Tim Macfariane

Principal, Dewhurst Macfariane and Partners
Consulting Civil and Structural Engineers
Dewhurst Macfariane and Partners

In the last one hundred years the means of production however have changed dramatically. Two hundred years ago a piece of glass 4 metres by 2 metres would have been cast from a hand fed crucible on to a flat metal bed and carefully annealed to avoid cracking during cooling. After it had cooled it would have been hand polished to remove the cast face imperfections to produce a flat transparent surface. Cast glass of this type and size can still be procured today. The price of a single sheet this size would be in excess of US$15,000.

Today giant float plants are producing a continuous ribbon of perfectly clear glass up to 25mm in thickness in widths of up to 3.6 metres and lengths of up to 12 metres or longer if it can be handled. The cost of a piece of glass 4 metres long by 2 metres would be in the region of US$500.

The annealed glass produced by the float process can then be further heat treated or tempered to improve its toughness thereby reducing its resistance to cracking. Sheets up to 7 metres long by 3 metres wide and 25mm thick can be tempered in this way. Another way to improve the strength of the glass is to use a chemical toughening process. Although this is an expensive operation its benefits are that odd shapes of glass which would break in the heat strengthening process can be processed and there is no heat distortion therefore very flat panels can be produced.

Annealed glass and tempered glass can be laminated into multiply panels up to 100mm thick using plastic inter layers that hold the glass together and following breakage prevents shards of glass becoming detached from the panel. Sentry Glass Plus is the trade name of an interlayer that can retain its significant tensile strength up to temperatures of 70 Degrees C and has such improved properties that it can allow the laminated glass panels to be designed as fully composite panels that would continue to perform structurally after all the glass laminates have cracked.

Shaping by bending or slumping float glass over moulds can create complex double curved panels and with current techniques the glass can be subsequently chemically toughened and laminated. To control light and heat transfer, glass panels can be coated with virtually transparent metallic coatings generally known as Low E coatings which perform as heat mirrors reflecting incoming or outgoing infrared radiation as required. High reflective mirror coatings are sometimes used in very hot climates but they can pose the problem of reflecting sunlight onto highways or other buildings and blinding the drivers or occuopants.

Other methods of controlling heat transfer include fritting the glass which involves screen printing an opaque glazed paint on to the glass and baking it at a high temperature to bond it to the surface. The frit pattern can be specifically developed for each application.

Coloured glass is also used to reduce heat gain and glare which can be a problem particularly for offices with computer screens that require lower light levels. Where a greater degree of transparency is desirable for example for museum cases or shop windows low iron glass is often specified. This glass is usually more expensive than the more common green tinted soda lime glass as its production requires higher melt temperatures.

Insulated glass units using one or two 8 to 16mm wide air cavities are commonly used to control thermal performance. Filling the cavity with Argon or Krypton improves performance and it is possible to introduce insulating translucent material or operable blinds in slightly wider cavities to further improve the desired shading or insulating characteristics. A very effective insulated glass unit which uses a vacuum space of less than 1 mm between the glass sheets, with the gap being maintained by tiny aluminium spacers set out in a grid pattern, has been produced by a Japanese company in a joint venture with an Australian University. The insulation values that they have achieved for this panel are impressive and if the cost of production can be competitive it will potentially save a significant amount of Energy.Although glass up to 3.6m wide and of lengths limited only by handling or cutting machinery could be made available the reality is that the majority of glass that is shipped from the float lines to the processing plants and workshops is transported in purpose designed trucks that are capable of handling maximum stock sizes of 6m x 3m. Subsequent processes as described above further reduce the available sizes of glass to common sizes of typically 4m x 2m. There are invariably individual processors and fabricators that can extend beyond these limits and new machinery is constantly being developed that will improve the range.

Innovation in Design
The structural properties of glass are well known and have been studied for many years by private companies such as Pilkington, Corning and Asahi Glass. There are however currently no National Standard to consult (although there is currently a European Code in draft form) therefore considerable effort and research is required to seek out and piece together any useful technical design information that is in the public domain. Independent Consulting Engineers have very little official technical data to work with which means that glass engineering is not taught in Universities and there are very few Engineering Consultancles that have sufficient knowledge to confidently develop a structural glass design. Although there will always be design innovation arising within the production and fabrication arm of the industry, where there are many talented Design Engineers practicing, the degree of innovation will be tempered by the practical and financial interests of the manufacturer. In order to maximise profit it is easier to make the same thing many times than to change production methods every time an Architect or Consulting Engineer has a bright new idea. Faster progress can be made if Consulting Engineers and Architects can develop designs that they are willing and able to take responsibility for. It small scale experimental ideas are undertaken initially by specialised small contractors with the flexibility to do something different the idea, if successful, will be developed and adopted eventually by the larger companies.

dewhurst Macfarlane and Partners's first Structural Glass Design was for a staircase in London for The Joseph Shop in Sloane St. (Plate 1). The designer was Eva Jiricna Architects and it was completed in 1990.The treads for this staircase were made from 19mm thick annealed glass which had a sand blasted top surface for slip resistance and visual relief. Below the glass there was a 15mm thick acrylic panel to act as a fail safe mechanism in case the glass cracked.The acrylic system works well for small spans but for longer spans the use of two or more pieces of glass 19mm or even 25mm thick may be required. This can be achieved by laminating two or more sheets of annealed glass together and ensuring that glass is supported on all four sides.

