Designing timber structures An introduction
Contents
Introduction
7
Chapter 1 Introduction to timber
11
1.1 Timber and the tree 1.2 Timber as a building material 1.3 Sawn timber production process 1.4 Sustainability 1.4.1 Why should we build with timber? 1.4.2 Forest stewardship 1.4.3 Timber certification
11 12 15 19 20 20 21
1.5 Strength 1.5.1 Timber species 1.5.2 Timber grain
23 23 25
1.5.4 Strength-reducing characteristics
1.7.1 Moisture content 1.8 Engineered timber products 1.9 Durability 1.9.1 Prevention 1.9.2 Species selection 1.9.3 Protection
37 38 42 42 44 45
1.11 Summary
48
Chapter 2 Simple timber construction
55
2.1 Timber joists and how they are used
55
2.3 Safety factors and limit state design
59
2.7 Design
74
2.7.2 Bending 2.7.3 Shear 2.7.4 Bearing 2.8 Summary
4 |
32
77 82 84 91
Chapter 3 Timber stud walls 3.1 What are timber stud walls and how are they used?
101 101
3.3.5 Permanent load
108
3.4.1 Understanding the timber frame stability system 3.4.2 Designing the timber frame stability system
113 115
3.7 Summary Chapter 4 Glued laminated frames 4.1 What is glulam and how is it made? 4.1.1 Typical glulam sizes
127 135 135 140
4.1.3 Hardwood glulam
142
4.3.2 Stability bracing
145
4.4.1 Designing flexural members 4.4.2 Designing columns 4.4.3 Designing glulam frame connections
148 151 153
Chapter 5 Cross-laminated timber 5.1 Introduction to cross-laminated timber
5.4.1 Analysis of one-way spanning slabs 5.4.2 Analysis of two-way spanning slabs
179 179
188 192
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5.5 Design for axial load 5.5.2 Axial capacity of walls
5.7 Summary
6 |
205 207
219
Chapter 6 Additional timber design considerations
239
Chapter 7 Further information
265
Glossary
270
Introduction Welcome to Designing timber structures: an introduction. We hope that you will both enjoy reading this book and find it incredibly helpful. Before we step into the wonderful world of timber, we wanted to answer a few simple questions.
Who is the book aimed at? This book is aimed at engineering students who are designing for the first time, or for those who want to learn more about designing with timber. It starts at the very basics (how to calculate a load, what is a factor of safety etc) and assumes no real knowledge about design. We do, however, assume that you have already studied structural analysis. As a result, we generally don’t provide the theory on bending moments, elastic deflections or on more complex continuous problems. If you are new to these concepts then you might want to seek out a book, course or online programme on structural analysis before you try and use this book. If you have carried out some structural design before (for example, in steel or concrete) this book will still be very helpful, but you might find that you can skip over some of the basic concepts, especially those covered in Section 2.2 and Section 2.4 of Chapter 2. If you have never designed with a material before then timber is a great place to start. Designing in any material is complicated. Steel buckles in a variety of ways and is often designed plastically. Concrete is a composite which becomes plastic due to the non-linear behaviour of the reinforcing steel. Timber is assumed to behave elastically, making it much simpler than steel or concrete but, and this is a big ‘but’, as it is a naturally occurring material it is highly variable and it is these issues that this book explores.
How is the book designed? Many books in engineering are designed as reference books which you can drop in to at any stage, pulling out the necessary information you desire. This book is not like that: it is instead designed to be a story book, taking you on an adventure. You can of course flip through the sections you are less interested in, but the book is designed to build up your knowledge steadily. Every chapter will assume knowledge from the chapters before, increasing in its level of complexity. • •
•
Chapter 1 will introduce you to timber and discuss the qualities of this material. Chapter 2 will introduce you to the very basics of design, working up to the design of the humble floor joist. On the way you will discover concepts like loading, partial safety factors and a series of constants all called k. Chapter 3 will introduce you to buckling and stability, including taking you through the design of a stud wall, so now you can design the floors and walls of a house.
Introduction | 7
• • • •
Chapter 4 will introduce you to glued laminated timber (glulam), an engineered timber product, and will discuss connection design and bracing. Chapter 5 will introduce you to cross-laminated timber (CLT), another engineered timber product and will cover slab design, the gamma method, vibration and continuity. Chapter 6 will finish the book off by discussing additional timber design considerations including fire, acoustics and working with existing buildings. Chapter 7 provides general and chapter-specific references and further information.
Reading a textbook isn’t always fun, so we have tried to make it as lighthearted and accessible as possible. We hope that this approach makes the book easier to read, while not diminishing the seriousness of the content. By the time you have finished reading this book you will hopefully be able to design whole buildings out of timber (or at least make a very good start) but if you make a mistake, the building could collapse. Of course, engineering is serious. However, it doesn’t have to be tedious, and we aim to convey the excitement and the sheer pleasure that designing in timber can bring.
