Sister Wood Joist with LVL
Sister Wood Joist with LVL
(OP)
Hello folks,
I've been working on some calcs and I was hoping to get your take on the following issue:
I have an existing 2X12 douglas-fir wood joists supporting the roof.
My clients want to convert the use of the roof, and want to strengthen the joists to accommodate the larger live and dead loads. Existing roofing is to remain.
I've been thinking to sister a smaller 1.75X9.5 LVL 2.0E joist to the 2X12 would be a good alternative - flush at the bottom face of the joists.
How would you calculate this system? How would you design for the shear flow into the lag screws?
- Rod
I've been working on some calcs and I was hoping to get your take on the following issue:
I have an existing 2X12 douglas-fir wood joists supporting the roof.
My clients want to convert the use of the roof, and want to strengthen the joists to accommodate the larger live and dead loads. Existing roofing is to remain.
I've been thinking to sister a smaller 1.75X9.5 LVL 2.0E joist to the 2X12 would be a good alternative - flush at the bottom face of the joists.
How would you calculate this system? How would you design for the shear flow into the lag screws?
- Rod






RE: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL
That could be tricky. However, there is a way to make the results more predictable by making the composite beam symmetrical. Create a flitch beam by sandwiching the 2 x 12 between a pair of LVLs. Use through bolts, not lag screws.
A traditional flitch beam has a steel plate sandwiched between a pair of wood beams, but there is no "rule" that says those two materials have to be used. The math is the same, just the material properties differ.
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RE: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL
You have to look at EI which will even things out a bit. I would think you would know the E of the DF a little better than you have stated. This is commonly done with nails, no lags or thru bolts req'd. It is unlikely you are transferring more than 100 plf thru your connection.
RE: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL
If you have end bearing at the LVL then just give it the relative portion of load as xr250 said. If you don't have end bearing it's a bit different.
RE: Sister Wood Joist with LVL
1) I feel that the simplified sketch below represents the real mechanical situation. Fastener shear, in a purely elastic system, would be a superposition of vertical shear transfer forces and VQ/IT forces.
2) In reality, dowel connections are pretty ductile. As a result, I think that it's pretty safe to simply design the fasteners for the vertical shear transfer force, ignoring VQ/It forces. Connection ductility effectively provides the horizontally slotted holes in Bookowski's analogy. From a member design perspective, this implies designing the two parts non-compositely which is usually the intent anyhow.
3) For the case under consideration, and indeed for most sistering scenarios, I believe that it would be un-conservative to design for VQ/It while ignoring the vertical shear transfer. As XR250 and RPMG have pointed out, VQ/It won't add up to much in these situations. Designing for horizontal shear only may produce a design that isn't even as competent as the two members acting non-compositely.
I have a tidbit of practical advice to offer as well. If nails are used as the fasteners, instead of lag screws or through bolts, I would ensure that they are clinched. The new side member will tend to laterally torsionally buckle on its own and it will be the fasteners acting in withdrawal that will prevent this.
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: Sister Wood Joist with LVL
Assuming (2)10s @ 12" o.c., you have about 50 plf or so withdrawal capacity - probably enough to keep the LVL stable. I wonder if the shear in the nail adds to the withdrawl capcity?
RE: Sister Wood Joist with LVL
The NDS gives an interaction equation for shear plus tension which seems to imply a reduction. Link
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: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL
I don't think that you would get quite the same load sharing in the absence of the little slotted holes. Imagine a free body diagram of the main member with just the inward pointing VQ/IT horizontal nail shears acting on it. It would arch the main member upwards without affecting the support reactions, a bit like PT balancing loads. The side member, however, would tend to resist the upwards arching of the main member.
The net effect of this is that the vertical load sharing that I indicated in my sketch (EI ratios) wouldn't be quite right. Somewhat more load would be carried by the main member. I know, it's a bit ridiculous how complex this gets. And we haven't even touched on baked in the cake, existing loads effects.
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: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL
Mike McCann, PE, SE (WA)
RE: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL
This statement is true if, and only if, the centroids of the main member and side member are at the same elevation. And in that case, there would be no VQ/It nail shears.
Because the centroidal axis of the side member is on the convex side of the main member centroidal axis, the radius of curvature is greater for the side member than it is for the main member given equal vertical displacement. Larger radius of curvature = less curvature = less load resisted by the side member. And that is consistent with my conclusion above that the main member will carry more than its EI ratio load share as a result of composite behavior and the VQ/It nail shears.
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: Sister Wood Joist with LVL
Nice... looks as though we're going to go all the way with our pedantry.
This speaks to SRE's comment at the top regarding symmetry I think. If the composite shear center were to land within the existing member, then I would argue that the induced torsion would be self rectifying as member twist would shift the point of load application to the shear center. Not so much if the shear center lands within the side member.
Theoretical fun aside, it might be prudent to have some roll blocking installed intermittently.
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: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL
Kootk - I see your point about the horizontal shear stresses induced with the ENA not aligning, however, I tend to agree with bookowski in that, practically speaking, the connection (if failed in horizontal shear) will likely fail with a yield mode that crushes the wood locally in the horizontal direction, thereby creating a horizontal slotted hole and then load sharing based on relative EI takes over.
RE: Sister Wood Joist with LVL
@Bookowski: I'm going to answer your last question this weekend if it kills me. I'm confident in what I've presented above but I'm having a heck of a time answering your particular question in a convincing manner.
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: Sister Wood Joist with LVL
RE: Sister Wood Joist with LVL