What size I-Beam to span a small creek?

You need to specify the max load that will cross this bridge - is it limited to cars, tractors, tractors with trailers, trucks? In addition, what is the wheel base of these vehicles?

From the above information, you can [I assume NO formal inspection & handover by local or government agencies here i.e. purely agricultural engineering!]

1. Set a factor of safety - say 2x

2. Assume point / uniformly distributed loads at the points of contact with the tyres - worst case when the vehicle is mid span

3. Allow for uneven distribution between the one set of beams - a wheel may not be dead centre of the two beams in parallel.

5.Calculate the required Z value [elastic modulus of steel]

6. Choose the section .

The maximum width of any vehicle crossing the bridge will be 8' 6". The typical traffic will be pickup trucks, cars and an occaisional tractor-trailer combo. The heaviest load that I ever anticipate will be cement trucks filled with concrete. Such a load will be approximately 72,000# spread across four axles (one a tag axle). Thanks Hammer ! George

ftp://ftp.dot.state.tx.us/pub/txdot-info/gsd/manuals/des.pdf

For design of this simple span bridge do you consider any floor planks or a concrete deck?

The wheel loads are considered moving loads passing throughout the bridge span. Consider the heaviest vehicle that will be allowed to pass and then determine the equivalent wheel loading from the AASHTO Specs. Each wheel will have a designated concentrated load P differing from wheel to wheel. If I can remember it right, for a three-axle wheel loading the front will be smallest and the second and third will be same value.

With that wheel loading determined, position each of the wheels to pass through the mid-span section of the beam, and then calculate bending moments for each positioning. The design moment will be the maximum of all the moments computed plus the maximum moment effects of the floor/deck including beam weight multiplied by a factor of safety (2.00 to 2.50 verify code requirements) for damping and/or vibration.

And when your bridge is already completed and operational, remember to put a LOAD LIMIT SIGN.

Hope this helps.

Honestly, employ an engineer to design the bridge. Your liability, should the bridge fail under a vehicule that is not yours (e.g. a cement truck), is quite extensive. Imagine the consequences.

Secondly, the advice given so far neglects the lateral torsional buckling of the I beams - 24 foot is a long way for an unstabilised I-beam - and also the deflection criteria - again 24 foot is a flexible length.

Thirdly, you need to consider the supports at either end, that means the supports themselves and also their foundations. If you are spanning a creek then the supports are likely to need some careful thought.

Don't do it yourself. Get an engineer.

If cost is an issue, and you say you intend to extend the I-beams beyond the end-dams, then you could contemplate this idea:

• Use an end design that converts the I-beams forming the bridge into CANTILEVERED sections from each side.
• Tie DOWN the BACK-END of your I beams at the top by using a section (perhaps steel cable) in TENSION, and also secured at the bottom of the end-dam
• Where the unsupported section of the bridge starts, use a vertical section in COMPRESSION, which also is secured at the bottom of the end-dam
• For a given bridge loading a cantilevered design could be the lowest cost option
• Encapsulate end steelwork in concrete to protect it

Do you have to have a bridge or can you use large concrete pipe. With a Bridge you will have much maintainance down the road.

Try getting hold an old 40ft flat rack shipping container (one of the containers that does not have any sides to it just a deep base, I have used these on many sites as bridges for typical sites loads of above 15tonnes per axle line. You would need a crane to set it, which is costly.

Another option would be to purchase a couple of surplus 30' bridge cranes in the 30T class. WHen we upgraded our shop, we had to give away the old 30T x 96' span crane, as we could not locate a buyer. You can likely sell the trolley/hoist assembly and keep the bridge beams.

Thanks for all the GREAT Suggestions ! NeilJ, I like your cantilevered suggestion particulary because the beams will extend past the concrete end-dams on each end. For clarity, the end-dams have been dug down to bed rock and will be poured with steel and re-bar. I think I understand what you are suggesting for the land-side of each end-dam. In cross-section, are you suggesting that a cable or steel will extend diagonally downward from the end of the I-beam to the base of the end-dam? I am confused re what you are suggesting for the unsupported section of the bridge. Are you suggesting having a steel beam angle downward from the center of the bridge to the base of the end-dam? Thanks NeilJ ! OMW7, Your advice is well-taken. I plan to see how far I get, then will have a friend take a look. Thanks for watching my back ! Thanks to all that have responded. Sincerely, GLD

Thanks GLD,

I hope you can see the simplified CAD sketch OK.

I think you got my drift... I was not clear on your end-dam details so I've left them out. The section to the right of upright A, and to the left of upright B is the unsupported section. So the two uprights A and B would be in compression from normal bridge loading PLUS the compressive load due to the tension in cables, shown here at 45 degrees. The overall effect is a pre-biased BOW (and two arrows) bending the bridge upwards, resulting in the two cantilevers facing each other.

Hope this is clearer. My gut feel is that after all the calcs are done, there should be less steel for a given load required.

Regards, Neil

Thanks Neil !! George

I've seen a number of bridges like this constructed from (surplus) flat-bed railroad cars, minus the wheels and trucks.

Get an old flatbed train car, we did this in Colorado years back, worked great. Otherwise, get an engineer or you will be sorry.

Lots of votes for the RR car. Any suggestions re where to find one ? Thanks, George

Why try to shade tree something like this? Someone could be killed! No qualified licensed professional engineer in his right mind would offer you a solution on a blog site. If the thing collapses with a loaded concrete truck valued at $250,000 + the liability of a personal injury do you really want to defend yourself by explaining that you went online so you could do this for cheap?

The only way that works is if you are judgment proof - i.e. financially worthless. If you have anything to lose, protect yourself. I don't care where you live - the stupid tax is really high.

Actually pay someone to do this right. If you cannot afford to do this right then sell the property and buy something you can afford to maintain.

