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Re: Slender Masonry Walls

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In a message dated 4/19/99 5:32:06 AM EST, KAMartin(--nospam--at) writes:

<< I recently completed a design for a masonry building with walls 27'-0" 
tall using 8 inch CMU.  I followed the Load Factor Design method of the 1997 
UBC and satisfied both the strength and serviceability requirements.  A 
senior coworker has questioned my design, primarily because the h/t ratio is 
over 40.  He says that the "slender wall" design approach is "new and 
untried" and "pushing the envelope".
 I have not heard of any problems associated with properly designed and 
detailed slender masonry walls.  I don't consider the design method 
particularly "new" as the publication that I reference was printed in 1986.  
Am I "pushing the envelope" designing a 27 foot tall wall with 8 inch block?
 --Kipp A. Martin

This may be a little late in being posted, but I don't think you are going to 
have a problem other than maybe the constructability based upon rebar size, 
spacing and experience of the mason.  I have gone as high at 30'-0" using 8" 
block wall which was reinforced using # 7 at 16 each face (staggering rebar 
at each face so effectively you have one bar each cell).  The heavy 
reinforcing was a result of considering roof diaphragm deflection due to 
seismic as an additional p-delta loading in addition to static p-delta 

The mason would have preferred using 10" block (which I am told costs the 
same as 12" block per CMU, but is cheaper than 12" when you consider that 
less grout is required) to make the grouting operation easier than when using 
the 8" wall.  We  used one curtain of horizontal reinforcing, staggering the 
horizontals between each vertical curtain.  What the contractor missed was 
staggering the rebar by lift, that is to say place all the horizontal rebar 
on one side for the first lift, and then place all the horizontal rebar on 
the other side for the second lift.  Offsetting the horizontals to one side 
or the other by lifts was necessary in order to get the pencil vibrator in 
the individual cells due to rebar congestion (remember that lap splices will 
be occurring in some cells for both vertical and horizontal steel). 

Michael Cochran