Minimum Horizontal Load for AISC LRFD design 1

[pba]

I'm setting out a basis of design document, without access to all the necessary codes.

We are to design a steel framed building which will be subjected to dead, live and wind loading. I'm not familiar with AISC LRFD design processes. Under British Steel design codes there is a Notional Horizontal Force, defined as 0.5% of the factored dead and live load. This force is applied in load combination where wind forces are not present.

The question is - Is there a similar requirement under AISC LRFD?

 

[scottiesei]

Where is your project? If ASCE 7 governs, then they have a minimum wind load to use. If a member is not exposed to wind, then the building code (IBC or whatever) has a minimum requirement (I think it's like 10psf or something). I am not aware of loading requirements in the AISC, just the design specs. Good luck!

 

[271828]

pba, it depends on which AISC LRFD Spec. you're using. The 13th Ed. Spec. is both ASD and LRFD. It allows you to use the Effective Length Method (Ch. C) or the Direct Analysis Method (Appendix 7).

The DAM requires either a notional load or direct modeling of out of plumbness and out of straightness.

The ELM sometimes requires a notional load and there is some confusion as to when.

AISC is putting out a new Stability Design Guide that covers this ad nauseam. I don't think it's out yet though, but it should be soon.

 

[WillisV]

pbs might be referring to the small notional "tie" forces that were put into effect in British codes following some progressive collapses in the many years ago.

And no, this is not explicitly covered within the AISC design specification. The closest thing that is currently in the U.S. codes would be within the seismic provisions (ASCE 7-05 Section 11.7.3 requires a minimum connection axial strength of 5% of the members weight).

Sooner then later provisions like this will be appearing as the fallout of the NIST report is dealt with in the codes.

 

[271828]

More:

The DAM notional load is defined in Appendix 7.3(2) as 0.2% of the factored gravity (vertical?!--what if you have wind uplift?!?!) load. This is based on an initial OOP of H/500. You guys in the UK must have a lot less confidence in your erectors, LOL.

You can also just push the initial geometry over by H/500 in your model. Some people are doing this instead of applying the notional load.

In some cases, you can get away with applying the notional loads only to gravity-only LCs.

The ELM confusion comes from C2.2a(3) which seems to clearly indicate that a notional load is only applied to gravity-only combinations. I know that a lot of people are doing this and it seems crystal clear to me. However, I was talking with someone "who would know" recently and they indicated that this is wrong and it's to be applied to all LC. I have seen the upcoming Stability DG, but I do not remember what they did with this. Their examples are 15-20 pages long, so I'm too lazy to dig through my copy to find the answer.

 

[271828]

Nah, Willis, I'd bet you five notional-dollars that he's typing about the stuff that I'm typing about, LOLOLOL.

This stuff has been common in Europe for a while, but AISC is just now catching on with Appendix 7.

 

[pba]

Folks,

Thank you one and all.

The purpose of the Notional Horizontal forces in the UK code are manyfold.

In this particular instance the NHF is described thus:

"To allow for the effects of practical imperfections such as lack of verticality, all structures should be capable of resisting notional horizontal forces, taken as a minimum of 0.5% of the factored dead and imposed load applied at the same level."

I will not quote further, but the NHF forces are applied together with factored vertical loads. So what used to be the pure gravity load condition now has a horizontal force component as well...

I've designed to both UK and US codes in the past, it never fails to amaze me that the end result is 99% the same.

Just to cloud things further the UK code uses the same 0.5% value for other purposes as well, including P-Delta checks, but that was not what I was after!

Tie forces do feature in the UK code but are substantially higher than the NHF described here - They are 50% of the factored floor load.

 

[271828]

What's the reasoning behind 0.5%? The AISC 0.2% is loosely based on the OOP tolerance. Surely European engineers don't expect the building to be initially OOP by H/200. That looks more like a drift limit than an initial OOP!

 

[pba]

Reasoning? Not sure there is any reasoning - This is a design code after all...

More seriously, it is difficult to compare the codes that closely.

UK uses 1.4D + 1.6L

US uses 1.2D + 1.6L + 0.5L on Roof

Actually the above is not strictly correct - That is the subject this thread:-

UK uses 1.4D + 1.6L + (0.007D +0.008L)acting horizontally...

 

[UcfSE]

So I suppose the short answer, and summary, is no AISC does not currently require that, but it is an option in the latest AISC spcification.

Interesting, I wonder where the 0.5% really comes from.

 

[J1D]

Canadian code uses 0.5% gravity load as the notional lateral load as well. It handles not only the geometric imperfection, also the residual stresses, and other things related.