Building a bridge to cross stream

[FlaDon]

A number of years ago I built two bridges from flatbed rail cars. I believe they were 10' wide and 40+ft long. The rail cars were about $3000 delivered about 70 miles. One we picked with a juice boom (crane) and one was skidded with two D-6 Cats, one shoving and the other winching from the opposite bank. One had poured concrete abutments and on the other I used discarded/surplus highway center barriers the county was selling cheap, bedded on compacted road base then backfilled to within a foot of the top. One bridge used 4x12 pressure treated plank and on the other I used 5" thick concrete running plank (not full width because of weight).

The bottom line for the bridges were in the range of $6000 each about 20 years ago. We put loaded concrete trucks and loaded logging trucks across these bridges. The rail cars are very stout (and extremely heavy...to move and place).

I may be able to find a picture and there is a slim chance I still have sketches and calculations squirreled away somewhere. I'll look if you are interested.

Railroad flatcars are built heavy as they are expected to last 40 years in rail service. Not only do the have to support the loads on them but also must handle compression and tension loads lenghtwise. Railroad passenger cars which have less end to end loading normally built in the Budd plant withstood 1 million pounds of compression with only 1/8" of vertical movement in the center of an 85' car.

 

[tree grower]

Anything about building bridges catches my eye. I have built a few--lots of fun. I am concerned about your proposed use of steel grating for a deck. If you build it correctly, the deck can actually add to the carrying capacity of the bridge. My biggest project was a 47' span using W24 x 180 beams 8' apart, and a 14' wide deck of 4 x 10 PT on edge. The 4 x 10s were spiked together every three ft using 12" spikes, so each spike went thru 3 beams. They didn't quantify the amount, but the engineers said that such a deck helps to spread the load. We put 100,000# log trucks over it, and it doesn't budge--over 1,000,000 board feet so far.

Your steel grate will not spread the load. See if you can find some rough-sawn hemlock 2 x 8s. Lay them on edge and use 20d spikes every 2 - 3 ft.

 

[spo307]

most likely use 3 x 10 pt lumber, when you say on edge u mean standing 8" up??

 

[LD1]

My biggest project was a 47' span using W24 x 180 beams 8' apart, and a 14' wide deck of 4 x 10 PT on edge. The 4 x 10s were spiked together every three ft using 12" spikes, so each spike went thru 3 beams. They didn't quantify the amount, but the engineers said that such a deck helps to spread the load. We put 100,000# log trucks over it, and it doesn't budge--over 1,000,000 board feet so far.

Just for fun I wanted to plug your bridge into beam boy to see the results. But beam boy doesnt list heavier than a w24 x 104 beam. So I had to search online to get the moment of inertia but I still couldn't find a 180# beam. Biggest I could find was a W24 x 164 with a mI of 5170.

Given your span of 47', the max allowable deflection is 1.56". Putting 50,000 lbs smack dab in the middle @ 23.5' only deflects that beam 1.2" with a bending stress of 16.7ksi. Well within the limmitations of the beams (assuming you only have two) even without the deck to make it stronger. And to boot, driving a 50 ton truck over it would never put 50000 lbs in the center of one beam, due to the length of the trucks and multiple axles. I'd say you gould probabally drive a 80 ton dozer over it and not have anything to worry about

 

[Danno1]

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You can put multiple loads on the beam in BeamBoy to simulate the actual axle loading.

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[LD1]

You sure can. You can do multiple point loads as well as evenly distributed loads. Along the lines of 100lbs per ft all the way across to take into account grating or the road surface

 

[LD1]

Here are some pics of a beam. It is a W24 x 164 beam. 47 ft long, 45000lbs @ 10' onto the bridge and 45000lbs @ 40 onto the bridge, with a evenly distrubited load of 200 lbs per ft. (which would total 9400 lbs)

The first screen is where you enter the load and support info and the second is the graph of everything calculated

 

[tree grower]

The bridge I built was done nearly 15 yrs ago, so the exact details of the beams are a little foggy, but the Beam Boy calculations confirm my own observations--there is nothing we plan to put on the bridge that will stress it. Also noteworthy is that the dry-laid stone abuttments that were in place were easily a hundred years old, and had not supported a bridge for many decades. Though subject to flooding at least twice a year, the engineers judged them to be sound and stable, so we just poured a concrete cap on them to support the beams.