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Experimental testing of high capacity screwed connections in Douglas-fir CLT

By Thomas Wright & Minghao Li, January 2021.

Download SWP-T119 (pdf)

Executive summary

This report presents experimental results of high capacity hold-down connections in Douglas-fir Cross Laminated Timber (CLT) using self-tapping screws installed with mixed angles (inclined and 90° angle to the timber surface).

This report consists of two experimental phases. In phase 1, a total of 28 small scale connection tests (50-100 kN capacity) were performed to identify the optimal threaded length of screws under withdrawal loads, the suitability of 45° washers to the mixed angle screw applications, and the optimal ratio of inclined screws to 90° screws. In phase 2, a total of 27 large scale connection tests (300-700 kN capacity) were performed to evaluate the performance of the mixed angle hold-downs performed in a CLT shear wall system. The tests also investigated the difference between single sided hold-downs and the double sided hold-downs, the optimal ratio between inclined and 90° screws, and the reparability of mixed angle screw connections after severe damage.

It was found that:

  • Fully threaded screws can provide higher stiffness and more load carrying capacity per fastener, tensile failure of the screw must be avoided to allow their use in this style of connection.
  • The 45-degree washers evaluated are suitable for mixed angle screw connection type if proper detailing allows for the formation of a plastic hinge at the head of the withdrawal screw.
  • The ratio of 1:2 (number of withdrawal screws to number of shear screws) was found to be the optimum ratio (compared to 1:1 and 1:1.5).
  • Double sided horizontal hold-down tests provided significant displacement capacity performance benefits over a single sided test, as that was not well horizontally constrained.
  • Tests prove the suitability of mixed angle screw hold-down connections to repair and both repair methods had broadly similar behaviour to the original connection.

The experimental results confirmed the suitability of Douglas-fir CLT and mixed angle screw installations for high capacity hold-down systems. These connection results will provide valuable technical information for engineers to design mass timber structures utilising Douglas-fir CLT in the lateral load resisting system to resist seismic loads.


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