Base Isolation for Multistorey Buildings: Seismic Response Analyses

Mitigation is through mounting the structure on an isolation system with considerable horizontal flexibility such that the motion is reduced during an Earthquake. As the isolator flexibility increases, building movement relative to the ground may become problematic when subjected to other vibrations such as wind loads. An isolation device's function is to support the superstructure while providing a high degree of horizontal flexibility. This gives the overall structure a long effective period and hence lower earthquake-generated accelerations and inertia forces.

Kelly (1990) gave a brief introduction to the response mechanisms of base isolated buildings through a two degrees of freedom linear system. The effectiveness of the isolation system to mitigate the seismic response is through its ability to shift the fundamental frequency of the system out of the range of frequencies where the earthquake is strongest. Also, it was demonstrated that the most important feature of seismic isolation is that its increased flexibility increases the natural period of the structure. Because the period is increased beyond that of the earthquake, resonance is avoided and the seismic acceleration response is reduced.

This study was carried out to investigate in more detail the effects of various structural parameters and ground motion characteristics on the response of base isolated multi-storey structures. The results were then used to develop two simplified analysis methods for practical design.

Several existing design codes and methods for both linear and non-linear base isolated structures are reviewed and discussed.

SEAONC requires the use of either static or dynamic analysis whereas UBC code has a number of differences in emphasis and has more restrictions on the use of the simple equivalent lateral force design procedure, requiring dynamic analysis under a wider range of circumstances.

A comprehensive design review of the current isolation design methodologies in practice and the limitations of these methods are outlined.

The essential basis of seismic base isolation is to support a structure while providing a high degree of horizontal flexibility. This gives the overall structure a long effective period and low accelerations and inertia forces generated by earthquakes.

Dynamic analyses are required in order to be able to predict the response of structures SUbjected to dynamic loading. In these analysis methods, the real structures are represented by appropriate analytical models which can be described mathematically.

• Lumped-mass model is used in this study because the mass of the system is assumed to be represented by a finite number of point masses

• The stiffness of the structure has a major effect on the design of a multi storey building. The stiffer the structure the shorter the natural periods of free vibration and the smaller the displacements under the earthquake excitation.

• Most building footings are square or rectangular and are partially embedded foundations. During dynamic loading the soil immediately below the footing foundation will undergo higher shear strains than does the soil remote from the footing base

• The actual soil beneath the foundation may be physically complex and no exact mathematical solution seems possible for the general problem of soil-foundation interaction

• The dynamic responses of foundation vibrations have been investigated for the embedded foundations

• Results from the analytical investigations and laboratory tests, however, indicate that under dynamic loading both the, stiffness and damping coefficients are frequency dependent.

• In reality, the footing foundations are not necessarily fully embedded but both arbitrarily shaped and partially embedded

• Seismic isolation below the structure provides flexibility which generally reduces the severity of earthquake attacks.

• For the design of the base isolated building, Skinner et al (1993) [S6] mentioned that the isolator deformations and struc~ural displacements must be considered and accepted in order to achieve the reductions in seismic response.

• Seismic isolation gives several benefits; firstly, isolation gives a large increase in the first-mode period and this may sometimes be used to reduce severe seismic response of the structure if the severity is caused by approximate tuning to the period of an unisolated structural first mode; secondly, hysteretic isolators may be used to confer ductility to brittle structures, thus enabling them to resist seismic loads and if the structure has high stiffuess and low damping, effective ductility can be introduced without large increases in structural deformations.

• The successful seismic isolation in the design of a seismically isolated structure depends strongly on the selection of an appropriate base isolation system. This will partly be governed by the nature ofthe design criteria.

• It is important to have a deep understanding of the influence of each parameter controlling the behaviour of the isolation device on the performance of the base isolated building during earthquakes.

• The selection of base isolation devices is generally decided by two steps; at the initial design stage, it is necessary to consider whether the addition of seismic isolation will prove to be a cost-effective means of providing appropriate levels of seismic resistance for a structure and its significant secondary structures and contents, the [mal decision to use seismic isolation must be made on a case-by-case basis.

Seismic isolation has been considered as a technique for problem structures or for equipment which requires a special seismic design approach, due to their function (sensitive or high-risk industrial or commercial facilities such as computer systems or nuclear power plants) their special importance after an earthquake (hospitals, disaster control centres such as police stations) or because of their inability to economically resist the imposed seismic effects (historic buildings)

The primary function of an isolation device is to support the superstructure while providing a high degree of horizontal flexibility. This gives the overall structure a long effective period and hence lower earthquake generated accelerations and inertia forces.

Many kinds of isolation systems have been developed to achieve this function, such as laminated elastomeric rubber bearings, lead-rubber bearings, yielding steel devices, friction devices (PTFE sliding bearings) and lead extrusion devices, etc ..

Seismic isolation gives several benefits; firstly, isolation gives a large increase in the first-mode period and this may sometimes be used to reduce severe seismic response of the structure if the severity is caused by approximate tuning to the period of an unisolated structural first mode; secondly, hysteretic isolators may be used to confer ductility to brittle structures, thus enabling them to resist seismic loads and if the structure has high stiffness and low damping, effective ductility can be introduced without large increases in structural deformations.

This research focuses on understanding the behavior of base-isolated buildings and increasing confidence in the behavior of substructures under the most credible ground excitation. It also seeks to evaluate the effect of using segmental buildings proposed by Cui (1995), where the isolation devices are placed at various levels in the building to reduce the displacements imposed on each device. Furthermore, it aims to ascertain the effects of added viscous damping based on the equivalent static method recommended by the New Zealand Standard Code of Practice for General Structural Design and Design Loadings for Buildings NZS 4203: 1992 [S8] and investigate the seismic responses of stiff and flexible buildings compared to structures without additional damping for different earthquake motions.

This research thesis aims to develop theoretical and analytical aspects of the behaviour of base isolated buildings.

The following outline describes the scope of the study:

• To evaluate the development, results and proper selection for further research on base isolated structures, a review of the current design methods and existing design codes are presented in Chapter 2.
• The principles of dynamic analysis, structure modelling and soil-footing foundation impedance are discussed in Chapter 3.
• The procedures of the analyses of the base isolated structures are outlined in Chapter 4. This chapter involves soil site model related, comparison of earthquake and wind loads, selection of base isolators, dynamic parameters of the base isolated buildings, earthquake excitations and the methods used in the analyses.
• Effects of the ground motion characteristics on the structural behaviour of base isolated multi storey buildings are studied in order to be able to select the right system for a particular type of ground motion so that the base isolation device will provide a guaranteed benefit. A range of earthquake records other than the N-S component of the El Centro 1940 are used as the basis of these analyses. The chosen earthquake records were scaled according to their 5% damped spectra to fit the design spectrum in Section 4.6.2.9 (b) (ii) ofNZS 4203:1992 code for the intermediate soil sites. The results of this investigation are reported in Chapter 7.
• The concept of introducing isolation systems to mitigate seismic effects is a well-known technique and the development of many practical base isolation devices was accompanied by proposed design methods for base isolated structures. An equivalent linear analysis was used by most proposed design methods for approximating the inelastic behaviour of the base isolation systems as it affects the response of the elastic superstructure. The main purpose of these methods was to assist the designers to design the isolated structures without depending on a series of deterministic inelastic time history analyses. In the following, several design methods and codes are reviewed from the numerous designs of base isolation systems.