Page 345 - Design of Reinforced Masonry Structures

COLUMNS 5.65

5. If the distance between the lateral supports of a pier exceeds 25 times its nominal thick-
ness (t), then the pier is to be designed as a wall element (see MSJC-08 Section 3-5,
discussed in Chaps. 6 and 7).
In addition to the abovestated dimensional limits, MSJC-08 specifies requirements for
longitudinal reinforcement for piers subject to stress reversals as follows:
1. The piers are to be reinforced symmetrically about their neutral axes.
2. The longitudinal reinforcement should comply with the following:
(a) One bar shall be provided at each end.
(b) The minimum area of longitudinal reinforcement shall be 0.0007bd.
(c) Longitudinal reinforcement should uniformly be distributed throughout the depth of
the pier.
It is noted that these requirements are predominantly seismic related and are intended
to provide the greatest ductility economically. Additionally, if a pier is a part of a special
reinforced masonry shear wall, it should satisfy strain compatibility requirements specified
in MSJC-08 Section 3.3.3.5.1, which are as follows:
1. For in-plane loads, where M /V d ≥ 1.0, the strain in the extreme tensile reinforcement
u
u v
must be at least equal to 4 times the yield strain.
2. For out-of-plane loads, the strain in the extreme reinforcement should be at least equal
to 1.5 times the yield strain.
The factored axial compression on piers is not to exceed 0.3A ′ f . According to the
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m
MSJC-08 Commentary, this is an arbitrary limit imposed due to the less severe require-
ments on the design of piers than for similar requirements for column. Indirectly, this
provision dictates the cross section of the pier

PROBLEMS
In each of the following problems, check if the column reinforcement complies with code
requirements with respect to size and longitudinal reinforcement. Provide 3/8-in.-diameter
lateral ties in each case and show reinforcement details with suitable sketches.
Problems for analysis
5.1 A 24- × 24-in. CMU column having an effective height of 24 ft is reinforced
with four No. 9 Grade 60 bars (one bar placed in each corner). Assume ′ f =
m
1500 psi. Determine fP for this column.
n
5.2 A 16- × 16-in. CMU column having an effective height of 20 ft is reinforced

with four No. 8 Grade 60 bars. Assume ′ f = 1800 psi. Determine fP for this
n
m
column. If the service dead load and service live load are 40 and 60 percent,
respectively, of the applied load, calculate the total service load, dead load, and
live load that can be supported by the column.
5.3 An 18- × 18-in. brick masonry column having an effective height of 22 ft is rein-
forced with six No. 9 Grade 60 bars. It carries a service dead load of 250 kips and
a service live load of 200 kips. ′ f = 2500 psi. Determine fP for the column and
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m
check if the column can support the imposed service loads. Could this column
carry the imposed loads if ′ f were to be lowered to 2000 from 2500 psi?
m
5.4 A 16- × 24-in. CMU column having an effective height of 24 ft is reinforced
with four No. 9 Grade 60 bars. Assume ′ f = 2000 psi. Determine fP for this
m
n
column. If the service dead and live loads are 30 and 70 percent, respectively,