Column Shortening in Tall Structures
The effects of column shortening, both elastic and inelastic, take on added significance and need special consideration in design and construction with increased height of structures, Differential column shortenings are magnified by the quest for optimum economy through use of highstrength materials and, in some instances, the use of compnsite structural systems.
These, in turn, change the initial pnsitions nf the slabs. As a consequence the partitions, mechanical equipment, cladding, architectural finishes, and built-in furnishings are also affected. The strains in the columns nf low as well as ultra-highrise buildings are similar if the stress levels are similar; however, the overall column shortening is cumulative and depends upon the height nf the structure. For example, in an 80-story steel structure, the total elastic shortening nf the columns maybe as high as 7 to 10in, (180to 255 mm) due to the high design stress levels of modern highstrength steels. By comparison, in an 80-story concrete building, the elastic shortening of columns wnuld amount to only about 2.5 in, (65 mm); however, the total length change of the reinforced concrete columns may be 7 to 9
in. (180 to 230 mm) due to shrinkage and creep.