Frame shortening in high-rise concrete buildings is a concern for both designer and builder, causing lost time and lost dollars. Yet much of the waste can be avoided with proper awareness on the part of the construction industry and design professionals. A 60-story high-rise concrete building can shorten as much as a foot during and after construction. As building columns shorten, the roof comes down toward the ground and each of the lower floors exhibits a similar and gradually smaller displacement from the design levels.
Concrete frame shortening is the result of elastic and inelastic concrete shortening (shrinkage and creep), and insufficiently precise concrete construction, particularly in the forming operation. Lack of foresight to allow for shortening in high-rise concrete structures has led to major or minor calamities in the curtain wall and mechanical construction industries.
Curtain wall fabricators and designers must be sure that exterior panels and their supports are planned and installed to accommodate concrete slab lowering and thermal expansion of the panels themselves. Also, not only do the panel attachments have to line up, but there must be sufficient space at the joints between the panels to compensate for future movement. Otherwise, the panels could fit tight during installation but pop off later. Mechanical trades have experienced problems ranging from splitting of vertical plumbing waste lines that were constructed in contact with concrete floor slabs, to elevator shaft guide rails punching through the elevator machine room slab at the top of the building from being installed too close to compensate. The general guideline is to make sure that in design and installation, the non-structural elements of a building are separated from the structural elements.