All, Looking for others opinion

[Structural Eng 7]

All,

Looking for others opinions/solutions for an issue that has come through my office. We are designing 200’ long trusses with WF top and bottom chords. The issue coming up is the lateral stability of the top chord since the joists supported by the top chord bear on the top flange. Typically for a beam one would assume that the joists laterally stabilize the beam (comp flange) however in this case the entire section is under compression. Would one think that additional framing or deeper wide flange would be needed to stabilize the whole section? We are thinking of adding beams at each panel point and then providing a horizontal truss frame to provide stability as opposed to utilizing the metal deck as this will help make erecting simpler and safer. Has anyone ran into this type of situation before?

Regards,

 

[hokie66]

The horizontal truss, with struts bracing the top chord at intervals, mid-depth of the wide flange, would be my choice.

Another way of dealing with lateral stability of the top chord is to use a wide flange section turned the other way, web horizontal. But that works better when supporting another type secondary framing other that bar joists, say a cold formed zed system.

 

[MIStructE_IRE]

I agree. I don’t believe that the top chord in your sketch is adequately laterally restrained and would also go for the horizontal bracing option as suggested above.

Don’t forget wind uplift/stress reversal - so you may need something similar on the bottom chord also

 

[MIStructE_IRE]

....when I say wind uplift..I just assumed that this is a roof! Perhaps its not?

 

[Structural Eng 7]

Gents, thanks for the replies.

Yes this is a roof and bottom chord has bracing. Main issue we are getting kick back about is in regards to lifting the trusses into place. Mid depth bracing is my preferred system just having issues with figuring out if they can install all joists on ground and still lift it with bracing threaded through the joists.

 

[CANPRO]

Could you extend the bottom chord of the joist to the web of the top chord? You could add some lacing to the underside of the joists to tie it all together. Might be a bit simpler than an independent horizontal truss.

 

[Shu Jiang PEng SE]

It is better to consider to use three dimensional triangular truss with two chords on the top.

 

[BridgeSmith]

Seems like you could add a gusset plate or kicker from the web of the WF to the joists to stabilize it, if it needs it. If the web members are rigidly connected to the top chord WF, and the joists are bracing the top flange of the top chord, it may be stable for lateral torsional buckling. It's hard to say without an FEA, since it's a complex system, though.

Maybe the configuration is something done in buildings, but it seems like much more fabrication than just using a single welded plate girder of the depth required, braced by the joists. If you need an open web, perhaps WT sections for the chords and double angles for the truss diagonals. That's just my 2 cents worth as a highway bridge design guy.

 

[oldestguy]

What about two channels, toe to toe?

 

[LECT12]

Your top chord can still roll, you need to provide stability to top and bottom flange. Add a stiffener and some diaphragm members for erection bracing. 200ft is a very long truss and you will need to provide lateral stability during construction anyway. Likewise do the same for bottom chord with some diagonal braces to prevent the bottom chord from kicking out during erection.

 

[BAretired]

The top chord should be a stocky member. If it is a WF shape, as Hokie66 suggested, the web should be horizontal. HSS sections also work well. Panel points of the truss should be located to match joist spacing.

I designed trusses for a local steel fabricator on an arena project with similar length trusses. The chords were HSS sections. For ease of handling, each truss was spliced once at midspan using bolted end plates.

BA

 

[KootK]

I second the vote for a stockier chord section but, even as is, I doubt that you currently have any issues with weak axis or torsional buckling. Consider:

1) The weak axis buckling effective length is the joist spacing.

2) The torsional buckling effective length should be the panel point spacing.

3) Any torsional buckling mode is really constrained axis buckling with that axis being the top flange. That's a higher energy buckling mode that makes for tricky math but a higher capacity.

With a sharp pencil, you'll likely find that the torsional buckling capacity of a wide flange is pretty significant. If you're already okay for negative bending lateral torsional buckling without bottom flange restraint, you might be okay.

Is your top chord panel point spacing exceptionally long? I'd normally expect raw economic considerations to drive you towards a stockier section.

 

[jayrod12]

Deeper individual sections will not really help when it comes to truss members, heavier ones work much better. I've got the drawings for our local NHL Arena and the trusses span 282 ft spaced at 36ft o/c and are made up of solely W14 members. the compression members at their highest demand are W14x370, Tension members are W14x233. In other words, crazy heavy but their failure mode is not weak-axis buckling. And also, for the record, the web of those W14 members is in fact oriented horizontally for the top chord, bottom chord it's oriented vertically to accommodate rigging loads.

 

[Structural Eng 7]

We have decided to provide WF compression struts to laterally brace the top chord at panel points and for erection and long term performance we are adding diagonal bracing between the top and bottom chord every few bays. We are also using a horizontal braced frame for lateral stability at the top and bottom chords.

I tend to agree with orientating the member as noted to avoid such issues with weak axis buckling. Our panel points are quite spaced at about 17ft to match end wall column locations and the architect made it a joy and threw some 30000lb rtus near 3rd span so there’s a large amount of extra load for one half of the truss top chord in bending so we decided to go with strong axis orientation to span. Lastly, again architect driven, a running track is supported off the bottom chord of the truss at one end.

We were originally thinking of using tie joists from the OWSJ but as of late we have been getting kick back from joist suppliers about strutting to stabilize members and they tend to throw a rediculous price into the project in these cases for some reason.

The cost for these trusses are rediculous according to some shops bidding the project so we are thinking there is going to be some cost savings measures at some point so we will likely relook at the design again to come up with some savings.

Has anyone ever seen a price per square foot for longer spanning roof structures like this in the recent past?