Ridge Beam Design

[jmme]

Can someone please confirm that I am designing the ridge beam correctly for the following?

26' wide building with a metal roof with a 12:12 pitch (unobstructed, slippery slope).

The rafter span is 18.5'.

My roof snow load is 17.3 psf which I got from the following:
Pf=0.7 x Ce x Ct x I x Pg
Pf=0.7 x 0.9 x 1 x 1 x 55
Pf=34.65 psf

Ps=Cs x Pf
Ps=.5 x 35.65
Ps=17.3 psf

If I add 10psf for a dead load my ridge beam will need to carry
W=(17.3psf + 10psf) x 18.5' = 505 plf.

The ridge beam will span 12 feet.

Based on these numbers a 1-3/4" x 11-7/8" Microllam LVL will work. However, in the iLevel sizing tables for ridge beams the smallest they list is a 3-1/2" x 9-1/4". Also, when I enter in my data to iLevel Forte is tells me a 3-1/2" x 11-1/4" lvl will work.

This makes me think my calculations are wrong or missing something. Can anyone let me know if my calculations are right?

Thank you.

 

[vato]

You're close with the 11 7/8 but probably busted on deflection.
Check that.
Or put a ceiling in (tension tie) and leave the ridge beam out.

 

[MiketheEngineer]

I would use 2 - 1 3;4 x 11.25 Microlams. No problem. Deflection a tad high but acceptable for a roof.

 

[Lion06]

Just remember, deflection of the ridge beam results in thrusts at the eave line. Maybe not for the statics to work out, but it's still there. So, if you have a soft ridge beam, you're getting more thrust (or movement if you don't take the thrust out) at the eave.

 

[BAretired]

In your calculation of W, you are using 18.5' as the tributary width. If the building is 26' wide, the tributary width is 13', so that W = 27.3 * 13 = 355 plf.

BA

 

[gte447f]

BA is right. You have an inconsistency between your rafter span and your building width. 18.5' appears to be your rafter length (i.e. on the incline @ 12:12 slope). The rafter span, in the horizontal plane, and consequently the tributary width to the ridge beam, is 26'/2=13'.

 

[kipfoot]

to add to BAretired's observation: if the 10 psf dead load is per sq ft of surface area, then the DL on the projected tributary width would be DL = 10/cos 45 = 14.1 psf. Then,

total uniform load on ridge beam = (17.3 psf + 14.1 psf) * 13' trib = 410 plf

 

[jmme]

Thanks for all the replies.

kipfoot, I should use the same 14.1 psf DL for my rafters then right?

 

[msquared48]

14.1 psf to the horizontal span, or 10 psf to the slope span.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask