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.

 

[hoshang]

ViaParadise said:​

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.

Can you elaborate more on this?

 

[BAretired]

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.

A Warren truss can have verticals if needed. Floor beams can be extended with diagonal braces to provide lateral support to the top chord.

Are you talking about a pedestrian bridge, road bridge or rail bridge? Can you provide a sketch showing the approximate dimensions of the bridge. So far, you have provided virtually no useful information.

 

[28516966]

In support of @Tomfh's statement about using inclined truss members to provide lateral restraint to the truss top chord, the Eurocodes give guidance for this situation - see Table D.3 on page 95 of the following: https://www.phd.eng.br/wp-content/uploads/2015/12/en.1993.2.2006.pdf. I have also personally checked footbridge designs of that form but I am unsure how well it would hold up to highway or rail loading before vertical posts as others have suggested become more economical.

I presume you are US-based so this may not offer much direct help in the way of a calculation reference unfortunately.

 

[hoshang]

Are you talking about a pedestrian bridge,

Yes

 

[BAretired]

You are too stingy with the information. Provide a sketch showing dimensions.

Top struts as per your suggestion would need to be too high. Normally, the top of truss is about handrail height.

You could use a Pratt truss or Warren truss with verticals moment connected to cross beams or you could extend the cross beams beyond the truss and laterally brace the top chord with diagonal braces from the ends of the beams.

 

[hoshang]

BA,
The pedestrian bridge is a two span truss, each 90ft long, 10ft wide, 7.5ft high. The floor would be checkered plate 1/8in thick. As a side note: what are the fabrication and erection loading for these types of construction?

 

[BAretired]

Thanks hoshang, in that case, top struts are possible and I believe, advisable.

 

[hoshang]

Hi,
what are the fabrication, erection, and installation loading for these types of construction?

 

[BAretired]

Hi,
what are the fabrication, erection, and installation loading for these types of construction?

I have no idea what you are asking. Maybe someone else can respond.