Bridge Load Limit

[Jw5875]

Lost my bridge several times over the past 12 yrs. and decided to do
a new design. Raised it and removed some rocks from under it for a
better flow of high water.
Bridge was originally on 4 - 17' x 10" x 5" x 3/8 beams with same dimensions of an
8" beam in the middle with a 17' span for the bridge.
Had several tri axles of gravel cross it at different times and just a little flex, due to
ends sitting on rocks.
Now have the beams on 2' x 2' x 6' concrete blocks at a 12' 9" span under the same beams.
Current driver feels bridge is not capable for a tri axle of 2A stone?
Can some one point me to the load capacity?
Pic 6 is flood damage.

Thanks,

JW5875

 

[Tomtint]

...Id drive over it.....

 

[Billrog]

Looks like a real nice bridge I wouldn't hesitate to drive over it either.

 

[bcp]

Your county may evaluate bridges, to see if they will send a fire truck across it.

Bruce

 

[Cord]

Should have buried another course of block below the water level. As constructed it won't take long for the foundation to wash out.

 

[LouNY]

We have several fields that we have to cross a small brook to get to,
were we have installed culverts in a few places we also maintain an actual stream crossing.
Heavy or loads too wide to fit will use the stream crossing our ground is quite stony so the crossings are very firm.

Looking at your bridge construction it certainly seems to be more then strong enough for that span.
I haven't looked recently but they used to be some design calculators on line for defection under load for steel or timber.

 

[BrokenTrack]

As close as I could come up with was: 7800 pounds per square foot.

Assuming half the tuck was on the bridge at one time (because a tri axle is longer than 17 feet), then 72,000 pounds divided in half=36,000 pounds.

Assuming 2 square foot for each tire for easy math, with 2 big front tires on the bridge (4 square feet)=31,200 pounds

Assuming 1/2 square foot for each rear tire for easy math, with 12 tires=46,000 pounds.

(Now these are VERY rough numbers. I am not sure exactly how the real weight transfers on a truck, but it seems it will hold a truck...barely because the front end would be less than half the weight of the loaded truck, but has less square surface of tires bearing down on the bridge. In the back end, the load is spread out over more square feet, but the weight is probably greater too.

This is done with 1/2 inch deflection being pass or fail.

 

[Jw5875]

Should have buried another course of block below the water level. As constructed it won't take long for the foundation to wash out.

We dug down with the excavator as far as we could, creek base is almost solid stone / rock.

 

[Redlands Okie]

Seems you already have a history of loads on a less well built bridge and it worked. Of course its the current drive you have to deal with. He is naturally concerned about damaging his equipment. And your bridge.

If you have a rock base under it that the blocks are setting on as it seems front the post how about some jack post set up under the bridge mid way for extra support and for the drivers price of mind. Usually available at box stores and lumber yards.

Just guessing at your height but the linked post are rated at 20,000 lbs at max height, each. around $60 dollars each with tax.


https://www.lowes.com/pd/Akron-56-in-Adjustable-Jack-Post/3201385

 

[Peter 315]

Almost 20k on the front axle and about 56k on the 3 rear axles gross of 76k that's what we run on are trucks, A 24 ton load + or -