Structural Design from First Principles

Michael Byfield

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781138503496
• eBook ISBN: 9781498741255
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

This enlightening textbook for undergraduates on civil engineering degree courses explains structural design from its mechanical principles, showing the speed and simplicity of effective design from first principles.

This text presents good approximate solutions to complex design problems, such as "Wembley-Arch" type structures, the design of thin-walled structures, and long-span box girder bridges. Other more code-based textbooks concentrate on relatively simple member design, and avoid some of the most interesting design problems because code compliant solutions are complex. Yet these problems can be addressed by relatively manageable techniques. The methods outlined here enable quick, early stage, "ball-park" design solutions to be considered, and are also useful for checking finite element analysis solutions to complex problems.

The conventions used in the book are in accordance with the Eurocodes, especially where they provide convenient solutions that can be easily understood by students. Many of the topics, such as composite beam design, are straight applications of Eurocodes, but with the underlying theory fully explained.

The techniques are illustrated through a series of worked examples which develop in complexity, with the more advanced questions forming extended exam type questions. A comprehensive range of fully worked tutorial questions are provided at the end of each section for students to practice in preparation for closed book exams.

Chapter 1

Limit state design

This chapter outlines the philosophy behind what Europeans call limit state design and Americans call load and resistance (safety) factor design. This is the method used by regulators to write modern structural design codes and it involves the application of partial safety factors to load and resistance. The ‘limit state’ is a condition beyond which a structure no longer fulfils the design intent, and there are two types:

1. Serviceability limit state (SLS) design: The structure must be fit for purpose under working loads. For most situations, this means the structure must remain elastic and not deflect excessively when supporting unfactored loads.
2. Ultimate limit state (ULS) design: The structure must be strong enough to support loads increased using (partial) safety factors. Unlike SLS design, the engineer can utilise the full plastic design strength if a material is ductile. For example, plastic design is allowed for some steel members, whereas brittle materials, such as wood, must be designed using elastic principles.

1.1 PARTIAL SAFETY FACTORS

The objective of ULS design is to ensure that

$$\text{Design}\text{strength}\ge \text{Design}\text{load}$$

which is expressed as

$$\frac{R}{{\gamma }_M}\ge S\times {\gamma }_F(1.1)$$

where

R is the resistance (strength).

γ_M is the material partial safety factor.

S is the estimate for load.

γ_F is the load partial safety factor.

The partial safety factors are normally based on proven work over many years. When new design equations are added to codes, they are tested for accuracy in the laboratory. It is important to remember that design resistances and loads are only approximations. The statistical uncertainty is modelled using the log-normal probability distribution function for both load and resistance, a process that is illustrated in Figure 1.1. The partial safety factors are selected to ensure that probability of failure is very small.

Figure 1.1 Probability distributions of resistance, load and resistance-load.

Lists of partial safety factors (γ-factors) are shown in Tables 1.1 and 1.2. It can be seen that different factors are applied to different types of load and ...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Contents
• Symbols and abbreviations
• Foreword
• Preface
• Author
• 1 Limit state design
• 2 Steel members in flexure
• 3 Buckling of steel columns and trusses
• 4 Buckling of arches
• 5 Buckling of thin-walled structures
• 6 Composite structures
• 7 Reinforced concrete beams and columns
• 8 Prestressed structures
• 9 Strut and tie modelling of reinforced concrete
• 10 Control of cracking in reinforced concrete
• 11 Timber beams, columns and trusses
• Index