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RE: Masonry Design

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I have the 5th edition of Amrhein's handbook.  The corresponding section
is 2-3.E:  Thanks for pointing it out.  I find it helpful:

"When the compressive strength of the masonry assemblage, f'm, is
specified, the component materials, grout and masonry units must be
stronger that the specified strength.    ...   It is recommended that
... the strength of the masonry unit and grout be 25 to 40 percent more
than the specified strength.   ...  When specifying ... specify a
minimum strength only, not a range of strengths.  This minimum strength
would be the average of three units tested with the strength of no
single being less than 20% below ..."

It seems that this 25 to 40% factor is written into the code's table.
If you enter the table with 1900 psi as the minimum average strength (of
three units) reading 1500 psi as the net compressive strength of the
masonry (f'm), you have a minimum average block strength 27% greater
than f'm. 

Alternately, if I enter the table with 2800 psi units, I'll read 2000
psi as the net compressive strength of the masonry (f'm).  Hence for
higher strength units the factor has increased to 40%.  Obviously if I
want to maintain the minimum average strength 40 percent greater than
f'm for the lower strength block (ASTM C90 minimum) as well, I could
reduce f'm from 1500 psi to 1360 psi for 1900 psi block units.

ASTM C 90 allows a single unit to test at 1700 psi.  This of course is
11% under the minimum average strength of 1900 psi and within Amrhein's
recommended 20%.

Ed Marshall, P.E.  

> -----Original Message-----
> From:	hsprague(--nospam--at) [SMTP:hsprague(--nospam--at)] On Behalf Of
> hsprague(--nospam--at)
> Sent:	Wednesday, August 12, 1998 5:23 PM
> To:	'seaint(--nospam--at)'
> Subject:	RE: Masonry Design
> 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)]
> 	Sent:	Wednesday, August 12, 1998 7:32 AM
> 	To:	seaint(--nospam--at)
> 	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
> 	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