Effective Seismic Weight?
Effective Seismic Weight?
(OP)
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?
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?






RE: Effective Seismic Weight?
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Effective Seismic Weight?
All mass must be accounted for.
By curiosity what is the explanation of your co-worker ?
RE: Effective Seismic Weight?
RE: Effective Seismic Weight?
If the mass get accelerated i.e : if the mass "move" WITH the structure, it must be included in the seismic mass.
RE: Effective Seismic Weight?
His defense is attached.
RE: Effective Seismic Weight?
that example is for computing the diaphragm force of an individual level, not the seismic base shear of the entire building.
RE: Effective Seismic Weight?
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 !
RE: Effective Seismic Weight?
RE: Effective Seismic Weight?
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
RE: Effective Seismic Weight?
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.
RE: Effective Seismic Weight?
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.
RE: Effective Seismic Weight?
EIT
www.HowToEngineer.com
RE: Effective Seismic Weight?
RE: Effective Seismic Weight?
If it weighs something - seismic will come into play - one way or another.
Deal with it!!
RE: Effective Seismic Weight?
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.
RE: Effective Seismic Weight?
Thanks again.
RE: Effective Seismic Weight?
If the wind load is higher that seismic... you are in big trouble !
RE: Effective Seismic Weight?
Well, I do feel slightly vindicated.
RE: Effective Seismic Weight?
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 !
RE: Effective Seismic Weight?
The mass should be taken entirely for seismic load
As for wind versus seismic it depends the type of structure and since we have important load located in a significant height the seismic could be governing....
RE: Effective Seismic Weight?
I saw a increase in seismic force only due to the green roofing that is now quite popular !
I would add that sometime, wind govern on a certain height and the seismic can govern on another !