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- To: <seaint(--nospam--at)seaint.org>
- Subject: More ? about screws
- From: "Nels Roselund, SE" <njineer(--nospam--at)att.net>
- Date: Tue, 9 Sep 2003 21:52:19 -0700
Are there any tests of strengths of sheet metal screws as fasteners for wood?.
Sheet metal screws are threaded from point to head and, except for the very shortest, who's length consists mostly of point, sheet metal screws are not tapered .
I would expect sheet metal screws to be stronger in pull-out than wood screws because wood screw threads are tapered, and their length is about 1/3 unthreaded shank, so that the sheet metal screws would have more threads in contact with the wood.
On the other hand, I would expect that wood screws are stronger in shear because of the unthreaded shank under the head, which provides more bearing on wood for bearing-controlled yield modes. For flexure-controlled yield modes, the fully threaded shank of the sheet metal screw is more flexible than the solid shank of the wood screw, so I would also expect wood screws to be stronger in shear for flexure-controlled yield modes.
Wood screws have the disadvantage of requiring 2-step drills for lead-holes for round-head screws, and 3-step drills for lead-holes for flat-head screws, while a round-head sheet metal screw requires only a single-size lead hole for a round-head screw -- so there is a significant difference in installation costs.
I have been toying with calculating strengths as follows:
For pullout, using the wood screw values of strength per inch of thread embedment should be should be reasonable, though conservative.
For shear, using the wood screw formulas in the NDS based on a screw diameter equal to the average of the base of thread diameter and the unthreaded shank diameter should provided reasonable values for all yield modes, whether bearing or flexure controls -- but I'm not sure whether the result is likely to be conservative or not.
Any thoughts Or know the source of test-data?
South San Gabriel, CA
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