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(A)(C)(B)▲▲Figure 14. Railway Bridge 3 field investigation: A) shear wavevelocity (V s ) profile; B) liquefaction assessment using V s data,comparing the cyclic resistance ratio CRR 7.5 to the Darfield(CSR 7.5 DAR) and Christchurch (CSR 7.5 CHC) cyclic resistanceratio; C) remediation of bridge using steel frame structure toprop abutments and tracks.ing in a train derailment soon after the event. The bridge wastemporarily remediated in the days following the event usinga steel frame between the abutments and stabilizing walls infront of the wingwalls as shown in Figure 14C.An SASW test was performed 20 m to the west of thebridge following the Christchurch event; the shear wave velocityprofile at this site is shown in Figure 14A. As indicated bythis V s data, the profile consists of ~4 m of a medium denselayer overlying a denser stratum. Using this shear wave velocitydata and the PGAs listed in Table 1, the CRR 7.5 for the siteand the CSR 7.5 induced during the Darfield and Christchurchearthquakes were calculated as outlined above for the otherbridges. The results are plotted in Figure 14B. As may beobserved from this figure, liquefaction is not predicted to occurduring either the Darfield and Christchurch earthquakes (i.e.,CSR 7.5 < CRR 7.5 ). These predictions are consistent with fieldobservations.Moorhouse Avenue OverbridgeThe Moorhouse Avenue Overbridge (Figure 15A) was constructedin 1960 and runs in the east-west direction (Figure 2).The bridge consists of 11 spans of T-girders that are supportedby dual reinforced concrete column bents.This bridge was not damaged during the Darfield earthquake.Following the Christchurch earthquake, the bridge wasout of service to all traffic for about a month due to damage sustainedduring the event. The damage was primarily to a singlecolumn where a deck expansion joint is located. There were nolinkages between sections of the deck as this position, while thecorresponding pier on the opposite side of the bridge did havethese linkages in place. The expansion joint detail extendedinto the column, increasing the slenderness of the piers (i.e.,these columns were of a size comparable to the other columnsalong the span, except that they were split down the middle bythe expansion joint). The columns also had widely spaced transversereinforcement. The damage was likely caused by a combinationof the high accelerations (estimated PGA = 0.41g) anda large velocity pulse, exciting the transverse response of thebridge and resulting in the flexural-buckling failure of the columnsshown in Figure 16.Upon first inspection the bridge had only suffered shearcracking in both columns, but several hours later the bridgewas inspected again and it was observed that the damaged columnshad started to buckle, putting the central span at risk ofcollapse. Temporary props were then put in place to providegravity support for the span until a rehabilitation plan could beimplemented. There was also evidence of concrete spalling andbar buckling at the abutment-deck interface.Port Hills OverbridgeThe Port Hills Overbridge was constructed in 1970 and runsin approximately the northwest-southeast direction (Figure2). The bridge, shown in Figure 15B, consists of a dual six-spanreinforced concrete voided-slab bridge supported by single pierbents and seat-type abutments. This highway bridge had beenrecently retrofitted, with spans and abutments linked togetherwith steel brackets and rods to form an integral system. Soilhad also been excavated from around the end piers so that theheight of the piers would be uniform along the structure.The bridge was not damaged by the Darfield earthquake,but was approximately 1.5 km from the epicenter of theChristchurch earthquake, with an estimated PGA of 0.68 g atthe site. Column damage developed during this event, with thecenter pier forming a plastic hinge at its base and two of thecorner reinforcing bars buckling over a length of 150 mm. Thisdamage was induced by ground shaking, which activated thetransverse response of the bridge. The retrofitted links betweenthe spans and the bolts connecting the span to the abutmenthad elongated, but the bridge was still able to service trafficwith the damage it sustained.Horotane OverbridgeThe Horotane Overbridge, constructed in 1970, is on StateHighway 74 and runs in approximately the northwest-south-Seismological Research Letters Volume 82, Number 6 November/December 2011 961