Earthquake-resistant
design
Seismic isolation
The seismic gap
limitation
Project objectives

Although methods of conventional earthquake-resistant design have substantially improved in the last decades, strong earthquakes still result in undesirable damage even in cases of buildings designed according to the most rigorous seismic codes.

The latter ensure the required strength and ductility to withstand cycles of inelastic deformations during severe seismic loads avoiding structural collapse and casualties, but allowing significant structural and non-structural damage as well as damage of the contents of a building. In high seismicity areas, this compromise is unavoidable, as it is almost impossible to build a conventionally fixed-supported low- to medium-rise building of reasonable cost to withstand severe seismic loads without inelastic deformations. Unfortunately, the fundamental frequencies of such buildings fall in the range of the predominant frequencies of earthquakes, resulting in amplifications of ground accelerations and large interstory deflections.

In recent years, there has been an increasing demand to minimize structural and non-structural damage, avoid functionality disruption and protect sensitive and expensive equipment in buildings even under extreme earthquake excitations. With conventional earthquake-resistant design, although providing flexibility to a structure reduces the floor accelerations, it also results in large relative displacements, causing structural and non-structural damage.

On the other hand, providing stiffness to a structure reduces the interstory deflections, but the floor accelerations approach the levels of the ground accelerations, which can be unacceptably large and damage the contents of a building. A different design approach is needed to rationally mitigate the damage caused by severe earthquakes, since it is difficult, with conventional earthquake-resistant design, to reduce both interstory deflections and floor accelerations.

 

Seismic isolation is an alternative earthquake-resistant design approach that reduces the induced seismic loads, in low- to medium-rise buildings, by avoiding resonance with the predominant frequencies of earthquake excitations. In order to seismically isolate a structure, flexibility is introduced together with an energy dissipation mechanism at the isolation level.

The increased flexibility avoids resonance with the earthquake excitation and significantly decreases the shear forces and the floor accelerations of the structure and, consequently, averts the possibility to damage the contents of the building. In addition, interstory deflections are substantially reduced due to the almost rigid body motion of the superstructure, which is relatively very stiff compared to the flexibility of the isolation system, minimizing structural and non-structural damage.

The deformations are concentrated at the isolators, which are specifically designed to withstand several inelastic cycles of deformation and accommodate the large relative displacements at the isolation level.

A limitation for the utilization of seismic isolation is the seismic gap that must be provided around the structure to allow the expected large relative displacements at the isolation level.

A major concern is the possibility of poundings during strong earthquakes, which may result in local damage, excitation of higher modes, increased floor accelerations and, consequently, damage of the contents of the building.

Therefore, there is a need to investigate the response of seismically isolated structures during poundings with adjacent structures due to strong earthquake excitations and understand how the maximum floor accelerations and interstory deflections of seismically isolated structures are affected by the various design parameters and conditions.

Although research work has been done for impacts between adjacent fixed-supported structures, limited studies have been conducted specifically for poundings of seismically isolated buildings with the moat wall or adjacent structures.

Hence, this aspect of seismic isolation deserves further research work as it presents issues that have not been thoroughly addressed in previous research studies. The size of the seismic gap, the type of the isolation system, the stiffness of the adjacent structure or moat wall, the flexibility of the seismically isolated superstructure, the location of the points of impact, the characteristics of the earthquake excitation, and torsional effects due to eccentricities are among parameters and conditions that may influence the way the effectiveness of seismic isolation is affected from poundings during strong earthquakes.

All these issues are suggested to be investigated using numerical simulations and parametric studies, in an interdisciplinary combination of structural engineering with computer-aided engineering. Furthermore, the effectiveness of certain measures that can be taken to mitigate the consequences of potential poundings during severe earthquakes, such as the usage of bumpers around seismically isolated buildings will also be assessed using numerical simulations.