ubc 1991 seismic loads

huangyhg

ubc 1991 seismic loads
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.
i believe that the 1991 ubc is a service level seismic load system.  in the 1997 ubc they switched to an ultimate load level for seismic.
the base shear given in equation 34-1 is then used in the load combinations of 2303 where no factored load combinations are offered.
jae is right.  it is at service level.  at 1991, there was no such thing called near source factors.  as a comparison, you can take the equation 30-5 in the 1997 ubc, assuming na = 1, then divide by 1.4, you will find that the value will be comparable to one using equation 34-1.
i'm having trouble proving to myself that the 1997 ubc eq 30-5 value divided by 1.4 is comparable with 1991 ubc eq 34-1 value as suggeted by whyun.  i seem to get a much higher value from ubc 1997 /1.4 with all other input data similar.
try an example, say light framed plywood shear wall structure, standard soil and importance factor of 1.0 in seismic zone 4.
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.