Time for a new bridge - how to engineer?

[paulharvey]

While the preponderance of the recommendations are for a culvert, my wife really likes the bridge option since that it what has always been there (since her grandparents' day). Local lumber yard has #1 used railroad ties, 7" X 9" X 8'. If I use them as beams, I shouldn't have to worry about rotting, and can afford as many as it would take to make the deck sturdy. One possibility would be to use 4" X 6" X 10' posts for the decking.

Anyone know how I can calculate how many beams I need to carry the load of a small, loaded gravel truck?

Thanks for all the information!

Careful. Those used rail road cross ties are often damaged/rotten/half split. They removed them for a reason. They're good for retaining walls, fence posts, ect; but don't assume 50 years old, half rotten cross ties are equal to new lumber.

I'm not trying to talk you out of a bridge. Just be careful with That plan. Really if you want a bridge, you could do two nice footers on either side, keeping span to say 48"; and use something like 4x6 pt as the deck. Of coarse, do some quick load calcs.

 

[RedNeckRacin]

I'm personally going to vote against using wood for your bridge beams. you say small truck but that's open to a lot of interpretation. Are we talking a one ton dump that could weigh 15,000lbs or are we talking a tandem that could weigh 54,000lbs? Personally, if your not going to use steel for the main beams, I think your going to have a tough time talking anybody but your friends or yourself into driving over a wooden bridge loaded. Have you looked at getting an old semi trailer?

 

[bcp]

I believe that #1 grade ties are the best of the landscaping grade ties. They are not good enough to be reused as a RR tie. I wouldn't use them for a driveway bridge.

A truck on a tie bridge would be supported by only one or two ties per side. Ties not under the wheels wouldn't make the bridge stronger, on a simple beam bridge design.

Bruce

 

[bcp]

I'm personally going to vote against using wood for your bridge beams. you say small truck but that's open to a lot of interpretation. Are we talking a one ton dump that could weigh 15,000lbs or are we talking a tandem that could weigh 54,000lbs?

Or a fire truck rushing to your house? Some fire departments or counties will examine private bridges to determine whether they would drive a truck across.

Bruce

 

[KennyG]

Here is a link to the standard wood beam table.

www.awc.org/pdf/WSDD/C2B.pdf

You will need to determine the actual clear span you need to bridge. Then you can find the capacity of beams. I usually use Fb=1000 for treated lumber. I vote against the used ties. You have no way to determine actual strength.

As an example, the table says a 2x12 spanning 8 feet will support 2636 lbs. If you want to support 10,000 lbs, you would need 4 of those. You need to design with blocking to keep them from rolling and it's always good to throw in a few extra for safety, given the relatively low price of treated lumber. If the design has the beams relatively close together, you can use 2x lumber for the deck also.

 

[Egon]

The railroad ties are not dimensioned correctly for bridge beams.

 

[2458n]

Farmer next door has a real Mill stream on his place. Mill is long gone but the stream still takes water off the river and runs across his property behind a barn etc. Anyways his grandson starts stockpiling concrete pipe of different sizes,along with a bunch of busted up slabs of old concrete. He ended up with a twelve foot wide crossover. Has withstood the river flooding, and some large loaded logging trucks. The old concrete and steel beams never would have handled the new traffic. Only thing it will see this summer is hay equipment.

 

[beeforty]

Or a fire truck rushing to your house? Some fire departments or counties will examine private bridges to determine whether they would drive a truck across.

Bruce

That is very true! Our local FD had a neighbors address 2 parcels away marked as "DO NOT CROSS BRIDGE" on the bulletin board, sure enough the house caught fire, they pulled up to the bridge and started dragging out hose. They were able to keep it from spreading up the hill but the house was a total loss. I will say that the bridge was one that I would be careful about walking over at night in the dark. Here in Santa Cruz Co. a bridge for a residence need to be certified for a minimum of 25 tons for fire trucks.

 

[LD1]

Here is a link to the standard wood beam table.

www.awc.org/pdf/WSDD/C2B.pdf

You will need to determine the actual clear span you need to bridge. Then you can find the capacity of beams. I usually use Fb=1000 for treated lumber. I vote against the used ties. You have no way to determine actual strength.

As an example, the table says a 2x12 spanning 8 feet will support 2636 lbs. If you want to support 10,000 lbs, you would need 4 of those. You need to design with blocking to keep them from rolling and it's always good to throw in a few extra for safety, given the relatively low price of treated lumber. If the design has the beams relatively close together, you can use 2x lumber for the deck also.

Those load tables assume even loads too.

While a loaded dual tandem truck may be more distributed, the front axle is not. And neither are any single axle trucks.

Personally I would want a minimum "point load" capacity 20T, which would be about 40t worth of distributed loading. (talking full sized trucks and fire trucks etc).

That would require 15 2x12's laminated together on each side under the tire tracks.

The 8x8's the OP first mentioned are good to about 5800# each for a distributed load. So a pair of them would handle most normal auto's up to a 1-ton truck but not loaded.

I wouldnt even trust 2 of them each side for a dump truck.

Have you considered steel beams?

A pretty common and not too heavy beam: W12x26 spanning 8' would be good for 20,000# point loaded in the middle. Add a second for the other tire track and you have a 20T point load and would likely handle anything you could think to drive across it. Including a fully loaded dump truck.

A 20' stick of this beam cut in half giving 1' over each footing, likely would cost ~$500 or so. Possibly add a third beam in the middle to help support some decking (which I wouldnt use 2x lumber. At least 3x or better).

Could also shop c-list. Not sure what it is like in your area, but I often see old beams from demo'd buildings or bridges for a few bucks more than scrap prices

 

[JeremyL]

OK, railroad ties are out. Actually the whole idea of re-building the bridge is out. No way I can engineer it for fire trucks and gravel trucks. A culvert is more practical and cost-effective. All the responses have been helpful in making this important decision. Now to plan for a pipe and quarry product.

- JeremyL