Confusion Regarding the Dead Load in ASCE Load Combination 5 and 7

[azureblue83]

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 = E[sub]h[/sub] +- E[sub]v[/sub] = pQ[sub]E[/sub] +- 0.2S[sub]DS[/sub]D
where Q[sub]E[/sub] is the effect of horizontal seismic effect, and 0.2S[sub]DS[/sub]D 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 0.2S[sub]DS[/sub]D? Do we use the building's D? Or, just the moment frame's self weight D?

 

[KootK]

@WannabeSE: do we disagree on some aspect of this? If so, can you elaborate? It's not clear to me as nothing in your last post seems to contradict any of my stuff as far as I can tell.



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.

 

[WillisV]

Let me clarify my previous statement: "D in all portions of all load combinations is the dead load directly tributary to the frame."

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.

 

[wannabeSE]

KootK
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.

 

[KootK]

This is as good a place as any for some pedantry.

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.

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.

 

[wannabeSE]

Now, I am really missing something. I can understand a horizontal forces in response to a vertical load. But, how do you get a horizontal load from a vertical acceleration?

 

[JAE]

With a leaning column - a sort of PDelta effect from the vertical movement creates a horizontal reaction.

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[KootK]

Now, I am really missing something. I can understand a horizontal forces in response to a vertical load. But, how do you get a horizontal load from a vertical acceleration?

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.

 

[RobertHale]

KootK, your interpretation is what I was trying to say. In my mind the additional vertical load from seismic accelerations has to be applied to the vertical imperfections captured by the "notional loads" or displacements. The vertical force is real, an approximation grant you, but a real force none the same. The initial structural imperfection is real (although the imperfection we model has a very low probability in my opinion). I see no way to negate or neglect this force.

 

[wannabeSE]

Is the seismic contribution to notional loads ever significant when compared to the seismic lateral loads? How often does .002(0.2S[sub]DS[/sub] D) exceed 1% of the seismic lateral load?

 

[KootK]

Is the seismic contribution to notional loads ever significant when compared to the seismic lateral loads?

To quote one of the most brilliant structural minds of our time...

It's also worth noting that 0.002 x 0.2 x SDS x D is about equivalent the wind pressure generated by the flap of a butterfly wing.

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.

 

[RFreund]

So is this agreed upon then? It appears Kootk's sketch on 22 Jul 15 21:03 follows what was done in an AISC seminar on Stability "Stability Design of Low- and Medium-Rise Steel Buildings (By Don White). Atleast I think it is similar, I haven't had time to really go through it.

EIT

http://www.HowToEngineer.com

 

[KootK]

I'd say so with the understanding that the "vertical seismic notional load" component is meaningless numerically and quite possibly not the intent of ASCE-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.