Self tappng screw failure 2

[diblazing]

I am involved with the design of steel and aluminum platform and have a fastener issue. I need to attach Al 5056 decking to a common a36 steel frame. I have a special aluminum extrusion that allows a piece of flat steel to be wedged into its sides and welded down to the steel frame. The aluminum decking can then be welded to the aluminum extrusion keeping the deck in place at the sides. I have currently been holding the center of the platform to the steel with zinc-plated self-tapping screws. The problem I am having is that the screws have been yielding and breaking in what looks like tensile failure. In order to change the design I need to know why is has been failing. I have looked into thermal expansion and galvanic corrosion but they both do not seem to be the problem. I think the failure could be due to cyclic loading; the aluminum extrusion that the decking is welded to creates a 5/16” gap between the decking and the steel frame. I have corrected this gap with a piece of steel flat but think that the screws could be failing do to a loads that are applied and released every time the platform is walked on. My question is, is this the right direction and do I solve it by simply beefing up my screws, using some sort of spring washer, or any other suggestion you might have. Thanks in advance for your time.

 

[CoryPad]

From your description, it does sound like you need to remove the gap. Screws work by generating preload (with a closed joint), so any steps to do this are necessary.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[diblazing]

Thanks corypad,
That’s what I thought needed to happen, it is just going to slow down my assembly time and add cost. I imagine I will have to drill though the steel spacers to create a firm joint around the entire fastener. Would it still fail if could place the screw as close to the spacer as I can without going through it? That would save me some serious time by removing any pre-drilling. Thanks again for your time.

 

[CoryPad]

Without knowing joint geometry, material properties, and force magnitudes, it is impossible to answer your question.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[griffengm]

Just how is the fastener failing? Breaking? If so, at head or next to the steel member? Is it stripping? If stripping, where? In the steel member, on the shank of the fastener?

Griffy

 

[dik]

Can you use pop rivets?

 

[DaleJ]

Your problem can be fixed by using a thermal drilled hole connection to hold the aluminum platform to the steel. It may also save you time and costs.

Check out this web site for more information:

http://www.frictiondrill.com

 

[scooter911]

Hilti makes a really neat grate fastening system, I saw it at the Offshore Technology Conference in Houston last year, try:

http://www.us.hilti.com/holus/modules/prcat/prca_navigation.jsp?OID=-11905

Good Luck

Scoot

 

[Kiwikid]

Rather than try and 'solid' fix the grating why not let it float. To keep it in place simply fix a beading around the edges to maintain its location.

Another option is to use clips with some degree of springiness to tolerate loading and unloading etc.

Other options are wire ties and stainless steel bands. Very cheap, very quick, very effective and inexpensive.

Screws are poor fasteners for anything that suffers from vibration or constant changes in loading especially if the structure has some degree of flexibility.