IBC / ASCE Seismic Load Effect "E" Question

[StructuralEd]

Understanding that the intention to combine the "horizontal" and "vertical" effects in the formulas ( E = pQe + .2SdsD) in each reference to attain the seismic "E" value to use in the load combinations, is it not redundant adding the .2Sdsx(D) back in when the Qe is the horizontal value, assumed to equal V, which based on the Dead Load in the first place?

 

[SteelPE]

I have always interpreted the additional .2SdsD as an increase or decrease in the dead load to account for the motion that might be up and down. On one and two story structures this load might be small, but if you were looking at a 10+ story building the load may become significant.

 

[JAE]

The second term (.25Sds) is an effect on the structure due to VERTICAL seismic motions.
The first term is the HORIZONTAL.

So they are not redundant in that they are forces applied in different directions at the same time and in the same load combination.

 

[StructuralEd]

It's peculiar syntax to mix directions within the same formula.

 

[Hokie93]

The equation as you typed it may be a bit peculiar but if you look at the equations in ASCE 7-05 section 12.4.2.3, it makes complete sense, at least to me. That is, 0.2S_DS is added to the dead load (i.e., it is a vertical load) and this load is acting along with the other loads, including horizontal seismic force effects, in a load combination. Note the 0.2 factor is different for the ASD load combinations.

 

[StructuralEd]

Yes, I see that.
It seemed like it could be a perpetual spiral...adding more dead load could add more lateral load being based on the dead load and so on.
Reading too far into it perhaps. Should be taken exactly as the formula is written.

Thanks.

 

[JAE]

The seismic horizontal load is based on the mass of the building.
That horizontal load is derived completely separate from the vertical seismic load.
They are then combined in the load combination. I guess I don't see how they could perpetually spiral.

One way to think about the combinations is not as an "formula" as you put it - but rather as a sum of effects on the structure.