ABSTRACT
Bridges risk facing unexpected loadings due to floods, landslides, tsunamis, earthquakes, blasts and various accidents. Displacement of bridges due to erosion and collapsing of abutments, lateral pressure generated by exceeding water levels and floating debris etc are common. Even though there are provisions for accidental loading used during bridge designs, managing the risk of damages completely is not possible (e.g. the tsunami damages in Sri Lanka in December 2002, Japan in March 2011).
Building new bridges generally consumes more money and time than repairing and retrofitting of damaged bridges. Therefore, the latter can be considered more sustainable. However, proper methods are necessary to assess the level of damages and to verify the fitness of such bridges prior to repair and retrofitting. In the assessment, there are two important criteria to consider. One is the amount of fatigue damage to the bridge due to usual past vehicle loading and hence the remaining fatigue life of the bridge. The other is the magnitude of damage caused to the bridge by the unexpected loading.
In order to determine the fitness of a damaged bridge for reuse, condition surveys are usually carried out. Using a validated numerical model of the damaged bridge (a finite element model, FEM), efficient decisions are possible as such models can be loaded with past loading histories as well as future expected loadings and then the stresses and deflection can be obtained from the FEM. Then the fitness of the damaged bridge can be verified and elements which need to be replaced or retrofitted can be determined.
The case study is about a 34 m long, 5.2 m wide, single spanned, double lattice girded, wrought iron railway bridge, located at Puttalam (Bridge No. 02 on the railway track between the Puttalam Cement Factory and Limestone Quarry, used for transporting limestone) which was built about 40 years ago and damaged and displaced from its abutments by floods. The bridge was then placed on temporary timber abutments for several years.
With the increase in cement production, the owners of the bridge wanted to use heavier locomotives on this railway track and also increase the number of trips. Therefore, there was a need for an assessment of bridges on this track in order to determine whether the bridges can be used further or should be demolished and new bridges built in its place.
In order to do the assessment, a condition survey was carried out on all the bridges on the track. One of the bridge was found weak as this had been damaged by floods. Then an analysis was done by modeling the bridge (FEM) by using general purpose SAP 2000 program and validating the FEM by using results of a field loading test (the bridge was temporarily erected on timber abutments for several years). Both static and dynamic loading tests were carried out using an M2 locomotive with 6 numbers of 13.16 ton axles for 5 different loading cases to measure the displacement, strain and acceleration at pre-determined (critical) members of the bridge. The future fatigue life of the bridge was estimated using the prescribed last method. The future life was found as 30 years with a factor of safety of 3. Further, using the validated model, the ability of the bridge for higher loading situations was confirmed.
The cost, estimated for retrofitting work and constructing new reinforced concrete abutments was much less than that for constructing a new bridge.
Therefore it was decided that rehabilitation of the bridge with necessary retrofitting work is more sustainable than demolishing it and constructing a new one. The bridge is now in use after being repaired, retrofitted and placed on new abutments.
© 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [Organizer Name].
