Page 301 - Bridge and Highway Structure Rehabilitation and Repair

276 SECTION 2 STRENGTHENING AND REPAIR WORK

6.10 RATING OF PRESTRESSED CONCRETE GIRDER
Simple Span I-Girder Bridge
The following steps are provided for ready reference. Prior to developing a software or for
solving the equations using hand calculations, equations need to be checked against the latest

version of applicable AASHTO LRFD Specifications or LRFR Manual.

Effective flange width (LRFD 4.6.2.6.1)
Select minimum of L/4; Spacing S or t 4 Greater of t or b /2
f
s
w
1.5
0.5
Ec 3 33000 (Wc) (fc1) (LRFD 5.4.2.4)
A. Dead load analysis for DC1, DC2, DW
M max 3 wL /8
2
B. Perform live load analysis
C. Calculate DF
1. One lane loaded: DF for fl exure
0.3
0.4
3 0.1
g 3 0.06 4 (S/14) 4 (S/L) 4 (K /12Lt )
g
m1
s
2
Longitudinal stiffness parameter K 3 n (I 4 A (e ) )
g
g
2. Two lanes loaded: DF for fl exure
0.2
3 0.1
0.6
g 3 0.075 4 (S/9.5) 4 (S/L) 4 (K /12Lt )
m2
g
s
3. One lane loaded: DF for shear
D 3 0.36 4 S/25.0
f1
4. Two lanes loaded: DF for shear
D 3 0.2 4 (S/12) 6 (S/35) 2.0
f2
Compute design live load moments for HL-93 (HS-20 truck, lane or tandem) and multiply
by DF.
Flexural resistance:
f 3 (1 6 K c/ d ) (f ) LRFD Eq. (5.16)
ps
pu
p
K 3 0.28 for low relaxation strands
f 3 270 ksi
pu
d 3 Distance from extreme compression fi ber to the centroid of prestressing tendons.
p
5. Distance to neutral axis c
c 3 (A f ) / (0.85 fc1 b 4 K A f /d ) LRFD Eq. (5-19)
ps pu
1
ps pu
p
a 3 c 1
6. M 3 A f (d – a/2)
p
ps
ps
n
7. Minimum reinforcement required to develop M r
Lesser of 1.2 M or 1.33 M (LRFD 5.7.3.3.2)
u
cr
M 3 ) M Eq. (6-4)
n
r
M 3 (f 4 f ) S 6 M (S /S 6 1) (LRFD 5.4.2.6)
bc
cr
b
pb
bc
r
d,nc
0.5
Modulus of rupture f 3 0.24 f 1) (LRFD 5.4.2.6)
r
c
f 3 P /A 4 P e/S b
pe
pb
pe
S 3 I/y
bc t
f 3 0 (No prestress)
pb
M d,,nc 3 0 (non-composite dead load moment)
8. Determine effective prestress force P pe
f 3 Initial prestress 6 Total prestress losses.
pe
f A 3 P pe
ps
pe