Fragility curves for non-ductile reinforced concrete frames that exhibit different component response mechanisms

Around the world, a large percentage of buildings in regions of high seismicity are older, non-ductile reinforced concrete. To assess the risk posed by these buildings, fragility functions are required to define the likelihood that these buildings will sustain damage and collapse under earthquake loading. This paper presents the initial phase of a research effort to develop fragility functions for non-ductile concrete frames using numerical simulation; the research presented in this paper focuses on development of the numerical model and application of the model to develop fragility functions for a prototype non-ductile concrete frame. To enable numerical simulation of concrete frame buildings, response models for beam–column joints and columns are developed to provide (1) appropriate simulation of component response and, thereby, reliable assessment of risk and (2) computational efficiency and robustness. These new models are developed using existing experimental data, build on response models proposed by others, and employ component and material models available in the OpenSees analysis platform (http://opensees.berkeley.edu). A new beam–column joint model combines a new expression for joint strength and newly developed cyclic response parameters; a new column response model includes a new shear-strength model and newly developed cyclic response parameters. Numerical models of a prototype non-ductile concrete frame are developed that include simulation of one or more of the following characteristics: (1) rigid beam–column joint, (2) nonlinear joint shear response, (3) nonlinear joint shear and bond–slip response, and (4) column shear failure. Dynamic analyses are performed using these frame models and a suite of ground motions; analysis results are used to develop fragility curves. Fragility curves quantify the vulnerability of the frame and provide understanding of the impact of different component failure mechanisms on frame vulnerability.This research was supported by the National Science Foundation under NSF Grant # 1000700.This is the accepted manuscript of a paper published in Engineering Structures (J-S Jeon, LN Lowes, R DesRoches, I Brilakis, Engineering Structures 2015, 85, 127–143

Tension-only ideal dissipative bracing for the seismic retrofit of precast industrial buildings

3siopenNew precast frame industrial structures are seismically designed according to reliable modern criteria. However, most of the existing built stock hosting many workers and both regular and strategic industrial activities was designed and detailed neglecting the earthquake load or according to outdated seismic design criteria and regulations. Its seismic retrofit is a main challenge for the Engineering Community and a critical objective for institutional and private bodies. Among the envisaged solutions, the introduction of dissipative braces appears to be promising, although mostly inapplicable for these buildings, due to the brace lengths required by their typical large dimensions and the related proportioning against buckling. In this paper, an innovative seismic retrofitting technique based on monolateral dissipative bracing is investigated. The device proposed in this paper, yet in phase of preliminary design and testing, dissipates energy through friction in tension only while freely deforming in compression, which makes the issue related to compressive buckling irrelevant. A numerical analysis is carried out to investigate the efficiency of the proposed device in seismic retrofitting of precast industrial frame buildings with the aim to explore its feasibility and to better orient the definition of the slip threshold load range and the future development of the physical device. The simplified Capacity Spectrum Method (CSM) is employed for the global framing of the structural behaviour of the highly nonlinear retrofitted structures under seismic actions. A numerical tool is set to automatically apply the CSM based on the definition of few main parameters governing the seismic response of precast frame structures. The efficacy of the CSM is critically analysed through the comparison with the results of a set of nonlinear dynamic analyses. A smart simplified design process aimed at framing the most efficient threshold slip/yield load of the device given an existing structural configuration is presented with the application of the CSM through the identification of the most efficient performance indicator related to either displacement, shear force, equivalent dissipation of energy or a combination of them.openDal Lago B.; Naveed M.; Lamperti Tornaghi M.Dal Lago, B.; Naveed, M.; Lamperti Tornaghi, M

The Case of the New Tagus River Leziria Bridge

A brief description of the New Tagus River Leziria Bridge composed by 1695 m North Viaduct, by 970 m Main Bridge and by South Viaduct with a length of 9200 m is presented. The observed thickness of the foundation alluvia material varies between 35m and 55m with a maximum value of 62m. Hundred eighteen boreholes were performed with a depth between 21m and 71m and eight boreholes were performed from a maritime platform. Standard penetration tests (SPT) were carried out in all boreholes 1.5 m apart. In addition CPTu tests, seismic cone tests, crosshole and downhole tests were performed. In three boreholes continuous undisturbed sampling with a triple sampler Geogor S was performed. Related with static laboratory tests namely identification tests, triaxial tests, direct shear tests and oedometer tests were performed. In addition for the dynamic characterization reasonant columns tests and torsional cyclic tests were performed. One of the most important considerations for the designers is the risk of earthquakes since Lisbon was wiped out by an 8.5 Ritcher magnitude earthquake in 1755. The seismic studies related to the design spectra were performed. The liquefaction potential evaluation was performed only by field tests taking into account the disturbance that occurs during sampling of sandy materials. In this analysis attention was drawn for SPT and CPT tests as seismic tests have only been used when soil contains gravel particles. The shear stress values were computed from a total stresses model, that gave results on the conservative side using the code “SHAKE 2000”. For the North and South Viaducts 1.5 m diameter piles were used and for the Main Bridge 2.2 m diameter piles were used. For the construction of the piles metallic casings were driven by a vibrofonceur or a hydraulic hammer and the piles length varies between 20 m to 56 m. Static pile load tests (both vertical and horizontal tests) were carried out on trial piles. In addition pile dynamic tests were performed. The construction aspects related with piles and bridge construction are addressed. To assess the integrity of the piles reception tests by sonic diagraphies (crosshole tests) were performed. Some problems that have occurred during piles construction in the Main Bridge, due to the gravel and cobbles dimensions, are described. The bridge was monitored with the purposes of: (i) Validation of design criteria and calibration of mental model; (ii) Analysis of bridge behavior during his life; and (iii) Corrective measures for the rehabilitation of the structure

