Pullout capacity in SDS into Aluminium 2

[m_struct]

Looking at fixing two pieces of aluminum together (aluminum cladding panel into a 4mm thick aluminum batten) with 12Ga self drilling screws. The screw manufacture published test data for pullout and shear data for into 1.6mm thick 450MPa (f_u = 480MPa) steel, 2.8kN. The aluminum is approximately 240MPa (f_u = 290MPa). Is it reasonable to calculate the pull out capacity of the screw into the aluminum by taking use ratio the yield strength x test data, ie 2.8kN * 290/480 = 1.7kN for a 1.6mm piece of aluminum?

Also can pullout capacity equations in steel members be used for aluminum?

Such as P =pi/4 * (D_major² - D_minor²) * t/p * f_u. Where t = material thickness, p = thread pitch, f_u = tensile strength, D_major = outer thread dia, D_minor = inner thread diameter.

 

[Ron]

No, you can't use steel pullout values for aluminum. It would be a lot more accurate to test the pullout in your materials. It is an easy test and considering the materials you described, the values will be fairly low.

 

[m_struct]

Thank you Ron. In terms fixing to aluminum, what screw material and coating is preferred? Stainless steel?

 

[NorthCivil]

what jurisdiction do you practice in? there should be a standard available where you can calculate strength of screws into aluminum.

you should double check where you got the strength values for that aluminum, seems high.

if you dont know what your doing, get help or leave it to the experts.

 

[GastonLagaffe]

If you're in Canada, CSA S157 does have a relatively easy formula to calculate screw pullout strength in aluminum. Depending on your thickness the strength is controlled by yield or ultimate strength. In the US you'd be looking at the Aluminum Design Manual (uses the same formula as S157) and in Europe you want Eurocode 9.

Looks like you're using 6061-T6, be careful about using the Fu = 290 MPa. Unless you're spec'ing ASTM B209 plate, I'd use Fu = 260 MPa which is what is used in basically every other 6061-T6 product. Though in my experience it's not uncommon for the MTRs to come out with Fu > 350 MPa.

We've almost always used stainless 304 fasteners. We'll use 316 if it's at very high corrosion risk but it's about twice the price. I think some curtain wall manufacturers use zinc-coated steel, which is probably fine too if the fasteners aren't directly exposed to weather. The stainless screws are a bit weaker and "stickier" though so you might need a bigger pilot hole to get them in without snapping, which you'll need to consider in your pullout calc.

 

[GastonLagaffe]

Also if this is for cladding, be 100% sure you're dealing with 6061-T6. Ask to see the MTRs. If you have the chance to visit the site, bring a piece of known 6061-T6 and see if you can scratch/gouge the metal. 6063-T5 is very common in the enclosure world and it's much, much weaker (Fy 110, Fu 150).

It happened to me once where I ordered some 6061-T6 tubing and was invoiced as such. This was for a research project so I did some coupon tests on there, and it failed at half the expected strength! We checked it out with the X-ray gun and sure enough we had a 6063 alloy. Not a fun "oopsie" to find out once you've built the thing!

 

[Ron]

Stainless steel screws are appropriate.

 

[m_struct]

Thank you for the heads up for different grades of aluminum grade. The Aluminum Design Manual seems to cover pull-out for rivets and tapping screws, but I do not see self drilling screws.

In this project, pull-out testing was done for 4.2mm rivets. The contractors has asked to use 12Ga self drilling screws. We have requested testing data for the 12Ga option. In the meantime, I was hoping to calculate the expected pull-out values.

 

[Settingsun]

Judging from your location (NZ), you probably want this:

Phi_sc = 0.50. Applies to self-drilling self-tapping screws.

 

[IFRs]

Have you looked in publications from The Aluminum Association?

 

[GastonLagaffe]

Self-drilling screws are just regular screws that drill their own pilot hole. I think you should be fine using the regular screw formulas, with your hole size taken as the drill point diameter (should be on the data sheet for the screws).

 

[IFRs]

From the Aluminum Design Manual 2005 Edition:

 

[dik]

Thanks for the information...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[m_struct]

In the Aluminum Association Aluminium Design Manual, it states "Aluminum can be coupled to magnesium, zinc, cadmium, and passive stainless steel in most environments without galvanic corrosion".

For wall cladding, in a really humid, coastal or over aggressive environments, a non-conductive washer or spacer is needed. I have heard of rubber, neoprene, or EDPM washer used. Is one preferred over the other?

Also, in a dry or normal climate (no electrolight), is it good practice to include a non-conductive washer or not necessary?

 

[GastonLagaffe]

I don't think a non-conductive washer will do much with self-drilling screws - the threads will still be in contact. You'd use those more typically in bolted connections between dissimilar metals. Often you might even put in a sleeve to make sure there's zero contact.

If you're going with stainless I wouldn't be very worried about it. Go with 316 if you really want to make sure.

From CSA S6-19: