Seismic Base Shear

[haynewp]

I got confused today when I noticed in the ICC seismic design guides that they calculate the base shear 2 different ways. In one example they only included the weight of 1/2 the height of the shear walls from the ground level up to the second floor, then in another case they included all of the shear walls that were parallel to the seismic force(north-south walls) but only included 1/2 the walls that were perpendicular to the seismic force (east-west walls not acting as shear walls for lateral in that direction).

So when you list the "Base Shear" for a building on your drawings, is that supposed to include:

1) All the walls from the ground floor up to the second floor (the entire weight of the whole building)

2) 1/2 the height of the walls from the ground floor to the second floor then the rest of the whole building?

3) All the walls acting as shear walls in an orthogonal direction with 1/2 of the walls that are not acting as shear walls in that direction plus the remainder of the building?

I looked in a different seismic design book I have and it used the entire building for calculating the weight for the base shear including all walls from the ground to second floor, and yet another book I had listed 1/2 the walls again.

In multi-story buildings, in the past I have disregarded the 1/2 the contribution of the walls from the ground to second floor because I didn't want what seismic force that is assumed going directly into the ground being factored into the floor distribution equation. (ASCE 7-05 eqns 12.8-11 and 12.8-12)

 

[271828]

"3) All the walls acting as shear walls in an orthogonal direction with 1/2 of the walls that are not acting as shear walls in that direction plus the remainder of the building?"

I agree with this approach from a fundamental vibrations POV. Other than shearwalls parallel to the load, only the upper half of walls from the fdn to the first level should be included in the mass.

 

[UcfSE]

I agree with 271828 and using point 3.

 

[haynewp]

Say if it is a braced frame or moment frame building with metal stud infill with brick (no shear walls either direction), would you then use only 1/2 the weight of ALL the walls from the ground to second floor since none of them are used as shearwalls, and are only adding mass that is going along for the ride?

 

[271828]

That's what I'd do.

I think about it like this:

Imagine you have a simply supported beam with both ends supported by vertical springs, uniform line mass, with goal being to calculate the natural frequency or period. Springs both end are same stiffness and the situation is symmetric.

One mode will be the entire beam displacing vertically in a rigid body manner. This natural frequency would be the same as if I had a 1-DOF spring and mass and used the mass equal to half the beam line mass.

This is the closest simple physical analogy I can think of, and I believe it to be a good one.

 

[JAE]

I've always used the upper half of the first level walls to lump the mass at the second level...assuming that the lower wall half seismic demand goes down into its supporting foundation. This is for just the design of the building's lateral force resisting system.

For listing the total base shear, and for checking the foundation for lateral resistance and stability, I use all the weight of the building since the foundation sees all that mass-acceleration.

I can't see including the lower half of the wall as though the seismic demand runs up-river and then back down into itself. That does'nt make any sense to me.

 

[haynewp]

Yeah seems a little weird. It Looks like Breyer is also using 1/2 of all the ground to second floor walls of this shear wall building example (I think it is masonry shear walls with weight summations starting on page 3.39), instead of using the full wall height for the parallel-to-seismic shear walls.

http://tinyurl.com/25p5gm

 

[msquared48]

I agree with JAE.

Mike McCann
McCann Engineering