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Withdrawal Behaviour of Self-tapping Screws in New Zealand Cross-Laminated Timber

Self-tapping Screws (STS) are commonly used in cross-laminated timber (CLT) construction. However, design
provisions with STS and CLT are currently not covered by NZS 3603:1993 or AS 1720.1:2010. Manufactured by
hardened steel, STS have high withdrawal strength and provide an efficient connection type. Previous research
and analytical design equations for STS connections were focused on European softwood species which often
have lower densities than New Zealand grown Radiata Pine and Douglas-fir. This paper presents an experimental
study to evaluate withdrawal properties of STS in New Zealand made Radiata Pine and Douglas-fir CLT. A total
of 202 withdrawal tests were performed with Ø8mm and Ø12mm STS in three-, five- and seven-layer CLT
specimens. The experimental results were compared with the analytical design equations in literature. It was
found that the design equations are generally applicable to the New Zealand CLT specimens. The penetration
length of the threaded portion of STS should be limited to 12d (12 times the screw diameter) to avoid brittle
fastener tensile failure. For partially threaded screws, increasing embedment length of the unthreaded portion
of STS could prevent timber surface splitting and the average withdrawal strength slightly increased by 10-15%.

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