The Now and Zen restaurant in St. Martins Lane, London (Plate 2) has glass pavement light measuring 3.6m x 1.0m wide. The make up is two sheets of annealed glass 19mm thick laminated with acrylic resin and designed to support a uniformly distributed load of 5KN/M2. The architect for the design was Rick Mather from London and it was completed in 1990. In this design we ensured that if both sheets of glass cracked there would be sufficient redundancy, because of the four sides support condition, to keep the panel in place. We have experimented with resin laminated and PVB laminated panels comprising two sheets of glass 19mm thick spanning between two supports only and found that if both sheets cracked at or near the middle of the span the panel failed.

In the late 90's Du Pont produced a laminating material with the trade name 'Sentry Glass Plus' which is stiffer than PVB and Acrylic resin and it retains its physical properties up to temperature of 70 degrees C. We carried out two sided support experiments with this material and found that for a four ply laminated sheets the panel did not collapse when all of the glass was deliberately broken. (Plate 3). We used the information from these tests to construct treads spanning 2.4m made from 4 ply annealed glass with a total thickness of 50mm. The top plate of the tread has an acid etched embossed surface which has very good slip resistance and is easy to clean. A number of staircase have now been built using this technique the most recent being a spiral staircase for the Apple store in Osaka Japan (Plate 4).

The knowledge gained from designing the stair tread and floor panels was then used to develop a lean-to portal frame using laminated annealed glass beams and columns.

The redundancy of the ply glass structural elements was achieved using triple glass beams and it was established that all of the elements could crack and the structural frame would stay in place. Simple mortice and tenon joints similar to that used in wood construction were developed for the beam to column connections. The lateral stability of the structure was provided by the glass wall panels and the structural silicone at the glass to glass connections.The largest structure built to date using this technique was the Broadfield House Glass Museum in Kingswinford England. The architect was Le Plan and it was completed in 1994. The roof beams which are spaced at 1m centres span 6m and the columns are 3.5m high (Plate 5 and 6).

was the Yurakucho Canopy at the Tokyo Forum in Japan where Rafael Vinoly Architects was keen to create an all glass cantilever strucure with a span of 10m. We solved this problem by proposing that the beams should be fabricated from four parallel rows of interlocking laminated and toughened glass blades. The bolted joint was designed to transfer the shear and bending forces between the elements and with the configuration we proposed we anticipated a maximum shear force in the region of 12 KN on each individual glass element joint. We conducted a series of tests to establish the actual breaking load and were surprised to find that a 19mm thick glass sheet with a 72mm dia hole was capable of transmitting forces of up to 120KN through the connection. The canopy which was completed in 1996 was designed for hurricane wind pressures of 5KN/M2 and for the most severe earthquake loading.(Plate 7 and 8).

The AUDI Cube in Riyadh Saudi Arabia which we designed together with Architect Nabil Fanous was completed in 1997. It is an 8m cube and is used as a library reading room. To connect the beams together we used friction grip bolts at four way junctions to create wall and roof planes which had beams that were two way spanning. The beams were fabricated from two sheets of PVB laminated tempered glass. Aluminium plates were introduced within the laminate layer at the bolt positions to avoid creep and loss of tension in the friction grip bolts. The 15mm thick tempered glass panels on the elevations are stacked one on top of the other and they are boned back with in-situ applied silicone to the beams and columns (Plates 9 and 10).

Recently we have been looking at the structural application of vertical glass wall panels. In Sate Fe New Mexico a house designed with the architect Mark du Bois employed a system of flat laminated using a 19mm toughened inner core with 10mm heat strengthened panels on either side. PVB was used as the laminating material and load tests up to six times working load were successfully carried out (Plate 11 and 12).

Working on Art installations has often given us some interesting challenges. The Ash and silk wall by Vong Phaophanit was an all glass construction with volcanic ash and orange silk laminated into the glass panels. This sculpture was erected in Greenwich outside London (Plate 13). The exhibition structure designed by Gerry Judah for a car exhibition in Birmingham England has vertical cantilever walls of laminated toughened glass and the cars are in turn bolted to and cantilevered off the glass panels.

Summary
In many ways the absence of building regulations and established codes has allowed us to apply engineering rules from first principles free from preconceptions and to many peoplees surprise, includeing our own at times, we have never failed to convince a regulating authority that our structural analysis was valid.

We are I feel at the very beginning of developing the full range of possibilities for this extraordinary paradoxical material and we are privileged to have worked with many Architects and Clients with the vision and courage to explore its boundaries. Personally although I enjoy working with the full range of structural material working with glass has given me a renewed vision of the role that Consulting Engineering can and should have in the design processs.