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Highway bridges constitute a large portion of the national wealth and build up the foundation for infrastructure development. For the last few decades researchers have made extensive effort to make bridges safe against devastating earthquake and keep them functional following a seismic event. Failure of bridges during past earthquakes (1971 San Fernando, 1989 Loma Prieta, and 1994 Northridge) have accelerated the need for bridges that can attenuate the seismic damage and reduce both long and short-term causalities following an earthquake. Bridge bents are one of the vulnerable elements in a bridge, failure of which can have catastrophic consequences. Bridge bents are reinforced concrete (RC) frames commonly used to support beams and girders. If a bridge bent can be made smart enough so that it can reduce the seismic damage and return to its original position after a seismic event, many of the issues associated with bridge management could be eliminated. The development and execution of smart and advanced materials in bridge construction not only can enhance the performance of bridge during seismic event but also can assure the post-earthquake recoverability. Shape memory alloy (SMA) is a smart material that has the unique ability to undergo large deformation, but can regain its undeformed shape through stress removal, which brings about an added advantage in seismic regions.
Reinforced concrete bridges located in high seismic region are designed to have high ductility which in turn increases the permanent deformations. In order to increase the post earthquake recoverability and reduce the maintenance and repairing costs, diminution of residual displacements is required. In the proposed study superelastic SMA will be used in conjunction with regular steel in the bridge bent. The objective of the proposed research is to develop smart bridge structures with (i) reduced risk of failure, (ii) enhanced deformation capacity, (iii) reduced residual crack sizes, and (iv) negligible/ reduced permanent deformation. This research will aid in developing performance- based design guidelines for SMA reinforced concrete bridge bent.
Literature review method and database used:
In order to prepare the literature review the application of SMA in structural applications was explored. Then the applications were categorized in different sections. As SMA is comparatively new as a construction material, how different researchers have incorporated it in their model was investigated. The database used for this literature review is compendex and web of science. Shape memory alloy, bridges were used as keywords for searching documents. Moreover as SMA is a relatively new material and considered as a smart material, journals such as Smart materials and structures, Journal of bridge engineering and Engineering structures were explored.
Shape Memory Alloy:
Shape Memory Alloys (SMA) is novel materials that have unparallel properties such as superelasticity (SE) and shape memory effect (SME). Superelasticity is the unique phenomenon that makes SMA a smart material. Due to its superelasticity SMA is able to regain its original shape upon stress removal even from inelastic region with significant energy dissipation. Superelasticity, negligible residual strain and excellent corrosion resistance have made Ni-Ti alloy most suitable for structural applications. The shape memory effect (SME) is temperature dependent. With this distinct property SMA is able to regain its original shape up to 8%starin with negligible residual deformation. SMA also performs well under cyclic loading with significant energy dissipation which makes it suitable for structural applications in seismic regions.
Applications of SMA:
Numerous researches have been carried out on applications for which shape memory alloys can be used for building and bridges in seismic regions. These include as reinforcing bar for concrete structures such as beams, columns and joints, as confining wires for retrofitting columns, and in bridge retrofitting, bolted connections, restrainer cable and dampers in bridges and prestressing strands.
Reinforcing bars in concrete structures:
Concrete, by nature is a very stiff material, and when exposed to seismic activity, especially at great magnitudes, can undergo severe damage, causing loss of life or millions in repairs. To manage this problem composites are added to strengthen the concrete and make it more malleable. The main goal of designing concrete structures for seismic design is to dissipate energy while combatting permanent deformations, maintaining the structural integrity of the members and avoid collapsing. An experimental investigation conducted by Saidi and Wang (2006) showed that column reinforced with SMA in plastic hinge region reduced seismic damage and dissipated adequate amount of seismic energy. Nehdi et al. (2010) proposed a hybrid beam-column joint reinforced with SMA in plastic hinge region which showed better performance under seismic loading. In an analytical investigation in an eight storey reinforced concrete frame Alam et al. (2009) demonstrated the efficacy of using SMA in plastic hinge region of the beam under seismic loading.
