Q.: The vertical distances between attachments of our precast exterior wall panels aren't matching those of the high-rise building for which the panels were made. A careful check shows that the spacings on the panels match the drawings, but those on the building don't. The building contractor claims that his records show the right spacings, but somehow we find they're all short. We plan to adjust the positions of the attachments on the remaining panels. Is this a common problem, and if so, what is the usual way for the precaster to work with the contractor to prevent it?
A.: The problem is probably related to frame shortening, and it is more common a problem than it should be. The person you should really have been working with is the designer. As the building gets taller, the frame shortens both elastically from load and inelastically through shrinkage and creep. If the designer has not allowed for frame shortening in his design he should be able to work with you to determine how much shortening to allow for. The shortening affects not only the attachments but the spacing between panels, so you may have to adjust the panel lengths to avoid the possibility that pieces may fall at some later time from spalling or buckling of panels. Most of the frame shortening occurs early in the life of the building but it can continue at a reduced rate for years.