Total Seismic Weight

[scanf1027]

I am determining the base shear for a single story structure and someone told me that I shouldn't include the parallel wall weights in this.
I also read in the Structural Engineering Reference Manual, 4th edition, Diaphragm Load Section that the seismic weight tributary to a diaphragm should not include the parallel walls.

This is confusing to me. I thought the seismic weight included all of the dead load of the structure, plus the additional items stated in the code.

 

[JAE]

Seismic weight includes all the weight of the structure if you strictly interpret the code language. All of the weight gets included in the determination of the base shear, V.
Then the base shear is distributed to the various levels based on the code vertical distribution factor.

For multiple story'd buildings this doesn't amount to a lot of load, and the static procedure just lumps all the mass in with each level according to the procedure.

However, common sense tells you that the lower half of a one-story building is not resisted by the upper diaphragm or lateral seismic system.

Also, for diaphragm design, per ASCE 7, the diaphragm force, Fpx, is calculated based upon wpx which is defined as the "weight tributary to the diaphragm at level x". This implies that only orthogonal walls - and NOT parallel walls) are included in the diaphragm force. (see ASCE 7-10, section 12.10).

 

[msquared48]

I have to disagree here to some extent...

I can understand the logic of not including the orthogonal wall weights for wood wall construction as they are light with respect to the lateral loads seen. However, I take issue, inspite of ACI, when it comes to Concrete and CMU walls. Their weight is frequently not insignificant.

As respects the orthoginal walls, it is true that the weight of the wall is already in the wall, and does not need to be transferred seismically through the floor or roof diaphragm to the wall in the same manner as the diaphragm and normal wall weights do.

In that fact, for the design of the floor or roof diaphragm and connections, I agree that the weights of the orthoginal walls do not need to be included. However, respecting the actual shear in the shear wall itself, the weight of the orthoginal walls should be included and proportioned according to relative shear wall rigidities.

It is more work, but it is also more correct. Not including these walls, particularly in a multi-story structure, will give results that are not conservative.



Mike McCann
MMC Engineering

 

[JAE]

Mike - I think I'm confused by your terminology here.

When you say "orthogonal" walls...are you speaking of the walls on the sides of the diaphragm that act as shearwalls? Or are you referring to the walls that are perpendicular (orthogonal) to the direction of loading?

The parallel (shearwall) walls on the sides, with the wall plane oriented parallel to the direction of loading, do not in any way shape or form contribute to the lateral force induced on the diaphragm. Their mass is beyond, or downstream, from the diaphragm.

 

[msquared48]

I am talking about the walls parallel to the force you are resisting, yes, the shear walls. Sorry for the confusion...

Mike McCann
MMC Engineering

 

[WillisV]

SERM is correct. Weight of upper half of walls that are perpendicular to the direction of the seismic force being investigated (those without the ability to act as shearwalls) is included in W because the seismic force due to the inertia of these walls must travel up through the diaphragm to the resisting shearwalls. Weight of shearwalls themselves (walls parallel to seismic force) need not be included in W because the walls can resist their own inertial effects. When designing the shearwalls though, the additional seismic force due to the wall's own weight should be applied at the center of the wall's mass vertically in addition to the seismic force being fed to the top of the wall by the diaphgram to get the total overturning force and shear.

With all that said, the large majority of engineers that I know just take the upper half of all of the wall weights so that you end up with the same seismic force in both directions - plus all canned software handles it that way which unfortunately becomes the de facto standard of care (RAM etc).

 

[JAE]

WillisV - I don't disagree with you - in fact I've designed numerous buildings doing just what you suggest. But don't you find it curious that the codes generally do not deal with that separation of the loads (lower half vs. upper half of walls)?

Per ASCE 7, W is the total weight of the structure - from the ground up. The code doesn't mention the lower half of the walls for one story buildings.

From that W (total structure weight) you get V, which gets distributed throughout the levels of the building - in a one story building it gets distributed just to the one story.

Just a strange anomaly in the code I think.

 

[hello12345hello]

There is one example in Seismic Design manual where it shows calculation for two different seismic base shear in Transverse direction and Longitudinal direction.
The example shows that if wall is parallel to base shear we have to include full weight of wall in that direction.

 

[msquared48]

"This implies that only orthogonal walls - and NOT parallel walls) are included in the diaphragm force."

I agree with that statement JAE, "diaphragm" emphasized;

As for the shear wall, however, for any story, it should see the upper diaphragm force, PLUS the seismic force due to it's own weight.

Mike McCann
MMC Engineering

 

[WillisV]

JAE - I would guess this is one example of the code actually NOT getting too far into the engineer's business and trying to over-complicate things by giving explanations such as the one I gave which would just serve to confuse most people. I think a little bit of engineering judgement as opposed to explicit code reading is the way to go this case.

Mike - read what I said regarding the design of the shearwalls - we ARE designing them to include the force associated with their own weight - just as a separate component. There is no reason to throw the seismic load due to the shearwalls onto the center of mass of the diaphragm by including it in the overall W just to then turn around and make the diaphragm deliver that force back to the shearwalls in which it originated.

Again - I generally don't sharpen my pencil this much, but it is certainly valid.

 

[RUS777]

To be convervative I would include entire building weight plus 25% of live load.