ASCE Structural Use of Glass

Structural Use of GlassStephen R. Ledbetter1; Andrew R. Walker2; and Alan P. Keiller3 Abstract: This paper introduces the materials, design methods, and details used to create glass structures from small unframed glazing screens to the largest glass walls and roofs. It includes information on the use of glass in floors, staircases, and bridges. The purpose of this paper is to give a state-of-the-art overview of current technology and to look at ideas being developed in the laboratory. Glass is an unforgiving material and the paper explains the measures taken to reduce risk of failure. DOI: 10.1061/!ASCE"1076-0431!2006"12:3!137" CE Database subject headings: Glass; Failures; Walls; Roofs. Introduction used structurally to create exciting buildings. It has the failing that it is a brittle material but engineers are learning to design within Glass has been known as a material for millennia and has been the necessary safety parameters. A further limitation to the use of used in buildings for centuries; yet only recently has it been used glass has been the ability to make structural connections. How- as a structural material rather than a transparent infill within a ever, many proven solutions are now available. supporting frame. The changing approach to the use of glass has been made Glass as Structural Material possible by the improved quality of glass, development of the The material properties of glass are similar to those of aluminum float glass and thermal strengthening !tempering" processes, and particularly when the glass is toughened !fully tempered" the availability of analysis tools. The use of glass to create large !Table 1". transparent screens, roofs, and floors has been driven by the ar- Work by Pye !1998" has shown that built-up glass beams can chitectural desire to achieve lightness of construction and trans- be constructed to deliver the same strength and deflection charac- parency and the availability of larger panes of glass. Structural teristics as similar aluminum constructions. The significant differ- engineers have responded and the design of glass structures has ences between glass and aluminum are as follows: now become the domain of the structural engineering consultant • The brittleness of the glass compared with the ductility of rather than the glass technologist employed traditionally by the aluminum; glass manufacturer. • The resultant influence of fracture mechanics on the failure of This development of the use of glass is directly analogous to glass; and the development of other structural materials, such as steel and • The time dependent failure stress of glass !particularly an- concrete in the early 20th century. Glass structures are still tested nealed glass". to check the validity of design assumptions. There are few struc- When building with glass it is always necessary to assume that tural codes for the use of glass and these deal with stresses in failure of a sheet of glass may occur and postfailure behavior edge supported glass comprising no more than two layers of glass governs the use of glass as a structural material. in a laminate. They do not cover point-supported glass or the The problems of brittleness of glass components may be over- detailing of glass connections. Additionally, many codes define come by: loads and safe use of glass as an infill material and building • The use of laminated glass so that there is some residual codes, such as the International Building Code !International strength provided by the unbroken layers of glass and/or the Code Council 2003" and the U.K. Building Regulations, refer to interlayer; the safe use of glass. • Using ductile interfaces at connections to avoid concentrated Glass is now seen to be a useful building material that can be loads; 1 • The provision of alternative load paths within a redundant Centre for Window and Cladding Technology, Univ. of Bath, structure; Claverton Down, Bath BA2 7AY, U.K !corresponding author". E-mail: • Bonding glass to a frame to ensure retention of the glass; and [email protected] 2 • Using nets, meshes or other building features to catch, or Centre for Window and Cladding Technology, Univ. of Bath, Claverton Down, Bath BA2 7AY, U.K. E-mail: [email protected] break-up glass, which falls. 3 Centre for Window and Cladding Technology, Univ. of Bath, The glasses used for construction are annealed, heat- Claverton Down, Bath BA2 7AY, UK. E-mail: [email protected] strengthened, toughened !fully tempered", and laminated. Note. Discussion open until February 1, 2007. Separate discussions must be submitted for individual papers. To extend the closing date by Annealed Glass one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- Annealed or float glass is glass that has been cooled gradually sible publication on May 6, 2004; approved on April 1, 2005. This paper from a high temperature during manufacture to minimize residual is part of the Journal of Architectural Engineering, Vol. 12, No. 3, stress, allowing the glass to be cut by scoring and snapping. It is September 1, 2006. ©ASCE, ISSN 1076-0431/2006/3-137–149/$25.00. the most commonly available type of flat glass. JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2006 / 137 Table 1. Properties of Glass and Aluminum Toughened glass Annealed glass !fully tempered" Aluminum 2 2 Strength 7 – 28 N / mm 59– 150 N / mm 130 N / mm2 Young’s modulus 70 kN/ mm2 70 kN/ mm2 70 kN/ mm2 Density 2.4 kg/ m3 2.4 kg/ m3 2.6 kg/ m3 Fig. 1. Toughened glass broken while subject to in-plane stress field Thermal coefficient 8.8" 10−6 K−1 8.8" 10−6 K−1 23" 10−6 K−1 of expansion Poisson’s ratio 0.22 0.22 0.34 annealed glass strength !described above" and the surface stress This glass is one of the weakest glass types and has a signifi- induced by heat strengthening. The failure stress varies as greatly cant potential to break when subjected to loads that have not been as for annealed glass. However, this variability is of less impor- designed for or when installed incorrectly. On breakage, the glass tance, as the usable stress is that in the untreated !annealed" glass tends to form sharp-edged, pointed shards. The sharp edges may plus the induced surface stress. The residual compressive surface cause cutting injuries, while the pointed shards may cause pierc- stress of the glass lies in the range 24– 69 N / mm2. The failure ing injuries. stress will depend on the condition of the glass surface, load The postfailure behavior of the glass will be dominated by its duration, environment, and the degree of tempering, as shown by lack of residual strength on breakage. The glass may not be able the level of residual surface stress. to resist loads potentially causing: In the event of heat-strengthened glass breaking, it will have • Full or partial collapse of the glass structure; similar breakage and postfailure characteristics to annealed glass. • Penetration of the glass structure; and Quality requirements for manufactured flat heat-strengthened • Glass fragments or shards to fall when it is used at height. glass are provided in ASTM C 1048 and EN 1863-1. ASTM C It is for these reasons that monolithic annealed glass is not used as 1048 defines heat-strengthened glass in relation to the level of a highly stressed structural glass. residual stress induced into the surface or edge of the glass during Annealed glass may be safe if suitably laminated but its low manufacture. EN 1863-1 requires a 95% characteristic strength of strength reduces its usefulness as a structural material. The 70 N / mm2 and defines heat-strengthened glass by its fracture strength of glass is limited by the presence of random defects and characteristics. is stochastic. A Weibull analysis may be applied to show the range of failure stress for glass elements !Carre and Daudeville 1999". The failure stress of an annealed glass is also dependent on the Toughened Glass „Fully Tempered Glass… duration of load. Sedlacek et al. !1995" shows that the relation- Toughened glass has been processed in the same way as heat- ship between failure stress and duration of load is strengthened glass but has been cooled more rapidly. The result- ing compressive surface stress is higher in magnitude than that in !nT = constant heat-strengthened glass. This gives the glass a bending strength where ! = failure stress !N / m2"; n = constant dependent on envi- higher than that of heat-strengthened glass. The high strength of ronment; and T = duration of load !s". the glass means it is far less likely to fail when mechanical or The constant n depends on the environment in which the glass thermal stresses are applied than either annealed or heat- is used. n = 16 is normally used for building design although much strengthened glass. higher values can be achieved under laboratory conditions. As with heat-strengthened glass the surface prestress has to be The