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bmc846

Out of Plane Anchorage Forces (Rigid Diaphragm)

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

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

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