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Fig. 1.15. Typical I-joists and their application.

Fig. 1.15. Typical I-joists and their application.

shrinkage, warping, splitting and checking, and are more efficient than solid timber for large spans and loads. They can be used as structural framing in floors, walls and in flat and pitched roofs. They are susceptible to shear buckling and are unstable until braced laterally. Compression flanges should be supported to prevent lateral deflection and buckling. Web stiffeners may be required at the bearing supports and positions of concentrated loads; service holes in the web should only be located in areas where shear loads are low.

I-joists can be designed using the rules in EC5 (see Chapter 7) and they are also available as proprietary systems. Manufacturers produce design guidance literature such as load span tables, permitted web hole requirements, joist hanger details, stiffener requirements, etc. In the United Kingdom, three main manufacturers or suppliers of I-joists are James Jones & Sons Ltd, iLevel TrusJoist (Weyerhaeuser) and Finnforest:

• James Jones JJI-joists® manufactured at the company's Timber Systems Division in Forres, Scotland, are available in a range of sizes familiar to the UK construction industry. Flanges are made with solid timber grade C24 and webs with 9-mm-thick OSB/3. Section depths range from 145 to 450 mm with flange widths from 45 to 97 mm all 45 mm deep.

• iLevel TrusJoist TJI- joists are manufactured in the USA and shipped to the United Kingdom with LVL flanges and 9.5 or 11.5mm OSB/3 webs. They are available in a range of sizes and depths and in lengths up to 15 m. Section depths range from 220 to 406 mm with flange widths from 38 to 89 mm all 35 mm deep.

• Finnforest FJI-joists1® manufactured at the company's factory in King's Lynn, England, have flanges produced from LVL, and a web of 10mm thick OSB/3. Section depths range from 195 mm to 400 mm with flange widths from 38 mm to 89 mm all 39 mm deep.

1.7.9 Thin webbed beams (Box beams)

Box beams comprise solid timber, LVL or glulam flanges with plywood or OSB webs. The webs are generally glued to the flanges on each side to form a box shape. Machine driven nails/staples can be used to aid fabrication.

Fig. 1.16. Box beam: (a) cross-section, (b) curved ply-box beam during construction (photo courtesy of Peter Yttrup and Fred Bosveld), and (c) tapered ply-box beam (photo courtesy of Catriona McLeod and Port of Brisbane Corporation, Australia).

Similar to I-joists, the larger parts of the cross-section (flanges) of box beams are at the top and bottom where the flexural stresses are highest. Plywood box beam showing veneer on its webs can be used as part of the aesthetic finish as well as the structure.

The hollow cross-section of the box profile also permits services to be run in the void inside the member giving a cleaner finish. It also gives the member torsional rigidity, which makes it more able to resist lateral torsional buckling or stresses due to eccentric loads.

Box beams are manufactured in depths up to 1.2 m. Web stiffeners are used to help control shear buckling of the web and provide convenient locations for web butt joints; see Figure 1.16. They are also located at positions of point loads to counter localised web buckling. In box beams, the web joint locations are ideally alternated from side to side and away from the areas of highest shear.

Unlike I-beams, which are factory produced in their final sections, it is not currently possible to buy box beams 'off the shelf'. Box beams are normally designed specifically for each contract requirement and assembled on-site.

The design of thin webbed joists and beams (I-joists and box beams) is covered in Chapter 7.

1.7.10 Structural insulated panels (SIPs)

SIPs are factory produced, pre-fabricated building panels that can be used as wall, floor and roof components in all types of residential and commercial buildings. They were developed in North America and have experienced wide-scale utilisation around the world. The biggest benefit with the system is that the structural support and the insulation are incorporated into a single system during manufacture. This enables high quality, more accurate thermal efficiency and a greater level of structural support to be achieved.

They are composed of a core of rigid foam insulation, which is laminated between two layers of structural timber panels (boards) by industrial adhesives. This process produces a single solid building element that provides both structural and insulation qualities. These panels are produced in varying sizes and thicknesses depending on the application and thermal/structural requirements.

The materials used to produce these building components can vary greatly in both the structural sheathing and the inner insulation core. Materials commonly used in the United Kingdom for the panels are OSB grade 3, or plywood combined with a variety of plastic foams including expanded polystyrene, extruded polystyrene, urethane and other similar insulation cores. Typical SIPs can be seen in Figure 1.17. Further information on the structural performance of SIPs may be obtained from the Structural Insulated Panel Association (SIPA) website and relevant research publications [22,23].

In Table 1.19 the available range of EWPs is summarised and their applications are outlined.

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