Efficiency of helical springs and viscodampers in base isolation

Earthquakes and Structures, 2018

In this paper, an analytical study was carried out to propose an optimum base-isolated system for the design of steel structures equipped with lead rubber bearings (LRB). For this, 5 and 10-storey steel moment resisting frames (MRFs) were designed as Special Moment Frame (SMF). These two-dimensional and three-bay frames equipped with a set of isolation systems within a predefined range that minimizes the response of the base-isolated frames subjected to a series of earthquakes. In the design of LRB, two main parameters, namely, isolation period (T) and the ratio of strength to weight (Q/W) supported by isolators were considered as 2.25, 2.5, 2.75 and 3 s, 0.05, 0.10 and 0.15, respectively. The Force-deformation behavior of the isolators was modelled by the bi-linear behavior which could reflect the nonlinear characteristics of the lead-plug bearings. The base-isolated frames were modelled using a finite element program and those performances were evaluated in the light of the nonlinear time history analyses by six natural accelerograms compatible with seismic hazard levels of 2% probability of exceedance in 50 years. The performance of the isolated frames was assessed in terms of roof displacement, relative displacement, interstorey drift, absolute acceleration, base shear and hysteretic curve.

Journal of Wind Engineering and Industrial Aerodynamics, 1998

The base isolation is a very effective technique for reducing the seismic forces through a decoupling of the structure motion from that of the soil. With regard to the earthquake, the insertion of a very flexible base-isolation system is generally favourable, particularly for reducing the ductility demand (i.e., for controlling the ultimate state). On the other hand, under wind a considerable increase of deformability of the isolated structurein comparison with that of the fixed-base structurecan lead to displacements at the top of the isolators and accelerations at the floor levels so large that serviceability limit states (particularly, the discomfort threshold) have to be carefully checked. The main purpose of this paper is to study the dynamic response of base-isolated structures subjected to strong earthquakes or wind loads in order to pursue an optimal design of the base-isolation system. For this purpose, a numerical investigation is carried out with reference to three-and five-storey reinforced concrete (r.c.) framed structures, isolated by high-damping-laminated-rubber bearings (HDLRB). The results, obtained by a step-by-step procedure, show a different behaviour of the test structures subjected to earthquake or wind when assuming different levels of deformability of the isolators within a wide range of variation.

Increasing buildings’ resistance to earthquake forces is not always a desirable solution especially for the building contents that are irreplaceable or simply more valuable than the actual primary structure (e.g. museums, data storage Centre’s, etc.). Base isolation and seismic dampers can be employed to minimize inter-story drifts and floor accelerations via specially designed isolation and dampers system at the structural base, or at higher levels of the superstructure. In this research, we’ll examine the response of buildings isolated using isolation system hybrid consisting of Lead-Rubber Bearings (LRB), Flat Sliding Bearings (FSB), with the addition of Rotation Fiction Damper (FD) at the base, then compare the results with buildings that have traditional foundation, in terms of the (period, displacement and distribution shear force and height of the building). It conducts TIME HISTORY seismic analysis for some varying height buildings (eight, twelve, sixteen, and twenty stories), with help of SAP2000 using an earthquake acceleration-time history for (El- Centro). The results show that the use of insulation system Hybrid has had a significant impact on improving the performance of origin in terms of reducing displacements and base shear with increasing height of the building, but has had a negative impact on the drift, which leads to an increase in drift with the increased flexibility of the building.

International Journal of Earthquake and Impact Engineering, 2016

The effects of low magnitude vibrations on the base isolated building are analysed and discussed with reference to two particular structures. The first one is a complex structure, composed by two blocks of three floors, rising up from a common base deck, which is supported by the foundation beams by means of isolation devices. During a moderate intensity earthquake, higher modes were excited, pointing out their very irregular behaviour. In the second case, which regards a building having a hemispherical shape, with three floors plus an underground one, the data recorded under ambient vibrations revealed the significant contribution of the isolation system. In both cases, the experimental behaviour was quite different from the expected one and this should be accounted for when designing base isolation systems.

Viscous and other damping devices are often used as elements of seismic isolation systems. Despite the widespread application of nonlinear viscous systems particularly in Japan (with fewer applications in the USA and Taiwan), the application of viscous damping devices in isolation systems in the USA progressed intentionally toward the use of supplementary linear viscous devices due to the advantages offered by these devices. This paper presents experimental results on the behavior of seismically isolated structures with low damping elastomeric (LDE) and single friction pendulum (SFP) bearings with and without linear and nonlinear viscous dampers. The isolation systems are tested within a six-story structure configured as moment frame and then again as braced frame. Emphasis is placed both on the acquisition of data related to the structural system (drifts, story shear forces, and isolator displacements) and on non-structural systems (floor accelerations, floor spectral accelerations, and floor velocities). Moreover, the accuracy of analytical prediction of response is investigated based on the results of a total of 227 experiments, using 14 historic ground motions of far-fault and near-fault characteristics, on flexible moment frame and stiff braced frame structures isolated with LDE or SFP bearings and linear or nonlinear viscous dampers. It is concluded that when damping is needed to reduce displacement demands in the isolation system, linear viscous damping results in the least detrimental effect on the isolated structure. Moreover, the study concludes that the analytical prediction of peak floor accelerations and floor response spectra may contain errors that need to be considered when designing secondary systems.

