Glulam Beams
7.1 INTRODUCTION
A glulam section is one manufactured by gluing together laminations with their grain essentially parallel. In the UK a horizontally glued laminated member is defined as having the laminations parallel to the neutral plane while a vertically laminated member has the laminates at right angles to the neutral plane. This leads to different strength values for horizontally and vertically laminated members made from the same species and grade of timber. Because Eurocode 5 does not differ- entiate between the orientation of the laminates in ascribing strength values to glulam, the European definitions are simpler – horizontal glulam is glued lami- nated timber with the glue line plane perpendicular to the long side of the glulam section while with vertical glulam the glue line plane is perpendicular to the short side of the section.
There are four basic reasons for laminating timber:
• sections can be produced very much larger than can be obtained from a single piece of timber
• large defects such as knots can be distributed throughout a glulam section by converting the solid timber section into laminates and forming a glulam section
• structural members of tapered and curved profiles can be produced easily (by laminating a previously two-dimensional curved glulam a three-dimensional portal or arch frame can be made, albeit at a cost!)
• members can be cambered to offset deflections due, say, to the self-weight of the structure.
The production requirements for glulam are now given in BS EN 386 ‘Glued laminated timber – Performance requirements and minimum production require- ments’. It is necessary to have a further three standards, BS EN 390 ‘Glued lami- nated timber – Sizes – Permissible deviations’, BS EN 391 ‘Glued laminated timber – Delamination test of gluelines’ and BS EN 392 ‘Glued laminated timber – Shear test of gluelines’ in order to achieve comparability with the now withdrawn BS 4169 ‘Glued laminated timber structures’.
In selecting the species of timber for laminating the two prime characteristics are the ‘gluability’ and dimensional stability with changing atmospheric conditions.
Many tropical hardwoods are difficult to glue because of natural oils and resins contaminating the gluing surface. Timber with large movement values such as Ekki 123
have to be used with caution and special care taken in the orientation of laminates to reduce the risk of glue line splitting. The most common species used in Europe is whitewood.
It is essential to control the moisture content of the timber at the time of man- ufacture. BS EN 386 specifies that the moisture content of the laminates should lie in the range 8% to 15% with the range between the highest and lowest laminates in the member being 4%. It is also prudent to have the moisture content at manu- facture as close as possible to the likely equilibrium moisture in service, particu- larly in heated environments, to avoid glue line splitting.
BS EN 386 gives the maximum finished thicknesses for laminates forming members to be used in the three service class environments. For the conifer species, in service class 1 the maximum thickness is 45 mm and the cross-sectional area 10 000 mm2 (say 45 ¥ 220 mm); in service class 2 the thickness is 45 mm and 9000 mm2; while the limitations in service class 3 are 35 mm and 7000 mm2. Where the cross-sectional area is greater than 7500 mm2it is recommended that a longi- tudinal saw kerf is run down the centre of the laminate to reduce the risk of exces- sive cupping. The requirements for the broadleaved species are more stringent.
The laminates have to be kiln dried to the required moisture content and it follows that there must be adequate storage space available to store the timber at all stages of the production cycle so that the costs of kilning are not thrown away.
The dried laminates will still be in the sawn condition so they are then planed on the surfaces to be bonded. These planed surfaces must be flat and parallel. It is normal practice to glue up the section within 24 hours of planing as there is a risk of timber distortion (cupping) and, in even the most benign environments, chemi- cal contamination of the surfaces.
The storage, mixing and application of the adhesive may appear to be straight- forward. Far from it. Getting the adhesive on to the laminates and laying on the next laminate (open assembly time), and then applying the clamping pressure within a further time limit (closed assembly time) can be an interesting logistical problem bearing in mind that there is also a finite time period from the time of mixing the adhesive to it becoming unuseable (pot life). These various time periods are influenced by the ambient conditions – for example, with a relative humidity of 30% (60%+is the norm) the open assembly time can be well under 10 minutes due to the rapid evaporation of the volatile constituents of the adhesive whereas the laminating team would be expecting at least 20 minutes in normal circum- stances.
