Seismic loading IBC 1

[dwaine]

Using the Simplified method in the IBC 2000 code would the "Effective Seismic Weight" ( W ) be Actual dead load of roof + actual dead load of full wall supporting roof? Do you add live load? Factor load? 1.2 etc.?? Thanks

 

[JAE]

E is based on the dead loads (not live loads). You do not factor the dead loads but use the actual weights. This will give you the E effect to use in your load combinations listed at the begninning of Chapter 16.

The location of the seismic force is always concentric with the center of mass of whatever dead loads you have. So at each level, your dead load, and thus your seismic force, would be based on the center of mass of the floor dead load AND the adjoining walls attached to that level.

 

[AlohaBob]

Check also IBC 1616.4.1

Minimum wieights for partitions, storage loads, operating weight of permanent equipment, snow loads, should be factored into the design W which includes dead load.

 

[pcronin]

JAE

Based on your description and my previous experience, I have used the "effective" weight of an exterior wall for a one story structure to be half the total weight of the wall. Meaning that since the force acts on the centroid of the wall, half of that force goes directly to the foundation system and the other half goes directly to the roof diaphragm. Is this what you have done in the past?

This assumption appears to be in conflict with one of the design software packages I use. The software now has a toggle to allow the self weight of walls to be included in the design. The program adds the total weight of the wall to the building weight multiplied by the seismic coefficient to determine the base shear. I feel this is overestimating the "effective" weight and being overly conservative with the design. Do you agree?

 

[JAE]

pcronin -

your first paragraph - yes, for a wall, the half story below, and the half story above (if there is one) get included in the mass of the floor...this is called a lumped mass system because you are lumping all the mass at one level, even though some of it occurs below and/or above the level...but the center of mass is still carefully considered.

Second parapgraph - perhaps the software is using the half below ....and .... half above concept I stated above...

For base shear, you have to include all the mass to get the correct base shear, the whole wall, the whole roof and all the floors. This gives you the total shear at the bottom of the building.

For analysis of the deflections, stresses, forces, etc. that follow the various load paths through the building, you would lump the masses as I mentioned above, and the lower half of the lowest level wall would be simply taken down through itself into the supporting foundation and not directed up to the floor/roof above and then back down.

 

[pcronin]

JAE,

So for a one story building, only half of the weight of the wall will make it into the roof diaphragm and back through the lateral system in the building. This is inconsist with a common design software package, which for the case I just reviewed (a warehouse with 12" CMU walls), caused the lateral load to be overestimated by over 60%.

Everyone should keep in mind that software packages are only a tool, and their results should be verified by hand calcs. For this case, the software was overly conservative, which of course will not cause failure, but costs your clients excess money by overdesigning the lateral system. Keep in mind that software packages may also underestimate the load and which can lead to overstressed conditions.

I will contact the software company to bring this to their attention for review. Thanks JAE.