Page 297 - Design of Reinforced Masonry Structures

COLUMNS 5.17

Example 5.2 Nominal strength of a square CMU column.
A nominal 24 × 24 in. CMU column having an effective height of 28 ft is reinforced
with eight No. 9 Grade 60 bars (Fig. E5.2). It carries a service dead load of 200 kips and
a service live load of 300 kips. ′ f = 1500 psi. Determine fP for the column and check
n
m
if the column can support the imposed service loads.
3''
8#9
23.625''

3''
3'' 3''
23.625''
FIGURE E5.2 Columns cross section
Solution
2
Given: A nominal 24 × 24 in. CMU column, h = 28 ft, ′ f = 1500 psi, A = 8.0 in. (eight
m
st
No. 9 Grade 60 bars), D = 200 kips, L = 300 kips.
Check dimensions and h/t ratio for code compliance.
Nominal column width = 24 > 8 in. OK
Nominal column depth = 24 in. < 3(24) = 72 in. OK
h/t = 28(12)/24 = 14 < 30 OK
For a nominal 24 × 24 in. CMU,

A = (23.625)(23.625) = 558 in. 2
n
2
A = 8.0 in. (eight No. 9 bars)
st
h = 28 ft b = t = 23.635 in.
r = 0.289t = 0.289(23.625) = 6.83 in.
12
h
= (28 )( ) = 49 .19 < 99
r . 683

For h ≤ 99,
r
⎡ ( ) ⎥ ( ) 2
⎤
2
49 19
.
h
1
C = 1 − 140 r ⎦ =− 140 = 08 .776 (5.13 repeated)
⎢
P
⎣
Alternatively, C can be found directly from Table A.16 as follows:
P
12
h (28 )( )
= = 14 .22
t 23 .625
By interpolation from Table A.16,
C = 0.880 – (0.880 – 0.862)(0.22) = 0.876
P