Page 293 - Bridge and Highway Structure Rehabilitation and Repair

268 SECTION 2 STRENGTHENING AND REPAIR WORK

Factors for operating rating: 3 1.25; DW 3 1.25; 3 1.35
LO
DC
Operating rating 3 Inventory rating 8 ( / )
LI LO
2. Design rating equation
Strength I limit state:
C 3 () ) () ) ()) R n
s
c
RF 3 [() ) () ) ()) R 6 ( ) (DC) 6 ( ) (DW)] / (LL 4 IM) (LRFD 6.5.4.1)
c s n DC DW L
Condition factor () ) 3 1.0 for no deterioration (LRFD 6.4.2.3)
c
System factor () ) 3 1.0 for a slab bridge (LRFD 6.4.2.4)
s

Resistance factor ()) 3 0.9 for flexure (LRFD 5.5.4.2)
IM 3 1.33
6.8.3 Beamless Reinforced Concrete Slab Bridge

Some of the older bridges constructed in the earlier part of twentieth century have smaller
spans less than 30 feet and were constructed in the pre-prestressed concrete era. Nearly all of
them were cast in place construction requiring heavy formwork. Both single and multiple spans
have been used. Original design in reinforced concrete was based on AASHO code, which was
popular at that time. The advantages were that design of beams and bearings was not required.
Due to live load restrictions they are commonly used for pedestrian bridges or are posted for
about 15 tons.
They are uneconomical since live load defl ection requirements lead to a small span/depth

ratio. Main reinforcement is placed parallel to direction of traffic. Distribution reinforcement is
required. Due to continuity in transverse direction, shear design is not required. Although there
are lesser local effects of shear distribution due to moving wheels than from vehicles which may
be stationary in a traffic jam, the live load impact factor is higher. Fatigue stress needs to be

considered since reversal of stress from moving vehicles will induce fatigue stress in bending.
6.8.4 Solved Example for Design of Single Span Slab Bridge:
Data: Single span 3 30 ft with uneven surface of deck overlay
Clear distance between curbs 3 20 ft including 5 ft wide, 5 in thick sidewalk
on deck slab.
Out-to-out bridge width 3 20 ft 4 2 8 1.75 ft (New Jersey barrier width) 3 23.5 ft
Slab thickness 3 18 in with 2 in overlay; f 1 3 0.30 ksi, f 3 30 ksi
c y
d 3 18 6 1.5 6 0.5 3 16 in
L/D 20 i.e. 30 8 12/18 3 20, hence assumed thickness is okay. (AASHTO
8.24.1.2.1)
Effective span 3 Clear span 4 Depth of slab or distance to center lines of bearings
Center-to-center distance
Effective width of slab E 3 4 4 0.06 S 3 4 4 0.06 8 30 ft 3 5.8 ft (AASHTO 3.24.3.2
Case B)
Dead load of slab and overlay 3 0.15 8 (18 in/12 4 2 in/12) 3 0.25 ksf
Manual of Condition Evaluation of Bridges (3.3.2.1 to 3.3.2.3)
Dead load of parapet 4 Sidewalk to be distributed over 0.5 k/ft / (23.5/2) 4
(0.15 8 5)/12 / (23.5/2)
= 0.043 4 0.005 3 0.048 ksf; use 0.05 ksf
Total DL 3 0.25 4 0.05 3 0.30 ksf