Concise illustrated guide to timber connections Preview by BM TRADA, Element and Warringtonfire

CONCISE ILLUSTRATED GUIDE TO TIMBER CONNECTIONS

concise illustrated guide to timber connections >

brings together architectural and structural considerations

illustrated with more than 100 connection details and assembly forms

follows the design principles in Eurocode 5

researched and written by experts in timber construction

concise illustrated guide to timber connections contributors: peter ross, patrick hislop hugh mansfield-williams and adrian young

Ever since man conceived structures bigger than a tree, connecting together pieces of timber has challenged the ingenuity of designers. It is a lightweight fibrous material whose strength-to-weight ratio compares favourably with concrete and steel. Nevertheless, savvy designers who appreciate timber’s many aesthetic advantages also understand the structural limitations that its organic nature impose. And that is the essence of timber connection design. Ross, Hislop, Mansfield-Williams, Young

This concise illustrated guide to timber connections illustates: > the main structural forms for which timber is suitable > a range of the common connections used to assemble these forms > what these connections might look like > the structural design constraints that affect configuration and appearance > good detailing practice. The book illustrates the evolution of timber structures from medieval times to today, describing the materials used in connections. It explains the three connection types – all timber, metal connectors and adhesives – used in the principal assembly forms of timber structures. Connection principles are illustrated for beams, trusses, arches, portal frames, braced and unbraced frames, curved and double curved lattices, platform frame, panel systems and pole structures. It summarises the key factors contributing to load transfer, durability and fire resistance.

BM TRADA, part of the Element Group, provides a comprehensive range of independent testing, inspection, certification, technical and training services. We help organisations to demonstrate their business and product credentials, and to improve performance and compliance. 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.

Exova BM TRADA

ISBN 978-1-900510-85-1 BM TRADA

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1 Introduction

together as well as connecting timber to other materials:

• all-timber connections • metal connections • adhesives.

Ever since man first created dwellings from tree trunks, the greatest challenge he faced was to devise a form of connection between the individual components which would result in a safe and stable structure. Even today, the connections remain the most critical aspect of timber design, and many contemporary structures exploit the connections for their role in the articulation of the assembly as well as their visual interest.

Chapter 5 Connections for various assembly forms contains examples of the more common forms of timber construction, as well as a few non-standard ones. The illustrations:

• define the common assembly forms, together with

Timber is the only organic framing material, and its appeal is in part due to the visual qualities of the surface figure, created as the saw intersects the naturally formed grain of the trunk. However, this grain also creates a material which is markedly anisotropic – along the grain its strength-to-weight ratio is better than mild steel, but it is much weaker across the grain, with a potential for splitting that can influence the performance of a connection. It is for this reason that connections dominate timber frame design and can often determine member sizes; something that rarely happens when designing in an isotropic material such as steel.

the mechanism for transferring load

• describe tried and proven connections for each •

assembly form, with guidance on their capacity to transfer tension, compression and bending moment highlight constraints such as edge distances for fasteners, durability and fire resistance.

Bear in mind these are illustrations of principles, not fabrication drawings. Chapter 6 Fire resistance and Durability explains the key requirements to ensure the structure is sufficiently durable for its environment and anticipated service life, and gives options for providing a specified period of fire resistance.

The gradual adoption of the Eurocodes and the development of timber supply chains that mirror those in the steel and concrete industries mean that timber design, once regarded as a specialisation, is now on a par with the other structural materials.

Finally, Chapter 7 Design summarises the principles of structural design of timber connections, referring to Eurocode 5. It includes a summary of the ‘vocabulary’ used in connection design.

Timber’s increasing popularity is also driven by its sustainability credentials and architects’ desire to express its use in the building by exposing the timber structure and connections. Hence, the design of timber connections from the visual as well as the structural point of view is becoming more important for both architects and engineers.

Key lessons for timber designers

• Connections, rather than the forces in the •

This book illustrates:

•

• the main structural forms for which timber is suitable • a range of the common connections used to assemble

•

these forms

• what these connections might look like • the structural design constraints that affect configuration and appearance

• good detailing practice.

•

Chapter 2 Evolution of timber structures traces the common assembly forms from medieval times to the modern era. Many are still viable forms for contemporary use, as shown by the popularity of frames based on medieval models and fabricated in unseasoned oak.

