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

 

[sandman21]

12.7 is for the effective seismic weight. You would need to add 25% of the storage LL to the seismic weight on the system.

 

[MotorCity]

When "L" is a storage load.......

Anywhere "L" appears in a load combination w/ "E", use 100% of "L".

Anywhere "D" appears in a load combination w/ "E", use "D + 0.25L"

 

[JAE]

So just to repeat sandman's statement in other words:

You calculate E based on all the dead load and 25% of the live load.
Then you apply your dead, live, wind, seismic loads as they are to the load combinations.

The 25% is NOT saying that: [blue]"when you have a code seismic event, only 25% of the live load is present".[/blue]
It IS saying: [blue]"when you have a code seismic event, including 25% of the live load in the effective seismic mass is an means of estimating the contribution of some of the present live load to the seismic demand."[/blue]





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[P1ENG]

sandman:
I don't know if you understood my question. If I had 1 kip dead and 4 kip live in a column and laterally had 0.1 kip from dead and 0.1 kip from 25% live then per a D+0.7E load combination my column would be 1 kip axial and 0.14 kip lateral. How can I get the extra 0.07 kip lateral from live load in a load combination that only contains dead load? I think, and Motorcity confirmed, the column should have 2 kip (1+0.25*4) axial load and 0.14 kip lateral (0.07[D]+0.07[0.25L]). I simplify, but I would also be considering the vertical components of seismic.

MotorCity:
That was the way I was leaning. Thanks for the confirmation. Have you got published guidance or is that your standard practice? Please rethink D+0.75L+0.525E though. Per what you said, that would become D+L+0.525E. I would think that the 0.75L of that load combination is inclusive of the 0.25L that acts as the seismic mass, so there is no need for that load combination to change.

JAE:
See my response to sandman. If you include 25% of the live load in the lateral/vertical seismic forces, then you have to include that same 25% in the load combinations you are using. D+0.7E becomes (D+0.25L)+0.7E. In my query, it does matter if some or all of the live load has to be present to get the 25% effective seismic weight.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[MotorCity]

I did not mean to imply that you replace "0.75L" w/ "L". I simply meant that you put the full, unreduced live load into the load combination. So if L = 100 psf, 0.75L = 75 psf. The 0.25L only affects the seismic weight. It has no effect on the actual magnitude of the live load or whether or not the live load is present.

 

[JAE]

If you include 25% of the live load in the lateral/vertical seismic forces, then you have to include that same 25% in the load combinations you are using.

No you don't.

Section 12.7 says ONLY - that you have to include 25% of the live load in the DERIVATION of W for determining your seismic demand, E.
This does not affect your live load at all - nor does it affect your inclusion of the full live load, or 0.75L, etc. in your combinations.
It ONLY affects how you get your E to use in the combinations.


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[P1ENG]

JAE:
I'm sorry, but I [somehow by conveying it tactfully on a forum] reject that. If you assume a portion of live load is present as a mass, then that same portion of mass is present for both lateral and gravity. It doesn't become weightless. The 25% live load mass has acceleration laterally and vertically from the seismic event; that mass will also have acceleration of gravity acting on it.

Motorcity:
I did not replace 0.75L with L. I did (D+0.25L) + 0.75L + 0.525E = D + L + 0.525E.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[MotorCity]

I understand the confusion, the requirement seems contradictory that you can have a part of the live load included in the seismic weight calculation "D + 0.25L" even though "L" itself may not actually appear anywhere in the load combination. However, realize that there may be more than one live load....a storage live load and a people live load. I think the rationale is that the odds of a storage live load being present during a seismic event is more likely that a people live load being present during a seismic event.

 

[JAE]

Keep in mind that the inclusion of 25% of the live load - as I stated above - is an estimate of the seismic contribution of some or all the live load present.

You could have 100% of your live load present during a code seismic event, but only 25% of it contributing to our code-estimated-static-equivalent seismic demand.
The other 75% could be jostled around, not stiffly secured to the floor, etc. and not contribute much of anything to the lateral demand.

Example - a floor with shelving and lots of weight - the floor shaking would cause the shelving to sway within the building - possibly in opposite directions/rhythm to the floor movement and actually reduce the seismic demand a bit. So correlating what live load I put into my seismic W has nothing to do with the live load I include in my various combinations.

