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A multi-span, curved, concrete box-girder bridge has been extensively instrumented by the California Strong Motion Instrumentation Program (CSMIP) in cooperation with the California Department of Transportation (Caltrans). On 28 June 1992, the bridge was shaken by the magnitude 7–5 Landers and magnitude 6–6 Big Bear earthquakes in southern California. The epicentres of these earthquakes were 50 and 29 miles (81 and 46 km) from the bridge, respectively. All 34 strong-motion sensors installed on the bridge recorded the response to these earthquakes and provided an insightful set of data. A striking aspect of the response is the presence of intermittent sharp spikes in nearly all of the acceleration records from sensors at the deck of the bridge. Among these the highest spike was 0.8g for the Landers and 1.0g for the Big Bear earthquake. The peak ground acceleration at the bridge site was only about 0.1g for both these earthquakes. With the aid of visual examination and simple analysis it is deduced that (1) the spikes were caused by forces generated at separation joints by impacts and stretching of the cable restrainers between adjacent bridge segments; (2) the forces of impacts and cable stretching are directly proportional to the size of the spikes and can be estimated by the use of a simple formula; and (3) the spikes travelled from their source to other locations on the bridge with the velocity of a compression wave propagating through concrete. 相似文献
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Robert G.Strom Renu Malhotra Zhi-Yong Xiao Takashi Ito Fumi Yoshida Lillian R Ostrach 《天文和天体物理学研究(英文版)》2015,(3):407-434
We review previously published and newly obtained crater size-frequency distributions in the inner solar system. These data indicate that the Moon and the terrestrial planets have been bombarded by two populations of objects. Population 1,dominating at early times, had nearly the same size distribution as the present-day asteroid belt, and produced heavily cratered surfaces with a complex, multi-sloped crater size-frequency distribution. Population 2, dominating since about 3.8–3.7 Gyr,had the same size distribution as near-Earth objects(NEOs) and a much lower impact flux, and produced a crater size distribution characterized by a differential –3single-slope power law in the crater diameter range 0.02 km to 100 km. Taken together with the results from a large body of work on age-dating of lunar and meteorite samples and theoretical work in solar system dynamics, a plausible interpretation of these data is as follows. The NEO population is the source of Population 2 and it has been in near-steady state over the past ~ 3.7–3.8 Gyr; these objects are derived from the main asteroid belt by size-dependent non-gravitational effects that favor the ejection of smaller asteroids. However, Population 1 was composed of main belt asteroids ejected from their source region in a size-independent manner, possibly by means of gravitational resonance sweeping during orbit migration of giant planets;this caused the so-called Late Heavy Bombardment(LHB). The LHB began some time before ~3.9 Gyr, peaked and declined rapidly over the next ~ 100 to 300 Myr,and possibly more slowly from about 3.8–3.7 Gyr to ~2 Gyr. A third crater population(Population S) consisted of secondary impact craters that can dominate the cratering record at small diameters. 相似文献
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Praveen K. Malhotra 《地震工程与结构动力学》1999,28(11):1309-1326
Ground motions affected by directivity focusing at near-field stations contain distinct pulses in acceleration, velocity, and displacement histories. For the same Peak Ground Acceleration (PGA) and duration of shaking, ground motions with directivity pulses can generate much higher base shears, inter-storey drifts, and roof displacements in high-rise buildings as compared to the 1940 El Centro ground motion which does not contain these pulses. Also, the ductility demand can be much higher and the effectiveness of supplemental damping lower for pulse-like ground motions. This paper presents a simple interpretation of the response characteristics of three recorded and one synthetic near-field ground motions. It is seen that for pulse-like ground motions—similar to any other ground motion—the Peak values of Ground Acceleration, Velocity, and Displacement (PGA, PGV and PGD) are the key response parameters. Near-field ground motions with directivity effects tend to have high PGV/PGA ratio, which dramatically influences their response characteristics. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
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Jatin Srivastava Dinesh Shukla Vishal Chand Ram Naraian Harish Chandra Anant R. Nautiyal 《洁净——土壤、空气、水》2010,38(8):771-774
The presence of arsenic (As) in water is of great public concern. Arsenic exists in three common valence states viz., As(0) metalloid arsenic, As(III) (arsenite) and As(V) (arsenate). Arsenite [As(III)] is the most toxic form among arsenicals which, predominates in anaerobic conditions, generally in flooded soils and in the water with high BOD. Experiments were conducted to investigate the effect of As(III) on the mycorrhization in vetiver (Vetiveria zizanioides (L.) Nash) grass in hydroponics. Studies showed significant alteration in the mycorrhizal colonization in the roots of vetiver exposed to higher concentrations of As(III) starting from 1.0, 2.0, 3.0, 4.0 to 5.0 mg/L prepared in 5% Hoagland nutrient solution without addition of phosphate ions. Considerable reduction in the mycorrhizal intensity (M) was observed in all the treatment sets as compared to the control suggesting a negative impact of the As(III) on the mycorrhizal association. Simultaneously, the study also showed that, As(III) is toxic to the vetiver plants having mycorrhizal association however plants with non‐mycorrhizal (cleansed) roots were found to be able to survive for a longer period exposed to As(III). 相似文献