•

Chapter 3 Materials used in connections describes the range and characteristics of timber and other materials used in framing.

•

Chapter 4 Types of connections sets out the principles of the three basic methods of connecting pieces of timber

members, usually govern the timber cross-section sizes. Four criteria should be addressed in connection design – ability to transfer load, appearance, durability and fire resistance. If connections are exposed to view, consider and exploit their visual impact. If connections are exposed to rain, ensure the durability of the timber by specifying a suitably durable species or preservation, and galvanised or stainless steel metal components. Detail connections carefully to avoid water traps and moisture build-up. For larger timbers, take account of possible movement across the grain caused by variations in the moisture content of the wood. For solid timber over 300mm × 75mm in cross-section, it is more difficult to obtain supplies of seasoned material. The usual solution is to use glued laminated timber (glulam), but sometimes it is possible to use thinner sections in pairs, which may influence the connection design. Seasoned timber is dimensionally stable in a modest moisture content range. However, if ‘unseasoned’ timber is used (such as green oak), the shrinkage that occurs as the timber dries out demands special attention.

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2 Evolution of timber structures

in oak, the most durable of the temperate hardwoods. The members are fabricated when the log is ‘green’ (freshly cut) because the timber is much easier to work in this condition, and oak dries very slowly.

No other building material has such a variety of historic styles. Their inclusion in this guide, however, is not merely on the grounds of academic interest, but because many of them are still valid models for contemporary structures. Frames of green oak, for instance, based on medieval models, are made around the country by small specialist firms, albeit with some design modifications to meet current standards of insulation and weathertightness.

The members are square, or nearly square. Section 4.1 All-timber connections contains a selection from the extensive vocabulary of connections in medieval construction. It is important that the connections are of traditional proportions and tight fitting, so that the material is held to line while it seasons, for oak has the highest shrinkage of all the commercially available timbers. In order to make these connections, the members were set in one plane.

2.1 Log construction

Connections work mainly in compression and shear, although an exception is a truss with a raised-collar tie (Figure 2.3), which is in significant tension under load. The medieval solution was to provide a multi-pegged brace, but a peg has a small tension capacity, and most modern frames have the joint reinforced with a concealed bolt (Figure 2.4).

In those areas of the world which are most densely forested with softwoods, there is a long history of log construction. The logs are trimmed and stacked one above the other, with a notched interlock at the corners (Figure 2.1). Thus the connections, even in this simplest of structures, are the key to the stability of the structure as a whole.

Figure 2.3 An exception to the rule – collar tie in tension

Figure 2.1 Log construction

2.2 Medieval construction In the UK, which was forested mainly with hardwoods whose growth is less disciplined than softwoods, the open frame developed as a material-efficient form of construction (Figure 2.2). Surviving examples are mainly

Figure 2.4 Early tension connection

2.3 17th to 19th Centuries In this period of emerging Classicism, roof structures were generally based on the king post truss (Figure 2.5) and concealed by a ceiling, now an essential requirement of upper floor accommodation. Walls, however, were increasingly in brick. The truss members were still set in a single plane, although now mainly in imported softwood. The queen post truss (Figure 2.6) rose in popularity because it required shorter span rafters, and the rectangular central bay made access easier, opening the way for a habital space in the roof.

Figure 2.2 The earliest medieval timber buildings relied on triangulated frames and joints in compression

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5 Connections for various assembly forms

Figure 5.39 Cruciform connection using Cowley Connector

Because of the difficulty of achieving adequate stiffness, these details are recommend for domestic-scale structures only, up to two storeys.

5.5 Connections for curved and double-curved lattices

Figure 5.36 Head-height braces resist lateral forces

Connection criteria

Apply the criteria given in Chapter 4 Types of connections, depending on whether the connections are all timber or with metal fasteners. Lattice connections are usually visible and in service class 1 or 2.

5.5.1 Lamella structures The lamella roof made its appearance in Germany in 1921 in a system patented by Friedrich Zollinger. The aim was to construct arch roofs using relatively short timber members, which interlocked to create a cylindrical surface (Figure 5.40). All members are identical and two modules long, with a central mortice and two end tenons. The rectangular system needs to be stabilised, either by triangulating members or by an overlying deck. Initially used for house roofs, it was later applied to larger structures with a span of 30m or more, although few examples now exist. It essentially forms an arch, which of course is primarily in compression, with secondary bending from asymmetric loading such as drifted snow.

