Page 293 - Design of Reinforced Masonry Structures

COLUMNS 5.13

The code’s minimum reinforcement requirement is intended to provide ductility and to
prevent brittle collapse of a column. The maximum percentage limit is based on experience
and to avoid possibility of congestion in the core, which may be problematic for grout con-
solidation. Four bars are required so that ties can be used conveniently around them to pro-
vide a confined core of grout. In all cases, longitudinal bars should be placed symmetrically
about both column axes as the design assumes implicitly that all bars are loaded equally.
Example 5.1 illustrates the provision for limitation on the maximum bar size. Although
the limitation of No. 9 size bar is arbitrary, there is little masonry work large enough to
accommodate bars larger than No. 9 bars. There is also a disadvantage with using larger
diameter bars. Because the larger diameter bars carry larger loads, they require longer devel-
opment lengths which are proportional to the square of the bar diameters. For example, a
No. 8 bar would require almost twice as long a development length as a No. 6 bar. In addition,
sufficient information is not available to justify the stress levels, which might occur around
fully loaded larger bars [5.3].

5.4.3.2 Lateral Ties (MSJC-08 Section 1.14.1.3) All longitudinal bars for columns are
required to be enclosed by lateral ties in order to develop confinement of masonry as shown
in Fig. 5.8. The MSJC Code requirements for lateral ties are modeled after those for rein-
forced concrete. Lateral ties perform three functions:

1. They provide the required support to prevent buckling of longitudinal bars acting in
compression.
2. They provide resistance to diagonal tension for columns subjected to shear, an action
similar to reinforced concrete columns [5.3].
3. They provide ductility.

Requirements for lateral ties for columns are modeled on those for reinforced concrete
[5.3]. Except for permitting ¼-in. ties outside of Seismic Design Category D, E, or F, they
reflect all applicable provisions of the reinforced concrete code (ACI-05). A summary of
requirements for lateral ties for masonry columns as specified in MSJC-08 Section 1.14.1.3
follows:
1. Lateral ties shall be placed in either a mortar joint or in grout; the choice would depend
on the type of masonry units used in construction. However, lateral ties in columns that
form parts of a LFRS in Seismic Performance Category D shall be embedded in grout.
2. All vertical reinforcing bars, whether in isolated columns, wall columns, or pilasters,
must be enclosed by lateral ties (bar or wire), which must be at least ¼ in. in diameter.
Note that ¼-in. diameter bars are available only as smooth bars, not as deformed bars;
however, they can be used for ties.
3. Vertical spacing of ties is to be the smallest of the following to comply with the code
requirements:
a. Sixteen times the diameter of the vertical bar
b. Forty eight times lateral tie bar or wire diameter
c. The least cross-sectional dimension of the column

Ties may be placed at desired spacings when using hollow column units (Fig. 5.4), in
which case ties would be embedded in grout. Constructibility issues should be recognized
when specifying spacing of lateral ties that would be placed in mortar joints. All masonry
units (clay or concrete) are available in certain standard thicknesses. Therefore, tie spacing
should preferably be a multiple of this dimension to match coursing; a different tie spacing
might require cutting the units at the jobsite, involving unwarranted additional costs and
time as well as resulting in an unsightly structure. For example, when using typical 8-in.