sanity check my bridge design

[tractchores]

I need to replace a failed bridge (rotted out after 30 years) on a property I'm purchasing. Due to restrictions in local rules I'm only allowed to replace the bridge (i.e. I can't make it a culvert). The crossing is 12' long over a V shaped ditch and small (6" wide, 1" deep) stream, the height is 3.5' in the center so not huge. The goal is to be able to drive across it with various vehicles, including my truck (~5,000 lbs) and a tandem axle trailer (~7,000 lbs). Usage will be minimal as this is a vacation home and the bridge just goes to a boat launch so access across the bridge is probably under a dozen times per year.

My thought is to create a bridge that is 10' wide. I would use 2x10x12 or 2x12x12 PT lumber sistered to form joists that consist of 2 boards apiece. I would do 4 joists (so 2.5' apart). I'll deck it with 2x6 PT lumber. The footing will either be what's there now (RR tie) or a 6x6 PT timber. I will add blocking for lateral stability in 2 places per beam and a small raised 2x4 strip on the edges.

Ground conditions are stable and its all gravel The stream does not get high so no risk of washing out or concerns about the footing supporting the beam weight. I believe that the 2x10 design will support close to 10,000 lbs live load and the 2x12 will be closer to 13,500 - though I'm not totally convinced I'm using the span tables correctly. I'm limited due to the local rules and can't use metal i beams or pour footers (basically I can "repair" but cannot replace or build a new structure).

Would love thoughts about if this design will support the weights I'm looking to use it for and if I'm missing something.

 

[LouNY]

I would place 2 joists in the area of where my tires would normally be, after decking with the 2x6 I would also run about 3 2x10x12's side by side for each wheel track,
I would be leery of just 2" stock for the decking. Many of the old bridges around here had 2x6 decking but the 2x6's where on edge then 3 wide planks flat for each tire track.

 

[jfh28]

I agree 2 x 6 flat spanning 30" is not good.....especially with the quality of todays lumber......even if it doesn't fail outright the bouncing you will get from the deflection will make you think twice about driving back over it again........also if you were looking at the typical loading tables for lumber the type of loads that occur in a floor situation inside a building which most tables are designed for are way different then the type of moving load you will encounter with a moving vehicle on a bridge.......it does sound like you are very limited with what you can do but I would get some professional advice on this......Jack

 

[Tractor Seabee]

I have used RR ties for stringers and sill supports. Place them with the narrow side up. Use only grade 1 or 2. They are pretty cheap and are up to 16' long. I use 3 stringers with the outer two at wheel spacing. 2" rough cut lumber makes good decking. My last one was 14' between sills. My 1 ton dually dodge with 2 yards of sand never deflected it or created even a creak. Individual axle load is your support criteria

Ron

 

[EddieWalker]

Here, RR ties rot out fairly quickly if they are touching soil. The rail roads put them on gravel so they never touch soil, and water drains quickly.

I would start out with the footings and go with either treated 6x6's, or ask your lumber yard if they have anything with a higher rating. Since you only want to do this once, paying a little extra for a higher treatment would be money well spent.

Where the tires line up, I would triple 2x12's to create beams, and use a lot of exterior liquid nails to increase their strength. Just screwing them together wont give you the same level of stiffness and load capacity as gluing and screwing them together. I wouldn't use a nail on anything outdoors. Then I would run a single 2x12 between the beams, and anther single 2x12 on the outside edge of either side.

Lay 2x6's on the flat to create your deck with 3 screws into each board and the beams. Then I would run a pair of 2x6's the length of the bridges over each beam so that's what your tires actually drive over.

 

[tractchores]

Thanks, some good thoughts here. I will definitely add some boards across the decking for where the tires go. This type of bridge is common in the area for logging roads so I may look and see how they build them too.

 

[CincyFlyer]

I would get some steel beams and wrap them in wood.

 

[Rustyiron]

I'd call 30 years of service from the original bridge a great success and rebuild it exactly as originally built.

 

[quicksandfarmer]

I'd call 30 years of service from the original bridge a great success and rebuild it exactly as originally built.

That was my first thought too.

 

[tractchores]

I'd call 30 years of service from the original bridge a great success and rebuild it exactly as originally built.

I’d like to, but I think it uses creosote soaked railroad ties (12x12) with just 2 stringers. Can’t get those anymore that I know of. I like the idea of putting a wood wrap on a steel beam though it’s probably not realistic. Fines are unlikely, but large!