What yield stress do i use for section capacity of a Parallel flange channel?

Question

When the yield stress of the web is more than the flange, so i use the smaller yield stress to calculate the sectiona capacity, or the larger one?
Im guessing the smaller to be safe.

Answer 1

Normally the stress is higher in the flange, since you use [ M*x / I ], and at the flange x is largest. Also, in reallife applications, it is always better to be more conservative, so use the smaller yield stress.

Answer 2

I'll warn you in advance I'm not a structural Engineer. But I understand that as the second answer said it is customary and I think theorectically correct to assume the bending stress, tension and compression is concentrated in the flanges and the shear stress is carried by the web.

I'm assuming the member you are looking at is a composite with different materials for the flanges and the web?

One other point. I'm hoping you are not designing to the yield point. Beyond 60% of the yield point you can't assume the beam will be elastic and you can't be certain about the strain (stretch) that will occur.

Sounds like to me you need to have this checked over by a real structural engineer. What I usually do is make my best guess at it and do all the grunt work. Then hire a consultant to review my design before I proceed.

Answer 3

The easiest way to determine section capacity would be to use the allowable stress design method. This is an elastic method, meaning the it assumes the section reaches capacity at the instant the outermost fiber reaches the yield stress. This will eliminate the need to worry about the different yield strengths. Normally, if the section and the compression flange is braced you can use 0.66Fy. Meaning your moment capacity would be M=0.66FySx. If your section is not braced to help prevent lateral torsional buckling, you will not get that high of an allowable stress.

Answer 4

Also remember as a rule of thumb, that the flange carries moment, and the web carries shear. A member may have a limiting moment when the shear is minimal or visa versa.

Answer 5

Ahhh, engineering, again? Mechanical Engineering is NOT my profession. But, knowing you, I'd say grease away!! You will regardless what others say! LOL ****Penny Laine, I think you're referring to phalanges. (fingers) I'm a nurse and that's the anatomically correct term for fingers. ;)