UBC 1991 seismic loads
UBC 1991 seismic loads
(OP)
I assume that the seismic load calculated by equation 34-1 in section 2334 is an "utimate load"? The reason I think this is because in section 2338 for non-building structures, it states specifically that "the minimum design lateral forces in this section are at a service level (rather than yield or ultimate level)". They don't make such a statement in section 2334.






RE: UBC 1991 seismic loads
The base shear given in equation 34-1 is then used in the load combinations of 2303 where no factored load combinations are offered.
RE: UBC 1991 seismic loads
RE: UBC 1991 seismic loads
RE: UBC 1991 seismic loads
Using the 1991 UBC, ZIC/Rw
Z = 0.4
I = 1.0
C = use 2.75
Rw = 8
Base shear V is 0.1375
Now try the 1997 UBC, 2.5 Ca I / R
Ca = 0.44Na = 0.44 (assuming type Sd soil and Na = 1.0)
R = 5.5
Base shear is 0.2 but this is at ultimate.
0.2/1.4 gives 0.1428 at allowable level.
Both codes gave similar results at around 0.14. You may want to try using different lateral systems with different Rw and R values. Result should be comparable.
1997 Code introduced the near source factors to penalize you if the building is near a seismic source.