Page 89 - Design of Reinforced Masonry Structures

MASONRY UNITS: APPLICATIONS, TYPES, SIZES, AND CLASSIFICATION 2.41

2.8 STRUCTURAL REQUIREMENTS FOR
MASONRY IN STACK BOND

From a design perspective, it is important to recognize these two types of bonds because
of the many ramifications when masonry is in stack bond. Design requirements are more
restrictive for masonry constructed in stack bond. The Code [2.3] specifies (1) special
requirements that must be met when masonry is laid in stack bond and (2) limitations on
strength when masonry is constructed in stack bond.
1. Special requirements: MSJC-08 Section 1.11 states that “for masonry in other than
running bond , the minimum area of horizontal reinforcement shall be 0.00028 times
*
the gross-sectional area of the wall using specified dimensions. Horizontal reinforce-
ment shall be placed in horizontal joints or in bond beams spaced not more than 48 in. on
center.” Furthermore, when stack bond is used for masonry walls in seismic regions, a
much greater area of minimum horizontal reinforcement must be provided as compared
to that for walls in running bond. MSJC-08 Code Sections 1.17.3 and 1.17.4 address
specific requirements for reinforced masonry walls in structures in various Seismic
Design Categories as follows:
a. Walls in Seismic Design Category D: Wythes of stack bond masonry to be con-
structed of fully grouted hollow open-end units, fully grouted hollow units laid with
full head joints, or solid units. The minimum area of horizontal reinforcement to be
0.0007 times the gross cross-sectional area of wall; maximum spacing of reinforce-
ment to be 24 in. (as compared to 48 in. for masonry in running bond).
b. Walls in Seismic Design Category E and F: Same as walls in Seismic Design
Category D (above) with these additional requirements: (i) if the walls are not a part
of lateral force resisting system, the minimum area of horizontal reinforcement to
be at least 0.0015 times the gross cross-sectional area of masonry, and maximum
spacing of horizontal reinforcement to be 24 in.; (ii) if the walls are a part of lateral
force resisting system, the minimum area of horizontal reinforcement to be at least
0.0025 times the gross cross-sectional area of masonry, and maximum spacing of
horizontal reinforcement to be 16 in.
2. Strength limitations: The Code [2.3, Section 3.1.8.2] places limits of strength of
masonry in stack bond. The modulus of rupture for masonry in stack bond when the
direction of flexural tensile stresses is parallel to bed joints is zero (Table 3.1.8.2.1,
Ref. [2.2]). This is the case when beams and shear walls are subjected to loads (beams
loaded transversely and shear walls carrying in-plane loads). This essentially means that
transversely loaded members such as beams and shear walls cannot be constructed of
masonry in stack bond.

The idea behind requiring some horizontal reinforcement as well as bond beams in stack
bond masonry is to provide continuity across the head joints and ensure that all masonry
units would act together as integral units rather than various stacks of masonry acting inde-
pendently. Furthermore, masonry in intersecting walls is required to be in running bond
(Ref. [2.2], Section 1.9.4.2).
In large wall areas and in load-bearing construction, it is advisable to reinforce the wall
with steel placed in horizontal mortar joints. Dimensional accuracy of units is very impor-
tant to maintain vertical alignment of the head joints.

*Masonry not overlapped a minimum of 1 ⁄ 4 in. of the unit length is considered to be laid in “other than running
bond.”