A simple approach for seismic retrofit of low-rise concentric X-braced steel frames Ernesto Grande ⁎, Alessandro Rasulo 1 Depart. of Civil and Mechanical Engineering (DICeM) — University of Cassino and Southern Lazio, Cassino, FR, Italy abstract article info Article history: Received 18 November 2014 Accepted 22 January 2015 Available online xxxx Keywords: Retrofit CBF DBD FE analyses Existing concentric steel braced frames (CBF) have been generally designed without considerations of principles based on plastic mechanisms. This leads to most of the inadequate responses shown by CBFs during recent earth- quakes. On the contrary, the seismic performance of CBF has been demonstrated to substantially improve when the design is specifically addressed to obtain resistant ultimate mechanisms characterized by the yielding of diagonals. The design approach suggested by numerous modern seismic codes such as the Eurocode 8 and the recent Italian guidelines NTC08 focuses on detailed design provisions aimed to assure new CBFs a ductile ultimate mechanism, characterized by the yielding (reasonably distributed along the building height) of diagonals, while the buckling of beams and columns is prevented. This approach could also represent a useful support in the definition of effi- cient retrofit strategies devoted to improve seismic performance of existing CBFs. In the past the authors proposed a simplified approach for the assessment of existing CBFs carried out considering a procedure developed in the light of the direct Displacement Based Design (DDBD) method and where the role of diagonals assumes particular relevance for defining the seismic safety level of CBFs. In the paper a retrofit procedure for CBFs is developed and applied to some cases of study. Non-linear time- history finite element analyses are presented in the paper with reference to the examined cases considering a set of spectrum-compatible accelerograms. The derived results are then presented by examining the ability of the proposed procedure to carry out suitable retrofitting interventions. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Concentric braced frames (CBFs) are one of the most used structural types for steel buildings. Indeed, the truss mechanism which is activated in the presence of lateral forces due to wind or earthquake loads, assures levels of lateral stiffness and strength significant higher in comparison to other structural types such as moment resisting frames. On the opposite hand, the possible occurrence of buckling phenom- ena in the members of CBFs, which are mainly subjected to axial forces, can lead either to a fragile global failure mechanism or to a significant reduction of the global ductility and energy dissipation. For those reasons, modern codes [1,2] suggest detailed design provi- sions aimed to assure CBFs a ductile ultimate mechanism, characterized by the yielding (reasonably distributed along the building height) of diagonals, while the buckling of beams and columns is prevented [3]. This clearly underlines the crucial role of diagonal members in CBFs which are pivotal in providing adequate levels of lateral stiffness, strength and ductility. In this context, the seismic evaluation of existing CBFs, designed in absence of seismic provisions or complying with poor seismic practice, is essential particularly considering that in many countries the revision of the design codes has lead to the increase of the seismic actions and that severe earthquakes, including the 1994 Northridge (California), 1995 Kobe (Japan), 1999 Chi-Chi (Taiwan), and 2011 Christchurch (New Zealand) events [4], have demonstrated vulnerabilities of existing CBFs. In the current literature many studies concerning retrofitting solu- tions for braced steel frames are available. In particular, the majority of researchers propose the substitution of diagonals with specials mem- bers or devices able to increase the global ductility and energy dissipa- tion of CBFs [5–9]. Differently, in this paper a framework for the retrofit is proposed in order to improve the seismic response of CBFs, applying it to a more tra- ditional technique: the simple substitution or enhancement of some of the diagonals. The goal is to carry out a design procedure fulfilling the Journal of Constructional Steel Research 107 (2015) 162–172 ⁎ Corresponding author at: Depart. of Civil and Mechanical Engineering (DICeM), University of Cassino and Southern Lazio, via G. Di Biasio 43, 03043, Cassino, FR, Italy. Tel.: +39 07762993478; fax: +39 07762993392. E-mail addresses: e.grande@unicas.it (E. Grande), a.rasulo@unicas.it (A. Rasulo). 1 Depart. of Civil and Mechanical Engineering (DICeM), University of Cassino and Southern Lazio, via G. Di Biasio 43, 03043, Cassino (FR) — Italy. Tel.: +39 07762993478; fax: +39 07762993392. http://dx.doi.org/10.1016/j.jcsr.2015.01.017 0143-974X/© 2015 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Journal of Constructional Steel Research