Oversize wood sheets better in earthquake

by Stephen Forgacs
Staff writer

UBC researchers have discovered that by using oversize wooden sheets in wall construction, they can as much as double the ability of many wood-frame structures to withstand wind and earthquake forces.

"We knew already that North American platform construction is an efficient method to withstand wind and earthquake forces," said Helmut Prion, an assistant professor with a joint appointment to the Dept. of Civil Engineering and the Dept. of Wood Science.

"The proof was in Kobe, Japan, where buildings with this type of construction withstood earthquake forces much better than traditional Japanese post and beam structures."

The standard method of residential wall construction in North America involves framing a wall using vertically-positioned and evenly spaced wooden studs, and then nailing sheets of plywood or oriented strand board (OSB) to the outside of the frame for racking strength and shelter, he explained.

Prion and Frank Lam, an assistant professor in the Dept. of Wood Science, found that by building a wall using standard construction techniques and a single large sheet of oriented strand board instead of multiple smaller sheets, they could double the load the wall can bear.

"What our research has shown is that by making this one change -- using considerably larger sheets -- we can greatly enhance the wall's ability to withstand earthquake and wind forces."

Oriented strand board is a building material that is similar to plywood but is made using small strands or flakes glued together to form a rigid sheet.

Lam and Prion are project leaders on a research team that includes research engineer Paul Symons, graduate students Henry He and Jennifer Durham, and visiting undergraduate students Henrik Magnusson from Sweden's Lund University, and Dominik Sieber from the Swiss Institute of Wood Technology.

The research team is halfway through the two-year project which has received support from the Structural Board Association and funding from the National Research Council's (NRC) Industrial Research Assistance Program (IRAP).

The team hopes to find ways to apply its research findings to the construction of larger commercial structures, such as warehouses and retail spaces, which at present usually require the use of large structural members for strength and support.

"This is an example of a very successful university-industry venture with government support through the NRC," Lam said.

"The wood products industry is growing very rapidly and is developing some very advanced products. But we're behind when it comes to using these products in a very efficient way."

Prior to the project undertaken by Prion and Lam, research efforts in this area had focused on walls built with standard size panels. Using standard construction techniques, a 7.2-metre by 2.4-metre wall would comprise six panels.

The joints between panels create discontinuities in walls that weaken their load-bearing capacity, Lam said.

"The idea that clicked in our minds was, why don't we build a wall using large sheets. It will reduce discontinuity in the walls."

The research team tested both standard walls and walls made of a single sheet using hydraulic jacks to apply a uniformly distributed vertical load along the top of a fixed wall, while applying pressure laterally in one direction across the top of the wall, or alternately pushing and pulling the wall from an upper corner.

"Almost all failures are in the nail connection, with nails breaking or bending and pulling out, or with the wood around the nail giving way," said Prion, adding that while large pieces of board have not been buckling under pressure, alternate framing methods may have to be considered to avoid stress concentration around windows and doors.

The oversize wood panels are harder to handle, Prion said, but the North American construction industry is already working with large construction components such as those used in prefabricated housing.