characteristic bending strength for flat annealed glass is overcome before a bending failure can occur. Again the failure 45 N / mm2 as stated in EN 572-1. stress is the sum of the failure stress of the untreated glass and the Annealed glass can however be processed into other glass residual surface stress. The residual compressive surface stress of types and products, which are stronger, have safer breakage char- toughened glass is over 69 N / mm2. acteristics, and/or have safer postfailure characteristics. These In the event of breakage, toughened glass generally breaks into glasses commonly include heat-strengthened glass, toughened small, relatively blunt, glass fragments called dice. These frag- !fully tempered" glass, and laminated glass. ments do not have sharp edges and are unlikely to cause deep Quality requirements for manufactured flat annealed glass are cutting injuries. However, they may form blunt-edged clumps. provided in ASTM C 1036, EN 572-1, and EN 572-2. These are less likely to cause piercing injuries than shards of annealed or heat-strengthened glass but may still cause injury. Toughened glass breaks into small semirounded dice with an Heat-Strengthened Glass aspect ratio of #1.0 when it fails as a result of internal loads !for Heat-strengthened glass has undergone a controlled heating and instance from inclusions" and impact loads, however the writers cooling process to give the glass increased resistance to mechani- have observed different behavior when large in-plane applied cal and thermal stresses. In this process, a permanent compressive stresses are present. In this case the fracture pattern showed the surface stress and a permanent tensile internal stress are induced underlying pattern of the applied stresses with cracks following in the glass. The compressive surface stresses give the glass a the lines of principal stress. Fragments may have aspect ratios as bending strength higher than that of annealed glass and reduce the high as 10 and dimensions of up to 6 mm" 60 mm !Fig. 1". likelihood of glass failure. The postfailure characteristics of monolithic toughened glass The surface prestress has to be overcome before a bending are dominated by the lack of residual strength on breakage. This failure can occur. The principle of superposition applies and the means the broken glass may not be able to resist residual loads, failure bending stress of heat-strengthened glass is the sum of the causing full or partial collapse of the glass structure and a poten- 138 / JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2006 tial for glass fragments to fall should it be used at height. These characteristics may require measures to be taken to reduce the risk of injuries on failure. Quality requirements for manufactured flat toughened glass are provided in both ASTM C 1048 and EN 12150-1. ASTM C 1048 defines fully tempered glass in relation to the level of re- sidual stress induced in the surface or edge of the glass during manufacture. EN 12150-1 defines toughened glass by its fragmen- tation characteristics, and also requires a 95% characteristic strength of 120 N / mm2. Laminated Glass Laminated glass is an assembly consisting of glass sheets joined together with one or more plastic or resin interlayers. The glass assembly can have performance advantages over monolithic glass, which may make it the most appropriate choice for some applications. The strength, breakage characteristics, and postfail- ure behavior of laminated glass are dependent upon the glass types, glass thicknesses, and interlayer types and thicknesses used in construction. Thin flexible sheet materials used as interlayers are commonly known as foils. Fig. 2. Postfailure flexibility of toughened–toughened laminate For structural applications, laminates are normally composed of: • Toughened glass to provide sufficient strength to resist applied loads; and • Heat-strengthened or annealed glass to govern the postfailure nated glass and the resulting toughened–toughened laminate is behavior. relatively flexible with little postfailure strength on breakage Framed glazing, on the other hand, does not normally have to !Fig. 2". The use of annealed or heat-strengthened glass for at withstand such high loads and many more combinations of glass least one ply will normally ensure that some strength is retained and interlayer types may be used to achieve other attributes such on failure. as security, blast resistance or acoustic performance. In addition, the interlayer may need to have sufficient adhesion Film interlayers in structural laminated glass, are normally to hold broken glass fragments on failure and have sufficient polyvinyl butyral !PVB" but thicker sheet interlayer materials are strength to resist tearing should failure occur. Further consider- available including ionoplast, which is stronger and produces a ation should be given to the possibility that the plastic interlayer stiffer laminated glass !Chaszar 2003". An ionoplast is a clear, may creep or deform at elevated temperatures over time if rela- tough ionically cross-linked ethylene copolymer. Unlike PVB, tively high temperatures are likely to be encountered during nor- ionoplast interlayers are supplied in rigid sheet form. Laminated mal building use. In such circumstances the laminate may sag and glass is produced by applying light pressure and moderate tem- come out of its frame or fixing. A number of interlayer types are perature to a sandwich of glass and interlayers. available and these can have significant differences in properties Poured !cast-in-place" resin interlayers can be used, but soft that may make them appropriate for particular applications. resins !polymethylmethacrylate" are primarily intended for en- Quality requirements for manufactured laminated glass are hancement of acoustic performance and may not give adequate provided in ASTM C 1172 and EN ISO 12543. postfailure strength. Some cast-in-place polymethylmethacrylate There are examples of laminates using plies other than glass, and polyester interlayers will provide a degree of post failure in particular the laminating of marble to glass. This is done to strength. Postfailure behavior depends on the glasses forming the give translucency, and for appearance, rather than strength. With a plies and the possible need to invoke membrane action. triple laminate it is possible, using a toughened central ply, to Under sustained load, laminated glass exhibits almost pure induce fracture in the central ply to give a degree of obscuration. layered behavior as the interlayer creeps and reduces the shear Glass laminates may also incorporate plastic sheets, such as poly- connection between the plies of glass. For shorter load periods, carbonate and polymethylmethacrylate, to give spalling resistance the interlayer provides more effective shear transfer. Behr et al. in the event of glass breakage. Such laminates have been used in !1993" and Norville !1999" have shown that for gust loading bullet-resistant glazing and overhead glazing. laminated glass behaves as monolithic glass of the same glass In external applications the edge of laminated glass may need thickness. to be protected from the weather or moisture to prevent delami- Laminated glass characteristically breaks into glass fragments nation. In addition, any sealants that could come into contact with that remain adhered to the interlayer and minimizes the risk of the laminate edge should be compatible to prevent delamination. cutting and piercing injuries from exposed glass edges. This pre- Small areas of delamination will not normally effect strength. vents glass fragments from falling from height on failure. The postfailure behavior of laminated glass is of importance. Design Stresses and Glass Selection In the unlikely circumstance of all the glass sheets breaking this may lead to the laminated glass falling from its frame or fixings The failure stress of glass depends on the loading rate. This is as a single object and causing serious injury. This is a particular commonly understood and different load factors are applied to concern where toughened glass forms all of the plies in a lami- long-term loading, such as snow, and shorter term loads such as JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2006 / 139 Table 2. Glass Thickness and Tolerance in Accordance with EN 572-1 always thought that the methods used would be prohibitively ex- Glass thickness !mm" Tolerance on thickness !mm" pensive for a short run of panels on a building. However, a num- ber of forming techniques have been developed for architectural Up to 6 ±0.2 glass. These include cold bending, sag bending and molding and Up to 12 ±0.3 their use is being driven by the trend toward free-form 15 ±0.5 architecture. Up to 25 ±1.0 The advantage of forming glass is that larger pieces of glass can be used than those required to