Pressure has to be applied to the glue lines while the adhesive is setting to obtain a close contact between laminates (the thicker the glue line the lower the glue line strength). There is also an element of flattening of the laminates required to remove minor cupping distortion. For laminates of the maximum thickness this glue line pressure is often taken as 0.7 N/mm2 (100 lbf/in2). Pressure can be applied in a number of ways – the most usual being either clamps or air bags. The need to main- tain the gluing surfaces in close contact during the setting of the adhesive rules out simply nailing the laminates together. There will tend to be some movement of the timber during the setting period and the withdrawal characteristics of even annular ring shank nails will not contain this movement. As a consequence the glue line is stressed perpendicular to the plane of adhesion, which is the worst possible direc- tion for an adhesive whether partially set or fully set and cured.
The fabricated member has to remain under pressure for a period of time depend- ing on the glue line temperature. Bearing in mind that the rate of setting of most adhesives used for glulam production is very slow below 15 °C and effectively ceases at 10 °C, it follows that the laminates themselves should be at least 15 °C at the time of gluing. There is no possibility of taking timber from an unheated storage shed in winter with the temperature at -5 °C and running it through the laminating process in an hour or two.
All the foregoing problems of control of the gluing process apply equally to any gluing on site which is why site gluing is rarely carried out. In one situation the site heating to achieve the requisite temperature only managed to set fire to the enclosing, protecting structure with dire consequences!
With temperatures raised to 30 °C it is possible to release the clamping pressure after 4 hours or so and gently move the laminated member to a storage area for the adhesive to cure and harden further. After perhaps 24 hours curing the section can be machined to remove any deviations caused by the different widths and straightness of the individual laminates and the relative slippage sideways during laminating. In this machining process the squeeze out of adhesive at each glue line is removed. A consistent squeeze out for each glue line along the length of the member is a good indicator of uniform spread of adhesive and adequate glue line pressure. By the time this machining process takes place this adhesive will be very hard and will damage the cutters of the planer or thicknesser. For this reason these cutters cannot be used to plane the laminates before gluing as any chips in the edge of the blades will result in ‘tramlines’ up to 1.5 mm high in the intended gluing surface.
It is normal for the standard glulam sections which can be obtained from stock- ists to be manufactured without a camber. Incorporating a camber into bespoke glulam simply requires a former to gently bend the laminates. The art in this pro- cedure comes in guessing the spring back on release of clamping pressure.
Glulam sections are built up from thin members, and it is therefore possible to manufacture complicated shapes, curving laminates within limits. The designs in this chapter (indeed in this manual) are limited to beams with parallel, mono-pitch or duo-pitch profiles. The method of building up laminations for beams with sloping tops is indicated in Fig. 7.2. Although the grain of the laminations which Glulam Beams 125
Fig. 7.1
occur near the sloping surface is not parallel to this surface, it is permissible to consider these laminations as full strength in the design.
When it is necessary, for appearance, to fit a lamination parallel to the sloping surface or the end of the beam (Fig. 7.3), provided it is correctly glued into place, it may be considered to add to the strength of the beam.
In no circumstances should two prefabricated part-sections which have been manufactured separately be glued together as sketched in Fig. 7.4 and treated as a fully composite beam. Each part is almost certain to have too much inertia to be held by one glue line once the clamps are removed. It has been proved by experi- ence, however, that a section which, for example, is too deep for the sanding machine, may be manufactured initially with a ‘dry joint’, the two pieces being separated after the glue has been cured; then each piece is sanded (not on the dry joint), glued, re-assembled and clamped. Where feasible, the method of adding to a glulam section should be by one laminate at a time, because great care and man- ufacturing expertise are required to obtain a successful member by the method explained above.