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Bridge Construction question

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Ron (Tex)

07-06-2005 11:37:59

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I was given a mobile home trailer frame that was 76 feet long. I plan to cut it in half so that I will have four I beams sections to place across a creek that is about 18 ft from one bank to the other. The frame is an I beam that measures 12 inches from flange to flange but is fairly thin guage steel. I would guess that the thickness is 3/16 inch.
Question: without any support in the center, how would one calculate the loading that can be safely placed over the center of the span?
If need to, I can weld sections of angle under the bottom flange to gain greater support.

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dr.sportster

07-07-2005 13:06:19

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Re: Bridge Construction question in reply to Ron (Tex), 07-06-2005 11:37:59

You better measure that thickness.It may be 9 gauge or10 gauge[no longer popular]9 gauge is .149" which would support a few bicycles.Cant you just get two used I beams from a demolition site?

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paul

07-06-2005 20:40:47

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Re: Bridge Construction question in reply to Ron (Tex), 07-06-2005 11:37:59

Dad built 2 bridges over the creek, I believe there are 8-10 solid heavy I beams on each, about 16' long. We run the tractors & combine over them, never bothered.

For the steel you have, & only 4 of them, I hope it's a walking bridge or maybe a 4-wheeler.

--->Paul

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Bus Driver

07-06-2005 18:27:53

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Re: Bridge Construction question in reply to Ron (Tex), 07-06-2005 11:37:59

One thing not mentioned by others is the need for bracing (in house construction we call it bridging) between the beams- any kind of beam. As the load increases and the beam begins to deflect downward, it also begins to rotate along the axis. If the rotation is controlled, the vertical sag is also lessened. It is critical that the ends of the beams be blocked from rotating- like the rim joist on a house. Intermediate blocking is aslo beneficial.

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Jon C -Florida

07-06-2005 18:47:26

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Re: Bridge Construction question in reply to Bus Driver, 07-06-2005 18:27:53

Bus Driver is absolutely correct.

In engineering lingo the "compression flange" (the top of the beam for simple positive bending) must be braced. Normally, but not always the decking acts as a top flange brace PROVIDED it is welded or otherwise firmly connected to the beams. Cross bracing, or welded "diaphrams" made up of channels welded to angle clips inside the webs of the beams is recommended at perhaps 4 to 5 foot centers.

Like Bus Driver says..its to prevent rotation or twisting of the beams and it will allow full design loads to be carried.....otherwise they will fail by buckling at much lower loads.

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DanOh

07-06-2005 17:36:42

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Re: Bridge Construction question in reply to Ron (Tex), 07-06-2005 11:37:59

First you need to determine what you want to drive over this bridge (i.e. car, truck & trailer, delivery truck, concrete truck, etc.) Then you should talk to an engineer (registered in your state) to do some design for you.

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Galen

07-06-2005 17:07:57

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Re: Bridge Construction question in reply to Ron (Tex), 07-06-2005 11:37:59

BIL built an 18' trailer out of MH beams, so he could haul around his yuppie tractor. You can see the trailer bow when he drives the yuppie on and the chains pull the sides in. Cheap CR*P. Go with something more substantial.

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RayP(MI)

07-06-2005 15:38:23

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Re: Bridge Construction question in reply to Ron (Tex), 07-06-2005 11:37:59

Used to work in a mobile home factory. Homes were constucted on an assembly line crossways of the conveyor. During lunch break, one of the guys liked to stretch out on the carpeted end of a trailer for a few zzzzz 's. Usually chose one with only frame and flooring applied. Had another wiseacre who loved to take a flying leap landing on the far end of the frame. Literally launched the sleeper! Those frames are so flimsy, I wouldn't consider using one for a bridge for a lawn tractor. Only because of the rest of the structure are those trailers even road worthy.

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geok

07-06-2005 15:23:42

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Re: Bridge Construction question in reply to Ron (Tex), 07-06-2005 11:37:59

If you are determine to use the m/h frame, you might consider reinforcing them with a back bone. A flat strap 3/8 X 3"welded to each end going under a 6 inch spacer in the middle. They use this method to reinforce long Gooseneck trailers.

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old

07-06-2005 15:14:36

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Re: Bridge Construction question in reply to Ron (Tex), 07-06-2005 11:37:59

I use a beam like that in my shop for the chain fall. I lift 2000lbs plus with it all the time. But for a bridge and haveing an 18 foot span I'd put a center post in to make sure it would go any where. Also the more beams across the better off you will be. I use them beams to build a lot of my sheds out of and just love the way the work for that.

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Jim@concordfarms.

07-06-2005 12:19:31

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Re: Bridge Construction question in reply to Ron (Tex), 07-06-2005 11:37:59

I don't think it would be safe. Mobile homes are very heavy, but the weight is very evenly distributed. The manufacturers don't make them any stronger than is necessary. If you built your bridge and there was an accident, you would be open to all sorts of liability suits and your insurance company would probably disappear. I'd go with a heavy duty flat bed semi trailer. Jim.

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Dell (WA)

07-06-2005 11:59:26

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Re: Bridge Construction question in reply to Ron (Tex), 07-06-2005 11:37:59

Ron..... ....round here, people like to use old Rail-Road flatbed cars without the expensive RR trucks (wheels). Check yellow pages or little nickle wantadds. Also old flatbed highway trailers (without expensive rear axles) are also good. Both of these "alternatives" make STRONG ready made bridges.

I wouldn't mucka'round with wimpy ex-mobilehome frames 'cuz you'll put more money into up-strength than you'll ever save in iron framework of a proper bridge deck..... ....Dell

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Truck

07-06-2005 11:45:00

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Re: Bridge Construction question in reply to Ron (Tex), 07-06-2005 11:37:59

Speaking from experience with m/h framing used to build cheap trailers, I think you are going to be sorely disappointed in the load capacity.

Best bet is to go find some used I beam or H beam and build a bridge that will outlast you and your kids.

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T_Bone

07-06-2005 12:27:07

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Re: Bridge Construction question in reply to Truck, 07-06-2005 11:45:00

Hi Ron,

I'm not engineering your bridge!!!

But W18-35 will support 50k/lbs with 18ft bearing points and for three 18ft beams at todays cost, $1040 new.

Still wanna use mobil home I-beam?

T_Bone

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