Page 302 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 6 APPLICATIONS OF BRIDGE DESIGN AND RATING METHODS 277

Total prestress losses 3 ( f pES 4 f pTL 6 f ) (LRFD 5.9.5.1)
pRI
Loss due to elastic shortening f 3 E f /E (LRFD 5.9.5.2.3a)
pES
p cg p
ci
2
f 3 (P /A 4 P e / I 6 M e / I)
D
cg p
i
i
For low relaxation prestressing strands, initial prestress 3 0.75 f (LRFD Table 5-7)
pu
P 3 f A / (f A 4 fyAs) (LRFD Table 5-12)
py
py
ps
PR
ps
Time-dependent losses for I-girder:
f pTL 3 33[1.0 6 0.15 (fc16 6.0)/6.0] 4 6.0 P 6 6.0 (LRFD 5.9.5.3)
PR
Relaxation at transfer f 3 log (24.0 t) [(f /f ) 6 0.55] f / 40.0
pj
pRI
py
pj
For As 3 0, P 3 1 LRFD Eq. (5-8)
PR
f 3 P /A 4 P e/S b
pe
pe
pb
Maximum reinforcement: (LRFD 6.5.6)
c/d 0.42 (LRFD 5.7.3.3.1)
e
9. Critical shear check (LRFD 5.11.4)
Shear check is not required if there is no visible sign of shear distress.
Critical location for shear occurs near the supports and it is greater of d or 0.5 d (Cot 0).
v
v
(LRFD 5.8.3.2)
Effective shear depth d is maximum of:
v
0.9 d ; 0.72 h; distance between resultants of tensile and compressive forces.
e
Assume 0 3 30 degrees; 0.5 d (Cot 0) 3 0.87 d d v
v
v
Minimum transfer length 3 60 8 strand diameters (LRFD 5.11.4)
If section is outside transfer length, full value of f is used in calculating shear resistance.
po
Maximum shear at critical section near supports:
Total shear 3 V Lane 4 V Truck 8 IMP
Nominal shear resistance 3 V 3 V 4 V 4 V LRFD Eq (5-66)
n
p
s
c
For straight tendons, V 3 0
p
Provide minimum transverse reinforcement:

10. Modified Compression Field Theory (MCFT)
V Lane 3 0.0316 (fc1) b S / f (LRFD 5.8.2.5)
0.5
y
v
0.5
V Lane 3 0.0316 (fc1) b d LRFD Eq (5-68)
v
v
V 3 A f d Cot 0 / S LRFD Eq (5-69)
s
v
v y
11. Simplified approach for shear design

0 3 45 degrees; 3 2
0.5
V 3 0.0316 (fc1) b d v
c
v
Table 6.11 Sample summary table of rating factors.
Design Load Rating
Limit State Inventory Operating Permit Load Rating
Strength I Flexure
Shear
Strength II Flexure
Shear
Service II
Fatigue Stress Ratio 5