See-through Structure

The developments in glass architecture over the last 40 years have resulted from improvements in glass technology and the growing confidence of both engineers and architects in using the material. This article traces the key steps in these developments, with particular reference to the available techniques for making joints in glass structures. Using metals and organic materials in these joints, an enriched vocabulary of glass architecture has become possible.

Modern glass technology could be said to have started with the invention of the float process by Pilkington in 1959. This enabled glass sheets to be produced with a high quality surface, on a continuous production line and therefore at a reasonable cost. This process effectively replaced the plate glass process, which involved polishing individual panes of glass one by one. High quality glass became readily available, as did more durable sealing materials such as silicone rubber, used for both gaskets and for wet seals.

Glass has a beguiling combination of weight and lightness: it has the density of concrete, yet can be totally transparent. It is not metallic, yet has the stiffness and (when tempered) the strength of aluminum. The key to the successful use of this remarkable material is to understand its properties and in particular the methods of making connections to it.

Although strong, glass is a brittle material, and is sensitive to surface imperfections. Glass is also sensitive to three further factors: the duration of the load; the level of pre-stress in the surface; and the area which is loaded. Glass can sustain short duration wind loads at relatively high stresses, whereas lower stresses are used for snow loads. Secondly, the higher the pre-stress in the surface (by heat or chemical tempering), the higher the load that can be carried. Finally, larger areas have slightly lower strengths overall than smaller areas.

Thus, an important feature of glass as an engineering material is its variability, yet that variability is invisible, hidden within its transparency. This probably explains why glass is perceived by the public as a “weak” material. Nevertheless, with appropriate factors of safety, and the use of lamination for important structural members, glass can be designed to be safe under working loads, and also “failsafe”–safe even after failure.

To distribute load, glass has traditionally been supported around its edges. This is as true for stained glass in a cathedral window as for a modern laminated glass floor plate or four-sided structural silicone window. Effectively, the connection between glass and its frame is a “bearing” connection. This is the case for both the way that self-weight is carried on the setting blocks, and also for the way that wind loads are transmitted through sealants or gaskets to the frame.

Such bearing technology is also typical of glass support systems, even as significant innovations took place in the early 1990s. This applies to the supports for the first large glass floor plates (London, 1990), first use of glass beams for permanent load (London, 1992), and first use of free-standing glass columns (Paris, 1994).

A major advance took place in 1975 in the Willis Faber Dumas (now Willis Corroon) building in Ipswich, where hung, bolted glazing was used for the first time, and also the glazing was stiffened against wind resistance by vertical glass fins. The corners of the glass panels were fixed to these fins by steel “patch” fittings, separated from the glass by a fibre gasket. These fittings, which had first been developed in the 1960s, were typically square or rectangular.

The technology of hanging glass in this way was greatly extended with the windows at the Parc de la Villette in Paris in 1986. In this structure the vertical load of the glass was taken by hanging each piece of glass from the one above–eight metres high in total–while the lateral loads due to wind were taken by horizontal cable trusses, pre-stressed to provide resistance to both inward and outward pressures. An articulated fixing (“rotule”) was used for the first time, to allow the corner of the glass to rotate, thus reducing the corner stresses.

Recent examples of such connections are in the glass enclosure to the Talk Zone pavilions* at the Millennium Dome in London, and the glass subway canopy to Jubilee Place at Canary Wharf, London, for which Yolles is currently the structural and facade engineer.

Although originally used only with single-glazed systems, bolted fittings have now been developed for use with insulated glass units, and this makes the technology feasible in the Canadian climate.

The use of glass fins to resist wind loads became well established in the late 1970s and 1980s. There are some excellent examples exceeding 10 metres in height in Toronto, notably at BCE Place and at One Canada Square. These fins are of annealed glass, as tempered glass is not available in such lengths.

Where such fins are tempered, the lengths are generally limited by the size of the tempering oven, with the maximum length typically about four to five metres, although it can be up to six or seven metres with special ovens. If longer fins or beams are required then spliced connections are typically used, derived from similar connections in steel beams. These connections are based on steel plates being tightly bolted on each side of the glass, with the pressure being transferred between steel and glass through an incompressible fibre gasket.

However, such systems have no built-in redundancy in the event of failure of the fin due to mechanical damage or spontaneous shattering due to nickel sulphide inclusions. In the late 1960s the process of heat soaking had been developed as a method of post-processing tempered glass in order to cause any such impurities to bring about failure of the piece while still in the factory. Nevertheless, the realization that, despite strict quality control, glass could fail due to an impurity which was invisible to the eye has led to the growing use of laminated glass as a “failsafe.” This has led to thicker and heavier members, which are not as elegant as the earlier examples.

