRP section. It was concluded that both adhesively bonded beams demonstrated higher strength and bending stiffness compared to the beam using PTFE plates. The beam composed with epoxy adhesive illustrated the best performance. For both adhesively bonded beams, initial failure was governed by the longitudinal shear strength of the GFRP profile. As a result, the load bearing capacity of these beams can be upgraded easily (e.g. by choosing RHS profiles with higher thickness). Moreover, thanks to the GFRP-section, the beams also demonstrated significant strength and stiffness after both glass panes were broken, indicating a significant robustness level. This beam concept was rather atypical as the web existed out of the combining material and glass was used for the flanges. In all other beam concepts, the reverse was true. The reason for this lies in the application purpose. Unlike all other cases in which the goal is to develop structural glass beams, this concept was tested with the aim to develop glazed façade units. Keeping this in mind, it is easy to see that the glass flanges actually build up a double-skin façade. Several of the investigated concepts above exhibited safe failure behaviour through post-breakage strength and ductility. However, this ductility is different from what is generally expected. Unlike steel, GFRP fails in a brittle way without any yielding. The ductility achieved by these concepts is addressed to the installation of a new load-transfer mechanism, namely the activation of the GFRP-components in the section. Ultimate collapse of these beams still occurred in a brittle way through cracking of the GFRP. So, it is stated that GFRP-glass composite beams possess a semi-ductile post-breakage behaviour, characterised by deterioration of the GFRP components through cracking (e.g. for the GFRP-H and GFRP-V beams, the GFRP breaks layer by layer) and/or increased glass cracking of the web, which resulted in higher displacements. So, the extent of ductility is rather limited for these beams. Furthermore, despite the advantage that GFRP can be produced transparent, the cost of this material is large compared to other, more frequent, materials such as steel, timber and concrete. Finally, GFRP also exhibits a time-dependent behaviour (creep), which should be assessed for these applications. 2.4. Combination of glass and metals The next example of composite beams is composed of glass and metals. Several different concepts were developed and tested. As a first concept, steel-glass hybrid I-section beams were investigated. These beams consisted of a glass web and steel flanges, connected through an adhesive. A first section was investigated by Wellershoff et al. (see Fig. 8(a)) [32–35]. The steel flanges were connected to the 20 mm thick tempered glass web through steel leg angles, bolted to the flanges and adhesively bonded to the glass web with a Polyurethane adhesive. A 3600 mm long specimen was examined through four-point bending, resulting in an initial crack strength of 137.8 kN (which is much larger than what would have been achieved with a similar reference beam made of glass only). The span of the beam was

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Fig. 8. Steel-glass composite I-sections according to: (a) Wellershoff et al. [32–35], (b–h) Abeln et al. [36].

3450 mm. The load–displacement diagram of this non-destructive test can be seen in Fig. 12 (a) (p. 24). The ultimate strength of the beam (corresponding to collapse) was estimated in the range of 375 kN to 425 kN. Next, Bucak et al. tested an I-section wherein the steel flanges were directly connected to the glass web using various adhesives [37]. Beam specimens of 6 m to 11 m were produced and tested through four-point bending. The glass web was typically realised as a three- or fivefold laminate, using heat strengthened glass. Because no information was found about the cross section’s dimensions, these sections were not added to Fig. 8. However, the concept is the same as in Fig. 8(b) and (g). To realise the bond between the steel flanges and the glass web, use was made of either an acrylate, polyurethane or a silicone adhesive. The specimens were tested in four-point bending, using spans of approximately 5 m and 10 m, for the small and long beams respectively. It was found that the stress distribution, and therefore the load carrying capacity, in the hybrid steel-glass beams was highly dependent on the stiffness of the adhesive. The highest load carrying capacity was found for the beams composed with the most stiff adhesive (i.e. acrylics). In the Innoglast project, an extensive experimental testing programme was performed on hybrid steel-glass beams [36,38–40]. A first experimental programme was executed at TU Dortmund, in which four-point bending tests were performed with a span of 4 m [36,38,40]. The glass web was composed of two laminated toughened glass panes. Two different glass-steel connections were tested. In the first one, the steel flange is directly connected to the glass web as depicted in Fig. 8(b). In this case,

