TM 5-809-3/NAVFAC DM-2.9/AFM 88-3, Chap. 3
CHAPTER 5
GENERAL CRITERIA FOR REINFORCED MASONRY
5-1. Introduction. This chapter provides the general criteria for the design of reinforced masonry using the
working stress design method. Generally, a running bond masonry pattern is the basis of the design and
reinforcing requirements contained herein. Running bond is the strongest bond pattern and will be used unless
a stacked bond pattern is essential to the architectural treatment of the building. Additional design and
detailing requirements for stacked bond masonry are contained herein.
5-2. Working stress assumptions. The assumptions for the working stress design of reinforced masonry
are the same as the assumptions used in the working stress design of reinforced concrete.
a. Basic assumptions. The basic assumptions are as follows:
(1) Plane sections remain plane after bending.
(2) Stress is proportional to strain which is proportional to the distance from the neutral axis.
(3) The modulus of elasticity is constant throughout the member in the working load range.
(4) Masonry does not resist tension forces.
(5) Reinforcement is completely bonded so that the strain in the masonry and the strain in the
reinforcement are the same at the location of the reinforcement.
(6) External and internal moments and forces are in equilibrium.
(7) The shearing forces are assumed uniformly distributed over the cross section.
b. Modular ratio. As per the basic assumptions above, the strain in masonry, ,m, at a given load is equal
to the strain in the reinforcing steel, ,s, at the same location.
fm
fs
' ,s '
,m '
(eq 5-1)
Em
Es
Where:
fm = The stress in the masonry, psi.
fs = The stress in the steel, psi.
Es = The modulus of elasticity of steel, psi.
The modular ratio, n, is given by the following equation.
Es
n'
(eq 5-2)
Em
The relationship between fs and fm is then,
E
fs ' n(fm) ' s (fm)
(eq 5-3)
Em
c. Transformed sections. When a masonry member is subjected to bending, the masonry above the neutral
axis of the cross section is in compression. The masonry below the neutral axis is assumed cracked. The
transformed section consists of the area of masonry above the neutral axis and n times the reinforcing steel
area below the neutral axis. The transformed area of steel in tension, Atrans, is--
Atrans = (n)(As)
(eq 5-4)
When the reinforcement and surrounding masonry is in compression, such as a column with a concentric axial
load, Atrans is one of the following--
(1) For long term loading conditions;
Atrans =(2n -1)As
(eq 5-5)
(2) For other than long term loading conditions;
Atrans = (n - 1)As
(eq 5-6)
Using n-1 or 2n - 1, rather than n, accounts for the area of masonry in compression being occupied by the
actual steel area.
5-3. Structural properties. The structural properties of hollow concrete masonry units provided in this
manual are based on the minimum dimensions given in ASTM C 90. These properties may also be assumed
for hollow brick masonry with the same minimum dimensions.
a. Unit types. It is recommended that open-end units, as shown in figure 5-1, be used in all masonry
construction. The open-end unit shown in figure 5-1 meets the requirements of ASTM C 90. The use of
5-1
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