Stelios AntoniouDr. Stelios Antoniou
Head of Structural/ Earthquake Engineering Department at Alfakat
Co-founder, Managing Director and R&D Director at SeismoSoft

Table of Contents

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.

Seismic Isolation

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.

fig-1

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

fig-2

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.

Type of Base Isolation Systems

Seismic isolation can be achieved with the use of various devices like rubber bearings, friction bearings, ball bearings, and spring systems. The isolator types that are mostly used in practical applications are: (i) elastomeric rubber bearings, (ii) elastomeric lead-rubber bearings, (iii) friction-based isolators and (iv) friction pendulum systems, FPS.

The elastomeric isolators (Figurea), which can be low (LDRB) or high (HDRB) damping depending the material used, are composed of a series of horizontal layers of elastomeric material (synthetic or natural rubber) interspersed by steel plates. These bearings are very stiff in the vertical direction, being capable of supporting high vertical loads with very small deformations, but flexible under lateral loads in both horizontal axes. The steel plates provide a significant contribution both to the vertical stiffness and to the lateral confinement preventing excessive lateral deformations, while limiting the lateral bulging of rubber. The key parameters for the design of elastomeric isolators are the maximum vertical load that can be sustained, the horizontal stiffness and the maximum permissible, relative horizontal displacement between the two ends of the bearing.

The elastomeric lead-rubber bearings (Figureb) have the same characteristics as the plain elastomeric ones, with the exception of a central cylinder at the core, which provides a source of larger damping due to the shear deformation of the lead material, since the plain bearings have limited energy dissipation capacity.

fig-3

Figure 3: (a) A typical rubber bearing and (b) a typical lead-rubber bearing [adapted from Pinho et al. 2019]

The friction-based isolators are sliding bearings that use sliding elements between the foundation and the superstructure. Depending on the shape of the interface between the sliding elements, they are divided into flat slider bearings and curved slider bearings. The sliding displacements are controlled by high-tension springs, laminated rubber bearings or the curved shape of the sliding surface, and these mechanisms provide a restoring force to return the bearing and the structure to their equilibrium position. The most common types in this category are the friction pendulum systems (FPS), which use the principles of a pendulum to elongate the fundamental period of the isolated structure. The FPS consists of a concave stainless steel surface covered by a Teflon-based composite material and a slider. During severe ground motion, the slider moves on the concave surface lifting the structure and dissipating energy by friction between the spherical surface and the slider. This isolator uses its surface curvature to generate the restoring force from the pendulum action of the weight of the structure on the FPS. Error: Reference source not founda shows the typical cross section of a FPS base isolator and Error: Reference source not foundb shows this isolator installed.

fig-4

(a)

fig-5

(b)

Figure 4: (a), (b) A typical FPS base isolator [Giarlelis et al. 2018]

The evolution of this system is the double-curved sliding pendulum isolator [Constantinou 2004], which combines two (rather than one) concave surfaces. It is able to achieve the double displacement with respect to the ordinary systems, and is suitable when the aim is to limit the size of the device.

Advantages and Disadvantages

Following a different approach from all other strengthening methods, base isolation leads to the significant drop of the seismic forces applied to the structure, and is one of the safest methods for seismic upgrading. Its main advantage is that all the strengthening works are carried out at the foundation level thereby reducing the disturbance to the residents during the construction works. This means that additional costs for non-structural damage and business interruption are also limited.

Even in the case of a large seismic event, the damage is small and concentrated at the isolators’ level; hence the repair can be done easily and without the need for the evacuation of the building. What is more, the vibrations at the upper part of the building are very small, with limited absolute accelerations and relative displacements. This is very important in the cases, where the value of the building content is very high. This value can be either financial (e.g. a very important industry that cannot afford to interrupt its operation for a large period of time), architectural (e.g. a historical building) or cultural (e.g. an important museum). Base isolation is particularly suitable for historical buildings, since the method is the least invasive and does not require interventions in the building superstructure that needs to be preserved.

However, base isolation is still a very expensive solution, especially for small to moderate-sized projects, i.e. buildings up to 10 floors high, and in such cases the increased level of safety is not fully justified by the increased cost. It is noteworthy however that in recent years the price of isolators has dropped significantly, and they are expected to further decrease in the near future, gradually making base isolation more appealing.