achieve the same overall de- gree of curvature with a faceted glass surface. An intermediate wind load. Load factors vary from code to code but typical factors approach has sometimes been used in which laminated glass is are: dead load= 2.50; static live load= 1.67; and dynamic load folded by cutting both plies of glass and relying on the interlayer !wind gust" = 1.00. to hold the two semiplates together. A silicone sealant is applied One glass manufacturer gives the following working stresses to the joint to protect the interlayer. This approach can achieve a for use with factored loads: given degree of curvature with pieces of glass of twice the size. Cold bending is achieved by bending the glass at site to fit a Load duration Permanent Medium Gusts curved or warped frame. Eekhout !2003" reckons, depending on Annealed glass 7 N / mm 2 17 N / mm 2 28 N / mm2 the glass type and thickness, that the forced out-of-plane move- Heat-strengthened glass 22 N / mm2 24 N / mm2 37 N / mm2 ment may be several times the thickness of the glass, the induced bending stresses take up only 25% of the available bending stress, Toughened 50 N / mm2 53 N / mm2 56 N / mm2 and the glass can be bent to a radius as low as several meters. For annealed glass load factors for different load durations When cold warping glass, for instance to give a progressively from three seconds to greater than one year are given in ASTM E sloped-back wall, the glass undergoes double curvature. If the 1300, Appendix X6. attempted curvatures are too great the glass is likely to buckle and The bending strength of glass may be reduced considerably by give discontinuous reflections. It is possible to cold-bend lami- surface and edge damage. Surface damaged glass is usually nated glass and insulating glass units !IGUs". In the latter case it rejected during the construction phase. Edge damage is more is claimed that the seals of the glazing units are unimpaired but it common and is more likely to remain undetected. Edges of may be difficult to achieve the normal manufacturers’ guarantees tempered glass are normally finished by arrising to reduce both of performance. the risk of edge damage during handling and failure during the Dodd !2003" describes a process of sag bending. This involves tempering processes. heating the glass and supporting it around its perimeter so that it When calculating the load carrying capacity of glass, it is sags to the required shape, quenching the glass can produce a necessary to assume the thinnest possible cross section for the heat-treated glass. However, it is difficult to control the exact glass. Allowable tolerances on glass thickness for flat float glass shape of the finished glass and the residual induced surface are given in EN 572-2 as shown in Table 2. Greater tolerances are stresses in the heat-treated glass. allowed by other standards including ASTM E 1300-03. Glass molding involves heating the glass and allowing it to When using glass with bolted connections it is normal to use settle over or into a ceramic mold. The ability to computer-cut heat-treated glasses to give the required local strength around the molds has made this a viable solution. It is now possible to go bolt. Holes are drilled prior to heat-treating the glass and these direct from computer to ceramic mold rather than producing an will cause different residual stresses at holes, corners and other intermediate form from which the ceramic mold is made discontinuities in the surface. This is explained by Carre and !Eekhout 2003". Daudeville !1999" and Daudeville et al. !2002". Postfailure Sag bending and molding are unable to produce glass of the behavior is also important and several designers prefer to design same high visual quality as flat float glass but will often produce with heat-strengthened laminated glass as the breakage pattern of glass of acceptable quality. Forming can be sufficiently accurate the glass is better able to prevent the fractured glass pulling off a to allow laminating with foils; Dodd !2003" describes a vacuum bolt. bag process for compressing the glass–interlayer–glass sandwich. The strength of heat-treated glasses is totally dependent on the Alternatively, a poured resin interlayer may be used if necessary residual induced surface stress which may be measured. Some to accommodate the varying gap thickness between the plies. manufacturers measure the surface stress at the time of production Single curvature glass of smaller radius of curvature can be as required by ASTM C 1048, but this is not common in all made by traditional methods of forming by rolling. countries. Stresses may be measured at site or fabrication plant using either a differential surface refractometer !DSR", grazing angle surface polarimeter !GASP", or edge stress meter. The DSR Framed Glazing and Beyond is the most expensive and the most accurate method for stresses over 70 N / mm2. The GASP is most accurate around stress levels Traditional use of glass in windows has comprised a frame to of 50 N / mm2. The edge stress meter is seldom used on site as it support and retain the glass. With the development of roof glazing can only be used if the edge of the glass can be accessed. this has led to the use of two-edge supported glazing as well as four-edge supported systems. Such glazing systems have used either a wet-applied bedding Glass Forming compound or a flexible gasket to support the glass and prevent point contact between the glass and frame, as neither is perfectly Traditionally glass has been used as flat glass or has been bent straight. This fundamental aspect of glass support continues with with a single degree of curvature. The automotive industry has the most advanced glass structures. been using double curvature glasses for half a century, but it was The term “structural glazing” was loosely used to describe 140 / JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2006  Fig. 4. Typical bolted connections for glass Fig. 3. Bolted connection with nylon interface wind load and the self-weight. When bolting through an IGU it is essential that the inner and outer panes are not drawn together and structural sealant glazing systems when they were first intro- distorted and that the IGU remains hermetically sealed. Typical duced. However, the glass behaves no differently than when in- details of bolted connections are shown in Fig. 4. stalled in a rebated frame. The term “structural” should only be Undercut anchors have been developed for fixing thin stone applied to the sealant joint which carries stresses resulting from panels. These will work with glass provided there is sufficient the wind load and self-weight of the glass. thickness of glass to accept the anchor. The load capacity of these In this paper, the terms “glass structure” and “glass supporting fixings is much less than for through-bolted solutions. Given the structure” are used. Glass structures contain glass elements that thickness of glass required and the load carrying capacity, it transfer loads other than those imposed directly on to the element. seems likely that these fixings will only be used for fixing internal Examples of glass structures are cantilevered balustrades, portal glass or decorative external glass veneers to facades. frames, bridges, and shells. Glass supporting structures range from simple rebated grillages !stick curtain walls" to cable trusses and nets used to support point fixed glass. In this case, the glass Frictional Connections carries only the load imposed directly on to it, i.e., wind, snow, self-weight. A further use of glass that concerns structural engi- Friction plates may be used to connect metal brackets to a piece neers is the construction of glass floors and stair treads. of glass. They are also used to connect glass sheets edge on by using patch plates that cover both pieces of glass. Metal plates are placed either side of the glass and clamped together to generate a normal force and a corresponding frictional Glass Connections load capacity in the plane of the glass. A suitable interface is required between the glass and the plates. A soft metal !pure The use of glass in any way other than framed glazing requires aluminum" or fiber-reinforced plastics are normally used to pro- glass connections. These fall into two categories: point connec- vide the required coefficient of friction and accommodate any tions !bolted and bonded" and continuous connections !bonded or lack of flatness in the glass and the metal plate. It is important that sintered". the material does not creep and allow the clamping bolt to relax. Bolted glass connections comprise either: The efficacy of any frictional interface will depend not only on • A bolt through the glass that bears on the glass; or the clamping force but also the area over which it is present. • Friction plates that are clamped on to the glass by bolts that do