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  • Nov 29, 2017

Can ASSY Screws Be Considered “Wood Screws” for the Purpose of Lateral Connection Design? (Canadian)

MTC Solutions

Clause 12.11 in CSA O86 pertains to wood screws that meet the requirements of ASME B18.6.1. The yield mode equations in Clause include a higher embedment strength in the main member (f3) where the failure mode is fastener yielding, which accounts for the rope effect. Commentary for CSA O86 notes that, although the formula for f3 is the same for both wood screws and nails, screws are expected to achieve higher withdrawal resistance ( and therefore, rope effect) by virtue of their threads. Additionally, the resistance factor for wood screws in Clause 12.4.2 is ø = 0.8, compared to ø = 0.6 for lag screws. According to ASME B18.6.1, tolerances for gauge 12 screws allow for a maximum outer thread diameter of 5.59 mm and a minimum body (or “root”) diameter of 4.19 mm for screws with rolled threads. The minimum size for ASSY® screws has a major outside thread diameter of 6 mm, with a root diameter of 3.9 mm (partially threaded) or 3.8 mm (fully threaded). Therefore, most engineered self tapping wood screws do not meet the size requirements pertaining to wood screws in CSA O86, and designers must not assign the outer thread diameter to the variable df in the yield limit equations of Clause for the purpose of lateral connection design. Laboratory confirmation testing at UBC using ASSY® screws has confirmed that lateral resistance design in accordance with CSA O86 Clause 12.6.6 is very conservative, with 5th percentile test yield values exceeding CSA O86 design values by a factor of at least two.

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