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[SEAOC] Gyp. board shear walls in LA

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I have recently been working on a seismic evaluations of both 2 and 3 story 
wood framed apartment buildings in LA.  I am curious as to the rational used 
in the drastic drop in the allowable shear value for gyp. board.  According 
to research the allowable shear value of a 5/8" blocked gyp. board shear wall 
with 6d @4"o.c. has been modified in recent codes as follows:

		Prior to 1988 UBC:		175plf

		1988 UBC thru current:		1/2 * 175plf = 87.5plf

		Current LA requirements:	30plf max

I understand that gyp. board will tend to crack and can therefore not handle 
cyclic loading very well.  Also, accoriding to some of the articles I have 
read if the nails are overdriven and break through the surface of the gyp. 
board there will be less capacity.

My question is with the current 30plf allowable in LA.  Is that the true 
capacity of the qyp. board?  If so was the original 175plf a complete over 
estimate of the gyp. boards real capacity?  If the 175plf was an overestimate 
then it was so by a factor of almost 6:1.

Any thoughts you might have on this topic would be greatly appreciated.

Stephen Fisher, PE

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