Study the influence of perpendicular to grain compression and creep in 4 to 8 Storey Lightweight Timber Framed Buildings
6th July 2018
The aim of this project was to validate the assumption used within the Australian Standard AS1720.1 for calculating compression perpendicular to grain for common timber species and develop practical methods to reduce compression deformation.
The project was successful in demonstrating that perpendicular to grain displacement of wall plates by studs can be reduced. It found that stud on stud connection was the best method as it removed the wall plates out of the load path altogether. The study also found where this is not possible, replacement of the wall plate with a stiffer timber such as a high density hardwood, softwood or cross laminated timber reduces this deformation.
The investigation also found that the method used in AS1720.1 to assign perpendicular to grain bearing capacities for various timber species by “strength group” or stress grade over predicted low to medium density timber species whilst under predicting high density timber species. Accordingly, it is recommended that perpendicular to grain bearing capacity be assigned by the timber species’ density and that the characteristic values of commonly used timber species be re-established.
It was also evident that AS1720.1 k7 bearing length factor was incorrect as it did not match the source characteristic value determined by AS/NZS 4063; as AS1720.1 increases the bearing capacity by 20% when it was not warranted. The research recommended that AS1720.1 method be changed to a serviceability check so that designers can limit the deformation required and also account for creep.
It was also found that the effective length in calculating the buckling capacity of a stud is effected considerably by the stiffness of the wall plate used. As the stiffness of the wall plate increased, the effective length shortened substantially increasing the capacity of the stud. Therefore, it is recommended that further work in this area be carried out as the current method contained within the timber engineering standard will heavily penalise wall systems that utilise stiff wall plates or stud to stud connections.