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Praveen K. Malhotra 《地震工程与结构动力学》2002,31(7):1441-1457
The seismic response spectrum defines the amplitude of the load, but it does not specify the number of cycles for which the load must be resisted by the structure. The amplitude by itself is not sufficient to evaluate the seismic resistance of a structure, because the structure's strength, stiffness and energy‐dissipation capacity reduce with an increase in the number of load cycles. This paper presents a cyclic‐demand spectrum, which, in conjunction with the amplitude spectrum, provides a more complete definition of the seismic load, hence a way to consider the degradation in strength, stiffness and energy‐dissipation capacity in a rational manner. Similarly to three amplitude parameters (peak ground acceleration, peak ground velocity, and peak ground displacement), three cyclic‐demand parameters are introduced for stiff, moderately stiff, and flexible systems. A design example is presented to illustrate the use of the cyclic‐demand spectrum. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
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We investigate the survivability of Trojan-type companions of Neptune during primordial radial migration of the giant planets Jupiter, Saturn, Uranus, and Neptune. We adopt the usual planet migration model in which the migration speed decreases exponentially with a characteristic time scale τ (the e-folding time). We perform a series of numerical simulations, each involving the migrating giant planets plus ∼1000 test particle Neptune Trojans with initial distributions of orbital eccentricity, inclination, and libration amplitude similar to those of the known jovian Trojans asteroids. We analyze these simulations to measure the survivability of Neptune's Trojans as a function of migration rate. We find that orbital migration with the characteristic time scale τ=106 years allows about 35% of preexisting Neptune Trojans to survive to 5τ, by which time the giant planets have essentially reached their final orbits. In contrast, slower migration with τ=107 years yields only a ∼5% probability of Neptune Trojans surviving to a time of 5τ. Interestingly, we find that the loss of Neptune Trojans during planetary migration is not a random diffusion process. Rather, losses occur almost exclusively during discrete prolonged episodes when Trojan particles are swept by secondary resonances associated with mean-motion commensurabilities of Uranus with Neptune. These secondary resonances arise when the circulation frequencies, f, of critical arguments for Uranus-Neptune mean-motion near-resonances (e.g., fUN1:2, fUN4:7) are commensurate with harmonics of the libration frequency of the critical argument for the Neptune-Trojan 1:1 mean-motion resonance (fNT1:1). Trojans trapped in the secondary resonances typically have their libration amplitudes amplified until they escape the 1:1 resonance with Neptune. Trojans with large libration amplitudes are susceptible to loss during sweeping by numerous high-order secondary resonances (e.g., fUN1:2≈11fNT1:1). However, for the slower migration, with τ=107 years, even tightly bound Neptune Trojans with libration amplitudes below 10° can be lost when they become trapped in 1:3 or 1:2 secondary resonances between fUN1:2 and fNT1:1. With τ=107 years the 1:2 secondary resonance was responsible for the single greatest episode of loss, ejecting nearly 75% of existing Neptune Trojans. This episode occurred during the late stages of planetary migration when the remnant planetesimal disk would have been largely dissipated. We speculate that if the number of bodies liberated during this event was sufficiently high they could have caused a spike in the impact rate throughout the Solar System. 相似文献
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The Haumea family is currently the only identified collisional family in the Kuiper belt. We numerically simulate the long-term dynamical evolution of the family to estimate a lower limit of the family’s age and to assess how the population of the family and its dynamical clustering are preserved over Gyr timescales. We find that the family is not younger than 100 Myr, and its age is at least 1 Gyr with 95% confidence. We find that for initial velocity dispersions of 50–400 m s?1, approximately 20–45% of the family members are lost to close encounters with Neptune after 3.5 Gyr of orbital evolution. We apply these loss rates to two proposed models for the formation of the Haumea family, a graze-and-merge type collision between two similarly sized, differentiated KBOs or the collisional disruption of a satellite orbiting Haumea. For the graze-and-merge collision model, we calculate that >85% of the expected mass in surviving family members within 150 m s?1 of the collision has been identified, but that one to two times the mass of the known family members remains to be identified at larger velocities. For the satellite-break-up model, we estimate that the currently identified family members account for ~50% of the expected mass of the family. Taking observational incompleteness into account, the observed number of Haumea family members is consistent with either formation scenario at the 1σ level, however both models predict more objects at larger relative velocities (>150 m s?1) than have been identified. 相似文献