Big design result difference by using UBC-Code and by using IBC & ASCE
Big design result difference by using UBC-Code and by using IBC & ASCE
(OP)
I have always had an impression that for a specific project, no matter what Code you are using, the final result designed by the two different Code should not off too far. However recently I am working on a non-building structure design that are not similar to building. I found a surprisingly big difference between the results by these two different design Codes.
The project is located at Fresno, California. The effective seismic weight of the structure is W=1273569 lb. The fundamental period is T=0.614.
By using UBC-97: from the seismic zone map, it falls in seismic zone 3, the control equation for base shear is:
V= (Cv*I*W)/(R*T) = 386230 lb session 1630.2 Eq.(30-4)
The redundancy factor for seismic zone 3 = 1.5.
By using IBC & ASCE7-05: Fresno, California has following value: Ss=0.507, S1=0.223, SDs=0.4713, SD1=0.2905, the control equation for base shear is:
V= Cs*W = 200078 lb ASCE7-05 Eq. (12.8-1)
Where:
Cs=SDs/(R/I) ASCE7-05 Eq. (12.8-2)
The redundancy factor for non-building structure that are not similar to building = 1.0 (see Clause 12.3.4.1).
Therefore the final seismic load effect on the structure for UBC design is about:
1.5*386230/200078=2.89 times as large as by IBC & ASCE7-05. The weight for this structure by UBC will also approximately doubled than by IBC & ASCE7-05.
Am I missed anything?
Thanks.






RE: Big design result difference by using UBC-Code and by using IBC & ASCE
1) I don't think you need the p factor in your UBC 97 calculations, see setion 1634.1.2.
This brings the forces down to a factor of 2. This does seem higher than I would expect but I think this discrepancy is because you are in Seismic Zone 3. I think the IBC forces are much lower than the UBC forces in Seismic Zone 3. Had your structure been in Zone 4 of UBC I would not expect such a discrepancy.
RE: Big design result difference by using UBC-Code and by using IBC & ASCE
http://www
As you can see he had similar findings. In Sacramento (Zone 3) he was finding the base shears to be about 60% of the 97 UBC. See Table 6 on page 5 of 26
RE: Big design result difference by using UBC-Code and by using IBC & ASCE
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Big design result difference by using UBC-Code and by using IBC & ASCE
RE: Big design result difference by using UBC-Code and by using IBC & ASCE
Another consideration is that it seems 99% of the thought that goes into the standard is for buildings, and when you get into the oddball stuff, you tend to have a lot less clarity in the standard and more variation in the approaches used.
RE: Big design result difference by using UBC-Code and by using IBC & ASCE
RE: Big design result difference by using UBC-Code and by using IBC & ASCE
Maybe you're running into that with Fresno.
RE: Big design result difference by using UBC-Code and by using IBC & ASCE
JStephen:
To compare apple to apple, for IBC & ASCE7-05, by using ASD, the seismic load does multiplied by 0.7. However by UBC97, ASD, the seismic load also need divided by 1.4 which is approximately same as multiply by 0.7.
The only place which can bring down UBC97 seismic load a little is that we really can use redundancy factor = 1.0 instead of 1.5 by applying UBC97 Eq(30-3). Then the seismic load by UBC97 is still about twice as large as by IBC & ASCE7-05.
RE: Big design result difference by using UBC-Code and by using IBC & ASCE
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com