Engineering Structures 31 (2009) 2345–2356 Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Pounding mitigation and unseating prevention at expansion joints of isolated multi-span bridges Shehata E. Abdel Raheem ∗,1 Structural Engineering, Civil Engineering Department, Faculty of Engineering, Assiut University, Egypt article info Article history: Received 15 August 2008 Received in revised form 12 May 2009 Accepted 14 May 2009 Available online 6 June 2009 Keywords: Pounding mitigation Expansion joint Seismic design Shock absorber Unseating prevention Restrainers Isolated bridges abstract Damage of adjacent bridge structures due to relative responses, such as pounding and unseating, have been observed in many earthquakes. The isolators in bridge structures are effective in mitigating the induced seismic forces. However, the deck displacement becomes excessively large when subjected to ground motion with unexpected characteristics. This increases the possibility of pounding; and contributes to the unseating of bridge decks and subsequent collapse. An analytical model of expansion joints, that takes account of the interaction between adjacent bridge segments and the effect of impact and restrainers, is developed and nonlinear time history analyses are performed on a typical isolated multi- span bridge using three standard ground motions. The numerical simulation results show that pounding between adjacent bridge segments could amplify the relative displacement, resulting in the requirement of using an unseating prevention system. Restrainers are substantially effective in reducing the relative opening displacements and impact forces due to pounding at the expansion joints. However, the impact and the stretch of cable restrainers at expansion joints results in a large lateral force transfer from one deck to the other, which, consequently, significantly changes the global response of the participating structural systems. Therefore, it is effective to provide a shock absorber for the mitigation of impact effects between bridge segments or at the restrainers’ ends. The sudden changes of stiffness during poundings can be smoothed by using a natural rubber shock absorber, which prevents, to some extent, the acceleration peaks due to impact. The reaction forces at the pier bases and the pounding forces exerted on the superstructure can be satisfactorily reduced. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Due to a lack of structural redundancy, bridges suffer se- vere damage that generally leads to catastrophic failure during earthquakes. For bridges with relatively short piers, the natu- ral frequency of vibration lies in the range of pre-dominant fre- quencies of the earthquake ground motions, particularly when founded on hard or medium soil. Merely increasing the strength of members will not be effective and uneconomical too, unless the transmission of the earthquake forces and energy into the structure is reduced. Therefore, base isolation devices may replace the con- ventional bridge bearings. Seismic isolation is an innovative earth- quake resistant design approach that introduces flexibility at the isolation level supplying a means of energy dissipation. Such isola- tion devices decouple the bridge deck from the bridge substructure ∗ Tel.: +20 11 785 7478; fax: +20 88 233 2553. E-mail address: shehataraheem@yahoo.com. 1 Formerly, Visiting JSPS Fellow, Bridge Engineering and Structural Design, Graduate School of Engineering, Hokkaido University, Japan. Visiting AvH Fellow, Fachgebiet Stahl- & Verbundbau, Universität Kassel, Germany. during earthquakes reducing the forces transmitted to abutments and piers. Thus, the bridge is protected against damage from the earthquake by limiting the earthquake attack rather than resisting it [1–3]. Expansion joints may be a weak point in an isolated bridge where a large relative displacement occurs between decks. The rel- ative displacement anticipated at an expansion joint in a standard bridge under a design ground motion could reach many times the standard clearance between decks. Such large relative displace- ments between the adjacent girders can not only cause poundings, but it could play major role in bearing damage, hence unseating failure of a bridge system and subsequent collapse [4]. Pounding between adjacent bridge segments could amplify the relative dis- placement resulting in the requirement of a longer seat width to support the deck [5,6]. Through numerous field observations af- ter damaging earthquakes and previous analytical and numerical studies [7–11], pounding has been identified as the primary cause for the initiation of collapse, and damage of adjacent superstruc- ture segments in highway bridges due to relative responses. Such poundings and unseating have been observed in many earthquakes in the past, e.g. during the 1989 Loma Prieta earthquake [12], the 1994 Northridge earthquake [13], the 1995 Kobe earthquake [3], and the 1999 Chi-Chi earthquake [14,15]. Pounding causes local 0141-0296/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2009.05.010