Report - PEER - University of California, Berkeley

4. MOMENT RESISTING FRAMES4.1 Ductile MRFThe storey drift factor calculated using equation (1) for a number of ductile, welldesignedMRFs can be correlated with damage as shown in Figure 3. The damageindex used is the final softening representing the effect of stiffness degradationfollowing the application of the load. This damage index was arbitrarily selectedbecause of its simplicity. Other damage indices could have been also used. In Figure3, zero damage index indicates no damage while 1 represents collapse. However inpractical terms, the actual failure of the structure occurs at damage index values of 0.7to 0.8. For ductile MRF, damage index values up to 0.2 represent repairable damage.The plot in Figure 3 using SDF on the horizontal axis can be compared with asimilar damage plot using the maximum interstorey drift shown in Figure 4. Thefigures are similar but not identical. Comparison between the two horizontal axes ofFigures 3 and 4 gives a rough relationship between the maximum interstorey drift andthe SDF values.The SDF for the ductile reinforced concrete moment resisting frames is plottedwith the ductility factor as shown in Figure 5. For SDF values from 0 to 0.2 thedamage as measured by the final softening damage index is light. Moderate repairabledamage is estimated for SDF values from 0.2 to 0.4. The start of yield as indicated byductility >1 from figure 5 corresponds to SDF of 0.4, damage index of 0.15 andinterstorey drift of 1.3. In the figure, the point marking the departure from ductilityfactor 1 is well defined. Past the yield point, damage increases and is consideredirreparable. When using a large sample of frames, the mean damage index at frameyield is closer to 0.2. The maximum interstorey drift limits corresponding to variousdamage states for a ductile MRF are listed in Table 2.Table 2. Drift ratio (%) limits associated with various damage levelsState of damage DuctileMRFNonductileMRFMRF withinfillsDuctilewallsSquatwallsNo damage 2.5 >0.8327