undersized ridge beam of double 2x10 needs reinforcement to strength of double 2x12 3

[Sambg]

The current ridge beam of a 12'x12' room with a vaulted ceiling is a double 2x10 but it needs a double 2x12'. The room has 2x8 joists at 24" OC and bearing points of two 2x6 on either end of the beam. The roof has been sheathed and strapped so taking out the current beam is a difficult path. What can be done to support the current beam to improve its capacity?

 

[phamENG]

The easiest is to put the correct size beam underneath it (taking into account the lack of bracing or providing some sort of bracing to make it work). The architect won't like it, but the only way to make it look like they intended is to rip it out and put the right size in.

 

[XR250]

That is my typical approach. I suppose one could epoxy and nail a flat 2x4 to the bottom to increase its capacity to almost that of an 11 1/4" member.

 

[jerseyshore]

Reinforcing wood is always tricky. You could do what XR250 said and do a homemade approach that would certainly help capacity although it would be tough to prove on paper.

You could also put a new 2-2x12 beam below it.

Not many other options because 2x8 rafters to 2x10 ridge leave you no space below to reinforce with steel angles or anything like that.

Prying the hangers off shouldn't be too bad and then cut out any toenails.

What I've done before is if it's easier for the framer to sawzall a beam out like this, but the cut is bigger than the double 2x,just put a triple beam back up there to fill up the space.

 

[XR250]

ou could do what XR250 said and do a homemade approach that would certainly help capacity although it would be tough to prove on paper.

I'm getting like 10d @ 3" O.C. in each member at the max shear which is doable and not taking into account the glue

 

[Sambg]

Thank you for this feedback! Laminating and nailing 2x4 as XR250 suggested to the bottom sounds like the easiest approach if I can get this calc shown to account for the load. If that is not possible, I think the next best approach is to sawsall-ing the old beam out and double or triple 2x12 in with new hangers and repair roof sheathing/strapping. Adding an additional double 2x12 under this current one is the back-up, though that changes the look of the room considerably.

The span is about 2 feet too long at 40psi snow load and 10psi dead load for the 2x10. Would diagonal bracing at each end of the beam reduce the 12' span enough to

 

[XR250]

Are you sure about the 10 ft. span capacity? I get 3,750 ft-lb for 50 PSF @ 6 ft. trib. Seems like a lot for current lumber design values. Makes my calc not work as great.
Diag. bracing just shoves the load into the posts which are generally not able to take the demand.

 

[Sambg]

I'm not sure. I was looking at a span table and could have misinterpreted. All I am sure of is the lumber supplier's spec for a double 2x12 for the current design and load for our area(attached). The only difference between their spec and the actual is the bearing points are two 2x6 not 2x4.

https://files.engineering.com/getfi...4e1b-94c4-6b4d8f1136b2&file=Gallaher_Beam.pdf

 

[jerseyshore]

That's SS grade which is pretty rare around here. Also the unbraced length is small with rafters coming into it so that helps capacity. Maybe if you use roof snow load vs ground snow you can get the 2x10s to work.

 

[XR250]

Yea, that is some stout stuff. All we usually get is #2 which is about 800-ish psi now.

 

[Sambg]

I wonder if they actually sell SS at the place that made the beam spec. Their prices are high enough but the lumber I've been getting sure doesn't seem that great.

 

[phamENG]

In these situations, your best bet is to discard the span tables. They are useful for laying out a simple framing plan, but when you start getting into this kind of stuff the work needs to be done by a competent structural engineer. If that's not you, you should find one and hire him/her.

Grade should be stamped on the lumber.

I really don't like the 2x4 on the bottom. It may work 'by the numbers' but a lot has to go right. Wasn't there a thread recently that beat up on any idea that that could work in reality? Between flexibility in the glue, nail slip, and generally sloppy work that often accompanies such things (even the best carpenter is only going to be but so good with a 2x4 trying to attach it to the bottom of a beam overhead and over that sort of length), any composite effects are effectively neutered and you'll likely get more than acceptable sag out of the assembly.

 

[jerseyshore]

I just hate having to rely on nailing to the small dimension on any piece of lumber. The risk for splitting is too high.

You should definitely get a local structural engineer to look at this.

That calc screams non-engineer who did whatever they needed to do to get to 100% capacity. Using SS dimensional lumber, double 2x12s, wrong unbraced length etc.

That ridge should probably be a double 9.25 LVL. But that's what you get when it's not an engineer designing it.

 

[milkshakelake]

Another way if you don't want beam on the bottom is to use a cold formed steel stud at the bottom, like 400S200-97 or something. Will need lots of screws, but there will be less nail slip and more quality control with a crappy contractor. An out of the box idea is to extend a 4x4 or 6x6 post within a nonbearing wall somewhere, though it only works if you're lucky and everything lines up below.

 

[Sambg]

I have a local structural engineer scheduled to meet next Wednesday. This is not my wheel house of engineering. I appreciate all the feedback and it's informing the plan with the structural engineer. I'm guessing we'll be supporting the structure, remove this beam, and install one designed to the space.
Kicking myself for taking a builder's word for it that they knew what to do without verification. Tough lessons to learn!

 

[JStructsteel]

Whats the beam failing in, shear, bending, deflection?

Is 10psf realistic?

How about a flat plate screwed to the bottom to create a composite section. I assume it will be hidden?

 

[Sambg]

I believe deflection. Code in north Idaho is 40psi for roof snow load the 10psi for dead load is just their estimate. We'll get a more precise estimate for the design solution