Countersunk Pull-Through

[IreneAdler]

Hello World.

We have this scenario as shown below. Given the thickness of the plate & diameter of the bolt. I am not worried much about the pull-through of the connection, but I have to show proof / document that it will not fail. Is there a code showing this ? I know there is this formula in ADM (Aluminum Design Manual), but it is only for thin aluminum plates & self tapping screws. I am not sure if it will be applicable to steel plates & bolts. I checked in AISC 360, but nothing about pull through of material connected to the head of the countersunk bolt. Hoping for your help.

Your responses are highly appreciated.

Cheers,
Irene Adler

 

[desertfox]

hi Irene

I think pull through will be the least of you problems, the joint is more liable to fail by joint separation, bolt thread stripping, bolt overload well before the head pulls through.
What loads are on the joint in service? how many CSK screws are they? what material grade are the plates made from and what grade of screw are you using?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[IreneAdler]

Hi desertfox,

for a bolt group of 4 CSK screws, there is a 54 kN force acting on it. Plates are A36 steel & screws are Gr. 8.8. I have a gut feel that governing failure won't be pull through, but I am just not familiar with the case of countersunk bolts.

 

[desertfox]

hi

Does the 54kn load act in the centre of the bolt group? I assume direction of loading is as per your sketch.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[SnTMan]

Sanity check please. 98 thousand pounds per bolt??

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[BrianE22]

More like 12,140 lbs/bolt.

 

[btrueblood]

The OP said 98 kN, i.e. kilo-Newtons, or about 22,000 lbs. For 4 bolts, assuming equal loading, about 5500 lbs per bolt.

 

[btrueblood]

Irene,

Is there a need to use flush/countersunk fastener heads (if not just switch to hex bolts and forget about it)? Could you use counterbored socket-head cap screws instead?

I say this not because I think you have a problem with pull-though, but because countersunk fasteners can be side-loaded by a bolt pattern (ok, countersink pattern) mismatch or that is not perfectly aligned, i.e. two or three bolts in the group may make contact during assembly, but the remaining fastener(s) see an offset in the centerline of the thru hole to the countersink in the plate.

 

[SnTMan]

Ooops, I now see my conversion utility was converting kN to "poundal" not "pound". I forget the conversion from there. Sorry to intrude

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[btrueblood]

No worries SNT, I have to do the conversion manually (writing it down on paper) so it comes out right. Never trust these computers.

 

[drawoh]

IreneAdler,

What kind of flat head screws are these? If they are hex socket, another failure mode is tensile failure of the head around the socket. I have asked about this. thread725-335040

--
JHG

 

[desertfox]

Hi Irene

Well a very approximate method you can use is divide the 54kN by the four bolts, this gives the external load seen by each screw. This load you have calculated is the minimum force each screw must be tightened too. Now you can use the formula :-
F= T/(0.2*d)

Using F for the load you obtained previously and transposing the formula above to find the screw torque T, d by the way is the screw diameter and the .2.is a friction factor for unlubricated threads.
So once you have the torque from the above I would multiply that torque figure by 1.5 to give you a safety margin and use this torque figure to ensure the joint doesn't separate.

There are a few other checks you should Cary out like checking the steel being clamped doesn't yield due to tightening, check the threads on screw for shearing failure and finally the tensile stress in the screw using the minor diameter of the screw.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein