3x3 or 4x4 square tubing bending strength

In summary, a cantilevered beam with a 10' span using square tubing is stronger than a 3"x3" 1/4" wall, but it will deflect less.
  • #1
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I need to erect a hoist beam with a 10' span using square tubing. Which is stronger, 3"x3" 1/4" wall or 4"x4" x 3/16"? Also, how much weight could I lift at the center?
 
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  • #2
To determine which is stronger, all you have to do is calculate the bending moment of inertia for each cross section. The higher the bending moment of inertia, the stronger the beam is in bending. To calculate the moment of inertia for a hollow tube, calculate the MOI for a solid tube, and subtract the MOI for the hole in the middle.

http://en.wikipedia.org/wiki/Second_moment_of_area" [Broken] (a.k.a. moment of inertia)

To figure out how much weight it can hold, you need to read up a bit on beam bending theory, and you will need a bit more information about what kind of material the tube is made out of:

http://en.wikipedia.org/wiki/Beam_bending" [Broken]
 
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  • #3
Most box tubing is A36. Other than that, you should be able to plug and chug with what M_E gave you
 
  • #4
Way over my head. Another option is to nail three 2x6x10 yellow pine boards together. I'm hoping this will support lifting 1500 lbs.
 
  • #5
Well it sounds to me like you will want to hire an engineer to design this "crane" for you then... especially since it sounds like a safety-critical item.

It does look like a 4"x4" hollow tube with 3/16" walls should be able to hold 1500lb without too much trouble, but it will depend and what kind of holes are drilled in it and how the beam is suppoorted at the ends.

... and a 4"x4" tube with 3/16" walls is about twice as strong in bending as a 3"x3" tube with .25" walls.
 
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  • #6
In terms of just what will bend less,
3x3x.25 Ix = 3.16 in^4
4x4x.188 Ix = 6.59 in^4

So the 4x4 beam will be deflecting less. I attached some quick calculations. The first calculation is calculating the load required at the center of the beam to get to yield (You don't want to go that high). Then using that load, a deflection at the center of the beam is calculated. These are pretty rough considering the end conditions, but they can be a guide for you.
 

Attachments

  • Quick Beam Calcs.pdf
    9.6 KB · Views: 4,664
  • #7
Thank you to everyone. This project is only temporary. I'm building a new floating dock on top of my old dock. I just need to raise and slide it ever so gently into the water. You know what they say. It only costs a little bit more to do it yourself.
 
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  • #8
Fred- It looks like you're off by a factor of 2 in your max force calculation, the beams should be able to put up with about twice as much force as you're quoting...
 

Attachments

  • Mathcad - Simple beam bending with stress, 3x3x.25.pdf
    29.1 KB · Views: 3,171
  • Mathcad - Simple beam bending with stress, 4x4x.188.pdf
    29.1 KB · Views: 1,775
  • #9
Oh yeah. I forgot the factor of 2 in my calculation for M. Whoops.
 
  • #10
"That factor of two will get you every time," a wise engineering professor once told me... :wink:
 
  • #11
Hi Folks, Is there a corresponding equation (or equations) for a cantilevered beam, and tables that someone can point to that have the values needed to run the computations for different sizes and wall thickness of square tubing (mild steel I think in my case)
 

1. What is the difference between 3x3 and 4x4 square tubing?

3x3 and 4x4 square tubing refer to the dimensions of the square tube, with 3x3 indicating a square tube with sides measuring 3 inches and 4x4 indicating a square tube with sides measuring 4 inches. This means that the 4x4 square tubing is larger and has a greater cross-sectional area than the 3x3 square tubing.

2. What is bending strength?

Bending strength, also known as flexural strength, is the ability of a material to resist deformation or breakage when subjected to a bending force.

3. How do you calculate the bending strength of 3x3 or 4x4 square tubing?

The bending strength of a square tube can be calculated using the formula: Bending Strength = Moment of Inertia x Yield Strength / Section Modulus. The moment of inertia and section modulus can be determined based on the dimensions of the tube, while the yield strength is a material property that can be obtained from testing or reference tables.

4. Which one is stronger, 3x3 or 4x4 square tubing?

Assuming both tubes are made of the same material, the 4x4 square tubing will be stronger due to its larger cross-sectional area. This means that it can withstand a greater bending force before deforming or breaking compared to the 3x3 square tubing.

5. What factors affect the bending strength of square tubing?

The bending strength of square tubing can be affected by several factors, including the material properties, dimensions of the tube, and the type of bending force applied. Other factors such as the presence of any defects or imperfections in the tube can also impact its bending strength.

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