Lateral and Torsion Bracing

[toluun]

Hello All,

I am getting a little confused by the bracing required during steel girder erection and was hoping for some clarification.

I know that during steel erection LTB can become a problem because the concrete deck has yet to be installed. I know that this restraint can be provided either by Lateral(plan) bracing at the top flange or K/X(torsional) bracing. Typically girders are erected in pairs with X-bracing preinstalled to mitigate this issue.

My question arises when there is additional diagonal lateral bracing installed in between one or more girder lines. I have included a picture below:

This bracing is usually connected to the top flange and I usually see this when large lateral forces are expected. Am I correct that this bracing is used to increase strength against lateral loads? How exactly is it adding more lateral strength?

 

[BridgeSmith]

What's considered lateral (horizontal) bracing is typically for strengthening the girders against lateral loadings, typically wind, bending the girders laterally over the entire span between piers. The X or K frames are typically for bracing the compression flanges against LTB, whether it be for the top flanges before the deck is placed, or the bottom flange near the piers for continuous spans.

In the layout shown, it appears the lateral bracing does not add any additional braced points for LTB beyond what's provided by the X or K frames.

The lateral bracing creates what amounts to an open web truss, with the girders as the flanges, the lateral bracing members as the diagonals, and the X or K frames as 'vertical' struts.

Rod Smith, P.E., The artist formerly known as HotRod10