Experimental Investigations on the Structural Behaviour of a Distressed Bridge

Distressed structures require necessary remedial measures in order to restore their original structural properties like strength and stiffness. Validating the effectiveness of the proposed qualitative remedial measure experimentally is of utmost importance as there is no well-established analytical method to verify the effectiveness of the same quantitatively. Prototype testing which would have been the best option for this purpose would not only prove costly but also be associated with numerous practical difficulties; hence model testing is resorted as the only option for the purpose. This paper presents one such typical experimental study on the structural behavior of a distressed bridge, mainly observed in the form of prominent tilt in the bearing plate in transverse and longitudinal direction on downstream side. The main focus of the proposed experimental investigation is to assess the structural behavior particularly the load carrying capacity. The extent of deformation of some models with specific structural arrangements and some models with specific need based remedial measures were also studied. This study also assessed the contribution of each remedial measure towards restoration individually and collectively.

Introduction

In the year 2008, a contract for laying 700mm diameter water supply pipeline project from water treatment plant situated at Rangil, Ganderbal to Shalteng area of Srinagar (Jammu & Kashmir, India) city was won by Pratibha Industries Ltd Mumbai. Along its alignment, the pipeline had to cross the largest river in Kashmir Valley called Jehlum (150m wide) at Palpora located in the outskirts of Srinagar. A pipe rack was constructed to carry this water supply pipe (diameter 700 mm) across the Jehlum river. The super structure of pipe rack is arch shaped truss girder bridge consisting of four equal spans of 37.5 m each as shown in Fig.1. Due to limited available time for the completion of the project, the fabrication work of truss girders was carried out in Jalandhar (Punjab) in parallel with the execution of sub structure works to ensure timely completion.

Four Span Pipe Bridge Tilt in Bearing Plate
Figure 1: A view of four span Pipe Bridge Figure 2: A visible tilt in the bearing plate

Placement of Cross Girder over Bearing Plate
Figure 3: Eccentric placement of cross girder over the bearing plate

Problem Statement

Due to lack of proper coordination between site and fabrication teams, the length of each span of the truss was fabricated 37 m only instead of the desired 37.5 m. Miserably inadequate non engineered measures were taken at site to make up for the deficiency in the desired span length. These non-engineered measures included the improper arrangement of bottom chord extension, highly insufficient channel section for top chord extension together with unsafe joint detailing. This totally non-engineered approach finally resulted in a structurally unsafe solution to the above quoted crucial problem of the structure. The proposed restoration scheme suitable for a structure in distress is governed by the nature and extent of distress the structure has undergone (Jagadish 1995, Limal et al. 2008, Peterson 2011, Edward et al. 2014). Before selecting the suitable and acceptable restoration scheme, it is important to critically examine the nature and extent of distress the structure has suffered (Arya et al. 1992, Mehta 2010, AASHTO 2011, Caglayan et al. 2012, Stamatopoulos G. N. 2013, Subramanian 2014). At times, it is also the imposed constraints from various considerations which play important role in the selection of suitable restoration scheme (Goel & Sanyal 1999, Ram Kumar et al.1999, Williams 2013, Subramanian 2014). To explore the various alternative restoration schemes for the distressed pipe rack in the present study, it was necessary to examine critically the nature and extent of distress. For this purpose few visits were paid to the site at a time when signs of distress were clearly noticeable as well as fabrication faults and errors were also visible. The distress in the pipe rack was observed in the form of noticeable tilt in one of the bearings and significant tilt in the girder supported by this bearing, as is clearly seen in Fig. 2. It was also observed that the end cross girders in all the spans were located about 250 mm away from the centre line of the bearings towards the river side, as seen in Fig. 3. In addition to this a prominent tilt measuring 1 in 9 in transverse direction and 1 in 45 in longitudinal direction was observed on the down-stream side of bearing in span 3 over Pier P3 as shown in Fig. 2. wide gaps were also observed at number of joints as well as non-uniform gaps were visible at expansion joints. The newly constructed pipe rack was thus severely in distress. The pipe rack in such distressed condition was vulnerable to total collapse particularly under seismic loading of zone-V, resulting in the disruption of essential water supply service to millions of people across the river. Considering its vital importance, an urgent need was felt to take appropriate remedial measures of the distressed pipe rack to restore its safe structural capacity particularly during worst seismic loading condition in zone-V.

NBM&CW September 2016

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