two steel grades were used, i.e. S235 and S355. In the second connection, the glass web is embedded in a U-profile which is welded to the steel flange (see Fig. 8(c)). Here, grade S235 was used. Furthermore, four different adhesives were used: a polyurethane, soft and stiff epoxy resins and a silicone adhesive. For all adhesives, a thickness of 3 mm was applied. From the bending tests it was concluded that the failure load increased with increasing connection stiffness. So, the stiffer the adhesive, the higher the failure load as illustrated in Fig. 12(b) (p. 24). A post-breakage performance of 132% was found for the silicone adhesive, whereas values between 180% and 188% were achieved using the polyurethane adhesive. The test with the soft epoxy yielded a value of 192%, which was significantly lower than the 316–404% of those with the stiff epoxy. Furthermore, the U-profile connection seemed to demonstrate higher stiffness than the direct connection (The difference on the graph is rather small, but this is due to an insufficient upper flange connection for the U-profile bonded beam. A higher failure load is expected for a correctly produced beam [36]). After initial glass breakage occurred, the tensile stresses had to be transferred to the steel flange at the weekend sections. The more stiff the adhesive and the larger the connected contact surface, the higher the transferable shear load. As a result, the U-profile connection together with the stiff epoxy yielded the highest performance. Yielding of steel was encountered for this latter case. In all other cases, shear failure of the connection was encountered at a smaller load. A second experimental programme was conducted at the Czech Technical University [36,39]. Here, four-point bending tests with a span of 4 m were performed on two types of beam specimens until

K. Martens et al. / Engineering Structures 101 (2015) 1–15

glass failure occurred (see Fig. 8(d) & (e)). The first type consisted of a double-layered toughened glass web made of 12 mm panes, which was embedded at both sides in a U-profile that was connected to a S235 steel flange. The horizontal contact between glass and steel was avoided by inserting a polyamide layer. The profile and glass were bonded at their vertical contact zones by filling the gaps with adhesive. The second type consisted out of a 19 mm thick toughened glass pane which was directly connected with two S235 steel flanges at the horizontal contact zones. A thickness of 3 mm was used for the adhesive. Three types of adhesive were used, i.e. an acrylic adhesive, a polyurethane adhesive and a polyurethane booster system (the booster component provides uniform hardening of the adhesive layer. Furthermore, the process of curing does not depend on air humidity and most important, curing is finished in hours and not in days as for other one-component adhesives). The tests resulted in the conclusion that higher load-bearing capacities were achieved for the U-profile connected beam. Furthermore, this capacity is dependent on the type of adhesive used. The stiffer the adhesive, the higher the load-bearing capacity. One can easily draw these conclusions from Fig. 12(c) (p. 24). As all beams yielded good results, it was concluded that the type of adhesive should be chosen in function of the temperature and elongation requirements and of the type of load acting on the real structure. Additionally, the beam connected with U-profiles using acrylic adhesive demonstrated residual carrying capacity and stability when both glass panes were broken. In addition, flexural and lateral torsional buckling tests and robustness tests were performed [36]. The buckling tests were conducted on beams with a section as illustrated in Fig. 8(f). A 12 mm toughened glass pane was directly connected to grade S235 steel flanges. Two adhesives were chosen, i.e. a medium stiff polyurethane and an epoxy resin with high stiffness. For the flexural buckling test, 1 m long specimens were produced and axially loaded. The supports were designed in such a way that the beam was able to buckle along its weak axis. For the lateral torsional buckling tests, 1.6 m long specimens were tested in three-point bending in which the supports could rotate about their vertical axis, enabling the beam to buckle about its weak axis. From the flexural buckling tests, it was concluded that the beams with the polyurethane adhesive buckled abruptly whereas the ones with epoxy adhesive buckled more gradually. This can be explained considering the buckling mechanism. For the polyurethane specimens, buckling of the glass web and steel flanges occurred simultaneously. For the case of epoxy, the steel flanges buckled first, followed by the glass pane. As a result, buckling of the whole beam section occurred in an almost ’plastic’ way, as the steel plastically deformed during the buckling of the glass pane. For both cases, a significant residual carrying capacity and ductile post-breakage behaviour was observed. The amount of lateral torsional buckling tests was too small (4 tests) to draw general conclusions. In these tests, the two beams with epoxy illustrated global stability failure. Those with polyurethane resulted in adhesive failure with extremely high rotations of the flanges so that the tests were stopped. For the robustness tests, 4.25 m long beams were produced in which the glass web was composed of 5 laminated HSG panes in longitudinal direction. Two types of joints were used for the contact of the glass panes. The first type was an abutment joint and the other one an overlapping joint. Two beam sections were chosen, as illustrated in Fig. 8(g) & (h). An acrylate and a polyurethane adhesive was used. Prior to testing, one of the glass panes was mechanically broken. Subsequently, four-point bending tests were performed on these beams in which a serviceability load was introduced for 48 h (to evaluate if the broken beams are still able to perform in a practical situation for two days, during which repair works should be installed after which the beams can be removed).

