Plywood Box Beam Construction
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Also Available:
* Fact Sheet on plywood box beams
* Newsletter article by Frame & Truss Manufacturers Association (FTMA)
Box beams are very versatile light-weight structural members that can also be used as a decorative feature.
Simplistically, the box beam is similar to the I-beam, but with the web split in half and put on each side of the flanges. The larger part of the cross-section is at the outside where the flexural stresses are highest. In this case, the side view of the box beam just shows the decorative face of veneer on the plywood, so the member can be used as part of the aesthetic finish as well as the structure.
The box shape (hollow cross section) has a number of other advantages. It allows wires and 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 direct torsion due to off-centre loads.
Unlike I-beams which are factory produced in their final sections, it is not possible to buy box beams “off-the-shelf”. Box beams are normally designed specifically for each job and assembled on site. The individual components of box beams are normal timber products:
* Top and bottom flanges can be made from sawn timber, LVL or for very large beams, glulam. The only face of the flange on view is the bottom face of the bottom flange, and it can be clad with plywood to match the sides of the beam. The side faces of the flanges is covered by the webs.
* Webs are normally made from plywood sheets supported on a frame that allows for transfer of shear from one sheet to another at a butt join in the web sheets. The plywood can have high appearance finish, if the beams are on display, or low finish if the beams are hidden behind other cladding materials.
Box beams with their high torsional stiffness can be used as rafters as they have high resistance to lateral-torsional buckling. They can be used as a long-span rafter, or incorporated into a protal frame with stiff knee joint connections (usually made with plywood gussets). Other uses include floor beams for long span members (either bearers or joists), and long span purlins in industrial buildings. The high strength to weight ratio of box beams makes the attractive in construction.
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| Fact Sheet: Plywood Box Beams | 299.2 KB | ||
| Plywood Box Beam Span Tables for Detached Housing Construction | 3.3 MB | ||
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