Screw tip protrusion

[NorthCivil]

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?

 

[Duwe6]

What you'll find is "Flush at a minimum". Thus, the condition you have described is the same, structurally, as the previous one with a bunch of point sticking out. Can't believe I'm saying this, but looks like you're wrong and the client has the correct view of the as-built condition.

If you were worried about vibration, threaded fasteners w/o locking devices should've been your last choice. Too late now.

 

[MintJulep]

faq725-1331

 

[NorthCivil]

@Duwe6, we (engineers) don't know everything, and aren't always right about it when we think we do. Our clients have a lot of valuable experience in their own right, and their opinions are valid too. That's why when I see something I don't like, I try not to come outright and say its wrong, just that it concerns me.

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.

 

[dhengr]

NorthCivil:
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.

 

[wannabeSE]

I believe AISI S100 requires screws to protrude three threads. But this standard is for cold formed steel.

 

[Duwe6]

"Flush at a minimum" is tucked into AISC and/or RCSC. I've also seen it in ASME, but I can't remember where. First place was Sect III, because that is what I started out on.

 

[NorthCivil]

The screw was self tapped through both items. the head sat on the aluminum.

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.

 

[Duwe6]

FYI - RCSC is free at "

http://www.boltcouncil.org/files/2009RCSCSpecification.pdf"

100 pages containing a LOT of valuable commentary. Strongly recommend everybody working with fasteners download it. Guarantee that you will learn something.

 

[dhengr]

NorthCivil:
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).

 

[hokie66]

I'm with your client on this. You have a screw attaching 1/8" aluminium to 1/4" steel, so the aluminium will fail first.

 

[BUGGAR]

If y'all are worried about injuring yourselves from exposed screw points, put wire nuts on 'em. I've even seen heat-shrink tubing used for this.

 

[Tunalover]

I think that what you want is an acorn nut to go on the exposed screw thread. Why do you call them screw "tips"? They're not self-tapping are they?

Tunalover

 

[Tmoose]

I wonder what method was used to shorten the screws. If someone grabbed the tip with a big pair of end cutting pliers and rocked them back and forth I'd be concerned that the screws might now be loose. Actually I'd somewhat concerned no matter what the method.

http://www.toolup.com/images/Product/large/148-10_lg.jpg

 

[tbuelna]

The requirement for having X number of screw/bolt threads extending beyond the female threads is for an inspector to be able to make a visual confirmation that the threaded connection has a sufficient engaged length. In aerospace there is a general rule that at least two complete threads must be visible beyond the locking feature of a nut or insert. In reality, once the screw or bolt installation has been inspected, there is no benefit provided by the threaded portion of the bolt/screw extending beyond the female threads. So grinding the screw ends flush would not seem to create an issue with regards to the tensile strength of the fastener installation.

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

 

[IFRs]

"The screw was self tapped through both items. the head sat on the aluminum." You may have little or no clamping force between the metals unless the screw stripped out the aluminum which is unlikely. The two metals may not be tight together unless they were clamped first. If water gets in between them, look for corrosion soon. If there is a gap you may have combined shear and bending in the screw. If the screw was stainless it may be 410 series which corrodes more than 300 series in certain environments. That being said, in my humble opinion, if the screw extended far enough outside the steel plate that it was fully engaged with the steel, I see no real harm being done here. The deformation of the screw body or threads may serve to lock it in place, allow for some mild corrosion which may further lock it in place, it sounds like the shear load may be small. Is there vibration? What is the shear load?