Pony Truss Problem

[cazkoop]

Our firm has been asked to solve a problem with a pony truss. It appears that one of the trusses has been impacted. The vertical members are bent in the direction of traffic, but the truss itself is leaning out from the bridge. Since it is a pony truss, the addition of ties at the top of the truss would restrict vertical clearance. Is there any other solution that would bring the truss back into alignment?

Thanks for any info.

 

[bridgebuster]

By impacted, I assume you mean it was hit by a vehicle resulting in the leaning.

Without a picture I can only guess...are any other truss members warped besides the one vertical? Is the entire plane of the truss leaning outward? Are the truss-floorbeam connections intact?

Just some thoughts ...perhaps the truss can be heat straightened. If the connections are distorted, possibly you could pull the truss back vertical with cables and replace the connections one by one.

 

[cazkoop]

You are correct, the impact was from a vehicle. Each of the four verticals is bent, but all connections are still intact. The extent of leaning is not major, and at first glance you may not even see it. Is heat straightening an expensive option? If it is heat straightened, will it stay straight, or will it try to go back to its current shape without top bracing?

Thanks for the help.

 

[bridgebuster]

Heat straightening would be less expensive that replacing the verticals. When done properly, it does retore a structure to its pre-damaged condition.

It seems to me the need for top lateral bracing is a somewhat different issue. Since the deformation was due to vehicular impact, top bracing won't prevent the verticals from becoming damaged. Although it might prevent lateral deformation of the entire truss. You would need to perform a 3-D analysis. Perhaps you should consider a guard/guide rail. If top bracing is a necessity, you could do it without reducing the clearance by putting stub columns on top of your truss. It would probably look ugly but it would work.

The AASHTO Guide Specification for Pedestrian Bridges has a good example for the analysis of a pony truss, particulary due to lateral loads.

 

[jike]

If top chord bracing is desired to prevent further lateral deflection, why don't you extend the bottom stringers outward and then brace up to the top chord with a diagonal?