Concrete Buildings in Seismic Regions

کتاب ساختمان های بتنی در نواحی زلزله خیز

Earthquake engineering is an independent scientific discipline that has come into being along with engineering seismology over the past 100 years and is therefore still evolving, as happens in every new scientific field. It started as a framework of codified rules for the seismic design of buildings at the beginning of the twentieth century, after the catastrophic Messina (Italy) earthquake of 1908. This design procedure was based, on one hand, on a framework of empirical rules for avoiding seismic damage observed in previous earthquakes and, on the other, on the simulation of the seismic action on a set of lateral forces equal to a percentage of the gravity loads of the building. This loading pattern constituted one additional load case, the ‘seismic loading’.

The above simulation was based on the fact that the acceleration of the masses of a building due to earthquake causes inertial lateral forces proportional to the masses of the building, and in this respect proportional to the gravity loads. The proportion of lateral seismic loads Hseism to gravity loads W resulted from the ratio of the peak ground acceleration (PGA) to the gravity acceleration g (ε = PGA/g = Hseism/W). A critical point in the above procedure was the assessment of the PGA of an earthquake that should be taken into account for the determination of the seismic lateral loading, since for many years up to 1940 there were no instruments capable of determining the PGA (strong motion instruments).

Therefore, for a period of almost 35 years after the San Francisco earthquake, qualitative observations were used, like the overturning of heavy objects located at ground level for the quantitative estimate of the PGA as a percentage of (gravity acceleration). These values were introduced as seismic coefficients in the various Seismic Codes in effect and ranged usually from ε = 0.04 to 0.16, depending on the seismic hazard of each region. At the same time, it was realised that the vibration of the superstructure of a building caused by seismic actions does not coincide with that of its foundation, particularly in the case of tall buildings, which are flexible. Thus, the first steps in structural dynamics were made early on, particularly for one degree of freedom systems, that is, single-storey buildings simulated by a concentrated mass and connected to their base by an elastic spring and a damper.

The aim of these first steps was the determination of the time history of the mass motion in relation to its base for a given time history of motion at the base. The problem of the resonance of the vibrating mass was one of the basic points of interest, since such a resonance resulted in an amplification of the acceleration of the mass in relation to that of the base motion. These research activities were carried out for simple forms of vibration of the base, for example, sinusoidal or ramp excitations, since these were functions that could easily be integrated without computer facilities. In 1940, the first accelerograph (strong motion instrument) installed at El Centro in California was activated by a strong earthquake. This was the first time-history acceleration record (Fintel and Derecho, 1974) known as the El Centro earthquake record.

This was the first big step in earthquake engineering, since an objective method was invented for recording strong ground motions. It should be noted that the first measurement of the PGA of an earthquake had been preceded during the Long Beach earthquake in 1933. Nowadays, there are thousands of such instruments installed all over the world in seismically sensitive regions, enriching the collection of records of strong earthquakes. The next big step was made 15 years later (in about 1955) with the development of computational techniques for obtaining acceleration, velocity and displacement elastic response spectra for a given time-history acceleration record. The use of acceleration response spectra enabled the easy and direct determination of the maximum acceleration that develops in the mass of a building of a given fundamental period for a seismic spectrum under consideration. The above step must be attributed mainly to the introduction of computer facilities in
engineering (Housner et al., 1953).

کتاب ساختمان های بتنی در نواحی زلزله خیز

کتاب ساختمان های بتنی در نواحی زلزله خیز

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