Seismic Isolation

This post aims to present main aspects of seismic isolation in the strengthening of a reinforced concrete building, the available types of base isolation, as well as their advantages and disadvantages.

Base isolation, also known as seismic isolation, is a state-of-the-art method that constitutes one of the most effective means of protecting a structure against earthquake forces. A collection of structural components, called the isolators, are used to decouple to a large extent the superstructure  from the base (foundation or substructure) that rests on shaking ground, thus protecting the building’s integrity [Constantinou et al. 1998].

When used for the seismic upgrading of existing reinforced concrete structures, seismic isolation is typically applied at the columns and walls just above the foundation level (see Figure and Figure ). If the building has a basement the options are to install the isolators at the top, bottom or mid-height of the columns and walls of the basement.

In the most common configuration, a diaphragm is constructed immediately above the isolators, in order to connect the columns and prevent their independent vibration during a large seismic event. Often a similar diaphragm is also constructed at the foundation level, right below the isolators.

Before cutting the columns for the installation of the bearing, hydraulic jacks are installed in symmetrical position in the entire building plan simultaneously, or around each column separately. The superstructure is lifted with the jacks by 1-2mm, in order to allow the decompression of the column, which is then cut with conventional methods, e.g. with a diamond saw. The bearings are installed and the gap between the concrete and the bearing is filled with non-shrinkable mortar or epoxy resin.

Usually, a large wall is also constructed in the perimeter of the building at the level of the isolators in order to prevent displacements that are larger than the isolators’ deformation capacity (Figure ). It is also noted that allowing large relative movements of the building with respect to the ground means that even the non-structural components (e.g. partition walls), as well as the components of the electrical and the mechanical installations system (e.g. cables, pipes) that cross the plane where the isolation system is installed must be altered, in order to be able to sustain the seismic movements without interruption of their operation.

Figure 1: Typical base isolation configuration [adapted from Pinho et al. 2019]

Figure 2: The base isolators under the Utah State Capitol building [Wikipedia 2021c]

The main characteristics of a seismic isolation system are the limited stiffness at the isolators’ level, which leads to the significant period elongation of the structure to fundamental periods of up to 2.5 sec or more. This leads to a significant reduction in the acceleration passed on to the superstructure, the inertia forces and the earthquake force demand. As a result, the lateral deformations, and the interstorey drifts are considerably smaller, leading to light or very light damage to the structural and the non-structural components even in very large earthquake events.

The main concept behind using base isolation for retrofit is that, instead of strengthening the structural members to withstand the imposed seismic action (as is done with all the other methods), base isolation takes the opposite approach, that is to reduce the seismic demand instead of increasing the capacity. Since controlling the ground motion that is imposed to the structure is impossible, the structural protection is done by modifying the demand by preventing/reducing the motions being transferred to the superstructure from the foundation level.

The fundamental principle is to modify the response of the building, so that the ground is capable of vibrating without transmitting significant motion and inertia forces to the superstructure. A complete separation would be possible only in an ideal, fully flexible system, and no acceleration would pass to the superstructure. However, in real world applications, it is necessary to have a system that is able to transfer the vertical loads to the base, as well as to resist the small lateral forces induced from the wind and minor seismic events.

With seismic isolation the achieved decrease in the seismic demand is usually very large, hence no other intervention is required in the superstructure, even if this is constructed without modern anti-seismic standards, adequate reinforcement or good detailing. Depending on the condition of the superstructure, the design can be carried out, so that it accepts limited inelastic deformations, or remains totally elastic. The main drawback of the method is that the site of the building should permit horizontal displacements at the base of the order of 200mm or more in every direction. Consequently the method is not suitable for buildings that are not open on all sides in their perimeter.

Seismic isolation was first introduced for the design and construction of new buildings, however nowadays it is gradually gaining ground for the protection of existing structures against seismic loading. With the rapid decrease in the cost of isolators the technique is gradually changing from an ‘exotic’ method suitable only for special applications, to one of the standard methods for seismic upgrade.

Today seismic isolation is considered one of the preferred methods for retrofit in the cases of historical building preservation that require minimum modifications, and for content protection, i.e. when the value (financial, cultural or architectural) of the contents of a building is greater than the value of the building itself, as for example in museums. Moreover, it constitutes a competitive method even in purely economic terms for medium to high-rise buildings, especially when one takes into account that all the retrofit works are carried out at a single level (typically the foundation or the ground level), which means that the disruption to the operations of the building and the cost for the business interruption are limited and considerably less with respect to other methods, e.g. jackets or new shear walls.

It should be noted that some of the most prominent U.S. monuments, e.g. the Pasadena City Hall, the San Francisco City Hall, the Salt Lake City and County Building or the LA City Hall were mounted on base isolation systems [Wikipedia 2021c], whereas base isolation has been used extensively in seismic upgrading of existing buildings in other countries, such as Italy and Japan.