Does the use of lag screws with shear plates affect installation?

Shear plates are designed to be installed with standard machine bolts and nuts, but they can also be used with lag screws.  One thing to take into consideration is how the use of lag screws affects installation.  When installing lag screws, you need to drill a pilot hole to prevent the head from breaking (Lag Screw Pilot Hole Diameters).  This causes an issue when installing shear plates because the size of the pilot hole for lag screws is different than the hole drilled for bolts. 

For example, The use of a  4” x ¾ dapping tool requires a 13/16” pilot hole, but the pilot hole for a ¾” lag screw is ½”.

Section 12.1.4.2 of the American Wood Council’s National Design Specification for Wood Construction reads as follows:

“Where lag screws are used in place of bolts, the hole for the unthreaded shank shall be the same diameter as the shank.  The diameter of the hole for the threaded portion of the lag screw shall be approximately 70% of the shank diameter”

For example, if you have a ¾” x 12” lag screw with standard 6” thread length, you would drill a 12” deep pilot hole ½” in diameter then re-drill the top 6” of the hole 3/4” for the shank of the screw.  However, the pilot bit on the grooving/dapping tools is 13/16” in diameter.  Therefore, you will need to check with your engineer to determine if the top portion of the pilot hole can be drilled large enough to accommodate the 13/16” pilot bit or if you will need a custom pilot bit made that will fit into the ¾” diameter hole.

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