Bridge from Mobile Home Frame?

Question

Bridge from Mobile Home Frame?

KenB2N (Computer)

(OP)

17 Aug 03 22:42

I need help determining if a mobile home frame that I have will be sufficient for a private bridge on my property.  I know nothing about bridge design, and I am having trouble finding folks that are knowledgeable on the subject.  I have a creek that I am trying to cross on my property.  The creek span is 40 feet.  Usually there is little water in the creek, but the creek bank is about 5 feet deep.  The trailer house frame is 46' long, so I know that it is long enough to go across the creek, but I don't know what type of load capacity it would have.  I would probably deck the frame with steel plate, but I have not decided anything yet.  I may find that this project is too difficult for the average joe.
The I-beams (2 x 46' long) measuere: Width= 2.5" Height: 8"
The thickness of the I-Beams are 1/4" thick.  The I-Beams are about 77" apart.  About every 46 inches there is a connecting truss of 2.5x1.5 angle iron connecting the two I-Beams.
What other information is needed to calculate the load capacity?  I would like to be able to walk safely across it, but I would also like to know if I can run a 4-wheeler across it safely.  Can someone please help me?  Thanks.  I can possibly provide more information if needed.  But I'm not sure what else is needed to calculate this?

oldhand (Civil/Environmental) · Answer 1 · 2003-08-18 05:20:46

Best advice is to employ a structural engineer. Bridge design is a specialist subject. Should you carry out the design and the bridge is not capable of carrying load you will be liable. The bridge should be capable of carrying normal highway loading for vehicles likely to access your home, i.e. garbage trucks, etc.

Qshake (Structural) · Answer 2 · 2003-08-18 11:27:23

Sounds like you're trying to gain some access to the woods on the other side for hunting or other recreation.  That said, you're not likely wishing to have any vehicles of substantial size cross the bridge (I use the term loosely to describe this situation), am I correct?

While I don't know much of anything about trailer homes, I do know about bridges.  Presently, the 8" beams seem light for a 46' crossing.  What kind of supports did the frame have while in the trailer?  Was it supported twice or three times or more?

The bracing is a good thing, but may not do enough since the beams are awefully small.

I recommend either finding competent structural or civil engineer with bridge experience to help out.  Or go to the local steel fabricator or contractor and ask what, if any, miscellaneous steel they wish to get rid of.  A lot of times a contractor will have lengths of really large (depth) beams that are rusted and look like crap but have saved a many a stream crossings.  These beams usually provide more than enough capacity when combined with the proper decking and bracing.

koodi (Civil/Environmental) · Answer 3 · 2003-08-22 20:54:48

KenB2N,
It sounds like there's a fifty-fifty chance it will work--and if it does, there's a fifty-fifty chance it will work well.

Streams move, particularly in storms.  So even if your guess that the bridge will not end up underwater is correct, it may move and take out one of the supports.  Then there are the supports how deep how big, what kind?

If you decide to get an engineer, don't assume a "structural" engineer will know how to design a bridge.  The title "structural" often means someone who is particular to designing buildings.

If you don't get an engineer: the bridge isn't high, and if you drive slow, chances are, if the bridge fails, you'll go uninjured and survive--just a little Auto damage.
Good Luck,

frank1371 (Military) · Answer 4 · 2003-09-23 14:36:10

I think your plan will work for light traffic. I would make sure the abutments are stable to prevent settling over time. I can point you to a military field design/classification system which you could use to check the math on the structural members if you are interested in doing it.

One other concern I would have would be high water. 5 foot bank heights is not really that high depending on where you are geographically.

Good luck

tincan (Civil/Environmental) · Answer 5 · 2003-09-26 13:11:42

The beams appear to be M8x6.5 (or similar) with a section modulus of 4.62 in^3.  Assuming an equal distribution to the two beams and fs=24ksi, the allowable moment without impact would be approximately 2(4.62)24/12 = 18.48 ft-kips.
Placing a deck on the axles 3/16" plate @ 7.7psf @ 6' wide = 46 plf, use 60plf total ==> MDL = 60(46^2)/8 = 15,870 ft-lbs.  This leaves 18480 - 15870 = 2,610 for the live load.

For a center loading, M = PL/4 = 2610 ==> P =226#.  This will be a moon-walk ride for the neighborhood kids.  Unless you're a GOOD welder and can assure continuity between the sections and the plate, I would not use composit action on the section,
Run, don't walk to your friendly neighborhood structural/bridge engineer for help.

Best of luck

twinsemi (Mechanical) · Answer 6 · 2003-09-30 01:29:22

Ken,
As you can see from the quick crunch of the numbers by tincan, the frame is most stressed by the weight of the 3/16" plate that is laid on it. (60#/ft) This is considerably more than any live load from a 4-wheeler. (am assuming as others have that you mean an ATV) Your problem thus becomes one of finding a lighter weight deck for the frame. Some additional angles or other small members to minimize the unsupported span. (46") Try to think like building a deck; you wouldn't put joists on 46" centers. Maybe expanded metal over a network of braces. Do you work for Boeing or Lockheed? Have any honeycomb aluminum or foam-cored CF? :)

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Bridge from Mobile Home Frame?