Quick question for the Engineers...

[nailit69]

Not sure if this is the right place for this question but here goes.   

I'm working on my cabin in Okanogan County, Wa. and looking to save some money by searching for a ridge beam on Craigslist, i've found a couple but not sure if the sizing is adequate.  Wondering if anyone can help me make an informed decision.  I found a 3.5"x13.5"x27'-0" glu-lam but not sure it's enough... i'm not looking to skimp on quality, just save a buck or two if possible.

I'm looking to span 25'-0" outside to outside (24'-1" inside). I'll be using 2"x12"x10' DF rafters @ 2'-0"o.c., 12/12 pitch with no intermediate support.  I'm at 2500' elevation so snow will be a factor but i'm planning on using metal roofing which (I thought I read/heard) can reduce snowload by 60%?  According to the county the snowload is 80/130.  What is the appropriate size for a beam in this situation?

Thanks in advance...

 

[Don_P]

Just a carpenter here but maybe some help. I think the 80/130 numbers you gave are wind/snow design loads? You are correct on the allowable deduction for a slick roof with no obstructions that would tend to hold snow. I use the full ground snow load because I've seen it all stick on a 12/12 metal roof before after freezing rain followed by snow, all it takes is once, but your call.

There is a code stamped usually on the top edge of the beam, something like 16F-V1 which relates to the design strength of the beam according to the species, grade, and layup of the beam... the strength varies according to what they glued together.

I believe you can download span tables from the (google this) American Institute of Timber Construction
If I remember right the publication is AITC-117 but the titles on their pubs page should tell. If you get stuck holler I do have an old '80's copy of their timber construction manual. You don't want to have to buy it for one beam. A retired engineer friend gave me his old copy last week in a box full of his manuals. I had my copy from back in the day so passed a few of his manuals on to our new engineer intern yesterday.

My gut says not enough but that is not the way to check a beam.

[MontanaJared]

I just did a lot of legwork on this topic...... my 24' with a support at 12' with 90lb snow load was 3.5x14 (for half your span), and that was an LVL. Don certainly knows more than me, but I can confirm that your beam is undersized.

J

[nailit69]

I thought that 3.5"x13.5" was a bit undersized just wanted to confirm.  What would the appropriate size be for the span?

[Don_P]

24' span, how wide is the building? 130psf snow?

[Don_P]

Here it is, these are Hahney's adjustments to snow load
Use:
100% for a roof near the ground with a slope less than 3/12
90% where the building is located in a densely forested or sheltered area
60% where the building is located in open terrain extending at least 1/2 mile
70% for all other structures


That result P_f can then be further modified. For a slippery roof when snow load is greater than 100 psf and slope is greater than 20 degrees the factor obtained from the following equation can be multiplied by P_f.

C_s= the adjustment to be multiplied by the normal roof snow load.
a= the roof angle in degrees

C_s=1-(P_f-20/ P_f)(a-20/40)

Off to try to build a temp roof, pouring a slab tomorrow and they just changed the forcast 

[nailit69]

The building is 14'x24' w/a 1'-0" popout for a 25' span overall sitting on a concrete foundation.

I don't recall where I came up w the 80/130 figure... the website wouldn't load yesterday so I was guessing from memory.  I'm looking at the design criteria/snowload figures on the county website again... ground snowload 30/105...wind speed 85 exp. C... seismic design Category C...

I'm a Carpenter too... 25 yrs... Desinged a few, built a few more, never did any real beam calcs though... anything that's not prescriptive or standard practice has always been engineered.  My general rule of thumb has always been 1" of 4"x beam for each 1'-0" of span for floors/doors... roofs are a different animal.   I can generally tell by looking if something isn't sized correctly and I didn't think 3.5x13.5 was going to be big enough ... just wasn't certain when it comes to roof loads and snowloads.  I'm shooting in the dark but guessing something in the neighborhood of 6"x16" seems more appropriate for the span.  Also a factor... there will be a transition to a 4/12 porch roof on each side spanning approx. 8'-0".  Is there any difference w/regards to sizing  using an LVL vs. PSL vs. GLB?

While I have your attention I have another question... with regards to the floor, is a 4x10 Doug Fir beam adequate for a 14' span?  I'm basically planning on dividing the 14'x24'  floor into 1/3ds' with the 4"x10" @ 8'-0" o.c (spanning 14') and then hanger the 8'-0" joists between them @ 16" o.c..  My current house has the same beam/joist sizing and span with little to no bounce... i'm guessing again but it seems adequate.

[Don_P]

That snow load number is greek to me  Severe winter, do you pile up 3' deep or 10' deep?
I keep my calcs here; timbertoolbox.com

I'll use this one for the ridgebeam;
http://www.timbertoolbox.com/Calcs/beamcalc.htm
I'll run through an example, you'll need to confirm and adjust some I imagine.
Load- 24'x14' x(30psf snow + 15psf dead load)=15120 lbs uniformly distributed load on the roof.
Span- 289"
Width- I'm going to go with LVL's, 1-3/4" width/ply, try a triple 5.25"
Depth- try a 16"
Fb- 2900 psi
E- 2.0
Fv- 285psi

I get a pass at 18" for a roof but it still sags about an inch at full load, judgement call, one good rule of thumb is to keep deflection under 3/4" on those longspan beams even if they "pass" in deflection by code.

LVL's have higher design values than glulam, I don't have PSL numbers at hand but they won't exceed lvl.
You'll need to check the supplier's design numbers or have them check (best) but those were typical LVL strengths. Glulam, the first part of that code 16, 20, 22 or 24 is the Fb number so 1600-2400psi in bending strength, the lvl is 2900psi. E, stiffness, what controls deflection, is typically 2.0 in LVL but is only solid wood stiffness in a glulam 1.3-1.8. Horizontal shear strength, the same 285 psi in LVL, 90-165 in glulam (this might be higher since my book was published... an interesting engineering math error that persisted for about 40 years and was caught around 2001. oops, it happens. Shear rarely controls though and never in a longspan beam.) You can do it in a glulam, it'll just need to be bigger than an LVL. You'll need that code and species along with dimensions. Typically a glulam is 3-1/8 (3.125 in the calc) or 5.25" wide, they start with 2x4's, 2x6's etc but resurface the thickness before laminating to get a fresh surface to glue to and then they plane the width of the beam after gluing to clean it up, that's why the 3.125 and 5.25" width on glulam.

Shed porch roofs, I lay them on the roof sheathing but run kickers up from the exterior wall top plate to the underside of the porch rafters to support them. This keeps from getting into engineer territory and reinforces the roof where the snow will pile up near and below the pitch break.

Floor girders, assuming nothing other than just floor loads;
http://www.timbertoolbox.com/Calcs/ddsimplebeam.html
Load- 14'x8' ( this beam is supporting the joists halfway to the adjoining beam on each side, so  a swath of floor 4' on each side of the 14' girder, so 8'tributary width)... so 14x8 x 50 psf (40lb Live load+ 10 psf Dead) 5600 lbs

Span-168
width and depth, start playing
click species and grade from the list
duration, 10
repetitive member, no if a solid sawn, yes if a built up (you get a 10% strength bump for 2 ply and a 15% for 3 ply or more ( better defect scattering vs a single solid sawn beam)
Click the appropriate depth adjustment
No
No

I was up to a triple 2x12 in #1 DF before it worked for me. In 2x10 it took a 4 ply Select Struc built up beam to get a pass. look at the bottom box as you mess around with sizes and grades and you can see how the base design values and strength adjustments play out.

what was the question 

Now if the rain will be kind till we get the trucks in and out tomorrow... third try at scheduling this has got to be the charm.