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RE: 1997 Wall Anchorage

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Where does 44% come from?  In the Seaoc method you described, the tributary weight is 24^2/(2x20)=14.4.  In the design manual example, the tributary weight is (20/2)+4=14.  I see this as only about a 3% difference.  I think either method is acceptable and both are used in practice depending on the engineer (hence the different methods indicated in each reference).  Use engineering judgement.
Chris Roper

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