Page 94 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 2 DIAGNOSTIC DESIGN AND SELECTIVE RECONSTRUCTION 69

2. Decks are generally made of timber or corrugated metal. Test results of the mechanical
properties of wrought iron show that average yield strength was 24.5 ksi but minimum
was as low as 18 ksi; while average ultimate strength was less than 40 ksi and minimum
was 26 ksi. Over the years there have been average reductions in the cross sectional area
of 25 percent.
3. For historic bridges, preservation is the preferred option. Structural procedures include the
following tasks:
• Preparing records of inventory
• Preparing CADD drawings and detailed documentation of members and connections
• Research into alternate materials and refurbishment methods
• Structural analysis and load capacity rating
• Replacing old deck and railing with new
• Replacing deteriorated components with new components
• Conversion into pedestrian bridges if necessary, while preserving the bridge
• Foundation strengthening or design and construction of new foundations
• Installing sensors for SHM.

2.6.5 Restoration of Historic Footbridges
1. Footbridges promise increased challenges to designers by incorporating more elegant and
modern materials, thoughtful designs, and complex engineering. Even railway bridges can
be included in this trend of illustrating and interpreting local heritage. Footbridge design
has become an aesthetically pleasing way for neighborhoods and communities to create a
desired landmark.
To strengthen masonry arches, a successfully used reinforcing anchor system process
involves:
• Maintaining the historical integrity of the bridge
• Simulation of the loading regime to specify a retrofitted reinforcing system

• Numerical modeling
• Designing the number of bars to be used and their location
• Dry or low volume wet diamond drilling techniques
• At selected locations, where a structure is in need of reinforcement, a steel bar enclosed
in mesh fabric sleeve is inserted
• A non-polymer grout is injected into the sleeve under low pressure.
2. Preserving historic railings that may not meet current standards on bridges that are listed or
are eligible to be listed in the National Register of Historic Places presents a special chal-
lenge in maintaining the appearance of the bridge. Original railing on a historic bridge is
not likely to meet:

• Current standards for combination traffic and pedestrian railings, e.g., a minimum height
of 42 inches and the minimum 6-inch limit for openings in the railing
• Current crash test requirements

• Current standards for railing height and for combination traffic and pedestrian railings.
3. Options for upgrading the railing include the following:
• Replace the existing railing with an approved, acceptable railing of similar appearance.

• Place approved traffic railing inboard of existing railing, leaving the existing railing in
place to act as dummy railing.
• Where an existing railing is especially decorative, remove the current railing and mimic
into a new acceptable railing.