What is the book and what is it not? You hold in your hands a bridge. Not a literal bridge, but a metaphorical one. Out in the construction world is a whole host of information on timber design (and steel design and concrete design and earth design and masonry design and straw design and … you get the picture) designed to be used by practising engineers. There are Eurocodes, British Standards, Published Documents (PDs) that provide background information to the Eurocodes, Wood Information Sheets (WIS), Non Contradictory Complimentary information (NCCs) which provide additional information not included in Eurocodes, Building Regulations, websites and books. These are all invaluable resources to practising engineers.
Further reading For more detailed EC5 guidance see: Manual for the design of timber building structures to Eurocode 5, IStructE/BM TRADA, 2020 Eurocode 5: timber design essentials for engineers
8 | Introduction
But what if you are not a practising engineer? What if you don’t know your stress from your strength, your characteristic from your design, your partial material factor from your partial load factor? Well, this book is for you. It will take you through four different designs and at the end of it you will hopefully know your kmod from your kdef, your ksys from your kcr. The aim of this book is not to tell you everything there is to know about timber, in fact quite the opposite. The aim of this book is to tell you enough about timber that you can access all these other codes, books and resources. This book should not be seen as a direct guide to BS EN 1995 (referred to as Eurocode 5 or EC5). There are other very good books that serve this purpose. While reference is inevitably made to some of the numbers from EC5 to enable design, this book tries to look beyond this to general engineering principles. In later chapters where the subject area becomes highly complex (such as the design of CLT structures) this book serves as an introduction which can lead you on to explore the many books dedicated to the subject of design for
these products. Suggestions for further reading are provided throughout to enable you to develop the design beyond the scope of this book. Finally, as with all design, the designer should take full responsibility for their design, ensuring that it is in line with the latest design standards and principles. While the authors have made every effort to ensure the correctness of the content of this book, it is provided for academic purposes only and all values should be confirmed from the latest codes of practice.
Introduction to BM TRADA Many of the references in this textbook relate to BM TRADA publications which you will find invaluable for timber design. References to BM TRADA books and information sheets are identified throughout this book with a . More information on specific BM TRADA publications is also given in Chapter 7.
Introduction | 9
CHAPTER 1
Introduction to timber Overview In this chapter we will: • • • • • • • •
explore the properties of timber consider how timber behaves look at timber’s strengths and weaknesses consider naturally occurring variations in timber introduce timber as a construction material answer questions around how timber is processed explain what timber products are available, including engineered timber introduce using timber as part of the larger conversation around circular economies, climate change and the environment.
1.1 Timber and the tree At the very start of the design process it is important to recognise the fact that timber derives from a living tree. Trees start out as a seed just like so many other plants. Many grow to saplings, before being eaten by wildlife. The few that remain are stunted in their growth down on the forest floor, as their parent trees above take most of the sun. But when a tree dies (or is felled) the sun comes pouring in and the young trees grow up to meet it. They may need to twist as they grow to maximize their exposure to the sun. Eventually they become mature trees, their canopies high above the forest floor, taking the spoils of the sunlight.
Key term timber refers to wood or wood-derived engineered products
While in a managed forest the process may be more controlled: trees are planted as whips, growing into saplings from which branches may be cut back (pruning) to improve timber quality; whole trees are removed (thinning) to open up the canopy and allow light in thereby encouraging a consistent rate of growth; trees are felled when they reach a certain size or maturity and new trees are planted to replace them: the natural process of tree growth has been the same for millions of years. Mature trees, large enough to be used for houses, ships, schools and offices have within them their ‘sapling self’. The branches of the juvenile tree become enveloped by new growth and at some point, may wither and die (or be pruned) as they become less useful to the tree. But still they reside within it. Likewise, the layers of the tree adapt and change. At the core of the tree is the heartwood and pith. The heartwood provides the living tree with most of its strength as it makes up most of the volume of the trunk. The sapwood is used to draw nutrients and water from the ground and deliver it to the branches and leaves, enabling photosynthesis. The outer bark of the tree acts as a protective layer, preventing insects and fungi entering but most importantly keeping moisture in and insulating against the cold and heat.
Figure 1.1 Penone, G. (2008) , sculpture on exhibition at the Art Gallery of Ontario Photograph
Chapter 1
11
CHAPTER 2
Simple timber construction Overview In this chapter we will: • • • • • •
apply the information learned in Chapter 1 to the design of beams and joists consider safety factors and limit state design look at how to calculate permanent action and variable action look at how to calculate bending moments, shear force and deflection design the floor joists of a small house present a worked example of a timber joist floor design for a house.
2.1 Timber joists and how they are used Before we start it would be useful to define what a beam and a joist are. A beam is an element that spans between supports and works in bending. A joist is really a type of beam but, and here is the key difference, a beam is a single element, whereas joists are a series of beams which run side by side to form a floor or roof. In this chapter we will design the floor and roof joists of a small house as a way of introducing the design of bending elements (beams and joists). In Chapter 4 we will look at the design of beams more specifically. To design a joist we need to do three things: 1. 2. 3.
