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Ridge Beam Design

Ridge Beam Design

Ridge Beam Design

(OP)
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.


   

RE: Ridge Beam Design

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.
 

RE: Ridge Beam Design

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

RE: Ridge Beam Design

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.

RE: Ridge Beam Design

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

RE: Ridge Beam Design

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'.

RE: Ridge Beam Design

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

RE: Ridge Beam Design

(OP)
Thanks for all the replies.

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

RE: Ridge Beam Design

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

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

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