Big design result difference by using UBC-Code and by using IBC & ASCE 1

[chrislaope]

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.

 

[jdgengineer]

Well I have a couple of thoughts...

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.

 

[jdgengineer]

Below is a link from an old SEAOC paper by Ghosh.

http://www.seaint.org/seaocconvention/convention1999/proceedings/impact of seismic.pdf

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

 

[msquared48]

Rho is included in UBC 97 for EQ forces 1630.1.1), but not for wind.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[jdgengineer]

Mike - sorry I didn't intend to say you don't need the tho factor merely that tho is allowed to equal 1 if you follow 1634

 

[JStephen]

I don't have the standard here in front of me- but if you apply the load combinations from the first chapter of ASCE 7, does the seismic get multiplied by 0.7 for allowable stress design?

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.

 

[jdgengineer]

Jstephen you do get to take 0.7 for allowable stress design. From what I remember UBC 97 was e first code to use strengthn level forces for earthquake forces. Had it been UBC 94 then there would have been this difference.

 

[JedClampett]

In my observations, the UBC Zone method was a sledgehammer. Sometimes areas were placed in zones due to political boundaries. For example, all of Yuma, AZ was in Zone 4, but using the mapped values the east side had barely Zone 2 accelerations.
Maybe you're running into that with Fresno.

 

[chrislaope]

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.

 

[msquared48]

That really depends on where you are though. I found the loads very comparable for my region. Not much change at all...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com