Exhibition Hall Truss

[dcw815]

I'm doing some preliminary sizing of steel trusses for an exhibition hall. One set of them spans 150' and has 30' tributary width with 300psf live loading (yes it's big) and a 5.5" slab on deck. Unfortunately, for various reasons, our firm wasn't able to be responsible for layout so I can only have control over member sizes.

Any creative ideas for the top chord? The factored compression in the top chord works out to 4900kips so wide flanges are getting ridiculous and there arn't common tubes that can handle that. Just wanting to get a few ideas bouncing around. Thanks.

 

[paddingtongreen]

Composite action?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dik]

If you can't use composite action, then you may have to weld several HSS Tubes or even WF beams together, or go deeper... Can you use a storey deep truss?

You can use a welded plate girder... one of my current projects uses one 8' deep with 4" thick top and bottom flanges... just need to get 'big'... It's a consequence of having a silly design criteria...

Dik

 

[slickdeals]

Composite action like dh and dik alluded to. Alternatively, you can try a tri-chord truss.

What is your truss depth? Ideally you want to be around L/8 to L/12, but you are dealing with some high loads.

 

[tony1851]

Why such a high live load?

 

[ishvaaag]

With big spans and small slab thickness composite action might not work (something like the steel deck systems trying to contain main member forces we have been seeing being discussed in this forum these days, may not work).

I found some cases when establishing the moment strength of composite beams in which the limit value did NOT exceed the bare structural steel moment strength. This I then reasoned was because the slab was adding as little stiffness that trying to count on it to stiffen the "hard" beam under it was like trying to do so with a slab of butter. Hence the steel moment strength was showing as the limit as well for the composite action. That was using the LRFD 1992 code.

If such is the case, to minimize steel aspect you may use first box chords or void chords with hidden steel at some material cost.

Of course when minimizing size of the chords their slenderness grows and bracing becomes very paramount; the secondary members plus slab system needs be able to provide the required bracing. That system is analyzable for the bracing forces plus those of the secondary system, or just making the complete 3D model showing the initial imperfections etc ... after direct analysis only what not properly addressed in the model needs be looked at from a bracing viewpoint, since all bracing forces will be already extant for what properly addressed in the model.

 

[dik]

or in lieu of a welded plate girder, a large welded wide flange (custom) for the chord members.

Dik

 

[msquared48]

Can you post-tension the bottom chord?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Archie264]

I know of a case where the entire truss and column supports were post tensioned. While it was for a basketball arena it didn't have loads that big. For what that's worth...

 

[SteelPE]

So it looks like your truss is approx 7 feet deep spanning 150' with a 4,900 kip top chord compressive load. Is this correct?

 

[slickdeals]

That's a shallow truss for the spans. Does it even work for deflections?

 

[dcw815]

The truss is 10' deep so L/15, not the best. The floor above is supposed to take full vehicle loads for car shows, concerts, semi's unloading equipment, etc. so they asked for 300psf. I like the composite action idea although the slab seems so small relative to the truss as ishvaag said. Post-tensioning might be a good idea. Thanks for all the help!

 

[dcw815]

Detailed design won't be for a while so it will be good to bring these ideas to the table.

 

[JedClampett]

Well, one good thing is if it can be designed, you'll make the AISC magazine.
This is one of those cases where you need the AISC Certified Erector and Fabricator.

 

[MiketheEngineer]

Maybe double trusses side by side or cut spacing in half (15') and install girders between columns??

 

[RareBugTX]

5.5" Slab thickness sounds way too skinny You would not be able to get any steel at all! What kind of load is that 300 psf?, Can it be shared by two trusses?

 

[RareBugTX]

Could you provide a sketch plan with loading,spans and spacing? I was just thinking for top chord compression members (near supports,4900 k) you could do built-up composite and then transition towards midspan with W's for both top, bottoms and diagonals.

 

[JAE]

I would not put semi-loading on a 5.5" thick slab on metal deck. You'd have serious long term punching shear issues - fatigue cracking in the concrete, etc.

 

[Michel60]

Good morning dcw,

Consider using jumbo W14 sections and grd. 65 steel as chords. Positioning the chords in a minor axis position simplifies connection design if you use W14 sections for the web members too.

Also is you proposed deck 5.5" fill above the deck or 5.5" overall? I agree with the others that 5.5" overall sounds a little too thin for the proposed loading and use.

regards,
Michel

 

[IsaacStructural]

How about massive tube sections? Valmont Industries makes some superstruct tubes that can go to just about any dimension you like. You could place two large rectangular tubes, say maybe 30x16x1 side by side and weld together.

Like Jed said, do something like this and you are SURE to get into one of the magazines!

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)