Page 246 - Design of Reinforced Masonry Structures

DESIGN OF REINFORCED MASONRY BEAMS 4.109

4.15 BOND BEAMS

4.15.1 Definition and Functions of a Bond Beam
“Bond beam” is a special term that is encountered frequently in the context of masonry con-
struction. A bond beam is not a beam in the conventional sense of a structural member that
typically supports transverse loads; for example, a beam supporting a part of a roof or a floor.
Rather, it is an integral part of a wall that performs the following important functions:
1. It ties the reinforced masonry structure together around its perimeter, resulting in stron-
ger unit.
2. It acts as a chord of a diaphragm in resisting lateral loads.
3. It acts as an effective restraint against wall movement and helps reduce the formation of
cracks.
To perform the aforestated functions, bond beams may be located at all floor and roof
levels, above and below the openings, and around the tops of walls and foundations as
means of tying all blocks together and distributing the loads to better advantage. Bond
beams located at the top of walls stiffen them.
In load-bearing masonry walls, bond beams are commonly placed in the top course of the
wall beneath the roof framing system to act as diaphragm chords to resist lateral loads due to
wind or earthquakes. In this case, the bond beam has to resist axial forces—compressive and
tensile—on the opposite edges of the diaphragm system. Because of the inherent weakness
of masonry in tension, adequate amount of reinforcing must be provided in the bond beam
to resist the tensile chord force. Diaphragm action is discussed in the next section.
Bond beams in concrete masonry walls can be accommodated either by using special channel-
shaped units called bond beam units (Fig. 2.31, Chap. 2) or by saw-cutting out of a standard
unit. Bond beam units are fabricated either with reduced webs or with “knock-out” webs that
are removed prior to their placement in the wall. The width and the depth of bond beam units
is the same as those of the other masonry units used in the wall in which they are placed.

4.15.2 Typical Reinforcement in Bond Beams
The size of reinforcing bars used in a bond beam depends on the thickness of the wall. It is
a common practice to provide horizontal reinforcement in bond beams as two No. 4 bars
in 8-in.-wide bond beams and two No. 5 bars in 10- and 12-in.-wide bond beams. After the
placement of horizontal reinforcement, the units are filled with grout or concrete. Upon
hardening of the grout or the concrete, the beam behaves as a solid reinforced masonry
member. Building codes in some earthquake-prone areas require a 16-in.-deep bond beam

with two additional No. 4 bars located in the top of the beam.
It is important to note that the horizontal bars in bond beams may act as structural
reinforcement, such as bond beam reinforcement in roof and floor diaphragms that resist
diaphragm chord tension. Consequently, they must be continuous or lapped, and bent
around corners as required by the applicable code. Continuity in bond beam reinforcement
is required even through the control joints, which provide complete separation between two
segments of a wall. Control joints are provided to permit free longitudinal movement, but
may need to transfer shear or lateral loads. Nonstructural reinforcement, such as horizontal
joint reinforcement, should not be continuous across the control joint.
In nonload-bearing masonry walls, bond beams may be placed in any of the top three
courses below the roof slab. In this case, it functions as a structural tie, integrating the
nonload-bearing walls with the adjacent walls, and also a crack control device. Also, in this