Val,

Here is a summary of the Gougeon Brothers screw pull tension tests. The screws were bonded into Sitka Spruce samples with 1/4 in plywood on the front face. Screws were set in oversize holes of 1/4 or 3/8 inch using un-thickened epoxy and silica (cabosil) thickened epoxy. There was no significant difference between thickened and un-thickened epoxy. Total failure occurred when the resin plug was pulled from the wood. They also recorded the load at first detectable movement. The low mean value for first detectable movement was 435 pounds with #8 wood screws, while the high mean value was 1094 pounds for #14 machine screws. The low mean value total failure was 781 pounds for #8 machine screws, and high mean total failure was 2198 pounds for a #14 machine screw. Looking at their table, it appears that the low mean values represent the 1/4 inch diameter holes, while the high mean values represent the 3/8 inch diameter holes. Larger screw sizes increased the load values, probably because to they were longer – length was not specified.

If you are concerned about secondary bonding to the end pour, use an oversize tap to thread the epoxy before bonding in the u-bolts. Of course, you would degrease your drills and tap before using them.

Sam