Sizing of Steel I-Beams for Bridge

[Green_in_MI]

Do the right thing and hire an engineer-safety is paramount. Having said that though, I would not even think of giving you internet advice on the construction because there is a LARGE chance in this scenario for an unfortunate ending. I noticed other posters said engineers will give you the run around when it comes to advice. They have a class in that subject called Confuse-em 101. /forums/images/graemlins/confused.gif The fact that even your supplier won't tell you anything ought to give you a big hint. Would a doctor provide even the slightest of do-it-yourself surgical advice over the internet?

 

[DocHeb]

<font color="blue">Just as a surgeon was finishing up an operation and was about to close, the patient awakes, sits up and demands to know what is going on.

"I'm about to close the site," the surgeon says.

The patient grabs the surgeon's hand and says, "I'm not going to let you do that! I'll close my own incision!"

The surgeon hands him the thread and says, "Suture self".
</font>

Do-it-Yourself Brain Surgery

Where There is No Doctor


It isn't the Confuse-em 101 class that makes folks reluctant - its the lasest edition of the book "Bridge-Builders Litigation for Dummies" that sits on every trial lawyers shelf. The bridge I want to build will take five steps to go across and will be four feet above the water. Without a structural engineer's input, it might get washed away in a 100-year flood, it might not survive earthquakes, it might tilt a little over the years from settling, but its just a little bridge. I figure I can't do much worse than the Michigan Department of Transportation, who's bridges are crumbling all around our state.

Actually, the best Internet sites for the most accurate medical information are eMedicine.com and WebMD. Neither, however, contain much information on Do-it-Yourself surgery. /forums/images/graemlins/grin.gif

 

[knucklehead]

Chris - what do you think about that second book you linked? You may PM me or reply here - I was one of the negative reviewers after reading the book.

I started a reply regarding your bridge, but got busy here at work and deleted it. I think since you are not building this for your main driveway, you don't really have to worry as much and can get by with common sense. I was going to get into why copying a trailer isn't the best way to go, as the trailer is more like a midspan supported (spring perches and axles) cantilevered structure, but I agree with the idea behind the suggestion. Around here the logging road crossings are bridged with two main beams of various material (steel, really big timber, etc), then a cross work of cants like 4x4's or 6x6's, then decked either with the same 4x4 material or planks. I was mostly going to suggest you do the same. It is a lot like a trailer, or a flat bed, in construction: two main beams, cross supports, and decking. I had a sketch but I have to wait to get home to scan it in. I think you can visualize, though. The other thing I was going to suggest is putting rub rails on the outer edges to keep the tires on the deck.

I like the span calcs BB_TX did but wonder if there is any safety factor in that. If not I'd go for the 12" beam for a 2x factor; it's nice he spec'd which beam with the weight per foot. That seems to be how they are catalogued. My whole perspective is that if cost is not greatly increasing with size, go for the next bigger size. You only have to get into serious calcs if you are trying to really optimize. I'd get an idea what the cost was shaping up to be before I tried to optimize. 16 feet isn't a big span between supports. You could probably even accomplish that with timber if you wanted to.

My biggest concern with your initial post was that you were intending to deck the beams directly, apparently with the 3" ash running across the beams. That seemed a little light, since you'd be limiting yourself to white-knuckling it across each time trying to stay on the beams. Put one end of a piece of that ash up on a cinder block and drive up on it with the tractor, and you'll get an idea of what it will feel like to get off the beam. You may want to go with the cross pieces and deck over with the ash. I'd space the beams out 6 feet if I used the 4x4's, too.

 

[BB_TX]

Mark- Here is what I am thinking for a 30' span. I will buy four 8" I beams(18.4lb/ft) x 40' long. I will cut each 30' long leaving four 10' pieces. I will space 2 of the 30' sections at approximately the rear tread width of my tractor. I will place the other two on the outside so that the total width is 8'. Then I will equally space the four 10' sections under and perpendicular to the 30' sections. I will weld the 10' sections to the 30' sections. I would also have some cross bracing to prevent tilting of the beams. That way the tractor load will concentrated directly above the inner beams but be shared across the four beams. I will deck it with 2x6 treated lumber.
By the calculations above, a 30' 8" I beam safe middle load would be ~2500 lbs. With four of those, it should be adequate for my ~3500 lb tractor + the weight of the building materials. I would prefer the 8" beams because of the total weight to work with, but I may consider 10" beams. A 10" beam would boost the per beam load to about 4400 lb. Or I could go with a heavier 8" beam.
I have used the lowest weight beams for calculations since I believe those are common stock items.
Opinions??

 

[MRHURT]

Well i might as well throw my hat into the ring as well, from the sounds of what you are looking to do with your bridge, you are under what i wanted to do with mine. What i calculated for mine was i used W class i-beams they are 12" tall and 26lbs per ft. The catalogs would list these as W12 X 26 i-beams. Now on a 20 ft beam, if i remember right you want to go 16 ft, mine in theory will hold 11,000lbs.
Now before anybody gets excited over that figure, that is before safety factors, etc. My 2 cents would be that two i-beams like these would do just fine for you. I have no idea how much steel cost in your area, but i paid a little over $150 for each of my i-beams. i didn't think that was two bad.

