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In 1988 the American Institute of Steel Construction changed the method from Allowable Stress Design (ASD) to Load Resistance Factor Design (LRFD) on which the building code is based. This text develops a treatment of steel which is behavior-oriented and explains the causation for the LRFD approach. Focuses on creating cost-effective solutions for designing situations efficiently; discusses problems engineers must face on a regular basis; and offers insight into potential areas of concern. Also covers earthquake resistant design procedures. Includes over 400 drawings and 36 photos.
From the Preface
The organization of this text and its content was developed to promote in students and the structural engineering profession a better understanding of how to utilize scientific principles effectively in the creative part of the design process. The organization of this text was adopted to allow the focus necessary to attain a fundamental understanding of the behavior of steel structures and the basic theory used to predict behavior, both of which are essential design tools, especially in this period of transitional design specification.
Load and Resistance Factored Design (LRFD) was introduced in 1986. The decades that follow its introduction will see a gradual transition from Allowable Stress Design (ASD) to LRFD. Practitioners and design offices have invested too much time and money developing procedures based on ASD Specifications to quickly, or completely, make the transition. The American Institute of Steel Construction has anticipated the gradual nature of the transition from ASD to LRFD by the publication in 1989 of a new ASD specification, which is completely rewritten in the contextual format adopted by the LRFD specification. This revised specification is accompanied by a new set of design aides, both of which are far more popular than their LRFD counterparts. Accordingly, the student trained exclusively in the LRFD arena may not be well prepared for that first job.
Motivation to change the practice will come from two sources: (1) economic incentive, both in the produced building and the design process, and (2) a change in the standard of care. This text has then been developed so as to present, in as concise and understandable a fashion as possible, the basic theory associated with each discussed concept. The focus is on the assumptions used in the codification process. The specific relationships, which codify the concept in ASD and LRFD specifications, are then developed. This intermingling of a strength- and stress-based criterion will, at least at first, tend to confuse the student accustomed to a precise step-by-step problem-solution procedure, but the graduate only armed with this level of knowledge will find that the computer accomplishes these programmable tasks in practice and, further, that the building design process will, in almost all cases, require the use of both stress- and strength-based procedures. The adopted approach should also help the transitioning practitioner rapidly differentiate substantive changes from translations and understand where a conversion from ASD to LRFD offers a significant economic advantage or is required to evaluate system behavior more appropriately.
Several traditional topical areas are not emphasized. This selective deemphasis was based typically on my perception that the material was either too basic to require focus, as in the case of tension, or well developed in other texts and manuals, as in the case of standard connectors, bolts, and welding. Material on the design of standard connections is best taught or easily understood from material available in Design Aides, Specifications, and Commentary. Where a considered concept requires an in-depth understanding of connector behavior, the specific connector is developed as a part of the development of the system and thereby allows for design continuity. Chapter 1 provides the reader with the basic background material necessary to an understanding of the forthcoming focal topics. Material covered includes the design and construction process, material characteristics, and the application of codified principles to the design of tension elements and connectors.
The student and the practicing professional who choose to adopt a strength-based criterion must be completely satisfied with the flexural behavior of a steel member or a system when strains approach and exceed yield. If the structure must withstand seismically induced ground motions, the resultant strains may considerably exceed yield.
Accordingly, Chapter 2 is devoted to developing an understanding of the flexural behavior of members. Determinate systems are first examined as they approach their strength limit state and experience strains in the elastic, inelastic, and plastic strain ranges. This basic building block is then extended to indeterminate beams and beam systems. Elastic and post yield behavior is studied. Post yield deflections are predicted as are the strain levels likely to be experienced in the beam. Composite construction and the design procedures developed in specifications are described from a performance perspective. System serviceability is discussed along with occupant comfort. The material is developed in a manner that enables the undergraduate student to understand basic behavior to any level of complexity considered appropriate by the professor. The graduate student can extend the basic theory into advanced topics such as post yield behavior, strain quantification and serviceability considerations. This cohesive treatment allows the graduate student to efficiently prepare for the development of advanced topics.
Stability is treated as a consonant subject in Chapter 3. The stability of a column subjected to axial loads is extended into plate stability. Torsion is studied in this section as a precursor to flexural stability. Frame and system stability considerations are also discussed. The development of the strength and stability of the beam-column and the frame conclude this section. This cohesive treatment of stability along with the adoption of strain instead of stress as the destabilizer allows the student and practitioner to understand how elastic stability theory is extended to predict stability in the inelastic and plastic material behavior ranges. Once again, the sequential treatment of increasing complexity allows for both undergraduate and graduate student use.
