Evaluation of site amplification, structural dynamic characteristics, and structural vulnerability rating of the city of Aqaba

Abstract

The potential to destroy any structure during earthquakes depends on the structural design, vulnerability rating, percentage of structural elongation as compared with the natural quiet conditions, the dynamic characteristics of structure itself, as well as the dynamic characteristics of the site. Thus, factors related to the dynamic characteristics of structure and the dynamic characteristics of the site are necessary in the design of resistant structures against earthquakes. In this study, recordings of free oscillation were made directly on 18 selected structures in the urban city of Aqaba, which represent the structural culture of the city and the whole Jordan in general. Ground ambient vibration records were conducted at 18 nearby location at a few meters away from each structure. Records were conducted during the maximal cultural activities using three-component seismometer of 2 Hz free oscillation. Analytical results of records obtained on structures, has given the following equation: \( f = {18}/N \), where f, is the fundamental mode of structure and N, is the number of stories. All structural records were conducted at the top level of each structure except at Shmesani 4 and 7. Horizontal sensors were oriented with respect to the longitudinal and transverse direction of structural horizontal projection. Damping factors for each structural longitudinal and transverse fundamental mode were calculated based on the obtained FFT spectrums of each orientation using half-width band method. Analysis of obtained records on structures in Aqaba show that most of them are of short periodic structures except a few multi-story tall buildings. Most of the results obtained on structures refer to damping factors that range between 0.05 and 0.208. Mathematical relation derived from this study above, in addition to the actual calculated damping factor, refers to non-compliance with the standards of earthquake-resistant design. Results of ground records analysis indicate to the westward and north-westward decrease of dominant frequency relative to the eastern and southeastern part of the study area, where the crystalline granite bedrock is much shallower reflecting the actual condition of surface geology. Striking that the H/V amplification was relatively higher in the scope of areas where the thicknesses of soft surface deposits in the eastern and southeastern part are much shallower relative to the western and north-western part of the study area. This can be interpreted as a result of much higher impedance contrast in the eastern and southeastern part relative to the western and northwestern parts of the study area.