Bulletin of the New Zealand Society for Earthquake Engineering

The effect of viscous, viscoelastic, and friction supplemental dampers on the seismic response of base-isolated building supported by various isolation systems is investigated. Although base-isolated buildings have an advantage in reducing damage to the superstructure, the displacement at the isolation level is large, especially under near-fault ground motions. The influence of supplemental dampers in controlling the isolator displacement and other responses of base-isolated building is investigated using a multi-storey building frame. The coupled equations of motion are derived, solved and time history analysis is carried out on a building modeled with fifteen combinations of five isolation systems and three passive dampers. The seismic responses are compared with that of the fixed-base and base-isolated buildings. Based on the results, it is concluded that supplemental dampers are beneficial to control the large deformation at the isolator level. Parametric study is conducted and ...

2008

The basic idea of seismic isolation is based on reduction of the earthquake induced inertia loads by shifting the fundamental period of the structure out of dangerous resonance range, and concentration of the deformation and energy dissipation demands at the isolation and energy dissipation systems, which are design for this purpose. In this paper, after a brief introduction, the energy approach to the earthquake resistant structural design is described. As a numerical example, a six story structure analyzed with five different seismic protection alternatives as fixed base, rubber bearing, friction pendulum bearing, additional isolated story and viscous damper. Three dimensional nonlinear time history analysis is performed on r/c building model for fixed base case with respect to the seismic isolation and earthquake protection alternatives. In determining the specifications of isolators and dissipaters, such device features are taken into consideration, which would transfer minimum ...

Smart Structures and Systems, 2018

Traditional base isolation systems focus on isolating the seismic response of a structure in the horizontal direction. However, in regions where the vertical earthquake excitation is significant (such as near-fault region), a traditional base-isolated building exhibits a significant vertical vibration. To eliminate this shortcoming, a rocking-isolated system named Telescopic Column (TC) is proposed in this paper. Detailed rocking and isolation mechanism of the TC system is presented. The seismic performance of the TC is compared with the traditional elastomeric bearing (EB) and friction pendulum (FP) baseisolated systems. A 4-storey reinforced concrete moment-resisting frame (RC-MRF) is selected as the reference superstructure. The seismic response of the reference superstructure in terms of column axial forces, base shears, floor accelerations, interstory drift ratios (IDR) and collapse margin ratios (CMRs) are evaluated using OpenSees. The results of the nonlinear dynamic analysis subjected to multidirectional earthquake excitations show that the superstructure equipped with the newly proposed TC exhibits a superior response with higher margin of safety against collapse when compared with the same superstructure with the traditional base-isolation (BI) system.

Earthquake Engineering & Structural Dynamics, 2003

This paper presents a two-dimensional numerical study on the nonlinear seismic response of buildings equipped with two types of energy dissipators: Constant Friction Slip Braces (CFSB) and Adding Damping and Stiffness (ADAS). Three types of reinforced concrete buildings with 3, 7 and 15 storeys, representatives of the short-medium-and long-period ranges, are considered. Dissipators are placed in steel diagonal braces in all the floors. The sliding threshold (or yielding) forces for each mechanism are selected using two different criteria: (i) they are taken as 50, 75 and 100 per cent of those generated by the equivalent static lateral forces recommended by the UBC-91 for a ductile moment resisting frame and (ii) they are constant in the whole building (this constant value is chosen equal to the maximum forces obtained with the previous criterion). The input consists of ten recorded earthquakes (normalized with respect to their Housner intensity) corresponding to medium and stiff local soil conditions. Average values on the ten registers are given for the maximum horizontal displacement, the base shear, the energy dissipated and the interstorey drift. The possibility of failure in some devices has been numerically simulated to assess the robustness of the system. The obtained results show that both devices are useful to reduce the response compared to the bare frame and that CFSB is more efficient than ADAS; for 7and 15-storey frames the lateral displacement with CFSB is even smaller than the one for the braced frame (rigid connections instead of dissipators). The conclusions are expected to provide simple design guidelines.

International Journal for Numerical Methods in Engineering, 1999

A ÿnite element formulation modelling hyperelastic quasi-incompressible rubber-like materials (elastomers) is developed which takes into account large displacements and large elastic strains as well as inelastic e ects. The capacity of laminated rubber-like materials to support high loads in compression and large displacements in shear is the principal reason for their use in devices for seismic base isolation of structures. The energydissipation capacity of these devices is increased by using high damping rubber, which is an elastomer incorporating carbon black particles, or having lead-plug insertion. The Ogden strain energy function has been used as a basis for the material model implemented in a total Lagrangian formulation, the strain being decomposed into its deviatory and volumetric parts and the pressure variable being condensed at element level. Mooney-Rivlin and neo-Hooke strain energy functions can also be used by simply changing the parameters of the model. The stress-strain hysteresis, which appears when these devices are subjected to dynamic or quasistatic cyclic loads, has been modelled by frequency dependent viscoelastic and plastic constitutive models. The bearings have been modelled by means of an equivalent single element capable of describing the composite behaviour of the actual isolation system. The proposed model is validated using available experimental results and it is proved to be a powerful tool in dealing with di erent bearings. Finally, results for a six-storey base isolated building subjected to the El Centro earthquake are given.