Structural Concrete - Hassoun

942 Chapter 22 Prestressed Concrete Bridge Design
Calculated Parameter
Summary of Shear Design with Three Different Approaches
Modified
Compression Field
Theory (MCFT)
Simplified MCFT with
Calculated Negative ε s
Simplified MCFT with
ε s
= 0
Cot θ 2.689 1.91 1.8
β 4.75 6.8 4.8
V c (kips) 164.7 238 166.5
V s (kips) 146.1 72.9 144.4
S (in.) 14 20 10
By comparing these three methods, it can be observed that the simplified MCFT, when ε s is
zero gives the most conservative stirrups spacing.
ANCHORAGE ZONE DESIGN (AASHTO 5.10.10)
Factored Bursting Resistance (AASHTO. 5.10.10.1)
The bursting resistance of the pretensioned anchorage zones provided by vertical reinforcement in
the ends of pretensioned beams at the service limit state shall be taken as
P r = f s A s
where
f s = stress in steel not exceeding 20 ksi
A s = total area of transverse reinforcement within h/4 of end of beam
The resistance shall not be less than 4% of the prestressing force before transfer:
F pbt = f pbt A ps = (218.7)(5.625) = 1230.2 K
P r = 0.04 ‘2367890-F pbt = 0.04(1230.2) = 49.2 K
( )
Pr
A s = in. 2 = 49.2 = 2.46 in.2
20 K 20
Prestressing force before transfer
Minimum bursting resistance
area of transverse reinforcement
within h/4 of end beam
Steel
within h = 18 in.
4
A v = 0.4 in. 2
Try #4 stirrup
N s = A s
= 2.46 = 6.151 Number of stirrups required within
A v 0.11
distance of 18 in.
CONFINEMENT REINFORCEMENT (AASHTO 5.10.10.2)
Confinement Reinforcement (AASHTO 5.10.10.2)
For a distance of 1.5 h from the end of the beam, reinforcement not less than no. 3 bars at 6 in. shall
be placed to confine the prestressing steel in the bottom flange.
1.5 h= 108 in.
Use seven no. 4 stirrups within 18 in. to resist bursting force.
REINFORCEMENT DETAILS
The details for the girder reinforcement is shown in Figs. 22.17, 22.18, and 22.19.