Page 299 - Bridge and Highway Structure Rehabilitation and Repair

274 SECTION 2 STRENGTHENING AND REPAIR WORK

Overhang 1140 4 0.833 X
4 ve moment 660 4 0.55 S
6 ve moment 1220 4 0.25 S
IM 3 33 percent of LL (3.6.2.1)
• Number of lanes
N 3 Int (roadway width / 3600) (3.6.1.1.1)
• Multiple presence factors
m 3 1.2 for one loaded lane
3 1.0 for two
3 0.85 for three
• Tire contact area 3 l 8 510 mm
Where l 3 2.28 (1 4 IM/100) P (3.6.1.2.5)
Wheel load is applied as a distributed load.
• Fatigue limit state: Not required for multi-girder applications (9.5.3)
• Empirical design: Alternate method to traditional method (9.7.2.4)

6.9 RATING PROCEDURE FOR REINFORCED CONCRETE T-BEAM BRIDGE

Refer to AASHTO LRFR Specifi cations.
1. Perform dead load analysis for DC and DW.
2. Perform live load analysis for design live loads.
3. Compute live load distribution factors. (LRFD Table 4 – 11)
• One lane loaded: DF for fl exure
0.4
0.3
3 0.1
g 3 0.06 4 (S/14) 4 (S/L) 4 (K /12Lt )
m1 g s
• Two lanes loaded: DF for fl exure
0.2
0.6
g 3 0.075 4 (S/9.5) 4 (S/L) 4 (K /12Lt )
3 0.1
m2 g s
• 6.9.1 Computation of Distribution Factors
• Comparison with LFD method for steel girders:
3 0.1
0.2
0.6
• DF 3 0.15 4 (S/3) (S/L) (Kg/12 L ts )
• Assume L 3 1000 ft, S 3 12 ft, ts 3 8 in, Kg 3 1317,726
• DF 3 1.773 wheels per beam
• LFD distribution factor 3 S/5.5 3 2.18 wheels per beam
• Reduction in BM or SF 3 1.773/2.18 3 0.81
• Reduction 3 19 percent for interior beam
• One lane loaded: DF for shear
g 3 0.36 4 S/25.0
f1
• Two lanes loaded: DF for shear
g 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.

4. Compute effective flange width (LRFD 4.6.2.6.1)
Select minimum of L/4; Spacing or t 4 Greater of t or b /2
s w f
5. Compute distance to neutral axis c
c 3 A f / (0.85 fc1 b); a 3 c
s y 1 1