6,892 research outputs found
Post-Earthquake Assessment and Numerical Modeling of Freestanding Heritage Structures
Historic and heritage structures are particularly vulnerable to earthquakes, where damage or collapse can not only lead to loss of a structure but also the loss of irreplaceable heritage. Many heritage structures can be classified as freestanding (detached) structures, including unreinforced masonry walls, classical multi-drum columns, and statue-pedestal systems. However, the seismic response of freestanding structures (sliding, rocking, rock-slide, overturning) is poorly predicted by existing methods due to geometric non-linearities as well as sensitivity to interface conditions and modeling parameters. Previous studies have focused on analytical modeling of simplified systems and/or experimentation under controlled laboratory conditions. In contrast, this paper presents the post-earthquake assessment of multiple statue-pedestal systems following the 2014 South Napa earthquake. The objective is to examine the seismic response of these complex freestanding structural systems, under real-world conditions, to elucidate key characteristics of the response and evaluate the influence of both physical and modeling parameters. In this study, the responses of the selected statues from the Napa area are numerically simulated under original ground motion records. The complex geometries of the statues are represented using meshes generated from lidar-based point clouds obtained during post-earthquake reconnaissance. The responses of the statues are simulated using the Distinct Element Method (DEM) where the statue and pedestal have been modeled as rigid blocks with deformation concentrated at the joints (i.e. interface of statue and pedestal or pedestal and ground). The study analyzes the results of the numerical simulations in comparison to the observed physical response during the earthquake event. Results emphasize the significant impact of ground motion parameters (e.g. directionality), the presence of soil, and modeling parameters such as contact stiffness
Efficient Intensity Measures of Slide-Rocking Structures for Precariously Balanced Rocks
Precariously balanced rocks (PBRs) and other fragile geologic features are important in both the engineering and seismological communities since they are indicative of the maximum ground motion at a site over the rock’s lifetime. Precariously balanced rocks are individual or stacks of freestanding rocks that tend to respond in rigid body modes when subject to seismic excitation – namely, rocking, sliding, slide-rocking, and free-flight, which can lead to overturning. The seismic response of freestanding structures, such as PBRs, is known to be extremely sensitive to small changes in geometry, position, and earthquake excitation. As such, deterministic methods are limited in their application to PBRs and reliable probabilistic relationships are necessary. Previous probabilistic studies on freestanding structures and PBRs have focused on a single response mode, such as overturning, and utilized a single intensity measure, typically the peak ground acceleration. To this end, this paper aims to identify optimal ground motion intensity measures (IMs) that correlate well with the multiple possible rigid body modes including rocking and sliding. In this study, structural parameters, including geometry and friction, were varied to induce each of the fundamental rigid body modes. The response of each structure was simulated to 3750 analytical pulse motions through numerical integration of the slide-rocking equations of motion within MATLAB. The results of the simulations were statistically analyzed to determine the optimal IMs in terms of both sufficiency and efficiency. A set of viable IMs are presented that have more robust probabilistic relationships with the dynamic response of a sliding or rocking block compared to previous IM studies
Effects of Friction on the Dynamic Behavior of Flexible Rocking Bodies
Flexible rocking bodies are freestanding structures that are free to deform and rock, potentially overturning, when subject to ground excitation. Prior research on the seismic behavior of this type of structure typically assumed sufficient friction at the ground surface to avoid sliding motion. However, experimental observations showed that this assumption was often violated and structures demonstrated non-negligible sliding during their responses. The overarching goal of this study is to evaluate the impact of sliding on the overturning response of flexible rocking bodies. To this end, a two-dimensional analytical model was developed in a Lagrangian formulation, which is presented in this paper. This model was subjected to one-cycle sine pulses of varying amplitude and frequency for several levels of base friction to quantify the impact of sliding on overturning. In general, the results highlight that sliding behavior reduces the overturning demand, however motion-to-motion variability was observed
3-D Reconstructions and Numerical Simulations of Precarious Rocks in Southern California
Reliable estimates of seismic hazard are essential for the development of resilient communities; however, estimates of rare, yet high intensity earthquakes are highly uncertain due to a lack of observations and recordings. Lacking this data, seismic hazard analyses may be based on extrapolations from earthquakes with more moderate return periods, which can lead to physically unrealistic earthquake scenarios. However, the existence of certain precariously balanced rocks (PBRs) has been identified as an indicator of an upper bound ground motion, which precludes toppling of the balanced rock, over its lifetime. To this end, a survey of PBRs was conducted in proximity to the Elsinore fault east of San Diego, CA. Each identified PBR is modeled using point clouds derived from ground-based laser scanning and images from an unmanned aerial vehicle. The resultant geometric reconstructions are then used in a probabilistic overturning analysis and compared to the anticipated seismic hazard at the site. Accounting for an estimated age range and 50% probability of overturning for the PBRs, approximately half of the surveyed PBRs indicate a potential overestimation of seismic hazard at the site
Robust nonparametric detection of objects in noisy images
We propose a novel statistical hypothesis testing method for detection of
objects in noisy images. The method uses results from percolation theory and
random graph theory. We present an algorithm that allows to detect objects of
unknown shapes in the presence of nonparametric noise of unknown level and of
unknown distribution. No boundary shape constraints are imposed on the object,
only a weak bulk condition for the object's interior is required. The algorithm
has linear complexity and exponential accuracy and is appropriate for real-time
systems. In this paper, we develop further the mathematical formalism of our
method and explore important connections to the mathematical theory of
percolation and statistical physics. We prove results on consistency and
algorithmic complexity of our testing procedure. In addition, we address not
only an asymptotic behavior of the method, but also a finite sample performance
of our test.Comment: This paper initially appeared in 2010 as EURANDOM Report 2010-049.
