Implementation of a simple novel Abaqus user element to predict the behavior of unbonded fiber reinforced elastomeric isolators in macro-scale computations

The need for seismic protection of structures located in earthquake prone areas of developing countries has motivated several investigations of low-cost seismic isolators in recent years. One of the most promising device is the Unbonded Fiber Reinforced Elastomeric Isolator (UFREI): in UFREIs no bonding or fastening is provided between the bearing and top–bottom supports and expensive thick steel plates are not required. The behavior of UFREIs is characterized by rollover and full-contact deformations under horizontal loads: the former decreases the effective stiffness of the isolation system, thus reducing the seismic demand, while the latter plays a key role in generating a hardening phase, thus limiting the horizontal displacement of the isolator under severe earthquakes. Such remarkable advantages highlight a great potential for world-wide UFREIs applications. However, until now there is no representative UFREI model available in structural analysis software codes. In this work, a comprehensive but simple UFREI model is implemented in an ABAQUS user element (UEL), taking into account non-linear, hardening and hysteretic behavior of the bearing: in addition, multiple DOFs are considered to simulate the complex 3D behavior of UFREIs, which is characterized by horizontal and vertical displacements, rotation and torsion. The effectiveness of the UEL model is evaluated performing 3D FE dynamic time history analyses on a rigid slab supported by four different types of UFREI. The results show that the UEL model can reasonably fit the behavior of the detailed FE model, significantly reducing the computational efforts of the analyses. The UEL model proposed in this study can be particularly suitable for 3D dynamic time history analyses of complex base isolated structures.