Need to build a 25' tractor bridge across creek

[deanfitz]

Thanks everyone for the advise. From what I've learned I may come out better going with steel.
OK, heres what I have found out.
During flood periods the whole area(app.1/2 mile from river) is under water in extreme high river. Heavily wooded, so the current is moving slow...maybe 1 mph. I have found 2 used 39' I beams measuring 12"Hx6 1/2"W x.46"T @ 30lbs a foot. Price-$936.00 w/o shipping. Will haul myself from P&W Ind. 30miles away from me. I'm thinking concrete footers at 12'w x 8'd x3' thick= 8 yds cement(1 truck)=around here$690.00 P&W also has 4" angle iron reasonably priced.
I can maybe use creosote boards to plank since sheet/grating steel is high priced. I can burn anchor holes in I beams for plank boards before hauling to bridge site.
This is a COMBINED VIEW of top & side design to give you a idea of what I'm thinking.
http://i785.photobucket.com/albums/yy139/deanfitz/BridgeSteelPlan.jpg

All advise welcome, Dean

 

[deanfitz]

I have talked & shown the site to a superintendent at Boh Bros. Construction, he's in charge of a 35 man highway/bridge crew and has built many small bridges. He has recommended steel I beams, also there will be little debris passing under bridge because of its location. 99% of the time the bridge will be 6-8' above water.

 

[LD1]

I would like to know for sure the exact spam the I-beams will cover. There is a pretty substantial difference in load capacity of a 25' span vs a 30' span.

I think the beams spaced for wheel tracks with something along the lines of good RR ties would be fine provided the beams are strong enough.

I cant remember off the top of my head bit I think those beams you mention @ a span of 30' are good for a point load of somewhere between 2500 and 3000lbs, but don't quote me on that. That is pushing the limits of what you want to use them for. Ofcourse a 25' span would be a greater load capacity.

I know personally I would feel a lot better with a 14 or 16" beam in the 30-40 lb per foot range. That would give you a good bit of saftey factor.

 

[RickB]

I think the beams spaced for wheel tracks with something along the lines of good RR ties would be fine provided the beams are strong enough.

30' of railroad ties laid side by side isn't exactly light.

 

[LD1]

30' of railroad ties laid side by side isn't exactly light.

You are correct. He will need to figure how much each tie (or whatever he uses for decking) weighs. Figure total weight to do bridge divided by length of span and compute the distributed load accordingly.

 

[Transit]

Two beams, you are relying on the strength of the planking to support the load crosswise. Three beams or four would be a better choice. I would weld cleats to hold the planks; cutting holes in beams can severely weaken them.

 

[deanfitz]

Here is a Top View of the Steel Plan
http://i785.photobucket.com/albums/yy139/deanfitz/SteelTopView-1.jpg

 

[deanfitz]

The span will be app. 25' in length.
I think you are right Transit, We will use cleats or angle to hold down planks. By the way, the planks will be 4"x12"s pressure treated with track runners on top.

 

[LD1]

Do you plan on running the 4" angle the length of the bridge parallel to the i-beams?? If so, it won't provide much support spanning that distance. And only using 2 beams spaced ~5-6ft appart, the 4x12 PT wont provide much support of 5000-6000lb truck/tractor if you happen to veer a little off the beams.

Just an idea I'll throw at you here: Span is 25'. You plan on grabbing the two beams you mentioned which are 39ft long. You will have roughly 14' left off the ends of each beam. you could either

A) weld them together and space the beams only 24-30" appart that way you'll have one in the middle to help support the ties. or

B) cut them down and use them perpindiculat to the main beams. You should be able to get at least 4. Space them ever 5' and then use the angle between each of them. Running parallel with the main beams.

 

[MikeD74T]

Railroad ties are grossly excessive for your application. since the beam spacing is (should be) under the tires the extra weight of the ties simply decreases the capacity of the bridge. 2" or 3" rough lumber is very adequate. Logging bridges in my area that carry over 100K are 3x12" cross planks with a pair of planks 3x24" running lengthwise under the wheels. These are to protect the cross planks from the tire chains. 3/8" carridge bolts will hold the planks, drill rather than torch the holes. For our snowmobile bridges we run a 3x8" on top of the steel then fasten the cross planks with log cabin screws. Gravity does most of the work. MikeD74T