Development of earthquake resistant design guidelines for bridges in Sri Lanka

Abstract

Sri Lanka is an island located on the Indo-Australian tectonic plate and the location of the country is far away from the plate boundary where inter-plate earthquakes are possible to occur. However, there are earthquake records during the recent past which have considerable magnitude. Moreover, intraplate earthquake risk is there which is possible to occur without prior warning. Therefore, Sri Lanka is no longer be considered safe from seismic threats. With the increasing demand to improve the road network in the country, it is necessary to reconstruct existing bridge structures. However, there are no seismic design guidelines to use for bridge design procedure in Sri Lanka. This study is therefore aims to formulate earthquake-resistant design guidelines for bridges in Sri Lanka. For the purpose of formulating the design guideline, bridge classification into three different important classes is proposed based on the relevant classifications in similar codes such as EN 1998-2:2005 , IS 1893-3:2014 and AS 5100.2-2004. Important factors and return periods are proposed based on the guideline given in EN 1998-1:2004. Further, Elastic response spectrums for rock or hard soil are selected based on the available response spectrum for Sri Lanka. However, there is no response spectrum defined for medium soil and soft soil. Therefore, it is proposed to use the response spectrum available in IS 1893-1:2002 for medium and soft soil since soil and rock types are more similar when both countries lie on the same tectonic plate. This study proposed a seismic analysis approach for bridges using either EN 1998-2:2005 or AS 5100.2-2004. Moreover, suitable design parameters such as peak ground acceleration values are proposed to select according to the available national data. iv There are three case studies carried out to illustrate the use of developed guidelines for seismic analysis of bridges in Sri Lanka. Three Case studies were carried out for bridges in the category of important class I, important class II and important class III by using the design codes of AS 5100.2-2004, IS 1893-3:2014, and EN 1998-2:2005. For case study 1, base shear values were calculated using the static lateral force method of analysis and three different design codes provided different results of 50kN, 51kN, and 36kN. For case studies 2 and 3, static analysis was carried out using selected three design codes, and response spectrum analysis was carried out according to EN 1998-2:2005 using the response spectrum defined for rock or hard soil of Sri Lanka. Base shear values of the static analysis results provide different values for three design codes. However, a comparison of the results of response spectrum analysis and fundamental mode method of analysis for Eurocode 8-part2 as the basis of analysis, provided similar fundamental period values and base shear values for both methods. Results of the case studies illustrate that when the design basis is Eurocode 8-Part 2, it provides an average or lower result of base shear value. Since Eurocode 8 provides an opportunity to use it with national choices, it is more suitable to use EN 1998-2:2005 with a national annex for bridge design. Therefore, a developed guideline with national parameters can be used for the seismic design of bridges in Sri Lanka.