base shear in dual system for a multistory bldg

[genzo]

g'day!
i have a 3D model of a multistory bldg in etabs with a dual sytem, a shearwall and moment resisting frames.
how would you check your base shear on moment resisting frames that these frames independently resist 25% of the base shear and 75% on the shear wall according to the UBC code?Can i directly use the horizontal reactions on their supports?

 

[Taro]

Save the model as a separate file and delete the shearwalls. Apply the 25% earthquake load to the moment frame system. Check the resulting forces in the moment frames versus the forces from the original model. Design for the worst case.

 

[erkansamhal]

Dear GENZO,

If you have a little time to check it by hand calculation for the shearwall using one of the familiar method, it will be more accurate..

See You,
ERKAN

 

[tacmena]

I don't agree with taro. If you check the frames "alone" probably they will deflect in a different way than linked to the walls.
On your original model you have to compare 25% of the base shear vrs the loads on the frames outcoming from a torsional distribution analysis. Usually the 25% requisite governs.
MMena

 

[Taro]

tacmena,

According to the SEAOC seismic design manual, "This 25-percent requirement may be interpreted in two ways
a. Q'E may be found by an equivalent lateral-force analysis of the independed moment-frame using 25 percent of the design base shear.
b. Q'E may be found by factoring the combined... system Q'E value such that Q'E corresponds to the action that would occur if the portion of the base shear resisted by the moment frame VF were to be at least equal to 25 percent of the design base shear V."

Either approach is valid. The purpose of a dual system is to provide a secondary load path to increase the reliability of the system when the primary system degrades. Either approach will work because the 25% force requirement is arbitrary and the force distribution will change from your initial elastic assumption anyway. Your force distribution at ultimate levels will not be accurate unless you perform a nonlinear analysis anyway.