The effects of pilot hole, screw types and layer thickness on the withdrawal strength of screws in laminated veneer lumber

The objective of this study was to evaluate the effects of screw type, pilot hole and layer thickness on the withdrawal strength of some screws in laminated veneer lumbers (LVL). For this purpose, three, four and five layered laminated samples were produced in different veneer thickness (4, 5 and 7 mm) of Uludag fir (Abies bornmülleriana Mattf.) and oak (Quercus petraea spp.) woods. And melamine-formaldehyde (MF) and phenol-formaldehyde (PF) adhesives were also used for laminated veneer lumber manufacturing. Withdrawal strength test was applied on the test samples. According to the test results, it was found that the highest withdrawal strength was obtained in oak samples having 4 mm veneer thickness bonded with PF for 3.5 × 16 screw as 23.14 MPa, the lowest was obtained for Uludag fir having 5 mm veneer thickness bonded with MF adhesive with 4.0 × 50 screw as 7.61 MPa. The use of proper size of pilot holes is advised in order to avoid splitting of the face during insertion of screws as well as to obtain maximum withdrawal strength.