9

Finally, the beams were unloaded and then loaded up to full collapse. For reference, tests with short duration were performed on beams without broken panels. From the latter tests, it was concluded that the polyurethane was too soft to sufficiently transfer shear forces between glass and steel. The beams with an U-profile connection illustrated better but insufficient results. As a result, no verification of robustness was done for this type of adhesive. Two test specimens were used for robustness verification. In the first one, the central glass pane was broken whereas the first pane near the support was chosen for the other. From the tests it was concluded that the hybrid beams meet all requirements regarding robustness, provided that an adhesive with a minimum initial shear modulus of 40 MPa to 50 MPa is used. Furthermore, the overlapping joint between longitudinal glass panes is recommended over the abutment joint as a higher web stiffness is achieved. High residual capacities were demonstrated by the tests. Another concept is the steel-framed glass beam. In this concept, a glass web is embedded in a surrounding steel framing. A first example of this concept is a high filigree glass-steel-system [41]. In this system, glass panes are used as compression stressed diagonal struts in a trussed framework. Tensile forces are transmitted by the surrounding steel members. To prove the effectiveness of the concept, a 4 m long glass-steel girder was built and tested through three-point bending as conceptually sketched in Fig. 9). The steel construction was built in normal modulus steel and the glass members were composed of laminated HSG panels (2  4 mm). The load–displacement diagram is given in Fig. 12(d) (p. 24). Considering the obtained load–displacement diagram, it was found that the girder illustrated a linear behaviour and that the glass panes closest to the supports failed due to flexural buckling at an ultimate load of about 135 kN. Optimisation of the components that compose these girders is possible and can be done for each specific application. Also, alternative materials for the strips (that hold the glass panes in place) such as timber, aluminium or GFRP can be used. Furthermore, alternative systems with tensile members made of round steel or cables have been developed too. A second example of steel-framed glass beams was developed by Absoluut Glastechniek, The Netherlands. A glass web is surrounded by a slender steel framing. The connection of both components is achieved through a structural sealant. This sealant also makes sure that there is no direct contact between glass and steel. In this system, longer spans can be created by bolting several framed glass segments together (see Fig. 10). This type of beams was experimentally investigated by Belis et al. [42]. Three-point bending tests were performed on 7.47 m long beams with a span of 7.4 m, consisting of two unequal segments of 4500 mm and 3000 mm long. The glass web was made of 2  12 mm laminated fully tempered glass (FTG). The structural

Fig. 9. Conceptual sketch of the filigree steel-glass girder in three-point bending according to Englhardt and Bergmeister [41].

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K. Martens et al. / Engineering Structures 101 (2015) 1–15

Fig. 10. Connection concept of the steel-framed glass beam according to Belis et al. [42].

sealant, ensured to hold the glass in place, to avoid direct contact between glass and steel and to guarantee load-transfer between both elements, was chosen to be 5 mm thick. In addition, one long-duration load-controlled test was executed wherein five point loads were introduced on the upper flange of the beam through application of mass elements. The investigation resulted in the following statements. To avoid squeezing of structural sealant at the zones with high compressive bending stresses, structural sealants with a higher Shore hardness should be applied in these specific regions. Doing so, the breakage of glass due to direct contact between glass and steel will be avoided. The lateral deformation of a glass panel that broke due to buckling was so large that, from a serviceability point of view, the beam had failed already prior to buckling. Furthermore, the broken glass panes remained well in place such that the failed beam was still able to carry its own weight. From the long-duration test, it was concluded that for this type of beam significant creep deformation could be expected. Especially in applications where high permanent loading is present, creep should effectively be taken into account. With the necessary improvements, this system illustrated adequate load-bearing capacity and eventually was applied in a public building in the Hague (The Netherlands) [42]. A third concept is the Hybrid pre-stressed steel-glass beam or ‘Trabes Vitreae Tensegrity’ (TVT), investigated by Froli & Lani and Mamone (see Fig. 11) [12,13]. It was developed on two basic

Fig. 11. Picture of the TVT Gamma beam.