Before the student or practicing professional can effectively design a complex building system, an understanding of system behavior must be acquired. Bracing systems are studied in Chapter 4 from the perspective of developing an understanding of how components affect behavior. Columns, beams, frame flexure, panel zone deformation and members of varying stiffness are all treated as independent variables. This segregation of variables allows the designer to assess how each component affects the behavior of a system and accordingly understand how to allocate material and optimize systems efficiently. Moment frame and braced frame systems, including eccentrically braced frame examples, are used to demonstrate how component characteristics can be used to optimize the design and performance of bracing systems. For the computerized designer who refuses to push a pencil, computer optimization procedures are developed and demonstrated. The development of a complex bracing system design using component influence procedures, based on manual as well as computer-based procedures is demonstrated in the design of a 60-story bracing system. An understanding of the elastic response of a bracing system to static loads is often not sufficient for the engineer charged with the responsibility for the design of a building in a seismically active area. A conceptual understanding of dynamic behavior is developed and extended to describe the influence of post yield behavior on the response of bracing systems. Response spectra design procedures are developed along with inelastic time history response prediction techniques. The focus here is not on developing an analytical methodology or the use of computer programs, but rather the development of a conceptual understanding of how to efficiently control the behavior of bracing systems. Chapter 4 is not really appropriate material for a basic undergraduate of graduate steel design class. Material contained in this chapter is, however, quite useful in structural engineering curriculums expanded to study the design of bracing systems, a subject that, though typically of interest to structural engineers, is often not taught from the design perspective or with sufficient continuity or depth to leave a lasting impression on the student.
Chapters 5 and 6 move into "hard core" designs. The various design criterion used currently in the seismic and wind design of bracing systems are described and these applications demonstrated by example. Chapter 5 is devoted to the comprehensive treatment of the design of a frame both to a seismic and a wind criterion. The material developed in Chapters 2, 3 and 4 are now devoted to the cohesive development of a frame design from the initial frame shaping and sizing process to the sizing of the anchor bolts. Prescriptive code compliance issues are identified and discussed. The inelastic response of the designed system to seismically induced ground motion is reviewed and compared to the response of systems of augmented strength.
Chapter 6 is more fun than work, for the objective here is to exploit the imagination and creativity of the designer. A variety of design problems are presented and solved to a conceptual level. This is quintessential engineering, for it challenges the engineer to solve problems by creating optimal structures from both a cost and behavior perspective that will behave as the designer intends, and survive in a hostile environment. This plethoric overview of design solutions is likened to reading the bridge problem in the daily paper, for it strives to stimulate creativity by implanting design concepts. Eccentrically Braced Frames (EBF) are increasingly used to brace buildings. EBF design concepts are appropriately treated in Chapter 6, for they allow the reader to explore the design process and understand how to create an EBF. Much of the engineers' efforts today are directed to revitalizing existing structures and this too, like EBF, is fertile ground for studying the design process. Chapters 5 and 6 can, at least in part, be integrated with entry-level graduate steel design classes or used in an expanded structural curriculum where the focus is the design of bracing systems.
Contents
Steel: Material Properties and Design. The Flexural Behavior of Stable Beam Systems. Stability. Behavior of Bracing Systems. Developing the Design of a Ductile Frame. The Artistic Aspects of Structural Engineering. Index.
About the Author
Dr Robert E Englekirk was born in New Mexico and raised in New Orleans. He has spent most of his adult life in southern California where he practices his specialty in structural engineering. He earned an undergraduate degree in civil engineering from Tulane University and earned his masters degree and PhD in 1970 at UCLA.
He has practiced structural design for over 30 years. For 25 years he has served as president and director of design of one of the largest consulting structural engineering firms in the United States (Robert Englekirk Consulting Structural Engineers Inc). He is also chief executive officer of Englekirk and Sabol (founded in 1979).
Licensed in over 20 states, Dr Englekirk has designed literally thousands of buildings of diverse functions - from shopping centers to multibuilding mixed-use high-rise complexes. He has received honors on many occasions by both clients and peers for his innovative and creatively produced designs, including the Lindau Award (1983) and the Turner Award (1986) for innovative and creative design.
Dr Englekirk has taught classes at the senior and graduate level in both steel and concrete design. He is currently an adjunct professor at UCLA.
In addition, he has coauthored two books - on concrete masonry for the Concrete Masonry Association of California and developed design aides to guide practicing professionals in the design of seismic bracing systems of concrete (published by the Portland Cement Association). He has also published his articles in more than 50 journals. |
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