Evolution, Monitoring and Predicting Models of Rockburst: Precursor Information for Rock Failure

Load/unload response ratio predicting of rockburst; Three-dimensional reconstruction of fissured rock; Nonlinear dynamics evolution pattern of rock cracks; Bayesian model for predicting rockburs

Soil-Structure Modeling and Design Considerations for Offshore Wind Turbine Monopile Foundations

Offshore wind turbine (OWT) support structures account for 20-25% of the capital cost for offshore wind installations, making it essential to optimize the design of the tower, substructure, and foundation to the extent possible. This dissertation focuses on monopile foundations, as the vast majority (approximately 75%) of currently installed OWTs are supported by monopile structures. The objective of this dissertation is to provide information on the behavior of monopile support structures to better substantiate design and planning decisions and to provide a basis for reducing the structural material costs. In pursuit of these objectives, research is presented on the topics of hysteretic soil-structure damping (referred to as foundation damping), cyclic degradation of soil properties, and the impact of marine growth on OWT monopile support structures. OWTs are lightly damped structures that must withstand highly uncertain offshore wind and wave loads. In addition to stochastic load amplitudes, the dynamic behavior of OWTs must be designed with consideration of stochastic load frequency from waves and mechanical load frequencies associated with the spinning rotor during power production. The close proximity of the OWT natural frequency to excitation frequencies combined with light damping necessitates a thorough analysis of various sources of damping within the OWT system; of these sources of damping, least is known about the contributions of damping from soil-structure interaction (foundation damping), though researchers have back-calculated foundation damping from “rotor-stop” tests after estimating aerodynamic, hydrodynamic, and structural damping with numerical models. Because design guidelines do not currently recommend methods for determining foundation damping, it is typically neglected. The significance of foundation damping on monopile-supported OWTs subjected to extreme storm loading was investigated using a linear elastic two-dimensional finite element model. A simplified foundation model based on the soil-pile mudline stiffness matrix was used to represent the monopile, and hysteretic energy loss in the foundation was converted into a viscous, rotational dashpot at the mudline to represent foundation damping. The percent critical damping contributed to the OWT structural system by foundation damping was quantified using the logarithmic decrement method on a finite element free vibration time history, and stochastic time history analysis of extreme storm conditions indicated that mudline OWT foundation damping can significantly decrease the maximum and standard deviation of mudline moment. Further investigation of foundation damping on cyclic load demand for monopile-supported OWTs was performed considering the design situations of power production, emergency shutdown, and parked conditions. The NREL 5MW Reference Turbine was modeled using the aero-hydro-elastic software FAST and included linear mudline stiffness and damping matrices to take into account soil-structure interaction. Foundation damping was modeled using viscous rotational mudline dashpots which were calculated as a function of hysteretic energy loss, cyclic mudline rotation amplitude, and OWT natural frequency. Lateral monopile capacity can be significantly affected by cyclic loading, causing failure at cyclic load amplitudes lower than the failure load under monotonic loading. For monopiles in clay, undrained clay behavior under short-term cyclic soil-pile loading (e.g. extreme storm conditions) typically includes plastic soil deformation resulting from reductions in soil modulus and undrained shear strength which occur as a function of pore pressure build-up. These impacts affect the assessment of the ultimate and serviceability limit states of OWTs via natural frequency degradation and accumulated permanent rotation at the mudline, respectively. Novel combinations of existing p-y curve design methods were used to compare the impact of short-term cyclic loading on monopiles in soft, medium, and stiff clay. Marine growth increases mass and surface roughness for offshore structures, which can reduce natural frequency and increase hydrodynamic loads, and can also interfere with corrosion protection and fatigue inspections. Design standards and guidelines do not have a unified long-term approach for marine growth on OWTs, though taking into account added mass and increased drag is recommended. Some standards recommend inspection and cleaning of marine growth, but this would negate the artificial reef benefits which have been touted as a potential boon to the local marine habitat. The effects of marine growth on monopile-supported OWTs in terms of natural frequency and hydrodynamic loading are examined, and preliminary recommendations are given from the engineering perspective on the role of marine growth in OWT support structure design