principles, namely hierarchy and redundancy. With a hierarchic organisation of the components, one can control the sequence of progressive failures, starting from the level with the weakest components. For this application, ductile materials were put at the lowest level, assuring that failure will start with large plastic deformations, creating a global ductile behaviour. Redundancy makes sure that, when a component fails, the other components take over this part of the load transfer so that the beam is still able to carry the load. This redundancy aspect should be respected at each level of the hierarchy, from the weakest to the primary strongest elements. In addition, prestress is used to assure its integrity and to increase the apparent tensile strength of the glass panes. The beam is composed of equilateral triangle glass panes, held together by a system of prestressed steel cables. To improve redundancy and lateral stability, the beam is composed of two parallel identical curtains, braced in the upper side by a horizontal truss and connected in the lower edge nodes by stainless steel tubes. The design of the different components can be done in such a way that the collapse of the beam is due to yielding of the steel cables and not due to the breakage of glass panes. In this way, a ductile failure mechanism is created. A 3300 mm long prototype of this beam was produced and subjected to static, quasi-static cyclic and dynamic tests. The glass panes consisted of two 5 mm chemically tempered glass sheets, laminated with PVB. From the experimental tests, it was found that the beam is able to dissipate energy without suffering any fracture. This phenomenon is illustrated by the load–displacement diagram in Fig. 12(e) (p. 24), in which a comparison is made between static and cyclic (loops) tests. The ability to dissipate energy can be allocated to the friction that is created due to the slip movements at the interface between glass pane and steel nodes and maybe also to the viscoelastic slip in the PVB interlayer. In the monotonic test, failure was initiated in the ductile component of the hybrid beam. Due to strain hardening of the lower cables, the load could be further increased until final collapse occurred when the upper parts of the midspan glass panels buckled. The post-failure performance is estimated to be 120%. The same tests were performed on repaired specimens, achieved by only replacing the broken glass panels with new ones. Post-failure performances for the repaired beams are estimated to be 105%. When comparing the three beams, it was observed that the stiffness properties were preserved fairly well and the dissipated energy increased with the number of repairs. Recently a larger beam prototype, TVT Gamma, was built. This beam had a length of 12 m and is the one illustrated in Fig. 11 [13]. Both webs and the upper flange consisted of 2  10 mm PVB-laminated heat-strengthened glass panels. The panes were again embedded in steel nodes and the post-tensioning system was realised with steel bars. Dynamic as well as quasi-static cyclic four-point bending tests were executed. The resulting load–displacement diagram for the quasi-static cyclic test is given in Fig. 12(f) (p.24). From the diagram, one can conclude that also the larger prototype is able to dissipate a significant amount of energy without suffering any fracture. As the load increased, the tensile steel bars yielded, resulting in the diminished slope of the diagram. Final collapse of the prototype was reached when a triangular glass panel ruptured due to buckling. Large displacements were observed upon final collapse, which indicates a ductile failure behaviour. The post-failure performance of this beam is about 118%, which is close to the value obtained by the smaller prototype. The application of prestress to glass beams was also investigated in [43–46,10]. The mentioned concepts are similar to the concept of prestressed and post-tensioned concrete and therefore are classified as post-tensioned glass beams. However, this topic is out of the scope of this article and is not further discussed here.

K. Martens et al. / Engineering Structures 101 (2015) 1–15

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Fig. 12. Load–displacement diagrams for metal-glass composite beams according to: (a) Wellershoff et al. [32–35], (b and c) Abeln et al. [36], (d) Englhardt and Bergmeister [41], (e) Froli and Lani [12] and (f) Froli and Mamone [13].

2.5. Combination of glass and plastics Finally, attempts have been made to combine glass and transparent plastics [47,48]. Initially, Veer developed laminated glass beams in which the interlayer was substituted by a thin layer of ductile polycarbonate foil. This concept can be seen as a variant on laminated glass and therefore is not further investigated in this review. In the research of Hildebrand & Werner, a T-shaped beam was developed as illustrated in Fig. 13. Despite their low Young’s modulus, plastic sheets are capable of transmitting normal and shear stresses when properly designed.

