Safety factor for Sliding with Seismic load combinations 1

[rob7204]

I have been trying to find if there is any reduction to the safety factor 1.5 for Sliding when seismic forces are involved. In AASHTO, the use of 0.75 reduction factor is permitted. Is there a similar reduction factor in IBC 2003? Please respond.

Thanks,

Robert

 

[sundale]

As I interpret the IBC, the seismic load "E" is an ultimate load, whereas all other loads are service loads. The basic (working stress) load combinations of IBC 1605.3 reflect this and the governing load combination for a footing subject to sliding will most probably be 0.6D+0.7E. The reciprocal of 0.6 is 1.67 is how the code implicitly takes care of the the old 1.5 safety factor (resisting sliding/causing sliding), I think.

To make your issue worse, IBC 1605.3.1.1 specifically says not to increase allowable stresses for these load combinations (i.e. don't bump up your passive soil resistance). To make matters better, you are allowed to ignore the vertical seismic component (0.2 Sds x D)in the special seismic load combinations for a foundation per IBC 1616.1.

Hope this helps.