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Re: Masonry half stresses and strength design

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The wall really isn't any "stronger", the same masonry, rebar etc is in the final product.  The difference is in the analysis you are getting a slightly higher permissible load by using a strength based calculation.  We have reviewed the differences in masonry design with strength vs. wsd many times and figure there really isn't much difference in the final product (small percentage differences).  You are correct that a strength based analysis will give slightly higher calculated capacity for tall walls, for smaller walls the difference is negligible.
 
The question becomes one of margins of safety.  The half stress concept provides a higher factor of safety in design for non-inspected walls to account for the inherent unknowns in material and workmanship.   The intent is for cases where the stresses are low, final design is typically governed by code minimums, and the cost of inspection is not warranted.  Because strength design will provide slightly higher allowable capacity, special inspection is required for walls designed on the basis of strength level calculations.
 
Though what you are proposing seems a logical extension of the half-stress concept, I would not do it.  You are reducing the factor of safety intended for non-inspected construction.  There is a wide variance in the quality of masons and masonry construction, and though inspection is a cost issue it provides a lot of benefit to the final product.  If the wall does not fall comfortably within the low stress intent of the half-stress provisions, provide inspection and design accordingly.
 
----- Original Message -----
Sent: Thursday, June 05, 2003 8:50 AM
Subject: Masonry half stresses and strength design

Strength design of masonry seems to give slightly more economical answer than working stress design, especially for tall walls.  I am evaluating an existing building that wasn't special inspected and would like to extend the half-stress concept to strength design.  I am doing this by using half of f`m for calculating moment capacity but allowing full steel stresses.  This effectively doubles the depth of the Whitney stress block in the moment capacity calc while using only half the allowable stress.  The impact of this approach seems to be a smaller penalty than using half-stresses for working stress.  Translation, I seem to be getting relatively stronger walls with strength design over working stress.
 
Has anyone else tried this or do you have any thoughts on this approach?  I realize this is inconsistent with the original design procedures but it seems rational to me.  Am I missing anything else?
 
Jake Watson, P.E.
Salt Lake City, UT