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# Vertical distribution for base shear determined by response spectrum

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## Vertical distribution for base shear determined by response spectrum

(OP)
Hey. I’m doing a response spectrum analysis as per section 12.9 of ASCE7-05. It does not address how to distribute the seismic base shear (like the equivalent lateral force procedure does). As I recall, there is some formula to use (i.e. F(level)=participation factor x mode shape vector x acceleration x etc.,etc.). Does anyone have an opinion on this? Does STAAD perform this for you?

### RE: Vertical distribution for base shear determined by response spectrum

The formulation u have mentioned is correct. Seismic base shear in RS is done based on the mass participation factor. So according to the time priod of the structure in different modes it (STAAD) reads corresponding acceleration from the graph you have entered into it and then simply gives u the base shear as = acc.*mass part(%)*weight for the particular mode. For all the modes listed it sums up based on the ABS/CQC/SRSS methods and then gives u the total base shear.
Be aware that STAAD gives all the forces (support reactions, member forces, etc etc) ignoring their sign...meas for a horizontal seismic force on a portal frame it gives both the support reactions +ve (insted of one +ve & other -ve).

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