FRP-Retrofitting RC Bridges for Stabilizationin Earthquakes

Transportation infrastructure destruction in earthquakes can be extensive, particularly when older RC bridges are not designed to current earthquake codes. Bridges constructed before the 1970’s, in Europe, Asia, and the USA, and not retrofitted, have a potential risk of failure, even in moderate earthquakes. Recent California earthquakes (last 45 years) caused multiple modes of failure in RC bridge critical structural elements. The primary causes of bridge collapses were insufficient ductility in the bridge columns, inadequate lap-splice length in the potential plastic hinge regions, and insufficient transverse reinforcement. Retrofitting existing RC bridge bents (columns and beams) is an efficient and cost effective rehabilitation technique to improve seismic performance.


Introduction
Transportation infrastructure destruction in earthquakes can be extensive, particularly when older RC bridges are not designed to current earthquake codes. Bridges constructed before the 1970's, in Europe, Asia, and the USA, and not retrofitted, have a potential risk of failure, even in moderate earthquakes. Recent California earthquakes (last 45 years) caused multiple modes of failure in RC bridge critical structural elements. The primary causes of bridge collapses were insufficient ductility in the bridge columns, inadequate lap-splice length in the potential plastic hinge regions, and insufficient transverse reinforcement. Retrofitting existing RC bridge bents (columns and beams) is an efficient and cost effective rehabilitation technique to improve seismic performance.
Major techniques for structural rehabilitation of RC bridges include encasing columns and beam column joints using steel, FRP, RC jackets, and adding new steel bracing structural elements [1]. Retrofitting circular/rectangular bridge columns by adding cylindrical or elliptical steel jackets in the plastic hinge region is an effective measure to enhance shear strength and flexural ductility. As an alternative to steel jackets, advanced FRP composite material have been developed for retrofitting concrete columns, resulting in significantly improved hysteretic response and increased ductility.
The unique properties of carbon FRP (CFRP) with its high strength, low weight, and ease of application provide an ideal material for strengthening RC bridge columns and beams for gravity-load designed multi-column bridge bents. Retrofitting older highway bridges to mitigate loss of service ability in seismic events is a costeffective option compared to the high cost of bridge replacement. Three earthquakes produced far reaching changes in California seismic codes and bridge design philosophy.

Sylmar earthquake
Occurred in southern California with a magnitude 6.6, lasting 12 seconds at a depth of 13km. Thrust faulting ruptured a 19km segment of the San Fernando fault with a maximum slip of 2 meters, causing twelve overpass bridges to fail falling onto freeways below. Damage to bridge structures varied from minor cracking to complete loss of bridge sections.

Northridge Earthquake
Occurredin the north-central San Fernando Valley, Los Angeles, magnitude of 6.7, with seven collapsed freeway overpasses, causing $50 billion in damage and 57 deaths. Bridges collapsed due to column failure from inadequate shear design, lack of flexural capacity and buckling of the compression reinforcement, ( Figure  1) High vertical acceleration caused column flexure/shear failures. The rate of vertical acceleration was the highest ever recorded in North America.

Materials and Methods
Following the 1994 Northridge earthquake Cal Trans (California Dept of Transportation) established the Seismic Advisory Board (SAB) making substantial efforts to improve bridge seismic structural codes. In their seismic design criteria the SAB specified ductile/seismic-critical RC bridge members sustaining damage during a seismic event cannot collapse or lose structural integrity. Since 1994, substantial efforts were made to retrofit RC highway bridges with a variety of structural improvements. This paper analyzes the application of CFRP composite material to strengthen and stabilize bridge bents and foundations for seismic loading.

Materials
The CFRP composite material for jacketing RC bridge columns and beams to strengthen columns or girders for shear, reinforced concrete slabs in flexure, and joints for external cap/column connections, has significant benefits over steel jacketing. The CFRP sheet material is a carbon fiber/epoxy resin composite with 48,000 unidirectional carbon fibers per tow with 6.5 tons per 25.4mm. CFRP design parameters have a modulus of elasticity of 65 GPa, a tensile strength of 628 MPa, ultimate axial strain of 10mm/m, and a layer thickness of 1.32mm [2].

Methods
In retrofitting RC bridge bents it is necessary to develop a higher base shear and moment capacity than the existing foundation and pile cap system. The performance-based design procedure includes a nonlinear static analysis of the bridge bent to determine column CFRP jacket thickness for plastic hinge confinement, shear strengthening, and lap splice clamping. The design increases displacement ductility of the bridge bent (column beam cap) developing a higher base shear and moment capacity than the existing foundation and pile cap system. The RC beam connecting pile cap completes the tension and compression load path, increasing shear and flexural capacity of the foundation. The CFRP jacket in the bent T-joints, (Figure 2) consists of three elements: (a) diagonal FRP composite sheets for resisting diagonal tension, (b) FRP composite sheets in the direction of the beam cap axis for shear strengthening and increased flexural capacity, and (c) U-straps anchoring the longitudinal column bars providing additional flexural strength [2]. An additional retrofit method involves application of near-surface-mounted (NSM) CFRP rods anchored into column footings to increase flexural strengthening [3].

Results and Observations
In-situ experiments of multi-column bridge bents demonstrate an external CFRP composite seismic retrofit combined with a retrofit of the foundation providing increased ductility and seismic performance. The applied CFRP layers strengthen the column lap providing additional clamping capacity, adding external stiffness to the lap splice, transferring the plastic hinge above the lap splice elevation, providing additional shear capacity, flexural stiffness, and confinement. With increased flexural stiffness the base moment demands are transferred more efficiently into the bent pile foundation system [1].

Conclusion
The bridge retrofitting program after the 1971 earthquake provided a new basis for designing and constructing highway bridges to survive moderate to major earthquakes. Cal Trans made significant changes in designing and constructing highway bridges. Retrofitting bridge bents with CFRP composites prove valuable in reducing column flexure/shear failure. FRP-structural systems applied to RC bridge bents provide reliable strengthening to mitigate the impact of earthquake loading and sustain bridge serviceability.