This paper presents a conceptual discussion on structural response to ground shocks. Numerical parametric analyses are performed on a simplified linear structural model to investigate the special features of structural response brought by short duration, large amplitude and high frequency excitations, which are the basic characteristics of ground shocks induced by blasting. Nonlinear finite element analyses on a 2-storey RC frame subjected to ground shocks are carried out to qualitatively understand building response to blasting. This study shows that maximum structural response to blasting depends primarily on the amount of impulse, and it generally occurs after the major ground shock has ceased. To capture the maximum response, it is hence necessary to consider additional time duration beyond the major ground shock period in blasting analysis. It is found that the response in the forced-vibration phase includes high frequency vibration modes with small displacement but large acceleration, thus inducing high inertial shear force. However, the free-vibration response is dominated by lower frequency modes with larger displacement but smaller acceleration. Hence, buildings subjected to strong ground shocks might experience a sudden shear failure of its components. Nevertheless, if a building's strength is enough to avoid the sudden shear failure during the major shock, it may be damaged after the ground shock during the free vibration, and the extent of damage depends on the ground shock magnitude.Keywords: Blasting induced ground motion; impulse; inertial force; shear failure; structural response * Corresponding Author, Fax: (+65)-67910046; E-mail: cdhakal@ntu.edu.sg 1. INTRODUCTION Storing ordnances in the form of weapon, ammunition, and explosive is an integral part of the defence strategy of each country. Accidental blasting of such storages may cause significant damage to nearby structures. Hence, it is necessary to regulate the construction of residential structures in the vicinity of ammunition arsenals or underground explosive storage facilities. In other words, the closest permissible distance of residential buildings from such magazines, termed as the inhabited building distance (IBD), should be clearly manifested in the specifications. In general, the current practice is based on NATO regulations [1]. Equations proposed in these regulations to recommend IBD were based on analyses and tests conducted between the mid-1950s and the mid-1970s. Obviously, there are uncertainties in the present state-of-the-art, and further research in this field is necessary to identify the areas of technical uncertainties and to determine which of these could lead to significant economic paybacks when the degree of uncertainties is reduced. Due to space, costs and safety issues, extensive experimental investigation of structural response and damage due to blasting is usually not feasible. That is why only a few tests [2][3][4] have been conducted, and experimental data in this field are scarce. This leaves numeric...
SUMMARYAn analytical model for high damping elastomeric isolation bearings is presented in this paper. The model is used to describe mathematically the damping force and restoring force of the rubber material and bearing. Ten parameters to be identiÿed from cyclic loading tests are included in the model. The sensitivity of the ten parameters in a ecting the model is examined. These ten parameters are functions of a number of in uence factors on the elastomer such as the rubber compound, Mullins e ect, scragging e ect, frequency, temperature and axial load. In this study, however, only the Mullins e ect, scragging e ect, frequency and temperature are investigated. Both material tests and shaking table tests were performed to validate the proposed model. Based on the comparison between the experimental and the analytical results, it is found that the proposed analytical model is capable of predicting the shear force-displacement hysteresis very accurately for both rubber material and bearing under cyclic loading reversals. The seismic response time histories of the bearing can also be captured, using the proposed analytical model, with a practically acceptable precision.
In this study, signal processing approaches and nonlinear identification are used to measure seismic responses of reinforced concrete (RC) structures using the shaking table test. To analyze structural nonlinearity, an equivalent linear system with time-varying model parameters, singular spectrum analysis to elucidate residual deformation, and wavelet packet transformation analysis to yield the energy distribution among components are adopted to detect the nonlinearity. Then, damage feature extraction is conducted using both the Holder exponent and the Level-1 detail of the discrete wavelet component. Finally, the modified Bouc-Wen hysteretic model and the system identification process are employed to the shaking table test data to evaluate the physical parameters, including the stiffness degradation, the strength deterioration and the pinching hysteresis. Finally, the identified stiffness and strength degradation functions from the test data of RC frames in relation to the degree of ground shaking, damage index and the identified nonlinear features are discussed. Based on the proposed method, both signal-based and modelbased identifications, the relationship between the damage occurrence and severity of structural damage can be identified. Figure 9. (a) Comparison of Inter-story drift ratio, the Holder exponent, Level-1 detail component and residuals estimated from SSA with respect to time from response data of RCF6. Correlation of singularities is identified among different analyses; (b) comparison of Inter-story drift ratio, the Holder exponent, Level-1 detail component and residuals estimated from SSA with respect to time from response data of RCF2. Correlation of singularities is identified among different analyses; and (c) comparison of the Inter-story drift ratio, the Holder exponent, Level-1 detail component and residuals estimated from SSA with respect to time from response data of RCF4. Correlation of singularities is identified among different analyses.To quantify the damage to these four RC frames, the hysteretic model parameters were evaluated from the load-displacement data of the RC frame. The original rough data were not used; rather, the SSA method was applied to filter out the high-frequency components (>20.0 Hz) of the recorded
SUMMARYAlthough Singapore is located in a low-seismicity region, huge but infrequent Sumatran subduction earthquakes might pose structural problems to medium-and high-rise buildings in the city. Based on a series of ground motion simulations of potential earthquakes that may a ect Singapore, the 1833 Sumatran subduction earthquake (Mw = 9:0) has been identiÿed to be the worst-case scenario earthquake. Bedrock motions in Singapore due to the hypothesized earthquake are simulated using an extended re ectivity method, taking into account uncertainties in source rupture process. Random rupture models, considering the uncertainties in rupture directivity, slip distribution, presence of asperities, rupture velocity and dislocation rise time, are made based on a range of seismologically possible models. The simulated bedrock motions have a very long duration of about 250 s with a predominant period between 1.8 and 2:5 s, which coincides with the natural periods of medium-and high-rise buildings widely found in Singapore. The 90-percentile horizontal peak ground acceleration is estimated to be 33 gal and the 90-percentile horizontal spectral acceleration with 5% damping ratio is 100 gal within the predominant period range. The 90-percentile bedrock motion would generate base shear force higher than that required by the current design code, where seismic design has yet to be considered. This has not taken into account e ects of local soil response that might further amplify the bedrock motion.
SUMMARYA representative attenuation relationship is one of the key components required in seismic hazard assessment of a region of interest. Attenuation relationships for peak ground acceleration, peak ground velocity and response spectral accelerations for Sumatran megathrust earthquakes, covering M w up to 9.0, are derived based on synthetic seismograms obtained from a finite-fault kinematic model. The relationships derived are for very hard rock site condition and for a long-distance range between 200 and 1500 km. They are then validated with recorded data from giant earthquakes on the Sumatran megathrust occurring since year 2000. A close examination of the recorded data also shows that spectral shapes predicted by most of the existing attenuation relationships and that specified in the IBC code are not particularly suitable for sites where potential seismic hazard is dominated by large-magnitude, distant, earthquakes. Ground motions at a remote site are typically signified by the dominance of long-period components with periods longer than 1 s, whereas the predominant periods from most of the existing attenuation relationships and the IBC code are shorter than 0.6 s. The shifting of response spectrum towards longer period range for distant earthquakes should be carefully taken into account in the formulation of future seismic codes for Southeast Asia, where many metropolises are located far from active seismic sources. The attenuation relationship derived in the present study can properly reproduce the spectral shape from distant subduction earthquakes, and could hopefully give insights into the formulation of future seismic codes.
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