# Calculating strength of steel square tubing

• gary d

#### gary d

Hello,

I'm enrolled in College in the civil engineering program and am looking for guidance with calculating strength of steel square tubing.

We are making a project that has parameters.

1- Must be made of steel
2- 4"x6"x36" is maximum OD of section.
3- 14 lbs is maximum weight
4- the load will be applied at the connection, from the top aimed straight down

We are working on a reasonable budget and I'm working on simple, strong, efficient, designs.
Two of these pieces will be connected to each other, so there will be a connection point.

So far I'm thinking 3x3" square tubing with a wall thickness of 11 gauge at 34" long should do the trick. Looks like 10.6 lbs (according to website lists)

What types of steel should I be considering?
How do I calculate the strength of steel square tubing?
How to I calculate the connection points?
Should I scratch the idea of square tubing and go to a more complex design, like truss design?

Thank you

In general, the word "strength" refers to the maximum load that can be carried. This is going to be determined primarily by
1) the geometry of the structural members;
2) the strength of the connections;
3) the yield strength of the material.

A structure fails when some component either yields or buckles. As a designer, you must consider both possibilities.

Remember that a local failure, such as a connection tear-out, is just as much a failure as collapse due to gross yielding. You can raise yield strengths (up to a point) by using more and more exotic steels, but this is often a question of economics. The structure has to be as strong as it needs to be, not as strong as it can possibly be.

I think this link should work for the pics

http://s1293.photobucket.com/user/Engineer-man/media/imagejpg1_zps798135ed.jpg.html?sort=3&o=0

http://s1293.photobucket.com/user/Engineer-man/library/?sort=3&page=1

Since the posting I looked into some calculations and basics. Looks like moment of inertia is critical.
Found a calculation that seams on track for my uses. For the triangular design in the photobucket pics,

I used 29500000 modulus of elasticity (E) for carbon steel (seams most common and cheapest,) 1000 lb for (F), (I) moment of inertia, 8.49023, 34 inches long (L), and 2 inches for (Y), which sounds like perpendicular distance to neutral axis... I might have the wrong.

According to an online calculator, I'm looking at .00327 in of deflection. But now to somehow calculate how the joinery will work out...

Is the triangular design with three pieces and a plate attaching to the next stronger then a square tubing and 3 sided plate?
And 1400 lbs will be the most weight loaded.

Thank you

You said that you did your calcs for 1000 load, but 1400 lb is the max load. Why? Why not use 1400 lb?

You said that the deflection was only 0.00327 in but this is only a part of the picture. What is the stress? What about buckling?

Your wording suggests that you really do not know what you are doing, yet you presume to do structural design. This is dangerous. You really need to involve a competent engineer for this work.