Volume 40, Issue 9 September 2005
Point to Point
Point Supported Glass …
Where’s it All Going?
by Bill Coddington
Architectural glass façades, both insulating and monolithic, traditionally have been supported by capturing the edges of the glass. As architects have expressed a desire to make the walls of the buildings more and more transparent, engineers have developed methods of reducing the size of the supporting structures. In the 1970s structural glazing was developed using “patch plates” in the corners of the glass lites. Often these patch plate systems were used in conjunction with structural silicone to bond the glass lites to the structure using glass or metal mullions.
In recent years, however, especially in Europe, it has become increasingly popular to attach the façade glass directly to the structure using bolted fittings, i.e. point supports. By connecting through holes in the glass, these systems increase the wall’s transparency and offer additional architectural opportunities in the detailing of the bolted connections and hardware. Attaching glass to metal fittings almost always requires fully tempered glass, in order to resist the concentrated stresses around the bolts connected through the holes in the glass. Because the glass is drilled prior to tempering, the holes need to be at least the glass thickness in diameter and must also be located a sufficient distance from the edges and corners of the glass lite to allow for successful tempering. ASTM E 1300-02, Standard Practice for Determining Load Resistance of Glass in Buildings, specifically excludes glass with holes and notches. Architects who want to incorporate a point supported glass façade in their design need to seek out engineers who are familiar with structural glass design.
Each point-supported glass system, whether a canopy or a vertical wall, requires you to consider three aspects. First, remember that you are furnishing a complete system and not just glass and metal. Second, you must use quality hardware that has been designed and tested for use in the specific application. Last, and often most important, the system must be reviewed by a competent engineer who has experience designing point-supported glass systems and understands how they are attached to the building structure. Even a small point-supported system may require extensive engineering analysis. In 2002, when I began my campaign to correct problems with point-supported glass in the U.S. marketplace, I saw many glaziers ordering hardware from one company and the glass from another without any consideration of the limitations of the individual components. Very tight fabrication tolerances for both the glass and metal are required. Without adequate engineering review, hazardous systems could be installed. A major failure could be a catastrophe to both the people directly involved and to the glass industry as a whole.
Traditional glass and metal curtainwalls are installed with the glass continuously supported on the edges. These systems have been thoroughly researched and tested, but point-supported glass has been used in the United States only on a limited basis. Glass that is supported only at bolted points does not behave structurally in the same manner that edge supported glass does. Building code officials and other government authorities in European and Asian countries have tested point-supported glass systems extensively, but U.S. authorities and engineers have not. Architects and authorities having jurisdiction and building code officials depend upon glass professionals to provide safe, quality installations. The design criteria and tests done overseas can often be used in the United States, but they need to be reviewed carefully to ensure that they are applicable to the installation conditions and building codes here. For larger projects, engineering and architectural firms have tested mockups of the specific system application thoroughly, but this is costly. Unfortunately, in some cases, the proper engineering and tests have not been done, especially for small projects such as glass canopies.
While tempered glass is very strong in bending, it is often deflection—not the structural strength of the glass—that becomes the limiting criteria in determining glass thickness. Most point-supported structural glass designs involve large, relatively thin lites of glass. However, if the glass lite deflects laterally by more than half its thickness, then the large deflection non-linear plate theory comes into play. The simple four-sided plate theory doesn’t take into account the membrane stresses that will occur under loading, especially near the holes at the corners of the glass. Often a finite element analysis is required to determine the amount of stress and deflection of the glass lite.
The movement capabilities and limitations of each hardware connection within the system must be evaluated by the structural engineer. Deflection of the glass lite can also reduce the relative distance between the support points. Allowance for this dimensional reduction must be designed into the supporting fittings by using either oversize holes or slotted connections. The designer can limit the glass deflection under load by specifying thicker glass, since the increased flexural strength of tempered glass does not change its stiffness. Building movement, thermal expansion and contraction of the system must be designed into the connections to prevent excessive stress on the holes in the glass.
Many architects and contract glaziers have wanted the industry to develop a comprehensive point-supported glass design guide. Numerous hardware systems are available and each system has different glass design criteria depending upon hole/point location, size and hardware flexibility. Each hardware system has different capabilities and limitations. If the hole in the glass is larger and further from the corner, the glass lite is stronger. If the hardware system is very rigid it may place more stress on the glass than another hardware system that deflects a lot. Engineers and consultants advocate a systems engineering approach rather than an item-by-item checklist.
After more than three years of effort, members of a task force sponsored by the Glass Association of North America have published a glass information bulletin on point supports, but this is not a design guide. Application of fundamental engineering principles with adequate design tools, (finite element analysis, in particular) proper determination and application of design loads, performance testing of the component assemblies and adequate provisions for movement can provide safe, effective installations. The structural performance of glass is a complex subject. The GANA Glazing Manual (2004) has an excellent section on design considerations using glass as a structural material. More testing and research, similar to what has been done in Germany, needs to be done in the United States. New designs with cable trusses and cable nets that allow the wall to deflect eight inches or more have been designed and installed.
An Internet site sponsored by the Glass Processing Days conference, a global industry forum, at www.glassprocessingdays.com, has many technical articles on point-supported installations worldwide, test results and design methods. The more I study them, the more I know that we in the United States need to do a lot of work to catch up with our European counterparts. In the mean time, I urge a cautious conservative approach. Architects are very interested in this kind of design so our industry will be asked to provide more and more of them in the future.
Bill Coddington owns the consulting firm of WS Coddington Consulting LLC in Perrysburg, Ohio. He retired from Oldcastle Glass in 2002.
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