Structural Concrete - Hassoun

3.14 Rectangular Sections with Compression Reinforcement 117
3.14.1 When Compression Steel Yields
Internal moment can be divided into two moments, as shown in Fig. 3.23. Let M u1 be the moment
produced by the concrete compressive force and an equivalent tension force in steel, A s1 , acting as a
basic section. Then M u2 is the additional moment produced by the compressive force in compression
steel A ′ s and the tension force in the additional tensile steel, A s2 , acting as a steel section.
The moment M u1 is that of a singly reinforced concrete basic section,
T 1 = C c (3.36)
A s1
f y = C c = 0.85 f ′ cab (3.37)
a =
A s 1
f y
0.85 f cb
′
(
M u1
= φA s1
f y d − a )
2
(3.38)
(3.39)
The restriction for M u1 is that ρ = A s1 /bd shall be equal to or less than ρ max for singly reinforced
tension-controlled sections, as given in Eq. 3.31a.
Figure 3.23
Rectangular section with compression reinforcement.