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Masonry Design

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I have a question regarding the determination of the design compressive
strength of masonry using the unit method rather than the prism test
method (assuming all necessary special inspections are done).

When using standard ASTM C 90, Type 1, Grade N, hollow load bearing
concrete masonry units with type S mortar, do you use the value of 1900
psi (average minimum net area strength for 3 units) or 1700 psi (minimum
individual unit strength), taken from ASTM C 90 - Table 2, in selecting
the net area compressive strength of masonry (f'm) from ACI 530.1-95,
Table 2 (or Table 21-D, 1994 UBC **) ?  I can't find anywhere in ACI
530.1-95, ACI 530-95, the 1994 UBC, or several textbooks that makes this
clear.  ACI 530.1-95 says that the cmu's are sampled and test in
accordance with ASTM C 140 - but it reports both values.  

I'm inclined to think that it was intended that the 3 unit average
strength be used since this corresponds to a line in the table -
yielding f'm = 1500 psi (rather than making everyone interpolated).  If
anyone knows somewhere where this is clarified I would appreciate being
pointed there.

** Both the 1996 BOCA and the 1997 SBC codes refer to ACI 530 for
engineered masonry design.

Ed Marshall, PE
Simons Engineering,