The propagation of fault-rupture towards a site at a velocity close to the shear wave velocity causes most of the seismic energy from the rupture to arrive in single/multiple large long-period pulse/s of motion that generally occurs at the beginning of the record. The radiation pattern of the shear dislocation on a fault causes this large pulse of motion to be oriented in the direction perpendicular to the fault, which is known as the directivity effect. This became evident from the ground records of numerous recent major earthquake events, which have caused extreme structural destructions in near-fault zones that are within 10 miles of the fault-rupture planes.
A methodology has been proposed to identify pulse-effects by the use of a discrete-time signal processing method in which a low-pass filter with a suitable cut-off frequency is applied to the Fourier transforms of the processed acceleration or velocity time history records. An important difference between the proposed approach and those of the other researchers is that no predetermined pulse shape is assumed in the analysis. The extracted pulses are then used to determine their effects on linearly-elastic or inelastic single degree-of-freedom (SDF) systems.
Quantitatively, these pulse-types are identified through a modified displacement response factor , Rd. A statistical study of the effects of dynamic magnification due to near-fault ground motions, using the modified displacement response factor, Rd, is presented. A new method is introduced to allow engineers to construct analogous displacement response spectra for an undamped linearly elastic SDF system subjected to long-period ground motions; these spectra can be constructed directly from the Fourier amplitudes of velocity time history.
Finally, this dissertation examines the displacement ductility requirements for a typical bridge bent subjected to such extreme loadings. The ductility requirement, as stated by prevailing design codes, may not be valid for such structures in near-fault zones. Due to the dominance of pulses, medium- to long-period structures are significantly affected, often resulting in high residual displacements (permanent deformations) after the cessation of seismic ground motions. A suitable balance between ductility and residual displacement should be a goal in the design of such structures.
|Advisor:||Lui, Eric M., Sangani, Ashok S.|
|School Location:||United States -- New York|
|Source:||DAI-B 71/11, Dissertation Abstracts International|
|Subjects:||Geotechnology, Geophysical, Civil engineering|
|Keywords:||Displacement ductility, Ground motions, Inelastic responses, Pulse effects, Seismic engineering|
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