Selection of the Most Unfavorable Real Ground Motions for Low- and Mid-rise RC Frame Structures

The goal of this article is to select those real (or recorded) ground motions capable of exposing the low- and mid-rise reinforced concrete frame structures to an extreme limit state. By performing correlation analyses, two optimal intensity measures are first selected to represent the ground motion damage potential. Then based on each record's damage potential, four subsets of strong ground motions, referred to as the most unfavorable ground motions, are identified and preliminarily confirmed to be applicable to the low- and mid-rise RC frame structures.     

Effects of Near-Fault Pulses on Nonlinear Soil-Structure Systems

ABSTRACT

This paper is focused on effects of near-fault pulse characteristics on seismic performance of soil-structure systems considering foundation uplifting and soil yielding. To this end, an extensive parametric study is conducted. Mid-to-high-rise buildings of different aspect ratios (SR) resting on shallow mat foundations are investigated. Different vertical load-bearing safety factors (F_S) of foundation as well as different soil types (i.e. soft to very dense) are considered in this study. Finite element method is used for numerical modeling. The underlying soil is simply modeled with a set of nonlinear springs and dashpots beneath the foundation. Mathematical near-fault pulse models of fling step and forward directivity are used as input ground motions. The results show that reduction in structural drift demands due to nonlinear soil-structure interaction (SSI) is more considerable in the case of short-period pulses compared to long-period ones. In more precise words, significant reduction occurs when pulse-to-fixed-base period ratio falls within 0.7–1.5 in the case of directivity pulses and 0.5–1.4 in the case of fling pulses. It is also demonstrated that the beneficial effects of nonlinear SSI reduce when the number of stories increases.     

Estimation of the Displacement of Viscously Damped Pinching Hysteretic Structures Subjected to Near-fault Ground Motions

To fulfill a displacement-based design or response prediction for nonlinear structures, the concept of equivalent linearization is usually applied, and the key issue is to derive the equivalent parameters considering the characteristics of hysteretic model, ductility level, and input ground motions. Pinching hysteretic structures subjected to dynamic loading exhibit hysteresis with degraded stiffness and strength and thus reduced energy dissipation. In case of excitation of near-fault earthquake ground motions, the energy dissipation is further limited due to the short duration of vibration. In order to improve the energy dissipation capability, viscous-type dampers have been advantageously incorporated into these types of structures. Against the viscously damped pinching hysteretic structure under the excitation of near-fault ground motions, this study aims to develop a seismic response estimation method using an equivalent linearization technique. The energy dissipation of various hysteretic cycles, including stationary hysteretic cycle, amplitude expansion cycle, and amplitude reduction cycle, is investigated, and empirical formulas for the equivalent damping ratio is proposed. A damping modification factor that accounts for the near-fault effect is introduced and expanded to ensure its applicability to structures with damping ratios less than 5%. An approach for estimating the maximum displacement of a viscously damped pinching hysteretic structure, in which the pinching hysteretic effect of a structure and the near-fault effect of ground motions are considered, is developed. A time history analysis of an extensive range of structural parameters is performed. The results confirm that the proposed approach can be applied to estimate the maximum displacement of a viscously damped pinching hysteretic structure that is subjected to near-fault ground motions.     

Floor Spectra of Inelastic RC Frame Buildings Considering Ground Motion Characteristics

A range of reinforced concrete frame buildings with different levels of inelasticity as well as periods of vibration is analyzed to study the floor response. The derived floor acceleration response spectra are normalized by peak ground acceleration, peak floor acceleration, and ground response spectrum. The normalization with respect to ground response spectrum leads to the lowest coefficients of variation. Based on this observation as well as previous studies, an amplification function is proposed that can be used to develop design floor spectra from the ground motion spectrum, considering the building’s dynamic characteristics and level of inelasticity.     

Practical Application of Digital Photogrammetry for Fieldwork in the American Midwest: An Example from the Middle Ohio Valley

The use of close-range digital photogrammetry for field documentation has been steadily increasing in the past half decade in several parts of the world. However, this technology has not been widely utilized in archaeological contexts in the American Midwest. We explore the utility of close-range photogrammetry in this region with examples from the Guard site (12D29), a Fort Ancient village located in southeastern Indiana. This article outlines the methods utilized for production of georeferenced 3-D models of several units excavated during the 2016 field season. These models as well as plan and profile orthophotos derived from them act as important supplements to standard photographs and drawings made in the field and easily integrate with the site GIS. Overall, we found close-range digital photogrammetry to be very useful to better document excavation details, doing so for limited cost and time expenditure.     

