Weight capacity of my concrete/steel bridge?

[2manyrocks]

It will outlast me and every car/truck/tractor I own unless I do something stupid and let someone drive over it with something too heavy.

 

[yooperdave]

I need a forestry mulcher to come in and do some clearing. The operator said his Cat 299d with the mulching head weighs 15,000 lbs. The last guy to bring a compact track loader said his weighed 10,000 lbs and was able to cross without any issues, but 15,000 lbs is a lot of weight.

The bridge is 10" thick of concrete with a 13 foot width x 12' length. It is sitting on three I beams. The upper flange is inside the concrete, but the exposed portion is 8 1/2" deep with a 4 1/4" flange that seems to measure 5/16" thick.

The concrete pad does not have any cracks. The beams need repainting, but are solid. I remember that mesh was placed in the concrete when it was poured, but I do not recall how much or if there is any rebar in the pad.

Any ideas as to whether 15,000 lbs is too much for my bridge?

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The bridge is required to have a load rating plaque posted in clear view. You should not have to reverse engineer as too many assumptions would be needed. BTW I was involved in a forensic analysis of an old railroad bridge deck that collapsed. There was a load rating sign, but the driver ignored the sign and went full throttle instead. The floor deck gave out and his truck and trailer plunged into the river. He was lucky to survive, but needed to change his clothes afterwards.

 

[3Ts]

I have a creek on my property that has vertical banks and is about 10' across. I had a forestry crew come in to harvest some of the trees with their shear and skidder which I'm sure (but don't know) weighed a lot more than 15,000#. They cut some of the trees and put them in the creek to cross it, then pulled them all out when they were done. Why can't he do something like that?

 

[3Ts]

The bridge is required to have a load rating plaque posted in clear view. You should not have to reverse engineer as too many assumptions would be needed. BTW I was involved in a forensic analysis of an old railroad bridge deck that collapsed. There was a load rating sign, but the driver ignored the sign and went full throttle instead. The floor deck gave out and his truck and trailer plunged into the river. He was lucky to survive, but needed to change his clothes afterwards.

There was a covered bridge in Ohio many years ago that cars drove across with a posted 5 ton limit. A single axle dump truck fully loaded decided to cross, but didn't make it and destroyed the bridge.

There has to be another way to cross that to use your bridge. The stakes seem to be very high, I personally, wouldn't do it.

 

[CliffordK]

The bridge is required to have a load rating plaque posted in clear view. You should not have to reverse engineer as too many assumptions would be needed.

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 may well depend on the bridge, and the purpose. A public access bridge may well be different from a private access bridge. If it is a main access bridge for a person's house, it might require a building permit filed with the county. That may be a place to look for information.

If it is just a bridge to a back pasture, or to some outbuildings, it is possible it was simply built on the "Good Enough" principle without permits.

There was a lot of rock work done for the foundation of the bridge with the cap being concrete. It is possible that the bridge was reworked at least once in its lifetime, with an early wooden deck that was replaced with concrete (and perhaps the I-Beams).

Now, weights?

15,000 pounds isn't a lot... maybe.

I'm seeing that UPS step vans are probably somewhere in the 10,000 to 20,000 pound range. Now, the front and rear axles will distribute the weight some, and it may only have one axle on the bridge at a time, but a lot of the weight will be on the rear axle. So, do you get UPS, Fedex, Box trucks, etc, bringing deliveries across the bridge?

 

[2manyrocks]

This concrete bridge replaced an earlier pole and plank bridge, but I don't know if the stonework was part of the earlier bridge or not. It might have been. There weren't any permits required at the time it was built, and it was used for access to a residence.

There are no delivery trucks using the bridge.

The neighbor had a 299d come in to do forestry mulching work. His bridge abutments are made of concrete block and one of them looks like it is trying to fall into the creekbed. I don't know if the weight of the machine did that or if the abutment was failing to begin with. One good gully washer rain and his bridge may end up in the creek.

 

[paulsharvey]

This concrete bridge replaced an earlier pole and plank bridge, but I don't know if the stonework was part of the earlier bridge or not. It might have been. There weren't any permits required at the time it was built, and it was used for access to a residence.

There are no delivery trucks using the bridge.

The neighbor had a 299d come in to do forestry mulching work. His bridge abutments are made of concrete block and one of them looks like it is trying to fall into the creekbed. I don't know if the weight of the machine did that or if the abutment was failing to begin with. One good gully washer rain and his bridge may end up in the creek.

So, not trying to spook you... But let's play it out; if the bridge gets damaged, you probably will not be able to replace it with another unpermitted crossing. You likely would need multiple permits, a civil or structural PE, and then atleast $50k for the construction. So, it's just not worth the risk over some mowing. Your existing is grandfathered in, protect it.

 

[ruffdog]

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....

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.

^^^agree.

 

[2manyrocks]

I would also suspect that the permitting process will require a lot of time and then getting the work done means I would be without access for months on end, best case.

Starting to think I'll cancel the entire mulching project.

 

[Chewwy]

So, not trying to spook you... But let's play it out; if the bridge gets damaged, you probably will not be able to replace it with another unpermitted crossing. You likely would need multiple permits, a civil or structural PE, and then atleast $50k for the construction. So, it's just not worth the risk over some mowing. Your existing is grandfathered in, protect it.

That depends entirely on where the OP is located. Non-public access bridges are not regulated at all in many locations.

A culvert with overfill is unlikely to require a permit or much, if any, engineering.