Structural Timber Design to Eurocode 5

Jack Porteous, Abdy Kermani

• Éditeur: Wiley-Blackwell
• Année: 2013
• ISBN:
  9781118597293
• Édition: 2
• Sujet: Technology & Engineering
• Sous-sujet: Construction & Architectural Engineering

Structural Timber Design to Eurocode 5 provides practising engineers and specialist contractors with comprehensive, detailed information and in-depth guidance on the design of timber structures based on the common rules and rules for buildings in Eurocode 5 – Part 1-1. It will also be of interest to undergraduate and postgraduate students of civil and structural engineering.

It provides a step-by-step approach to the design of all of the commonly used timber elements and connections using solid timber, glued laminated timber or wood based structural products, and incorporates the requirements of the UK National Annex. It covers:

• strength and stiffness properties of timber and its reconstituted and engineered products
• key requirements of Eurocode 0, Eurocode 1 and Eurocode 5 – Part 1-1
• design of beams and columns of solid timber, glued laminated, composite and thin-webbed sections
• lateral stability requirements of timber structures
• design of mechanical connections subjected to lateral and/or axial forces
• design of moment resisting rigid and semi-rigid connections
• racking design of multi-storey platform framed walls

Featuring numerous detailed worked examples, the second edition has been thoroughly updated and includes information on the consequences of amendments and revisions to EC5 published since the first edition, and the significant additional requirements of BSI non contradictory, complimentary information document (PD 6693-1-1) relating to EC5. The new edition also includes a new section on axial stress conditions in composite sections, covering combined axial and bending stress conditions and reference to the major revisions to the design procedure for glued laminated timber.

Chapter 1

Timber as a Structural Material

1.1 INTRODUCTION

Timber from well-managed forests is one of the most sustainable resources available and it is one of the oldest known materials used in construction. It has a very high strength to weight ratio, is capable of transferring both tension and compression forces and is naturally suitable as a flexural member. Timber is a material that is used for a variety of structural forms such as beams, columns, trusses, girders, and is also used in building systems such as piles, deck members, railway sleepers and in formwork for concrete.

There are a number of inherent characteristics that make timber an ideal ­construction material. These include its high strength to weight ratio, its impressive record for durability and performance and good insulating properties against heat and sound. Timber also benefits from its natural growth characteristics such as grain patterns, colours and its availability in many species, sizes and shapes that make it a remarkably versatile and an aesthetically pleasing material. Timber can easily be shaped and ­connected using nails, screws, bolts and dowels or adhesively bonded together.

The limitations in maximum cross-sectional dimensions and lengths of solid sawn timbers, due to available log sizes and natural defects, are overcome by the recent developments in composite and engineered wood products. Finger jointing and various lamination techniques have enabled timbers (elements and systems) of uniform and high quality in any shape, form and size to be constructed; being only limited by the manufacturing and/or transportation boundaries.

Timber structures can be highly durable when properly treated, detailed and built. Examples of this are seen in many historic buildings all around the world. Timber structures can easily be reshaped or altered, and if damaged they can be repaired. Extensive research over the past few decades has resulted in comprehensive information on material properties of timber and its reconstituted and engineered products and their effects on structural design and service performance. Centuries of experience of use of timber in buildings has shown us the safe methods of construction, connection details and design limitations.

This chapter provides a brief description of the engineering properties of timber that are of interest to design engineers and architects, and it highlights that, unlike some structural materials such as steel or concrete, the properties of timber are very sensitive to environmental conditions; for example moisture content, which has a direct effect on the strength and stiffness, swelling or shrinkage of timber. A proper understanding of the physical characteristics of timber enables the building of safe and durable timber structures.

Fig. 1.1. Cross-section of tree trunk.

1.2 THE STRUCTURE OF TIMBER

Structural timber is sawn (milled) from the trunk of the tree, which provides rigidity, mechanical strength and height to maintain the crown. Trunk resists loads due to ­gravity and wind acting on the tree and also provides for the transport of water and minerals from the tree roots to the crown. Roots, by spreading through the soil and acting as a foundation, absorb moisture-containing minerals from the soil and transfer them via the trunk to the crown. Crown, comprising branches and twigs to support leaves, provides a catchment area producing chemical reactions that form sugar and cellulose that allow the growth of the tree.

As engineers we are mainly concerned with the trunk of the tree. A typical cross-section of a tree trunk, shown in Figure 1.1, illustrates its main features such as bark, the outer part of which is a rather dry and corky layer and the inner living part. The cambium, a very thin layer of cells underside the inner bark, is the growth centre of the tree. New wood cells are formed on the inside of the cambium (over the old wood) and new bark cells are formed on the outside and as such increase the diameter of the trunk. Although tree trunks can grow to a large size, in excess of 2 m in diameter, ­commercially available timbers are more often around 0.5 m in diameter.

Wood, in general, is composed of long thin tubular cells. The cell walls are made up of cellulose and the cells are bound together by a substance known as lignin. Most cells are oriented in the direction of the axis of the trunk except for cells known as rays, which run radially across the trunk. The rays connect various layers from the pith to the bark for storage and transfer of food. Rays are present in all trees but are more pronounced in some species such as oak. In countries with a temperate climate, a tree produces a new layer of wood just under the cambium in the early part of every growing season. This growth ceases at the end of the growing season or during winter months. This process results in clearly visible concentric rings known as annular rings, annual rings or growth rings. In tropical countries, where trees grow throughout the year, a tree produces wood cells that are essentially uniform. The age of a tree may be determined by counting its growth rings [1, 2].

The annular band of the cross-section nearest to the bark is called sapwood. The central core of the wood, which is inside the sapwood, is heartwood. The sapwood is lighter in colour compared to heartwood and is 25–170 mm wide depending on the species. It contains both living and dead cells and acts as a medium for transportation of sap from the roots to the leaves, whereas the heartwood, which consists of inactive cells, functions mainly to give mechanical support or stiffness to the trunk. As sapwood changes to heartwood, the size, shape and the number of cells remain unchanged. In general, in hardwoods the difference in moisture content of sapwood and heartwood depends on the species but in softwoods the moisture content of sapwood is usually greater than that of heartwood. The strength and weights of the two are nearly equal. Sapwood has a lower natural resistance to attacks by fungi and insects and accepts preservatives more easily than heartwood.

In many trees and particularly in temperate climates, where a definite growing season exists, each annular ring is visibly subdivided into two layers: an inner layer made up of relatively large hollow cells called springwood or earlywood (due to the fast growth), and an outer layer of thick walls and small cavities called summerwood or latewood (due to a slower growth). Since summerwood is relatively heavy, the amount of summerwood in any section is a measure of the density of the wood; see Figure 1.1.

1.3 TYPES OF TIMBER

Trees and commercial timbers are divided into two types: softwoods and hardwoods. This terminology refers to the botanical origin of timber and has no direct bearing on the actual softness or hardness of the wood as it is possible to have some physically softer hardwoods like balsa from South America and wawa from Africa, and some physically hard softwoods like the pitchpines.

1.3.1 Softwoods

Softwoods, characterised by having naked seeds or as cone-bearing trees, are generally evergreen with needle-like leaves (such as conifers) comprising single cells called tracheids, which are like straws in plan, and they fulfil the functions of conduction and support. Rays, present in softwoods, run in a radial direction perpendicular to the growth rings. Their function is to store food and allow the convection of liquids to where they are needed. Examp...