Summary
• A finger-joint has been developed that can be used for all geometric forms.
• Glued joints are strong and low-cost.
• Flexible manufacturing methods have been developed.
• A glued truss is visually attractive and has good fire resistance.
Finger-joints have long been used for splicing sawn wood and, to a lesser extent, L-joints and E-joints. Now, however, a finger-joint has been developed which can be used for all geometric forms, including T, L, K and E-joints, giving those designing in timber much more freedom of choice in terms of structure and geometry.
Glued finger-joints are also universal as far as loading is concerned. The joints can take tension, shear loads and bending moments and are also suitable for joining thick wooden components from 50-250mm and thin ones from 10-40mm.
A wide range of recent developments in gluing wood have also opened up new opportunities. Reliability has improved and costs have come down. Gluing wood with a high moisture content and cross gluing, in which the grain of the wood runs at right angles at the glued interface, have both become possible. Hardening times for adhesives have reduced from hours to minutes. Adhesives behave like filler and work even if there are gaps in the glued surfaces. The required gluing pressure has reduced to a tenth of what it used to be.
Hundreds of laboratory tests have been carried out on new finger-joints and the results have been convincing. The potential of these finger-jointed products is enormous and production line development, type approval and marketing all lie ahead. The new joints are well suited for all wood products where the joints have to be strong and low-cost, making them ideal for trusses.
Flexible manufacturing methods have also been developed. The timber components are cut to length and the fingers machined, then the timber components are glued together in a jig. Directly after assembly the truss is put into intermediate storage for a few hours for the adhesive to harden, although if rapid-hardening adhesive is used this storage phase is not required.
Quality control
The trusses are test loaded for quality control purposes. In the early stages of manufacture all the trusses are test loaded, but, in time, the number of test loadings may be reduced. This ensures the quality of the design programme, the design, the joints and the timber.
Glued trusses have cost benefits as glue is a relatively cheap way of joining sections of timber.
In addition, for several reasons, less timber is needed for a glued truss and the cost of the diagonals is lower as the tension diagonals and a part of the compression diagonals are made of boards which are about half the thickness of the top and bottom chords. The top and bottom chords are also smaller since the stresses in a glued truss are dispersed more economically due to the completely stiff joints.
The timber used in glued trusses does not have to be planed as the timber components are machined to precise dimensions at the glued joints and the cross-section of the components between the joints can vary.
At points where the dimensioning is critical because of the deflection of the truss, the amount of timber used in a glued truss is 20% less for this reason alone, because the displacement of the glued joint is negligible.
In manufacturing a glued truss there is next to no wastage caused by cutting the wood, as short cuttings can be spliced further. The labour costs involved in manufacturing a glued truss are also apparently lower, as there are about 60% fewer components to be handled.
There are also benefits other than cost. For example, the glued truss can be very visually attractive. If the timber used in a glued truss is selected not only for its strength but also for its appearance, or if it is to be painted and any glue flashes are removed, the truss can almost look like a piece of furniture.
The fire resistance of a glued truss is also impressive as the glued joints char in the same way as the timber and stand up to fire longer. A glued truss is moisture resistant and can generally be used in damp conditions – outside, for example.
Roofs built of glued trusses do not need any bracings to be fitted to the diagonals on site. This is a result of the diagonals in the glued truss being fixed stiffly at the ends, which increases the compression strength of the struts. Although the diagonals in a glued truss are mostly 20-25mm thick boards, the diagonals critical for buckling are sufficiently thick to stand up to buckling.
A roof built of glued trusses may demand as little as half the timber demanded in an equivalent roof built with other standard methods due to lower amounts used in each truss and less wastage.
Glued truss applications
Structures that can be built using glue-joint technology include:
• very shallow trusses, with a mid-span height of as little as span/15;
• trusses intended for large loads or long spans on commercial or industrial buildings;
• boomerang beams or very low scissors trusses;
• in trusses where the wooden components are finished in paint, varnish or anti-fungal treatment, for example, the timber can be finished cost-effectively before it is cut to length as cutting and gluing the joints does not damage the finish;
• trusses intended to be visible, fire-resistant or for use in damp conditions;
• shuttering trusses for in situ concrete bridges. For this type of use, the minimal deflection of the truss and the ease of dismantling are fundamental;
• packaging trusses – the packaging becomes extremely strong;
• trusses where there is a large opening in the middle, for example in usable attics or for installing transverse pipe work; there is a large bending moment at the edges of the opening, which the glued truss can stand up to well;
• trusses where there is a bend in the upper or lower chord; in glued trusses, a re-entrant bend may be incorporated into the lower chord, for example, so that transverse plumping can be located without breaking the moisture and air barrier;
• glued finger-joint technology can be used to manufacture 300-500mm high trussed beams and lintels which can act as strong primary beams carrying large loads or as light secondary beams. In both cases, the trussed beam contains about half the volume of timber of an equivalent laminated timber beam;
• trusses with a curved top or bottom chord, or where there are ornamental patterns at the eaves. The glued-truss machining unit is capable of machining curved finger grooves in the wood. The same unit can machine curved components or any ornamental pattern without incurring any significant cost increase.