Moreover, the glass sheets can be coupled over the plastic sheets in a shear resisting way with appropriate adhesive techniques. Doing so, the cross-section should have an increased normal force and moment-resisting capacity while the coupling element is not in danger of brittle fracture since it is made of ductile plastic. The transmission of shear forces between web and flange occurs only through the plastic. To evaluate this concept, a 1 m long T-beam was tested in 3-point bending. The plastic was chosen to be an UV-stabilized polycarbonate, able to resist temperatures between 100 °C and +120 °C. The annealed float glass panes were bonded to the plastic with an acrylic UV-hardening adhesive. In

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K. Martens et al. / Engineering Structures 101 (2015) 1–15

Fig. 13. T-shaped plastic-glass composite beam according to Hildebrand and Werner [48].

Fig. 14. Stress-time curve for a plastic-glass composite beam subjected to threepoint bending (based on strain gauges applied at the bottom edge of both vertical glass panes – left and right – near the centre of the span) according to Hildebrand and Werner [48].

Fig. 14, the stress-time curve is given for the three-point bending test. The tensile stress in the glass was measured at the bottom side of the T beam’s web, at midspan. A maximum load of 1000 N was obtained after 125 s. 75 s later, both glass sheets of the web broke. The residual capacity of the plastic was sufficient to avoid complete collapse of the beam. A second specimen was exposed to 25 temperature cycles according to standard DIN EN ISO 9142 Aging Cycle D3 (exposure of 15 ± 1 h at +40 ± 2 ° C – change within 60 ± 20 min to 20 ± 3 °C, exposure of 2 ± 1 h – change within 80 ± 20 min to +70 ± 2 °C, exposure of 4 ± 1 h – change within 60 ± 20 min to +40 ± 2 °C). A crack in the centre of the flange glass pane developed and grew in longitudinal direction up to 150 mm before both beam ends. This crack steadily opened and closed through the subsequent cycles. Moreover, the effects were amplified by the different heat expansion coefficients of glass and plastic. It is stated that constructive solutions should be found which neutralise these effects.

3. Discussion In Table 1, an overview of the concepts mentioned above is listed. With respect to the used glass, the investigated sections and dimensions, it is clear that a large variety exists. Thermal treatment, lamination and thickness are the main parameters of interest here. Generally concluding, it is recommended to use laminated annealed float glass (ANG) panes because they possess residual stiffness and an additional load-transfer mechanism in the post-fracture state. After the glass web has broken, the large glass shards (typically for ANG) are held together by the laminate

interlayer and so are able to transfer shear and compression stresses. This phenomenon has been observed in several investigations. On the contrary, heat-treated glass (HSG and FTG) disintegrates into very small pieces, making it impossible to transfer loads. It should be noted that the additional load-transfer mechanism of ANG was more significantly encountered when a stiff interlayer was used. With respect to the choice of adhesives in the investigations, it is concluded that a broad range is covered regarding stiffness properties. An evaluation of all test results leads to the conclusion that the post-fracture performance (i.e. the ratio of post-breakage strength to initial failure strength) is highly dependent on the type of adhesive. Better performance is achieved for beams that are built up using stiff adhesives (i.e. epoxies and acrylics). Nevertheless, if ductility is more important than post-breakage strength, soft adhesives (i.e. polyurethanes and silicones) can be used in some concepts. Furthermore, it is clear that a carefully designed connection concept (i.e. geometry) is also key to a successful composite beam design. When initial glass failure occurs, the beam has to be able to transfer the tensile stresses to the other material. In almost all concepts, this is done by shear from web to flange. So, the connection should be able to transfer these shear stresses. If not, composite action will be limited and the beam will fail prematurely. As an example, during the Innoglast project, different connection concepts were tested and it was concluded that the U-type connection performed best. The most investigated beam section is the I-section. This is logical if one considers the high effectiveness of this section for beams subjected to bending, which is generally accepted in structural engineering. Moreover, as glass is relatively expensive, choosing an I-section with a glass web will minimise the amount of glass needed for the beam. Regarding the height-to-span ratio, it is stated that more than half of the investigations used a value between 0.07 and 0.09. For structural glass beams, these values are rather common. This also enables some comparison between the test results of the beam concepts. Looking at the post-breakage performances in the table, it is stated that composites using steel and timber generally achieve better performances than those using GFRP. The main reason for this is not directly related to the materials, but to the level of development of the beams. The timber and steel composite beams are already well developed and tested, resulting in optimal designs for these beams. Considering the amount of developed GFRP composite beams and the number of experimental investigations on them, it is stated that an optimal beam design has not yet been achieved. The material GFRP is a high-strength material and therefore, the authors believe that similar performance as timber and steel composite beams is possible to achieve. Also high values of height-to-span ratios were used. These tests seem less significant from a practical point of view considering the higher relative glass costs (which stimulates low repeatability of tests and minimum variation of parameters. Nevertheless, dimensional variety of beams is important in order to evaluate whether scale effects are present. Regarding the overall load-carrying behaviour, it is concluded that the different materials used to produce a composite glass beam also impose important differences in the way residual load-carrying capacity is provided. In the case of steel-glass composite beams, the post-breakage phase is characterised by a significant load increase until yielding of the steel occurs. From this moment, the beam can be further loaded up to reinforcement rupture, after which it will collapse. So, post-breakage strength is determined by the steel’s ultimate strength and ductility is provided in two phases, namely the phase up to steel yielding (where glass cracking further lowers the beam’s stiffness) and the one after this phenomenon. Timber and GFRP are not classified as ductile

