compression of bottom chord. Truss

[Bertil B]

Hello!

Typical case. Upper chord is braced in both directions due to roof sheeting and the web. But the bottom chord is not braced at all.. Since it is operating mainly under tension.

I'm struggling to find a method of verifying the stiffness of the bottom chord under compression. We have had this case on several occasions, and different answers each time. Some say that the bottom chrod needs lateral bracing. And some say, the axial compression is so small it can be neglected.....

This typical case is generating uplift for 1/3 of the truss. So that 1/3 of the lower chrod is under compression and 2/3 is under tension. (max compression force in bottom chord is 10kN)

Upper chrod HEA 160
Bottom Chord HEA140

Any suggestions?

 

[jayrod12]

Make sure it meets the slenderness requirements for elements in compression, otherwise bracing would be required.

 

[Bertil B]

But what about the length coeff? Is there a guidance when there is no lateral constraint besides the stiffnes of the web?

 

[jayrod12]

People here may disagree, but I'd argue if only a portion of the chord is in compression, then if you took a decent distance past the point of inflection as your length you'd likely be in good shape. In your example I may use 1/2 the chord length as the number.

Don't you need minor bracing to meet the tension slenderness values anyway?

 

[hokie66]

Design for compression like any other strut. This is usually controlling in high wind areas.

 

[KootK]

For starters, you'll like this: Link

The bottom chord does need bracing, not just under compression but, also, under tension. Typical ways of dealing with that:

- Let the bottom chord span laterally between the far ends of the truss and tie into some form of lateral restraint.

- Let the bottom chord span laterally between at least two lines of bridging that brace the bottom chord laterally. Pretty much every OWSJ ever.

- Let the self weight of the bottom chord itself do the bracing acting essentially as ballast. Sounds nuts but lots of large scale trusses depend on this whether it's acknowledged by their designers or not.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

People here may disagree

Strongly. After all we've been through dispelling the myth of IP bracing in beams?? Surely. You jest. There's gain to be had if a strut is not compressed all the way but that warrants more rigorous evaluation if it's to be relied upon.

Interestingly, I've seen a number of OWSJ in the field that have single rows of bridging and the ends of the bottom chords not retrained laterally in any way. Me no understando.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

It probably goes back to something like your third point. Generally speaking, I wouldn't provide a new design in that way, but analyzing something existing I may go that route, mainly because it may have been designed that way back when the use of the IP in beams was still acceptable.

 

[Bertil B]

Thank you all, for the quick reply.

I see how the truss need lateral restrain. Its the combinations that gives me the compression. 1*SW(roof)+1*SW+1,5*Wind. And this is standard in all our cases. If we set the self weight of the steel deck to be 0,6kN/m^2 we will have no compression at all.. So it is realy in the middle of tension/compression zone.

For this project we have included the weights from the lamps that will hang from the lower chord. And the railings for theese lamps is also giving lateral restrain.

We tried model one truss as a shell and analysed the critical buckling force, but we got uninterpreted results. Anyone having experience with this?

 

[hokie66]

With compression in the middle, and tension at the ends, I assume this means the ends of the chords are rigidly connected to the columns, so the truss forms part of a rigid frame. Correct? Where the chord is in compression only some of the time due to uplift, you still have to design it as a column.

 

[Bertil B]

only the upper chord is pinned at the column. That is why i cannot determine the real buckling length. The compression is occuring at the end of the lower chord. Please see attached img.

 

[hokie66]

Those numbers make no sense to me. Is that the whole truss or half? Why is no support shown at the right end? Have these numbers been spit out by some computer program? How do they compare with hand computations? This is a simple Warren truss, so easily analyzed at each joint.

 

[Bertil B]

Someone who has read this?

http://www.sciencedirect.com/science/article/pii/S1644966514001198