The Tall Buildings Reference Book by Parker David Wood Antony

Author:Parker, David, Wood, Antony
Language: eng
Format: epub
Publisher: Taylor and Francis

17.4 The John Hancock Center, Chicago

Image: Skidmore, Owings & Merrill LLP/Ezra Stoller © Esto

A stiff shear system is necessary for the entire building to act as a single giant beam rather than an aggregation of individual elements or subsystems. Because it is not possible to create a completely rigid shear system, there exists a phenomenon called “shear lag.” This occurs when the overturning strains and stresses are not distributed linearly from the center of the tower, resulting in a less effective use of the tower’s vertical elements to resist the overturning moments on the structure. This is evident in Figure 17.2, where not all of the columns in the “flanges” are uniformly stressed.

Although efficient and stiff shear resisting systems can reduce shear deformations to a small portion of the target deflections, it is not practical to do the same for the flexural deformations. The deflection of a steel I-beam without holes is almost totally attributable to the flexural bending. For a given layout, this deflection can generally only be reduced at the cost of increasing the size of the columns and/or walls. Beam deflection can be halved by doubling the area of the cross section, for example. The great expense of reducing deflections by increasing the cross-sectional area of the vertical element places very practical limits on reducing flexural deflections.

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