Differential Axial Shortening of Wood/CMU Structure 2

[JohnRwals]

Hi!

I have seen wood frame Apt building wrapping precast concrete parking structure.
Five story huge CMU walls are laid out as an exterior walls close to the parking structure.
I am curious how axial shortening of wood frame (dotted lines) can impact exterior CMU walls.
Is sliding joint connection used between wood frame and CMU walls?
As interior wood frame settles down, how can finish levels around doors be adjusted like in detail "A"?

JRW

 

[milkshakelake]

Axial shrinkage along the grain doesn't really happen. It's on the order of hundredths of an inch. So it won't affect the CMU wall in the direction of the dotted lines.

Shrinkage happens perpendicular to the grain. This affects mostly top plates and bottom plates. So the shortening will happen on the wall to the right. This will cause the joists to sag on that side and crack the interior/exterior finishes. As for how to connect joists to a CMU wall to prevent that from happening, I'm not sure. I've been thinking about that for a while and haven't found a solution. Maybe frame another bearing stud wall next to the CMU to keep the shrinkage the same as on the right.

Speaking of sliding joints, if you're bearing on CMU, you will need anchorage to prevent sliding. You might also need to fire cut the joists; check your local code, or Google it.

 

[dik]

... also should have a water barrier between the CMU and the lumber.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[jayrod12]

I believe you're asking how the shortening of the wood building is accommodated at the connection to the wall. This has been discussed previously on this site. The main points are to limit the number of plates/joists as the shrinkage perpendicular to grain is the more substantial concern (compared to parallel to grain as indicated by milkshakelake). Often I've seen engineered lumber plates on the lower floors to mitigate the overall shortening. Framing perpendicular to the masonry wall allows the floor to hinge/slope gradually (but you need to be careful with this method as you could end up with substantial slopes in some cases). Also as you've indicated in your sketch, often the openings between the garage and the wood framing are only made at the lower levels to eliminate the potential for substantial difference in floor levels between the two different structures.

 

[masonrygeek]

Here is an article that is somewhat related to that being asked:

https://www.gobrick.com/docs/defaul...above-30-feet-with-wood-framing.pdf?sfvrsn=0.

This is for brick veneer over tall wood frames. The differential expansion/shrinkage is worse in this case and is mostly addressed by allowing each material to move independently and then making sure that any elements that go through both walls are detailed for the higher amounts of movement. Of course, you always redesign the wood apartment as loadbearing CMU and then you don't have this issue.