Application in bridges:
Bridges are considered to be one of the most vulnerable structures during an earthquake event. In an attempt to make bridges serviceable after seismic events and reduce their repairing cost researches have been carried out to establish the feasibility of using SMA in bridge construction. Saiidi et al. (2009) experimentally investigated the effectiveness of reinforced concrete bridge columns with superelastic shape memory alloy (SMA) reinforcement and engineered cementitious composites (ECC) in plastic hinges. Their results showed that the inclusion of SMA reduced the residual displacement by a substantial amount compared to regular steel reinforced column. In an analytical investigation Roh and Reinhorne (2010) introduced SE SMA bar at the base segment of precast segmental bridge pier to improve the energy dissipation capacity. Andrews et al. (2010) experimentally and analytically demonstrated the feasibility of wrapping bridge columns using SMA in order to increase the strength and ductility of bridge column.
Application in retrofitting of existing structures:
Insufficient ductility, defective construction, inadequate seismic detailing makes a structure seismically deficient and raises the need for retrofitting. As SMA possesses some unique properties now a dayâ€™s it is considered as a potential candidate to be used in seismic retrofitting of structures. Soroushian et al. (2001) practically demonstrated the use of SMA bar for the rehabilitation of a shear strength deficient bridge in Michigan. This technique of SMA post tensioning resulted in successful reduction in the crack width. In an analytical study on multi-span simply supported bridge with SMA restrainers by DesRoches and Delemont (2002) demonstrated that SMA restrainers can effectively reduce the relative displacements of hinge compared to the regular steel restrainers. In an analytical study Andrews and Shin (2008) compared the seismic performance of bridge column retrofitted with SMA ring and FRP sheets. Their study asserted the supremacy of SMA over FRP in damage reduction and reduced lateral displacement under dynamic loading.
Application in seismic isolation and dampers:
Dampers are considered as passive control devices to protect structures during a seismic event. A wide variety of dampers such as visco-elastic dampers, viscous-fluid dampers, frictional dampers, hysteretic dampers, tuned-mass dampers etc are widely used. But these dampers have some disadvantages such as ageing and durability, high maintenance cost and little or no recentering capability. Experimental and theoretical studies conducted by Witting and Cozzarelli(1992), Aiken et al.(1993), Hodgson and Krumme(1994) Dolce et al.(2000) have demonstrated the potential of SMA to surpass those disadvantages with adequate structural control. An intelligent damping device composed of SMA was developed by Adachi et al. (2000). They conducted experimental program and reported its effectiveness in reducing seismic force and regaining original position resulting from SMAâ€™s SE and SME. A smart damper based on SMA wires was proposed by Han et al. (2003) for structural vibration control. They carried out experimental investigation and concluded that SMA dampers effectively reduced structural response with adequate energy dissipation capacity. Mekki and Auricchio (2011) incorporated SMA as an energy dissipating device in cable stayed bridge and revealed that SMA performed better in reducing vibration as compared to conventional dampers. Isolators are filter devices placed in between the superstructure and substructure to reduce damage during an earthquake. Graesser and Cozzarelli (1991) developed a one dimensional constitutive model of SMA and experimented its possibility to be used as an isolation device. Wilde et al. (2000) developed an isolation device composed of laminated rubber bearing and SMA. They found SMA based rubber isolation device very effective in reducing deck displacement and showed good damping property. Casciati et al. (2007) developed an innovative isolation device comprising of three inclined SMA bars with two disks and one vertical cylinder held up by three horizontal cantilevers. Billah et al. (2010) assessed the effectiveness of SMA isolation systems in reducing structural seismic responses has through a finite element analysis on a two span continuous reinforced concrete (R/C) bridge and compared the performance with high damping rubber bearing and lead rubber bearing. They showed SMA bearings satisfactorily restrained the deck displacement and the relative displacement between the deck and the pier for strong ground motion.
Current seismic design guidelines demand the structural members and systems to be ductile enough with enhanced deformation capacity, and ability to reduce permanent displacement. This research will be an important step toward implementing performance-based earthquake engineering for avant-garde design of reinforced concrete bridges. Moreover, this study will assess the seismic vulnerability of SMA RC bridge bent and compare it with conventional RC bridge bent. Bridges designed based on guidelines of the proposed research will offset substantial amount of repair and maintenance costs that is incurred with conventional highway bridges and will generate sustainable highway bridges in Canada.