Plywood box beams: a fast, flexible construction alternative

Plywood box beams are a great alternative to steel beams and other engineered wood products used commonly in construction. They are fast, flexible, strong and lightweight – but according to research funded by Forest & Wood Products Australia, they are also vastly underutilised.

A study into large span first and second storey timber and wood products for detached housing by key researchers from the Timber Development Association of New South Wales (TDA NSW) in partnership with Engineered Wood Products Association of Australasia (EWPAA) aims to educate builders in how to use long span timber structures. Its findings could encourage greater use of plywood within Australia’s construction industry.

“Builders willing to try something different have been surprised by how easy the beams are to construct from standard builders’ tools,” says Simon Dorries, general manager, EWPAA. “Not enough builders know about this method but they are happy when they use it.”

Plywood box beams have many benefits. Plywood sheets on either side of a horizontal flange and vertical stiffeners provide structural support and stability. They also have an attractive finish when viewed from the side.

Their high torsional stiffness and resistance to buckling means they can be used as rafters or as floor beams for bearers or joists. They can also double as walls to save space and avoid height restrictions, acting as the structural element as opposed to having a wall with bracing.

Plywood box beams particularly suit spans in the four to eight metre range most common in housing construction, their span increasing with beam depth. According to Andrew Dunn, chief executive of TDA NSW: “Solid timber can span so far, glulam can span so far, steel can span so far. There is probably a bracket, a niche span, in between where the box beam is at its ideal.”

It also helps that using plywood box beams avoids the health and safety concerns and costs of steel and cranage, props and welding – and of having to call in extra trades onsite.

“If an engineered wood beam that does the same job can be provided onsite at equivalent or, more importantly, lower cost than alternatives, people will use them,” says Mr Dorries.

The researchers also suggested ways in which frame and truss manufacturers could add to the beams’ wider acceptance. They found that builders and designers deal directly with manufacturers as a “holistic design service”.

Therefore, timber industry representatives believe that if manufacturers could create customised solutions and prefabricate beams offsite in their factories that involve normal wood and are then supplied as part of a complete package of value-added engineered wood products, builders would welcome them.

“Manufacturers could use existing machinery, in theory using a nail gun rather than nail plates,’ says Mr Dorries. “It is equipment they would be expected to have.”

The findings of the study are being widely publicised within the industry. This information has been published in the Frame & Truss Manufacturers Association Australia (FTMA) newsletter and appears in the January / February 2010 edition of Broadcast Builder.

As part of a growing range of resources, Wood. Naturally Better.™ has developed the Plywood box beam construction for detached housing guide to complement the researchers’ findings. This guide compares different timber beams, including laminated veneer lumber (LVL), glue laminated and plywood box beams, and steel beams in housing construction.

The guide also includes new span tables, developed by the EWPAA, that are now in line with current calculation techniques, are designed to limit state format, wind speeds up to N3 and the latest Australian timber structure and design and wind load standards. They cover MGP 10, MGP 12, F5 and LVL 10 stress grades for flanges and stiffeners as well as common locations, such as for lintels, bearers, hanging beams and strutting beams.