Either the splice plates have to be of sufficient stiffness to distrib- not make contact with the glass. ute the load into the interface material or oversize washers/nuts have to be used to increase the effective clamping zone. Bearing Connections While frictional connections work well with monolithic glass, The brittle nature of the glass makes it imperative that the load there is an inherent problem with their use on laminated glass. from the bolt is distributed into the glass and that bearing at a The plastic interlayer will creep under the high compressive point is avoided. Countersunk bolts may be used with a ductile stresses induced by the clamping forces in the bolts. With time, interface normally a soft metal cup placed under the countersunk this creep leads to relaxation of the clamping force and loss of the head. Nijsse !2003" has used a nylon or similar interface between frictional forces. the shank of the bolt and the glass. This not only distributes the load evenly into the glass but may also be a site-drilled hole Bonded Point Connections positioned to allow for construction tolerances !Fig. 3". Some bolts are articulated by means of an integral ball and socket. Metal fixings may be bonded to glass to provide point connec- Retention of a single monolithic piece of glass is straightfor- tions. The principal concerns are durability and load carrying ca- ward and requires only a through bolt. However, when connecting pacity. Bonded point connections are particularly sensitive to to laminated glass or an IGU it is not so simple to ensure that all applied moments, but viable designs have been developed by Ee- layers of glass are retained. Connecting only to the inner most khout !2003". Durability tests have been undertaken by layer of glass relies on the integrity of the PVB or resin interlayer Eekhout to show that bonded joints can have an acceptable ser- !in the case of laminated glass" or the edge seal !in the case of an vice life. He has used bonding technologies developed for the IGU" to retain the external layer of glass and transfer any negative aerospace industry. JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2006 / 141 Continuous Connections Glass-to-glass connections can be made by sintering the glass together. This is a process of fusing the glass using powdered glass and the application of heat. Sintering has been used in the manufacture of glazing units !particularly evacuated glazing units" but is of little use in the field of structural glazing as it is Fig. 5. Locations of fixings to minimize stresses in glass not compatible with the use of laminated or heat-treated glasses. !F = fixing" Glass may be bonded to glass or to metal. Soft sealants and adhesives are used to bond glass into frames. They may be ad- equately strong in tension but are generally too soft in shear to The wall may be hung from a truss or roof beam or stood from provide an adequate shear connection to develop composite ac- the base. Standing the wall from the base may make for ease of tion between two components. Stiffer adhesives such as modified construction and avoids the need for a large load carrying truss or epoxies are sufficiently stiff in shear to provide a shear connection cantilevers at the roof. However, it is not feasible for the tallest between two components and allow the development of built-up walls where it is necessary to avoid compression and buckling in sections !Ledbetter and Pye 1999". Blandini !2003" has the components. investigated the bonding of glass using acrylics, epoxies, and Yoxon !1987" designs fins on the basis of preventing local polyurethanes. edge buckling of the fin, which will not occur provided the stresses are limited by limiting the moments to Componentry Et3 M max # 6!1 + $" This term is used to include all of the metal brackets, plates, where M max = maximum bending moment in the fin !N m"; washers and so forth that are incorporated into the glass wall or E = Young’s modulus !N / m2"; t = fin thickness !m"; and structure. $ = Poisson’s ratio. Components that come into contact with glass have to be made to higher tolerances than are normally associated with construc- tion projects. Components are generally made by casting or ma- Point-Fixed Glass chining from the solid. Where metal components are connected only to glass they may Point supports have been used for several decades and are gener- fall when the glass breaks. It may be advisable, in some cases, to ally through-bolted connections. However, there are recent ex- provide light suspension cables to prevent heavy brackets falling amples of the use of bonded connectors and undercut anchors. when glass fails. Whichever form of connector is used, the issues that have to be considered are the same. They are: • Allowance for movement; Fin Walls • Allowance for tolerance; and • Retention of all glass layers/plies. These were the first form of frameless glazing and are still com- Movement of the glass arises due to temperature changes and monly used as a means of achieving transparency in glazing imposed loads. The movements to be considered are an in-plane screens. They comprise one-way spanning glazing supported on expansion/contraction and dishing of the panel predominantly due glass beams or fins. The glazing is either attached continuously to to wind or other imposed loads. the fins using a soft silicone sealant or is connected intermittently Dishing of the glass due to temperature movement may occur, using bolted connections. for some glass combinations, because of different uniform isotro- Whether a soft sealant joint or a bolted connection is used, the pic thermal strains at each surface, which result in the glass dish- fin and the glazing panel do not act compositely. The structural ing to form a spherical surface. model is that of a glass plate supported on its edges by glass Any restraint of the glass will lead to induced stresses in the beams. The connection between the two is capable of transferring glass. It is important that point fixings are designed to limit these both positive and negative wind load, but there is no effective induced stresses. Point fixing of glass to a fixed plate !clamping" shear connection between the different components. prevents out-of-plane rotation at the corner, but can be acceptable Fin walls may be no more than one story in height with the for smaller plate sizes. Leading glass manufacturers give guid- fins provided simply to stiffen the glazing panels and give accept- ance on glass size and appropriate positioning of fixings. The use ably small deflections. However, they are commonly constructed of soft washers or thicker interface materials will allow some to five or more stories in height. The length of individual pieces out-of-plane rotation at the fixings. of glass in a fin is limited by the size of glass available and the Some designers articulate the point fixings to ensure that the allowable depth of the fin as the aspect ratio attainable with glass can freely rotate out-of-plane at the fixings. This is de- toughened glass is limited. However, individual pieces of glass scribed by Rice and Dutton !1999". These generally involve the may be connected end-to-end to form a long fin spanning from use of a ball joint. Note that the point of rotation has to lie in the top to bottom of the wall and carrying the wind loads by bending plane of the glass so that the arm connecting to the support struc- action. This is an approach adopted for glass roofs as well as ture cannot impose rotation on the glass. walls. Alternatively, the sections of the fins may be supported by Dishing can occur unrestrained provided: some other structural element such as an orthogonal cable truss or • The fixings have sufficient articulation; and struts back to the primary structure !see glass supporting struc- • All the supports for a single sheet of glass lie nearly on a circle tures below" which carries the wind load. !and therefore on the sphere" !Fig. 5". 