Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
(OP)
Load Combination 5 and 7:
1.2D+1.0E+L+0.2S
0.9D+1.0E
where D is defined as simply "dead load" and E = earthquake load, calculated as following:
E = Eh +- Ev = pQE +- 0.2SDSD
where QE is the effect of horizontal seismic effect, and 0.2SDSD is the vertical seismic effect.
Now, suppose designing a moment frame that is for lateral load only, i.e. it is not taking any dead/live load except its self weight, but it is taking seismic lateral load due to building's dead load. What "D" should be used for the 0.2SDSD? Do we use the building's D? Or, just the moment frame's self weight D?
1.2D+1.0E+L+0.2S
0.9D+1.0E
where D is defined as simply "dead load" and E = earthquake load, calculated as following:
E = Eh +- Ev = pQE +- 0.2SDSD
where QE is the effect of horizontal seismic effect, and 0.2SDSD is the vertical seismic effect.
Now, suppose designing a moment frame that is for lateral load only, i.e. it is not taking any dead/live load except its self weight, but it is taking seismic lateral load due to building's dead load. What "D" should be used for the 0.2SDSD? Do we use the building's D? Or, just the moment frame's self weight D?






RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
1.2D1+1.0(pQE + 0.2SDSD2)+L+0.2S
where D1 is the moment frame's self weight dead load; and D2 include the frame's self weight plus the part of the building's dead load that the frame is taking lateral load from.
Interesting.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
Just like you would use the seismic load (from the attached tributary dead loads) that are applied to the frame.
I guess I'm not sure why the question at all?
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RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
In ASCE 7-10 12.4.2.3, they seem to say that the D should be the same D:
"...the following seismic load combination...shall be used in lieu of the seismic load combination in Section 2.3.2 or 2.4.1"
5. (1.2 + 0.2SDS)D + pQE + L + 0.2S
..."
Here, you see they are suggesting the D being the same for 1.2D and 0.2SDSD.
But we just discussed a case where the D can be different.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
There is no difference between them.
One is a gravity effect and one is the vertical effect of the seismic event on that dead load.
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RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
I've always understood that the 0.2SDS factor on D was intended to deal with the vertical accelerations of the seismic event and how they add to or subtract from the static dead load D.
In your sketch, you are attempting to take a 0.2SDS amount of dead load and create a lateral load effect on the frame.
Isn't it correct that the typical rho x QE seismic demand value, along with the direct design method, already takes notional load effects into account?
Now if you had some purposely angled columns in the framework then I could see the vertical seismic effect on D creating additional lateral effects on the frame via 0.2SDS(D).
Your sketch is dealing with someone modeling the frame as a 2D entity and then attempting to manually bring in loads to it to account for the combinations.
If you had a full 3D model of the entire building, you would simply apply the loads to the model, apply the combinations (1.2D+1.0E+L+0.2S or 0.9D+1.0E) with E = Eh +- Ev = pQE +- 0.2SDS(D), and get your results.
There would be no need to add fictitious rigid members with phi angles.
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RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
Me neither. I welcome the debate.
In my mind it does. The whole building tributary vertical seismic load (WBTVSL) generates a lateral load on any out of plumb columns that rely on the moment frames for lateral stability. The WBTVSL also adds to the P-delta burden on those same columns. If this effect is accounted for elsewhere in the process, then that's news to me. I feel that it should be accounted for somewhere, however.
One interesting feature is that the vertical seismic loads are transitory, cyclical, and short lived. Perhaps they could disappear before P-delta buckling gets rolling? But then who knows how fast a thing buckles?
It definitely takes notional loads into effect. But notional loads only deal with the destabilizing effect of the 1/500 out of plumb imperfection. They don't include the lateral loads that result when the WBTVSL acts on those out of plumb columns. They also don't capture the contribution of the WBTVSL to the system's P-delta burden.
Your approach, like mine, already includes the destabilizing effect of the WBTVSL on the P-delta burden.
[1.2D + 1.0E] = [1.2D + 1.0Eh + 1.0Ev] = [1.2D + 1.0Eh + 1.0 x 0.2SDS x D] = [(1+0.2SDS)D + 1.0Eh].
As your 3D model moved laterally, [0.2 x SDS x DL_[whole building]] would be contributing to P-delta instability I think. The difference with my approach is that I would add (0.002 WBTVSL) to the notional loads.
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RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
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RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
The notional loads applied to the frame will have a dead load component. And that component will, in general, include contributions from dead loads that are not directly tributary to the frame in a gravity sense.
In a second order P-delta analysis (DAM), dead loads not directly tributary to the frame in a gravity sense will generate lateral dead loads on the frame as those non-tributary dead loads shift laterally in space.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
(1.2 + 0.2SDS)D + ρE + L + 0.2S
(0.9 - 0.2SDS)D + ρE
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
for the 1.2D, the D is the dead load the member is supporting.
for the 0.2SDSD, the D is the dead load that's causing the seismic lateral load.
They DO NOT have to be the same D. If they are not the same D, (1.2 + 0.2SDS)D + ρE + L + 0.2S does not apply.
For a simple example:
The floor dead load are supported by Beam A and B, not by 1 and 2.
Designing Beam 1 and 2, the 1.2D is only their self weight. But the 0.2SDSD is from half of the floor dead load plus the beam self weight.
(1.2 + 0.2SDS)D + ρE + L + 0.2S in this case I think do not apply.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
For vertical load applied to any member, the D used for [0.2 x SDS x D] is the same D as that used for regular gravity design.
It is only the lateral and stability loads that need to be based on the larger D that generates the seismic load. At least that's my take on it.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
Vertical seismic forces are proportional to the dead loads on each members. The vertical seismic force is applied to all framing not just the lateral framing.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
The "D" in question is the gravity dead load directly tributary to the member being designed for the load effects resulting from the load combinations. It is the SAME D between the 1.2D and the 0.2SdsD. NOT DIFFERENT. The thought that you must carry a different "D" through the load combinations for the vertical seismic effect is ludicrous. It is a member force check, so certainly the forces on the columns should be based on the tributary areas to those columns. The "D" should be collected to the members (beams, columns, etc. separately) and then multiplied by 0.2Sds for that particular member to get the vertical seismic effect on that particular member in the load combination.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
Conceptually, we do not disagree. It is semantics. When discussing horizontal seismic loads, I think it is confusing (and technically wrong) to call the effective seismic weight a dead load. Perhaps, I am being pedantic.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
I think that I see the source of the confusion now. At no point in any of my comments above was I discussing seismic loads resulting from horizontal accelerations. Rather, I was referring to the vertical and horizontal loads resulting from vertical accelerations. The code defines seismic loads due to vertical accelerations in terms of dead load and I followed suit. Why there's not some "other stuff" in the effective vertical seismic mass, I can't say.
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RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
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RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
Under vertical acceleration, all of your columns would, on the up cycle, have extra compression in them related to vertical seismic acceleration, right? Now tilt those same columns by 0.2% and you've got a lateral load component that compression.
In my opinion, there are two aspects to this:
1) The lateral loads mentioned above become part of the notional load demand.
2) The vertical loads generated by vertical seismic acceleration add to the P-delta demand and affect the lateral load design of vertical bracing elements that way.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
To quote one of the most brilliant structural minds of our time...
No, it would never be more than 1%.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
EIT
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RE: Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.