Using friction connections in laminated glass poses particular technical challenges: the high tensions in the bolts give rise to high stresses in the interlayer material, which would tend to creep, thus relieving the tension in the bolts and greatly reducing the moment capacity of the joint. At the joint, therefore, the interlayer must be replaced by something that will not creep, and aluminum has been found to be excellent for this purpose. Although harder metals, such as steel, cannot be used in direct contact with glass, pure grades of aluminum are sufficiently soft to prevent the stress concentrations that can cause failure.

This technology was successfully employed on the 14 metre by six metre glass subway canopy* at Buchanan Street in Glasgow, Scotland, completed in 1999. Here a series of glass portal frames at 1.7 metre centres support the side and roof glazing. The verticals of the portals are of triple-laminated tempered glass cantilevering from ground level, and the horizontals are of triple-laminated annealed glass, spanning six metres. The verticals are clamped at their feet by stainless steel angles. At this position, thin layers of aluminum are laminated into the glass. There are no metallic components at all above ground level, so the glass rises cleanly out of the black granite plinth.

Bolted fixings generally have now become commonplace, with systems having been developed by a number of glass manufacturers. However, a major development from this is the use of highly-stressed bolted connections which transfer loads in the plane of the member. In this way a member can be extended by bolting together a number of smaller members. The loads are transmitted from steel to glass through a hard plastic material, such as Delrin. Lack of fit between holes in adjacent glass panes in laminated glass can be accommodated using cams in the steel bushes.

The first example where this technology was used was in 1996
for the Yurakucho Canopy in Tokyo, Japan. Here the connections were used to transfer shear loads between glass panes, and thus a large cantilevered beam was built up out of smaller components.

Another example of highly loaded connections is in the Transport Stack* at the Discovery Centre in Birmingham, England, where large pieces of laminated tempered glass cantilever up from the ground at various angles. These pieces support motor vehicles, which apply large moments out of the plane of the glass. The fixings used are the double-cam type, so that the connection with the glass can be accurate and tight, in order to avoid stress concentrations.

Although glass panes are usually fixed by four-sided framing, by four-sided structural silicone or by bolting near the corners, the glass can also be fixed using small plates at its corners, thus removing the visual intrusion of bolts. An example of this is the atrium wall of the Kempinski Hotel in Munich, Germany, completed in 1994, where the wall structure is an orthogonal cable net. Special fixings were developed for the glass/cable connection.

This principle was taken further in the 50-metre diameter gable walls* at the Kimmel Center in Philadelphia, USA, completed in 2001, where the structure was composed of vertical cables only, pre-tensioned by hanging cast iron weights. The fixings were carefully detailed to avoid clamping the corners of the glass, which would have caused excess stress. Lateral movement under wind is relatively high for these kinds of facades, with deflections in the order of 900 mm being possible under maximum loads. These deflections produce relative shear movements between adjacent cables, due to the differing cable lengths. The connection details need to be able to accommodate these kinds of movements.

The final result is walls with a dynamic character as the wind pressures change. This creates an interesting situation: a wall composed of a material with the same density as concrete can nevertheless move in response to small variations in wind pressure.

The development, in the early 1990s, of glass beams to carry permanent loads for the first time used mortise and tenon type details to seat triple-laminated glass beams on columns, in a way reminiscent of carpentry. There is an interesting analogy here with the early development of iron structures. In the Iron Bridge at Coalbrookdale in England, the first iron bridge in the world, completed in 1779, the connections between the iron pieces were all based on carpentry. Thus, the technology developed for one material was used for another, before the joining technology for the new material had been developed.

Although connections in glass are at present extrapolated from those in timber or steel, it is possible that the future of glass joining technology lies in bonding, using high strength adhesives, or in high-energy fusion techniques. Bonding technology, using glues, is already well established in glass furniture design, and research is being carried out on structural uses. Large areas might be enclosed by laminated glass shells, formed of pieces of glass bonded together at their edges. Chemical tempering is another way of significantly increasing the strength of glass. Although expensive at present, ways will probably be found to make this process more economical.

Glass can be an important contributor to energy saving in buildings, if the “winter garden” effect is used, whereby the sun’s energy can be enticed in and then retained within an insulated glass skin. It also provides the benefit of increasing natural daylighting, thus reducing the energy needs of artificial lighting. With careful engineering, glass structures can be designed to be affordable, strong, failsafe, durable and exhilarating.

Jonathan Sakula is a Director of Yolles in London, England and works on a number of faade and structural glass projects in Europe and Canada. See Web site at www.yolles.com

*The author was responsible for these projects prior to joining Yolles.

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