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HOW2->figure out load capacity of a tubular steel framed box ?

What would be an adequate design of a 3" tubular steel frame to safely transport a 10Ton live load that bears on a narrow area of approximately

25 x 1 [feet] that is the bottom-most plane of the "box" ?

The box is about 32 x 10 x 4 feet, l x w x h. The load would bear on the bottom plane which is about 32 x 10 feet of welded 3" tubing, comprised of 4 long tubes and numerous welded tubes comprising the width.

To be specific, I own a tubular [3"] steel trailer on which I wish to transport a 10Ton keel sailboat whose keel is 25 x 1 feet, approx., though the 1' dimension is not planar but rather conical, so the actual bearing surface is somewhat less, in other words the keel does not have an exactly flat bottom.

The trailer has air ride & brakes [hydraulic] so should cushion the load considerably. Also there will be stanchions to support [laterally, primarily] non-linear horiz. forces, as opposed to the vertical gravitational force about which this query is concerned.

The trailer is not to be built, but rather exists. I'd like to reinforce by weldment whatever is necessary to have a margin of load bearing error as is commonly used by competent mech. engineers.

Is the trailer so lightly built that I'd be better off discarding it and starting afresh with an I-Beam type trailer or do I not properly understand the strength of such a tubular trailer ? It was formerly used to haul several pontoon aircraft at a time and the overall tractor-trailer rig was rated at 50,000 GVW and I'm surmising that the trailer load itself was rated at appox. 10Tons, but it may be that that accrues only from the fact that the tractor was rated as capable as safely being able to transport a net 10Tons by DOT. I don't know.

Also, what references are recommended in such matters ?

Most appreciatively, Courtney Thomas

Reply to
Courtney Thomas
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A 10 ton load uniformly distributed along a 25 ft X 1 ft area is to be supported by four 3 inch diameter steel tubes, length 32 feet.

Need to know:

thickness of tubes Material Support points along the 32 feet.

Regards

Brian W

Reply to
Brian Whatcott

Thank you for your interest.

The trailer, I'm told, was built some 20 years ago as a car hauler but was modified to haul pontoon planes by Kenmore Air a Seattle Co. and resultantly had the top cut off it for that purpose. The reason I mention all this is that they indicate the orig. manufacturer is unknown, so, I don't know how to determine the material.

I will be going over tomorrow to work on the hydraulic lift and will count the number and measure... the support points.

I assume using a micrometer to measure thickness will suffice for your thickness request.

Do you have any suggestion regarding determining "material" ?

Appreciatively,

Courtney

Brian Whatcott wrote:

Reply to
Courtney Thomas

If no other reference to material is available, look for warning signs like "Do not weld this structure" [ denotes heat treated stuff...]

and just confirm its steel not aluminum with a magnet. A micrometer will be good for wall thickness. But even a steel rule is better than nothing. The support locations are used to find the longest unsupported length, which controls stress and deflection.

Brian W

Reply to
Brian Whatcott

Brian,

Went over to the trailer today so have some data....

The tubular steel is 2 1/2" not 3", though some of the side structure is 3".

The wall thickness of the tubes is 3/16". It's definitely steel in that there's a little surface rust wherever paint is abraded.

There are 4 approximately equally spaced uninterrupted longitudinal bottom members including the two outside ones running then entire 25' length of what I'll call.... the usable net cargo area.... which does not include the extended structure that goes over the dual wheels of the single axle.

There are 5 bottom laterals, including both ends, at approximately 6' intervals, ..... each traversing the 8' width of the trailer. They are cut as necessary to enable welding to the longitudinals.

There are also two welded diagonal steel strappings that run the length of the trailer that I would estimate to be 1/4" in thickness and not more than 5/16".

The sides, front and rear are 37 1/2" high from the bottom tubing.

The sides are heavily reinforced with welded diagonal tubing structures.

The changed dimensions are 25' x 8' x 3', i.e. l x w x h for what I'll call the keel bearing area. Succeeding this area is the structure to which the axle/brakes/wheels are attached, which would support the bottom but not be directly load bearing.

The preceeding portion that connects to the 5th wheel is a little less than 4' and is made of heavier stuff than the load bearing area.

Hopefully this is enough to come up with at least a rough approximation of what the trailer can safely transport without further modification.

I look forward to hearing from you.

Cordially,

Courtney

Brian Whatcott wrote:

Reply to
Courtney Thomas

A single 2.5 in diam tube with 3/16 in walls that's 30 feet long, and loaded evenly along the central 25 feet deflects 8.2 inches when loaded with a distributed 330 lbs and its self weight.

Four such tubes would carry only 1320 lbs. But the vehicle on which they are mounted is said to have carried a car let's say 2T, and you would like to haul 10T.

The side rails are evidently making the structure much stiffer and stronger but you have not described the side rails in much detail.

You mentioned the height of the rail is a little more than 36 inches. I'll estimate what tubes at 36 inches above the base rails would provide 2 then 10 T capacity...

Each side could carry 7350 lbs (and meet code!) if it consisted of an upper and lower tube of 2.5 diam 3/16 wall spaced 36 inch apart and was provided with struts and diagonals of the kind you suggest it has already. The load estimate then jumps to 2 X 7350 + 660 lbs = 15360 lb These rails would sag considerably less than one inch at this load.

If the cross section of the side rail tubes were increased by

20/15 i.e 33%, then the trailer would carry the 10T load, so long as bumps were avoided. The cross section might be suitably increased by welding in bars along the top and bottom rails that I mentioned above, of 1.362 X 0.33 = 0.45 sq in cross section, i.e 1 X 0.5 inch bars along the entire length of the upper and lower rails. [your risk, of course...]

Brian Whatcott Altus OK

Reply to
Brian Whatcott

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