ASCE 7-05 Load Combo 2.4.1 (6) 2

[jike]

Load Combo 6, does not make sense! It adds in LL but then uses E based on sustained load not necessarily the full LL.

Would it make more sense for Combo 5 and 8 to be based upon E1 (Eq based upon a portion of DL only which is sustained) and combo 6 based upon Eq using full DL+LL?

 

[ishvaaag]

5. D + H + F + (W or 0.7E)

In this you count on full wind or the probabilistic earthquake (once thought it is ASD). I find a bit rare that no part of the live load be present for an hypothesis like this.

6. D + H + F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or S or R)

Then when live load is present the former (in LC 5) wind or earthquake actions are reduced to a 75% of value.

8. 0.6D + 0.7E + H

Seems to be a case aimed at the case where sheer weight would be equilibratng tumbling action by the earthquake and so it is better only a quite conservative 60% of the assumed dead weight is present for the effect. This may well create tensile action at columns and requirements of anchors at foundations.

In all what I see more rare is that no part even small (sat 20% of live load) is present in LC 5.

Purportedly the set of load combinations and checks are run together for a target statistical reliability. I frankly can't believe this is the case when different institutions with different aims and agendas are doing their work, in terms of decisions, independently. It may be done but I don't think the coordination is as good. And when done in one of such setups, imagine, maybe, who knows, eurocodes ... for now the consensus even if according to some plan is taking quite a long time to come. Nor that I wish it comes soon ... ways of construction and awarenesses of circumnstances are affected not only by materials buy by local practices, some born out of very practical (and economical) reasons and maybe tight regulation may be one more for the worse that we may be already seeing.

In all, that I think that from engineering judgement these things are not as well founded as they should, at whichever its current state, and so I think many critics from many viewpoints may be perfectly reasonable, and arguing against such criticism quite to be promoting some technical faith. We have enough faith in technicalities already (should I add science?) to push just more.

 

[ishvaaag]

errata:

paragraph 7 (say 20% of live load)

paragraph 8 end of 3d line from bottom by materials, but

 

[JoshPlumSE]

Well, I'm not sure that they're really saying that the live load is going to be there at the same time as the earthquake load.

Rather, they're saying that a reasonable design will result when you combine your normal earthquake load with these other loads.

It is much simpler to use one EQ load for the various seismic load cases rather than calculating a different value for each one:
1.0DL + 0.7EQ
0.6DL + 0.7EQ
1.0DL + 0.7LL + 0.75*0.7EQ

Afterall, how "accurate" is the seismic load in the first place? Not very. We're really just trying to come up with a reasonably safe (and simple) procedure for designing for this extremely complex load effect.

 

[JAE]

jike,

I'm not sure what the problem is that you see.

All the values of E are always based upon the full DL plus any special live loads (such as 25% storage, etc. - see definition for W in section 12.7.2).

The code recognizes that a major seismic event statistically "won't" happen with full LL - thus the 0.75 factor on the LL and the seismic together.

Load combination 5 is for full DL with full seismic. The live loads many times are "fluid" in that they aren't fixed to the floors and thus actually serve to dampen the seismic demand to some extent. People loads also move around and don't add to the mass-accelleration on the building.

 

[jike]

Thanks Josh and JAE:

I think both of you sort of answered different parts of the question. Josh saying combos not necessarily reflect reality but it is a way to design reasonably safe structures. I am all for simple and not trying to make our design life harder than it actually is. JAE saying that we are designing for a lesser seismic event with full LL because it is highly unlikely that both full LL and full seismic event will occur at the same time.

Excellent insight!

 

[jenofstructures]

Im not really good in codes, but as i can see, some of these load combinations does not have live load because they assumed that there will be no live loads (no people around) during earthquake

but of course we know that earthquake happens unexpectedly
buildings will be operational with that time, thats why we assumed a 75% of live load

i think its alright that there is a 75% in earthquake since according to Section 9.5.2.7 of ASCE, E = ?Qe + 0.2 SdsD
since ? is sometimes 1.5,then there would be a bigger factor used in designing

in additional, we design buildings on almost equal to minimum rebars so that during failures, we can know already if the building is in failure because of this, cracking of concrete first will see before the building collapse

i wish i shared the right info on these

 

[ishvaaag]

I also wish to remark that even if not as clearly stated as by me in my post above, if this was a peer review of the load combinations by ASCE (or whatever) there is a general feeling that they are not by themselves entirely corresponding to some precise pre-qualified level of safety and reliability, but, contrarily, simplifications conducive to some approximation of the same, sometimes to be found in disagreement of actual customary use and other codes and practices, and hence, as a corollary, I would love this thing so interpreted anytime conflicts develop around a safety design issue. In all, I am saying that 5% or 10% above or under the safety stated by typified analyses in the codes may through engineering judgement be entirely justifiable, because the codes are not themselves precise models in safety and reliability.