Link to the abstract at EURANDOM repository:
http://www.eurandom.tue.nl/reports/2010/049-abstract.pdf Link to the paper at
EURANDOM repository: http://www.eurandom.tue.nl/reports/2010/049-report.pd
Damage Assessment of a Sixteen Story Building Following the 2017 Central Mexico Earthquake
The 2017 M7.1 Central Mexico Earthquake caused significant infrastructural damage in the Mexico City area. The earthquake contained a significant pulse in the long period, resulting in numerous buildings severely damaged or collapsed. This paper discusses a reinforced concrete building which was still partially occupied post-earthquake. The building’s interior walls were examined to have substantial damage, including some extensive cracking. In January 2018, the authors visited the structure and collected detailed assessment data. The data collection included ground-based lidar scans and recorded ambient vibrations of the damaged structure using accelerometers. Eleven scans were collected from the four exterior facades to create a three-dimensional point cloud of the building. The collected point cloud data were used to measure and quantify the permanent deformation of the structure at three corners as well as to generate depth maps of two parallel exterior walls. The measurements based on the lidar point cloud data are accurate with an error of 2 mm at 10 meters, enabling high resolute and accurate assessments. As for the accelerometers, one setup with sixty minutes of ambient vibrations data collection was performed. Twenty unidirectional accelerometers were installed on the basement, ground, second, fourth, eighth, tenth and roof floors at southwest and northeast corners to capture the torsional and translational acceleration structural response. The collected data can be used to perform system identification throughout operational modal analysis to demonstrate the dynamic and modal properties of the structures. Both the lidar and system identification sensing techniques provide essential input to establish and calibrate a detailed finite element model. The outputs are used to validate through the comparison of modal frequencies obtained in operational modal analysis method. Besides, the finite element model also provides a detailed response and insight to understand performance under future earthquakes
Commissioning of the CMS High Level Trigger
The CMS experiment will collect data from the proton-proton collisions
delivered by the Large Hadron Collider (LHC) at a centre-of-mass energy up to
14 TeV. The CMS trigger system is designed to cope with unprecedented
luminosities and LHC bunch-crossing rates up to 40 MHz. The unique CMS trigger
architecture only employs two trigger levels. The Level-1 trigger is
implemented using custom electronics, while the High Level Trigger (HLT) is
based on software algorithms running on a large cluster of commercial
processors, the Event Filter Farm. We present the major functionalities of the
CMS High Level Trigger system as of the starting of LHC beams operations in
September 2008. The validation of the HLT system in the online environment with
Monte Carlo simulated data and its commissioning during cosmic rays data taking
campaigns are discussed in detail. We conclude with the description of the HLT
operations with the first circulating LHC beams before the incident occurred
the 19th September 2008
The Apollo ATCA Platform
We have developed a novel and generic open-source platform - Apollo - which
simplifies the design of custom Advanced Telecommunications Computing
Architecture (ATCA) blades by factoring the design into generic infrastructure
and application-specific parts. The Apollo "Service Module" provides the
required ATCA Intelligent Platform Management Controller, power entry and
conditioning, a powerful system-on-module (SoM) computer, and flexible clock
and communications infrastructure. The Apollo "Command Module" is customized
for each application and typically includes two large field-programmable gate
arrays, several hundred optical fiber interfaces operating at speeds up to 28
Gbps, memories, and other supporting infrastructure. The command and service
module boards can be operated together or independently on the bench without
need for an ATCA shelf.Comment: Submitted to the Proceedings for TWEPP 201
Electron Antineutrino Search at the Sudbury Neutrino Observatory
Upper limits on the \nuebar flux at the Sudbury Neutrino Observatory have
been set based on the \nuebar charged-current reaction on deuterium. The
reaction produces a positron and two neutrons in coincidence. This distinctive
signature allows a search with very low background for \nuebar's from the Sun
and other potential sources. Both differential and integral limits on the
\nuebar flux have been placed in the energy range from 4 -- 14.8 MeV. For an
energy-independent \nu_e --> \nuebar conversion mechanism, the integral limit
on the flux of solar \nuebar's in the energy range from 4 -- 14.8 MeV is found
to be \Phi_\nuebar <= 3.4 x 10^4 cm^{-2} s^{-1} (90% C.L.), which corresponds
to 0.81% of the standard solar model 8B \nu_e flux of 5.05 x 10^6 cm^{-2}
s^{-1}, and is consistent with the more sensitive limit from KamLAND in the 8.3
-- 14.8 MeV range of 3.7 x 10^2 cm^{-2} s^{-1} (90% C.L.). In the energy range
from 4 -- 8 MeV, a search for \nuebar's is conducted using coincidences in
which only the two neutrons are detected. Assuming a \nuebar spectrum for the
neutron induced fission of naturally occurring elements, a flux limit of
Phi_\nuebar <= 2.0 x 10^6 cm^{-2} s^{-1}(90% C.L.) is obtained.Comment: submitted to Phys. Rev.
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