| Author |
Message |
William C. Pegues, FCSI, CCS
Senior Member
Username: wpegues
Post Number: 869
Registered: 10-2002
| | Posted on Thursday, August 23, 2012 - 02:58 pm: |    |
Opinions requested.
We have recently received a comment to our curtain wall section to specify specific products by specific manufacturers for screws related to curtain walls. Specifically, 3 manufactures and their 3 products were called out where 'self drilling screws' may be required.
Questioned, the response was:
"The fasteners listed are only case hardened at the tip (not fully case hardened). In the past there have been negative experiences when specific screws were not specified, because fully case hardened screws were used. These screws are then subject to failure by having their heads shear off as a result of being fully case hardened."
I myself don't mention anything about self drilling requirements or case hardening. The statement about fasteners that I use is more similar to what appears in Arcom's MasterSpec referencing the material, head type, etc.
Has anyone heard about this kind of failure before?
Thanks!
William C. Pegues, FCSI, CCS, SCIP Affiliate
WDG Architecture, Washington, DC | Dallas, TX |
Richard L Matteo, AIA, CSI, CCS
Senior Member
Username: rlmat
Post Number: 534
Registered: 10-2003
| | Posted on Thursday, August 23, 2012 - 03:34 pm: | |
Who made the original request, and who made the response? Contractor or CM?
By specifying specific fasteners, aren't you treading on "means and methods"? I wrote a lot of curtain wall specs for a firm back East and don't remember specifying a specific fastener, other than the material. By the way, what material are we talking about?
Also, I'm not sure you can just "case-harden" the tip of the screw, in any even maybe they need to back off the torque on the screw gun. My personal experience has been that any screw head will shear off it the torque on the drill is to high. I even had the tip of the screwdriver bit shear off because it wasn't case hardened.
I generally leave the selection of fasteners up to the manufacturer by calling for his standard. |
William C. Pegues, FCSI, CCS
Senior Member
Username: wpegues
Post Number: 870
Registered: 10-2002
| | Posted on Thursday, August 23, 2012 - 04:06 pm: | |
Richard,
This was response from a review by a curtain wall consultant reviewing our documents. My typical paragraph is more like you say:
Fasteners: Stainless steel Type 300 series, selected to prevent galvanic action with the components fastened. Where exposed in finished surfaces, use countersunk Phillips heads with color to match adjacent surfaces.
So its pretty open to means and methods - the entire curtain wall spec is all performance based.
This actual project is in Dallas, TX.
William C. Pegues, FCSI, CCS, SCIP Affiliate
WDG Architecture, Washington, DC | Dallas, TX |
Richard L Matteo, AIA, CSI, CCS
Senior Member
Username: rlmat
Post Number: 535
Registered: 10-2003
| | Posted on Thursday, August 23, 2012 - 04:15 pm: | |
William - I googled stainless steel fasteners and found that stainless steel screws cannot be case-hardened anyway. Also, I would be more specific with the Type 300 series stainless steel. Type 316/316L are more corrosion resistant than Type 302 or 304
Some curtain wall consultants are good, but as a co-worker once said "they can't write specs" |
William C. Pegues, FCSI, CCS
Senior Member
Username: wpegues
Post Number: 871
Registered: 10-2002
| | Posted on Thursday, August 23, 2012 - 04:29 pm: | |
Agree on the stainless designation.
while you were googling that I did the same with one of their recommended 'screws'.
http://www.elcoconstruction.com/Products/Metal-Fastening/Self-drilling-screws/Dril-Flex-R
That's its web site, there are 2 other manufacturers that they list. These are Hex head - not available in phillips or in countersunk configuration. They are also alloyed steel and a proprietary corrosion resistant coating.
Funny, though these bear no resemblance at all to the basic requirements for stainless steel and need for exposed fasteners to be countersunk phillips, all they did was tack on the that I list out the product names.
That would sure look good under close examination!
Thanks for the bit about the case hardened not applicable to stainless. That was their major concern that the heads shear off on fully case hardened screws. So, it is essentially a non-issue.
William C. Pegues, FCSI, CCS, SCIP Affiliate
WDG Architecture, Washington, DC | Dallas, TX |
Richard L Matteo, AIA, CSI, CCS
Senior Member
Username: rlmat
Post Number: 536
Registered: 10-2003
| | Posted on Thursday, August 23, 2012 - 04:35 pm: | |
I would be leery on the steel alloy/corrosion-resistant coating with aluminum. I generally only specified aluminum or stainless steel fasteners with storefront and curtain wall, preferably stainless. Also, given the firm I was working for, when we specified exposed fasteners, hex head were not acceptable, we preferred countersunk oval head phillips. |
William C. Pegues, FCSI, CCS
Senior Member
Username: wpegues
Post Number: 872
Registered: 10-2002
| | Posted on Thursday, August 23, 2012 - 04:45 pm: | |
As you can see from the above, stainless is where I go. I was floored when I saw it was a coating. Regardless that they tout 8000 hours of salt exposure - thats on a 'newly minted' sample - and maybe not even a sample as applied to the screw but on flat metal. And certainly not after it has been driven into and through the aluminum and whatever 'self drilling' into the substrate may have occurred.
