1

I am removing a center post on my patio cover to create a clear span of 15'. The current beam is 2 2x8s with three 4x4 posts. I have had a engineer friend calculate the load for the roof (pic attached). His recommendation is to replace the beam with 3 2x12s to support the roof loads and eliminate the middle post. Posts will become 6x6s.

My question is: What would be a suitable beam replacement for the suggested 3 2x12s? My goal is to find something that will be less tall than the 11-1/4" of the 2x12 beam since the existing patio cover beam is pretty low.

My thought is a wood I-joist or LVL, but I haven't worked with them before and am having trouble finding specs.

Thank you.specs

  • 1
    There's no such thing as a wooden I-beam. There are I-joists, but they carry relatively little load. LVL (laminated) beams have a substantially higher load rating than solid wood. Two or three 9-1/4" LVL beams should do, but I'm not someone well versed in load tables, so I can't offer specifics. – isherwood Jul 13 at 19:31
  • 2
    Why not buy your engineer friend another dinner/pizza/beer and ask him for another option? Ask her to spec out an LVL and/or steel. (Steel for that span would likely be pretty darn small, though it'd be a royal pain to get up there.) – FreeMan Jul 13 at 20:45
  • Anything you use has to be approved for exterior exposed to rain, right? – Jim Stewart Jul 13 at 21:18
1

You can do it with LVL.

Grabbing the first LVL spec sheet I came across (Boise Cascade, from https://www.bc.com/versa-lam-lvl-span-size-chart/), I find a Versa-Lam LVL 2.1E 3100 has an allowable bending stress of 3100 psi. That will get you above the 2930 psi from your friend's calculations. So let's start there.

That LVL comes in (among other sizes) 3.5" x 7.25", which is really close to your existing doubled 2x8. In fact, it's enough bigger that it'll give you I (=bh3/12)) of 111.1 in4, up from the 90.4 in4 of your current beam.

Bending

So then your bending stress (fb=MC/I) becomes (74390 * 3.625)/111.1 = 2426.2 psi. That's below the 2930 psi allowable, so that will work.

Shear

The allowable shear for that LVL is 4821 pounds -- well above the 1725 pounds that you need to carry.

Deflection

The LVL also has a higher modulus of elasticity (E) than typical dimensional lumber, but will have a smaller I than the triple 2x12, so let's check.

Deflection = 5WL4/384EI = 5 * 20 * 172.54 / 384 * 2,000,000 * 111.1 = 1.04 inches. That's slightly over the 0.96 inches allowed. You could go up one size (to a 3.5 X 9.25 or to a 5.25 x 7.25) and that would get the deflection back under 0.96 inches.

| improve this answer | |
0

I think your friend may have miscalculated. I’ll make a few assumptions and you tell me if I’m wrong.

The clear span distance between your posts is 14’-1”.

Your friend says the 2-2x8 beam is “No. 1 pine” (probably Lodgepole Pine, not Ponderosa Pine, Sugar Pine or Idaho White Pine). Although, the allowable stress loads do not match the WWPA Grading Rules for the “new” sized lumber.

So, I’m guessing this double 2x8 was installed prior to about 1970, when the milling of joists, etc. went from 1 5/8” x 7 1/2” to 1 1/2” x 7 1/4”.

Your friend used the following loads:

Snow Load: 30 psf

Dead Load: 10 psf

———————————-

Total Load: 40 psf

The tributary load is 6 foot (which means your roof is 12’ deep with 1/2 that depth attached to the building and 1/2 resting on the new beam.)

Therefore, the load on the beam is: 6’ x 40psf = 240 plf

OPTION #1: Therefore, you could use 3-2x8’s Lodgepole Pine , Grade: No.1, for a net clear span of 14’-1”, which equals about 260 plf which is greater than the required 240 plf.

In addition, if your snow lasts less than 3 months, you can increase the allowable stress values too, which gives you a greater factor of safety.

OPTION #2: You could use a 3 1/8” x 9” glu-lam beam. (I used fb=2200psi for bending.)

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.