Page 131 - Bridge and Highway Structure Rehabilitation and Repair

106 SECTION 1 ADMINISTRATIVE ISSUES

3.14.2 Seismic Ranking and Prioritizing

1. Retrofit priorities are defined in FHWA Seismic Retrofitting Manual for Highway Bridges
and will depend upon:
• Structural vulnerability (Some components are more vulnerable than others, such as girder
connections, bearings, seat width, piers, abutments, and soils.)
• Seismic and geotechnical hazards
• Importance factor.
2. The rating system:
• The quantitative part consists of the seismic rating (bridge ranking) and is based on the
structural vulnerability or seismic hazard.
• The qualitative part consists of an overall priority index consisting of importance factor,
remaining useful life, non-seismic deficiencies and redundancy.

3.14.3 Seismic Retrofi t Goals
The primary goal of seismic retrofitting is to minimize the risk of unacceptable damage

during an earthquake. Damage is unacceptable if it results in the collapse of all or parts of the
bridge or loss of use of this vital transportation route.
Bearings, sliding plates, and anchor bolt nuts may exhibit moderate to severe rust and mate-
rial loss.
The old approach for continuous girders was to provide two lines of bearings on each pier. It
is important that all bearings allow movement during a seismic event and therefore will require
regular maintenance. At the time of original old bridge constructions, there were no seismic
criteria in effect. Hence, it is important to analyze the seismic vulnerability of bridge using
three-dimensional finite element software, such as SAP 2000 or ADINA in order to minimize

the risk of:
• Structural damage
• Loss of life
• Collapse of all or part of bridge
• Loss of use of a vital transportation route (essential route).
AASHTO minimum seat width requirement to be satisfied—formula revised to include skew

of support S measured from line normal to span L.
2
N 3 (200 4 0.0017L 4 0.0067H) (1 4 0.000125S )
3.14.4 Seismic Retrofi t Process
1. The following issues need to be addressed:
• Preliminary screening—inventory
• Detailed evaluation
• Computed vulnerability rating
• Seismic ranking evaluation
• Design of retrofi t measures.
2. Evaluate and upgrade the seismic resistance of existing bridges.
Examples of seismic retrofi ts are:
• FRP wrapping
• Column strengthening and jacketing
• Substructure stabilization/repairs/foundation improvement
• Bearing strengthening/use of restrainers
• Seat width improvement/bearing seat retrofi t
• Bearing replacement/using elastomeric pads or isolation bearings