Page 128 - Design of Reinforced Masonry Structures

3.22 CHAPTER THREE

TABLE 3.8 Protection of Steel reinforcement (MSJC-08 Code Section 1.15.4, [3.2])

Condition Bar size Required cover, in.
Masonry face exposed to Larger than No. 5 2
earth or weather No. 5 and smaller 1½
Masonry not exposed to All sizes 1½
earth or weather

shell, mortar, and grout surrounding the bar. At bed joints, cover includes total thickness
of mortar and grout from the exterior of the mortar joint surface to the nearest surface of
steel. Table 3.8 presents mandatory code requirements for the protection to steel reinforc-
ing bars [3.2].

3.7 MODULUS OF ELASTICITY
OF MASONRY MATERIALS

3.7.1 Modulus of Elasticity of Steel Reinforcement
The value of the modulus of elasticity of steel, E , is commonly taken as 29,000 ksi (MSJC-
s
08 Code Section. 1.8.2.1, [3.2]).

3.7.2 Modulus of Elasticity of Masonry
The value of the modulus of elasticity of masonry, E , has been found to be dependent on
m
the 28-day compressive strength of masonry (f′). This relationship is analogous to the modu-
m
lus of elasticity of concrete which depends on the 28-day compressive strength of concrete.
Accordingly, the values of modulus of elasticity of masonry are expressed as a function of the
28-day compressive strength of masonry prism as follows [MSJC-08 Code Section 1.8.2.2]:
Clay masonry: E = 700 f′ m (3.1)
m
Concrete masonry: E = 900 f′ m (3.2)
m
The above values of the moduli of elasticity are carryover from 1999 and 2002 MSJC
Codes. In earlier masonry codes, the value of modulus of elasticity was specified as 1000 f′.
m
Research has indicated a large variation in the relationship between the compressive
strength of masonry and the modulus of elasticity [3.39, 3.40], and values of modulus of

elasticity lower than 1000 f′ may be more typical. Values of E given by Eqs. (3.1) and (3.2)
m
m
are higher than indicated by the best fit of data relating the modulus of elasticity of masonry
to the compressive strength of masonry. This is justified in view of the fact that actual
compressive strength of masonry significantly exceeds the specified compressive strength,
particularly for clay masonry. Readers should refer to Commentary ([3.4] Section 1.2) for
a discussion on this topic.
Users of some of the earlier design codes such as 1997-UBC [3.41] would note that
the aforestated values of the modulus of elasticity of masonry are different than those in
1997-UBC, which specified the same value for both clay and concrete masonry, given by
6
Eq. (3.3) and its maximum value limited to 3 × 10 psi:
E = 750 f′ m (3.3)
m