Page 453 - Forensic Structural Engineering Handbook

13.4 MATERIAL-SPECIFIC FORENSIC ANALYSES

Very high slump grout can have very high compressive strength, because the water-cement
ratio is reduced by absorption of masonry units. A cited reference provides wall section
properties for a wide array of masonry walls. Estimates of typical masonry material prop-
erties in lieu of testing are available.

Clay Brick Masonry
Weight of Clay Masonry. The weight of clay brick masonry can vary by more than 30
48
percent from the minimum design dead load required by ASCE 7-05. Variations in clay
brick masonry weight have implication not only for structural and acoustical design, but
also for design of heating, ventilating, and air conditioning systems.
Incorrect assumptions about wall weight can result in excessive or inadequate seismic
loads on masonry walls; insufficient resistance to uplift; increased structural framing and
foundation costs; reduced factors of safety in shear walls; increased summer air condition-
ing costs; overestimates of acoustical transmission loss; and inefficiency in passive heating
and cooling systems. Designers should be aware that masonry wall weight is variable and
should consider that variability where appropriate. The density of brick masonry has a
3
3 33
3
3
mean value of 116 lb/ft (1858 kg/m ) with a standard deviation of 12 lb/ft (192 kg/m ).
Brick masonry wall weights are given in Table 13.1.
Clay Brick Masonry in Compression. The mean expected 28-day compressive strength
of masonry f′ —measured perpendicular to bed joints and built with standard modular clay
m
1
brick [nominally 2 / in (57 mm) high], inspected workmanship, and ASTM C 270, type
4
63
2
S mortar, without air entrainment —is estimated by the author as f ′ (lb/in ) ≈ 0.283 (f ′ +
u
m
8380), where f ′ is the brick compressive strength in pounds per square inch.
u
That strength is reduced about 29 percent by use of type N mortar, about 27 percent by
uninspected workmanship, and about 15 percent by increasing the mortar air content from
5 to 18 percent. Mortars without air entrainment typically have air contents of 5 percent
or less. The specification for type N masonry cement (ASTM C 91, Specifications for
63
Masonry Cement ) permits an air content of 22 percent, which may result in typical mor-
tar air content of 24 percent. Use of type M mortar increases strength about 6 percent. Use
5
3
of 3 / -in-high (92-mm) high clay brick with / -in (10-mm) mortar joints rather than a
8
8
1
2 / -in (57-mm) high clay brick increases strength about 22 percent. Other things being
4
equal, use of high air entrainment and uninspected workmanship combine to reduce the
compressive strength of clay brick masonry about 36 percent. Clay brick masonry in com-
pression has a mean cracking strength of 47 percent of its f ′ ultimate strength with a coeffi-
m
cient of variation of 15 percent. Frequent application and withdrawal of load may cause
fatigue and strength reduction and therefore may increase cracking probability. The com-
pressive strength of brick masonry is typically significantly greater than the building
code–assumed values. The ratio of saturated to air-dry strength of clay brick is reported to be
0.83. One hundred cycles of freezing and thawing reduce the compressive strength of clay
brick by an average of 11 percent with a standard deviation of 13 percent. 57
Elastic Modulus of Clay Brick Masonry. Masonry elastic modulus in compression is
rather constant over the stress range from 5 to 33 percent of the masonry’s ultimate
strength. Within that stress range, the mean chord modulus of elasticity E of solid clay
m
2
brick masonry with a prism aspect ratio of 5 is 464 f′ (lb/in ) with a standard deviation of
m
2
185 f ′ (lb/in ). The modulus E is reduced about 24 percent when compressive stress is par-
m
m
allel to bed joints. The modulus of elasticity of masonry is typically significantly greater
than the building code–assumed values.