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 The specific compressive strength and the modulus of elasticity of the
mortar may be assumed to be equal to that of the unit.
(b) Joint reinforcement shall conform to the requirements of
ASTM A82 and, therefore, it will have a minimum ultimate (fu) and yield
(fy) stresses equal to 80 ksi and 70 ksi respectively. Reinforcing bars
shall conform to ASTM A615 (Grade 60) and have minimum ultimate stress
(fu) of 90 ksi and minimum yield stress (fy) of 60 ksi. The modulus of
elasticity of the reinforcement is equal to 29,000,000 psi.
(2) Dynamic Strength of Material. Since design for blast resistant
structures is based on ultimate strength, the actual yield stresses of the
material, rather than conventional design stresses or specific minimum yield
stresses, are used for determining the plastic strengths of members.
Further, under the rapid rates of straining that occur in structures loaded
by blast forces, materials develop higher strengths than they do in the case
of statically loaded members. In calculating the dynamic properties of
concrete masonry construction, it is recommended that the dynamic increase
factor be applied to the static yield strengths of the various components as
follows:
Concrete
Compression, axial or flexure
1.25 f'm
Shear
1.00 f'm
Reinforcement
Compression, axial or flexure
1.10
fy
(3) Ultimate Strength of Reinforced Concrete Masonry Walls.
(a) The ultimate moment capacity of joint reinforced masonry
construction may be conservatively estimated by utilizing the horizontal
reinforcement only and neglecting the compressive strength afforded by the
concrete. That is the reinforcement in one face will develop the tension
forces while the steel in the opposite face resists the compression
stresses. The ultimate moment relationship may be expressed for each
horizontal joint of the wall as follows:
EQUATION:
Mu = Asfdydb
(126)
where,
As = area of joint reinforcement at one face, in2
fdy = dynamic yield strength of the joint reinforcement, psi
db = distance between the centroids of the compression and
tension reinforcement, in
Mu = ultimate moment capacity, in-lb/joint
2.08-206
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