Page 291 - Design of Reinforced Masonry Structures

COLUMNS 5.11

research findings. In addition, there are practical and constructibility considerations, such as
ensuring that required cover is provided to reinforcement, that there is no congestion (crowd-
ing) in the cells due to too many closely placed reinforcing bars that would impede flow of
grout, and spacing of ties (to match coursing).
It is noted that per MSJC-08 Code Section 1.17.4.4.2.2, neither Type N mortar nor
masonry cement is permitted to be used to construct elements (such as columns) that
participate in lateral force resisting system (LFRS) in Seismic Design Category D, E, and
F (discussed in Chap. 7). There is no such restriction for LFRS in other Seismic Design
Categories.
The compressive strength of masonry is limited to 4000 psi for concrete masonry and
6000 psi for clay masonry. The nominal yield strength of steel reinforcement is specified
to be 60,000 psi. The MSJC Code [5.6] is specific to these limitations for design pur-
poses because these are the material strengths of masonry structural components on which
research has been conducted.

5.4.1 Dimensional Limits

General provisions for reinforced masonry columns are specified in MSJC Code Sections
1.14.1 and 3.3.4.4 [5.1] as follows, which are judgment-based and intended to prevent local
instability or buckling failures:
1. Minimum side dimension shall be not less than 8 in. nominal.
2. The nominal depth of a column shall not be less than 8 in. and not greater than 3 times
its nominal width.
3. The distance between the lateral supports of a column should not exceed 30 times its
nominal width, that is, the effective height–to–least nominal lateral dimension ratio, that
is, h/t ratio, should not exceed 30.

The minimum column width limitation to 8 in. is based on experience with masonry
columns. The h/t ratio limitation of a maximum of 30 is based also on experience; sufficient
data are not available to justify a higher h/t ratio [5.3].

5.4.2 Cross-Sectional Areas of Masonry Columns

Masonry column cross sections are generally square or rectangular. It is a common practice
to express their cross sections (sizes) as thickness times depth, for example, 24 × 24 in,
16 × 24 in., etc. The thickness (t) of a column cross section is the smaller dimension, which

is typically used in determining design parameters such as h/r ratios. As noted in earlier
chapters, concrete masonry units (CMUs) are manufactured, typically, in 6-, 8-, 10-, 12-,
or 16-in. nominal widths, the actual width being 3 /8 in. smaller. Columns constructed from
masonry units are specified by their nominal size, the actual size being 3 /8 in. smaller on
each side. For example, a nominal 24 × 24 in. column constructed from CMUs would actu-
2
ally be 23 5 /8 × 23 5 /8 in. (so the cross-sectional area = 23.625 × 23.625 = 551.14 in. , not,
2
24 × 24 = 576 in. ), a nominal 16 × 24 in. CMU column would actually be 15 5 /8 × 23 5 /8 in.
2
2
(so the cross-sectional area would be 15.625 × 23.625 = 369.14 in. , not 16 × 24 = 384 in. ),
and so on. On the other hand, when clay masonry (bricks) is used to build columns, the
nominal size, expressed in whole numbers (e.g., 16 × 16, 18 × 18, 24 × 24 in., etc.) and
the actual size are generally the same. In all cases, only the net cross-sectional areas (A )
n
should be used in design.