ارزیابی عملکرد لرزه ای احتمالی یک برآمدگی RC موجود با ستون های قاب پورتال طراحی شده برای بارهای گرانشی Probabilistic seismic performance assessment of an existing RC bridge with portal-frame piers designed for gravity loads only

1. Introduction Widespread damage, partial and global collapses of bridges have frequently been surveyed in the aftermath of moderate-tohigh magnitude earthquakes worldwide [9,26,13,65,41,40,64,74,72]. Most of the existing highway bridges, especially reinforced concrete (RC) bridges, were built without seismic details during the 60’s and 70’s; hence, their structural performance tends to be inadequate under earthquake ground motions [59,68,43,14,52,38,15]. Brittle failure due to limited shear capacity is a common damage pattern experienced by non-ductile reinforced concrete (RC) bridges (e.g. Fig. 1), especially those having portal-frame piers with short elements. Structural failures can also be caused by the inappropriate connection between piers and footing, for an insufficient anchorage or lap splicing. Such damage pattern is exacerbated for RC bridge structures with smooth steel reinforcement, which is commonly found in existing RC bridges, especially those that were designed for gravity loads only as also proved during pseudo-dynamic [1] and dynamic shake table experimental tests [23]. The present analytical work assesses the earthquake performance of an existing RC bridge with portal-frame piers designed for gravity loads only, which is typically found in seismic-prone regions in the South of Europe. Novel research is focusing on existing sub-standard European bridges [51]; nonetheless, to Authors’ knowledge, the present work is an early attempt to assess the Italian bridges with portalframe piers designed for gravity loads. The novelty of this work is the investigation of adequate formulations for the evaluation of the structural capacity of the elements of the piers according to existing structural codes and assessment guidelines [12,30,29,63]. Ductile and fragile components are assessed at two limit states: the significant damage limit state (DLS) and the collapse limit state (CLS). Moreover, a new simplified definition of the damageability to account also for the shear strength, based on the modified compression field theory, is proposed. Towards this aim, an existing RC Italian bridge located in the Emilia-Romagna region has been selected as case study. The seismic response of the sample bridge has been investigated through advanced dynamic nonlinear analyses. To evaluate the record-to-record variability, according to the Pacific Earthquake Engineering Research (PEER) approach [16,50], two advanced structural analysis methods, i.e. the Cloud analysis [44,34,42,25,37] and the Incremental Dynamic Analysis (IDA) [69,70], have been employed. Both procedures are based on the selection of a suite of ground motions that are applied as they are in the case of Cloud analysis and are repeatedly scaled in the case of IDA. Performing both Cloud analysis and IDA facilitates the comparison of the two procedures, allowing also the identification of pros and cons of both methods. IDA is time consuming but it is suitable for a comprehensive structural assessment since it allows the evaluation of the relationship between engineering demand parameters (EDP) and intensity measures (IM) considering a large range of IM. On the other hand, it is nowadays common practice using automatic routines that perform structural analyses to prioritize the inspections on critical elements of the communication infrastructures, thus facilitating the emergency management [46]. Within such framework, Cloud analysis is very efficient since uses un-scaled ground motions, and requires relatively low computational efforts.