Confusing beam sizing results 2

[EngineerofSteel]

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.

 

[EngineerofSteel]

More info... I just analyzed the same structure, without any changes except increasing beam size to W12x14. This beam size also fails the Bending ratio test. (B.Ratio>1). AND it has an I greater than the W6x12. So, the Y-Y moment of inertia is not the relevant factor... it seems to be that the web depth is too great. Which makes no sense to me.
My experience is in hydraulics, not structural. A basic concept may be what I am missing.
Thanks, DD

 

[JAE]

What software are you using?
What lateral unbraced length are you using?

 

[UcfSE]

Yes, what unbraced length do you have? That makes all the difference. For long unbraced lengths, shorter relatively heavy sections work better. You also have to check deflection.

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).

 

[SlideRuleEra]

Not using any software, if you look at the "allowable moment in beams" graphs (page 2-175 in AISC 9th Ed., ASD), an A-36, W10x12 "beats" a W6x12 on allowable moment all the way out to an unbraced lenght of just over 8 feet.

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.

http://www.SlideRuleEra.net

 

[EngineerofSteel]

I am using RAM Advanse. I left the unbraced length as the effective length. (25.25' on the longest span) The only "bracing" are the purlins (16 gauge 8x2.5 Z)spaced evenly at 4.59 feet o.c.
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

 

[JAE]

Lateral bracing of flexural members is concerned with prohibiting either rotation of the memeber or lateral translation of the compression (top) flange.

So the purlins most likely would provide the bracing required.

 

[UcfSE]

The purlins may or may not provide bracing depending on how they are connected to the beam and to what they are connected and if they have the strength and stiffness to take a bracing force. Usually if they are connected to a diaphragm then they will work as braces if the connection to the beam is good. They need to be connected to the flange they are bracing. That isn't for every case but to not do so would require more checks. If you have uplift the compression flange is now the bottom flange and your unbraced length may change from that you use for gravity loading.

 

[SlideRuleEra]

Compression flange bracing is what is important. The previous designer probably considered the purlins as adequate bracing since the ASD graphs shows that, for a range of allowable moments, a W10x12 is the most economical section for an unbraced length of 4.6 ft.

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".

http://www.SlideRuleEra.net

 

[EngineerofSteel]

GREAT!!!

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