Earthquake input energy

The performance of the present isolation system can be investigated with evaluating the input energy to the structure due to the earthquakes (Figure 6-22 to Figure 6-27).

Table 6-8 Maximum input energy by different earthquakes (N-m)

El-Centro Kobe Manjil

Fixed base 15.40 97.45 20.80

Base isolated 11.07 29.38 13.77

A significant reduction in maximum input energy to the structure is observed when a strong ground motion – Kobe – is considered. It can be concluded that the performance of the isolation system in terms of reduction in the input energy is better when considering strong earthquakes in comparison to moderate earthquakes.

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Figure 6-22 Input energy trend over time due to El-Centro earthquake on fixed-base structure (N-m)

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Figure 6-24 Input energy trend over time due to Manjil earthquake on fixed-base structure (N-m)

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Figure 6-26 Input energy trend over time due to Kobe earthquake on base-isolated structure (N-m)

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6.7 Conclusion

Numerical assessment of capability of the proposed isolation system was presented in this chapter. The ability of the new system to protect structural and non-structural elements from seismic hazards through its implementation on a structure were checked. It is found that the new isolation system is able to greatly reduce the structure accelerations, story drifts and base shears, while keeping acceptable displacement at base. The proposed isolation system is highly effective when both moderate and strong motion are considered.

It was clear from results that a larger period of vibration and a faster attenuation of acceleration amplitude accompany the isolated structure. Although, the best performance of the isolation system is obtained when strong motions are considered, the results confirmed that the efficiency of the present system against earthquake attacks does not highly depend on the type of input earthquake. It should be noted that, one of the drawbacks of current base-isolation systems is dependency on the input shocks characteristics.

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7

Conclusion and future

works

This chapter concludes the thesis with a summary of results, the main contributions and proposes topics that may be considered for future works.

7.1 Conclusion

Results from the experiments and numerical simulations reveal that the present isolation system is a very efficient tool for seismic protection of building structures under different earthquake features. The new isolation system exhibits a robust performance for a scaled model of a typical five-story building. The behaviour of the structure isolated by the new proposed system is relatively independent of the frequency content and the amplitude of base excitation.

As the core of the idea for the new proposed isolation system was to make use of air- bearing solutions, the air-bearing units were explicitly designed, analysed and optimised through numerical simulation and further investigated through experimental tests. The experimental tests on the air-bearing devices confirmed the best shape for the nozzles of bearing in order to handle larger loads.

The purpose of the experimental study on the scaled structure was to determine whether observations agree with or conflict with the predictions derived from the hypothesis established earlier and its subsequent analytical model. The experiments confirmed that

172 the isolation system is able to lengthen the fundamental period of the structure and add significant level of damping to the whole system (structure and its isolation). It was also concluded that the isolation system reduces the level of acceleration exerted to the structure subject to different applied impulse loadings. In addition, the isolation system provides the structure with a reduction in the level of story drifts which is a known cause of damages to the structures.

The computer investigation was conducted in order to simulate the behaviour of the structure with two different base configurations (fixed base and base isolated) and subject to different earthquakes; El-Centro 1940 as a moderate and short excitation, Kobe 1994 as a strong and near-field ground motion, and Manjil 1990 as a strong ground motion with long duration. The performance of the isolation system was confirmed appropriate in terms of lengthening the fundamental period of vibration of the structure, great attenuation of structural acceleration, significant reduction in story drifts and a reduction of input energy to the structure in all cases. This is a great achievement that the structure isolated by proposed isolation system is working quite independently from the characteristics of input earthquake. As discussed in the literature review, one of the drawbacks of current isolation systems is that they are not efficient in moderate earthquakes and their performance is highly dependent on the input earthquake characteristics; whereas, the new proposed system is able to protect a structure regardless of the characteristics of the ground motion.

Measurements in engineering research works are usually accompanied by estimatipon of their uncertainty. The value of the measured frequencies and periods by experiments concur with the values obtained from the analytical model and numerical simulations. There are only around 10% and 3% differences for fixed-base structure and base- isolated structure, respectively, between experiments and computer modelling. This stems from the uncertainty in the stiffness and damping characteristics of the structure and its isolation system. A small variation in stiffness produces quite a large difference in measures of dynamic parameters.

There are some limitations in computing equivalent damping values using experimental data which arises because linear behaviour for a dynamic system is normally assumed.

173 When the given dynamic system is behaving highly nonlinear, a substantial error could be introduced into the damping estimation. Nonetheless, it is routine to assume linear viscous damping in experimental data analysis.

7.2 Contributions

This research aimed to develop an innovative isolation system that is efficient and has some particular capabilities which are not offered by contemporary isolation systems. The proposed isolation system works as a semi-active isolation system which can be triggered by the signals received from metrological centres as an early warning. The isolation system, then, employs air-bearing solutions to decouple the super-structure from its base at the time of ground motion. The air-bearing system is accompanied by a re-centring mechanism such as rubber-bearings or friction pendulum for self-re- centring. As the horizontal stiffness at the base of a structure is reduced the fundamental period of the structure is shifted to a greater one and the accelerations exerted in the structure are reduced. Below is the list of features provided by the new system:

 Great decoupling system

 Lengthening the vibration period to the designer desire  Involving a wide range of base stiffness

 Great level of energy dissipation  Self-re-centring system

 Suitable for all range of earthquake from moderate to strong motions  Improvement of damping mechanism