Structural Performance of Timber Connections with Long Self Tapping Wood Screws

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Long self-tapping wood screw (STS) is an important recent European invention intended to reinforce the inherent weaknesses of wood such as the tension and compression perpendicular to the grain strengths and longitudinal shear strength. These screws are widely used in Europe to reinforce large timber/glulam beams that have suffered from excessive cracking. Besides the role of reinforcements, these screws can also be used to provide primary load transfer in timber connections. In N. America, these screws are not yet popular; code provisions for lag bolts have the closest resemblance for the applications for STS. However, these provisions ignore the most important feature of these connectors, namely, their withdrawal capacity and stiffness applied at an angle to the wood grain; therefore, these provisions are not appropriate. The extensive database developed in Europe on STS for different brands of proprietary STS products, each having its own technical approval, can be applied mainly to European species (Spruce and Pine). Preliminary evaluation of the European design provisions indicates that they are not suitable for N. American species where the knot structures in the wood and density range are very different from European wood. Also there is no information on the application of these screws in application of structural composite lumber such as laminated veneer lumber and parallel strand lumber; therefore, significant work is needed to understand the behaviour and quantify the performance of STS for applications in N. America.