Need some strength of materials advice.

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I have some trailers that I am rebuilding for resale, and the bodies are all attached to the frames with a 4"x6" oak runner on each side.  The Oak runners are 8ft long,and supported on each end, and the bodies rest on them in four places evenly spaced down the runner.  I need to pull the oak runners out and replace with either a steel channel or tubing and remount to the frame.  Question is I am having trouble deciding what size material I should go with.  I'm trying to swing this as cheaply as possible in order to maximize my profits, so just over engineering isn't the answer.  The maximum load on these trailers is 12,000 pounds.  Any help you guys might offer is greatly appreciated.  ~JacksonI'm a Lover, Fighter, Wild horse Rider, and a pretty good welding man......
Reply:You mean a piece of oak between the body and frame?  Like on a truck body?Just replace it. Or turn it over DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:No, this is a different setup all together.  These are grain wagons, and the 2 oak runners actually support the load of the trailer not just used as a spacer.  The oak runners are supported  on each end, and the body or the trailer rests on it.   basically the main supporting beams.I'm a Lover, Fighter, Wild horse Rider, and a pretty good welding man......
Reply:So the Frame rails are WOOD?Real world weldin.  When I grow up I want to be a tig weldor.
Reply:Well one of the more difficult parts is replacing that oak knowing that one of it's most redeeming qualities pliability.  Going to be a challenge.
Reply:Originally Posted by David RSo the Frame rails are WOOD?
Reply:I keep thinking there is some reason why that oak is there other than cost or convenience. Could be wrong.  Shock absorption?? Do they shake these things apart running wide open out in the fields and on the back roads?
Reply:Maybe some Hemlock wood ? I would say Sandy - to allow it some flex.John -  fabricator extraordinaire, car nut!-  bleeding Miller blue! http://www.weldfabzone.com
Reply:Originally Posted by MicroZoneMaybe some Hemlock wood ? I would say Sandy - to allow it some flex.
Reply:Hammack,Really not enough info to help much.Maximum compression load will occur at the two ends of the beam, where it's supported.  Need to determine how many square inches of bearing surface is utilized at each end of the beam.  In the case of the oak you're replacing I would assume it's 4" wide by a yet to be determined length.  Does the oak beam just sit in a saddle attached to the frame?Also, with a trailer, you have got to design the beam to account for "live loads" rather than just figuring the weight that will be carried (static load).Has the oak just rotted or is it failing from loads?There's a poster over on the Miller Boards (Dabar) who is very experienced in trailer fabrication.  You may want to PM him and get his suggestions.A couple of photos would help us to help you.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:Back on topic, grain isn't the heaviest material there is by a long shot so that helps.
Reply:As far as I know, 1 lb. of wheat is 1 lb. of wheat. John -  fabricator extraordinaire, car nut!-  bleeding Miller blue! http://www.weldfabzone.com
Reply:Yeah, about that heavy grain ...What weighs more, 12,000 lbs of grain, 12,000 lbs of feathers, or 12,000 lbs of lead?  Dang, Micro got his response in there as I was typing!A quick calc for two 8ft long beams holding up 12,000 lbs says you need minimum 6x5x1/2 steel.  That will give you a deflection of about 1/4 inch in the middle, and at least a little positive factor of safety.  For dynamic loads (bouncing through the fields, etc), I think you need more steel than that though.You 'make' it, you are liable for it.  Pay a PE to do a few quick calcs and stamp it.  Or overbuild it to tomorrow and hope it's strong enough.This advice worth what you paid for it.Last edited by MoonRise; 08-30-2007 at 01:21 AM.
Reply:Yeah, about that heavy grain ...
Reply:Without the info I outlined in the previous post, and considering the timeline you're trying to meet, I'd be on the phone to the local recyclers/scrap yards looking for 4"Wx6"H I-beams.I don't think I'd be comfortable recommending 2x6x3/16" box for that job.  From the picture shown, the bearing plates don't look that large.  I think the point loading from the crossmembers would crush 3/16" box.Just my .02 based on a minimal amount of info.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:The Oak has rotted the trailers I have are 25+ years old.  They are now manufacturing the trailers utilizing all steel.   The oak runners are 4"x6" and 8ft long They rest on a 4" wide rail at each end.  They have a single 1/2 bolt that goes thru the side of the runner and thru a tab (you can see the tab in the pic right over the front wheel) to hold them on the frame.  I do have some 6" "I" beams in stock and could use those, so that is also an option.  I'll try to get some more pics today of it.I'm a Lover, Fighter, Wild horse Rider, and a pretty good welding man......
Reply:You say they are now making them with all steel, unless they are a totally different design can't you just look at a new one and see what they are using now for the runners.
Reply:Originally Posted by MoonRiseYeah, about that heavy grain ...What weighs more, 12,000 lbs of grain, 12,000 lbs of feathers, or 12,000 lbs of lead?  Dang, Micro got his response in there as I was typing!This advice worth what you paid for it.
Reply:Originally Posted by Hammack_WeldingThe Oak has rotted the trailers I have are 25+ years old.  They are now manufacturing the trailers utilizing all steel.   The oak runners are 4"x6" and 8ft long They rest on a 4" wide rail at each end.  They have a single 1/2 bolt that goes thru the side of the runner and thru a tab (you can see the tab in the pic right over the front wheel) to hold them on the frame.  I do have some 6" "I" beams in stock and could use those, so that is also an option.  I'll try to get some more pics today of it.
Reply:Originally Posted by 737mechanicYou say they are now making them with all steel, unless they are a totally different design can't you just look at a new one and see what they are using now for the runners.
Reply:Ahhh, looking closer at the pic and enlarging it a little bit and it not being 1AM, I see now that the loading is not -just- in the center of the beams.In the attached pic, I originally saw that 12,000 hopper sitting in the middle of the beam (load on the cyan arrow and it's symetrical cross-member toward the aft of the trailer).Now I see that the load can rest on the blue arrow -and- the cyan arrow, and their symetrical cross-members toward the aft of the trailer.If you can get the majority of the load weight on the front and rear cross-members (blue arrow) and just some in the middle of the beam (cyan arrow), then you may be able to squeak by with 2x4x3/16 steel tubes.Getting the majority of the load over the front and rear axles/cross-members makes the bending in the middle of the beams MUCH better, but would tend to try and 'taco' the hopper.You could get the majority of the load over the axles by shimming under the front and rear hopper cross-members so that the middle of the hopper isn't touching the beam to start with.  Pad it up about 1/8 to 1/4 inch.Note that if the user overloads the hopper (it rains and fills with water, or rains and soaks the grain, or the hopper gets filled with something denser than grain, like sand, gravel, mud, etc) then all bets are off.This advice still worth what you paid for it, objects in mirror may be closer than they appear, or they may be on fire and flammable objects are also inflammable, mumble fine-print mumble, etc, etc. Attached Images
Reply:Cyan ? Actually that is #OOFFFF according to HTML language. John -  fabricator extraordinaire, car nut!-  bleeding Miller blue! http://www.weldfabzone.com
Reply:Hammack,Sorry.  Engineers, with limited information,(I had not seen photo when I wrote original post) always tend to "overdesign".After seeing how the load is distributed, you may well be able to get by with your original thoughts.  I would recommend, though, that you cap the beams with thicker material to "spread" the load on your 3/16" material.  As it is, it appears that the cross supports (hopper) are only about 2" bearing on the load carrying beam.  Also will depend on how "rigid" the hopper support frame is.LaterSyncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.