A prediction model for long term creep and creep-rupture behavior of OSB was developed by the late Senior Research Scientist, Dr. L. Palka at Forintek Canada Corporation, Vancouver, BC, Canada. By using one minute destructive ramp load test results, the "Palka Model" allows the prediction of time-to-failure and time-dependent creep deformations under given sustained loads. Verification testing of the model was performed on time-to-failure at the 75% sustained load level and on creep deformation behavior at the 25% and 50% levels. The verification tests gave encouraging results which showed a reasonable agreement between the test results and the Palka Model predictions. The Palka Model is recommended for use as a first approximation prediction tool for long term creep behavior of OSB. The model prediction technique appears ideal to assist in product development decisions and also in evaluation of load duration and creep factors in timber design codes.
This addendum completes the presentation of the data and research findings for the project entitled "Development of a high performance OSB panel." Most of the short-term test results and preliminary long-term creep data were summarized in a previous report (Project No. 38-43-M-410) which was submitted to the Canadian Forest Service. This addendum to that report includes new data on rolling shear, revised data on shear-through-thickness, and creep data collected during a load duration of nine months, followed by a three-week creep recovery. In addition, the effects of parameters such as strand length and climatic conditions on the engineering properties are discussed for OSB, waferboard, and plywood panels. Utilization of long strands improved the majority of the properties of OSB panels produced with commercial strands. This improvement, however, was not observed for the OSB panels produced with laboratory strands.
Cross-laminated timber (CLT) products are used as load-carrying slab and wall elements in structural systems, thus load duration and creep behaviour are critical characteristics that should be taken into account in design. Given the nature of CLT with orthogonal arrangement of layers and either mechanically fastened with nails or wood dowels, or bonded with structural adhesive, CLT is more prone to time-dependent deformations under load (creep) than other engineered wood products such as glued-laminated timber.
Time-dependent behaviour of structural wood products is accounted for in design standards by providing load duration factors to adjust specified strengths. Since the Canadian Standard on Engineering Design in Wood (CSA O86-09) does not deal with CLT, it does not provide load duration and service condition factors. Until this can be rectified, two options are proposed for adopters of CLT systems in Canada. These include not only load duration and service factors, but also an approach to accounting for creep in CLT structural elements. The proposed recommendations are in line with the specifications in CSA O86-09 and Canadian National Building Code.