Calculate the load acting on the joist. Analyse the joist to work out the stresses and strains that it experiences. Design the joist to show that the joist is adequate to carry the load (1) and can resist the stresses and strains (2).
This chapter is therefore structured by doing each of the above in turn. Timber joist floors are frequently used, in many types of construction, especially smaller-scale residential construction; being easy to install, light in weight, and environmentally friendly (see Section 1.4 Sustainability). They can be supported by masonry walls, as is typical in most UK housing stock, or off timber stud walls in what is commonly referred to as platform construction. Timber joists themselves support the floor deck, which can be made from oriented strand board (OSB), plywood, particleboard or, in the case of older houses, floorboards (see Section 1.8 Engineered timber products for more detail). This decking can typically span no more than 1m, so joists are required to increase the span typically up to 5m between supports. If larger spans are required then joists can span onto beams which can span greater distances.
Figure 2.1 Floor joists in an existing house
Key term platform construction the method of construction where floor structures bear onto loadbearing wall panels, thereby creating a ‘platform’ for construction of the next level of wall panels
Chapter 2
55
CHAPTER 6
Additional timber design considerations Overview In the introduction we suggested this book should be read very much like a story. But it is probably fair to say that this book is actually more like the first book in a series. We have been through the design process, starting with the simplest of joists and ending up at quite a complex engineered material (CLT). Yet there is still so much more to learn about timber design. This chapter is designed to start a conversation on these other topics and will hopefully encourage you to go and find out more. You will consider: • • • • •
how timber reacts in fire acoustic allowances the importance of detailing how to care for timber on site how to assess existing timber buildings.
6.1 Fire Every building must be designed for fire. There are several different strategies and approaches but all buildings in England must be designed to meet the requirements of Part B of the Building Regulations 2010 (England). If you are building in Wales, Scotland or Northern Ireland, you will need to check Approved Document B (Wales), Section 2, Technical Handbook (Scotland) or Technical Booklet E (Northern Ireland) for specific fire design requirements in these locations. If you are designing further afield then other local regulations must be complied with. Building regulations cover a wide variety of different aspects of building design, including energy efficiency in Part L and disproportionate collapse in Part A. Part B covers fire. To provide practical guidance on how to meet the requirements of each part of the regulations, the Government provides guidance in Approved Documents. For fire, there are two documents, one for residential and one for other building types. Both documents are intended to provide guidance for more common building situations and cover a variety of different conditions. For the design of a building you will need to determine the structural fireresistance requirements. Fire-resistance ratings are commonly used for this purpose; a fire-resistance rating is the approximate amount of time that the fire will burn for until it burns out. Different building types have different fire-resistance ratings which are given in detail in Approved Document B of the Building Regulations 2010 (England) and are summarised in Table 6.1.
Web links Approved Documents are available for free download from: www.gov. uk/government/collections/ approved-documents
Further reading For more information on Building Regulations in Wales, Scotland and Northern Ireland, please see Chapter 7
There are a few things to note. First, we would not recommend building basements out of timber. There are a variety of reasons including effects of moisture, but generally basements are constructed from concrete (and occasionally steel inside a concrete box) and we would suggest that this practice is continued. This includes the ground floor slab of a basement.
Chapter 6
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Designing timber structures An introduction
A comprehensive introduction to designing and engineering with timber Progressive structure builds up knowledge and understanding End-of-chapter worked examples consolidate learning and demonstrate engineering principles Highly illustrated with over 250 drawings and photographs This book is aimed at engineering students who are designing for the first time, or for those who have begun their journey into design and now want to learn more about designing specifically with timber. It starts by introducing timber as a sustainable material; a key consideration in our current climate emergency. Students then move through timber design basics such as deflection, strength and buckling to enable the design of a timber floor and stud wall. Two chapters focus on core engineered timber products: glued laminated and cross-laminated timber. Finally, fire, acoustics, detailing and additional considerations around the use of existing timber buildings are introduced.
Designing timber structures: an introduction aims to provide students with a solid understanding of the key basic principles of designing with wood. Regular further reading references and links to online material and sources provide students with the opportunity to access the broad and complex spectrum of materials available for practising engineers.
Dr James Norman and Dr Andrew Thomson are passionate advocates of timber in engineering design. James is Associate Professor in Sustainable Design at the University of Bristol while Andrew has been a Teaching Fellow at the University of Bath. Both worked extensively in private practice before moving into teaching and drew on that wealth of sustainable design experience to bring Designing timber structures: an introduction to life. BM TRADA, part of the Element Group, provides a comprehensive range of independent testing, inspection, certification, technical and training services. We help our customers to make certain that the management systems, supply chain and product certification schemes they operate are compliant and fit for purpose.