Weight capacity of my concrete/steel bridge?

[Jims1025R]

An interesting discussion. Though cribbing sounds like a good idea, the arrangement of the bridge construction makes this a little less ideal. The beams will flex under load, as they should, the concrete not so much. Cribbing would certainly prevent a bridge collapse, but might not prevent the bridge from being damaged. If the cribbing is not really highly compressed in place, the beams might flex down a little under load and allow the concrete portion to crack. If you're going to crib it, make the cribbing really tight!

I like the fording idea under the circumstances described....

 

[bigtiller]

Wave a metal detector over it to get some idea of the amount of steel in the bed.

 

[2manyrocks]

An interesting discussion. Though cribbing sounds like a good idea, the arrangement of the bridge construction makes this a little less ideal. The beams will flex under load, as they should, the concrete not so much. Cribbing would certainly prevent a bridge collapse, but might not prevent the bridge from being damaged. If the cribbing is not really highly compressed in place, the beams might flex down a little under load and allow the concrete portion to crack. If you're going to crib it, make the cribbing really tight!

I like the fording idea under the circumstances described....

Thank you for this warning.

 

[Egon]

It looks like a nice bridge. There are too many unknown variables to contemplate a proper load rating.

I doubt if an engineer would touch it.

 

[mikester]

Get a copy of your operator's insurance coverage and workman's comp, make sure it's adequate, and let him decide if he uses the bridge.

I wouldn't touch the contractor if he has less than $5M GCL. Ask for a letter from his insurance proving coverage on your property.

 

[oosik]

There is no question. No matter the load on the bridge - it will deflect a certain amount. The question being - will it deflect so much as to cause the concrete to crack OR cause some other type of more serious failure.

I would look back into the history of the bridge. Was it designed by an engineer. Was a permit secured to build the bridge and did it indicate max load. Is the person who designed the bridge still around. What were his design parameters and what was the bridge's designed max loading.

 

[MattB4]

There is no question. No matter the load on the bridge - it will deflect a certain amount. The question being - will it deflect so much as to cause the concrete to crack OR cause some other type of more serious failure.

I would look back into the history of the bridge. Was it designed by an engineer. Was a permit secured to build the bridge and did it indicate max load. Is the person who designed the bridge still around. What were his design parameters and what was the bridge's designed max loading.

Presumably the deflection that happens now with a 10K load is not enough to damage the bridge. By shoring the center of the span you effectively double the ability of the bridge to sustain weight. But not being perfect load bearing for temporary placed piers a safety margin of 1/2 would allow you to support the 15K.

Likely the bridge would support the 15K load (especially a distributed load from a tracked machine) without shoring up but it would add peace of mind.

 

[2manyrocks]

All parties involved in building the bridge are deceased, and there are no records to be found. Not knowing how much steel is in the concrete, there's no certain way of calculating the weight capacity.

When the mulcher operator told me his machine weighs 15,000 pounds, my first instinct was that is too much weight. I now realize I should have had more discussion with him when he was here to look at the job.

If he doesn't feel comfortable fording the creek or it rains a lot before he comes to do the job, then I will call the job off. He has operated his machine for enough hours and he has enough experience that he is likely the best one to decide if he can safely ford the creek or not without getting stuck.

It's not a good plan for me to guess about how much weight this bridge will carry because the consequences of being mistaken are too expensive and too time consuming to deal with. There is no other vehicle access across the creek.

 

[EddieWalker]

I would trust your gut and not risk damaging the bridge for something that's not as important as the bridge is. Let the skid steer operator figure out how to get across the creek if he wants the job, or hire somebody with a smaller skid steer.

 

[MattB4]

All parties involved in building the bridge are deceased, and there are no records to be found. Not knowing how much steel is in the concrete, there's no certain way of calculating the weight capacity.

....

It's not a good plan for me to guess about how much weight this bridge will carry because the consequences of being mistaken are too expensive and too time consuming to deal with. There is no other vehicle access across the creek.

You sorta are guessing how much the bridge will support every time you use it now since you have no idea of its design capacity and age related reduction. This becomes more and more of a problem as the bridge gets older. Since this is the only access for vehicles you may want to start investigating upgrades or replacement.