Seismic Evaluation of Fiber Reinforced Elastomeric Isolator with Different Irregularity Conditions in Building

The growth in seismic isolation technology has led to the development of numerous innovative and unique base isolation devices. Fiber reinforced elastomeric isolators (FREIs) are low cost alternative to conventional steel reinforced elastomeric isolators (SREIs). FREIs are light in weight, easy to manufacture and cheaper than SRFIs. Fiber reinforced elastomeric isolators (FREIs) comprise alternating bonded layers of elastomer and fiber reinforcement sheets. A Partially Bonded Fiber Reinforced Elastomeric Isolator (PB-FREI) is achieved by partially bonding the top and bottom surfaces of the bearing to its contact supports. This study investigate the performance of partially bonded fiber reinforced elastomeric isolator under stiffness irregularity and mass irregularity conditions. Keywords— Elastomeric isolator; mass irregularity; stiffness irregularity

INTRODUCTION Fiber-reinforced elastomeric isolators (FREI) are a relatively recent development in the field of base isolation. The benefit of FREI is that they have the potential to be a lighter and less expensive alternative to current rubber isolation devices, such as low-damping rubber and lead-plug rubber isolators. Traditional isolators are comprised of layers of rubber and rigid steel shims that provide the bearing with vertical stiffness. By replacing the heavy, steel shims with lighter layers of fiber that have an elastic modulus similar to that of steel, studies indicate that FREI are able to satisfy vertical stiffness requirements for an isolator. In addition, the use of fiber as reinforcement introduces an additional flexural type response component into the isolator that affects its horizontal stiffness. Fiber reinforced elastomeric isolators (FREIs) comprise alternating bonded layers of elastomer and fiber reinforcement.
The prospect of a lighter and cheaper alternative to current rubber isolation devices prompted research into using FREI to isolate low rise masonry buildings and bridges. There are three types of application for fiber reinforced elastomeric isolators: Bonded FREI, Unbonded FREI and Partially Bonded FREI. In a bonded (B)-FREI, two thick steel mounting plates are bonded to the outer rubber layers at the top and bottom of the isolator. During installation, the top and bottom mounting plates are bolted to the superstructure and substructure, respectively. In a UB-FREI, the isolator is placed between the substructure and superstructure without any bonding or fastening provided at its contact surfaces. As such, the shear loads at the bearing contact surfaces are transferred through friction only. In a PB-application, only portions of the contact surfaces of the bearing are bonded to the upper and lower mounting plates. A significant amount of the tensile and shear forces can be transferred via the partial bond. The bearings were made from alternating layers of cold vulcanized Neoprene and carbon fiber fabric.

II. OBJECTIVES
• To investigate the performance of FREI under stiffness irregularity • To study the behaviour of FREI under mass irregularity conditions III. SUMMARY OF LITERATURE REVIEW Fiber reinforced elastomeric bearings (FREB) comprise alternating bonded layers of elastomer and fiber reinforcement layers. A PB-FREB is achieved by partially bonding the top and bottom surfaces of the bearing to its contact supports. Studies were mainly carried out on the results of a comparative experimental study between UB-FREB and PB-FREB. [1] A comparison of the seismic response of steel frames by using different types of bracing systems were mainly carried out. [2] FREI matches the behavior of an SREI, Consequently, the FREI could replace the conventional SREI for seismic isolation with low-cost manufacturing and lightweight installation. [3] By increasing the maximum imposed shear strain, the area of the isolator surfaces in contact with the concrete blocks decreases and, as a consequence, the isolator average horizontal stiffness also decreases. [4] IV. NUMERICAL ANALYSIS The analytical study includes the development of finite element models to evaluate the performance of a partially bonded fiber reinforced elastomeric isolator with different irregularity conditions in a building. ANSYS workbench 16.1 software was used to perform the seismic evaluation of partially bonded fiber reinforced elastomeric isolator with different irregularity conditions in building.

A. Material properties
The material properties of the partially bonded fiber reinforced elastomeric isolator are tabulated in Table 1. was used to model the partially bonded fiber reinforced elastomeric isolator. Length and width of the partially bonded fiber reinforced elastomeric isolator was 100mm. A FREB pad of 100×100mm comprised of 4 inner layers of elastomer of 4.5mm thickness, and two outer layers of 2.5mm thickness is modelled. The total height of the isolators is approximately 25.5mm. The total thickness of the rubber layers is tr =23mm. The shape factor of the isolators is S1=5.56. The aspect ratio is approximately S2=3.92. The elastomeric laminated rubber isolators, a cover rubber layer is bonded along the perimeter of each isolator to provide protection against oxygen and ozone attacks. Two square end steel plates of 100mm width are partially bonded across an area of 50×50mm at the center of the outer elastomer layers. Fig 1 presents the isometric view of partially bonded fiber reinforced elastomeric isolator.

V. RESULTS AND DISCUSSIONS
The results of numerical analysis such as isolation displacement, top storey displacement and shear stress on corner, exterior and interior under mass irregularity is shown in Table II. The storey displacement graph of 4 storey and 3 storey mass irregularity are shown in Fig.4. The storey displacement graph of 4 storey and 3 storey stiffness irregularities are shown in Fig.5. The results of PB-FREI with stiffness irregularity is shown in Table III Table  II. The shear stress is maximum at the isolators on corner and shear stress on exterior is minimum.  VI. CONCLUSIONS In the present study, partially bonded fiber reinforced elastomeric isolator were introduced in the building and the behaviour of PB-FREI under mass irregularity conditions were studied. Four storey building and three storey building were considered. Further the performance of PB-FREI under stiffness irregularity conditions were also studied. From the result obtained from this paper it can be concluded as follows: • The displacement is maximum at the top storey of the building • The top storey displacement is more than the isolation displacement. • Shear stress is maximum at the isolators on corner and shear stress is minimum at the isolators on exterior.