142 / JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2006  Four fixings placed symmetrically on a rectangular plate al- ways meet this condition, as do three fixings in a triangular plate. Use of more than four fixings, typically six fixings at the corners and midsides of a rectangular plate, do not meet this criterion and particularly high stresses may be induced around the midside fix- ings. The stresses induced in the glass by any combination of fixings can be checked by finite element analysis. In-plane expansion/contraction may be accommodated by the use of sliding connections remote from the glass/bolt interface such that the bolted connector can move with the glass indepen- dently of the supporting structure. This arrangement may also accommodate tolerances on the glass size and position of holes. The tolerance on position of the supporting structure connection points in-plane can also be accommodated by these means. Out-of-plane tolerance of the supporting structure connection points can normally be tolerated by the glass, which is flexible in bending, but allowance for adjustment in this direction may be required to achieve the required fit and appearance, particularly for a highly reflective wall. Fig. 6. Suspended glazing assembly with each mullion supporting Supporting Structures several bays of glazing Glass supporting structures have to have structural integrity and stability under the loads applied to and by the glass. The other structure or floor slabs. Variations on this include providing a equally important requirement is that the supporting structure catenary on either side of the glass, one to carry positive wind should be sufficiently stiff to limit the movement at the glass load; and one to carry negative wind load !Fig. 7". connecting points. This is necessary so that the connections be- Deflection of the primary supporting structure due to the tween the glass and the supporting structure can accommodate the weight of the glass occurs progressively as the glass is installed. movement and not transfer loads to the glass. The magnitude and timing of these deflections will affect the Glass is a thin sheet material and is relatively flexible in bend- position of the glass fixings at the time of glass installation and ing but stiff in plane. Glass may therefore accommodate out-of- the position of the glass elements at the time the glass-to-glass plane movements of the supporting structure but will not tolerate joints are sealed. It follows that the stresses within the glass ele- smaller in-plane movements. For this reason, it is possible to ments may depend on both the stiffness of the primary structure attach glass to a cable net provided only that it is stiff in-plane and the sequence of construction. It is important that the structural and contains diagonal stiffening cables as well as orthogonal engineers responsible for the primary structure and those respon- cables. sible for the glass work closely together and with the building Glass connections can be fixed directly to the primary struc- contractors. ture or to brackets attached to it. However, it is more normal to It is possible to impose the final displacements on a supporting use a secondary structure of the forms described below. structure such as a truss by pretensioning against a temporary Grillages of mullions and transoms may be used to support dead weight or anchorage and releasing the pretension to balance glass walls and roofs. These are traditionally used in the form of the weight of the glass as it is installed. With pretensioned cable rebated mullions and transoms. Similar grillages may be used to trusses it is necessary to ensure that they remain in tension as the support point fixed glass by bolting or welding brackets !spiders" building moves and this is normally achieved with pretensioning to the frame. These support systems are only suitable for spanning springs orthogonal to the plane of the glass as described by Cole small distances of two or three storeys in height. !1999". These springs are pretensioned to induce a level of stress Trusses may be used to span from foot to head of a glazing in the cables that is sufficiently high, even after movement of the screen and these are suitable for larger spans. Additionally, a primary supporting structure, to carry the wind load on the wall. welded truss may be constructed as a square or triangle in cross section or with projecting arms. This allows more than one verti- cal line of fixings to be supported from a single framing element !Fig. 6". Cable trusses and catenaries are frequently used to provide supporting structures that do not detract from the overall transpar- ency and “lightness” of the wall. Either the glass is supported on a pretensioned cable truss that spans from the head of the wall to its base and supports the self-weight of the glass along with posi- tive and negative windload, or the glass is supported on hangers that carry the self-weight back to a truss or beam at the head of the screen and the wind load is then transferred to a cable truss or similar that spans from one side of the screen to the other. A catenary alone can be used to carry positive wind load while ties are used to transfer the negative wind load back to the primary Fig. 7. Lateral support systems for glass walls JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2006 / 143  Cable nets (cable lattices) may be used to support glass. Large out-of-plane deflections can be tolerated, but measures must be taken to ensure that the lattice is stiff against in-plane shear and that no stresses are induced in the glass as a result of in-plane deflection of the lattice. For this reason a cable lattice may contain additional diagonal cables. Glass is relatively flexible out-of-plane and can accommodate deflections caused by mild warping of the plane of its connecting points. Overall deflections are generally controlled only to reassure people adjacent to the Fig. 8. Built-up laminated beam !exploded view" wall or roof. The Hotel Kempinski at Munich Airport has a cable lattice supported glass wall 25 m high and 40 m wide that may deflect up to 0.9 m out-of-plane. Extension of the catenary cable stabilized, unless the stresses are limited as described above for will not causes stresses in the glass elements provided the glass glass fins in a fin wall. fixings permit movement, but will lead to an opening of the joints Longer beams may be constructed by connecting glass plates between the glass or a large in-plane movement of the glass at the end to end using patch plates to create moment resisting connec- boundary of the glass screen. tions. Veer et al. !2001" describe the construction of beams by laminating pieces of glass with the discontinuities in the plies staggered and the laminating foil running continuously for the length of the beam !Fig. 8", but account must be taken of creep in Glass Assemblies the interlayer and not all interlayer materials are suitable for this application. Glass assemblies are structures in which the glass carries loads Glass cantilevers, portals, and frames can be constructed other than those applied directly to it or its self-weight. In glass provided that moment resisting joints can be constructed. As with assemblies failure of a glass element will lead to load transfer and beams, this can be achieved with patch plates. However, it is changed loads in the other glass elements. In this case, redun- more normally achieved by interleaving the pieces of glass on dancy should be the primary design concern and there is a need to either side of the joint, to create a tenon or finger joint, and using distinguish between the serviceability and safety conditions. friction connections or through bolts. There are examples of these Suspended glass walls are constructed by hanging a top tier joints being made by using a continuous laminate interlayer to of glass from a supporting truss or beam. Successive lower tiers connect the fingers of the joint in the same way that Veer, are then hung from the tier above. The glass plates may be con- Riemslag, and Ting constructed a beam. nected one to another by patch plates or bolted connections. Out- The moments that can be carried are limited by the depth of of-plane loads still need to be carried by glass fins, a horizontal the glass members and bolt spacing. Macfarlane !1997" overcame cable truss or catenary as described above. this by overlapping blades of glass by half their length to increase Failure of a glass component will lead to a load transfer into the spacing between the bolts. This allowed the Yurakucho station the adjacent glass elements, and the glass and connections must canopy in Tokyo to achieve a cantilevered roof projecting 10 m. be designed to accommodate this. It is more normal today to use Built-up sections may be constructed by the use of point con- cables as hangers to suspend each piece of glass separately. This nectors or continuous connections. Glass sheets may be formed reduces the risk from failure of a structural glass element and into “T” beams, stiffened plates, hollow square sections, and so leads to a more robust design. on. Sections may be built-up simply so that one component sta- Glass beams are used to support roofs, act as fins in fin wall bilizes another against excess deflection or buckling, in which construction, and construct short span bridges. Monolithic glass case simple intermittent bolted brackets will suffice. can be used for spans in excess of 6 m. However, the use of stock Fully monolithic behavior is possible if a shear connection is sizes of glass will limit the length to 6 m. When heat-treating made between the different component parts of the built-up sec- glass, in-plane distortion of the glass may occur if