Time for a new bridge - how to engineer?

[JeremyL]

This old bridge needs replacing after a hard winter outside of Bowling Green, Kentucky. It usually has only this amount of water trickling under, but in a big, hard rain there is enough water to wash over the surface.

I want to building it so that a gravel truck can cross it with much needed loads for my driveway. I am thinking about treated 8" X8"s. How many would I need and at what spacing? Then treated 2X boards across the 8X8s. I will have no idea what the footing is like until I do the demolition. It is 8' long and 10' wide.

Suggestions? Links to plans or articles?

Thanks, JeremyL.

 

[MNwoodchuck]

Couple of culverts, side by side? Assuming you can fill enough over the top.

1999 JD 4600 MFWD
460 loader
Curtis cab

 

[bcp]

I've always preferred large culverts or a concrete ford instead of a bridge. Use a bridge when the crossing is too high above the water for culverts or ford, or there is too much water flow.



Bruce

 

[paulharvey]

An 8x8 wouldn't be better than multiple 4x8s spaced closer. Minimize the clear span as much as possible. The SPC (southern pine council) has or used to have good free info on beam strengths.

Another thought could be to find someone with small dump truck (8 or 12 ton) and make more trips, but expense will add up fast (if a mini wheeler is $60/hr; I doubt a tandem would be any less, and a 8 or 12 ton would be maybe $50/hr for truck and driver).

I do agree with the last post; maybe a double barrel 24" RCP. There about 32" OD; maybe close to 36" OD at the bell. Plenty strong, easy to install if you have a hoe or even a loader; and should last 50 years. Make it at least 3 joints of pipe (24 ft); 16 ft gets pretty narrow at times. Price I believe should be in the neighborhood of $10/inch/8 ft pipe (~$240/pipe; X3 pieces, X2 runs=$1440) so far from cheap.



Edit: I found a price on "allcostdata.info" that lists $43.92/lf; total installed with $25.30/lf as the material price. $25.30 ×24×2=$1214.40

Re-Edit: their labor rates aren't for small home owner projects, but part of large scale projects; don't expect someone to mobilize in and lay that pipe for $800, unless they are hungry and close by.

 

[HCJtractor]

Looks like an easy place for a culvert. Seems a better choice than a bridge

 

[vtsnowedin]

Looks like an easy place for a culvert. Seems a better choice than a bridge

Agreed. The size needs to come from the drainage area upstream and your ability to let over flows wash over the road at a relief point. Just a 24"if you can relieve it and maybe a 30 or 36 inch if not.

 

[RedNeckRacin]

I agree with the culverts. Some pipe and some stone and call it good. I would do a rough flow calculation for the pipes size to oversize them a bit. Also, make sure that you can get enough cover over the pipes so that they do not crush with your max anticipated loading. This usually requires so many inches/inch of culvert size but I would have to look it up to make sure I didn't quote it wrong.

I think calculating the drainage area is a waste of time for a driveway or at least severely overkill. Figure out the max flow in the flood time and find the flow of the culverts to match. 36" is way to big. You will not be able to get enough cover over the pipe to protect it from stone truck deliveries. Using multiple small pipes is fine if they are sized correctly. And no, two 12" pipes do not equal a 24" pipe,

 

[vtsnowedin]

The area of the waterway under the present structure appears to be a triangle about eight feet wide and three feet deep. 1/2 b*h = 12 sq. feet. A 36" pipe has a cross sectional area of 1.5 *1.5 *3.14159=7.07 sq ft. But a smooth bore pipe will flow more water then an open channel.
Another thing is you can look at the nearest pipe downstream and upstream and see how big they are.

 

[paulharvey]

The area of the waterway under the present structure appears to be a triangle about eight feet wide and three feet deep. 1/2 b*h = 12 sq. feet. A 36" pipe has a cross sectional area of 1.5 *1.5 *3.14159=7.07 sq ft. But a smooth bore pipe will flow more water then an open channel.
Another thing is you can look at the nearest pipe downstream and upstream and see how big they are.

He's right, but consider what type of pipe you will use. 36" rcp isn't do able with a tractor mounted back hoe, and might not with a JD-310 sized machine.. probably want an 10 ton track hoe for concrete that size. ABS/HDPE/CMP/ACCMP that size will need pretty good coverage to avoid damage by a real dump truck.

Edit: of coarse you could use Elliptical pipe (squash pipe, ercp) and I think 36" nominal would be 22" x48"?

 

[KennyG]

Culvert does make sense. If you really want a bridge, build it higher ramping up on both sides. Pour substantial concrete footings and tie the beams to it somehow to keep it from floating. Use 2 x 12's for beams, not square sections, as many as required for your design load using beam tables.