Sister Wood Joist with LVL 3

[RM87]

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

 

[KootK]

Continuing the overthinking category.....the different sections and EI will cause lateral torsion in the weaker member.

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.

 

[bookowski]

I still don't see how the presence or absence of horizontal slotted holes would change your argument. In either case the side member takes the same shape, agreed it's a larger radius or curvature relative to the main member but isn't that case in both scenarios?

 

[Lion06]

Just one thing no one mentioned that will certainly affect your calculations is that Emin applies only to column buckling calculations, not to deflection checks (or load sharing based on relative EI).

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.

 

[KootK]

@Lion06: I agree completely. And great catch with Emin

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.

@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.

 

[bookowski]

It's ok, I think I'm convinced - after that last post I realized what I was missing. In either scenario the sistered member takes the same shape (in terms of curvature), I was thinking that this defined the internal forces which is not correct. This is only correct in the absence of add'l external forces (i.e. nails pulling on it). If they are perfectly composite then the sistered member takes this curvature but has additional elongation as well (as it gets pulled by the main member and this assumes bottom flush), kind of like a P in addition to the M. So same curvature but in one scenario the member is longer. I think that's it... You could look at a fully composite shape and work out the internal moment in the sister, then look at the sister alone given only the curvature and compare the two. That should jive with what you had and what I have above.

 

[bookowski]

and to clarify - when I say the sistered member gets longer that is along the bottom face... parts of it get shorter (at its top).