Confusing beam sizing results
Confusing beam sizing results
(OP)
I am designing a simple pole barn. It is a duplicate of an existing barn. As an Ag building, the DL is just 3.5 PSF and LLr is 10 PSF. The longest span is 25.25 feet. Trib widths are 30'. The previous engineer (with a 40 year history), used a W10x12. I put this into my software, and the software tells me the beam has a high bending ratio (>1) on the X-X axis (the long axis). Then, it optimizes to a W6x12 (!!). This baffles me. I want to guess the 6x12 has a better I value in the Y-Y axis... and it does, but only 2.99 over 2.18 for the 10x12.
There is no X-bracing.
There is no X-bracing.





RE: Confusing beam sizing results
My experience is in hydraulics, not structural. A basic concept may be what I am missing.
Thanks, DD
RE: Confusing beam sizing results
What lateral unbraced length are you using?
RE: Confusing beam sizing results
What you should look at is the rT value if you are using allowable stress design. The rT value for a 6x12 is 1.05", 10x12 is 0.96" and 12x14 is 0.95". That's about 11% difference in favor of the w6. The rT value is used in the ASD equations to find Fb, the allowable bending stress. The higher the rT the higher the Fb basically, for a given unbraced length and all else being the same.
For LRFD you will be looking at the Iy value, or you can compare X1 and X2. X1 for the w6 is much larger than that for the w10 and w12 you mention. This comparisson is of course for a long unbraced length (elastic LTB).
RE: Confusing beam sizing results
Since steel is often assumed to be sold by the pound, a W10x12 and W6x12 would (theoretically) cost the same. As a bonus, delection would be less with the W10. If the unbraced lenght IS less that 8 ft. (or so), sounds like the original designer used (good) engineering judgement to select a deeper beam.
www.SlideRuleEra.net
RE: Confusing beam sizing results
But, adding lateral braces is an option available.
I am using allowable stress design.
Your explanations are good & clear. If my unbraced length exceeds 8 ft, I should look for a more shallow beam. If less than 8 ft, I will need deeper beam sizing.
Which leaves this question: Do purlins attached at the top (compression) flange qualify as lateral bracing? I am under the assumption the lat. braces must join at the lower flange (this info from an "old hand", not engineer.)
Thanks, DD
RE: Confusing beam sizing results
So the purlins most likely would provide the bracing required.
RE: Confusing beam sizing results
RE: Confusing beam sizing results
Your design/construction may differ from the original in small, but important details; be sure to analyze what you plan to do. Don't just copy and build the old design on "faith".
www.SlideRuleEra.net
RE: Confusing beam sizing results
I corrected my compression flange braced width to 4.59'. Now W10x12 is "OK" under all load combinations (DL+LL & DL + WL for Ag with light-gage metal roofing). So, I am pleased. I sweated this one all week, trying to make it work.
Muchas Gracias, Spasibo Balshoi, and Doomo Arigatoo!
The model, with full effective length as the braced distance of the compression flange, withstands the Wind Uplift force, without even the DL, so W10x12 is good, thanks for the reminder, UcfSE.
-DD