Author: Dr Noel O’Dowd | Size: 3.7 MB | Format:PDF | Quality:Original preprint | Publisher: Imperial College London | Year: 2002 | pages: 107
Fracture mechanics concerns the design and analysis of structures which contain cracks or flaws. On some size-scale all materials contain flaws either microscopic, due to cracked inclusions, debonded fibres etc., or macroscopic, due to corrosion, fatigue, welding flaws etc. Thus fracture mechanics is involved in any detailed design or safety assessment of a structure. As cracks can grow during service due to e.g. fatigue, fracture mechanics assessments are required throughout the life of a structure or component, not just at start of life. Fracture mechanics answers the questions: What is the largest sized crack that a structure can contain or the largest load the structure can bear for failure to be avoided? How long before a crack which was safe becomes unsafe? What material should be used in a certain application to ensure safety?
Studies in the US in the 1970s by the US National Bureau of Standards estimated that “cost of fracture” due to accidents, overdesign of structures, inspection costs, repair and replacement was on the order of 120 billion dollars a year. While fracture cannot of course be avoided, they estimated that, if best fracture control technology at the time was applied, 35 billion dollars could be saved annually. This indicates the importance of fracture mechanics to modern industrialised society.
The topics of linear elastic fracture mechanics, elastic-plastic fracture mechanics and high temperature fracture mechanics (creep crack growth) are dealt with in this course. The energy release rate method of characterising fracture is introduced and the K and HRR fields which characterise the crack tip fields under elastic and plastic/creep fracture respectively are derived. The principal mechanisms of fracture which control failure in the different regimes are also discussed. In the later part of the course, the application of these fracture mechanics principles in the assessment of the safety of components or structures with flaws through the use of standardised procedures is discussed.
The approach taken in this course is somewhat different from that in Fundamentals of Fracture Mechanics (FFM) as here more emphasis is put on the mechanics involved and outlines of mathematical proofs of some of the fundamental fracture mechanics relationships are provided. There is some revision of the topics covered in FFM, particularly in the area of linear elastic fracture mechanics though the approach is a little different.
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Flexibility in Engineering Design (Engineering Systems)
Author: Richard de Neufville, Stefan Scholtes, | Size: 2.8 MB | Format:PDF | Quality:Unspecified | Publisher: The MIT Press | Year: 2011 | pages: 311 | ISBN: 9780262016230
Project teams can improve results by recognizing that the future is inevitably uncertain and that by creating flexible designs they can adapt to eventualities. This approach enables them to take advantage of new opportunities and avoid harmful losses. Designers of complex, long-lasting projects--such as communication networks, power plants, or hospitals--must learn to abandon fixed specifications and narrow forecasts. They need to avoid the "flaw of averages," the conceptual pitfall that traps so many designs in underperformance. Failure to allow for changing circumstances risks leaving significant value untapped. This book is a guide for creating and implementing value-enhancing flexibility in design. It will be an essential resource for all participants in the development and operation of technological systems: designers, managers, financial analysts, investors, regulators, and academics. The book provides a high-level overview of why flexibility in design is needed to deliver significantly increased value. It describes in detail methods to identify, select, and implement useful flexibility. The book is unique in that it explicitly recognizes that future outcomes are uncertain. It thus presents forecasting, analysis, and evaluation tools especially suited to this reality. Appendixes provide expanded explanations of concepts and analytic tools.
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Funding for Phases I and II of the SAC Steel Program to Reduce the Earthquake Hazards of Steel Moment-Frame Structures was principally provided by the Federal Emergency Management Agency, with ten percent of the Phase I program funded by the State of California, Office of Emergency Services. Substantial additional support, in the form of donated materials, services, and data has been provided by a number of individual consulting engineers, inspectors, researchers, fabricators, materials suppliers and industry groups. Special efforts have been made to maintain a liaison with the engineering profession, researchers, the steel industry, fabricators, code-writing organizations and model code groups, building officials, insurance and risk-management groups, and federal and state agencies active in earthquake hazard mitigation efforts. SAC wishes to acknowledge the support and participation of each of the above groups, organizations and individuals. In particular, we wish to acknowledge the contributions provided by the American Institute of Steel Construction, the Lincoln Electric Company, the National Institute of Standards and Technology, the National Science Foundation, and the Structural Shape Producers Council. SAC also takes this opportunity to acknowledge the efforts of the project participants - the managers, investigators, writers, and editorial and production staff - whose work has contributed to the development of these documents. Finally, SAC extends special acknowledgement to Mr. Michael Mahoney, FEMA Project Officer, and Dr. Robert Hanson, FEMA Technical Advisor, for their continued support and contribution to the success of this effort.
