Investigation on the self tapping screws capability to induce internal stress in timber elements

In refurbishment operations of existing timber floors, the possibility of hogging the existing timber beams, in case of the presence of excessive permanent midspan deflections, could prove of some interest. In the case of sagged timber floors which cannot be buttressed due to heritage issues, the possibility of cambering a timber beam by simply putting another wooden beam on the top of it and inserting screws inclined at 45° relative to the beam axis was experimentally investigated, with some promising results. The cambering procedure proved to be more effective when the fasteners are inserted starting from the internal part of the beam, permitting to obtain significant values of upward deflection (an upward deflection of about one three-hundredth of the total beam length was observed): the values could possibly be increased by reducing the screw spacing or by using fasteners able to generate a greater pressure. The effectiveness of this method is based on the capability of self tapping screws to induce internal stress in timber elements during the drilling procedure: the horizontal component of the resultant pressure yielded by the inclined screws is directly related to the possibility to hog the composite system. The aim of the experimental campaign described in this paper is to investigate the values of the internal stress induced by different types of fasteners during the drilling procedure, studying the influence of different parameters such as screw angle with respect to the grain direction, initial pressure, head penetration length, threaded part length, connector typology, wood density, time dependence.