Load combinations and 0.75 factor

[mrMikee]

I have a question about load combinations for use with allowable stress design and the application of the 0.75 factor for seismic loads when combined with live loads. Per sections ASCE 2.4.1 and IBC 1605.3.1:

Equation 6 in ASCE 7 is: D + 0.75L + 0.75(0.7E)

IBC Equation 16-10 is: D + L + 0.7E

The IBC goes on to say that the 0.75 factor can be applied to variable loads (wind and seismic) when combined with dead loads. My interpretation is that the IBC equation would become the same as the ASCE equation. However the IBC continues with "the 0.7 factor on E does not apply for this provision," but the commentary also says these reductions are similar to the ASCE. Does anyone know what the IBC is trying to say here?

Carrying this on to the next step brings me to section 1620.3.2 of the IBC concerning the direction of a seismic load where the 100%/30% rule is discussed. Using equation 6 from above (including the 0.75 factor) and assuming seismic loads are Ex and Ey I would get the following load combination:

D + 0.75L + 0.75(0.7Ex) + 0.225(0.7Ey)

Actually this load combination depending on the symmetry of the structure and applied loads could under certain condition result in a total of 8 equations.

Is there anything wrong with this logic?

Regards,
-Mike

 

[aggman]

Right or wrong we have been using the following equations.

DL + 0.7*EQ
DL + 0.75*(LL + 0.7*EQ)

When we were using ASCE 7-98 they laid out the load combinations as the IBS 2003 does now, which I always found confusing as to applying the reductions. When the ASCE 7-02 came out the just wrote down the equations which simplified the problem. If I remember reading somewhere that the IBC basicly wants you to design using the ASCE 7-02 (I think in Alan Williams book Wind and Seismic Engineering) and that if you follow its provisions then you are satisfying code requirements.

 

[mrMikee]

Thanks aggman,

That's basically the conclusion I've come to. However, occasionally when I'm reading through the code it bothers me that the IBC is trying to say something about the 0.7, but I have no idea what it is. The 0.7 is used to bring strength based seismic loads into the ASD equation set and has nothing to do with load reductions. The 0.75 should be applied to the seismic load which is 0.7E as you show in your 2nd equation.

The second part on my post is about the 100/30 rule which I am concluding from the above becomes 75/22.5 because of the 0.75 factor. I'm just working thru the math here in order to get from basic load cases to the load combinations.

Thanks,
-Mike

 

[mrMikee]

My conclusion is that the IBC seismic equation wih the 0.75 applied becomes the equivalent of ASCE equation 6.

I assume that there isn't anyone seeing a problem with this.

Thanks,
-Mike

 

[AggieYank]

ASCE 7-02 Load Combinations are the IBC combinations with the reduction factors already in them.

It appears the IBC wants you to be a little conservative, but does allow you to use the ASCE equations.

I am not sure what the IBC statement "The 0.7 factor on E does not apply for this provision" means.

I found a great page dealing with changes to the IBC for the years 2004/2005 (coming out in the new IBC?). In this article, it doesn't have the "The 0.7 factor on E does not apply for this provision". It was either useless, wrong, or too confusing to even be there.

http://www.iccsafe.org/cs/codes/2004-05cycle/ProposedChanges/IBC-S1.pdf

 

[AggieYank]

http://www.iccsafe.org/cs/codes/2003-04cycle/pdf2/IBC-S.pdf

The ICC deleted that confusing sentence in the 2003-2004 cycle apparently. Go to page 9, and they simply deleted the "The 0.7 factor on E does not apply for this provision".

Basically, ASCE 7-02 has the IBC 2003 equations with the reductions already in place.

 

[mrMikee]

AggieYank,

That's an interesting link. It hadn't occurred to me to check drafts of the upcoming code. I had basically come to the same conclusion in the past, but during the process of setting up a new analysis program with my standard set of load combinations I started to think about the IBCs wording again. My concern was that a misinterpretation of the " 0.7 factor on E does not apply for this provision" could result in a substantial underdesign. They were saying that something doesn't apply. Regardless, I'm with you and aggman on this.

Thanks for the help. You’re assistance is appreciated.

-Mike