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REPORTS n The 4km-long bridge was partially constructed at the time of the Nepal earthquake SHAKEN, NOT STIRRED A bridge under construction in India was subjected to seismic forces from the earthquake in neighbouring Nepal. Morgan Trowland and Raj Singh report on how its actual response compared with that predicted by design codes he earthquake in Nepal earlier this year wrought havoc over hundreds of ten years due to safety concerns as a result of its dilapidated state. miles; although its impact on its neighbour, India was not so severe, it gave Robazza explains: “Bihar, the state we’re working in, is the most impoverished state in one seismic engineer the opportunity to directly witness the response of a India and the Indian Federal Government provided the funds for the bridge in the hope partly-built bridge. PhD student Brook Robazza had studied in Vancouver, that the link would boost trade in Bihar and neighbouring Uttar Pradesh, improving the Canada, but was working for Infi nity Engineering in a remote part of northern economy of both states. Poverty is apparent throughout the region and a major reason TIndia at the time. He had never experienced a real earthquake when he was sent on for this is the lack of adequate infrastructure. It is therefore very rewarding to be a part his fi rst assignment. Robazza was providing on-site support for geometry control and of the team building this bridge which will not only improve accessibility to the region, implementation of the Infi nity Engineering’s design of the Arrah-Chhapra Ganga River but also bring prosperity through increased trade and commerce.” Bridge in the state of Bihar. The bridge – currently in its fi fth year of construction – will However, the job is not a simple one. Site conditions such as excessive scour, high provide an important trade and transportation link across the Ganges between the water currents, and monsoon winds, in addition to its location in an active seismic zone, villages of Arrah and Chhapra. It is slated to replace the aging and extremely congested create a challenging environment. Gandhi Setu Bridge near Patna, which is expected to be decommissioned within the next The four lane, 4km-long bridge is being procured through a design-build contract. Bridge bearings Seismic protection Bridge expansion joints Kåordbrua - Norway - Kåordbrua IDEAS, ENGINEERING AND MANUFACTURE Agom International srl - Via Mesero, 12 - 20010 - Ossona - (MI) -Italy - www.agom.it - [email protected] - Tel +(39) 029029111- Fax +(39) 029010201 Bd&e | ISSUE 80 | 2015 www.bridgeweb.com 53 n REPORTS With respect to the design, Infinity value-engineered an alternative extradosed partially-completed structure has less stiffness, but similar strength to the service superstructure of 16, 120m-long spans, enabling the construction duration of the bridge condition due to the freedom to rock while the cantilever tips are disconnected. This to be reduced, compared to a traditional variable-depth segmental reference concept additional flexibility causes the natural period of the partially completed structure to be for the 1,920m-long river crossing. The 25m-wide superstructure consists of a constant- longer and the force effects to be lower, thus they do not govern the design. depth single cell precast segmental box modified to accept extradosed cables, while the Another consideration in the design was plan torsion. The major difference between substructure has double-bladed piers on sunken well foundations. the partially-completed structure standing on the day of the earthquake and the At the time of the earthquake, the bridge was in a half-completed state. The permanent structure is the former’s sensitivity to plan torsion. substructure was complete, but the superstructure in a state of partial construction The mechanism resisting this was equal and opposite shear forces along the strong through balanced cantilevering. Robazza was sitting in the site office in Chhapra when he axes of the pier blades. Due to the short 3m distance between pier blades, this mode noticed his desk begin to shake and the ceiling fans sway. He was feeling the shockwaves has a very long natural period. However, with the structure’s mass being symmetrical at of the 7.9 magnitude earthquake some 300km distant in Nepal. Despite the distance, the all stages of construction, it was anticipated that there would be little excitation of this strength of the earthquake and the fact that softer soils in the Arrah-Chhapra region mode by horizontal or vertical shaking. tend to amplify rather than dissipate the shaking meant the experience on the ground The US Geological Survey measured the spectral acceleration at the site on 25 was still quite violent. April at around 2% and 10% in the longitudinal and transverse directions respectively. Robazza ran to the bridge where he was able to witness first-hand the behaviour of The intensity of shaking was close to the construction earthquake as specified by the structure