SDPWS SECTION 4.2.5 AND CITY OF LOS ANGELES PLAN CHECK CORRECTION 1

[Adamian]

In the City of Los Angeles:

"Supplemental PLan Check Corrections Sheet for Structural Design - General"

http://www.google.com/url?sa=t&rct=...33obf7XDsOqnFEEBf-2yNw&bvm=bv.47008514,d.cGE.

"H. Wood Design, item 15" does not allow wood panel diaphragms used to transmit lateral forces unless it s constructed of straight sheathing.
Where is the conflict with the SDPWS 4.2.5 ? Why a rigid diaphragm not allowed?
What is "transmitting lateral force by rotation" in this particular case?
Any clarification will be greatly appreciated.

 

[JoshPlumSE]

"Transmitting lateral force by rotation" probably refers to a case similar to Figure 4A from SDPWS which shows an open front structure with walls on three sides and the front face free of any lateral force resisting system. Essentially an extreme torsion irregularity and a soft story combined together in one system.

In retrospect, it's not surprising that these types of structures performed poorly in Northridge. The only way these worked on paper was by assuming a rigid diaphragm and accepting that the diaphragm rotation would allow the perpendicular walls to resist the torsional force. The problem is that the lateral drift at the open face of the structure would get very large and degrade the capacity of the vertical load carrying system.

 

[Adamian]

Thank you for the clarification.
My project is a proposed deck attached to an existing two story wood framed house.
Steel posts with grade beam could be the next choice, but the owner/builder wanted to use wood posts.
I assume that Figure 4B will apply. Is it required that the entire diaphragm is rigid?

 

[JoshPlumSE]

Rotation is the only way to obey statics if you don't have a lateral resisting system on that front face.

I don't have any real experience on these types of projects, so I'm not familiar with current industry practice. But, my tendency would be to use some type of a lateral frame (moment frame?) on the front face instead. Maybe a Knee brace system or something along those lines. Either way, I would make sure that your columns are designed to withstand the P-Delta effect of the expected drift... preferably using the expected INELASTIC drift. Not usually done, but since there is so little redundancy in this type of system and they have a history of failure.

 

[Adamian]

My loads are low in my opinion,
15'x48'x20 psf = 14.4 K
so the seismic lateral force is less that 5K.
And the beam+post connection will give some resistance.
Thank you very much for your comment.