Report - PEER - University of California, Berkeley

Other factors such as the applied load whether shear or flexure, confinement andshear span influence the structural deformations. An important factor in the behaviourof columns and walls is the effect of the axial load. The increase in the axial loadincreases the shear resistance of the member. In addition, it was found experimentallythat the increase in axial load reduces the lateral drift.Although the performance objectives and the description of the associateddamage may remain unchanged, it is clear that several sets of drift definitions arerequired to establish the limits for various structural systems and elements such as:• Reinforced concrete moment resisting frame (MRF)(a) Ductile well designed frames according to current codes. Theestablished drift limits can be included in the code provisions.(b) Existing frame with nonductile detailing designed to earlier codes. Theestablished drift limits can be used in the evaluation of the lateral loadcarrying capacity of existing structures.(c) Moment resisting frame with masonry infills.• Structural walls(a) Flexural structural walls of aspect ratio (height/length) > 1.5.(b) Squat walls with predominantly shear behaviour of aspect ratio < 1.5.3.2 Interstorey Drift Distribution and DamageThe roof drift is a useful simple measure of the overall structural deformation that isroutinely calculated. It can be determined from nonlinear dynamic analysis, pushoveranalysis or the response of an equivalent single degree of freedom representation.Roof drift calculated using the gross section inertia is almost half the drift calculatedusing the cracked section inertia. Roof drift can be related to damage. However, theroof drift does not reflect the distribution of damage along the height of the structureand does not identify weak elements or soft storeys. The interstorey drift can bedirectly used in the design and serviceability check for beams and columns of theframe and can be correlated to damage at the floor level. A well-designed MRFstructure would have an almost uniform interstorey drift distribution along its height.In this case, the relationship between the roof drift and the maximum interstorey driftis linear with approximately 38 o slope as shown in Figure 2. For existing nonductilestructures and poorly designed frames such as those with a soft storey, the maximuminterstorey drift of the soft storey may indicate collapse while the roof drift willcorrespond to lower damage level. Therefore, the damage to the MRF can beconsidered influenced by two drift parameters: (a) the interstorey drift; and (b) itsdistribution along the height of the structure.325