Design of re-centering spring for flat sliding base isolation system: Theory and a numerical study Sanjukta Chakraborty, Koushik Roy, Samit Ray-Chaudhuri ⇑ Indian Institute of Technology Kanpur, UP 208016, India article info Article history: Received 19 March 2015 Revised 25 July 2016 Accepted 26 July 2016 Keywords: Seismic isolation Nonlinear spring Sliding bearing Time history analysis Optimal design abstract This study addresses the design of nonlinear elastic springs that render an effective re-centering mech- anism for a flat sliding base isolation system. A nonlinear stiffening behavior of the elastic spring offers added advantage of re-centering mechanism. The proposed spring-sliding system works in a similar fash- ion as that of the flat sliding base isolation system supplemented with re-centering mechanism for small to medium level of shaking. For high intensity shaking, the proposed system minimizes the peak bearing displacement in addition to keeping the bearing residual displacement close to zero. To demonstrate the concept of the proposed isolation system, a numerical study is conducted with a steel moment-resisting frame when subjected to ground motions of varying hazard levels. It has been found from this study that the proposed spring-sliding device is effective in limiting the peak bearing displacement and making the residual bearing displacement negligible for varying hazard levels. It has also been observed that the presence of nonlinear spring is in general beneficial in reducing the horizontal peak floor acceleration in comparison to the sliding only for ground motion with moderate hazard levels (i.e., 10% in 50 years). Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction The seismic design philosophy for civil engineering structures has undergone a series of modifications based on the performance of constructed facilities under various earthquakes. In the past, the seismic design of a structure was governed by a minimum level of lateral strength requirement. This concept was however modified later by introducing ductility in the design procedures. Earth- quakes such as the 1994 Northridge and 1995 Kobe further demon- strated that buildings designed as per contemporary codes may undergo severe damages and the economic loss arising due to damage of structures and/or loss of functionality may be beyond the acceptable level. It was therefore reasoned that the ductility- based criteria may alone not be sufficient to guaranty the desired performance of a structure during earthquakes. As an improve- ment, the performance-based design philosophy has evolved. In performance-based design of a building, a pre-specified level of performance (performance objectives with an acceptable damage level) is envisioned when subjected to an event of specific hazard level FEMA-356 [1], FEMA-349 [2]. As per FEMA-356 [1], (i) for a minor intensity shaking, normally, a negligible structural damage without hindering the functionality of the system is acceptable and the desired performance level is known as ‘Immediate Occu- pancy’; (ii) in case of moderate shaking, the damage includes major structural damage without collapse, minimal falling hazard and the performance level is known as ‘Life safety’ [1]; (iii) for a strong shaking the extent of damage can go up to a severe structural dam- age with probable falling hazard and the performance is the ‘col- lapse prevention’. For a structure, these performance objectives can be achieved by proper design that may include application of innovative devices such as isolation bearings. For seismic design of structures under strong events, modern design codes envision significant energy absorption through inelastic action of structural components and additional damping devices, if any. Seismic base isolation is a different approach where the force transferred to the superstructure can be minimized. Hence, in this approach, a structure can be protected from severe ground excitations without increasing the force resisting capacity of different structural components. Although the concept of base isolation is not new [3,4], seismic base isolation is getting signifi- cantly popular among researchers and engineers since the last four decades or so. A large number of publications highlighting the con- ceptual and experimental works demonstrates the developments in this area that are also obvious from an increasing number of structures isolated seismically throughout the world [5–30]. Traditionally, the seismic isolation devices are passive in nature. These devices can broadly be classified into spring like isolation bearings or sliding isolation bearings or a combination of the two http://dx.doi.org/10.1016/j.engstruct.2016.07.049 0141-0296/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: sanjukta@iitk.ac.in (S. Chakraborty), koushik@iitk.ac.in (K. Roy), samitrc@iitk.ac.in (S. Ray-Chaudhuri). Engineering Structures 126 (2016) 66–77 Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct