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RE: 1997 UBC Masonry Wall Design

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O.K., some edits and corrections of my previous post.

Thanks to:
Parker Resnick, S.E., Steve Rex, S.E. and Larry Hauer for comments.

First of all, even though I originally stated that Na=1, I used 1.3 in the
numbers I provided.

Also, I did not consider the cutoff at the bottom of the wall which is equal
to 0.7*Ca*Ip*Wp. This cutoff is a uniform load distributed over the lower
0.367*hr section of the wall.

So, let's try this again:

hr=27'-0"
Top of parapet = 29'-6"
Na=1.3
Soil Type SD
Ip=1.00
Ca=0.44*1.3=0.572

For the lowest 0.367*hr section (9.90 ft,),

Fp=0.7*Ca*Ip*Wp/1.4

Fp=0.7*0.572*Wp/1/4=0.286*Wp

At the roof level, ap=1, Rp=3, hx=hr

Fp=ap*Ca*Ip*(1+3)/(1.4*3)*Wp=1.0*0.572*1.0*(1+3)/(1.4*3)*Wp=0.545*Wp

For the parapet, ap=2.5, Rp=3

At the roof level:

Fp=2.5*0.572*1.0*(1+3*27/27)/(1.4*3)*Wp=1.362*Wp

At the top of the parapet:

Fp=2.5*0.572*1.0*(1+3*29.5/27)/(1.4*3)=1.456*Wp

Based on medium weight 12" block, grouted at 16", Wp=94 PSF

I get a maximum bending moment in the wall of 3,200 ft.-lbs. (including some
P*e effects of the roof load) and a wall tie force of 863 lbs (not including
the 1.5 factor for flexible diaphragms.).

In comparing with the 1994 UBC,

Between supports: Fp=0.3*Wp

At the parapet: Fp=0.8*Wp

The maximum bending moment in the wall is 2,690 ft.-lbs. (1997 code
represents an increase of 19% including the effects of Na=1.3) and a wall
tie force of 564 lbs./ft. (1997 code represents an increase of 53%).
However, the 1994 UBC is a LOT simpler.

Comments?

Bill Allen, S.E.
ALLEN DESIGNS
Laguna Niguel, CA