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

In summary, the load strength of a 3" x 3" 3/16" wall (.188) steel tube compared to a 1/8" thickness is 41% greater. The tube can safely support a 431 pound tongue weight, but the deflection is fairly significant (about 2.16").
  • #1
Dbb3
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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
 
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  • #2


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.
 
  • #3


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").
 
  • #4


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
 
  • #5


Dear Dan,
Thank you for reaching out to us for assistance with your project. I am happy to provide you with some information and guidance on the load strength of steel tubing.

Firstly, it is important to note that the load strength of a material is directly related to its cross-sectional area. This means that a larger cross-sectional area will have a higher load strength compared to a smaller one. In this case, the 3" x 3" tube will have a larger cross-sectional area compared to the 1/8" thick tube, giving it a higher load strength.

To determine the exact load strength of the 3" x 3" tube, we need to consider a few factors such as the type of steel used, the manufacturing process, and any additional treatments such as galvanization. However, as a general guideline, a 3" x 3" steel tube with a 3/16" wall thickness can typically handle a load of up to 10,000 lbs. This is based on the assumption that the tube is made of mild steel and has been properly manufactured and treated.

On the other hand, a 3" x 3" tube with a 1/8" thickness may have a load strength of around 6,000-7,000 lbs. This is significantly lower than the 3/16" tube due to the smaller cross-sectional area.

In terms of your specific application, it is difficult to determine the exact load strength needed without knowing the specific details of the trailer and boat being used. However, if the current 3/16" tube has been working well for you with heavier boats, it is likely that it would also be able to handle the load of a 1/8" thick tube. However, it is always recommended to err on the side of caution and consult with a structural engineer or conduct further testing to ensure the safety and durability of your trailer extensions.

I hope this information has been helpful to you. If you have any further questions, please do not hesitate to reach out. Best of luck with your project!

Sincerely,
 

1. How does the thickness of a 3x3 tube affect its load strength?

The thickness of a 3x3 tube plays a crucial role in determining its load strength. Generally, the thicker the tube, the higher the load capacity. This is because thicker tubes have a higher resistance to bending and buckling, making them able to support heavier loads without deforming.

2. What is the difference between a 3/16" and 1/8" thick 3x3 tube in terms of load strength?

The difference between a 3/16" and 1/8" thick 3x3 tube may seem small, but it can have a significant impact on load strength. A 3/16" thick tube has a higher load capacity compared to a 1/8" thick tube. This is because the thicker tube has a larger cross-sectional area, allowing it to withstand more weight before deformation occurs.

3. How does the shape of a 3x3 tube affect its load strength?

The shape of a 3x3 tube also plays a role in determining its load strength. A square or rectangular tube has more area and weight-carrying capacity compared to a round tube of the same dimensions. This is because the corners of a square or rectangular tube help distribute the load more evenly, preventing it from collapsing under heavy weight.

4. What other factors besides thickness can affect the load strength of a 3x3 tube?

Besides thickness, there are other factors that can affect the load strength of a 3x3 tube. These include the type and quality of the material used, the manufacturing process, and any additional reinforcements or modifications made to the tube. These factors can all impact the overall strength and durability of the tube under different load conditions.

5. How can I determine the load strength of a specific 3x3 tube?

The load strength of a 3x3 tube can be determined by using engineering calculations and formulas specific to the type of material and shape of the tube. It is important to consult with a qualified engineer or use reputable online resources to accurately determine the load capacity of a specific 3x3 tube before using it for any structural purposes.

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