# Out of Plane Anchorage Forces (Rigid Diaphragm)



## bmc846 (Apr 10, 2012)

I have read through several texts looking for a definitive answer on when to use ASCE Ch. 12 vs. Ch. 13 and am more confused than when I started. As this exact topic was my "needs improvement" last October I'm fairly concerned with it. I'll do my best to explain what I'm seeing.

Text 1: *2009 Design of Reinforced Masonry Structures by CMA of California and Nevada*

Structural walls supported by diaphragms that are not flexible, the out-of-plane forces are given by ASCE 7 sections 12.11.1 and 12.11.2. Non-structural walls connected to non-flexible diaphragms are given by ASCE chapter 13. Section 3.9.2 and 3.9.3 for those who have the book.

Text 2: *Seismic and Wind Forces 3rd Ed. by Alan Williams*

Anchorage for a rigid diaphragm shall resist the horizontal forces determined from ASCE Eq. 13.3-1. Page 96 for those who have the text.

My Assumptions / Confusion?

My interpretation of the code falls directly in line with the statements from text 1. I was shocked when reading through text 2 because there is no discussion or any way I can read the code that says all connections to rigid diaphragms should be design per ASCE Ch. 13. I felt that a flexible diaphram should have higher anchor loads than a rigid diaphragm due to acceleration. How are others interpreting this?

Also, could a shear wall possibly be considered a non-structural element for out-of-plane loading? I felt that if it is load bearing it will be structural regardless of loading direction but in a gravity supported frame system it may not.

Any help would be appreciated as this is not something I commonly work with.


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## McEngr (Apr 10, 2012)

bmc,

It sounds like you're saying that chapter 13 is required for rigid diaphragms and section 12.11 is used for flexible based on item 1 and 2 above.

"Non-structural walls connected to non-flexible diaphragms are given by ASCE chapter 13." - a non-flexible diaphragm would be a rigid diaphragm... no inconsistency with the Alan Williams book.

Whether the wall is structural or non-structural is not the issue as much as it is that the diaphragm in the Williams book is not flexible.

See attached spreadsheet. It might help.

WallLateralForce-IBC2006.xls


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## McEngr (Apr 11, 2012)

bmc, please let me know if this is helpful. Game day is approaching...


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## bmc846 (Apr 11, 2012)

My biggest concern is that it seems Williams is saying that "all" walls to rigid diaphragms fall under the guidelines of ASCE Chapter 13. Then CMACN states that only "non-structural" walls to rigid diaphragms fall under the guidelines of ASCE Chapter 13 but they conveniently do not cover this in their example problems.


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## bmc846 (Apr 11, 2012)

McEngr,

I've have gone through the excel sheet you provided (thank you for that by the way) and it matches with how I planned on approaching this type of problem. I was just confused when reading the CMACN information. I just want to be prepared if I show up and IL doesn't allow all of my text books. I don't like the fact that Williams book on Wind and Seismic has the word "examples" in the title.


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## McEngr (Apr 11, 2012)

My understanding is that all rigid diaphragms have to be checked for the worst-case. My spreadsheet was borrowed from a PhD in California, fwiw.


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## rexman (Apr 12, 2012)

According to ASCE 7-05 section 11.2, a Structural Wall is defined as: "Walls that meet the definition for bearing walls or shear walls." My understanding is that this would mean that the anchorage should be designed in accordance with section 12.11.


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## hank89 (Apr 12, 2012)

McEngr:

2006 IBC Structural/Seismic Design Manual Problem 35 Commentary "...For rigid diaphragms of SDC's C, D, E and F, the seismic anchorage forces are given in 12.11.2"

I haven't seen any references indicating using chapter 13 for structural walls with rigid diaphragms.


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## McEngr (Apr 13, 2012)

Engineering-international.com has software saying otherwise. I just hope it's not on the exam in the morning. Yikes!


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