Masonry Magazine October 1975 Page. 14
SHEAR TESTS
The allowable shear stress for masonry is generally
determined by performing shear tests on small scale
specimens constructed in the laboratory. These
specimens should be constructed using "inspected
workmanship" as previously described. Specimens for
both tests should be cured for 28 days prior to testing.
The E 72 method of test calls for three 8-ft by 8-ft
(2.438 mx 2.438 m) specimens. The panel can be
instrumented as shown in the standard and the horizontal
deflection plotted against the load applied in graph
form as described in the standard. It has been common
practice within the masonry industry to slightly modify
this test. In lieu of the instrumentation shown in the
standard, a series of strain gages are placed to measure
horizontal displacement of the panel under load. These
gages are placed along the vertical face of the panel
where tie downs and load devices do not occur. The
horizontal displacement or strain is then taken at
various load levels. The strain is the calculated average of
all dial readings at a particular load. The shear stress is
calculated by dividing the horizontally applied load by
the panel width times the panel thickness. From these
data stress-strain curves may be plotted. The
instrumentation should be removed at approximately 75
to 80 percent of the calculated load and the specimen
tested to failure. Data pertinent to the determination of
allowable design stresses are (f') ultimate shear stress,
a plotted stress-strain curve and the modulus of rigidity
at predetermined stress levels, usually 20 percent and
50 percent of ultimate shear stress.
The E 519 method of test also specifies three speci-
mens. Instrumentation of the specimens is provided
along the vertical and horizontal diagonals, as shown in
Fig. 8. The vertical diagonal instrumentation measures
the shortening along that diagonal. The horizontal in-
strumentation measures the lengthening along that
diagonal.
The calculations for shear stress for specimens con-
structed of solid units shall be based on gross area,
while the shear stress for hollow unit specimens shall be
based on net area. The shear stress shall be calculated as
follows:
S = (0.707 x P) / A (7)
where:
S = shear stress on gross or net area, psi (kgf/cm²)
P = applied load, lb (kgf)
A = average of the gross or net areas of the two contiguous upper sides of the specimen, sq in. (cm²)
Formula (8) shall be used when specimens are built
of solid units and formula (8a) shall be used for speci-
mens of hollow units.
A = (tx 1) + (tx 1) / 2 (8)
A=(txl) Av (8a)
where:
(tx/),... (tx/) = thickness and length or gross area of the two upper contiguous sides of the specimen, sq in. (cm³)
(tx/) = thickness and length or gross area of upper side of specimen built of hollow units, sq in. (cm²)
A, = area of voids of the upper side of specimen built of hollow units, sq in. (cm³)
The shear strain shall be calculated as follows:
y = (AV + ΔΗ) / g (9)
where:
y = shearing strain, in./in. (cm/cm)
AV = vertical shortening, in. (cm)
ΔΗ = horizontal lengthening, in. (cm)
g = vertical gage length, in. (cm)
AH must be based on the same gage length as AV.
The modulus of rigidity shall be calculated as fol-
lows:
EG = S / Y (10)
where:
EG = modulus of rigidity, psi (kgf/cm²)
The modulus of rigidity is calculated for predeter-
mined stress levels, usually at approximately 20 per-
cent and 50 percent of ultimate load.
The allowable shear stress should be determined by
dividing the ultimate shear strength of the specimens by
a safety factor selected by the designer, when E 72 or
E 519 are used to determine allowable design values.
The safety factor used should be based upon the design-
er's experience, type of workmanship expected, type of
loading the masonry will be subjected to, or as recom-
mended in the design standard or Recommended Prac-
tice for Engineered Brick Masonry.
DETERMINATION OF f'
At present, only one method of test is available in
ASTM standards for determining the ultimate and de-
sign flexural tensile strengths for masonry. This
method of test is covered in ASTM E 72. Recently
ASTM adopted E 518, Standard Methods of Test for
Flexural Bond Strength of Masonry. This test, how-
ever, is to be used only as a compatibility test for brick
and mortar or as a quality control test and should not be
used for determination of flexural or transverse design
stresses. ASTME 518 will be more fully discussed in a
subsequent issue of Technical Notes 39 series.
ASTM E 72 provides four methods for testing the
large scale panels. The specimen may be tested in either
a horizontal (Fig. 10) or a vertical position (Fig. 11). In
addition to these methods, the orientation of the
masonry panel itself within the loading frame will have