Volume 43, Issue 1 - January 2008
Architects' Guide to Glass
By Mark DuBois
The clients are collectors of contemporary art who are particularly interested in the aesthetics and evolution of the modern glass house.
I proposed load bearing glass as a way to take greatest advantage of the vistas, and to explore the ability of glass to simultaneously serve as an architectural enclosure and a structural element.
The concept of the glass house has been of great interest to architects and engineers throughout the 20th century. Where traditional architecture emphasizes enclosure, shelter and protection against the world outside, the glass house speaks to the importance of light, transparency, the flow of open spaces, and a strong connection between interior and exterior. These qualities were made possible by the development of the slab and column construction system popularized by Le Corbusier in his Domino House of 1914. By eliminating traditional bearing walls he opened the door for the “free plan” and the “free façade,” and thus the possibility of significant expanses of glass and a degree of openness that was not previously possible. Iconic structures such as Mies van der Rohe’s Barcelona Pavilion and Farnsworth House and Philip Johnson’s Glass House are highly refined expressions of this quest for openness. In all of these examples the structural columns were clearly expressed and the glass walls were conceived as infill elements that were secondary to the structural system.
The sculptural clarity of the Barcelona Pavilion of 1929 was an important reference point for much of 20th century design and for this project. The ground plane, the glass and stone walls, and the floating roof plane are masterfully configured to read as pure, independent elements. The design includes eight slender steel columns that allow the glass and stone wall planes to be located as desired in this “free plan.”
The thinness of the columns and their highly reflective stainless steel surface suggests that they were intended to recede if not disappear. This has the effect of making the wall planes the dominant elements. There is an inherent ambiguity here: Is the weight of the roof being borne by the thick stone walls or the slender columns?
The Santa Fe residence is organized around a series of parallel concrete walls that serve as an armature around which the house is assembled. Spaces are created by using large expanses of floor-to-ceiling glazing around and between the concrete walls.
Due to the experimental nature of the glass design, it was decided to limit the glass bearing system to the most important location in the house—the living room—which is oriented towards the big views to the west and north.
The space measures 25.5 feet east–west by 33.5 north–south. The south wall is almost 16 inches thick concrete and the east side has steel framing concealed inside a fireplace chimney enclosure. Floor-to-ceiling glass was planned for the north and west sides of the space in order to create an expansive glass room. It was important to have no visible structure in this corner in order to create an experience of pure space, form and materials.
Glass Bearing System
The first concept was a load bearing L-shaped glass column located in the corner, in plane with non-bearing glass panels that would enclose the space.
An alternative configuration was a cruciform column. Options of two or three layers of laminated glass were considered, and the issue of annealed or tempered glass was examined.
Annealed glass was preferred because a cracked ply could retain some strength as part of the laminated assembly, whereas a tempered glass ply would have no residual strength.
We discussed whether this columnar element should be made to disappear into the non-bearing glass, or whether it would be psychologically reassuring to make it visible, perhaps by means of a dichroic coating. Because the clients were interested in issues of space and perception in contemporary art and architecture, it was agreed to make the load-bearing column blend into the glass walls.
From the earliest discussions it was agreed that a thorough mock-up and testing program would be a necessary part of the process. It was important to appraise the client early on that it would be necessary to build full-size mock-ups and do extensive testing, and that the process of designing load bearing glass would be very different from traditional structural engineering. Local glass installers were brought into the process at this stage to ensure that they were willing and able to meet all the project requirements. While the budget for the project was generous, there was an ongoing effort to develop details that would be relatively easy to fabricate and cost effective.
Further research into the corner column concept revealed complex connection issues that would make fabrication and installation difficult. A cam/bezel connection was proposed in order to distribute the load into each ply of a multi-ply bearing element evenly. The concentrated load on a corner column and the absence of any redundancy would have required a very high safety factor.
We decided to make the longer west wall into a multi-panel bearing wall. The 28-foot length (less then the overall room size of approximately 33.5 feet due to a door) allowed the load to be distributed over a large bearing area. The multiple panels provided redundancy in case of failure. A continuous steel beam was located above the bearing wall in order to simplify the roof framing and provide an easy place to make the connection details at the head. There is a cantilevered roof overhang on the exterior side of this exposure which reduces the torsion on this beam. The glass would work in compression so that the controlling characteristics would be its flexibility and slenderness.
