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LTB of Bridge Beams

LTB of Bridge Beams

LTB of Bridge Beams

(OP)
I know this topic has been beaten to death...but I couldn't find something addressing my exact situation.  I am designing a small pedestrian bridge.  Steel I-beams with composite "wood" decking (2x8).  The deck planks are bolted to the top flanges of the beams.  Since the planks are discreet, I do not believe they brace the top flange.  Any restraint provided by a particular deck plank cannot be transferred to the ends at the abutments.  So I intend to prevent torsional rotation by means of a stiffener plate between the flanges.  Question is this...how do I determine the depth/thickness?  Actually wondering also if x-bracing is better than the plate idea.

RE: LTB of Bridge Beams

Vertical stiffener plates between flanges don't help with LTB.

You need to prevent twist in the beams and the resulting lateral translation of the compression flange.

For a bridge like this with two beams, one idea is that you create a series of X-bridging between the beams to avoid twist.  This is a relative brace (see AISC Specifications, Appendix 6).

Alternatively, you could create a horizontal series of diagonals, essentially creating a horizontal truss down the length of the bridge.

RE: LTB of Bridge Beams

Agree with JAE.  Use a horizontal truss.

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