Seismic Load Effect E (IBC 2003) 1

[aggman]

For an upcoming job my company will hopefully be designing some non-building (silos/ hoppers/ etc.) in a location where seismic design will control. I don't have that much experience with the IBC, but the seismic load effect E = rho*Qe +/- 0.2*Sds*D is confusing me. My structures are pretty straight-foreword 2D frames and approximately 4 stories tall. I understand the rho and the Qe and how to apply the forces (Equivalent Lateral Force Procedure). I also understand that the 0.2*Sds*D is the vertical component. What I don't understand is how to apply the vertical component. Should this be added or subtracted to your reactions as a vertical load? I am just unsure how to apply this force correctly and can't seem to make any sense out of the commentary in IBC or ASCE 7. Thanks in advance for your help.

 

[JAE]

Please check out this forum:


thread176-98527

as this issue was discussed. If you still have questions after reading that thread, just post here again.

 

[aggman]

I see now what this means. Just to make sure I am clear I would guess that the moment and shear values in your example would only see the effects of the horizontal shear force? It would seem that the axial component would not increase those values unless one column displaced with respect to the other. I know this probably seems like dumb questions, but my effective weight on each frame is 675 kips so seismic loads are fairly significant in my Ss = 155% and S1 = 46% region. 95% of the weight is at the top of the 48' tall frame also.

 

[JAE]

Yes, the best way to envision it is to think in terms of Load Cases vs. Load Combinations -

I haven't gone back to re-read that thread, but I think the example I gave was such that you would set up your individual load cases (dead, live, seismic) and then combine them to form combinations where the dead load seismic component would simply be included whenever there was an E in the combo. So E in a code combination is a load combination that includes the lateral seismic forces as well as a percentage of the dead load.

So I might have a combination of

0.9D + 1.0E that would look like:

0.9 x dead loads + 1.0 x (rho(QE) +/- 0.2(SDS)(dead load))

and that reduces to:

(0.9 +/- 0.2(SDS)) x D + rho(QE)

So the combination creates an effect (a shear, moment, or axial) in your frame that is the result of the dead load (vertically down) plus the lateral seismic (QE) plus a percentage of the dead load (vertically down).

 

[mrMikee]

aggman,

As an aside, I am curious how you handled the orthogonal loads here.

I see that the seismic acceleration values are fairly high which I assume puts this structure in a seismic category requiring orthogonal effects per IBC 1620.3.2. Orthogonality causes the problem to become 3D at least from the standpoint of loads, so I am curious how you did the analysis. The IBC permits the use of the 100%/30% rule and I suppose could be a 2D analysis, but a 3D design might take care of the bookkeeping that results from the many load cases involved. But then, the SRSS method could be used eith 2D or 3D too.

Thanks,
-Mike

 

[aggman]

mrMikee,
I modeled the frames as 2D including the effects of 100%/30%. If we get the job I will likely be doing dynamic (3D) analysis on the structure, but I felt doing a 2D frame and the ELFP was adequate for bidding.

 

[mrMikee]

aggman,

Thanks for the info. I've used SRSS but I'm not sure it was that much better compared to the 100%/30%. Most of my designs have been four column structures where 2D analysis would generally have been sufficient, but I used a 3D analysis mostly to handle the many load combinations and the corresponding combined stress calculations.

I wonder however, how many engineers still don't check orthogonal loads?

Thanks,
-Mike

 

[stanier]

Excuse my ignorance. Could you explain "orthogonal" loads? I am not a structural engineer but want to understand seismic engineering so follow these threads to the best of my ability. The term is not common to me.

 

[mrMikee]

stanier,

The most severe column loads are not necessarily when seismic forces are in the direction of the principle axes in plan view, but could be from another direction where the combined effects of two orthogonal directions are considered. For example, you would check seismic loads from the north and east separately, and then from the north-east. This effect is most significant in columns that share orthogonal lateral force systems, in other words, columns that get seismic load from either direction. When a structure lacks symmetry the analysis can get to be messy because you need to check 8 directions.

If you have access to the IBC spec this is all better explained in figure 1616.4 and section 1620.3.2.

Regards,
-Mike

 

[stanier]

Thanks Mike. I dont have access to IBC as I am based in Australia and we have our own building code, although I dont think it covers siesmic in so much depth.

My interest is in the behaviour of piping systems subjected to seismic loading. There appears to be even more complexity here depending upon the code used, size of piping and the behaviour of the structure that supports the pipes.

If readers know af a good piping book that covers seismic design I would be very interested.