Widening of existing motorway bridges: can pilegroup retrofit be avoided?
The expansion of current motorway infrastructure often involves widening of existing bridges, calling for pier and foundation retrofit. While pier retrofit is relatively straightforward, pilegroup strengthening can be a challenging, costly, and time–consuming operation. A non-negligible excavation is necessary to reach the existing pilecap, followed by construction of additional piles, and of a new pilecap that needs to be connected to the existing one by doweling. Such major operation can be avoided by developing more rational methods to assess pilegroup moment capacity, and by taking advantage of nonlinear foundation response. Largely thanks to the inherent conservatism in pilegroup design, allowing such nonlinear response may offer a viable design alternative. Within this context, the paper presents a comparative assessment of current design practice, which is based on “elastic” foundation design, to an alternative design approach that allows nonlinear pilegroup response. This allows the conventionally-defined (elastic) moment capacity of the foundation to be temporarily exceeded and the loads to be redistributed between piles during seismic shaking. Inspired from the widening of an existing motorway bridge in Switzerland, a case study is conducted employing 3D finite element (FE) modelling. According to conventional “elastic” design, the initial pilegroup requires retrofitting. Such retrofit may be avoided by allowing full mobilization of pilegroup moment capacity. This is demonstrated by comparing the seismic performance of the widened bridge with the two design alternatives: (a) the pilegroup retrofitted according to the “elastic” design approach; and (b) the initial foundation without any retrofit measures. For moderate (Swiss) design–level seismic shaking, the performance is almost identical. For stronger shaking, substantially exceeding the design limits, the performance of the un-retrofitted foundation is advantageous, as it reduce structural damage by dissipating energy at the foundation level, at the cost of increased – but totally tolerable – settlements.
Prof. Ioannis Anastasopoulos has been Full Professor of Geotechnical Engineering at ETH Zurich since 2016. He specializes in geotechnical earthquake engineering and soil–structure interaction, combining numerical with experimental methods. He holds a PhD from the National Technical University of Athens (NTUA), an MSc from Purdue University, and a Civil Engineering Diploma from NTUA. His research interests include the development of innovative seismic hazard mitigation techniques, faulting and its effects on infrastructure, site effects and slope stabilization, railway systems and vehicle–track interaction, seismic response of monuments, offshore geotechnics, and earthquake crisis management systems. He has been involved as a consultant in a variety of projects of significance in Europe, the US and the Middle East. His consulting work ranges from special seismic design of bridges, buildings, retaining walls, metro stations and tunnels, to harbour quay walls, and special design against faulting–induced deformation applying the methods he has developed. He currently has sat on the panel of Géotechnique and of the ICE Geotechnical Engineering Journal, and currently serves as Associate Editor of Frontiers in Earthquake Engineering. He is the inaugural recipient of the Young Researcher Award of the ISSMGE in Geotechnical Earthquake Engineering, and winner of the 2012 Shamsher Prakash Research Award.