Providing a balance

November 2012 » Columns » COMMENT
Daniel A. Cuoco, P.E., F.ASCE

This month's article on the new United States Institute of Peace building, entitled, "A cornerstone for peace," by Cristobal Correa and Stephen Curtis, mentions an interesting design condition that I have seen sometimes overlooked by structural engineers. This condition involves detailing the proper boundary supports when connecting major structural assemblies together in order to allow movement while at the same time providing adequate stability and load transfer.

The USIP building consists of three independent structures with a glass roof spanning between them. As described in the article, the glass roofs do not structurally link the buildings, thereby minimizing forces introduced into the roof structures due to lateral movement of the buildings under wind or seismic loads. Of course, horizontal restraint is provided for the roof structures at certain locations in order to provide lateral stability.

In general, when dealing with the support of large glass roof structures, there needs to be a balance between providing enough flexibility to minimize or eliminate forces due to thermal restraint and providing an adequate load path to ensure lateral stability. A simple example that shows how to accomplish this is to have a roof structure with one support fixed in all lateral directions and the other supports either fixed in one lateral direction or allowing movement in all lateral directions.

The selected locations of fixed and moving supports will dictate where, and in which direction, lateral forces will be applied to the roof support structure below. (In some cases, the stiffness of the support structure must be considered, but is ignored here for simplicity.) Once these forces are calculated, they can be incorporated into the design of the support structure. Problems can arise, however, when the roof structure and the support structure are designed by two different entities.

I recall a project that involved a long, glass-covered space frame canopy over a street in a major city. The canopy was to be supported on widely spaced columns along the long edges of the canopy, on both sides of the street. The canopy structure was to be designed on a performance basis and bid packages were sent to a number of proprietary space frame suppliers. The project's structural engineer-of-record designed the foundations for the anticipated canopy loadings, and the foundations were constructed and in-place when the space frame contract was awarded.

Upon analysis of the space frame and computing the reactions to be imposed on the existing foundations, it was discovered that the structural engineer-of-record had designed the foundations based upon the assumption that all of the space frame supports would be capable of resisting lateral loads. The foundations were thus designed for lateral load and overturning assuming that all of the foundations participated when the canopy was subjected to wind and seismic loads. However, this assumption also resulted in a build-up of extremely high thermal forces that were not considered in the design of the foundations. Releasing some of the supports to allow thermal movement resulted in higher lateral forces due to wind and seismic loads, which could not be resisted by the foundations. The end result was that major reinforcement of the foundations would have been necessary, at a cost of many millions of dollars, and the entire project was scrapped.

Of course, there are many cases where roof structures are designed on a performance basis by a specialty contractor and the support structure is designed by the structural engineer-of-record. In such cases, it is important that the structural engineer-of-record determine the boundary conditions for the roof structure (e.g., fixed supports, sliding supports, etc.) and specify these requirements in the performance documents for the roof structure. Lateral loads from the roof structure will then be directed to the proper support structure locations.

Daniel A. Cuoco, P.E., F.ASCE,
dcuoco@zweigwhite.com

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