Ridge beam undersized--any ideas on how to reinforce it now that its in place?
Ridge beam undersized--any ideas on how to reinforce it now that its in place?
(OP)
Members-
I was called to look at a 2-2x12 ridge beam spanning 11'9" on a mostly finished porch roof yesterday afternoon. The rafters are 2x8, and the roof pitch is 5:12. The issue is that the inspector noticed a difference in depth of the 2x12's, one is 11-1/2" and the other about 11". He said that the Contractor needed an Engineer to determine if the ridge beam was adequate.
I ran StrucCalc on it and it is undersized by 6% for Moment. Required depth is 11.58"
I am not coming up with any solutions to alleviate the issue without replacing the existing 2x12's with an LVL set.
I know this Board has many folks who must have come across something like this in their travels-- so does anyone know of a solution that could correct the the the existing 2x12's without taking them out?
DusterMick
I was called to look at a 2-2x12 ridge beam spanning 11'9" on a mostly finished porch roof yesterday afternoon. The rafters are 2x8, and the roof pitch is 5:12. The issue is that the inspector noticed a difference in depth of the 2x12's, one is 11-1/2" and the other about 11". He said that the Contractor needed an Engineer to determine if the ridge beam was adequate.
I ran StrucCalc on it and it is undersized by 6% for Moment. Required depth is 11.58"
I am not coming up with any solutions to alleviate the issue without replacing the existing 2x12's with an LVL set.
I know this Board has many folks who must have come across something like this in their travels-- so does anyone know of a solution that could correct the the the existing 2x12's without taking them out?
DusterMick






RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
I may bolt/screw something to the bottom......
XR250- Just lazy I guess.......or old. LOL
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
Adding a steel strap underneath as a possibility?
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
With that said, the analysis is not significant to 3 digits (106%), and I wouldn't reinforce this beam, unless I believed the beam was undersized to begin with.
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
Dave
Thaidavid
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
I'll reply after I see what the situation truly is.
Thanks for all your valuable input and responses!!
Dustermick
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
Appearance matters - since it appears they are visible. A sideplate of 8x1/4 plate would be easier to mount, to reinforce the member against vertical loads (across the center of the span ??)
But the true calculation needs to be correct as well - i am VERY surprised that a 1/2 of wood on a doubled 2x12 member actually lets you fall under the Code strength requirement! Check very, very carefully EVERY assumed condition for the loading.
I would assume a nailed-together-doubled-2x12 member would be OVERSIZED by at least 30% to 50% based on minimum stress levels for commercial wood and nails.
So why is this doubled member only 102% as strong as design theory says it should be?
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
But in all honesty, any engineer that has worked with wood more than a few times won't even take a second glance at an existing condition of 106%.
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
Or, to look at it another way, is there enough interaction between the sheathing and the beam to contribute 6% worth of capacity? Who knows? But the likelihood of it probably falls within the same realm of statistical uncertainty as guided the development of the proscribed imposed loads.
My mentor's mentor had a slide rule with a broken cursor on it. He used it to design many buildings, explaining to his protege that that was all the accuracy he needed...or could rely on.
I'd leave the beam alone.
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
While I agree the beam is okay and isn't worth strengthening, I'd try to justify a reduction in loads to bring the member closer to capacity so you can seal the letter and feel better about it. If they ripped a 2x14 down to a 2x12, then that is a whole different issue.
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
I've been thinking about this too and I would be willing to write and seal such a letter. And, rather than discussing difficult to quantify bonus items such as diaphragm action etc, my preference would be to simply state that I feel that a 6% calculated overstress is acceptable given the degree of accuracy inherent in our work. I feel that such a response would be the most honest. I would, of course be curious to see how such a letter would be received by the reviewing authority.
I feel that it's a logical error to make arguments such as "loads are less than we calculate" and "wood is way stronger than we assume". It is my understanding that these things have been statistically calibrated to produce, to the best of our ability, a uniform level of risk among our various construction materials. Just because wood has a higher COV than many of our other materials, that doesn't justify FOS encroachment in my opinion.
Of all the "leave it alone" arguments, I like this one the best. Unfortunately, round would put us at 110% instead of 100%. Doh.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
I wonder if the statistical calibration takes into account the inherent redundancy in light framed, wood structures? Probably not.
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
For most of us structural types - we tend to always try to analyze a structure based on very discrete load paths that are mapped out in our minds, and also in our calculations, such that we tend to ignore other load paths that do exist. It is just easier and more conservative to map out the "load follows stiffness" path and ensure that the path is adequate.
One of the features of wood framing with sheathed walls, floors and roofs is that the load, in reality, gets spread out in a lot of different ways.
For this particular case, my first thought was: "why don't we look at the fact that a ridge member also has sloped rafters that may be propping it up".
But with a discrete load path we'd also reason" "nope - that can't work because there isn't an eave beam that can take the horizontal thrust of the rafters."
But in reality - the sheathing on the roof does act as a sort of large leaning beam or diaphragm that does inhibit the vertical deflection of the ridge beam to a degree.
The problem is - how do we quantify HOW MUCH it helps? The steeper the pitch the more it helps. It also has to have adequate connection to an adjoining roof or system to prevent deflection as well.
But give 1.06 as a unity on this design - and knowing,...knowing...that the sheathing does help - I would be more inclined to write the letter at 1.06 and say its acceptable.
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RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?
There is the repetitive member factor. Of course that would just be a drop in the redundancy bucket.
Back in my metal plate connected wood truss days, the hot topic was coming up with ways to account for system behavior in component design. It went nowhere because, frankly, it's just do damn hard to come up with something that is reliably quantifiable and/or easily applied.
While there is surely some low hanging redundancy fruit out there to be picked, until we can figure out what to do with it, I don't feel that we have any business relying upon it. We still don't even have a live load reduction protocol for stud walls do we?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Ridge beam undersized--any ideas on how to reinforce it now that its in place?