Strength of Square Steel Tubing

[Windsor EDD]

Hi I'm a high school student and need help with the calculations for a bike design. I am required for the design to use 1.5" mild steel square tubing at 1/16" thick for it. I need to know what the strength of the tubing is before i can make or tweak the design. Can anyone help with my predicament?

 

[SteamKing]

The most common (and probably cheapest) square structural tubing is made from A36 carbon steel with a maximum yield stress of 36,000 psi.

There may be stronger grades, but you should probably check with (or google) steel suppliers.

 

[SteamKing]

Additional info and clarification:

A36 steel has a minimum yield stress of 36000 psi

Your design should have a safety factor so that maximum tensile stress in bending is less than 21,600 psi (same for axial loading)

Max shear stress should be kept below 14,400 psi.

 

[Windsor EDD]

Thanks, also to find the strength of the tubing I'd multiply the area of the tube by the Maximum Yield Stress right? Or would there be some other geometry that would affect the outcome?

 

[SteamKing]

It's a little more complicated than that. If your tube is purely in tension or compression due to a load applied axially, then the stress = applied force / area of the tube. Note that the area of the tube is not 1.5 x 1.5, but the area of the metal only.

If your tube has a bending moment or a twisting moment applied, then different formulas will apply. Also, combining several tubes to make a frame work requires special analysis to determine stress.

 

[billblack]

What you need is a good simple mechanics of materials or strength of materials book, but it is hard to pick out 'just" what you need (which is all about beams) to find the answer without working through a bunch of material.
if you can find a copy of "Strength and Stiffness of Engineering Systems" by Leckie and Bello and published by Springer in 2009 -the e-ISBN # is 978-0-387-49474-6 - Chapter 6 will be of great interest to you.
You will also need a few good free body diagrams to find the forces acting on each portion of the bike frame.
It's interesting stuff, just stay focused on each part till you get to the next one. A good book on the design process is "The Mechanical Design Process" by Ullman. This is a cool project. Lots to learn! :)

These are just some Mechanics//Strength of Materials resources that may be of help for this or similar problems:
The MIT MofM course in Opencourseware: http://ocw.mit.edu/courses/material...11-mechanics-of-materials-fall-1999/index.htm

Engineer's Edge calculators: http://www.engineersedge.com/mechanics_material_menu.shtml
Videos: http://sbainvent.com/strength_of_materials/#.UMqa2-TWLvk
Some good discussion from a while back @ homebuiltairplanes: http://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/3372-round-vs-square-tube-strength.html
and here: http://answers.google.com/answers/threadview/id/523190.html

 

[Windsor EDD]

Thank you I will be sure to check that out, and I like this project a lot because I learn new things everyday. Also do you by any chance know if it is possible to get triangular tubing for the bike or is that not a good idea?

 

[billblack]

The strongest tube will have a round cross-section. I have never seen triangular steel tube offered for sale. A triangular cross section would be very expensive to produce and mechanically inferior to a square tube.
For more information about what is available you might look at some of the online metals sellers like
www.speedymetals.com
www.metalsupermarkets.com
and my personal favorite: http://www.aircraftspruce.com/menus/me/index.html

and since weight is important to bicycle designers as well as aircraft designers:
http://exp-aircraft.com/library/heintz/material.html