Page 253 - Bridge and Highway Structure Rehabilitation and Repair

228 SECTION 2 STRENGTHENING AND REPAIR WORK

For collision load combination with partial live load:
7 R 9 [ Q 4 Q 4 Q
n (CIP DC1) (CIP DC1) CIP (Precast DC1) (Precast DC1) Precast DC2 DC2 DC2
4 WS Q )] 4 [ CV 0.5 Q ]
WS
CV
CV
WS
5.8.4 Service (see Tables 5.6b and 5.9a)
For service I with live load and partial wind:
R 9 [Q 4 Q 4 Q 4 Q )] 4 Q 4 [ Q ]
S CIPDC1 Precast DC1 DC2 WS LL wind wind
For service II and III, with factored live load and no wind:
R 9 [Q 4 Q 4 Q 4 Q )] 4 Q ]
S CIPDC1 Precast DC1 DC2 WS LL LL
For service IV, with no live load and reduced wind:
R 9 [Q 4 Q 4 Q 4 Q )] 4 Q 4 [ Q ]
S CIPDC1 Precast DC1 DC2 WS LL LL wind wind
For fatigue with partial live load and impact:
7 R 9 [ (1 4 I) Q
n LL HL93 HL93
7 R 9 ( CIP DC1- 4 Precast DC1- 4 DC2 4 ) Q / i
WS
i
n
DC is dead weight of component; CIP is cast in place construction; precast is factory made
component.
5.8.5 Fatigue (see Table 5.9b)
5.8.6 Defl ection (see Table 5.9c)

5.9 LRFD T-BEAM BRIDGE DESIGN
5.9.1 Summary of Design Method

A general method of solution based on LRFD sections of 2007 specifications is presented
here. Steps shown are for both primary and secondary loads. An excel Spreadsheet or Mathcad
program may be developed for ready use of large number of equations given in AASHTO Speci-
fications. For small spans : 40 feet, reinforced concrete T-beam design is suitable. Interior and

exterior beams need to be designed separately.
5.9.2 Structural Planning and Girder Design Data
1. Data required:
• Width of bridge
• Span lengths-(continuous spans are small depth)
• Skew angle
• Girder spacing
• f 1, f y
c
• Live load.
2. Develop general plan, elevation, and section, units in mm or inches.
3. Develop typical section and design basis on a trial basis:
• Use top flange thickness from deck design

• Check maximum slab span : 20 8 t
s
• Check web thickness
b 3 2 8 (conc. cover) 4 3 d 4 2 (1.5 d ) (AASHTO Sec. 5.10.3.1.1)
b
b
min
4. Check minimum depth of beam:
h min 3 0.065L for continuous spans (Table 2.5.2.6.3-1)
5. Reinforcement limits:
• Minimum reinforcement: Use the lesser from
7M 9 1.2 M cr
n
7M 1.33 8 Factored moment required for strength I limit state (5.7.3.3.2)
n
• Minimum reinforcement: A 0.03 f 1 / f 8 Area of x-section
y
s
c
• Crack control: f Z / (d A) 0.33 0.6 f (5.7.3.4)
s
y
c