圆钢钉和自攻螺钉钉入角度对规格材握钉力性能的影响

Effects of Angles on the Screw and Nail Withdrawal Strength in Dimension Lumber

【目的】基于规格材上不同角度钉入圆钢钉和自攻螺钉的握钉力性能测试,研究不同钉入角度、木材密度和木材径、弦向对握钉力性能的影响,为木结构钉连接设计提供更完善的科学依据。【方法】以落叶松和白云杉规格材为研究对象,参考握钉力性能试验国家标准,选取国产直径2. 5 mm的圆钢钉和直径4. 0 mm的自攻螺钉,长度均为50 mm,在满足国标最小钉边距、端距和间距的要求下,与木纤维分别成90°(横纹)、60°、45°和0°(顺纹)钉入规格材,在自主设计的多角度握钉力试验夹持装置上以3 mm·min-1恒定速度拔出并得到破坏荷载,比较分析不同条件下的握钉力。【结果】1)随着钉入角度减小,钉拔出时的荷载-位移曲线峰愈加尖锐,自攻螺钉握钉力大于白圆钢钉,密度较大的落叶松的握钉力大于白云杉,且对自攻螺钉的握钉力刚度明显大于白云杉,但相同角度条件下拔出时的荷载-位移曲线特征基本相似;2) 90°钉入时圆钢钉和自攻螺钉的握钉力均大于0°,但从90°到0°圆钢钉的握钉力先增大后减小,而自攻螺钉则相反;3)白云杉径面自攻螺钉的握钉力大于弦面,其圆钢钉的握钉力没有规律,落叶松径面圆钢钉的握钉力小于弦面,其自攻螺钉的握钉力没有规律。【结论】钉与木纤维之间的角度对规格材握钉力具有显著影响,随着角度减小,规格材对自攻螺钉的握钉力先降低后增大,而对圆钢钉的握钉力则相反。规格材径面和弦面的握钉力没有确定关系,在木结构握钉力设计值计算时无需考虑年轮角度参数。欧洲木结构设计规范(BS EN 1995-1-1:2004)推荐的计算公式为螺钉握钉力随着钉入角度减小而减小,测试结果证明该公式在0°时握钉力估算过于保守。端面钉入的自攻螺钉被拔出时,荷载-位移曲线和破坏特征均表明其为明显的脆性破坏,在实际工程中木构件端面上使用自攻螺钉连接需要有效的增强措施和强度验算。

【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(2.5 mm)and self-tapping screws(4.0 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.