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K. Martens et al. / Engineering Structures 101 (2015) 1–15 Table 1 Overview of investigated concepts. Material

Glass

Connection/contact

Section/type

Span (m)

Height-to-span ratio (mm/m)

Test method

Post-fracture performance

Refs.

Timber

ANG (10 mm) ANG, HSG, FTG (4 & 6 mm) FTG ANG (2  6 mm)

PU PU

I I

4.0 2.0

62.5 75

4PB 4PB

2.63 3.0

[14] [15]

PU P, PU, S

I I, R

6.0 0.65–3.20

97.83 846.15–171.88

4PB 4PB

[16] [17]

ANG (8 mm) ANG (10 mm) ANG (8 mm) ANG, HSG (8 mm)

S A, S S A, E, S

I I I I

4.32 3.5 1.5 4.32

55.56 70.86 200 55.56

4PB 4PB 4PB 4PB

1–1.5 1.3–1.85 (3.2 m span) 3.25 2.3–2.6 1.5–2.7 1.50–2.0

RC & PC

FTG (3  8 mm)

Concrete

I

7.8

71.79

4PB

[26]

GFRP

ANG (19  8 mm)

E

2.33

72.96

4PB

[27,28,9]

ANG (9  8 mm)

E

2.33

72.96

4PB

0.9

[27,28,9]

ANG (12 mm) ANG (12 mm) ANG (12 mm) TG (10 mm)

PU, E PU, E PU, E E, A, PTFE

Horizontally layered Vertically layered I I I I

2.5 (prestressed beam) 1.05–1.45

1.50 1.40 2  1.40 0.46

82.67 85.0 85.0 134.78

4PB 4PB 5PB 4PB

1.53–2.0 1.65–2.82 1.3–2.02

[29] [30] [30] [31]

FTG (20 mm) HSG – laminates TG (2  12 mm) TG (2  12 mm) TG (19 mm) TG (12 mm) TG (12 mm) HSG (3  8 mm) HSG (2  12 mm) HSG (2  4 mm) FTG (2  12 mm) CTG (2  5 mm)

PU A, PU, S E, PU, S A, PU A, PU PU, E PU, E A, PU A, PU Bolts/POM blocks Bolts/Sealant Bolts/Aluminum sheets Bolts/Aluminum sheets

I I I I I I I I I Truss Truss TVT

3.450 5–10 4.0 4.0 4.0 1.0 1.6 4.0 4.0 4.0 7.4 3.3

117.68

2.72–3.08

69.31 82.5 79.0 266 166.25 80 80 250 71.62 173.33

4PB 4PB 4PB 4PB 4PB FB LTB 4PB-R 4PB-R 3PB 3PB & LD St, StCy, Dy

1.05–1.20 (St)

[33,34] [37] [36,40,38] [36,39] [39] [36] [36] [36] [36] [41] [42] [12]

TVT

12.0

89.92

StCy 4PB, Dy 4PB

1.18 (StCy 4PB)

[13]

T

<1.0

>80

3PB & TC

Steel

HSG (2  10 mm) Plastics

ANG (3 mm)

A

1.32–4.04 2.5–3.10 1.45–1.79

[18] [19] [22] [23]

[48]

