Sizing of Steel I-Beams for Bridge

[BB_TX]

Chris-I have been looking to do the same thing with a 30' span over a creek. I found a book on structural design that gives some tables to do some calculations. The basic formula is W=(1795*A*D)/L where W is the greatest safe load in pounds in the middle of an I beam, A is the cross sectional area of the I beam in sq. in., D is the depth of the I beam in inches, L is the length of the beam in feet. Here are some typical I beam numbers;
8" I beam (18.4 lb/ft) A = 5.34
10" I beam (25.4 lb/ft) A = 7.38
12" I beam (31.8 lb/ft) A = 9.26
(these are the lightest weight beams for each height. you can get heavier beams with greater A)
If you run the numbers for a length of 16', then you see the greatest safe load in the center for;
8" = 4792 lbs
10" = 8279 lbs
12" = 12466 lbs
That load would be any materials that went into building the bridge plus any weight you put on the bridge.
I am not a structural engineer so I am not sure what all else should be considered. But it is a starting point. I am still working on it.

 

[rtimgray]

Just be careful when someone mentions using a 12" or 16" or whatever denomination I-beam, because depth is only one factor for the beam.

I-beams come in (at least) three different sections: The regular I-beam that has a relatively narrow flange section (i.e. about 4" for a 10" tall beam); these are called "S" beams - S meaning Standard. The next is a wide flange section (about 10" flange for a 10" tall beam); these are called "W" beams - W meaning Wide Flange; There are also "M" beams which look like wide flange beams but are actually used for vertical pilings - I really don't know what the difference is.

W-beams will span a much greater distance and support a much larger weight than their respective S-beam counterparts.
Another big factor is WPF or weight per foot; all steel beams are available in many different weights per foot, reflecting relative strength and span ability; of course, there are common and uncommon weights, common being cheaper.

What you want to do can certainly be done, but just be careful because a steel beam is not a steel beam is not a steel beam.

 

[Gatorboy]

Bill,

Thanks for that link -- there is alot of useful information to be had.

 

[dummy]

These guys Corps of engineers are supposed to be the ultimate authority on bridges. Send them an email and see what kind of response they give you. Not sure if MDE will be more help or hindrance. While you're designing, how about one like the attachment?

 

[Gatorboy]

I am staying away from the Army Corps of Engineers (ACE). They are the ultimate DELAY and RED TAPE machine.

One thing I have learned is to design a bridge that doesn't touch the water -- that way the ACE doesn't get involved. From what I hear, getting a permit from the MDE is a cakewalk over getting one from the ACE. In addition, if I design it myself to be built by myself, the MDE is more lenient than if some Engineering firm submitted the plans.

I guess when I get to that stage, I'll let you all know how I make out.

 

[Egon]

Usually The section modulus required for the beam choice will determine the type of beam chozen or specifically designed.

Egon

 

[yooperdave]

Is hiring a licensed Structural Engineer considered a bad thing? /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif

Someone with a bigger tractor may also wish to use your bridge someday

Considering the importance of this structure, soundness and safety should be worth a few dollars. There are many details that if not accounted for properly will severely limit the best of intentions.

Good Luck

Yooper Dave

 

[MarkV]

DocHeb,

Here is a low-tech way of thinking about it. For a short span like 16' and the 5000 lbs weight limit you mentioned take a look at a 12,000 lbs equipment trailer. Seems to me that the one I was looking at was built with 6" channel so I would sure think an 8" I beam would be more than enough.

MarkV

 

[DKinWA]

Obviously engineers have their place just like attorneys, but I don't want to spend the money to hire a structural engineer for a small utility bridge. As a biologist I work with a lot of engineers when permitting stream crossing structures for the state and most of them aren't cheap. When I was looking into designing a small bridge, the engineering costs were more than the materials for the bridge. This was to be a simple short span bridge to drive my pickup and tractor across.

I know there are a lot of variables involved in bridge design work, but I really don't believe that it's that difficult to design a small utility bridge. Most of the bridges folks are talking about are short single spans that don't involve piers or piling or high traffic volumes, etc. I also think most of us don't want to just guess on a design, and we'd like to use sound design principles to design a bridge. When I've asked engineers in the past about this, I always get the run around about all the variables and safety ought to be worth hiring an engineer, etc.

I don't mean to imply that anyone can build a bridge and I'd be the first one to hire an engineer for any bridge that is used for anything but a low volume, light load bridge crossing. It just seems to me that simple bridges like this are a matter of determining the varibles and plugging them into known formulas.

"I'm not an engineer, but I did stay at a Holiday Inn once"

 

[Egon]

When designing something like a bridge always Put "Third Party Liability" at the top of the priority list.

Egon