Another problem of the method is that it cannot be applied to any structural configuration. It requires that the building under consideration is free to move in any direction in its perimeter, and this is not possible when the building is in contact with other properties at any of its sides. Therefore, the method cannot be applied to a large proportion of the building stock, such as buildings in city centres and other densely populated areas.

Last but not least, base isolation is a new method that is quite different in its philosophy and its details with respect to the other existing methods for retrofit. It requires specialized knowledge and expertise both on the design and on the construction level. Because the number of projects carried out up to now employing the technique is relatively small and confined to larger, more prominent buildings and landmarks, this expertise is currently limited, and possessed only by large design or construction firms (which wrongly further corroborates the conviction that seismic isolation is currently a method for large and important projects only).

Design Issues

The main characteristics of any isolation system are the energy dissipation to control its lateral displacements, the lateral rigidity under low lateral load levels, such as wind loads or minor earthquakes, and the resistance (strength and stiffness) to vertical, gravity or live, loads. These have to be considered along with the total structural mass, and with some relatively simple and mundane calculations for SDOF oscillators the designer determines the basic parameters of the isolators and the new dynamic characteristics of the building, so as to achieve the required increase in the period of vibration and the consequent reduction of the applied inertia force. Usually, a fundamental period of 2.50 sec or larger is sought for in both horizontal directions, so that the system is most effective.

Besides the main prerequisite that the building should be open on all sides of its perimeter (usually at least 200mm are needed), the base isolation technique is suitable when the subsoil does not produce a predominance of long period ground motion and the lateral loads due to wind are less than 10% of the weight of structure. Ideally, isolation is most effective with buildings that lie on stiff soil or rock.

Considering that the isolation systems are almost always nonlinear and often strongly nonlinear, an equivalent linear static analysis is commonly utilized only in the preliminary design phase employing effective bearing properties, whilst the final design is usually performed with nonlinear dynamic time-history analysis using the exact dynamic and hysteretic characteristics of the isolators and the components of the superstructure.

During recent years many National Standards and guidelines on base isolation have been developed and published worldwide, e.g. specific chapters in ASCE-41, EC8, Part-1 and TBDY, NZ draft guidelines [NZSEE 2019]. Most of these documents are for the design of new construction, though implicitly it is assumed that they can also be employed for the upgrade of existing structures. To my knowledge only the American ASCE 41 guidelines have a dedicated Chapter on the evaluation and retrofit of buildings using seismic isolation systems. Gradually, as the technology becomes more common, the methodologies and requirements for the design of seismic isolation in the seismic upgrade of structures will become better-known and more standardized worldwide.

References
  • [ASCE] American Society of Civil Engineers. 2017. Seismic Evaluation and Retrofit of Existing Buildings (ASCE/SEI 41-17), 2017, Reston, Virginia.
  • Constantinou M.C. (2004), “Friction pendulum double concave bearing,” Technical Report, University of Buffalo, State University of Buffalo, NY.
  • Constantinou M.C., Soong T.T., Dargush G.F. (1998), “Passive Energy Dissipation System for Structural Design and Retrofit”, Multidisciplinary Center for Earthquake Engineering Research MCEER, Monograph No. 1, Buffalo, NY.
  • EN 1998-1, 2004: Eurocode 8: Design of structures for earthquake resistance -Part 1: General rules, seismic actions and rules for buildings.
  • Giarlelis C., Keen J., Lamprinou E., Martin V. and Poulios G., 2018b. The seismic isolated Stavros Niarchos Foundation Cultural Center in Athens (SNFCC). Soil Dynamics and Earthquake Engineering 114 (2018) 534–547.
  • NZSEE, 2019. Guideline for the Design of Seismic Isolation Systems for Buildings. Draft Report, June 2019. New Zealand Society of Earthquake Engineering, Wellington.
  • Pinho, R., Bianchi F. and Nascimbene R. (2019). Valutazione sismica e tecniche di intervento per edifici esistenti in c.a. Maggioli Editore. In Italian.
  • TBDY, 2018. Türkiye Bina Deprem Yönetmeliği, TurkishSeismic Building Code. Disaster and Emergency Management Presidency, Ankara. In Turkish.
  • Structural Assessment, Strengthening & Retrofitting carried out using SeismoSoft Earthquake Engineering Software.

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