Figure 5.37 Head-height brace strengthened

Figure 5.38 shows how the essential beam/column fixing could be made with steel gusset plates, if it is visually acceptable. The same detail could be used for a central splice between twinned members. If the bolts were recessed, there would be no exposed metal, and the joint would be fire resistant, although the detail is difficult to extend to a four-way beam connection.

Figure 5.38 Junction stiffened with gusset plates

If a four-way connection is needed (with lateral resistance in both directions), the Cowley Connector offers a solution, as shown in Figure 5.39.

Figure 5.40 Principle of the lamella roof

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7 Design

metal plate connection shown in Figure 5.8. Generally, the larger the plate, the less it behaves like a true pin, and the engineer should consider this before adopting a simplified ‘pinned’ analysis.

7.1 Connection vocabulary Joint and connection Although these terms are often used interchangeably, ‘joint’ simply means the junction between two or more elements of a frame, while ‘connection’ means the facility for transferring loads between elements.

Moment-resisting connection When a connection is restrained so that one element cannot rotate in relation to the other(s), it is said to be ‘moment-resisting’. The bending capacity of the joint is the measure of the resistance to rotation. Momentresisting connections are essential where there is no bracing or sheathing to restrain a frame from collapse. The most common moment-resisting connection is the knee in a portal frame, such as that shown in Figure 5.23.

Loads, forces and bending moments A load can be applied either directly as a force (that is, a load trying to move an object in a single direction) or eccentrically to apply an additional moment (which tends to rotate the object). Thus a wind load attempts to move a frame sideways, and to turn it over.

Single and double shear When dowel-type fasteners transfer load from one element to another, the load is said to be transferred in ‘shear’; that is, across the dowel, not along its length. If there are only two members, the load transfer is said to be by ‘single shear’ because there is only one plane between the elements. Single-shear joints are normally made with staples, nails, screws or bolts. Because the line of force in each element is offset from the other, the forces are eccentric, in which case bending moments arise in the members from the offset forces (see single shear joint in Figure 7.1). This is usually small enough to be neglected, but the engineer should check its effect in thick sections, or if the forces are large.

These forces and moments (described in Eurocode 5 as ‘actions’) produce responses in the frame elements (the ‘effects of actions’). These responses are either axial (compression or tension), or across the line of the element (shear and bending moment). Figure 5.13 shows the loading on an arch, with the gravity loads producing compression along the line of the member, and the (eccentric) wind load resulting in shear and bending moments across the member. They result in connection loads, which are predominantly in compression, with some shear. If the connection is detailed as a ‘pin’, it can take no bending moment.

If there are three elements, the load transfer is said to be by ‘double shear’ because there are two planes between the elements. Double-shear joints are normally made with bolts and dowels. Because they are balanced, no significant bending moments arise in the elements.

Deformation and slip All frames suffer ‘deformation’ to some degree under load. This term is used in the Eurocodes as a generalised definition of movement, which includes the deflection of beams and the side sway of frames. Deformation is largely a result of the elastic response of the frame elements to the loads, but connections made with metal fasteners also slip by some amount as they take up the applied load. For conventional triangulated frames these movements are generally small, but in the single-shear connection shown in Figure 7.1 (a fastener group taking the central bending moment in a lapped beam), the slip contribution to the total deflection could be significant. Connections made with adhesives, however, are assumed to be rigid. For this reason a joint should not be designed to rely on the combined strength of adhesive and metal fasteners, as the slip necessary to mobilise the fastener resistance will not occur (until the glue fails).

Figure 7.1 Single- and double-shear joints

Pinned connections Pinned connections are intended to transfer only axial and shear forces, not bending moments. An ‘ideal’ pinned connection does actually contain a pin (a heavy dowel connector), such as shown in Figure 5.17. In this case, one element is free to rotate in relation to the other.

Edge and end distances, and spacing In order to avoid splitting along the grain when the fastener takes load, Eurocode 5 specifies minimum edge and end distances for a single fastener, measured between the centre of the fastener and the outer profile of the timber, and given in terms of the fastener diameter d. For a group of fasteners, minimum spacings are given, along and across the grain (see Section 7.3 Structural design of connections with metal fasteners).

Many connections actually have a limited momentresisting capacity but are (for the purpose of the engineer’s analysis) assumed to be pinned, such as the 33

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