Page 44 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 1 OVERVIEW OF BRIDGE ENGINEERING 19

Superstructure

Figure 1.8 Minimum horizontal clearances to abutment walls measured from the edge of lane.

5. Entry or exit ramps connect two levels of traffic moving approximately at right angles. For
safety reasons both entry and exit ramps are located adjacent to the right lane which carries
slower traffic. A ramp has traffic moving in a single curved direction while a bridge has traf-

fic moving in both directions. An acceleration lane is for transition from a slow speed entry
ramp merging into fast moving traffi c. Similarly, a decelerating lane serves as a transition
between a fast lane and slow speed exit ramp.
1.6.3 Horizontal and Vertical Under Clearances
1. If existing horizontal or vertical clearances are not adequate, the existing bridge needs to
be replaced with a new bridge that has higher clearances. As an alternate, posting for ver-
tical over clearance or under clearance is required in keeping with agency requirements.
AASHTO specifications have defined minimum horizontal and vertical clearances to bridge

substructure and superstructure. These may be modified by state and local codes.
2. The minimum horizontal clearance between the edge of the lane and the concrete face of
the abutment or pier is applicable. A commonly used minimum horizontal clearance is 30
feet to abutment face from the edge of the travel lane (Figure 1.8 and Table 1.3) and 16 feet
6 inches for minimum vertical clearance (Table 1.4) from the top of the road surface and
minimum 23 feet from the top of the rail (Figure 1.9). Older bridges were designed for lower

Table 1.3 Minimum horizontal clearances.
Serial Minimum
Number Bridge Type Located Over Horizontal Clearance Remarks
1 Highway Intersection 301-02 AASHTO code governs

2 -do- Waterway N/A Due to soil erosion, horizontal
clearance is diffi cult to maintain
3 -do- Railroad -do-
4 Railroad Intersection 141–02 AREMA code governs

5 -do- Waterway Varies for each river. Due to soil erosion, horizontal
Determined by Coast Guard clearance is diffi cult to maintain
6 -do- Railroad 141–02 -do-
7 Pedestrian Intersection 141–02 Less maintenance due to low
live load fatigue
8 -do- Waterway N/A -do-
9 -do- Railroad 141–02 -do-