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Be Aware and Prepared: Understanding Building Movement Can Help Avoid Costly Mistakes
By Stewart Jeske
Building structural movement is one of the most misunderstood items in the glazing industry—and it can be very costly.
What You Need to Know
Building structural movement refers to how structural beams and floor systems move differentially with respect to upper or lower floor levels of the building. This differential structural movement must be accommodated by façade systems including glazing systems. As an example, if everyone on the third floor of an office building ran over to look out the curtainwall, the third floor spandrel beam would move down under the load and the second floor spandrel beam would not. Accommodating building structural movement is an extremely important requirement indicated in most all glazing specifications and if not understood could lead to serious problems.
Two Problems
The problem often starts in two ways that involve responsibilities ignored by the architectural-engineering (AE) design team. The first is failing to provide documents that clearly indicate the required structural movement to be accommodated. Delegated design of the glazing system places the design responsibility for the glazing system, including building structural movement, on the glazing contractor. However, architectural and structural plans rarely indicate by specification or detail the actual structural movement to be accommodated. When pressed by request for information (RFI), the design team typically responds, but many times the actual movement is too much and does not match the basis of design anticipated by the AE design team.
Failure to understand the inherent limitations of standard manufactured glazing systems is also often ignored by the AE design team. A little known fact is that standard stick-built curtainwall systems (without modifications) may accommodate about ¼-inch maximum vertical building structural movement. The limitations can often be found in notes on shop drawings produced by system manufacturers. Architectural details of storefront systems, for example, may be shown shimmed tight at the head without a receptor system. Such details are common in contract drawings for areas that require a receptor head detail to accommodate structural vertical movement.
Real-Life Example
I was asked by a glazing contractor to write a letter of review associated with design deficiencies by the AE design team to account for building structural. My scope was only for review, not curtainwall design. The glazing contractor had bid on curtainwall of a 15-floor multi-use commercial building. The details and specifications indicated a standard stick-built curtainwall system as a basis of design. The contract specifications included the standard excerpt under performance requirements for the curtainwall to be designed to account for movements of the supporting structure.
The movement to be accommodated was not included in contract documents, leading the manufacturer performing delegated design to submit RFI for the actual building structural movement. The structural engineer of record answered the RFI indicating that the system should be designed to accommodate approximately 1.7 inch of building structural
differential movement between floors.
Had this not been caught, the curtainwall could have been built with far too little capacity for the building structural movement, likely resulting in leaking damage. After much discussion, the manufacturer provided a significant stack joint at each floor line that posed a significant cost increase to the glazing contractor. The glazing contractor requested change orders to cover the increases and a significant battle followed with the glazing contractor finally getting the cost increases covered. It’s important to be aware of the issue and ask for the missing information early on.
Stewart Jeske is president and owner of JEI Structural Engineering in Kansas City, Mo.
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