Bridge beam diaphragms

[71corvette]

I am currently working on a rehabilitation project on a bridge that consists of a concrete deck supported on rolled girders. The bridge has minimal skew, is not on a radius, and is non-composite. We are currently considering having the contractor temporarily remove several of the bridge girder diaphragms to facilitate some of the repair work.

I have always been under the impression that, in scenarios similar to these, that the diaphragms serve little purpose once the deck has been placed and cured. We are thinking of leaving the diaphragms in place in the negative moment regions to provide the necessary bracing, but in the positive moment regions it seems like they could be removed without affecting the structures capacity.

I’d like to hear what others think.

Thanks,
Tim

 

[MichSt]

Diaphragms do provide lateral support of the compression flange as you mentioned. But, they also function to transfer wind loads on the fascia girders to all the girders and they also help in the distribution vertical dead and live loads.

If you can’t avoid removing the diaphragms during repair, I would have the contractor replace any diaphragms that are removed. You could replace with new diaphragms if the existing are in bad shape or consider reinstalling the existing diaphragms if possible.

How many diaphragms are you talking about??? I would weight the cost of replacing the diaphragms versus the over all project cost. If the diaphragms are a small percentage, I would just replace them with new diaphragms.

That’s my $0.02

 

[miecz]

Be careful that the negative moment region doesn't extend (even a little bit) into the region where you've removed diaphragms. I had a bridge where the negative moment region extended entirely across the interior span, due to the moving live load. Even if the negative moment is small, a compact section check (Eq 10-96, 17th Ed) could downgrade the carrying capacity of your girder by as much as 40%. Some bridge rating programs (BAR7), will downgrade the entire girder, if one section is found to be non-compact.