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This book offers a broad overview of the potential of continuum mechanics to describe a wide range of macroscopic phenomena in real-world problems. Building on the fundamentals presented in the authors’ previous book, Continuum Mechanics using Mathematica®, this new work explores interesting models of continuum mechanics, with an emphasis on exploring the flexibility of their applications in a wide variety of fields.
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Author: Carl H. Popelar, Melvin F. Kanninen | Size: 7 MB | Format:DjVu | Quality:Unspecified | Publisher: Oxford University Press, USA | Year: 1985 | pages: 578 | ISBN: 9780195035322
This book presents an extensive, unified, and up-to-date approach to the still developing subject of fracture mechanics from an applied mechanics perspective. Progressing from the simple to the more advanced topics, it goes beyond the well developed area of linear elastic fracture mechanics to consider the dynamic and elastic-plastic regimes, and in doing so, extends the subject into a broader range of realistic engineering applications.
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The precise characterization of ground motions incorporating site, source, distance and other effects and the accurate prediction of seismic demands at the component and system level are essential requisites for advancing performance-based design and evaluation methodologies. This research effort focuses on issues related to ground motion characteristics with particular emphasis on near-fault records and its interrelationship with seismic demand and ultimately in developing enhanced procedures for estimating deformation demands in structures for performance-based evaluation. Recent earthquakes have revealed an enhanced level of hazard imposed by ground motions recorded in the vicinity of causative faults associated with directivity effects. Both forward-rupture directivity and fling effects produce ground motions characterized by a strong pulse or series of pulses of long period motions. To highlight their potential damaging effects on building structures, the energy content of near-fault records were investigated by devoting special attention to forward-rupture directivity and fling effects and the influence of apparent acceleration pulses. A new demand measure called the effective cyclic energy (ECE) is developed and defined as the peak-to-peak energy demand imparted to structural systems over an effective duration that is equivalent to the time required for reversal of the system effective velocity. This energy term led to the evolution of a non-dimensional response index ( y cff ) as a new descriptor to quantify the destructive power of near-fault records. Based on validation studies conducted on numerous instrumented buildings, the ECE spectrum is proposed to estimate the input energy demand of multi-degree-of-freedom (MDOF) systems without performing nonlinear-time-history (NTH) analysis. In the final phase of the study, a new pushover analysis methodology derived from adaptive modal combinations (AMC) is developed to predict seismic demands in buildings. This procedure integrates concepts built into the capacity spectrum method recommended in ATC-40 (1996), the adaptive method originally proposed by Gupta and Kunnath (2000) and the modal pushover analysis advocated by Chopra and Gael (2002). A novel feature of the procedure is that the target displacement is estimated and updated dynamically during the analysis by incorporating energy based modal capacity curves in conjunction with constant-ductility demand spectra. Hence it eliminates the need to approximate the target displacement prior to commencing the pushover analysis. The methodology was applied to several vertically regular instrumented steel and reinforced concrete (RC) moment-frame buildings, and also validated for code-compliant vertically irregular steel and RC moment frame buildings. The comprehensive evaluation study including individual and statistical comparisons with benchmark responses obtained from NTH analyses demonstrate that the AMC procedure can reasonably estimate key demand parameters such as roof displacement, interstory drift, plastic rotations for both far-fault and near-fault records, and consequently provides a direct reliable tool for performance assessment of building structures.
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Design of a high-rise office building~ like any engineering design. is a complex
multidisciplinary process with the objective to discover~ detail and construct a system to
fulfill a given set of performance requirements. The success of this process is highly
dependent upon the cooperation taking place between the members of the design team.
