The use of base isolators for the anti-seismic design of structures has attracted considerable interest in recent years. The main aim here [1] is to separate the structure from ground motions ensuring flexibility as well as energy dissipation’s aptitude through inserting isolation system between foundation and the superstructure. Conversely, in conventional anti-seismic design which resistance is obtained through ductility whereby the structure is allowed to deform in the non-elastic range [2]. Therefore, this type of design means that even though the collapse of the structure can be avoided, significant structural damages may be caused in the case of major earthquakes. Consequently, the new concept allowing input energy dissipation appears to have an important potential in preventing earthquake damages to structures and their internal equipments. The concept to isolate the fixed base has been suggested in last century. Early as 1909 [3], a British medical doctor had performed for a patent on separating a building from the ground by a layer of talc or sand. Nevertheless, it was only in the last three decades that this design concept has received serious attentions. Also the first modern application of this technology was carried out in 1969 by using rubber isolators in a school building in Skopje, Macedonia [4]. The basic concept of this system is to uncouple the structure motion from the soil’s one by inserting between the foundation and the superstructure isolation devices that have a very important horizontal deformability and a very high vertical stiffness [5]. Seismic isolation can be achieved by increasing the natural period of vibration of a structure via use of rubber isolation pads [6] (Fig. 1). Consequently, the seismic effects are decreased which leads to significant reductions in seismic response variables such as floor accelerations, inter-storey drifts, and base shear forces [7]. On the other hand, as the flexibility of the base dissipaters increases, base displacement becomes larger [8]. That is why it is interesting to incorporate an automatic re-centring device. Different kinds of rubber base-isolation systems can be used [9], in particular, the lead-rubber base isolator (LRB), also provides energy dissipation and re-centering capability. But above all, the elastomeric bearings filter the ground motion, leading to a low frequency in the fundamental mode and, then, to a low pseudo-acceleration spectral value for most of the expected ground motions. Thus, a significant decrease in the floor accelerations and inter-storey drifts is obtained. This dissipater and its effects on the seismic structure response are the subject of this study.
Fig.1
Last years, extensive works have investigated the performance of this device to enhance seismic response of buildings. Mkrtychev et al. [10] examined the efficiency of lead rubber bearing system with different buildings heights at multi-component seismic impact. A seismically isolated monolithic ferro-concrete with five, nine and 16-storey buildings were considered. The analysis of the problem was achieved by a direct integration of the motion equations for an explicit scheme in the software package (LS-DYNA). The calculations were conducted considering the nonlinear nature of lead rubber isolators. An analysis of the effectiveness of buildings with and without the isolation was carried out. Zordan et al. [11] proposed a new approach to reduce the required computational time, called the equivalent linear method (EL). In the aforementioned method, the nonlinear response of dissipater can be adequately modelled using a fictitious viscously damped elastic structure. An investigation of existing expressions providing the state of research was conducted and then, an improved formulation has been proposed for equivalent linearization of structures fitted with lead rubber isolators (LRB). The new presented model predicts displacements that are similar with those obtained with nonlinear time history analysis (NLTH). Some approaches based on algorithms were developed to identify the nonlinear properties of rubber devices in base isolated buildings using only partial measurements of structural dynamic responses [12]. The first algorithm is used in the case that mathematical models are available for the base dissipaters. However, a second algorithm was proposed for the general case where it is complicated to implement a mathematical model describing the nonlinear behaviour of the rubber base isolators. The nonlinear behaviour is considered as “fictitious loading” on a linear building under a strong seismic hazard. The concept is based on the sequential Kalman estimator for the dynamic responses and the least-squares evaluation of the “fictitious loading”, to identify the nonlinear strength of rubber base isolator. The results of analyses demonstrate a good accuracy of the two proposed methods. Islam et al. [13] presented a paper where they studied a design of base isolation device for multi-storey buildings under medium seismic risk. Authors examined the dynamic response through automated nonlinear models. Lead-rubber bearing and high damping rubber bearing have been considered for the study. The nonlinearities of the aforementioned isolators have been duly chosen. Linear static, linear dynamic and nonlinear dynamic analyses due to site-specific earthquake signal were achieved on buildings with and without the isolation dissipaters. In [14], the influence of the soft-storey behaviour on concrete buildings which are fitted with lead-rubber base device was studied on four different structural models. Time history analysis on these frame systems was realised using Ruaumoko software, and effect of soft-storey behaviour on the structural response in fixed base and LRB base isolated systems was examined. Estimation of the frame system’s period, storey accelerations, inter-storey drift ratio, base shear, and plastic hinges distribution and their damage conditions was performed. The results show that LRB device can be beneficial to improve both structural responses. The rubber being very influenced by temperature, several works have investigated the effect of this parameter on the variation of its mechanical properties [15-16]. Casciati et Faravelli [17] conducted an experimental study where they examined the response of new base isolators compared with that of 10-year-old devices. A new base-isolation technique was investigated experimentally by OH et al. [18]. They considerate a laminated elastomeric base isolation and U-shaped hysteretic energy dissipating devices called UH dissipaters. Results obtained from a shake table tests show that the base isolated dampers provided better seismic response compared to the fixed-base frame. In [19], authors considered in their study a smart lead rubber bearing as new types of dampers which are characterized by the use of a shape memory alloy in the form of wires. These devices have the aptitude to improved performance in terms of re-centering ability and energy dissipation capacity.
This study aims to investigate the effect of the lead rubber bearing isolation on steel structure response subjected to bidirectional seismic ground motions. Fast Nonlinear time history analyses are carried out considering a Bouc-Wen nonlinear model for base isolation device. Finally, in order to identify plastic behaviour of frame elements, nonlinear dynamic analyses were applied to the structure.
2. Lead core laminated rubber bearing
The laminated rubber bearing system (Fig.2) in which a central lead core is used to reduce the base relative displacement and providing an additional mean of energy dissipation was proposed by Robinson in 1975 [20]. This device has been put in function for the first time in New Zealand. It is an isolator that present a description of a Low damper rubber bearing (LDRB) [21-24], with a bar of lead in its centre. The dissipater as shown in figure 2 is composed of alternated layers of rubber and steel, which contributes on one hand, to ensure the stability and the support to the structure and provides on the other hand, its isolation from vibrations. In addition the core lead inserted inside aims to increase the damping effect and confer a nonlinear behaviour and a flow state in shear, forced by the metal frets. The flow starts at about 10Mpa.
Fig. 2
The performance of LRB to improve dynamic response under a variety of conditions was reported in [3]. The rubber provides the flexibility for the lateral displacements of the isolator while the yielding property of the lead core works as a mechanism for dissipating energy and hence reducing the lateral displacements of the damper. The mechanical behaviour of this isolator is equivalent to a hysteretic device [25]. The schematic model of the LRB base isolation damper is presented in figure 3.. In [26], it was suggested that the restoring force generated by the hysteretic behaviour of the LRB isolator’s lead core may be approximated by wefts hysteretic model.
In [27], the influence of base isolators hysteresis loop’s shape on the response of multi-story structure for various bi-linear systems under different seismic signals was studied. Results showed that the equivalent linear elastic-viscous damping model of a bi-linear hysteretic system overestimates the base design displacements as well as underestimates the superstructure accelerations. Therefore, response of damped structure is significantly affected by the shape of hysteresis loop of isolators.
Otherwise, the analysis conducted in [28] on LRB base isolated frames under far-fault and near-fault ground motions concluded that near-fault sites induce strong ground motions with unwanted effects on the base isolation system and on the response of the superstructure. In order to reduce these effects, a reinforcing through supplemental viscous damping on the existing LRB system represents an effective design strategy.