(RC = reinforced concrete; PC = prestressed concrete; GFRP = Glass fibre reinforced polymer; ANG = annealed float glass; FTG = fully tempered glass; HSG = heat strengthened glass; TG = toughened glass; CTG = chemically tempered glass; PU = polyurethane; A = acrylic; S = silicone; E = epoxy; P = polymer; PTFE = polytetrafluoroethylene; POM = polyamide; I = I-section; R = rectangular section; Layered = rectangular layered section; TVT = Trabes Vitreae Tensegrity; T = T-section; 4PB = four-point bending test; 5PB = Statically indeterminate five-point bending test; FB = flexural buckling test; LTB = lateral torsional buckling test; 4PB-R = four-point bending test regarding robustness; 3PB = three-point bending test; St = static test; StCy = quasi-static cyclic test; Dy = dynamic test; StCy 4PB = quasi-static cyclic four-point bending test; Dy 4PB = Dynamic four-point bending test; TC = thermal cycling test). The post-fracture performance is calculated as the ratio of ultimate failure load and failure-initiating load (e.g. glass cracking).

materials, instead, once the maximum strength is reached, they will fail more abruptly. For the case of timber, failure will occur by gradual breakage of the fibres in one section. A typical G0FRP section will fail even more abrupt by the development of cracks. As a result, composite glass beams using these materials have a different post-breakage behaviour compared to those using steel. Here, ultimate collapse of the beam will be brittle. However, as the ultimate capacity of the composite beam is higher than its initial failure strength, significant displacements are possible due to the increased development of successive glass cracks, that weaken the beam’s section. This gives these types of beams a semi-ductile behaviour. Specifically, the GFRP-H and GFRP-V beams possess high ductility. This is provided by the redundancy of their layered sections. Each layer will successively break, giving rise to the saw-tooth load–displacement curves. The concrete-glass composite beam should have a combined behaviour of the aforementioned materials. However, this was not encountered in this case as the beam failed due to the lack of residual stiffness of the tempered glass web, therefore, this beam illustrated an unsafe failure behaviour. Plastics (here polycarbonate) have a low Young’s modulus and therefore exhibit high deformations when loaded. Therefore, the plastic-glass composite beam should possess a post-breakage phase characterised by high vertical displacements. However, in

the single experiment mentioned here, the plastic was only able to carry the beam’s selfweight. So, no definite conclusions about the load-carrying behaviour of composite glass beams using plastic can be stated up to this point. when more optimally designed, maybe these kind of beams also can achieve higher post-breakage strength values.

4. Proposals for further research Most of the time, investigations were carried out through four-point bending tests. On the one hand, this is good as this enables comparison between the beam concepts. On the other, very little information about lateral and lateral-torsional instability of such beams is currently available. Experimental data about this failure mechanism would however be useful to completely evaluate the post-breakage behaviour (for which composite glass beams are developed). It is however expected that most composite glass beams will perform very good in lateral and lateral-torsional buckling as they possess a higher lateral stiffness compared to regular glass beams made of (laminated) glass only. Also the amount of dynamic tests should be extended, as this type of loading is encountered in a lot of practical applications (e.g. wind loads on

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glazed façades, pedestrian loads on footbridges etc.). A lot of these beam concepts have a high level of inherent robustness. This is proven by the high post-breakage performance and ductility values. However, specific robustness tests including initial glass damage and impact tests should be performed for all beam concepts as well. Glass is rather easy to damage (scratches, impact, etc. due to vandalism), making it impossible to apply composite glass beams in practice without guaranteeing safe load-bearing behaviour in this case. Moreover, robustness is a hot topic in structural engineering and is becoming an important requirement for the structural system. Furthermore, it is also stated that interlayers, adhesives and plastics are typically susceptible to a lot of environmental factors (i.e. temperature, humidity, UV-radiation etc.), creep and relaxation. However, an insufficient amount of investigations has been done to assess the influence of these effects on the behaviour of composite glass beams. The behaviour of commonly used interlayers and adhesives is known to be time and temperature dependent. This will directly or indirectly influence the behaviour of the composite beam. For the case of concrete-glass beams the chemical compatibility of concrete, interlayer and glass should be assessed thoroughly as direct contact between them can potentially cause problems on the long term. Especially the alkaline character of concrete may cause problems. Plastic-glass composite beams are also expected to be very vulnerable to temperature and time effects. It is stated that additional experimental testing programmes are required to assess these influences. Finally, application of these beam concepts in statically indeterminate systems should be investigated. For other construction materials (i.e. steel and reinforced concrete), statically indeterminate beams have proven to possess structural system robustness through stress redistribution capacity. Moreover, a more optimal design of the beam’s section is possible compared to the statically determinate case as maximum bending moments are distributed over supports and fields instead of having a single maximum moment in the field.