Thanks for the assist.
William C. Pegues, FCSI, CCS, SCIP Affiliate
WDG Architecture, Washington, DC | Dallas, TX |
Richard L Matteo, AIA, CSI, CCS
Senior Member
Username: rlmat
Post Number: 537
Registered: 10-2003
| | Posted on Thursday, August 23, 2012 - 05:01 pm: | |
Actually it was only an 800 hour salt spray test. Like you said, when the coating is scraped off during installation, what happens? |
William C. Pegues, FCSI, CCS
Senior Member
Username: wpegues
Post Number: 873
Registered: 10-2002
| | Posted on Thursday, August 23, 2012 - 05:17 pm: | |
haha, yes, after posting I noticed I hit the 0 too many times -grin!
William C. Pegues, FCSI, CCS, SCIP Affiliate
WDG Architecture, Washington, DC | Dallas, TX |
Wayne Yancey
Senior Member
Username: wayne_yancey
Post Number: 538
Registered: 01-2008
| | Posted on Thursday, August 23, 2012 - 05:29 pm: | |
Metals used for curtain wall fasteners can be either carbon steel or stainless steel, depending on where the anchor is used in the wall.
The use of carbon steel fasteners should be limited to the wall anchors, and to frame assembly if they will be located to the interior side of the air barrier. All carbon steel fasteners must be plated or polymer coated to provide protection against corrosion.
All fasteners located to the exterior side of the air barrier should be stainless steel. Stainless steel should also be used for pressure plate fastening.
My CMHC Best Practice Guide for Curtain Wall states to use Type 316 stainless steel, however with the following caution:
"Stainless steel has a high coefficient of friction and low thermal conductivity; both factors tend to locally heat screw threads and cause seizure. This can lead to shearing off of self-tapping fasteners during power installation."
"Aluminum fasteners, with the exception of some riveting applications, are not appropriate for use in curtain walls."
I have never received factual or anectodal evidence about shearing off of self-tapping fasteners during installation. Seems like a means and methods issue.
From now on I will spec only non-bending nail fasteners when installed with a hand held hammer, swung in an arc past the right or left ear and nearly hitting the fingers. I will add something about removing the fingers before impact with the nail head.
After 45 years this week I have encountered two items I have never encountered before nor been on my radar. The second is what is the flame spread rating of stainless steel. Really!
Wayne |
Richard L Matteo, AIA, CSI, CCS
Senior Member
Username: rlmat
Post Number: 538
Registered: 10-2003
| | Posted on Thursday, August 23, 2012 - 05:54 pm: | |
I agree with Wayne on everything down to his last sentence. As I mentioned in my post to Willam, I think the installers crank up the torque on the screw guns, causing the screws/fasteners to shear off.
As for his last sentence, it must have been a request from a building official. They don't seem to understand that there are no flame spread ratings for most if not all metals.
I used to get comments like that from plan reviewers a lot. |
Mark Gilligan SE,
Senior Member
Username: mark_gilligan
Post Number: 503
Registered: 10-2007
| | Posted on Thursday, August 23, 2012 - 06:17 pm: | |
For regular steel screws it is possible to only case harden the tip. Requiring that the screw not be fully case hardened is a legitimate concern. I consider this an engineering not a means and methods issue.
I see no need to specify specific manufactures and specific screws. Specify the properties you want and let the contractor select the manufacturer and specific screw. |
John Bunzick, CCS, CCCA, LEED AP
Senior Member
Username: bunzick
Post Number: 1438
Registered: 03-2002
| | Posted on Thursday, August 23, 2012 - 06:18 pm: | |
I know that the drill-points of self-drilling screws are harder than the shanks. They have to be to drill, but the metal is more brittle. I think they fuse the two parts together using different alloys, probably prior to machining. All of the reputable manufacturers of fasteners have this type, so I think the issue of shearing the heads is strange. Maybe it was a one-off problem and I hate to specify around those. I've never had that problem on a project, nor heard of it.
I think that there's a potentially bigger issue with hydrogen-assisted stress corrosion cracking. I never did get the chance to complete my research on this, but a good place to start is the article "Making the Right Connections" in August 2008 Specifier. Here's a link (if it works): http://www.kenilworth.com/publications/cs/de/200808/files/64.html |
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