UBC 1991 seismic loads

[PAeng22]

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.

 

[JAE]

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.

 

[whyun]

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.

 

[PAeng22]

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.

 

[whyun]

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.