Smooth Acceleration Spectra for Pulse-Like Ground Motions

ABSTRACT Idealization of acceleration response spectra is the basis for construction of target spectra for seismic design and assessment of structures. The adequacy of current methods to reasonably idealize (or smooth) the acceleration spectra of pulse-like and nonpulse-like ground motions is examined in this study. The influence of separated pulses on different regions of acceleration response spectrum is first investigated using wavelet transform. One representative method is selected as the benchmark to examine the effectiveness of the Newmark–Hall-based methods to smooth the acceleration spectra of pulse-like and nonpulse-like ground motions. Presented are some important insights into why the plateau (or amplification factor) associated with the constant-acceleration branch may be underestimated and the ending cutoff period Tg be overestimated by Newmark–Hall-based methods. This study highlights the intrinsic characteristics and the importance of the constant-acceleration branch, based on which a two-step procedure is proposed to idealize the acceleration spectra. The results show that the proposed methodology can accurately identify the constant-acceleration branch regardless of the influence of pulses on the descending branch of acceleration spectra.

The Characteristics of Seismic Response on Hard Interlayer Sites

Based on the engineering geological data of a nuclear power plant site, nine engineering geological profiles were created with hard interlayers of different thicknesses. The equivalent linearization method of seismic motion segment-input used for one-dimensional nonlinear seismic response analysis was applied to study the effect of the interlayer thickness on the peak acceleration and the acceleration response spectra of the site seismic response. The results showed that there was an obvious influence of hard interlayer thickness on site seismic responses. With the increase of hard interlayer thickness, the site nonlinear effect on seismic responses decreased. Under the same thickness of the hard interlayer, the nonlinear effect of the site was strengthened with the higher input peak acceleration. In addition, the short-period acceleration response spectrum was found to be significantly influenced by the hard interlayer and showed that the longer the period, the less influence of the hard interlayer on the acceleration response spectrum coordinates. Moreover, the influenced frequency band was wider with the increase in the thickness of hard interlayer.

ACCELERATION RESPONSE SPECTRUM AND EFFECTIVE DURATION OF LEBAK EARTHQUAKE JANUARY 23, 2018 IN JAKARTA REGION

Jakarta is the capital city of Indonesia with a very high population density low to the upper distribution of buildings which causes many areas of Jakarta to have a high vulnerability to natural disasters, one of which is an earthquake. One of the earthquakes felt this year was Lebak Banten Earthquake January 23, 2018, with a magnitude 6.1 at 13.34 local time. The depth of the earthquake was 61 Km at -7.09o S - 106.03o E, in the South Indian Ocean of Java Island. The epicenter was 43 km from Cilangkahan Village, Malingping Sub-district, Lebak District, Banten. We analyze the ground motion characteristics in the Jakarta area using 3 component acceleration data at Jakarta City Hall Office (JAKO) and Tanjung Priok Maritime Station (JATA) with dynamic statistical analysis method. The effective duration and earthquake response spectrum are determined using the acceleration data. In this study, the active period of the earthquake was calculated using the Trifunac and Brady method. The results of the analysis show that the most significant acceleration spectrum at JAKO station is found in component E of 0.07742 g in the period of 0.54 s while for the most significant acceleration spectrum JATA station found in component N of 0.04572 g in the period of 0.58 s. The effective duration obtained at JAKO stations was 56.76 s and JATA station 63.47 s. These results indicate that in the case of the Lebak earthquake, the further region from the epicenter of the earthquake has an effective duration which is relatively longer.

Dynamic response of a structure due to different vibration sources

The aim of the work is to acquire the answer for a question if, and to what extent the dynamic excitation forced by underground movement of metro carts or another vibration source impacts the condition of the existing building. To get that assessment, a corresponding numerical model of the tunnel and soil and the structure is created. As input, the acceleration response spectrum computed for the Gulf of Aquaba earthquake is used, as well as the quasi-static axle loading of typical metro carts. The output is shown as the time-history response of the ground-level nodes that will later be used to assess the influence of vibrations with the help of Ciesielski (SWD) scales.

Pump Tower Loads in Spherical Tanks Onboard LNG Carriers Scaled by Characteristic Vessel Response

Fatigue of the pump tower structure is an important design aspect for spherical cargo tanks onboard liquid natural gas carriers. Current state-of-the art methodology for assessing these loads is based on a series of model tests carried out about 10 and 40 years ago. The last campaign recorded tower loads for several tank filling ratios, vessel headings and sea states. Long-term distributions of loads were established, and this required the development of a scaling methodology to estimate short-term load distributions for sea states outside the range of tested conditions. However, the scaling approach proved to be inadequate when put into use in projects. The present paper proposes a new and enhanced scaling method for short-term distribution of loads on the pump tower involving re-assessment of the model test results combined with new statistical treatment. The dynamic loads on the pump tower causing fatigue damage can be separated into loads due to sloshing and inertial loads. The sloshing loads are seen to correlate well with the amount of energy in the acceleration response spectrum of the tank in a frequency range in vicinity of the lowest natural sloshing frequency. A characteristic response variable is defined as the integrated energy in the acceleration response spectrum over one half octave band around the sloshing resonance frequency, and a functional relationship between the loads on the pump tower and this parameter is established. The results show that this model describes the observed pump tower loads well, especially for filling heights 30 to 70%. Some more scatter is observed for very low or high filling levels, but as these cause lower loads this is not of concern. The underlying model tests primarily focused on high sea states in order to reflect global trade. The proposed scaling methodology allows for robust extrapolation to lower sea states in a more accurate way than was previously possible. This is particularly useful to avoid overly conservative fatigue-load estimates for other applications such as FSRUs and FLNGs in more benign conditions.

A Metric to Screen Acceptable Velocity and Displacement Time Histories of Modified Ground Motions

Ground motion modification is extensively used in practice to modify a seed acceleration time history in intensity and frequency content until its acceleration response spectrum matches a target spectrum. However, the decision to accept or reject a modified motion commonly relies on a subjective process where the time histories of the modified motion are visually compared to those of the seed motion. Various metrics were used to quantify the similarity between the modified time histories and their scaled counterparts for hundreds of modified ground motions from three different earthquake scenarios. Of the metrics considered, the inverse modified RMSE metric for time histories ( imRMSE t) was found most appropriate as it resulted in the least amount of dispersion in the goodness-of-fit values with respect to spectral mismatch. The imRMSE t was then found to be correlated to qualitative rankings assigned to the modified time histories through a visual assessment. The correlation between the quantitative imRMSE t values and qualitative rankings is used to establish threshold values to screen modified velocity and displacement time histories that are likely acceptable or likely unacceptable.

Bi-Directional Pseudo-Acceleration Response Spectra

In seismic design, excitations are usually considered separately in two perpendicular directions of structures. In fact, the two components of ground motions occur simultaneously. This paper clarifies the effects of bi-directional excitations on structures and proposes the response spectra called “bi-directional pseudo-acceleration response spectra”. A simplified analytical model of a two-degree-of-freedom system was employed. The effect of directivity of ground motions was taken into account by applying strong motion records in all directions. The analytical results were presented in the form of the acceleration ratio response spectrum defined as the bi-directional pseudo-acceleration response spectrum normalized by a pseudo-acceleration response spectrum.