Determining Effective Width
Determining Effective Width
(OP)
Often I have to come up with an "effective width" to check a connection capacity. In determining the effective width, I feel like i'm doing just slightly better than just pulling a number out of a hat.
Here's a real application I am working on right now that made me post this:
A lateral load being resisted by a W18 beam's bottom flange.

If the load is light enough, I will justify the web resisting the moment (force times 18") in weak-axis bending (section modulus taken as effective width * thickness^2 / 6). Thinking a 1 to 1 force distribution is logical, the effective width would be 18" * 2.
Does this seem practical, and do you guys do the same? Don't want to go too far out on a limb with my "engineering judgement".
Here's a real application I am working on right now that made me post this:
A lateral load being resisted by a W18 beam's bottom flange.

If the load is light enough, I will justify the web resisting the moment (force times 18") in weak-axis bending (section modulus taken as effective width * thickness^2 / 6). Thinking a 1 to 1 force distribution is logical, the effective width would be 18" * 2.
Does this seem practical, and do you guys do the same? Don't want to go too far out on a limb with my "engineering judgement".






RE: Determining Effective Width
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Determining Effective Width
If the lateral load is occurring along the length of the beam, my preference is to span the bottom flange between regularly spaced kickers or roll beams. This keeps the deck from having to resist the applied moment which would require reinforcing for the negative moment region.
RE: Determining Effective Width
For comparison, you could try applying the concrete traffic barrier formulas which are based on the governing yield line mechanism. I reckon you'll get a much larger number than 45 degrees but that relies on deflection to form the mechanism which may not be acceptable in this case.
RE: Determining Effective Width
RE: Determining Effective Width
Dik
RE: Determining Effective Width
RE: Determining Effective Width
Even if we pretended the lower flange wasn't spanning like that, there isn't a failure mechanism caused by just failing the top edge of the web in bending. You need a couple of hinges stopping load from transferring further down the web, and likely a hinge at the lower flange to web interface. Your ultimate failure won't happen until a total hinge length much longer than 2xD forms. I think you'd likely get a capacity of at least three times that using a plastic analysis. Of course, that neglects deflection as a concern, and doesn't account for interaction with shear or moments in the other direction.
Given the weakness of the member in that direction and the high flexibilitiy, I agree that it likely makes sense to be pretty conservative, and if you need to get more aggressive, to create stiffer and more definite load paths using stiffeners or other methods.
RE: Determining Effective Width
It is specifically meant for analyzing the bottom flange of an beam which supports an underhung crane/hoist, but I think it applies anywhere a concentrated load bends a plate. Dowswell concluded the yield line in this situation is parabolic.
Anyway, calculating the effective width using 45 degree angles is conservative.
DaveAtkins
RE: Determining Effective Width
Do yield line analysis if you care about it. You'll find you pickup a fair length of web, due to the stiffness of the flange.