3 x3 tube, 3/16' vs 1/8 thick load strength

[Dbb3]

3" x3" tube, 3/16' vs 1/8" thick load strength

Hello,
Any help would be appreciated. After calling umpteen places, and searches, I've found this site, which is a gold mine of info, and thanks for that.

I am trying to figure out what the load strength of steel tubing, that is 3" x 3" 3/16" wall (.188) compared to same 3" x 3" at 1/8" thickness.
I will be using the tubing in manufacturing trailer extensions for launching of boats/sailboats, etc, and currently use 3/16" with no problems. It's pretty heavy duty after galvanized, and have had no problems with even heavier boats, 8000 lbs (10% of weight actually on tongue) using the extensions, and was hoping to be able to use 1/8", if not for the longer ones (12" then the 7' and 9' size.
picture of how it's used at www.extendahitch.com

Boats usually only have 10% of the boat weight on the tongue, and these aren't used for towing, strictly for launching at ramps.

Just not sure of the weight a 1/8" 3 x 3 steel tube will handle.

Any help would be greatly appreciated.
Thanks
Dan

 

[hotvette]

Be very careful. The bending strength of a beam is inversly proportional to the area moment of intertia which varies a great deal with wall thickness. You can use this online calculator:

http://www.engineersedge.com/calculators/section_square_case_10.htm

In your case of a 3" x 3" hollow rectangular beam, the bending strength of a beam with 3/16" wall is 41% greater than a 1/8" wall (if I did the calcs right).

The calc is straightforward. For a solid square beam of dimension h, the area moment of inertia is h⁴/12. If hollow, just subtract the area moment of intertia of the 'space'. You can confirm with the online calculator.

 

[Q_Goest]

I get the same number as hotvette (41% increase in moment of inertia, I) for a round tube or a square tube. Note the square tube is MUCH stronger than the round one. I'm assuming it's square, right?

Question is, what is the tongue weight either can support safely? I'm assuming this extension is rigidly mounted to the trailer and the tongue is simply extended by this new piece of tube. For carbon steel, I'll assume there's nothing particularly special about it (basic, low grade CS and not 4140 or something special) then the maximum allowable stress is on the order of 25,000 psi. A36 has a yield around 36,000 psi so this gives a safety factor of not quite 1.5. I think we can allow this relatively high stress because the chance of damage or injury due to failure is very small as long as it isn't being used to tow a boat around a parking lot at more than about 15 mph. Also, the stresses are relatively well understood (unless there's something strange going on at the connection point to the boat trailer) and also because tongue weight decreases with this extension, but we won't take that into account so that adds some conservatism. Fatigue shouldn't be an issue unless you would expect this to be used many thousands of times. For your average boat owner that shouldn't be an issue. Regardless, those are all things you should consider and the allowable tongue weight changed as necessary. Needless to say, if it's made for towing on the street, this is way too dangerous.

The original (3" square? x .188 x 9 ft long) can safely support a 431 pound tongue weight, but the deflection is fairly significant (about 2.16"). It might look a bit over stressed. Note that tongue weight will change depending on how far away it is from the trailer wheels. The farther away the hitch is from the wheels, the lower the weight on the tongue will be. Nevertheless, let's ignore this since it gives us a conservative answer if we do.

Compare to the alternative (3" square x .125 wall x 9 ft long) which can safely support 306 pound tongue weight. Again, deflection is high but it doesn't change (2.16").

 

[Dbb3]

Thank you both for your replies.
Yes, the tube in connected securely to the tongue (via a 2' 3 1/2" sq. bracket.) So, in essence, a 9' model will have 2' inside the tube, 7' extended outside tubing bracket. It's strictly for switching over at staging area, and backing down ramp, and vice versa at boating day's end, retrieving boat from water, and switching back to towing coupler.
Thanks again
Dan