Skewed Slab-on-Girder Steel Bridge Superstructures with Bidirectional-Ductile End Diaphragms Oguz C. Celik, A.M.ASCE 1 ; and Michel Bruneau, F.ASCE 2 Abstract: Specially designed ductile end diaphragms of steel bridge superstructures have previously proved, both theoretically and experimentally, to dissipate seismic input energy, protecting other substructure and superstructure members. Although ductile dia- phragms have been introduced in the latest AASHTO guide specifications as a structural system that can be used to resist transverse earth- quake effects, no guidance is provided on how to implement these ductile diaphragms in skewed bridges. To address this need and to resolve some shortcomings of the known end diaphragm systems (EDSs), two bidirectional end diaphragm configurations, namely, EDS-1 and EDS-2, with buckling restrained braces (BRBs) are proposed and numerically investigated. Bidirectional end diaphragm is a new concept, and can be implemented both in straight and skewed steel bridge superstructures to resist bidirectional earthquake effects. To assess the relative effectiveness of the proposed systems and to investigate how various parameters relate to seismic response, closed-form solutions are developed using nondimensional bridge geometric ratios. Numerical results indicate that skewness more severely affects end diaphragm behavior when φ ≥ 30°. Also, comparisons reveal that although both end diaphragm systems can be used with confidence as ductile seismic fuses, each of the two systems considered have advantages that may favor its implementation, depending on project-specific constraints. DOI: 10.1061/(ASCE)BE.1943-5592.0000141. © 2011 American Society of Civil Engineers. CE Database subject headings: Bridges, skew; Bridges, steel; Buckling; Bracing; Energy dissipation; Ductility; Diaphragms; Earthquakes. Author keywords: Skewed steel bridges; Seismic retrofit; Buckling restrained braces (BRBs); Hysteretic energy dissipation; Ductile end diaphragms; Bidirectional earthquake effects. Introduction The recently published AASHTO Seismic Guide Specifications for LRFD Seismic Bridge Design include provisions for the design of steel bridges having specially detailed ductile diaphragms to resist loads applied in the bridge transverse direction (AASHTO 2009; closely following the recommendations in MCEER/ATC 2003). Previous studies (e.g., Zahrai and Bruneau 1999a, b; Carden et al. 2006) support that significant energy can be dissipated in ductile bridge-end diaphragms, while reducing seismic demands in other substructure and superstructure elements. Past research has shown how shear panel systems (SPSs), steel triangular plate added damping and stiffness devices (TADAs), eccentrically braced end diaphragms (EBFs), and buckling restrained braces (BRBs—also called unbonded braces) could be detailed to provide an appropriate seismic performance. Ductile performance requires special detailing such as that evinced by bridge-end diaphragm damage in prior earthquakes (Bruneau et al. 1996; Itani et al. 2004). Currently, ductile diaphragms have to be combined with other lateral load resisting strategies to address seismic excitations along the bridge’ s longitudinal axis. Also, AASHTO (2009) pro- vides no guidance on how to implement ductile diaphragms in skewed bridges—even though steel bridges with skewed super- structure geometries are commonly encountered at highway inter- changes, river crossings, and other places because of alignment limitations. This paper investigates the seismic response of the proposed concept to implement ductile end diaphragms in skewed bridge superstructures, at the same time resisting bidirectional earthquake excitations—implementation in nonskewed straight bridges being a special case of the general formulation. Here, BRBs are used as the diaphragm ductile seismic fuses. BRBs have been implemented in many buildings in Japan and in the United States on account of their stable unpinched hysteretic characteristics, ease of design, and ability to eliminate seismically induced structural damage and provide satisfactory seismic performance. BRBs have also been used to retrofit the Minato Bridge in Japan (Kanaji et al. 2003), the world’ s third-longest truss bridge, using a concept similar to the ductile cross-frame system developed by Sarraf and Bruneau (1998a, b) and analogous to the ductile diaphragm concept. Here, two proposed end diaphragm systems (EDSs) (i.e., geometrical layouts) are considered, namely, EDS-1 and EDS-2. Although they are considered here in the perspec- tive of new bridge design, the information presented is also applicable to existing bridges for seismic retrofit purposes. Results from parametric studies are used to formulate design recommendations. 1 Professor, Div. of Theory of Structures, Faculty of Architecture, Istanbul Technical Univ., Taskisla, Taksim, Istanbul 34437, Turkey; formerly, Research Scientist, Dept. of CSEE, Univ. at Buffalo, Amherst, NY 14260. E-mail: celikoguz@itu.edu.tr 2 Professor, Dept. of CSEE, Univ. at Buffalo, Amherst, NY 14260. E-mail: bruneau@buffalo.edu Note. This manuscript was submitted on February 12, 2009; approved on April 15, 2010; published online on May 29, 2010. Discussion period open until August 1, 2011; separate discussions must be submitted for in- dividual papers. This paper is part of the Journal of Bridge Engineering, Vol. 16, No. 2, March 1, 2011. ©ASCE, ISSN 1084-0702/2011/2-207– 218/$25.00. JOURNAL OF BRIDGE ENGINEERING © ASCE / MARCH/APRIL 2011 / 207 J. Bridge Eng. 2011.16:207-218. Downloaded from ascelibrary.org by Istanbul Teknik Universitesi on 01/21/14. Copyright ASCE. For personal use only; all rights reserved.