Abstract:

Objective: By testing the withdrawal strength of round nails and self-tapping screws those were driven into larch and spruce dimension lumber under different angles, the effects of driving angle between axial and wood fiber, wood density, and tangential face and radial face on the withdrawal strength were studied. Method: Round nails(Ø2.5 mm)and self-tapping screws(Ø4.0 mm), were respectively driven into the larch(Larix gmelinii)and spruce(Picea glauca)lumber, and the withdrawal strength was tested following the Chinese national standard, meeting the minimum requirement of edge and end distance and spacing of nails or self-tapping screws, nails or self-tapping screws were driven into dimension lumber at 90°, 60°, 45° and 0°, respectively, and then pulled out at constant speed of 3 mm·min^-1 using self-designed nail/screw withdrawal strength test holding device until the failure load. The influences of different parameters on nail/self-tapping screws withdrawal strength were estimated through calculation and comparative analysis of failure load. Result: 1) With the decrease of the driving angle, the load-displacement curve became more sharp, and the withdrawal strength value of self-tapping screw was obviously greater than that of round nails. The withdrawal strength of value nail/screw in larch with higher-density was greater than that in spruce, and the withdrawal stiffness of self-tapping screw in larch was obviously larger than that in spruce. But the shapes of curves in different angles were nearly the same. 2) The withdrawal strength of nail/screw which were driven at 90° was greater than that at 0°, but from 90° to 0°, the withdrawal strength increased firstly and then decreased, and the variation trend of withdrawal strength of self-tapping screw showed opposite. 3) The withdrawal strength of self-tapping screw driven in the radial face of spruce was greater than that in tangential face, but that of round nail was not regular. The withdrawal strength of round nail in tangential face of larch was smaller than that in radial face, but that of self-tapping screw was not regular. Conclusion: The angle had a significant influence on withdrawal strength of nails and screws. With the decrease of angle, screws withdrawal strength decreased firstly and then increased, the trend of round nails was opposite. There was no definite correlations between withdrawal strength in tangential face and radial face, and the parameter of driving angle with tree ring should not be considered in the calculation of withdrawal strength design value in timber structure. The formula recommend in European code for design of timber structures(BS EN 1995-1-1:2004)showed the withdrawal strength of screw fall off with the decrease of driving angle, but the test result indicated that the estimation of withdrawal strength at 0° was too conservative. When pulling out the screws driven in the end face, the damage was obvious brittle fracture. So an effective reinforcement and calculation of strength was necessary for the self-tapping screws connection in timber end face in practical timber structural engineering.

Key words: driving angle, self-tapping screw, withdrawal strength, nail, dimension lumber