ASCE 7-05 defined the base for seismic design as:
“The level at which the horizontal seismic ground motions are considered to be imparted to the structure”.
Most of the times this is misinterpreted or misunderstood that where the base is considered when determining the height of building for seismic calculations specially when the site is sloped or if the basement is open from one side.
Read the following article published in December 2009 issue of Structure magazine, this article will answer lots of questions.
Per ACI 318-08 section 184.108.40.206 – “Grade beams designed to act as horizontal ties between pile caps or footings shall be proportioned such that the smallest cross-sectional dimension shall be equal to or greater than the clear spacing between connected columns divided by 20 but need not be greater than 18 inches. Closed ties shall be provided at a spacing not to exceed the lesser of one half the smallest orthogonal cross-sectional dimension and 12 inches.”
But ACI is not telling us that what force these tie beams should be designed for, so here is the IBC 2009 section to tell us that:
IBC2009 section 1809.13 for shallow foundations and 1810.3.13 for deep foundations:
for SDC C, D, E or F, ties shall be capable of carrying, in tension or compression, a force equal to the lesser of the product of the larger pile cap or column design gravity load times the seismic coefficient, Sds,divided by 10, and 25 percent of the smaller pile or column design gravity load. (there is more in this section so please read the code before applying this to your case)
In simple words Tie beam force; FT = Larger of ( Pu_large x Sds /10 , Pu_small x 25%)