Sub-Diaphragm - Partial Length CMU Wall

[sgs114]

Hello,

I have a project with a rectangular building foot print of 66' x 120'. It has a wood diaphragm on the roof and wood (shear)walls in the long direction. In the short direction I have (2) 20' long CMU shear walls and wood framed walls for the remainder. The CMU shear walls are not centered in the building footprint. We are in SDC D, so my question relates to wall anchorage and sub-diaphragm. Currently I have no continuous cross ties in my diaphragm, so I am assuming my minimum sub-diaphragm length = 66/2.5 = 26.4'. If I do add a continuous cross tie at the mid point of my diaphragm it would not even bear on the CMU wall, and it's required anchorage force would be minimal since it would just be the trib of a wood wall, right? I guess my main question is how do you determine required sub-diaphragm length in partial length CMU wall buildings? For my instance, would it be acceptable to say my sub-diaphragm only needs to be the length of my wall (not the distance between the diaphragm chords) divided by 2.5? That doesn't seem right either, but the alternative isn't very intuitive to me at least. All comments appreciated.

SGS

 

[KootK]

I'm game for a deep dive on this but, first, I'll need a plan sketch at least roughly to scale.

 

[sgs114]

Plan is below:

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1596033362/tips/Plan_f3tkhz.pdf[/url]

Seems like maybe the cross tie is purely a diaphragm element and having it anchored to my wood wall for out of plane isn't specifically required. It is just used to distribute the forces to the diaphragm?

 

[KootK]

Thanks for the sketch, it's a great help. Note the following regarding the use of sub-diaphragms in this context:

1) Their purpose is to transmit wall out of plane anchorage forces into the diaphragm via discrete ties without the diaphragm separating in tension along the way.

2) In this context, the code does not allow for a tie that does not run the full width of the associated diaphragm from chord to chord (interestingly, this is still ubiquitous in steel deck roofs). In a theoretical sense, this is the right thing to do. And, given some failures of this nature during past earthquakes, there are apparently good practical reasons to enforce this as well.

4) As a nod to practicality, the code allows you to designate sub-diaphragms, complete with their own chords, that allow your ties to not have to run the full length of the main roof.

So the strategy is:

A) Run your wall anchorage ties the full width of the sub-diaphragms.

B) Collect those tie forces and send them out into a smaller number of larger ties that do run the width of the full roof (your girder line).

C) Sleep easy that you'll not tear the walls away from the building via a diaphragm direct tension failure.