 

[kensfarm]

I was wondering if anyone has just gone out and looked at bridges that are already built.. similiar to the dimensions/length that you want to build. Take your camera and measuring tape.. and check out/measure the structures they've used.. could save a lot of time.

 

[knucklehead]

Bill - I'm not a bridge designer; so I'll avoid going as far as giving you the online thumbs up or down on your beam plans. I have knowledge of the fundamental principles and calculations involved in this type of problem, and experience more on the wood framing side of building stuff. I'm personally thinking about beam specs in garage or barn floor applications. I sketched your description, and as with my reply to Chris, I think the thing you guys want to keep in mind is weight distribution. I will say that tying the span beams together, if done well, is certainly going to increase stiffness and distribute weight somewhat. I still have concerns with the decking.....

As Chris is also apparently thinking, it sounds like you are planning to deck directly across the beams. That means you will have a PT 2x6 spanning the tread width (5 feet?) on it's flat side, right? That's probably gong to be a little whippy - as I said with Chris, do a little back lawn science - set a piece of your decking material across blocks set 5 feet apart and jump on it with just your body weight. But not too hard! Even though the plank is fastened to the beams, it is still just one piece and it will move independently of the rest of the planks. Only if you used tongue & groove stock would you get any sort of weight distribution over the entire deck.

Going even spacing on the four beams still only gives you 32" spacing to support your decking. You wouldn't do that in household construction, much less for a shed or barn - why on a bridge? I wouldn't count on staying over the beams. I'd add cross pieces of 4x4 16" on center and deck over that for better weight distibution on the deck planks and on the beams.

You also need less beam if you spread the load. I'd spread the load with 8" beams; I haven't the time or desire to dive into beam calcs right now as I am on nights and kinda crispy in the cranium, but my general comment is the same as to Chris, check the prices, and buy more beef. If you have to squeeze the design, get someone who specializes in this. I think you're headed in the right direction in your thinking. You could actually weaken the beams if you are not careful welding, especially since you are working on the tension side (bottom) of the beam. You are adding more torsional stiffness than along the bridge length, also.

I hope someone who builds with steel will add their expertise. Maybe those comments about liability are keeping the guys who "wrought" stuff out of steel silent, huh? /forums/images/graemlins/grin.gif

I gotta go get some more sack time....you sure you want advice from a burned out nightcrawler?

 

[rambler]

For the original poster: To avoid paying the engineer, buy 4 I beams instead of 2 (maybe an extra $300?). Using the 4 beams, you double your strength, plus when your whimpy wood weathers & breaks through, the inner beams will catch your tractor. I would _not_ want a 2-beam bridge as you describe. Bigger beams get too heavy to handle, while 4 will help your cross planking span issue. Buy 4, it's not much extra $$$ & will give you _much_ piece of mind.

For BB_Tex with the 30' span: Yikes, that gets too long to engineer yourself! No way would I walk across the thing with 2x pressure treated lumber. You need real bridge decking for something that size. Pressure treated 2x pine is in no way, ever, good enough for a 2' span between your beams. It's out there in the weather - it will not last & keep it's strength. Heck most house decks are required to have 16" spans or less, and that's not for vehicle traffic!!!!!

I do like the 4 beam idea. I'd be careful welding the cross members tho - you can greatly weaken an I-beam by welding across the I and setting up a stress line that will snap.

I'm a simple dirt farmer, so anything I say here really is of no help to anyone, and should not be given any weight. I don't know what I'm talking about.

Dad made 2 bridges across our ditch, 4" planks across 8 big I-beams that he bought for scrap. Bridges are about 16' long, 16' wide. Handles the loaded combine fine, I don't believe dad ever checked with an engineer.

Now, for those of you who say your bridge will only handle your little tractor - well, these bridges also took the fertilizer spreader truck, whick is _way_ more load than we ever planned for. The guy never asked, he just drove across. Kinda shocked us, but it worked - bridge held.

Don't build a whimpy bridge for 'just my little tractor' as we all _know_ something bigger is going to cross them.

--->Paul

 

[rtimgray]

Have you considered going with a concrete deck for your bridge? It will be (obviously) considerably heavier in dead load than a wooden deck, but with proper reinforcing it will add alot of strength to the bridge and weather will have little effect on it.

We (me, brother, dad) built a bridge spanning the creek going into mom & dad's property back when I was a kid (about 20 years). The span is about 20'. We started with large spread footers dug out in the creek bank below the running depth of the creek, came up with retaining walls (I think about 2' thick) and placed seven - 16" wide flange beams (don't know the weight - they were used, but I don't think it mattered with that many of them).Then we put on a steel deck and a couple of rebar mats and 6" concrete deck. It has handled fully loaded concrete trucks, dump trucks hauling dozers and backhoes, all kinds of tractor traffic, etc. It has wored well with almost no maintenance (after a few years with some real gully-washer storms, we had to go down in the creek and pour some additional concrete around the footers where the creek had eroded the surrounding earth).

I'm not sure if that is a viable option for your span, though. Just a thought. Good luck.

 

[DocHeb]

Actually, I have thought of using a Spancrete Panel, but then I'd have to have a crane available at the site. That isn't feasible.