Structural screws or through-bolts in post bases? The manufacturer's load table decides

Simpson's 2026 connector catalog lets the smaller ABU adjustable post bases go on with either nails or bolts, never both. On an ABU44Z for a 4x4, twelve 0.162 x 3 1/2-inch nails are rated for 1,900 pounds of uplift and two 1/2-inch bolts for 2,300; the download is 7,570 pounds either way. On the ABU66Z for a 6x6 the nailed version rates higher, 2,475 pounds of uplift against 2,190 bolted. The bolts have to be structural through-bolts of at least ASTM A307 Grade A, not lag screws or carriage bolts. The ABU88Z, ABU1010Z and ABU1212Z may instead take eight 1/4 x 3-inch Strong-Drive SDS screws for the same table load. The lighter ABA bases take only nails or Simpson's Strong-Drive SD screws.

Screws are catching on because they skip the hardest part of bolting. Paul McEntee, who runs engineering research and development at Simpson, wrote that boring straight through a post so the bit lines up with the steel hole opposite takes a skilled hand, while proprietary screws drive in without predrilling. Their ratings come from testing under the ICC-ES AC233 criteria.

The catch is that only the named fastener earns the number. Simpson says its SD connector screw is the only screw approved in place of nails in its connectors and that no other screw should be substituted. SD screws must be turned in, not hammered, nails and screws cannot be mixed in one connector, and nail size means both diameter and length.

The catalog adds a limit that no fastener fixes: post bases do not stop a post from rotating at its foot, so Simpson does not recommend them for posts with no support at the top, such as fences or unbraced carports. The published uplift values already include the increase for wind or earthquake loading.