That's my professional advice.

Bob, Your advice is well-founded and I will not proceed blind. The reason that I have posted this question is that it is very interesting to learn more about the subject before I simply hand it off. Also, the creek is very small. At the highest point, the bridge will be 4' above water, and the water is only inches deep after a rain. Re cement truck value, I will use a cement truck owned by my company, a truck that works very well even though it only cost $5,000. Thanks for watching my back ! George

I think the engineering has already been done on concrete pipe

A 24' span is a significant bridge. What you are describing is harder than you imagine. Get a structural engineer. If you want to save some money, ask him to design it for a temporary mid span support, like timber bracing. Add that support when the the concrete truck is to pass over it. I've seen this done a lot on rural roads when the bridge can only handle 8 tons or so and a contractor needs to take a bulldozer across. They would simple place timbers under the bridge for a few hours and cross without incident. Smetimes they would place then as close as 3' center to center.

I hope this helps.

Hire a Civil Engineering firm. It is likely that there are more issues to be considered, like potential effects on waters of the US, and State Environmental Protection. Just crossing a 24' wide stream could cost you more than the property is worth (I know of a guy who installed a 5' high dam in a small ephemeral stream, after the fines, removal of the dam and repairs to the stream system, it was estimated to cost nearly 3/4 of million dollars.) Also there are foundations to consider. What will the foundations to support the bridge look like and will they be adequate. Structural loading and adequate protection of users to be considered, and liability. You not only could be fined, but if someone was injured and the bridge was not engineered or quality assured, you could go to jail for many years. (Better to let the structural engineer go to jail, if he fails to properly design the structure.)

someone can get hurt from just walking thru your property and sue you. This is why you have choice when crossing these type bridges. Go with the flat rack ocean container they will carry 80000 lbs. or if you can get a rail car flat they can carry 200000 or more. The main thing is make sure you have solid concrete end mounts for these bridges. and POST your bridge with a Caution!!!. There are thousands of bridges in the usa rated at 40k and 80k trucks used them every day. Changing water flow and building dams is a different story then building a small bridge on private property. Dams affect property above and below your property but a bridge is confined to your own property.

Not always, it could be navigatable waters, environmental issues, wildlife issues and insurance considerations. It is true people can get hurt any time but if you are negligent then that becomes a different story. We cannot live in fear but we must be diligent. The States may be different but even on private property we need 6 permits to construct a bridge. Check first.

Opps!

I forgot to mention that it is extremely expensive to buy "NEW" steel I beams. My recommendation is to search for "USED" I-beams by contacting salvage and recycling companies and scrap yards especially aouund major cities that are demolishing old stuctures to make way for the new. The "old" I-beams are "REALLY STOUT". Just remember the taller the beam the stronger. Also remember you should contract a professional welder who has some experience with beams and not just some guy who can weld etc. You must cross brace your I-beams so that they do not flex. Talk to a steel fabricator and ask around. Get two to three opions and take an average.

And again, that is all I have to say about that. I am no expert.

We built a bridge using four 60' x 5/8" web with a 12" x 3/4" flange I-beams which were 30" tall to span our creek. We used galv steel decking and poured 6" of reinforced concrete on a 13' 8" wide deck. A bit fuzzy on the specs of the two concrete abutments dug into the creek banks and protected by concrete and rip rap but to say the least the abutments were significant and set back from the force of flood water and high enough to clear significant moving flood water.

This bridge can carry cement trucks, gravel trucks, propane trucks, trucks with dozers on it. The times that I take my 4x4 one ton across it I cannot even get the bridge to vigrate (jump up and down yadda yadda yadda).

We never did the math, but seems that it is working for us.

Spent a lot of time in libraries trying to research bridges and did not come up with much other than it is better to span the waters than to place pilings or piers in the creek bed. Pilings are subject to "scour" or undermining from flowing water and a BIG PROBLEM. Who wants to have to re-build a bridge? So forget about pilings in the creek bed and go for "free span".

Another comment about "free span" is that this is the better way to go as I believe that there may be regulations concerning pouring concrete into creek beds or should I say "Fill" in to creek beds (pilings) as opposed to setting foundations on dry land and spanning a creek.

And that is all I got to say about this.

Opps!

I forgot to mention that it is extremely expensive to buy "NEW" steel I beams. My recommendation is to search for "USED" I-beams by contacting salvage and recycling companies and scrap yards especially aouund major cities that are demolishing old stuctures to make way for the new. The "old" I-beams are "REALLY STOUT". Just remember the taller the beam the stronger. Also remember you should contract a professional welder who has some experience with beams and not just some guy who can weld etc. You must cross brace your I-beams so that they do not flex. Talk to a steel fabricator and ask around. Get two to three opions and take an average.

And again, that is all I have to say about that. I am no expert.

Hello. Thanks everybody for great information. I'm noting the above successful bridge specs. Can you clarify for me these specs. Are you saying 60 feet long? With a 5/8" web, and a 12 inch wide beam, 3/4 of an inch thick? And are you saying this beam is 30 inches tall? As in almost a yard? These are huge beams. I've seen much less of a beam carry all the loads you speak of. But I like the over kill of yours. Can I ask something else to everybody on here, thousands of people need this basic bridge. We seem to all be trying to cross a creek the same size, the same distance off the water, and driving the same loads (concrete trucks) across these bridges. Why isn't there a "boxed" solution to this. I would fully expect to find plans, fully spec'ed out for purchase. Beam loads are a given, loads are a given, and footers could be specified to exact design. Can anybody speak to this? Thanks.

<...cost is an issue...>

What is the cost of not doing it (rhetorical question - NNTR)?

CR4 doesn't really get involved with money. Commercially sensitive information varies with so many factors and is best kept concealed.