Truss Top Chord Lateral Support

[hoshang]

Hi all,
For a Pratt truss, lateral support is provided by a transverse U-frame consisting of the floor beam and truss verticals. What about a Warren truss? It doesn't have verticals.

 

[Tomfh]

Works the same, the members are just inclined.

 

[duckhawk]

Sounds like you are talking about a "pony" truss where the top chord does not have direct lateral bracing, but is indirectly laterally braced by vertical truss members.

For a pratt truss there are papers written how you can calculate a modified K factor for the top chord to use based on the stiffness of the vertical+floor beam+vertical U-frame like you mentioned.

This paper is helpful to understand the top chord of a pony truss with a U-frame. (by Derek Matthies)

Lateral Buckling Analysis of A Steel Pony Truss | PDF | Buckling | Truss

Lateral Buckling Analysis of a Steel Pony Truss

www.scribd.com

For a warren truss, I have not seen any papers that try to tackle this yet. I think I remember one of the pratt truss papers saying the diagonal vertical members don't do much to provide lateral bracing to the top chord. This is probably why most warren trusses have direct lateral braces.

Unless there is literature showing how to calc a modified K factor for the top chord for a warren truss without direct lateral bracing, it would probably be wise to stay away from that design.

Here is an old warren "pony" truss. They have clearly added straight up and down vertical truss members to indirectly laterally brace the top chord.

 

[hoshang]

Tomfh said:​

Works the same, the members are just inclined.

Do you have a reference on your approach?

duckhawk said: Unless there is literature showing how to calc a modified K factor for the top chord for a warren truss without direct lateral bracing, it would probably be wise to stay away from that design.​

My thought is to add top struts as well.

 

[duckhawk]

My thought is to add top struts as well.

That looks like the only way to take care of laterally bracing the top chord of a warren truss.

 

[ViaParadise]

If you have transverse floor beams you can use those as well. If your deck isn't attached to the compression members then you may want to consider compression bracing. Appendix 6 of AISC LRFD goes over it and it's pretty straight forward. For linear consideration, unless your web members are moment connected to the top chord then they don't provide out-of-plane resistance. But you could also just add a moment connection out-of-plane at midspan and half that unbraced length. Honestly, i would check non-linear, may be fine under larger deformations.