Making the connection

18 February 2010

TRADA president Peter Ross is co-author of Timber in Contemporary Architecture – a designer’s guide published last month to mark TRADA’s 75th anniversary. In this extract from the book, he examines metal connections

As a building material, timber is relatively lightweight and easily worked. In consequence, there are many ways of connecting timber members, both to each other and to other components of the building frame. These methods can be grouped under three broad headings – all-timber connections; metal connections; and adhesives.    
Timber is a markedly directional material, and the strength of a connection is usually determined by the timber properties in the weaker direction (ie perpendicular to the grain) in compression, shear, or tension.

From the 17th century onward, the all-timber frame gradually gave way to a standard construction of masonry walls with only the intermediate floors and roof trusses in timber. Larger spans and shallower pitches – the latter enabled by the use of slate covering – produced higher loads in the truss rafters, resulting in a gradual introduction of metal strapwork to reinforce their connections to the bottom tie.  

From the inventiveness of the industrial revolution emerged the first patents for the mass production of nails, screws and bolts. This resulted in a dramatic reduction of their costs, which led in turn to a fundamental change in truss construction. Throughout history, truss members had been set in a single plane, with their ends shaped to fit one another. If, instead, they were lapped, members could simply be nailed or bolted together. As reliance could now be placed on the fasteners acting in shear, the joint itself would now be equally strong in compression and tension, the
latter being the weakness of the traditional joint. The principle can be extended to join three or more layers of timber together (3.8a). However, it will be seen that the fasteners are limited to the overlap areas of the timber, and this may in turn limit the overall joint capacity.

As a further development of this idea, profiled steel flats can be fabricated, matching the member profiles, either as splice plates (3.8b) or flitch plates (3.8c). In these cases, the timber members return to a common plane, and a flitch plate may be virtually concealed within the timber profile.

An additional advantage of this system is that the steel ‘arms’ along each timber member may be as long as is necessary to accommodate the required number of fasteners, and so these joints do not impose a limit on the assembly capacity. They are effectively fabricated steel components, which connect together the separate timber members. Many contemporary structures use this form, clearly expressing the separate functions of the connecting node (in steel) and the frame members (in timber). An example are roof node connections at Portcullis House in London.

Purpose-made steel components are also a useful way of distributing a stress over a timber section at a point of load concentration, such as the base of a three-pinned arch. Here, an inverted steel T-section has been let into the base of the glulam, and bolts inserted through the web, with the heads recessed and plugged. A similar detail is used at the crown.   

A post-war development, which achieved almost overnight success, was the pressed metal plate fastener (often referred to as a ‘gangnail’ from the first proprietary product). As the names suggest, the ‘nails’ are first pressed out of a plate. Two plates are applied to a joint as splices, and pressed home. The plates are used principally in the fabrication of roof trusses, which are assembled on a flat bed equipped with a travelling hydraulic press.

Production rates for components using small fasteners speeded up with the introduction of gun-nails and machine-driven screws, which, with thinner shanks, avoided the need for pre-drilling. In joints such as 3.8c dowels could be substituted for bolts, and would be visually much less evident. Dowels are normally inserted into tight-fitting holes to hold the members together, which also reduces the tendency of the joint to slip under load.

For a large fastener group, this would require extreme accuracy in the drilling of the components, leading to the development of ‘self-drilling’ dowels, capable of cutting through timber and up to 15mm of steel, in two or perhaps three layers. Thus an in-line connection can be made between two members, with blank flitch plates installed in end slots and the dowels installed by drilling. The dowel diameters lie between 5mm and 7mm, with a maximum length of 220mm. Using multiple plates maximises the joint capacity.

It is said that if you meet a 50-year-old timber designer, he has spent 25 of them working on the connections. Their form is critical, and it is a good idea to start with the joints, rather than leaving them to the end of the design process.

Connection detail at Portcullis House, Westminster, London
3.8a N-truss with lapped members through-bolted
3.8b Truss with in-line members, joined with splice plates and bolts
3.8c Truss with in-line members, joined with concealed flitch plates and bolts