Understanding and Using Structural Concepts

Citation

Ji, T., Bell, A. J., & Ellis, B. R. (2015). Understanding and Using Structural Concepts (2nd ed.). CRC Press.

Chapter Summary

The second edition runs to about 390 pages with 555 black-and-white illustrations. An overview chapter is followed by three parts: Statics (twelve chapters), Dynamics (seven chapters) and Synthesis (four chapters), the last of which the publisher identifies as new in this edition. Almost every chapter in the first two parts follows the same four-step pattern: definitions and concepts, theoretical background, model demonstrations, and practical examples. The demonstrations are simple physical models intended for use in ordinary class teaching; the practical examples show the concept at work in engineering structures.

Chapter 1: Overview of Structural Concepts

A short introduction that asks what structural concepts are and why they are worth studying, describes approaches to learning them, explains how the text is organised, and gives advice on how to use the book.

Part I, Chapters 2–7: Equilibrium; Centre of Mass; Effect of Different Cross Sections; Stress Distribution; Bending; Shear and Torsion

The first half of Statics covers the basic ideas of a first course in structural mechanics: the conditions of equilibrium, the centre of mass and its role in stability, how the shape of a cross section affects stiffness and strength, how stress is distributed, and the behaviour of members in bending, shear and torsion. Each topic gets its own chapter of about ten to eighteen pages, so concepts usually met inside longer chapters of a structural analysis textbook are here isolated and demonstrated one at a time.

Part I, Chapters 8–13: Span and Deflection; Direct Force Paths; Smaller Internal Forces; Buckling; Prestress; Horizontal Movements of Structures Induced by Vertical Loads

The second half of Statics moves from member behaviour towards design thinking. The chapter on span and deflection relates the length of a member to how much it deflects. Two chapters, on direct force paths and on smaller internal forces, set out criteria (the section headings speak of "Definitions, Concepts and Criteria" and "Concepts and a Criterion") for judging which structural arrangements carry load more efficiently. The part closes with buckling, prestress, and the less familiar topic of horizontal movements caused by vertical loads.

Part II, Chapters 14–17: Energy Exchange; Pendulums; Free Vibration; Resonance

Dynamics begins with the exchange between kinetic and potential energy, then uses pendulums as the bridge to free vibration and resonance. The same four-step chapter pattern carries over, so dynamic behaviour is introduced through models before its use in real structures.

Part II, Chapters 18–20: Damping in Structures; Vibration Reduction; Human Body Models in Structural Vibration

The rest of Dynamics deals with how vibration is dissipated and controlled, ending with a chapter on modelling the human body in structural vibration, that is, treating the people who occupy a structure as part of its dynamic system. This chapter uses "Demonstration Tests" in place of model demonstrations.

Part III, Chapters 21–24: Static and Modal Stiffnesses; Static and Dynamic Problems; Experimental and Theoretical Studies; Theory and Practice

The Synthesis part changes format: each chapter has its own sections and a summary. Chapter 21 defines static and modal stiffness and derives, verifies and applies a relationship between them. Chapter 22 links static and dynamic quantities: maximum displacement, buckling load, tension force and periodic loads are each related to the fundamental natural frequency or to equivalent static loads. Chapter 23 classifies how experiment and theory relate through six models (comparison, integration, verification, explanation, creation and extension). Chapter 24 considers theoretical and practical sources of structural concepts and how to bridge the gaps between theory and practice.

Key Concepts

Chapter 1 and Part I: Statics

• Structural concept: a principle of structural behaviour that can be understood physically and then used in analysis and design; Chapter 1 discusses what counts as one.
• Equilibrium and centre of mass: the conditions for a body at rest and the point through which its weight acts, the basis of stability.
• Cross section and bending: how the shape and orientation of a section govern a member's resistance to bending, shear and torsion.
• Direct force paths: the idea that a structure carries load more efficiently when forces travel by shorter, more direct routes to the supports.
• Smaller internal forces: arranging a structure so that its internal forces are reduced, presented with a criterion for comparing alternatives.
• Buckling and prestress: instability of compressed members, and introducing forces in advance to improve a structure's behaviour.

Part II: Dynamics

• Energy exchange: the conversion between kinetic and potential energy that underlies oscillation.
• Natural frequency and resonance: the frequency at which a structure vibrates freely, and the large response when a load acts near it.
• Damping and vibration reduction: the dissipation of vibrational energy and measures to limit unwanted vibration.
• Human body models: representing people as dynamic systems that interact with the structures they occupy.

