Load carrying capacity of 15x50 s beams?

[rfc143]

Our private road is serviced by a bridge built in the 80's that consists of (6) 20' 15x50 S beams, covered with 4x6 hemlock cross members and topped with 2-1/2" x 12" planking. The bridge is about 15' wide. I inspected the underside and found several of the cross members broken but more important one of the beams' web where the beam sits on the abutments rusted out enough to allow the flange to bend upwards. There's some spalling of the concrete immediately beneath the failing beam.

We're thinking of leaving all the existing beams in place, but putting five new beams between the existing beams and replacing all the wood with Pressure Treated lumber. In the meantime is there a way to calculate what the safest maximum load on such a bridge might be so that we can post it until we get it rebuilt?

 

[MintJulep]

We know we need a structural PE to look at it; we're doing that. But until he/she comes, I feel we should alert the others to the potential problem and recommend some upper limit in the short term until we get a plan in place

Who are "we" vs "the others"?

 

[Ron247]

things don't fall down, until they do. That's my Yogi-ism.

It has to be said, "You just like being the Berra of bad news".

 

[phamENG]

It has to be said, "You just like being the Berra of bad news".

You hit that one out of the park, Ron.

 

[LittleInch]

Lets get practical for a minute:
So if we number these beams 1 to 6 from L to R as you look at the bridge in either direction, one of beams 3 or 4 is now basically failed. Lets say beam 4.

Of more interest to me is where are these cross beams "broken". In between beams 3 and 5 or somewhere else?

If between beams 3 and 5, then you need to restrict travel to only load beams 1 to 3. Add cones, barriers, bags of dirt, whatever to prevent travel over beams 4 to 6 and make the edge beam 3 the edge of the allowable crossing.. That only gives you 9 feet, but then the bigger 8 ft trucks will need to drive slowly.

If the broken cross beams are some where else then you're in serious trouble and continued use is wholly at the drivers risk. Post "Weak Bridge" , "Use at your own risk". My guess is the garbage truck and Propane lorry won't risk it. But if they do they can't say they weren't told.