the aspect ratio tion. Using a modified epoxy adhesive, Ledbetter and Pye !1999" is too large. In practice glass elements with aspect ratios up to 20 were able to construct a T beam with deflections under load only may be toughened without distortion. Such components may ap- 1% greater than those predicted for a fully monolithic beam. pear appropriate when used as the web of a beam, but can have a It should be remembered that sections might be built up using large visual impact when used merely as plate stiffeners. glass and other materials such as metals, e.g., a glass plate stiff- Beams may be built-up sections !see below", but are normally ened by aluminum webs. Freytag and Sparowitz !2003" produced simply several pieces of glass each acting independently, side-by- a glass plate with an edge flange of pretensioned ultrahigh side. Failure of one piece of glass will then be accommodated by strength concrete. This has been tested as a single beam element load transfer to the remaining members. Macfarlane !1997" de- and suggestions have been made for its incorporation into built-up scribes using four pieces of glass in parallel and designing with a sections. factor of safety of eight. In this way, three of the four pieces of glass can fail and there will still be a factor of safety of two. This was viable economically and aesthetically and gave confidence in Glass Columns the early use of bolted glass. Lower initial factors of safety are used today as both engineers and clients become more familiar Columns and struts may be constructed as built-up sections as with the structural use of glass. However, it is still essential to described above. The Glass Pavilion Rheinbach, described by design on the basis of at least one glass element failing. The roof Wellershoff and Sedlacek !2003", uses glass as the only vertical glazing or floor may be used to stabilize one edge of the beam elements to support a steel roof of dimensions 32.5 m " 15 m. against buckling. For walls, the load is predominantly wind load- An elegant solution has been derived by Veer and Pastunink ing that is reversible, so both edges of the beam would have to be !1999" who created a cylindrical glass strut from two concentric 144 / JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2006  Eekhout !2003" describes bonding a metal fin !square in cross section" to the edges of glass plates so that they may be connected by clamping together adjacent fins. Blandini !2003" describes the development of a prototype shell in which the plates of glass are simply edge bonded one to another with acrylic. In practice the durability of such a joint must be assessed taking full account of the bonding, the bonded materials, and the environmental conditions. Fig. 9. Cross section through one end of prestressed cylindrical glass Glass Floors strut Glass is increasingly being used for floors to give architectural expression, to provide diffuse lighting below mezzanine floors glass tubes between which they poured an ultraviolet !UV" cured and to provide decks on bridges within atria. Glass for use in resin. Earlier attempts to use a poured resin had failed due to floors is normally subjected to larger loads than that used in the shrinkage of the resin, leading to voids in the resin and cracking building envelope and has to have a postfailure behavior that will of the inner cylinder due to induced tensile stresses. More re- support loads for an adequate period of time so that people do not cently, cylinders have been fabricated using PVB interlayers. fall through the fractured glass plate. Similar considerations apply Laminated glass cylinders may even be prestressed by means of to stair treads and trafficked roofs. The design should contain at metal caps at each end and a central prestressing tendon !Fig. 9". least one more layer or ply of glass than the design load requires They may be used as short columns and as struts to carry wind to provide redundancy in the event of a glass pane becoming loading from a glass façade to a primary structure a meter or more damaged and failing. Laminated glass with plies of heat strength- behind the glass wall as described by Lowings !1999". ened or annealed glass is normally used. These glasses fracture to produce large shards that remain interlocked postfailure to give greater residual strength than does toughened glass. Balustrades Traditionally, glass in floors was supported on all four edges on a metal frame or grillage and the dimensions of the plates were Glass balustrades may be constructed as freestanding glass canti- severely limited. Today, floor glass may be supported on two levered from the base or by using glass as an infill to a frame. edges or point supported and larger plates of glass can be used. They will normally use a safety glass to reduce the likelihood of Elegance and transparency can be maintained by the use of glass breakage on impact. beams to support the floor. This has been used to good effect in The basic design criteria are strength to ensure that people do the creation of short span glass bridges !O’Callaghan 2003". not fall from height, and limited deflection to reassure people who Glass floors carry predominantly self-weight and applied ver- lean against the balustrade. Some building codes require a con- tical loads !distributed loads and point loads" and may be de- tinuous metal rail along the top edge of the glass that remains in signed as asymmetric plates. In particular, it is possible to use an place should the glass elements break. This is good practice as it acrylic sheet as the lower ply in a laminate to give better postfail- provides load sharing to adjacent glass panes and will generally ure strength, for the same overall thickness, than would be provide better retention of a fractured glass pane. Consideration achieved using glass as the lower ply. should also be given to the potential for penetration by pedestri- The use of glass in stair treads is one of the most problematic ans including children and for glass to fall from height, on glass uses of glass. The treads span in one direction and the span is balustrade breakage. If the risk is considered unacceptable then this may necessitate the use of laminated glass !Keiller et al. dictated by the width of the staircase. Early glass stair treads used 2005". acrylic panes under annealed glass panes. The use of stronger Freestanding glass balustrades are constructed by either ionoplast laminating sheet interlayers has further facilitated the clamping the glass to the edge of the floor slab or by inserting the use of glass as stair treads. Such laminates can provide the nec- glass in a slot and embedding it in an epoxy polysulfide com- essary strength, postfailure strength, and stiffness. Stiffness is a pound. A metal edge plate is normally fixed to the edge of the particular issue when designing glass stair treads without stiffen- floor slab with a continuous spacer to form the required slot. ers and supported only along the short edges, for if they are too flexible they may “bounce” underfoot and disconcert the pedestrians. Glass Shells Glass, particularly when wet, offers a very low coefficient of friction, which causes an unacceptable loss of slip resistance. Glass has been used to construct shell structures for many years, Glass used to be sandblasted or acid etched to resolve the mod- but there was always a metal frame to make the structural con- esty problem associated with glass floors. This also offered a nection between the glass plates and to make a weather seal. Shell slightly improved slip resistance. Today it is possible to apply a structures run the spectrum from simple geodesic domes to mod- ceramic frit to surfaces to improve slip resistance and meet con- ern “free-form” architecture. Folded plate structures and single cerns about modesty. Consideration should be given to fritting curvature walls containing glass elements can be constructed walk-on roofs to improve slip resistance and this may also serve using the same techniques. to reduce solar transmission through the roof glazing. Fritting as The ability to bond glass to itself and other materials opened with many surface modification techniques may lead to dirt accu- up the possibility of constructing glass shells in a different way. mulation on the glass and to difficulties in cleaning. JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2006 / 145 Construction Process Postfailure strength is also important when considering compo- nents subject to wind load as the response to a failure then de- Unlike most façade construction, the complex cladding/structure pends on the anticipated wind load in the following days. interactions described above require close collaboration between the project structural engineer and the cladding engineer from the Progressive Collapse outset. Macdonald and Carfrae !2004" describe the