Although present-day engineering computer technology allows for precise analysis and
design of the different subsystems of an high-rise building. it does not readily provide
insight for choosing among alternatives of these subsystems to arrive at the best overall
design.
This research study presents a computer-based computational method for optimal
cost-revenue conceptual design of high-rise office buildings. Specifically, a Multicriteria
Genetic Algorithm (MGA) is applied to conduct Pareto optimization that
minimi~es capital and operating costs and maximizes income revenue for a given
building project, subject to design constraints imposed by building codes and fabrication
requirements.
The conceptual design process involves the coordinated application of
approximate analysis, design and optimization. To commence the design process, a
population of different alternative designs are generated. Using approximate analysis and
design based on pre-developed data bases, the values of the conflicting cost-revenue
objective criteria are established for each design. Then, a MGA is used to explore the
design space and find improved designs having enhanced values of the objective criteria.
The results, for a given building project, is a set of Pareto-optimal conceptual designs that define the "trade-off' relationships between the three competing objective criteria to
minimize capital cost, minimize operating cost and maximize income revenue. The
corresponding three-dimensional criteria space is populated by feasible conceptual
designs that are 'equal-rank optimal' in the sense that each design is not dominated for all
three objective criteria by any other feasible design possible for the building. Life-cycle
costing is introduced to investigate the profit potential of building designs over time. The
conceptual design of four example office buildings are conducted from a variety of
viewpoints to illustrate the capability of the computational procedure to create
comprehensive computer-generated colour graphic representations of optimal costrevenue
trade-off relationships for office buildings taking into account architecturat
structural, mechanical and electrical systems. While this study focuses on office
buildings and corresponding cost-revenue criteria, the proposed computer method for
conceptual design is directly applicable to any type of artifact and related objective
criteria.
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Linear and Non- Linear Flexural Stiffness Models for Conc Walls in High-Rise Buildings
Author: Ahmad M. M. Ibrahim | Size: 5.76 MB | Format:PDF | Quality:Unspecified | Publisher: Ahmad M. M. Ibrahim | Year: 2000 | pages: 167
In the seismic design of high-rise wall buildings;, the fundamental period of the building and the building drift are usually determined using linear elastic dynamic analysis. To carry out this analysis, designers need to assume a linear flexural stiffhess of the wall sections that account for
cracking. The commentary to the 1994 Canadian concrete code (CPCA 1995) suggests a stiffness value of 70% of the gross moment of inertia (I g) of the wall section. The commentary to the 1995 New Zealand Standard (NZS 3101 1995) suggests much lower stiffhess values. A wall subjected to axial compression of 10% of fc! Ag is suggested to have half what is recommended in the CPCA Handbook (i.e. 0.35 lg). The NEHRP Guidelines for the Seismic Rehabilitation of
Buildings (FEMA 273) suggests stiffness values of 0.8 Ig and 0.5 Ig for uncracked and cracked concrete walls, respectively. While it is not clear which of the recommended stiffness values should be used, it is certainly clear that the choice of stiffness value will have a significant
influence on the predicted period and drift of the building.
The actual influence of cracking on the flexural stiffness of a concrete wall subjected to seismic loading is nonlinear. Nonlinear static analysis is increasingly used to capture this influence provided that an appropriate nonlinear model is used for the materiaL In this thesis, a simple nonlinear flexural (bending moment-curvature) model for concrete walls
in high-rise buildings is proposed. To validate the model, a 40ft high slender concrete wall was constructed and tested under simulated earthquake loading. Results from the test were compared with the proposed model and showed good agreement. Based on the proposed piece-wise linear model, a general method to determine the linear "effective" flexural stiffness of concrete walls was developed. Results from the general method for the effective flexural stiffness showed that the large variation in effective stiffness that is recommended by various design guidelines does actually exist for different wall configurations under certain conditions. The general method presented in this thesis gives the appropriate stiffness for a particular wall considering all important parameters that influence the stiffhess. A study was conducted to examine the influence of a variety of parameters on the stiffness of concrete walls and a set of simplified
expressions are proposed for the effective flexural stiffness of concrete walls.
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