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[P1ENG]

Motorcity:
Right. So I am assuming people load is not present when altering my load combinations. The 25% of live load is assumed connected to the structure by inertial affects or attachment. If you have to have 100% of the live load to obtain the 25% inertial loads assumed by the effective weight, then you should have to include that 100% of live load in your gravity components of the load combinations. If you only have to have 25% of the live load to obtain the 25% inertial loads assumed by effective weight, then you should have to include that 25% of live load in your gravity components of the load combinations.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[P1ENG]

JAE:
Except that by your admission, you have a minimum of 25% of the live load present (i.e. the condition where 100% of the live load is seismic mass) or a maximum of 100% of the live load present (i.e. the condition where 25% of the live load is seismic mass). You have an upper and lower bound of gravity influenced mass contributing to the seismic forces. I still maintain that mass cannot be ignored in the load combinations.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[JAE]

If you had a huge steel ball hanging from a chain from your roof - a pendulum - it would contribute very very little to seismic mass.

If you have to have 100% of the live load to obtain the 25% inertial loads assumed by the effective weight, then you should have to include that 100% of live load in your gravity components of the load combinations.

I'd agree with that.

So what I think you are suggesting is that:
With load combinations that include seismic, E, and Live Load, L, my E would include 25% of the live load and the L would include 100% of the live load.
With load combinations without Live Load, L, my E would be based only on the DL in deriving W, and not include the 25% Live Load?

Is that what you are saying?
I might agree with that in logical terms but need to think about it.
I'm not sure the technical-logical flow of the ASCE 7 text suggests this, though.



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[Celt83]

P1Eng it sounds like you may be mixing sections.

for derivation of E:
v = Cs W

W should be all structure dead load (D) + 25% of of storage live load x storage floor area + partition weight or 10 psf + weight of permanent equipment + 20% of snow load where Pf exceeds 30 psf.

when checking a member:
D+E combo should only include the applied dead load and seismic load (storage live load component as well as partitions and excess snow are baked in)


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[P1ENG]

JAE:
I am suggesting the following load combinations to be used with storage live loads:

E=seismic forces inclusive of effects of 25% of storage live loads
Ed=seismic forces only of dead load (25% of storage live loads ignored)
[ol 1]
[li]D+0.7Ed[/li]
[li](D+0.25L)+0.7E <--- 25% live load present to produce 25% effective seismic mass[/li]
[li](D+L)+0.7E <--- 100% live load present to produce 25% effective seismic mass[/li]
[li]D+0.75(L+0.7E)[/li]
[li]0.6(D+0.25L)+0.7E <--- 25% live load present to produce 25% effective seismic mass[/li]
[li]0.6(D+L)+0.7E <--- 100% live load present to produce 25% effective seismic mass[/li]
[/ol]

I don't think 100% of the seismic load must be present in order to get the 25% effective seismic weight [else we would have a standard load combination of D+L+0.525E instead of D+0.75L+0.525E]. I think more realistically 75% of the live load would have to be present to obtain 25% effective seismic weight which corresponds to the already defined live load of D+0.75(L+0.7E). Obviously the upper/lower bounds of the live load required to obtain the 25% contribution would affect the above load combinations.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[P1ENG]

Celt:

when checking a member:
D+E combo should only include the applied dead load and seismic load (storage live load component as well as partitions and excess snow are baked in)

Baked in to what? E? D? or both? I am trying to argue that W should at least replace D in the load combinations.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[MotorCity]

On behalf of structural engineers everywhere......please, no more code mandated load combinations

 

[Celt83]

P1ENG:

I think that goes directly against what AISC instructs you to do for derivation of the seismic weight.

Open Source Structural Applications:

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

 

[P1ENG]

Motorcity:
I agree! I don't know that I want these mandated, but applicable via engineering judgment. I want the extra weight. How much extra weight I can legitimately use depends on how the 25% of live load effective mass is figured. That is what I am trying to find out here. My arguments I feel are valid. If you are going to say some mass is present in the seismic force calculation, whether it helps you or hurts you, that same mass that affects the seismic is also there having gravity act on it.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[Celt83]

Celt:
Quote (Celt83)
when checking a member:
D+E combo should only include the applied dead load and seismic load (storage live load component as well as partitions and excess snow are baked in)
Baked in to what? E? D? or both? I am trying to argue that W should at least replace D in the load combinations.

It's baked into E
My understanding is its trying to capture static masses so partitions, permanetly attached equipment, a portion of the storage room live load.

I can see the rational on the portion of storage room live load being that likely the items being stored will shift as the building moves to and fro, or the probability that the 125 psf applies everywhere as there is likely access walkways, etc.

Open Source Structural Applications:

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