need help to determine twisting of angle with an applied force

[tdw123456]

I am looking for help to determine how much an aluminum angle will twist. The aluminum angle is 6061-t6 material 1/2" wall x 4" legs x 35" long. 1" from the bottom, one leg of the angle is secured to an inmovable object. 1" from the top of the other leg 30 pounds of force is applied at a 8" radius.
The question is how much will the angle twist

 

[racookpe1978]

1/2 inch angle iron leg thickness on a 4x4 angle? (Then again, MetalsDepot has an 8 foot length available at $198.00. Plus shipping.)

Go back to your text book, calculate the modulus. You've got all the other values.

 

[tdw123456]

"Go back to your text book, calculate the modulus."

That doesn't help at all! It's 1/2 inch angle 6061-t6 aluminum (not iron) leg thickness on a 4x4 angle. Ad the point is that I can't find a standard formula for torsion for an angle section.

 

[GregLocock]

Find out where the centre of torsional resistance of the section is. I think you structural bods call it the shear centre. then work out the moment of the force about that point, then work out the torsional constant of the section, call it J.

Then theta=L*M/G/J

This is first year undergraduate stuff from your structures lectures, if that helps you to find it in your textbook.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[rb1957]

don't know what books you're looking at but twist of an open section is a simple expression, should be in every book ...
theta = 3TL/(Gbt^3) ... b is the length of the flanges (8" in your case).

 

[tdw123456]

Thanks people,
I had already approximated the answer using theta=L*M/G/J but that is really an expression for round solids.
I am under the impression that theta = 3TL/(Gbt^3) is for thin section materials but I'll try it and see if the resule is reasonable.

 

[CANPRO]

I usually turn to Blodgett when I need to deal with torsion and rotation. Its an old book, but I find it to be a useful reference. It uses the same formula for theta, except they use the torsional resistance 'R' instead of J. There is a table that shows rotation calculated using R and J and compared to measured values. The angle of rotation, according to this table, is underestimated when using J.

 

[rb1957]

i'd consider your section to be thin.

in future, if you know the basics i think it helps limit the -ve comments if you say so. maybe "should i use 3TL/... here ?" rather than "what should i do ?". the former implies some knowledge, the latter implies none.

 

[tdw123456]

Thanks for the help everyone. Results submitted, boss happy.