Screw tip protrusion
Screw tip protrusion
(OP)
Have a job where component connects to structure. Specified a l=1+1/4", #12 screw, drilled and tapped through the component, 1/8th aluminum, to the structure, 1/4" steel angle.
Went to site to inspect, and another trade has come along and trimmed all of my screw tips. now we have the screws, fastening the component to scructure, but with no tip protruding past the steel angle.
Told client to re-fasten but he's giving me a hard time about it. connection is mainly in shear. clients opinion is the screw is through the component, and through the structure; the tip would be doing nothing on the other side if it was still intact. I'm not a fan of it, I'm thinking that over time, vibrations may loosen the screw. I'm trying to find some code/literature on it.
suggestions?
Went to site to inspect, and another trade has come along and trimmed all of my screw tips. now we have the screws, fastening the component to scructure, but with no tip protruding past the steel angle.
Told client to re-fasten but he's giving me a hard time about it. connection is mainly in shear. clients opinion is the screw is through the component, and through the structure; the tip would be doing nothing on the other side if it was still intact. I'm not a fan of it, I'm thinking that over time, vibrations may loosen the screw. I'm trying to find some code/literature on it.
suggestions?





RE: Screw tip protrusion
If you were worried about vibration, threaded fasteners w/o locking devices should've been your last choice. Too late now.
RE: Screw tip protrusion
RE: Screw tip protrusion
In regards to the "flush at minimum", did you get that from any published standards that apply to structural?
@Mintjulep, thanks for the link, I did look it over before posting. However in the structural field I really hesitate to cross over code from bolts to screws. Most of the litt in the link relates to how far the threads should extend past the nut, which in our case, we don't have.
Thanks for the input guys.
RE: Screw tip protrusion
I don’ t know exactly what you mean by a screw, a wood screw, sheet rock screw, self tapping sheet metal screw, all with a pointed end; or a machine screw of some sort where you did actually have to tap the .25" angle for the screw to make the connection. The 7/8" projection would annoy almost anyone, particularly if they caught their clothing or arm/leg on it. I don’t quite understand what your detail or situation is, but why not take a center punch and punch a couple dimples btwn. each screw and the angle iron. This should provide some control over the potential of vibration loosening the screws.
RE: Screw tip protrusion
RE: Screw tip protrusion
RE: Screw tip protrusion
I'm not especially keen to buy a bunch of american codes that may or may not contain what I need - I have nothing to refer to in my canadian code.
I looked into the ICC reports for hilti and simpson self tapping screws - they both are rated for min 3 threads protruding past the supporting steel, so I'm taking that to be the standard in this application. Contacted a hilti rep and he mentioned the potential problems - including heat generated during the cutting of the steel (i imagine they were cut off with a light angle grinder), which could affect the integrity of the steel.
Thanks all for your valuable contributions.
RE: Screw tip protrusion
RE: Screw tip protrusion
In good part, the reason for the 3 threads min. is that the cutting tip isn’t out of the base metal until about that length; and you are finally started to engage the full shank dia. and the full thread, from the standpoint of full tensile strength and holding power. The shear at the faying surface is a slightly different matter. The screws are much harder than the alum. and the steel angle for that matter and their strength or hardness probably wouldn’t be changed much back at the faying surface. You said the loads were low, primarily a shear loading, and didn’t say much about it being a significant vibrating load. If it was a vib. problem you probably should have used bolts (machine screws) and nuts, and you might still add a few of these for good measure, at stiff spots in the system. The ref. sources you used are probably as good as any to start to formulate an engineering judgement. I would still consider just using a center punch to form a couple dimples btwn. the stl. angle and the screw at the thread joint btwn. them. This will inhibit the screws from turning out (loosening). Try a couple and see if they don’t turn out harder and if that won’t start to settle your concerns. I don’t have a code section to point you at for the 3 thread min., but in the testing they have done they wanted the full shank dia. and full threads engaged, that’s how the 3 threads min. was arrived at. You might actually have bigger problems with the alum. and the screw (or bolt) and stl. angle in contact with each other, from the dissimilar metals standpoint. Google electrolytic corrosion (galvanic corrosion).
RE: Screw tip protrusion
RE: Screw tip protrusion
RE: Screw tip protrusion
Tunalover
RE: Screw tip protrusion
http://www.toolup.com/images/Product/large/148-10_...
RE: Screw tip protrusion
Having said all that, in the OP it was noted that these screws were mainly loaded in shear. It is poor design practice to put the threaded portion of a screw/bolt body in shear. So for this reason you may wish to re-think your fastener installation.
Good luck to you.
Terry
RE: Screw tip protrusion