Bedenik / Besant Analysis of Engineering Structures

1. Auflage 1999
ISBN: 978-0-85709-972-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 392 Seiten

Reihe: Woodhead Publishing Series in Civil and Structural Engineering

This text delivers a fundamental coverage for advanced undergraduates and postgraduates of structural engineering, and professionals working in industrial and academic research. The methods for structural analysis are explained in detail, being based on basic static, kinematics and energy methods previously discussed in the text. A chapter deals with calculations of deformations which provides for a good understanding of structural behaviour. Attention is given to practical applications whereby each theoretical analysis is reinforced with worked examples. A major industrial application consisting of a simple bridge design is presented, based on various theoretical methods described in the book. The finite element as an extension of the displacement method is covered, but only to explain computer methods presented by use of the structural analysis package OCEAN. An innovative approach enables influence lines calculations in a simple mannger. Basic algebra given in the appendices provides the necessary mathematical tools to understand the text. - Provides an understanding of structural behaviour, paying particular attention to applications, and reinforces theoretical analysis with worked examples - Details the methods for structural analysis, based on basic static, kinematics and energy methods

Autoren/Hrsg.

Bedenik, B.

Besant, C B

Leseproben

1 Introduction
Publisher Summary
Structural analysis is a science that ensures that the structures are safe and fulfill the functions for which they were built. Safety requirements must be met so that a structure is able to serve its purpose with the minimum of costs. Structural concepts arise from the work of engineers from different fields with a common aim that the structure is functional, aesthetic, and economic. Detailed planning of the structure usually comes from several studies made by town planners, investors, users, architects, and other engineers. In general, an architect is responsible to the investor and a structural engineer works in collaboration with the architect as an equal partner in a project. In some structures, such as industrial halls, bridges, and sports halls, a structural engineer has the main influence on the overall structural design and an architect is involved in aesthetic details. The process of analysis has to be repeated until the structure as a whole is optimal from all points of view, followed by final analysis and dimensioning. Structural analysis is a science, which ensures that structures are safe and fulfill the functions for which they were built. Safety requirements must be met so that a structure is able to serve its purpose with the minimum of costs. Structural concepts arise from the work of engineers from different fields with a common aim that the structure is functional, aesthetic and economic. Detailed planning of the structure usually comes from several studies made by town planners, investors, users, architects and other engineers. In general an architect is responsible to the investor and a structural engineer works in collaboration with the architect as an equal partner in a project. In some structures such as industrial halls, bridges and sports halls, a structural engineer has the main influence on the overall structural design and an architect is involved in aesthetic details. After the preliminary design of the structure, an approximate analysis of loads and stresses in all elements must be carried out including the determination of deformation in individual elements as well as in structure as a whole. This preliminary analysis is a check to show where and how the structure can be improved and reduced in costs. It is possible that the initial design proves to be uneconomic and the structure has to be changed in individual elements or as a whole. The process of analysis has then to be repeated until the structure as a whole is optimal from all points of view, followed by final analysis and dimensioning. The whole process can be divided into: – initial design – preliminary dimensioning – optimisation (when necessary, change of individual elements of the structure or change of the structure entirely must be made) – final analysis and dimensioning.
Figure 1.1 An example of CAD design (top picture) and photograph (bottom picture) of the sports stadium in the city of Maribor It is obvious that the processes of structural analysis and design are closely related, since each change in element dimensions influences the optimal structural shape, weight and stiffness. These quantities are known only after the elements have been designed. Thus, analysis and design are mutually interacting and the process is called structural analysis. 1.1 Types of Structures
Structural analysis deals with a number of different structures: – Buildings (residential, industrial) – Bridges – Underground structures, tunnels – Industrial structures, power stations, reactor containers – Planes, missiles – Vehicles (automobiles, railcars, ships) – Machines, cranes, elevators, aerials, electricity pylons Structures can be divided according to the nature of their components into three main classes: – Linear or uniaxial members: truss elements, beams, columns, arches and their combinations. Elements of this type are simple to analyse and are therefore suitable for elementary presentation of structural theory. It is possible to idealise even complex structures as assemblies of such members. – Two-dimensional elements such as plates, shells and walls. Although the analysis of such elements has been considered as a branch of the theory of elasticity, modern computational methods facilitates analysis to any degree of accuracy. – Three-dimensional elements such as machine parts, pressure vessels, soil and rock foundations. Some structural joints must also be included as such elements in a detailed stress analysis using the theory of elasticity or plasticity. Although there are several computer programs available today, in practice it is common to analyse structures using very simple models consisting of linear elements by the elementary methods presented in this book. 1.2 Loads
The nature and magnitude of loads must be determined before a structure can be analysed though these are only crude approximations in the initial design. The most important loads are: – Dead load (D), which can be exactly determined only after the structure has been designed. It is obvious, the smaller the ratio of the dead load to the other loads, the more efficient is the structure. Some structures, such as long span bridges, can carry dead loads many times higher than live loads. For such structures a shape optimisation has to be performed to gain an optimal and efficient structure. – Live load (L) is the useful load carried by the structure. If it is caused by human activities it should be determined by the use of probability theory. Building Codes (i.e. Eurocode 1) determine the most unfavourable cases that can occur in a lifetime of the structure. In bridges, the live load is moving, and an analysis has to determine the most unfavourable position of vehicles using influence lines (covered in Ch. 9). – Wind, earthquake and aerodynamic forces. Effects of these forces must be calculated including dynamic effects as they act in cycles and cause inertia forces in the structure. The field of structural dynamics, which is not included in this text, is rapidly developing and full dynamic analysis is possible using appropriate computer programs. In building frames equivalent static forces are taken into consideration although it is known that interaction between a forced vibration and properties of structures exist. It is known that a stiffer and heavier structure carries higher dynamic forces than a slim and light structure. This has been proved in recent earthquakes, where slim and economically reinforced concrete structures underwent only slight damage, and oversized and therefore minimally reinforced structures were heavily damaged or collapsed. – Earth pressure, gas and liquid pressures. Earth pressure varies between the extreme active and passive cases and is dependant on soil-structure interaction. Gas and liquid pressures are well known, controlled and act hydrostatically on a surface. – Self-strains due to supports settlements, pre-stressing, creep, shrinkage of concrete, welding and temperature gradient. Beside active loads, a change of length or misfit of structural elements can take place causing huge stresses in a structure as a whole or in an individual element. Specification of loads is usually included in building codes, but it is the structural engineer who has to find the most unfavourable combination of loads, which can also be time dependant as with creep or relaxation in pre-stressing steel.
Figure 1.4 Viaduct ‘Crni Kal’ – total length 1067 m CAD simulation and Finite element model (Courtesy of Ponting Ltd.) The probability of maximum loads due to several causes ocurring at the same time will decrease with the number of loads considered. In fact, a loading case of maximum normal force and maximum bending moment acting simultaneously is not always critical. The concrete column in Fig. 1.5 is carrying both a compressive normal force and a bending moment. It can be observed from the interaction curve, that at the same reinforcement ratio at constant bending moment M, by increasing the normal force from N1 to N3 the element goes from the unsafe through to the safe and again unsafe condition.
Figure 1.5 Interaction curve for a concrete column The determination of loads acting on a structure is a complex and difficult task and is readily underestimated in practice. Loads determined by building codes are approximate...