Masonry Magazine December 2000 Page. 32
can suddenly fail, causing the anchor
to pull out of the masonry. This does
not happen with headed anchors.
MSJC CODE REQUIREMENTS
AND TEST RESULTS
The current MSJC Code contains a
single set of equations for calculating
the allowable capacities of anchor
bolts. It does not distinguish among
plate, bent-bar and headed anchors.
The equations are based on research
results for all types of anchor bolts.
Because the Code allowable loads are
For Mason's
Hydrated Lime...
Contractors and Architects Rely
on MIRACLE MORTA-LOK®
* Type "S" hydrated mason's lime
* Outstanding workability saves time
and money
* A unique combination of strength
and elasticity
* Meets all national specifications
(ASTM, UBC, BOCA, SBCC)
* Available nationwide
ROCKWELL lime company
4110 Rockwood Road Manitowoc, WI 54220
Phone: 1-800-558-7711
intended to be safe for all types of an-
chors, they are very conservative for
headed anchors.
Research studies show that headed
anchors in masonry, loaded in ten-
sion, fail either by yield and fracture
of the anchor shank, or more typically
by breaking out a piece of masonry
(Figure 2).
Figure 2
Breakout
failure typical
of headed
anchors loaded
in tension
Bent-bar anchors, in contrast, typi-
cally fail at much lower loads, when
the anchor shank breaks or straightens,
pulling out of the masonry (Figure 3).
Figure3
Pullout failure
typical of
bent-bar
anchors in
tension
The pullout capacity of bent-bar an-
chors is highly variable, and depends
on the surface roughness of the anchor
and the presence or absence of bond-
reducing coatings such as oil or grease.
Differences in performance be-
tween headed and bent-bar anchors
are recognized in other design provi-
sions. For example, the draft MSJC
strength design provisions include
different formulas for calculating ten
sile anchor capacity as governed by
yield and fracture of the anchor
shank, by tensile breakout, and by
pullout. The pullout failure mode ap
plies to bent-bar anchors only. To see
how significant the differences are,
see Figure 4, in which the tensile ca
pacities of 1/2 in. diameter headed and
bent-bar anchors are compared using
the proposed MSJC Strength Design
provisions, assuming 36 ksi anchors
and masonry with a specified com-
pressive strength, fim, of 1500 lb/in..
In that graph, pullout capacity is cal-
culated assuming that no special
measures are taken to remove oil
from the anchor shank. The figure
shows that the capacity of bent-bar
anchors is always controlled by pull-
out, and is quite low. In contrast, the
capacity of headed anchors is con-
trolled by breakout, is significantly
higher, and increases with embed-
ment depth. As embedment depth in-
creases further, a headed anchor will
eventually fail by yield and fracture of
the anchor steel. As can be seen from
Figure 4, headed anchors can have
more than 4 times the capacity of oth-
erwise identical bent-bar anchors.
Accordingly, the MSJC is consider-
ing separating the allowable-capacity
equations for headed and bent-bar
anchors to reflect the higher capacity
of headed anchors. When this is
done, a given number of bent-bar an-
chors could in many applications be
replaced by a fewer number of
headed anchors.
SAVE
TIME
& LABOR
Original Drawing 1990
INCREASE
PRODUCTION
NEW 1 piece st
1997
VOLCANO ROD
Electric Sand Pile Heater
1500 Watts 120 Volts
VOLCANO ROD is a specially designed electric
heater that provides warm water and sand by the
time you arrive at the job site Volcano Rod wil
not scorch sand. It maintains temperature ata
constant of not over 200°F. This unit is
thermostated and will cycle on and off
intermittently
COMPACT-Volcano rod is only 16"x 38" long and
one inch in diameter. It is easy to transport &
can fit behind the seat of your truck
RUGGED-Volcano Rod is designed for strength and
durability. This unit is backed by a 60 day parts
and labor warranty
To order send check or money order for
$199.95 phe $9.50 shipping and handling
charge to:
World Masonry Technologies, INC.
8620 Wea St.
P.O. Box 322
De Soto, KS 66018
Figure 4 Comparison of tensile ca-
pacities of 1/2 in. diameter headed
and bent-bar anchors
Even before the allowable-stress
provisions of the MSJC Code are up-
dated in this way, however, designers
and contractors are encouraged to
use headed anchors instead of bent-
bar anchors. Calls to various distribu-
tors show that the two types of an-
chors cost nearly the same, and that
their installation expenses should
32 MASONRY-NOVEMBER/DECEMBER, 2000