Foundation, Concrete and Earthquake Engineering

Seismic Response of Flexible Structures

Excessive structural Flexibility

Excessive flexibility of the building to seismic loads can be defined as the susceptibility to large lateral distortions between different stories, or "drift". The main causes of this problem reside in excessive distance between the support elements (clear spaces or clearances), their vertical clearance, and their stiffness. Depending on the degree, excessive flexibility can have the following consequences:

• Damage to nonstructural elements attached to contiguous levels;

• Instability of the flexible floor or floors, or the building in general;

• Not taking advantage of available ductility.

Excessive flexibility of the diaphragm

An excessively flexible floor diaphragm involves non-uniform lateral distortions, which are in principle prejudicial to the nonstructural elements attached to the diaphragm. Additionally, the distribution of lateral forces will not be in accordance with the stiffness of the vertical elements (see figure 1).
Figure 1. Rigid and flexible behavior of the floor diaphragm

There are several reasons why there can be this type of flexible performance. Among them are the following:

• Flexibility of the diaphragm material. Among the usual building materials, wood or steel decking without concrete are the most flexible.

• Aspect ratio (length/width) of the diaphragm. The greater the length/width ratio of the diaphragm, the greater the lateral distortions may be. In general, diaphragms with aspect ratios greater than 5 may be considered flexible.

• Stiffness of the vertical structure. The flexibility of the diaphragm should also be judged in accordance with the distribution of rigid vertical elements in the plan. In the extreme case of a diaphragm in which all elements are of equal stiffness, better performance is expected than when there are major differences in this respect.

• Openings in the diaphragm. Large openings in the diaphragm for purposes of illumination, ventilation, and visual connections between stories cause flexible areas that impede the rigid assembly of the vertical structures.

There are multiple solutions to the problem of excessive flexibility of the diaphragm, depending on its cause. Measures used to stiffen the diaphragm where large openings occur should be carefully studied; other options include segmentation of the building into blocks.

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