Report - PEER - University of California, Berkeley
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Report - PEER - University of California, Berkeley

Report - PEER - University of California, Berkeley Report - PEER - University of California, Berkeley

peer.berkeley.edu
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12.07.2015 Views

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

6Roof drift %4200 2 4 6 8Maximum interstorey drift %Figure 2. Relationship between maximum interstorey drift and roof drift ofwell-designed 3, 6, 9, and 12 storey MRFs subjected to several ground motionrecords.To take into account a measure of the storey drift distribution along the height ofthe structure, a representative factor is proposed. The factor is called the Storey DriftFactor (SDF) and can be calculated by the formula:1 nn2 2∑(Si− S)∑(Si)2( n −1)i= 1 i=1SDF =(1)Swhere n is the number of storeys, S i is the maximum interstorey drift of floor i, andS is the mean value of the maximum interstorey drift ratios. A value of the SDF = 0indicates equal interstorey drift along the height. A value close to 1 represents the casewhere the overall drift is caused by few storeys (e.g., soft storey).Global damage can be related to damage at the element and storey levelsusing any damage index such as final softening. The results of the analysis of 10ductile moment resisting frames are summarized in Table 1.Table 1. Final softening damage index associated with various damage levelsState of damage Element Storey GlobalNo damage

6Ro<strong>of</strong> drift %4200 2 4 6 8Maximum interstorey drift %Figure 2. Relationship between maximum interstorey drift and ro<strong>of</strong> drift <strong>of</strong>well-designed 3, 6, 9, and 12 storey MRFs subjected to several ground motionrecords.To take into account a measure <strong>of</strong> the storey drift distribution along the height <strong>of</strong>the structure, a representative factor is proposed. The factor is called the Storey DriftFactor (SDF) and can be calculated by the formula:1 nn2 2∑(Si− S)∑(Si)2( n −1)i= 1 i=1SDF =(1)Swhere n is the number <strong>of</strong> storeys, S i is the maximum interstorey drift <strong>of</strong> floor i, andS is the mean value <strong>of</strong> the maximum interstorey drift ratios. A value <strong>of</strong> the SDF = 0indicates equal interstorey drift along the height. A value close to 1 represents the casewhere the overall drift is caused by few storeys (e.g., s<strong>of</strong>t storey).Global damage can be related to damage at the element and storey levelsusing any damage index such as final s<strong>of</strong>tening. The results <strong>of</strong> the analysis <strong>of</strong> 10ductile moment resisting frames are summarized in Table 1.Table 1. Final s<strong>of</strong>tening damage index associated with various damage levelsState <strong>of</strong> damage Element Storey GlobalNo damage

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