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RE: 1997 Wall Anchorage

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Paul.
 
I have approached this by calculating Fp coefficient at midheight between Roof Elevation and slab level ( for a 1 story structure) when designing the wall (tilt-up or cmu using the slender wall design parameters). However, using that coefficient is incorrect for wall anchorage because the code plainly says to use the connection height with respect to the roof height - they are one in the same as you know. I have then calculated the reaction, using statics, using this roof height coefficient applied as a uniform load along the height of the wall, to get my reaction at the wall anchor. You can't to my knowledge, use a trapezoidal distributed load that increases along the height of the wall as I believe you are suggesting. Of course, the result is a huge difference as you have discovered. If anyone has a published contradiction to this, please let me know as well. I think that when they say mass distribution it is likely that as wall thicknesses change, the load should be adjusted with respect to the mass, (i.e. 12"cmu courses and thins to 8" cmu).
 
-gerard
SJ,CA
-----Original Message-----
From: Paul Feather [mailto:pfeather(--nospam--at)san.rr.com]
Sent: Thursday, September 27, 2001 4:39 PM
To: SEAOC List
Subject: 1997 Wall Anchorage

Hello All,
 
I need help with an interpretation.  Under 1633.2.8.1, out of plane anchorage to flexible diaphragms, the directive is to use section 1632 with Rp = 3.0 and ap = 1.5.
 
Section 1632 provides the Fp equations with upper and lower bounds.  According to the SEAOC Blue book, for walls Fp average is used with the lower bound controlled by Fpmin (hx = 0 controlled by .7Ca) and Fp calculated at hx = hr.  Amplification due to parapet is accounted for in the statics.
 
In applying 1633.2.8.1, our interpretation is the force at the roof calculated in the same way, Fp average derived from Fpmin and Fp calculated at hx = hr, with ap = 1.5.
 
A recent comment is that this is incorrect and that according to the Seismic Design Manual, Fp anchorage is calculated as Fp at hx=hr applied to half the wall height plus parapet as a straight tributary force.  This completely neglects the provisions in 1632 where the forces shall be based and distributed in proportion to the mass distribution of the element.
 
The calculated difference for a 24 foot wall with hr = 20 ft. is 44%.
 
Which is correct?  What was the intent of the code?  I do not see anywhere in the code or the Blue book where this alternate method of calculation is specified, only the provision for the higher force at roof level (2.0Ca)
 
Paul Feather