importance of All structures should be designed to prevent progressive collapse. construction sequence for a glazed cable net. Green !2004" de- Glass structures may be particularly susceptible to this from of scribes the need for integrated design where it is essential for the failure due to the brittle failure mode. This means that there is a cladding contractor and main contractor to exchange information complete lack of residual strength of monolithic glass panes after on structural stiffness, not simply loads and deflections. fracture leading to transfer of the entire load to other components. Progressive collapse can occur to a glass structure or to a suspended glass wall. Ideally, failure is limited to a single com- Design Criteria ponent that has sufficient residual strength, and no load is trans- Structural design commonly considers a serviceability condition ferred to other components. An example is the use of multiple and an ultimate condition. Under the serviceability condition the plies in a laminated glass beam. If one ply fails the remaining structure is required to meet the design requirements !weather plies can carry the load so that beam can continue to support a tightness, appearance, etc." under the application of the design floor or roof. In the event that load will be transferred to other loads. In the ultimate condition the structure is required to with- components they must be designed to carry those loads. For ex- stand a higher load safely although the serviceability of the struc- ample, in the case of a suspended glass wall it should be designed ture may be impaired. A further requirement is that if failure does to work with at least one plate of glass missing. occur the failure mode should minimize the consequences of failure. General Safety Design of glass structures follows these principles but requires Even when failure of the glass does not cause unacceptable risks particular regard to the failure mode because of the brittle nature from its inability to withstand loads, it will affect the serviceabil- of the material. The brittle nature of glass means that stress con- ity of the building and may present a risk of injury to the building centrations as a result of poor workmanship or unforeseen design users. It is therefore good practice to carry out a risk assessment situations are more likely to lead to fracture of the glass and when selecting glass, particularly where it is to be used at height. failure of individual panes can occur at loads below the design Safety in this context relates to the potential for: load. Other construction materials that exhibit a degree of • Glass to fall—falling glass may injure people below; plastic behavior can deform locally to redistribute local stress • Other components such as metal brackets to fall; concentrations. • People to fall against or through the broken glass; and Glass which has been designed to withstand the design loads • Objects penetrating the glass to fall. can also fail for other reasons including; These risks will depend on the location of the glass and in loca- • Impact; tions where people are unlikely to be present no action may be • Thermal stresses; and required. Where people are likely to be present the risks may be • Inclusions. reduced by The consequences of such failures need to be considered. • Selection of glass which will break safely; • Laminated glass which will be less likely to fall than mono- Serviceability Condition lithic glass; Glass design is usually governed by limiting deflections. These • Laminated and toughened glass which are less likely to cause limits have been set to avoid disconcerting onlookers who may severe injuries on contact; and see the glass deflect directly or may see the distortion of reflected • Designing glass to reduce the risk of glass failure !see below". images in the glass. A full discussion of these issues is given by Keiller et al. !2005". For glass walls and roofs other serviceability criteria also apply. These include water tightness and air tightness, acoustic performance, and so on. Testing of Glass Structures and Elements Ultimate Condition The prefailure behavior of glass is well understood and knowl- In the ultimate condition it is essential to ensure that the glass and edge of interlayers and adhesives is often sufficient to allow de- structure remain safe and that glass breakage does not itself cause sign by calculating performance under normal service conditions. a hazard. This is achieved by ensuring adequate residual strength However, new materials and componentry are often used, neces- and overall integrity/stability of the structure. sitating testing to help prove performance in the service It may be desirable to test a structure or subassembly to deter- condition. mine what damage occurs under impact and to determine what Improved reliability of postfailure behavior is the most com- happens when a particular component, or ply, within a laminated mon reason for testing structural glass. It is not easy to calculate glass, fails. the postfailure strength of fractured glass components or to be assured that glass will be retained in place after failure. Smith and Postfailure Strength Dodd !2003" give a good overview of project testing, and a case Postfailure strength is important for all nonredundant glass com- study of project testing is given by Clarke !2004". ponents in a structure. Particular concerns arise with floor plates, Tests normally seek to fail a component while it is under load and stair treads, where it is unacceptable for a pedestrian to fall or prior to loading. This is done by: through. Similar considerations apply to barriers subject to impact • Increasing the service load until failure occurs; although in this case there may be no residual load after impact. • Inducing failure in a toughened glass ply; and 146 / JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2006 • Causing damage by impact while the service load is applied. may also be important, since windows that face the sun for pro- Type testing of an assembly such as a rooflight, on a product longed periods are more likely to encounter significant thermal or project-by-project basis, is appropriate to show the failure stresses. Finally, ensuring careful handling and installation of mode and postfailure strength; however, there is also a role for glass panes reduces damage to the pane edges and ensures ad- proof testing. When making semistandard components such as equate clearance for glass movement. The location and orienta- stair treads or floor plates it is relatively economical and efficient tion of glazing may limit the choice of glazing materials that can to test every one on a simple test frame. This approach is appro- be used. priate for materials with variable properties and is adopted for the stress grading of timber. In the case of glass, proof testing will identify any manufacturing flaws such as edge damage, poor Inclusions bonding of the interlayers or of stiffening webs. Proof testing Small impurities or inclusions can be introduced into the glass gives greater confidence in the components and allows the engi- from a variety of sources during manufacture and may, in some neer to design to lower factors of safety. This may make projects cases, increase the risk of failure. The most common example is economically viable and may allow the development of lighter- nickel sulfide !NiS" inclusions !CWCT 2002". These inclusions weight glass constructions. occur in their % phase in annealed glass but change to the & phase during toughening. The & phase is smaller in volume and when the glass cools, it contracts around the inclusion. Thereafter, the & Reducing Risk of Glass Failure phase inclusions gradually revert to the more stable % phase and, on expanding, may exert sufficient force on the surrounding glass Glass failure is of concern as it may impair or lead to the failure to cause failure. To cause failure, inclusions need a minimum of the structure and allow glass to fall and injure people. This, in theoretical diameter of 58 'm and will typically have an average turn, may lead to financial losses, from reduced use or function- diameter of around 150– 200 'm. They must also be located ality of the building, repair costs, personal injury liability, in- within the internal tensile zone of the toughened glass. Glass fail- creased insurance premiums, and loss of confidence in a building ure may occur at any time after manufacture with incidents re- or its environment. It is therefore good practice to reduce the ported to peak after around 4 – 5 years and occur up to 20 years likelihood of glass failures to a level that is as low as reasonably after manufacture. Failure is theoretically possible in heat- practicable !Keiller et al. 2005". strengthened glass but requires much larger inclusions than those The principal causes of glass