Unbraced Length - Footbridge

[MontrealEng]

I am designing a footbridge in an industrial environment. The span of the bridge is 9650mm, the width 900mm. The diagonal bracing connected to the compression flange of the main beams definitely provides lateral bracing, but do the intermediate transverse beams also provide lateral support (assuming they are connected to the web of the main beam, T.O.S. being equivalent for all members)?

Referencing my attachment, is Lu 1608mm or 3216mm?

http://files.engineering.com/getfil...0-4c2a-81ca-59c4488c8946&file=Foot_Bridge.JPG

Thank you,

Carl, ing. jr.

 

[ishvaaag]

When pursuing the prevention of LTB at discrete points you can follow different schemes. You can use a Relative Lateral Brace System (as exemplified in Galambos V, Example 12.4) where the panel points of bracing trusses at the compressed flange prevent the LTB, or you can use the stiffness itself of the transverse members, in what is called Torsional Beam Bracing (as exemplified by example 12.5 in the same text). For this the transverse beams and stiffeners to which they are attached need to be of stiffness enough to correctly prevent the LTB, as shown in the example.

Upon verification of the adequacy of the torsional bracing system, Lb could be taken 1608 mm.

I attach a screen printout of a Mathcad 2000 worksheet I made following the torsional bracing example. If interested, I may post a zip with the worksheet.

 

[ishvaaag]

I look again to your jpg and provided the truss shown is bracing the top flanges, and is adequate to provide the bracing, Lb can be taken 1608 mm irrespective of the ability of the transverse beams of torsionally restraint the LTB.

This because the orthogonal joints are as much panel points as where the diagonals meet.

 

[MontrealEng]

Ishvaaag,

Thank you very much for your help.

Carl

 

[BAretired]

I have read through your question several times and I must admit, I do not have the foggiest idea of what you are asking. Would you kindly elucidate?

BA

 

[hokie66]

I think I agree with ishvaaag, provided the picture shown is a plan of

 

[hokie66]

...the top chord bracing truss.

 

[JAE]

Both the diagonals and the orthogonal cross beams provide brace points as they both are part of the triangulated system. The braces are relative braces and not nodal braces.

 

[BAretired]

Okay, now I get it. I thought I was looking at an elevation. I agree that the unbraced length is 1608mm.

And thanks, JAE. Now I know the difference between relative and nodal braces.

BA