Effective Seismic Weight? 1

[SteelPE]

Believe it or not I am having another disagreement with someone in my office on how to calculate the effective seismic weight of a building. This disagreement involves what to do with the walls that run parallel to the assumed direction of the seismic load. I have always included the weight of these walls when calculating the base shear on the structure. The only time I don’t include them in the seismic weight of the building is when the wall is a structural element (for example CMU).

The reference project is a steel framed building with a brick façade. I included the weight of the entire perimeter (because I don’t want to drive lateral loads through a veneer) while he is saying I don’t need to include the walls that run parallel to the seismic load. Applicable code is IBC09.

How do others handle this situation?

 

[msquared48]

Include all the weight of the3 brick veneer no matter what the direction. It still contributes to the overall weight of the building whch is what you are calculating.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[PicoStruc]

I approve what msquared48 said.

All mass must be accounted for.

By curiosity what is the explanation of your co-worker ?

 

[PicoStruc]

Note : CMU wall must be included in mass too !

 

[PicoStruc]

A simple way to explain that to your co-worker :

If the mass get accelerated i.e : if the mass "move" WITH the structure, it must be included in the seismic mass.

 

[SteelPE]

Well then, when applying the load to the LFRS do you include the weight of the walls that run parallel to the LFRS (I guess this is the real question)?

His defense is attached.

 

[peal724]

tell your co-worker "close but no cigar". the language is there that supports his argument, but it's not the right argument.

that example is for computing the diaphragm force of an individual level, not the seismic base shear of the entire building.

 

[PicoStruc]

I agree with peal724.

This commentary to calculate the force in the diaphragm. That means the diaphragm is idealized as a beam. Walls at each end are like support in this kind of simple calculation. The calculation is relative to the diapragm side.

But for the base shear calculation of the building, all wall get accelerated !

 

[SteelPE]

What about calculating the loads to the LFRS? Do you include the weight of the walls that are parallel to the LFRS when calculating the story shear?

 

[msquared48]

I would certainly think so!

Although the diaphragm may not see the shear load from the walls parallel to the direction of force, the wall itself that is parallel to the load does see those shears directly that the diaphragm does not see, so the effect to the LRFS is the same - use all the weight!

Confusing enough?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[JAE]

I agree with the others. The overall base shear of the building is based upon the mass that is attached and tributary to that level.

What your co-worker is confusing is that in some cases, one-story shear wall type buildings, the mass of the side shearwalls (parallel to the direction of loading) does not deliver lateral thrust to the building level (i.e. diaphragm) and then jump back into itself and go down to the ground.

But you still have to take account of it in your base shear in that the upper part of that wall is attached to the roof, and is accelerated, with the overall building.

The example you posted is a diaphragm calculation and for that you certainly don't throw the side walls into the diaphragm loading.

 

[nac521]

If you look at Volume 2 of that 2006 Seismic Design Manual, the one story light framed construction, one story masonry shear wall, and tilt-up wall panel examples use only the walls parallel to the direction of seismic loading (different base shears for each direction of loading). The multi-story examples use the total perimeter wall load for seismic weight.

 

[RFreund]

How about when designing say a rectangular building with masonry shear walls. The seismic weight used for base shear (which gets distributed to each level) was based on walls perpendicular to the shear walls you are designing. Then you add in the weight of that wall (and wall above) when designing that shear wall. Meaning that the weight of a shear wall on the other side of the building (parallel the wall your designing) would have no effect on the wall your designing, no?

EIT

http://www.HowToEngineer.com

 

[PicoStruc]

RFreund, For designing the wall, my understanding is to include all the mass. (considering mid height each side of the floor)

 

[MiketheEngineer]

IF it is there - it weighs something.

If it weighs something - seismic will come into play - one way or another.

Deal with it!!

 

[PUEngineer]

I agree with what I think is the consensus.

The effective seismic weight should include all tributary heights of walls per each level for base shear calculation.

It appears that the article in the defense of your coworkers position, as others have stated, is referencing diaphragm force calculations. In that case I would agree that parallel lateral resisting walls could be ignored from calculation of the w sub px used for the proportioning of story forces into diaphragm forces. I don’t believe parallel walls that aren’t shear walls should be ignored from the diaphragm force calculation.

 

[SteelPE]

We are all in agreement now. I just wanted to make sure I was doing my calculation correctly. I have heard numerous times from other people (engineers included) that they "have never seen seismic base shears so much larger than the wind base shears". I was beginning to think I was doing the calculation wrong and that is why I asked my coworker what his opinions were.

Thanks again.

 

[PicoStruc]

SteelPE.. Where i live it's the composite opposite.

If the wind load is higher that seismic... you are in big trouble !

 

[SteelPE]

Like I said, this has been building for years. Everyone who I have ever talked to has said that wind loads are on par with seismic loads. Every building I have ever done with the exception of one has had the seismic load larger than the wind load by a considerable amount. The one was a small building that was located on a site class A (per the geotech) with foam panel siding. I know you can reduce your seismic base shear by increasing R but my clients are used to dealing with R=3 systems and don't want to deal with the detailing requirements with R>3.

Well, I do feel slightly vindicated.

 

[PicoStruc]

LOL... i meant Complete instead of Composite... I work too much

But, Of course, the wind versus seismic comparison is only function of where the building is located... It change place to place !

We cannot generalize on that ! Just in canada, Eastern/Western versus Central Canada are two different world !