A Comparative Earthquake Analysis of Structures By a Wave Propagation Method

Abstract eng:
The excitation and response of structures in epicentral regions of shallow focus earthquakes is quite complicated in which the horizontal and the vertical ground motions are almost simultaneous. In these cases the response of a structure cannot be simulated by a single time history that horizontally and uniformly strikes it. In the present communication it is tried to simulate the input motion by a transient ground motion, in the form of a wave that is traveling along the foundation of the structure with a certain group velocity. The ground particles perform oscillations in the vertical plane, that results from the convolution of horizontal and vertical components of a recorded ground motion. The motion, from the footings of the columns is propagated into the body of the structure as longitudinal and shear waves, producing displacements and member forces. This type of excitation is applied, with a variety, at a time, of group wave velocities 200 and 800 m/sec, predominant frequencies of particle oscillation 20, 5, 3, 2 Hz and a ratio of vertical to horizontal particle acceleration is equal to V/H=0.59, 2.0 and 3.0. The reference time history is that of the Cephalonia 2014 ground acceleration record, while the amplitudes and the predominant frequencies are scaled accordingly. For each one of these types of the horizontal only component time histories, the respective acceleration response spectra are defined and, for the structures mentioned below, the respective structural response is calculated. Each one of these corresponds to the respective reference structural response, and the displacements of various levels and member forces at selected critical cross sections are called “reference values”. In order to illustrate the effect of the wave type excitation and the resulting structural response, a set of two pairs (a three and an eight storied) of reinforced concrete plane frames are selected. The two pairs are identical to each other except for the clear beam span that in one set is 4.0 m and in the other is 8.0 m. In the same levels and cross sections, with those mentioned above, of the load bearing system, the resulting displacements and member forces are measured, that are called “test values”. The ratios between the “test” and the respective “reference values” are calculated and presented graphically in well understandable charts. From the comparison between “reference” and “test values” it is concluded that the said ratios may reach very high values.

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