Hamid Toopchi-Nezhad

Parametric Study on the Response of Stable Unbonded-Fiber Reinforced Elastomeric Isolators (SU-FREIs)

Tests on stable unbonded (SU) square carbon fiber-reinforced elastomeric isolator (FREI) bearings were conducted to investigate their lateral and vertical response. The bearings are intended for seismic isolation of low-rise buildings including those of ordinary importance. To simulate the in-place application of SU-FREI bearings, the contact surfaces of the bearings were not bonded to the platens of the test machine. This unbonded application permitted stable rollover deformation to occur which enhances the bearings isolation efficiency. The bearings were shown to safely sustain large lateral displacements. When subjected to large lateral displacements, their originally vertical faces completely contacted the horizontal surfaces of the upper and lower platens, which created a stiffening response and ensured the stability of these very large displacements. The sensitivity of SU-FREI bearings to lateral displacement history and vertical pressure applied on the bearings were investigated. Regarding the latter parameter, it was found that the effect of variations in vertical pressure on the lateral response can be neglected when the SU-FREI bearings are subjected to relatively light vertical pressures such as considered for low-rise buildings.

Stability of fiber-reinforced elastomeric bearings in an unbonded application

Stable unbonded fiber-reinforced elastomeric isolators (SU-FREIs) are a viable device for seismic mitigation purposes in low-rise structures. SU-FREI bearings consist of alternating layers of elastomer and fiber fabric with shape factor and aspect ratio such that stable rollover occurs during lateral displacement. This study investigates the stability of SU-FREI bearings through experimental testing. Two experimental test procedures are employed. The first investigates stability during incrementally increasing axial loads under lateral cyclic excitation, and the second investigates the ultimate shear properties of SU-FREI bearings through monotonic lateral displacement under design axial load. Dynamic stability testing reveals that the tested SU-FREIs remain stable at axial loads significantly greater than the design load under a wide range of cyclic lateral displacement amplitudes. In addition, an increase in axial load results in lower effective stiffness and greater effective damping in the SU-FREI bearings. Although rollout instability was not encountered during ultimate shear property testing, shear tests provide insight into the stiffening effect encountered as the originally vertical faces of the SU-FREI bearings make contact with the top and bottom contact supports.

Simplified analysis of a low-rise building seismically isolated with stable unbonded fiber reinforced elastomeric isolators

The seismic response of an ordinary low-rise base isolated (BI) structure, employing stable unbonded-fiber reinforced elastomeric isolator (SU-FREI) bearings, is predicted by using two different simplified analytical models. Subsequently, the accuracy of the two models is evaluated by using measured test results from a shake table study. Two models simulate the nonlinear experimental lateral load–displacement hysteresis loops of these bearings. The experimental hysteresis loops were obtained from cyclic shear tests on prototype bearings under a constant compression load. Because of the nonlinear lateral response behavior of the SU-FREIs, these models are employed in an iterative time-history analysis approach, enabling the model variables and the calculated peak lateral displacement of the bearings to converge to their unique values. Analysis results show that the presented simplified models may be used effectively in seismic response prediction of ordinary low-rise buildings that are seismically isolated...

Application of an Extended Bouc-Wen Model in Seismic Response Prediction of Unbonded Fiber-Reinforced Isolators

The focus of this article is on seismic-response prediction of Stable Unbonded-Fiber Reinforced Elastomeric Isolators (SU-FREIs). The lateral load-displacement of a SU-FREI can be characterized with a gradual softening that is followed by stiffening. An extended Bouc-Wen Model is developed to simulate the response behavior of SU-FREIs under seismic events. To examine the accuracy of model, the response of a base isolated structure to different input earthquakes were simulated and compared with the results of a previous shake table study. Results of this study indicate that the proposed Bouc-Wen Model is robust and reasonably accurate in seismic analysis of SU-FREIs.

Influence of thickness of individual elastomer layers (first shape factor) on the response of unbonded fiber-reinforced elastomeric bearings:

The objective of this paper is to study the influence of thickness of individual elastomer layers (first shape factor) on the vertical and horizontal responses of fiber reinforced elastomeric bearings that are not bonded or mechanically fastened to their top and bottom supports. By definition, the first shape factor, S1, is the ratio of the plan area to the perimeter stress-free area of a single elastomer layer within the bearing. Results of a comprehensive 2D-finite element study on a large group of bearings with different shape factors of 10, 15, 20, 30, and 40 suggest that the shape factor S1 is a critical parameter in controlling both the vertical compression modulus of the bearing and the level of stress in the fiber-reinforcing layers. Furthermore, the secant horizontal stiffness and the stress demand in the bearing elastomer material are to a limited extent influenced by the parameter S1.