Seismic: Vertical Distribution of Load

[ichibansan]

Equivalent Lateral Force Procedure...

Seismic Base Shear V= Cs x W

Where:

Cs = variable depending on accelerations,response modification, etc.

W = the TOTAL dead load (plus applicable portions of other loads)

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Fx = Cvx * V

Fx = Horizontal seismic force induced at any given level above ground

Cvx = Variable dependent on a story's weight and height above ground

V = Seismic Base shear, described above

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With all that said, knowing V is the shear at the base of the structure due to the TOTAL "weight" of the structure, this means that the lowest half story of structure weight (that portion of the structure from ground level halfway up to first elevated diaphragm) is included in V, correct?

If that's the case, isn't this mass getting dumped directly into the ground level? Why would this portion of structural weight be getting distributed to elevated diaphragms?

What am I missing here?

 

[JStephen]

I'm not sure I fully understand your question. But maybe this will help.

Consider a simply supported beam with uniform loading. The reaction at each end is 1/2 of the total load. However, the reaction on one end is NOT due just to the load on that end. If you remove the load from one half of the beam, the reaction on that end doesn't disappear.

It sounds like a similar issue with the force distributed to the mass in your case- you are assuming that the bottom half of the mass ought not to have any effect at the top- which is not the case.

 

[whyun]

ichibansan,

As far as I know, the seismic load from the lowest half is NOT a part of W. This load is tranferred directly to the base.

 

[SacreBleu]

whyun,
That is the way have always calculated it. However, when I looking at a CMU or concrete shearwall building, I add-in an extra seismic load due to the self-weight of the entire shearwall, including its lower half. This load was not included in seismic vertical distribution. It is added-in while calculating the force for the design of the shearwall.

 

[whyun]

SacreBleu,
You can put entire wall seismic force at center of wall (midheight) or upper half seismic force at top of wall. End result is the same. What do you think?

 

[haynewp]

The force from the lower half of the wall that goes directly into the ground should not be getting distributed into upper levels.

For the design of the shearwalls and foundation, it should of course be included.

 

[SacreBleu]

whyun,
The seismic force generated by the vertical shear distribution was already was applied to the top of wall.

If I am adding-in the seismic force generated by the lower half of the wall, I suppose it would be sufficient to apply that load at mid-height.