Evaluation of performance of non-invasive upgrade strategy for beam–column sub-assemblages of poorly designed structures under seismic type loading Saptarshi Sasmal 1, * ,† and Dikshita Nath 1,2 1 CSIR-Structural Engineering Research Centre, CSIR Campus, Taramani, Chennai 600113, India 2 India and Vellore Institute of Technologys, Vellore, Tamil Nadu, India SUMMARY Beam–column sub-assemblages are the one of the most vulnerable structural elements to the seismic loading and may lead to devastating consequences. In order to improve the performance of the poorly/under-designed building structures to the critical loading scenarios, introduction of steel bracing at the RC beam–column joint is found to be one of the modern and implementable techniques. In the present work, a diagonal metallic single haunch/bracing system is introduced at the beam–column joints to provide an alternate load path and to protect the joint zone from extensive damage because of brittle shear failure. In this paper, an investigation is reported on the evaluation of tae influence of different parameters, such as angle of inclination, location of bracing and axial stiffness of the single steel bracing on improving the performance through altering the force transfer mechanism. Numerical investigations on the performance of the beam–column sub-assemblages have been carried out under cyclic loading using non-linear finite element analysis. Experimentally validated numerical models (both GLD and upgraded specimen) have been further used for evaluating the performance of various upgrade schemes. Cyclic behaviour of reinforcement, concrete modelling based on fracture energy, bond-slip relations between concrete and steel reinforcement have been incorporated. The study also includes the numerical investigation of crack and failure patterns, ultimate load carrying capacity, load displacement hysteresis, energy dissipation and ductility. The findings of the present study would be helpful to the engineers to develop suitable, feasible and efficient upgrade schemes for poorly designed structures under seismic loading. Copyright © 2016 John Wiley & Sons, Ltd. Received 3 June 2015; Revised 13 February 2016; Accepted 22 February 2016 KEY WORDS: non invasive upgrade; beam–column sub-assemblage; experimental investigation; nonlinear analysis; cyclic behaviour; fracture energy; plastic hinge; energy dissipation 1. INTRODUCTION Large number of gravity load designed (GLD) structures prevail around the world in all the seismic zones. These were constructed prior to the implementation of the ductile detailing in the earthquake codes/standards. Structural ductility is directly proportional to member ductility. The earthquake forces appear to be cyclic in nature. These forces tend to apply and re-apply over the component of the structure and store an amount of energy in the system. As it has been widely reported in the literature, typical structural deficiencies of existing reinforced concrete frame systems are most often related to: (i) inadequate confining effects in the potential plastic hinge regions, (ii) insufficient amount, if any, of transverse reinforcement in the joint regions, (iii) low amount (nominal) of longitudinal and transverse reinforcement in columns and beams, (iv) improper and inadequate *Correspondence to: saptarshi@serc.res.in, † E-mail: sasmalsap@gmail.com Copyright © 2016 John Wiley & Sons, Ltd. EARTHQUAKE ENGINEERING & STRUCTURAL DYNAMICS Earthquake Engng Struct. Dyn. (2016) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/eqe.2730