Joist Girder Reactions 1

[jberg]

With the 2005 AISC specification recommending using the Direct Analysis Method for steel frame design, I was wondering how other engineers are specifying end reactions on joist girders that are part of a moment frame for lateral force resisting system.

I have seen load diagrams with the end reaction moment(s) shown for each load case (Live, Wind, Seismic, etc.), but with a 2nd order analysis, I do not believe it is possible to just post the load case results and have the joist designer combine them per ASCE 7, or the applicable design code(s).

Any comments or suggestions would be greatly appreciated.

Thanks in advance,

JWB

 

[JoshPlumSE]

The the Joist girder is a simply supported beam, then it should not particpate in the lateral force resisting system and can be replaced in the analysis with the equivalent load.

That means you are neglecting any stiffening / bracing effects that these members impart on the membrs they frame into... at least you're neglecting them from the analysis. That should be conservative.

In your case, however, it sounds like these members are part of a moment frame and would participate in the lateral force resisting system. I would think this would have to be modeled in the lateral analysis... if only with a member with "equivalent" properties.

 

[JAE]

I think the moments that you indicate are for the purpose of defining to the joist manufacturer the maximum strength required. As long as your indicated end moments do this then you have ensured adequate strength correct?

 

[jberg]

JAE,

You are correct, it is for defining the requirements to the joist manufacturer. What I am struggling with how to present this and accurately capture the required capacity. One could list the moment reactions from analysis, broken down to ASCE 7, eqn. 1, 2, 3, etc. (for example). I just was not sure if this is what other engineers are doing.

JWB

 

[JoshPlumSE]

JWB -

You're correct, the non-linear nature of the analysis will means that a pure super-position of individual load reactions (DL, LL, WL) will not give the same value as the final ASCE load combination.

Not sure how other engineers are handling that. To me it probably makes the most sense to still list individual reactions. But, then you might supplement that with a "Maximum Total Reaction" from your controlling load combination.

 

[JAE]

One thought would be to list the individual end reactions for all the load cases (DL, RLL, SL, WL, etc.) and then bump up the WL or E case to get you to the maximum end reaction in the combinations. So your plan notes might look like this:

Joist Girder JG1 end reactions
DL - 10.2 kips / 0 ft-kips
RLL - 10.2 kips / 59.5 ft-kips
SL - 15.5 kips / 90.5 ft-kips
WL - 16 kips / +/- 86 ft-kips

Note: WL values include additional magnitudes due to second order effects.