5. Conclusions An overview of a fairly complete collection of composite glass beam concepts was given in this paper. Generally it can be concluded that a lot of concepts were developed, using different materials, that illustrated significant post-breakage strength and ductility when subjected to bending, making them suitable for structural applications. However, some of the examples above may not be suitable for practical applications, e.g. because of reduced transparency. For example, the concepts in which the glass is combined with GFRP-layers (see Section 2.3) have an opaque look instead of a transparent one. Furthermore, the combination of glass and concrete (see Section 2.2) imposes durability problems for the glass and interlayer, as they can deteriorate due to the alkaline character of concrete (which has not yet been investigated properly). However, choosing cement with lower alkaline-level could yield an improvement in this regard. Furthermore, the choice of combining material deflects the load-carrying behaviour of the composite glass beam. Steel-glass composite beams adopt their ductility from steel yielding, whereas timber-glass, GFRP-glass and plastic-glass composite beams will exhibit a semi-ductile behaviour characterised by increasing damage and brittle collapse. The concrete-glass composite beam should possess an intermediate behaviour (this should be verified by experimental tests using an ANG-laminated glass web). No conclusions could be drawn for the plastic-glass composite beams at this stage, as a more optimal design is needed to achieve higher post-breakage strength.

The concepts using timber, concrete and steel yield better results with respect to post-breakage strength as it is larger than the initial glass failure load. However, despite their semi-ductile behaviour, one can state that the glass-GFRP composites illustrate significantly larger deformation capacity (GFRP-H and GFRP-V) compared to the other combinations. Hence, the optimal choice to make will be dependent on the practical application of the composite glass beam. Taking all considerations into account, one can conclude that the glass-timber and glass-steel composite beams prove to be very promising as they already are well developed. Furthermore, the Innoglast project [36] resulted in guidelines for the design of glass-steel composite beams, which should encourage their application in practice. Such guidelines should be established for the other concepts too in order to reach practical application. Moreover, all of these independent guidelines should be assembled and included in general codes (e.g. Eurocodes). However, first a general code on structural glass should be produced. When all of this is established, engineers all around the world will feel much more comfortable in applying these solutions in their projects. Regular structural glass beams (made of laminated glass only) are currently applied in small-scale projects (e.g. canopies, one-storey facades, etc.). Additionally, high redundancy and safety factors are included in their designs, leading to economically irresponsible structures. Due to the safe failure behaviour of composite glass beams, these measures can be revised thoroughly. Furthermore, the latter beams can be applied on a much larger scale (e.g. skybridges, floor and roof systems, large fins for glazed facades, etc.). In conclusion, the composite glass beam (actually, the hybrid glass beam) is the solution to create large-scale glass structures in a safe and cost-effective way. In the domain of safety and robustness of structures, one can conclude that composite glass beams impose a significant enhancement of the robustness on element level. Although this is a huge step forward for the application of structural glass beams in practice, one should realise that this is not the final step. To ensure application in practice, also the dynamic behaviour, environmental influences and long-term behaviour should be assessed for such beams. Moreover, also the robustness on a system level should be assured. This could be done by developing robust connections between the glass beams and their surroundings and investigating the possibilities of application in statically indeterminate systems. Finally, as the production of (laminated) glass and transport infrastructure impose dimensional limits on these glass beams, safe head-to-head connections of beams to create larger spans seem desirable. Acknowledgements The ‘‘Agency for Innovation by Science and Technology in Flanders (IWT)’’ is gratefully acknowledged for supporting this work. References [1] Palumbo M, Palumbo D, Mazzucchelli M. A new roof for the XIII th century Loggia de Vicari ( Arquà Petrarca -PD Italy ) based on structural glass trusses: a case study. In: Glass processing days; 2005. p. 1–3. [2] Orlando M, Cagnacci E, Spinelli P. Experimental campaign and numerical simulation of the behaviour of reinforced glass beams. In: Glass performance days; 2009. [3] Olgaard AB, Nielsen JH, Olesen JF. Design of mechanically reinforced glass beams: modelling and experiments. Struct Eng Int 2009;19(2):130–6. [4] Louter PC. Fragile yet ductile, structural aspects of reinforced glass beams [Ph.D. thesis]. TU Delft; 2011. [5] Invernizzi S, Trovato D, Hendriks M, van de Graaf A. Sequentially linear modelling of local snap-back in extremely brittle structures. Eng Struct 2011;33(5):1617–25. http://dx.doi.org/10.1016/j.engstruct.2011.01.031.

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