AS/NZS 4600:2005 - Clause 5.4.2.5 Screws in shear

[Toby43]

Hi all,

This clause states
"The nominal shear capacity of the screw shall be determined by testing in accordance with Section 8 and shall not be less than 1.25 Vb where Vb shall be calculated in accordance with Clause 5.4.2.3." CL5.4.2.3 relates to tilting and hole bearing, which depends upon member thickness and ultimate tensile stress (UTS). This 1.25Vb limit would seem to prohibit the use of screws in "thick" steels with high UTS - For example a #12 screw (5.5mm (0.216inch) diameter) in Grade 300 40x40x3 Equal Angle with UTS=440MPa would provide Vb of approx. 20kN. A #12 screw alone is approx. 10kN (ICC report for ITW Buildex), thus violating CL5.4.2.5.
I assume this clause relates to ductile performance of the connection and this would lead one to assume screws cannot be used for members with such attributes, yet I have come across some existing connections utilizing screws in "thick" steel, in which the connection capacity exceeds seismic demand ("elastic" - no force reduction for ductile response) and wind loading demand, yet does not satisfy CL5.4.2.5. Furthermore, the text "Cold-Formed Steel Design" (4th ed.) by Wei-Wen Yu and LaBoube, which to seems to form the basis for a majority of AS/NZS4600 Provisions, does not seem to provide such a limit.
I am currently reviewing a significant number of these connections and am reluctant to condemn them without due consideration of the intent of CL5.4.2.5. I would appreciate anyone's experiences/opinions on this matter.

Thanks in advance
Toby

 

[Backcheckrage]

What is the actual connection? Is it resisting seismic actions?
Section AS4600 5.4.2.5: If i'm understanding this language correctly... it sounds like it's trying to say "hey, when you test your screw in shear, make sure the testing regime is such that you have selected strong enough base metals thick enough to prevent screw tilt or bearing phenomena from occurring".

The reason I say this is because Section 5.4.2.3 is generic and the Vb value from that section is a moving target, so therefore it's just saying "keep your screw lab tests clean and uncontaminated from tilt/bearing phenomena"....?

Otherwise, I actually don't know what it's trying to say.

 

[Backcheckrage]

Let me add to my comment above:

I think AS4600 5.4.2.5 is merely attempting to guide the testing on the shear capacity of the SCREW ITSELF. Not some sort of assembly, this is why it's demanding the test setup is such that the testing load doesn't get close to Vb. And in this case 1.25xVb.

 

[Toby43]

Thanks Backcheckrage. Didn't think of it from that angle, yet it makes sense.