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RE: Masonry Design

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This is a problem area for masonry.

Check Amrhein, "Reinforced Masonry Engineering Handbook", Section 1.4.  The edition I have is the 4th.

Amrhein recommends that each component test 25% to 35% higher than the specified strength.  The minimum component strength is defined as the average of 3 units tested with the strength of no single unit 20% below the specified minimum average.

It is interesting to compare this methodology to the ACI 318 section on standard deviation and default standard deviation.

Amrhein's method accounts for the masonry code's lack of guidance regarding standard deviation as compared to ACI 318, Section 5.3.2.

In concrete, the default standard deviation is much higher.  A 3,000 psi concrete must crush at 4,200 psi unless a standard deviation is established.  This results in a 40% higher tested strength over the specified strength.

Amrhein's method would require a 3,000 psi grout to crush at 3,700 psi for a 25% higher tested strength.

Considering the care in measuring proportions observed by masons and in block plants, I am inclined to use the upper end of Amrhein's recommendations.

Harold Sprague, PE
Krawinkler, Luth & Assoc.


-----Original Message-----
From:	Ed Marshall [SMTP:elmarshall(--nospam--at)HASimons.com]
Sent:	Wednesday, August 12, 1998 7:32 AM
To:	seaint(--nospam--at)seaint.org
Subject:	Masonry Design

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.

Thanks,
Ed Marshall, PE
Simons Engineering,
Atlanta

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