Calculating max load of square tube (steel)

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Discussion Overview

The discussion focuses on calculating the maximum load that a rack made of 3X3X1/4 steel tube can support, specifically addressing bending and compression loads. Participants explore the necessary formulas and parameters for determining the distributed load capacity of the structure.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant seeks assistance in calculating the maximum distributed load for an 8ft long and 4ft wide rack made from steel tube, providing initial values for moment of inertia, section modulus, and modulus of elasticity.
  • Another participant suggests calculating the bending stress in the beam and references a Wikipedia page for guidance.
  • A participant expresses uncertainty about whether yield strength is the correct measure for determining the maximum load and mentions the assumption of fixed ends for the tubes.
  • One reply indicates that calculating the maximum bending moment and applying the bending stress equation is the correct approach, providing a specific formula for maximum stress in a simply supported beam.
  • A participant inquires about the value of 'q' in the context of the load calculation.
  • Another participant confirms that finding an allowable maximum stress and solving for 'q' is the next step.
  • A later reply clarifies the distinction between area moment of inertia and body moment of inertia, emphasizing the relevance of the area moment of inertia for this calculation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the specific values or methods for calculating the maximum load, and multiple approaches and interpretations of the problem are presented.

Contextual Notes

There are unresolved assumptions regarding the application of yield strength and the conditions under which the load will be applied, as well as the definitions of moment of inertia used in the discussion.

drewjohn
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I'm trying to calculate the max load of a rack made of 3X3X1/4 steel tube. It is 8ft long and 4ft wide. I just want to calculate the max load (distributed load) in bending (prob for just the 8ft sides) and also in compression for the legs. i can't find the right formulas. can anyone help?

so far i have

MOI (I)=3.495 in^4
Section modulus=2.330 in^3
MOE (E)=29,000,000 psi ( i think this is correct but I'm not sure. just want to use low grade steel)
 
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I'm not sure if that is what I'm looking for. I need to know how much weight i can put on this rack (distributed load). i don't know if that that be yield strength or what? i want to assume fix ends on an 8ft square tube (actually 2 8ft tubes parallel).
 
No, that's what you're looking for. It's a textbook problem really. Calculate the maximum bending moment in your beam. Then apply the bending stress equation to find the maximum stress.

Via http://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html the maximum stress in a simply supported distributed loaded beam is:
<br /> \sigma = \frac{y q L^2}{8 I}<br />
Where y is the perpendicular distance from the neutral axis, q is the load in force/length, L is the length of the beam, and I is the area moment of inertia of the cross section.

Be careful though, depending on the application. Yield strength is the strength assuming one cycle. If this beam is to be loaded and unloaded, then you should calculate a fatigue life, and then still use a safety factor.

Good luck,
 
but what i want to know it q. right?

would be 1.5?
 
Yes, find an allowable maximum stress and solve for q.
 
just as an addition to the topic...don't confuse the moment of inertia given with the body moment of inerita...this is the area moment of inertia whose unit in SI units system is bisqaure meter( m ^ 4)...the other moment of inertia of the body whose unit in the SI unit is (N.m) isn't applicable here
 

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