To prevent gross errors in modeling adversely affecting the results down to an unsafe base shear (e.g. analytical model periods are almost always higher than the the actual period of the building taking into account the stiffening effect of the facade and beams taken as pinned in the model but really aren't etc).   

Thanks WillisV! I suppose that's a reasonable concern. Any idea where the number 85% comes from?

Setting an absolute minimum like 85% for all structures doesn't really correlate to the adverse effects of modeling. It affects structures with long periods far more than it affects structures with short periods. I'd like to know the reasoning of the code writers.

It seems like a more reasonable approach could be to decrease the modal period of the structure by 15% and calculate an (increased) base shear based on that period. This could then be used as a minimum.

 

Dear amt55,
            if you are working with periods that are before the plateau it isn't reasonable to decrease the period. Regards,

Sebastian