Structural Concrete - Hassoun

19.5 Design of Flexural Members 757
Figure 19.7
Factored moment capacity of prestressed concrete beams.
19.5.2 Rectangular Sections
The nominal moment capacity of a rectangular section may be determined as follows (refer to
Fig. 19.7):
M n = C
(d p − 1 )
2 a = T
(d p − 1 )
2 a (19.34)
where T = A ps f ps and C = 0.85 f cab. ′ ForC = T,
a =
A ps f ps
0.85 f cb ′ = ρ p f ps
0.85 f c
′ d p (19.35)
where the prestressing steel ratio is ρ p = A ps /bd p ,andA ps and f ps refer to the area and tensile stress
of the prestressing steel. Let
( ) fps
ω p = ρ p ≤ 0.32β 1
f ′ c