Seismic Stability Coefficient- Total Vertical Design Load

[frv]

In reference to ASCE 7-05 section 12.8.7, I've found this thread related to my question, but I'd like some clarification: thread507-298607

I've seen a few example floating around that exclude roof live load snow load does not control; is this correct? The definition reads "total vertical design load". This is not the effective seismic weight. Shouldn't the total vertical design load for purposes of this section be either all of the load at each level above the story under consideration with a 1.0 load factor or, at the very least, the ASD gravity load combinations (in which case the gravity load would be either D+L or D + 0.75(L + Lr))

 

[frv]

Sorry.. I meant that to read.. "..exclude roof love load when snow load does not control.."

 

[JoshPlumSE]

I believe this section is intended more to ensure post seismic event stabilty of the structure. And, that's why it's using the lower loading.

Essentially, it wants to make sure that the residual deflection that remains after the seismic event is not so severe that it would cause the structure to collapse.

Though it wouldn't hurt to take a look at the FEMA / NEHRP documents (or the old SEAoC Blue Book) to see if they have a more definitive explanation.

 

[frv]

Thanks, Josh.

The section is attempting to quantify P-Delta effects and whether or not they can be ignored in the analysis. Theta is the ratio of secondary moments to primary moments. The code assumes if this ratio is less than 0.10, P-Delta effects may be neglected. Obviously, including the roof live load would be more conservative. In reality, the numbers I've run indicate that it makes little difference whether or not you include the roof live load.

The problem is that I'm going to sit for the 16 hour exam this October and you don't get credit for conservatism. This seems exactly like the type of problem where they would include one answer including the roof live load and one ignoring it. There is only one correct answer, so I'm trying to figure that out.

 

[JoshPlumSE]

Sorry, the limit on theta < 0.25 is the portion of the provisions that is intended to protect against post-event stability failures. That's what I was focusing on for some reason. I must need caffeine!

To me, the basis for the Px value would be the axial force for the LC that you are considering.... Where you can use a 1.0 factor when it is exceeded in your load combination.

Example: Your actual load Combinations may be:
1.2DL + 1.0EL + 0.2SL

Personally, I would use the axial force from that load to determine my Theta. However, the code would allow you to use the following for the calculation of Px:

1.0 DL + 1.0EL + 0.2SL

I don't see any Load Combinations which combine Roof Live Load and Seismic Load. So, I would not use the other (non seismic) ASD load combos.

As you point out, the code only says that Px is the "total vertical design load at and above level x". So, you could make an argument that you might have to use the axial force from the other (non-seismic) ASD Load Combos as well. But, this is the seismic chapter and using those other LC's seems odd.