Load Combinations When Using 25% Storage Load For Seismic

[P1ENG]

ASCE 7-10, section 12.7 says to use 25% of the live load for live loads that are considered storage. My question is what happens to the load combinations (ASD for the sake of this conversation)? I have to have live load in order for 25% of that load to be considered seismic mass, so D+0.7E no longer applies unless it were (D+0.25L)+0.7E -OR- (D+L)+0.7E. In order to know which of the latter (2), I have to know if the requirement of 12.7 considers only 25% of the live load to be present and 100% of that load is seismic [(D+0.25L)+0.7E] -OR- if 100% of the live load is present but only 25% of that load is seismic [(D+L)+0.7E].

Then what do I do with D+0.75(L+0.7E)? This load combination already has a live load component. I would tend to think in either condition (100% of 25% -OR- 25% of %100) I wouldn't modify this one.

This matters to me because I have relatively light construction (mezzanine) that is a cantilever column system. I have large moments at the base but not a lot of weight if I only use actual dead loads in the load combinations. The extra gravity load by including all/some of the live load would greatly help with footing sizes due to overturning safety factor.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[Celt83]

Moving (i.e. sliding on the floor) storage items would not be producing the inertial forces and would be like any other live load...

We are in agreement on this point. My thought process on the timetable of a design seismic event is that at t=0 that storage live load is static and "tightly" packed and at any time > period/2 all of that storage live load are now fluid particles that have individual accelerations in various directions that do not necessarily align with the acceleration vector of the building structure.

This is a good discussion and I can see your point and logic behind your proposal, I just have a feeling that your proposed load combination will fall somewhere in between the code load combos ie. the 0.6D+0.7L will produce a higher overturning ratio than your combo....but maybe your (D+.25L,storage) + .75(L,storage+0.7E) will produce a more conservative design condition for main seismic force resisting members.



Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[bones206]

Interesting discussion. Does anyone know the basis of the 25% storage live load rule? Based on empirical studies or just a rule of thumb passed down through the ages?

 

[P1ENG]

bones,
The ASCE commentary mentions very little about the 25% other than it is to provide inertial forces of "tightly packed" stored items.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[bones206]

Here's a video of a shake table test on a storage rack. If you watch in slow motion around the 20 sec mark, you can see where the heavy pallets start bouncing and create a downward acceleration greater than gravity. Along the lines of what Celt83 said, including this downward acceleration of the live load as part of the "vertical seismic effective weight", would likely be conservative for member design. But I wouldn't include this effect in any code-mandated load combination where it would reduce the conservatism of that particular combination.