failure are as follows: normally seen in failed toughened glass; NiS induced failure does not occur in annealed glass. Impact This risk of failure is significantly reduced by heat soaking the glass after toughening, however, the process does not completely Failure is caused by a sudden localized contact with a force that is eliminate the risk of glass failure due to the presence of these sufficient to overcome the strength of the glass. This contact force inclusions. To ensure the process is carried out effectively, heat may be caused by contact with a hard body or a soft body, and soaking should be to a recognized standard, such as EN 14179-1. may be caused accidentally, deliberately or by the forces of This standard is intended to reduce the incidence of nickel sulfide nature. failure from about one inclusion in 4 t for glass that is not heat Hard body impacts are caused by stones and larger objects that soaked to one inclusion in 400 t for heat-soaked glass. may be thrown, dropped or carried by extreme winds. Soft body impacts are caused by human bodies and objects such as footballs. Malicious Damage The likelihood of failure is most commonly reduced by using a stronger glass type or thicker glass, which is more resistant to Any glass structure may be deliberately attacked but can be de- impact, and by the use of barriers in front of the glazing that signed to minimize the potential for glass failure. It may, how- reduce the risk of contact. ever, not be possible to protect the glass fully from failure during malicious attack for reasons of cost and the ability to predict the means, methods, and determination of the attacker. It may then be Thermal Stress necessary to minimize the likelihood of failure by restricting the Failure is caused by a temperature difference between the center ability of people to carry out their malicious deed or from gaining and edge of a glass pane, which induces stresses within the glass. access to the glass. The main types of attack include the They are produced when the center of the glass is heated, as by following: sunlight, and the edge of the glass remains relatively cold, such as when shaded by the frame. The resulting expansion of the center Small Projectiles of the glass produces a tensile stress on the glass edges, which if Glass may be deliberately subjected to impact by small projec- sufficiently high will overcome the strength of the glass and cause tiles, or pellets, from air rifles and shotguns. These impacts can failure. Under normal conditions, this failure mechanism does not cause failure if the projectile is fired from a sufficiently close affect heat-strengthened and toughened glass. range. The nature of the failure pattern depends upon several The likelihood of failure can be reduced by using glasses that factors including the glass type. The characteristic failure pattern absorb less solar energy or glazing frames and surroundings that in annealed glass consists of a Hertzian cone expelled from the heat up or cool sufficiently to reduce temperature differentials. opposite side of the impact. For toughened glass the whole pane Removing shading devices or other building features that may breaks into dice. With laminated glass the glass will fracture ra- lead to the center of the glass pane heating up faster than the dially from the point of impact. edges or produce shadows that form temperature differentials on Glazing can be protected from projectiles by a variety of the glass may also reduce thermal stresses. Glazing orientation means. These include: JOURNAL OF ARCHITECTURAL ENGINEERING © ASCE / SEPTEMBER 2006 / 147 • Stronger glass types and/or thicker glass, which are more re- that prevents or minimizes the likelihood of flying glass frag- sistant to impact; and ments, such as laminated toughened glass. Performance against • Protective plastic sheeting or films, which minimize the like- blast load depends on ductility of the interlayer. Ionoplast is a lihood of damage. stronger but stiffer material and preliminary tests suggest that it performs less well than PVB under blast loading. Scratching Glass can be mechanically damaged by abrasions and scratches. This damage can be caused deliberately by objects such as glass Conclusions cutters and carbide or diamond pointed instruments. The damage may take the form of random lines or graffiti. In all these cases, The use of structural glass allows the creation of transparent and the glass may be weakened by the damage, but will not often fail. translucent structures. This has the advantages of creating build- The degraded appearance of the glass may, however, necessitate ings with a “lightness of appearance” and internal spaces that the glass being repaired or replaced. Repair may be accomplished benefit from the resulting natural daylight. by polishing and possibly filling in any damage with transparent Glass has become an accepted structural material, although resin, but this may not restore surface strength and can conceal a few codes or standards exist to govern the design and construc- latent weakness that will lead to failure. tion of anything but the simplest of glass structures. In the ab- Glazing can be protected from scratch damage by a variety of sence of codes, engineers must design on the basis of experience means. These include: and testing. Over the last decade, sufficient information has come • Barriers, such as wire grills, which inhibit contact with hard into the public domain to allow engineers to design glass struc- pointed objects; and tures to meet serviceability conditions. However, postfailure per- • Adhesive films, which protect the glass surface and can be formance is less well understood and testing is often required to removed and replaced, at some cost, in the event of damage. demonstrate performance in the ultimate condition. In extreme cases, where the scratches are very deep they may Glass is not an intrinsically ductile material and is prone to cause immediate glass failure if the surface compressive layer is failure from concentrated loads. Ductility is achieved by laminat- penetrated or when a load is subsequently applied. ing it with plastics. Concentrated loads are avoided through cor- rect detailing and workmanship or by protecting the glass. Deliberate Impact Ultimately, any structural use of glass must consider the pos- Glass may be deliberately subjected to impact by large projectiles, sibility of failure of any glass element and its postfailure behavior. such as stones or by pointed objects, such as center punches. Glass structures may become unsafe due to lack of integrity of the Glazing can be protected from deliberate impact by barriers structure, loss of containment, or falling glass. However, glass and adhesive films as above, and also by the use of: can be used safely to create roofs, walls, balustrades, floors, stairs, • Stronger glass types and/or thicker glass; and and bridges and to create architectural structures with transpar- • Plastic glazing, such as polycarbonate, which has a very high ency and lightness. resistance to impact. Failure of Heat-Treated Glass References Heat-strengthened and toughened glass has undergone a special heat treatment process that induces a compressive surface stress Behr, R. A., Minor, J. E., and Norville, H. S. !1993". “Structural behavior into the glass. Each surface compressive layer as a rule of thumb of architectural laminated glass.” J. Struct. Eng., 119!1", 202–222. is 1 / 5 of the glass thickness, and the core tensile zone is 3 / 5 of Blandini, L. !2003". “Structural use of adhesives in glass shells.” Glass the thickness !GANA 2004". Should the compressive surface processing days 2003, Tampere, Finland, 183–185. layer be penetrated by impact or mechanical damage the tensile Carre, H. and Daudeville, L. !1999". “Load bearing capacity of tempered stresses present are sufficient to initiate cracks that will cause structural glass.” J. Eng. Mech., 125!8", 914–921. glass breakage. In theory, the compressive surface layer does not Centre for Window and Cladding Technology !CWCT". !2002". “Glass in have to be completely penetrated since the application of subse- buildings: Breakage—The influence of nickel sulfide.” Bath, U.K. quent loads to the glass may produce sufficient stress to overcome Chaszar, A. !2003". “Hybrid laminations for structural glass.” Glass pro- the remaining compressive stress leading to failure. cessing days 2003, Tampere, Finland, 416–418. In a 6-mm-thick pane, the surface compressive layer is ap- Clarke, S. !2004". “Proving that an unsealed envelope can be safe for proximately 1.2 mm-thick but for structural glass of thickness 15, secure delivery.” Proc., Int. Conf. on Building Envelopes Systems and 19, or 22 mm the compression zone is correspondingly thicker Technologies 2004, Sydney, Australia, 337–342. 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