TM 5-809-3/NAVFAC DM-2.9/AFM 88-3, Chap. 3
kd = 0.308(5.81 in) = 1.79 in
Note that kd is greater than the face shell thickness, therefore the actual design section would be a T-section.
The following will show that the difference generated by assuming a rectangular section is negligible.
(450 lb/in 2)(0.308)(0.897)(24)(5.81 in)2
'
Mrm
2(12)
= 4,196 ft-lb/S . 4,147 ft-lb/S (table B-4)
Reinforcing steel resisting moment is Mrs:
FsAsjd
Mrs '
12
(24,000 lb/in 2)(0.44 in 2)(0.897)(5.81)
Mrs '
12
Note that the difference between the T-section analysis moments from table B-4 and the computed
rectangular section moments is negligible (approximately 1%).
(c) To illustrate the derivation of the table values, a T-section analysis will be performed.
np % (ts/d)2
kT '
np % (ts/d)
(21.5 0.0032) % 1/2(1.5/5.81)2
'
(21.5 0.0032) % (1.5/5.81)
= 0.312
6 & 6(ts/d) % 2(ts/d)2 % (ts/d)(1/2pn)
jT '
6 & 3(ts/d)
6 & [6(1.5/5.81)] % 2(1.5/5.81)2
jT '
6 & 3(1.5/5.81)
(1.5/5.81)[1/2 0.0032 21.5)
%
' 0.902
6 & 3(1.5/5.81)
AFj d
MrsT ' s s T
12
(0.44 in 2)(24,000 lb/in 2)(0.902)(5.81)
MrsT '
12
= 4611 ft-lbs . 4603 ft-lbs (table B-4)
450[1 & 1.5/(2 0.312 5.81)](24)(1.5)(0.902)(5.81)
MrsT '
12
= 4147 ft-lbs . 4147 ft-lbs (table B-4)
Note that since wind loadings are a part of the loading combination, the resisting moments of the wall cross
section may be increased by 33%. Thus, the design resisting moments for the masonry and the reinforcing
steel, respectively are:
MrmT = 1.33(4147 ft-lb/S) = 5,516 ft-lb/S
MrsT = 1.33(4611 ft-lb/S) = 6,133 ft-lb/S
Note: The masonry resisting moment controls the design:
MrmT = 5,516 ft-lb/S > Mmax = 4,364 ft-lb/S
....O.K.
Axial Load Check: For the 12-inch CMU wall with reinforcing spaced at 24 inches o.c., the effective area
in compression, Ae, is 68 in2/ft and the weight of the wall, W2, is 102 lb/ft2.
The axial compressive stress in the wall, fa, is determined as follows:
P % (w2)(h & x)
fa '
Ae
(1500 lb) % (102 lb/ft 2)(24 ft & 13.2 ft)
' 38.3 lb/in 2
fa '
68 in 2
The allowable axial compressive stress in wall is Fa:
6-6
Previous Page