Part III: Synthesis

• Static and modal stiffness: two measures of stiffness, one under static load and one associated with a vibration mode, and the relationship between them.
• Static–dynamic relationships: links between displacement, buckling load or cable tension and the fundamental natural frequency.
• Experiment–theory relationship models: six ways in which experimental and theoretical studies support each other.

Critical Analysis

Strengths

• Physical demonstrations. The publisher's description emphasises simple demonstration models that can be used in conventional teaching, and almost every chapter has a model-demonstration section. Peter Bullman, reviewing in The Structural Engineer, noted that the authors are known for using physical models to demonstrate structural concepts and that the book makes their ideas more widely available.
• A predictable structure. The repeated pattern of definitions, theory, demonstrations and practical examples lets a reader take each concept in isolation and move from intuition to theory to real structures. Martin Williams (University of Oxford), in an endorsement on the publisher's page, observed that many concepts that students find confusing can be understood in simple physical terms without recourse to mathematics.
• Statics and dynamics together. Seven chapters on dynamics, including damping, vibration reduction and human–structure interaction, go beyond most introductory structural texts. Messaoud Saidani (Coventry University) singled out this joint coverage and the treatment of how experiment and theory relate as unusual among structural analysis books.
• Research-based material. Ji and Bell are senior lecturers at the University of Manchester who have taught structural analysis and design at all levels (Ji researches structural dynamics and structural concepts), and Ellis is a former Building Research Establishment technical director whose work included testing and monitoring structures in situ; the publisher notes that the book incorporates teaching material based on current research. The Synthesis part and the human-body chapter reflect this.

Limitations

• A supplement, not a course text. Williams describes the book as a supplement to the more analytical structures books; it does not develop analysis methods for indeterminate structures or matrix methods, and the theory sections are short backgrounds rather than full derivations.
• Wide intended audience. The publisher addresses students in all years, graduate students, architecture students and consultants. Part III in particular, on modal stiffness, static–dynamic relationships and the theory of experimentation, assumes more than a first-year reader will have.
• Few chapters on materials and codes. The contents show no chapters on specific materials (steel, concrete, timber) or on design to codes; the book is about concepts across structures rather than member design.

Where It Fits in the Course

In this course the book sits between two kinds of reading. J. E. Gordon's Structures: Or Why Things Don't Fall Down is a non-technical account for general readers and beginners, and is the gentler place to start; Ji, Bell and Ellis take the same intuitive approach further, adding definitions, short theory and engineering examples for each concept. Hibbeler's Structural Analysis and Megson's Structural and Stress Analysis are the analytical course texts that develop methods for determinate and indeterminate structures with worked examples. This book is best read alongside them, a chapter at a time, as the physical explanation of what the calculations mean.

The two more specialised course texts connect to particular parts. McGuire, Gallagher and Ziemian's Matrix Structural Analysis formalises the stiffness ideas that Part I and Chapter 21 treat physically. Chopra's Dynamics of Structures develops in full the vibration theory that Part II introduces through pendulums, resonance and damping; Part II and Chapter 22 make a useful preview before a structural dynamics course.

How to Use This Book

The book suits civil and structural engineering students from the first year onwards, architecture students with a technical interest, and engineers who want to strengthen their intuition. Part I needs only school mathematics and the elementary statics taught early in an engineering degree; Part II assumes basic mechanics; Part III is best left until a reader has met both structural analysis and structural dynamics.

A practical plan:

• Start with Chapter 1, which explains the organisation and how to use the book.
• Read Part I alongside a first structural mechanics or analysis course, matching chapters to lectures: Chapters 2–7 with statics and mechanics of materials, Chapters 8–13 with the start of design and stability.
• Use the four-step pattern actively. Read the definitions, then try to predict the outcome of each model demonstration before reading it; build the simple models where possible; then look for the concept in the practical examples and in buildings and bridges nearby.
• Read Part II when vibration first appears in the curriculum, or before a structural dynamics course. Chapter 20 is optional unless the reader is interested in how occupants affect the vibration of a structure.
• Treat Part III as later reading. Chapters 21–22 suit students who have studied dynamics; Chapters 23–24 suit those starting project or research work that combines tests with analysis.

Chapters are self-contained, so a reader can go straight to a troublesome topic such as shear, buckling or resonance. Keep an analytical textbook at hand for the calculations the book deliberately keeps short.