The tensile strength of a material is the value at which the material fails when subjected to a tensile force. (Tensile means a force pulling the wood fibers lengthwise, as opposed to a compressive force.) There are a lot of dissimilar types and load cases for wood, so there is not one answer to this question. For example, wood used outside will fail at a lower load than wood inside. Wood is also anisotropic, i.e. it has dissimilar strength in dissimilar directions.