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?

 

[SWComposites]

Is there a way to calc it? Maybe. Depends on the condition details and a lot of other stuff.
Is anyone on here going to do it? No.
Does the bridge currently have a load rating?

 

[phamENG]

Based on your description of the condition, it sounds like it's on borrowed time already. The rusted web and bent flange suggests that one of your primary girders has already failed at the bearing, and is probably being held by an unreliable mechanism. In other words, determining exactly what or how much or when it will let go is as accurate as a literal crap shot.

If you plan to continue using that road, "until we can get it rebuilt" probably needs to be "now".

For a more nuanced answer, you need to hire a local engineer to come out and assess it in person.

 

[LittleInch]

So the beam are 3 feet apart. If one fails then with the broken criss members you could easily break through.

Which beam is broken?

What is the condition of the rest?.

But a 55 year old bridge has done its job. I doubt anyone will "rate" this bridge. If you can find the beans in decent condition limit the load to those and have a width restriction and limit to 2 tonnes.

What short of traffic does that bridge see?

 

[JAE]

You should hire a structural engineer now.