Page 292 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 6 APPLICATIONS OF BRIDGE DESIGN AND RATING METHODS 267

If RF 1 for the legal loads, the bridge needs to be posted for the lower live load. For a
new bridge, rating is generally not a problem since the rating factor is 9 1.
For rating, the load factors are different from those used for design. Load factors used are
higher for rating, and the approach is more conservative.
However, for an existing deteriorated bridge the moment of resistance is lower. Older bridges
were designed for lower truck loads and need to be evaluated against new truck types.
The following equations for slab bridge design or rating may be programmed using Excel
spreadsheets or Mathcad:

Using standard definition of nominal moment of resistance, M 3 A f (d 6 a/2)
y
s
n
s
a 3 c 1
c 3 A f / (0.85 f 1 b)
s
1
y
c
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)
r
n
1.2 M 3 1.2 (f f ) S 6 M (S / S 6 1) (LRFD 5.4.2.6)
cr
r 4 pb
bc
b
d,nc
bc
M 3 Non-composite dead load moment
d,nc
f 3 Compressive stress in concrete due to effective prestress in the prestressed
pb
tensile zone
S 3 Uncracked section modulus neglecting steel S 3 I/y t
bc
bc
I 3 Moment of inertia of uncracked section
y 3 Distance to extreme tension fi ber from neutral axis of uncracked concrete
t
f 3 0.24 (f 1) 0.5
r
c
Maximum reinforcement: For ductility, limit neutral axis depth. (LRFD 6.5.6)
c/d 0.42
e
Rating of interior strip:
Calculation of equivalent interior strip width ‘E’ (LRFD 4.6.2.3)
L 60 ft
1
Number of loaded lanes 9 1
E 3 [84 4 1.44 (L W1) ] 12 W/N LRFD Eq. (4-18)
0.5
1
L
W is lesser of 43 ft or 60 ft.
1
One lane loaded
0.5
E 3 10.0 4 5.0 (L W ) LRFD Eq. (4-18)
1
1
W is lesser of 43 ft or 30 ft.
1
Longitudinal edge strip width (LRFD 4.6.2.1.4b)
which supports one line of wheels 4 a tributary portion of design lane load
= $ (Width of strip under the barrier/parapet 4 E/2 4 12) (LRFD 4.6.2.1.4a)
E or 6 ft.
6.8.2 Rating Method
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 Specifi cations.
1. General load rating equation (LRFD 6.4.2)
RF 3 [C 6 ( ) (DC) 6 ( ) (DW) ? (P)] / (LL 4 IM) (EQ 6-1)
DC
DW
L
P
Factors for inventory rating: 3 1.25; 3 1.25; 3 1.75
DC
LI
DW