Because the goal was to create a pure plane of glass, we needed to conceal all connection hardware at the top and bottom of the bearing wall by burying it in the ceiling and floor.
Further study determined that the deadload of the roof was sufficient that there would be no requirement to resist uplift, thereby eliminating the need for any type of connection that would resist tensile forces. Making the connections to take purely compressive loads allowed the connection details to be greatly simplified. Throughout this process Dewhurst Macfarlane worked closely with the local structural engineers to coordinate the glass wall with the design of the roof. They worked, for example, to ensure that the roof diaphragm was stiff enough that lateral loads would not be introduced into the glass bearing wall.
Options for the number of panels were studied. Lamination could be easily done up to 2 m (6.5 feet) in width, so a minimum of five panels would be required. Our final selection was to use seven panels each approximately 1.2 m (approximately 4 feet) wide. This was based on ease of fabrication and visual appearance. The joints between panels were approximately .4 inch wide and were filled with clear silicone.
The decision was made to use three-ply glass panels with a ¾-inch fully tempered ply in the center, ¼-inch fully tempered plies on each side, and 0.06 inch PVB interlayers. The use of seven panels created a high degree of redundancy, and the center ¾-inch ply had enough compressive strength to meet the safety factor requirement of three. Because the outer ¼-inch plies were not needed for load bearing, they were designed to be ¼-inch shorter than the center ply, thereby ensuring that all the load was transmitted through the center ply. This amount was determined to be adequate to compensate for up to 1/8-inch of slippage during the lamination process. This greatly simplified the fabrication of the panels and the connection details. Due to the reliance on only the center ply, heat soaking was specified for these pieces of glass, and only two glass manufacturers would provide heat-soaked glass.
A maximum deflection of L/100, or approximately 1.37 inch over a 11.5 feet height, was considered the maximum that the client would find acceptable, and a deflection of between L/120 to L/150 was the target range.
The option of creating an insulating load bearing glass was studied. This would have been accomplished by making one of the two glass faces longer and load bearing, and using the other to create the insulating cavity. However, the spacer bars and adhesives would have created black stripes that would have changed the appearance of the glass wall significantly. Creating load bearing insulating glass would have added a significant degree of complexity to the project and compromised the aesthetics.
A deep U-shaped shoe that ran the full width of the glass panels was developed for the head and sill connections. The shoes were designed to fit snugly against the glass panels with neoprene spacers on each side. Because purchasing a shoe of the correct dimensions was not possible, one of precisely the right width was fabricated by using two steel angles that were machined and fixed together. Each shoe has two threaded rods, located at the quarter points, to transmit the load into the roof steel or foundation and allow precise leveling of the shoes to ensure very even load distribution across the end surfaces of the ¾-inch ply.
To ensure the even distribution of the roof load, a stack of spring washers was used at each of the head connections. A system of nuts on the threaded rods allowed the height of the stack of spring washers to be carefully equalized. The exact height of this stack could then be checked several months after installation to ensure that the panels were still being uniformly loaded.
A number of construction sequence issues were reviewed with the contractor before the installation was started. The glass was planned to arrive as late as feasible to reduce the possibility of damage by other construction. The roof steel was to be erected at or slightly below its final elevation and held in place with shoring.
The shoes would be installed onto the glass panels in the field just prior to craning the panels into place. When the glass panels were installed, they would then be very gradually tightened up against the roof connections to slowly and evenly transfer the load onto the glass. Because the 8.5 foot roof cantilever could not be installed until after the glass was in place, particular attention was paid to the protection of the installed glass.
The preparation was thorough and complete and the glass installation was completed in less than a day.
Mark DuBois, AIA, is with Ohlhausen DuBois Architects, New York City. This material is adapted from a presentation made at GPD 2007 in Tampere, Finland. Electronic versions of the original presentation are available at www.glassfiles.com.