Multi-criteria optimization of lateral load-drift response of posttensioned steel beam-column connections Saber Moradi PhD 1,2 , M. Shahria Alam PhD ⇑ School of Engineering, The University of British Columbia, 1137 Alumni Avenue, Kelowna, BC V1V1V7, Canada article info Article history: Received 12 July 2016 Revised 6 September 2016 Accepted 4 October 2016 Keywords: Posttensioned steel connection Beam-column connection with bolted angles Finite element modeling Self-centering Multi-criteria optimization Response surface method abstract Posttensioned (PT) elements in steel buildings can substantially mitigate permanent seismic damages and the associated post-earthquake repair costs during earthquakes. In this paper, a response surface methodology (RSM) is used to predict and optimize the lateral response characteristics of PT steel beam-column connections with top-and-seat angles. The monotonic lateral response characteristics con- sidered in the study include: initial stiffness, load capacity, and ultimate drift of PT connections, as well as load and drift levels corresponding to the gap-opening (decompression) in PT connections. Based on the results of finite element simulations and extensive sensitivity studies, six influential parameters are con- sidered as input variables in this study. These parameters are posttensioning strand force, beam depth, beam flange thickness and width, span length, and column height. By using a desirability approach, the lateral response of PT steel beam-column connections is optimized. The optimization studies aim at maximizing the initial stiffness, load capacity, and ultimate drift of PT connections and/or minimizing the amount of steel in the beam section, which contributes to the final cost of frame structures. The multi-criteria optimization studies reveal the regions of factor space where optimal conditions are achieved. The optimized solutions are then confirmed by performing simulation runs with the optimal factor combinations. Among the results, it is shown that damage occurs earlier in PT connections with deeper beams and greater posttensioning strand forces. The dominant limit state for the PT connections was beam local buckling starting at early drifts of 1.2%, whereas the first occurrence of angle fracture was at about 4% drifts, and two limit states of strand yielding and bolt extensive yielding were not observed in the analyzed PT connections. Crown Copyright Ó 2016 Published by Elsevier Ltd. All rights reserved. 1. Introduction Welded beam-column connections in steel moment resisting frames suffered brittle fractures during the 1994 Northridge earth- quake [1]. Following this unexpected damage, researchers aimed to improve the seismic performance of steel moment frames. In an effort to avoid possible defects and uncertainties associated with the welding at the beam-column interface, several connection details have been proposed. These modifications that are primarily intended to shift the location of plastic hinges away from the column face include the use of reinforcing plates and reduced beam sections [2]. Despite improved seismic performance, these connections sustain permanent damage to main structural mem- bers under strong earthquakes – due to yielding and local buckling of beam sections, steel buildings suffer permanent deformations under earthquake excitations. Large residual deformations, in turn, increase repair costs and result in large economic losses associated with demolition and collapse [3]. To prevent residual deformations in steel buildings, researchers have investigated several self-centering systems by which the structure can return to its original position following an earth- quake. One self-centering technique is to use posttensioned (PT) strands/bars in buildings. Posttensioning offers an efficient and cost-effective strategy for eliminating permanent deformations. The self-centering capabilities – restoring forces – are provided by high-strength steel elements while supplemental energy dissipating devices or fuses are often incorporated in the structure to dissipate energy. The response of such self-centering buildings is characterized by a gap opening mechanism occurring at the http://dx.doi.org/10.1016/j.engstruct.2016.10.005 0141-0296/Crown Copyright Ó 2016 Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: School of Engineering, The University of British Columbia, 1137 Alumni Avenue, EME 4225, Kelowna, BC V1V1V7, Canada. E-mail addresses: saber.moradi@ucla.edu, sbrmor@gmail.com (S. Moradi), shahria.alam@ubc.ca (M.S. Alam). 1 Address: Postdoctoral Scholar, Civil & Environmental Engineering Department, The University of California, Los Angeles, 2541 Boelter Hall, Los Angeles, CA 90095- 1593, United States. 2 Former affiliation: Ph.D. Candidate, School of Engineering, The University of British Columbia, 1137 Alumni Avenue, EME 3213, Kelowna, BC V1V1V7, Canada. Engineering Structures 130 (2017) 180–197 Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct