Page 93 - Bridge and Highway Structure Rehabilitation and Repair

68 SECTION 1 ADMINISTRATIVE ISSUES

Table 2.2 Notable differences in old and new materials and methods.
Old/Existing
Construction Replacement Widening Rehabilitation
Inspection and N/A for original Defi ciencies must be Not a defi cient bridge Modern inspection
structural evaluation construction established methods with ultra high
speed 3D laser techniques
Materials Traditional materials Modern materials used Partly traditional and Special materials used
used partly modern
Construction techniques Traditional construction Formwork, pile bent, -do- Modern techniques used
used precast components
Geometry and vertical Substandard 16 feet 6 inches Substandard Limited changes
underclearance standard cross slope
and grade

Structural system Non-redundant through Redundant system Widening not Non-redundant through
girders possible girders

Live loads Live load to be checked HL-93 Live load to be checked Live load to be checked by
by rating. bridge may be by rating. rating.
posted.

Seismic design N/A Seismic zone and multiple N/A (same as existing) Seismic retrofi ts
spans to be considered in may be required
design
Scour analysis/ River characteristics Based on hydraulic Improves narrow River maintenance is
countermeasures have changed studies openings and reduces required for bridges over
design scour scour critical rivers
Foundation scour Riprap Gabions, grout bags, Gabions, grout bags Gabions, grout bags
sheet piles
Bridge security N/A Full security Limited security Limited security

Environmental issues Monitoring agencies Stream encroachment Stream encroachment Some air, water and noise
such as EPA were not and other construction and other construction control requirements need
required permits required permits required to be complied with

need to be constructed, and the entire superstructure needs to be transported as one piece
requiring a single operation.
• The superstructure may be dismantled piece by piece and reassembled at another loca-
tion. One example is that of the historic London Bridge, which was relocated to the U.S.
at a considerable expense. In other situations when huge reservoirs are created to prevent
inundation, selected bridges downstream may be removed to an alternate site.
• Application of high capacity cranes, dollies, and heavy duty trucks needs to be investigated
for feasibility of moving and transporting the superstructure to a new location.

2.6.4 Structural Data for Preservation Design
1. The majority of historic bridges are over 100 years old. They are special bridges (such as cable
stayed with towers) or made of wrought iron, cast iron, and mild steel, with through Pratt,
Pony, or Warren truss superstructures and stone masonry arches. Wrought iron structures
represent an important era in the development of structures in the U.S. Figure 2.15 shows a
historic bridge.