Advances in Geotechnical Earthquake Engineering

This book sheds lights on recent advances in Geotechnical Earthquake Engineering with special emphasis on soil liquefaction, soil-structure interaction, seismic safety of dams and underground monuments, mitigation strategies against landslide and fire whirlwind resulting from earthquakes and vibration of a layered rotating plant and Bryan's effect. The book contains sixteen chapters covering several interesting research topics written by researchers and experts from several countries. The research reported in this book is useful to graduate students and researchers working in the fields of structural and earthquake engineering. The book will also be of considerable help to civil engineers working on construction and repair of engineering structures, such as buildings, roads, dams and monuments

Measuring residual strength of liquefied soil with the ring shear device

Natural and constructed slopes may contain zones of loose granular soils capable of liquefaction. Liquefied soils behave like heavy fluids and consequent rapid flowslides can produce great damage. The residual strength (Sur) of the liquefied soil can be estimated by back-calculation from field case histories; however, very little confirmation laboratory testing has been conducted thus far. A reliable laboratory measurement technique is needed to independently verify Sur values used for mitigation design. A ring shear device (RSD) designed and built at the University of New Hampshire (UNH) allows for residual strength testing under controlled strain rates and infinite total strain. The Sur of a fine sand, Ottawa F-75, was analyzed using the RSD. These results were verified by comparison to residual strength values obtained by geotechnical centrifuge testing. This study indicates that the UNH RSD can be a reliable tool for estimating the residual strength of liquefied soil

From plastic hinge to shell models: Recommendations for RC wall models

The severe damage and collapse of many reinforced concrete (RC) wall buildings in the recent earthquakes of Chile (2010) and New Zealand (2011) have shown that RC walls did not perform as well as expected based on the design calculations required by the modern codes of both countries. In this context, it seems appropriate to intensify research efforts in more accurate simulations of damage indicators, in particular local engineering demand parameters such as material strains, which are central to the application of performance-based earthquake engineering. Potential modelling improvements will necessarily build on a thorough assessment of the limitations of current state-of-the-practice simulation approaches. This work aims to compare the response variability given by a spectrum of numerical tools commonly used by researchers and specialized practitioners, namely: plastic hinge analyses, distributed plasticity models, and detailed finite element simulations. It is shown that a multi-level assessment—wherein both the global and local levels are jointly investigated from the response analysis outcomes—is fundamental to define the dependability of the results. The latter is controlled by the attainment of material strain limits and the occurrence of numerical problems. Finally, the influence of shear deformations is analysed according to the same methodological framework

Performance Based Seismic Assessment of Masonry Infilled Steel Frame Structures

Steel framed structures constitute a considerable proportion of residential and commercial structures in earthquake prone regions. In such structures, typically, masonry infills are implemented as walls and partitions. However, in common practice, the influence of the infill panels on the performance and resistance of the building is mostly ignored, not just at the design stage, but also during assessment. Despite the possible strength enhancement that infill panels can bring to the structure for modest earthquakes, they may put the building at high risk of heavy damage if their impact is overlooked, and the interaction not properly designed, as seen in the 2003 Bam earthquake and many other destructive seismic events. Following the performance-based seismic assessment methodology, the dissertation focuses on evaluating the seismic performance of existing masonry infilled steel frames. The seismic response of several building typologies, designed according to common practice, is assessed through nonlinear dynamic methods. Detailed three-dimensional numerical models of selected index buildings are developed, capable of simulating the impact of masonry infill walls along other critical elements such as the beam-column connections, according to available empirical and experimental data. In order to measure the seismic vulnerability, along with possible losses and life cycle costs, analytical fragility functions are derived for the structures, while considering the hazard characteristics of the location under study. The derived fragility functions will help enrich the limited library of existing function dedicated to both bare and infilled steel structures. The outcome is of great importance for insurance valuation, as well as managing disasters and performing strengthening if necessary

The Evolution of Geotechnical Earthquake Engineering Practice in North America: 1954-1994

This paper traces the evolution of geotechnical earthquake engineering practice in North America from 1954 to 1994. The development of the state-of-the-art has been shaped strongly by four areas of practice: assessment of seismic hazard, estimation of liquefaction potential, seismic response analysis of earth structures and seismic safety evaluation and remediation of existing dams with potentially liquefiable zones. Evolution of practice in each of these areas will be traced and the current state-of-the-art evaluated. Present capabilities in practice will be illustrated by examples from the areas of seismic response of dams, liquefaction potential and seismic safety evaluation and remediation of potentially liquefiable embankment dams