Page 63 - Design of Reinforced Masonry Structures

2.16 CHAPTER TWO

3
is deeper than ⁄ 8 in. Void areas may be cores, cells, deep frogs, or combinations of these.
ASTM C652 [2.15] defines hollow brick as a clay unit masonry whose net cross-sectional
area (solid area) in any plane parallel to the bearing surface is less than 75 percent of its
gross cross-sectional area measured in the same plane. The extent of void area of a hollow
brick is separated into two classes: H40V and H60V. Bricks with void areas greater than
25 percent and less than 40 percent of the gross cross-sectional area are classified as H40V.
Bricks with void areas greater than 40 percent, but less than 60 percent of the gross cross-
sectional area are classified as H60V.
Strict edge distance requirements are imposed by ASTM Standards for creating hollow
5
spaces within the bricks. Core holes may not be less than ⁄ 8 in. from any edge of the brick
except for the cored-shell hollow brick. The minimum shell thickness required for a cored-
2
1
shell hollow brick is 1 ⁄ 2 in. The minimum edge distance for cores greater than 1 in. in
2
cored shells is ⁄ 2 in. Cores not greater than 1 in. in shells cored more than 35 percent may
1
3
not be ⁄ 8 in. from any edge. Cells may not be less than ⁄ 4 in. from any edge of the brick,
3
except for double shell hollow brick. The thickness of webs between the cells should not be
less than ⁄ 2 in., ⁄ 4 in. between cells and cores, ⁄ 4 in. between the cores.
3
1
1
Hollow bricks are identical to facing bricks except that they occupy a larger core area.
Most hollow bricks are used in the same application as the facing brick. Hollow bricks with
very large cores are used in walls that are reinforced with steel and grouted solid. The two
most common types of hollow brick units used in reinforced masonry construction are the
stretchers and bond beam units which are easily reinforced. The stretcher units, typically
used in wall construction, are placed so that cells align vertically to permit placement of
vertical reinforcement and grouting (Fig. 2.6). Bond beam units are placed to align horizon-
tally, which permits placement of horizontal reinforcement and grouting (Fig. 2.7).
2.5.2.2 Types of Hollow Bricks. As discussed in Refs. 2.29 and 2.30, four types of
hollow bricks are specified by ASTM C652 [2.15], which are described as follows. Each of
these types relate to the appearance of the unit. Dimensional variation, chippage, warping,
and other imperfections are the qualifying conditions of the type. The most common type,
Type HBS, is considered standard and specified for most applications.
• Type HBS: Hollow bricks for general use in exposed exterior and interior masonry
walls and partitions, where wider color ranges and greater variation in sizes are permitted
than are specified for Type HBX.
• Type HBX: These are to be used for the exposed exterior and interior masonry walls
where a high degree of mechanical perfection, narrow color range, and minimum permis-
sible variation in size are required.
• Type HBA: Bricks are manufactured and selected to produce characteristic architectural
effects resulting from nonuniformity in size, texture, and color of individual units.
• Type HBB: Hollow brick for general use in masonry walls and partitions where texture

and color are not a consideration, and a greater variation in size is permitted than is speci-
fied for Type HBX.

2.5.3 Relationship Between Unit Strength of Clay Masonry Units and
Specified Compressive Strength of Clay Masonry, ′ f m
The specified compressive ′ f of a masonry is greatly influenced by the variability in
m
mortar strength, grout strength, and their interaction in generating compressive strength.
As such, the relationship between the masonry unit strength and the compressive strength
( ′ f ) of the masonry assemblage varies considerably, depending on the properties of
m