in a live test — something few seismic engineers have the opportunity to the IRC (half of design levels). At these levels some fine cracking was expected at the experience. What he found was the construction crews in a panic — many still on the top and bottom of pier blades but these cracks were anticipated to close up, as the bridge at the time — and heavy equipment rolling around. He watched the piers sway side reinforcement was not expected to yield. to side from a safe distance. Putting these expectations to the test, Robazza gathered eye witness accounts from The Arrah-Chhapra Ganga River Bridge is located in India’s second most active workers on the bridge once the shaking ceased. The amplitude of the oscillations was seismic zone, hence the permanent structure was designed for ground motions even described as up to 350-500mm longitudinally by people inside the bridge segments of stronger than those experienced. The design incorporates IRC:6-2010, which defines the pier which had the most erected segments — 24 — at the time of the earthquake. the required level of strength, amounting to horizontal force coefficients of 5% and There were also oscillation amplitudes of 100-150mm in the transverse direction 23% in the longitudinal and transverse directions respectively. These force coefficients observed at the top of the towers. The bridge appeared to behave in only longitudinal were derived using force reduction R-factors of 2.5 and 1.5 respectively. However, the and transverse mode shapes with a minor display of torsional motion, likely due to piers were ultimately stronger than 5% in the longitudinal direction. This was a result the directivity of the ground motion acting primarily along the longitudinal axis of the of a requirement for higher reinforcing content to control cracking during long- bridge. term shrinkage, creep, and thermal contraction of the superstructure, which, being By climbing on to a pier to inspect the connections for damage, Robazza saw that the continuous, imposes displacements on the piers. bridge had performed as expected. Despite the vulnerable state of the bridge at that Moreover, the bridge was configured with units of 360m-long continuous stage of construction, only a few hairline surface cracks appeared at the construction superstructure to monolithically connect three piers. This configuration is naturally more joints as evidence of the turbulence. And since the ground vibrations did not reach the robust than simply-supported spans on bearings, providing more redundancy with three maximum threshold, there was no permanent damage. While not of structural concern, piers working together in parallel. the cracks are nonetheless being repaired by epoxy injection. To give the bridge a good chance of withstanding greater earthquake displacements In an event of an earthquake that would test the maximum seismic thresholds of than the design level events, the detailing requirements of the American Association the bridge (Mercalli Intensity VII or more) more severe damage would be anticipated. of State Highway & Transportation Officials were followed for ductile piers. This meant Significant longitudinal movement of the bridge, involving hinging of the pier blades, ensuring that the concrete of the piers was well confined by vertical bars at close would be expected. The pier blades would show severe cracking in their top and spacing restrained by closely spaced stirrups hoops. These hoops were anchored with bottom zones where the vertical reinforcing bars would have yielded back and forth. 135° bends so that they continue to provide confinement after the loss of the cover At higher levels of shaking, spalling of concrete cover and initial buckling of bars in concrete. Additionally, the stirrup hoops were most closely spaced over one sixth of the the hinge zones of the pier blades would be likely to occur. In such a case, there is a pier height at the top and bottom where hinging is expected and compression strains on high probability the structure would be left with a permanent offset in the longitudinal the concrete will be most extreme. The hoops were designed to provide shear strength direction. However, little effect would be noticeable on the superstructure apart from in excess of the upper bound bending strength to ensure brittle shear failures would pounding damage around the expansion joints. be suppressed. Having tall piers, it was natural to curtail some longitudinal reinforcing Even in this irreparably damaged state, the bridge’s capacity to carry traffic loads in the mid-height regions, but this was done to ensure that hinging would still occur in would not be significantly diminished and it would continue to serve its lifeline function. the intended locations: the top and bottom of the piers where stirrup confinement is The lasting problem would be corrosion of the bars in the badly-cracked regions of the greatest and short column effects would not amplify the hinge rotations.

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