Experimental assessment of self-tapping screws for the reinforcement of multiple holes in laminated veneer lumber beams

Abstract

In recent times, there is increased focus in the built environment on the use of low carbon materials. In tall timber buildings, service ducting needs to be facilitated. Multiple holes in elements may be required in buildings where a high demand of services exist. Limited investigations have been conducted on laminated veneer lumber (LVL) beams in comparison to the literature on timber and glued laminated timber beams and no experimental testing has been conducted where multiple holes in LVL have been reinforced. Such openings are of particular concern when positioned in high shear zones. Therefore, an experimental programme in which seventy-five LVL beams were monotonically tested in three-point bending was conducted. The programme included standard beams, beams with a single round hole, and beams with double round holes for three different hole diameters and two holes spacings. The openings were concentrically positioned and were reinforced using internal screw reinforcement in which three configurations were examined. The programme was designed to evaluate the guidance that is currently available which has been developed based on testing with glued laminated timber. Throughout, it was seen that the self-tapping screws orientated at 60° were the most effective in comparison to when the reinforcement was orientated at 45° or positioned vertically. For LVL beams with single holes and up to hole diameters of 0.45 diameter (d) which far exceeded the current guidance of 0.3d maximum hole size, the reinforcement when positioned at 60°, achieved midspan failures like standard beams. The failure loads of such reinforced beams were in the same range as the standard beams. The screw reinforcement when orientated at 60o, was most effective for the double holed beams when a hole size of 0.35 d for both spacings (0.5 d and 0.75 d) was studied. The load carrying capacity was also in the same range as the standard beams but the failure mode was hole edge failure. Strain profile readings indicated the effectiveness of the reinforcement such that strains at the soffit of the beam where two holes were reinforced with screws exceeded the strains at the soffit of the standard beams.