An approach to mapping buried prehistoric palaeosols of the Atlantic seaboard in Northwest Europe using GPR, geoarchaeology and GIS and the implications for heritage management

Widespread layers of well-preserved organically and archaeologically rich palaeosols dating to the Bronze Age and Iron Age are known from across the Atlantic seaboard of Scotland, including the Western Isles, Orkney and Shetland. The survival of these soils has been facilitated by an overburden of Aeolian sand forming dunes above them. Whilst these soils display archaeological importance, they are constantly under threat from erosion through storm damage due to their exposed coastal position. This presents certain challenges with regard to heritage management, since their extents are largely unknown, normally only being identified when exposed by wind and storm erosion. Consequently, following such damage mitigation can only be reactionary in response to archaeological deposits being uncovered, and left open to threat, Without accurately mapping the extent of these important deposits, archaeological landscape management is compromised.This paper presents a case study aimed at enabling proactive management of these deposits through rapid three-dimensional mapping undertaken using a combination of borehole-calibrated GPR survey and GIS modelling. The results demonstrate how effective baseline data can be generated to highlight areas of greater or lesser risk thereby providing the potential for quantifying and predicting the effects of damage from future storm events.     

Improving the interpretability of 3D GPR data using target–specific attributes: application to tomb detection

Three–dimensional (3D) ground–penetrating radar (GPR) represents an efficient high–resolution geophysical surveying method allowing to explore archaeological sites in a non–destructive manner. To effectively analyze large 3D GPR data sets, their combination with modern visualization techniques (e.g., 3D isoamplitude displays) has been acknowledged to facilitate interpretation beyond classical time–slice analysis. In this study, we focus on the application of data attributes (namely energy, coherency, and similarity), originally developed for petroleum reservoir related problems addressed by reflection seismology, to emphasize temporal and spatial variations within GPR data cubes. Based on two case studies, we illustrate the potential of such attribute based analyses towards a more comprehensive 3D GPR data interpretation.     

An Improved Algorithm for Selecting Ground Motions to Match a Conditional Spectrum

This paper describes an algorithm to efficiently select ground motions from a database while matching a target mean, variance, and correlations of response spectral values at a range of periods. The approach improves an earlier algorithm by Jayaram et al. [2011]. Key steps in the process are to screen a ground motion database for suitable motions, statistically simulate response spectra from a target distribution, find motions whose spectra match each statistically simulated response spectrum, and then perform an optimization to further improve the consistency of the selected motions with the target distribution. These steps are discussed in detail, and the computational expense of the algorithm is evaluated. A brief example selection exercise is performed, to illustrate the type of results that can be obtained. Source code for the algorithm has been provided, along with metadata for several popular databases of recorded and simulated ground motions, which should facilitate a variety of exploratory and research studies.     

MOUND BUILDING AT LAKE JACKSON (8LE1), TALLAHASSEE,FLORIDA: NEW INSIGHTS FROM GROUND PENETRATING RADAR

Abstract

The Lake Jackson Mounds site (8LE1), located near Tallahassee, Florida, has long been considered to be a frontier Mississippian center. This assertion is primarily based on elaborate burial goods recovered during salvage excavations in the 1970s. Ground penetrating radar (GPR) on the two largest intact mounds at Lake Jackson revealed new information about their morphology and construction histories. These findings demonstrate that mound-building practices at the site were distinct from earlier, local Woodland mound-building traditions, and more similar to those of other Mississippian centers, such as Etowah and Moundville. Lake Jackson revitalized mound building in the Tallahassee area under the influence of external connections with groups in the Mississippian interaction network. These findings show how mound building was an integral practice for expressing and expanding Mississippian ideologies and rituals. This work also shows the utility of GPR in exploring mounds' morphologies and construction histories.     

SIMULATION OF THE FOURIER PHASE SPECTRUM FOR THE GENERATION OF SYNTHETIC ACCELEROGRAMS

This study presents the application of a new method for generating synthetic accelerograms based on statistical distributions for Fourier phase differences and Fourier amplitudes as functions of earthquake magnitude, hypocentral distance and site geology. Two important characteristics of the methodology are that it requires a small number of input parameters and that ground motion time histories can be simulated without any specific modulation function. Two areas with different tectonic patterns (North-Eastern and Central Italy) were selected for the application. The results of our analysis are reliable in the case of Central Italy because the data set is large and quite uniformly distributed, while for North-Eastern Italy our results should not be used for distances greater than 30 km.