8.2.Computation of earthquake action 1.Damping ratio h shear force APoportionofthotatoraloadsharedbybracingand 封 )25 percent of the total base shear.and rted by frame which are 6 6 Seismic design flow chart 8.2.3 Computation of earthquake action 1. Damping ratio n By the experience, the seismic design code recommends that: n when the action of frequently occurred earthquake is to be computed, the damping factor can be taken as 0.04 if the building is less than 50m, n while 0.03 if the building is more than 50m but less than 200m, n and 0.02 when the building exceeds 200m. n when expected rare earthquake is considered, the damping factor is taken as 0.05. 2. P-Delta effect : Second order effect To any story, if the second order moment which is computed by the product of gravity load over the story and the storey drift is greater one tenth of the overturning moment which produced by storey shear force timing the storey drift, P￾Delta effect should be considered in the computation results. c) Additional moment due to second order effect of vertical load b) Storey moment by horizontal load a) Analsis model Figure 8.14 Second order moment a) 3. Lateral displacement and the limitation of the storey drift The storey drift limitations for middle and high rise building steel structures are 1/250 when considering the frequently occurred earthquake; and n 1/50 when considering the rare occurred earthquake respectively, specified by design code. Mc1 Vb2 Vc1 Mb1 hb a)Moment and shearinpanel zone A Shearinpanel zone Vc=(Mb1+Mb2)/hb-(Vc1+Vc2)/2 Vb=(Mc1+Mc2)/hc-(Vb1+Vb2)/2 A Vb1 Mb2 Vc2 hc Mc2 column beam b)Distortingofpanel zone A Figure 8.15 Distorting of panel zone 4. Proportion of the lateral load shared by bracing and frame Design code asks the lateral load shared by frame is not less than the minimum value of the next two: 1) 25 percent of the total base shear, and 2) 1.8 times of the shear supported by frame which are computed by elastic analysis. 支撑 支撑 支撑