I wish to tap an M24 thread through 2

[Ginger]

I wish to tap an M24 thread through a 12mm thick piece of steel. This piece of steel is then fitted tight against a concrete surface.

An M24 bolt is then screwed into the hole and stops when it meets the concrete so I only have the end of the bolt being held by the thread.

Is there a method of calculating the capacity of this connection in tension or is the best solution to test a connection to failure?

Regards Andy Machon

 

[dik]

Not sure that I understand the question.

How is the plate attached to the concrete? independent of the bolt? Does the bolt act as a cantilevered stub, projecting from the plate?

Are you using the bolt to attach the plate to the concrete and there's a threaded insert in the concrete?

If so, I'd likely consider the diameter of the bolt based on the root diameter and treat it as a 'rod'. How far away from the face of the plate is the load applied? Moment and shear with some stress riser caused by the thread root.

Is the load cyclical and the bolt subject to fatigue?

 

[diamondjim]

Ginger,
Assuming the steel is 100 psi, a general rule
for depth of material is 1.1 times the diameter
to ensure full capacity of the bolt. (Grade 8)
You might check Machineries Handbook, I know the
general formula is in there, but lengthy.
This formula is saying the bolt will break or rupture
before you strip the threads if you follow the
guidelines. Just out of curiousity, are you using
these as leveling or jacking screws? Another thought
would be to weld a thick nut to the plate.

 

[Ginger]

Dik / Diamondjim

Thanks for your quick response. With regard to the situation I have, imagine if you will a 1.5 metre thick concrete wall with a 9m diameter circular hole cast through it (through which we are launching a tunnelling machine). I wish to bolt to the face of the wall a 9.1m internal diameter by 3m long steel tube for launch sealing purposes (which carries its own weight by feet seated onto the floor).

My solution for fixing the can to the wall is to roll a 200 x 150 x 12mm thick angle section into a 9m ID ring, insert one leg of the angle up the hole until the other leg hits the concrete face and forms a circular flange at the concrete face(when viewing the wall in front elevation). This flange will have approx 42 No M24 tapped holes in it. This is now the locating flange for the steel can to be bolted to.

The can (when viewed in cross section) may try to fall away from the wall and the M24 bolts holding the can to my flange will use their tensile capacity to hold the can onto the flange. This limited length of bolt held into the 12mm thick steel flange is what provides the tensile reaction force (and therefore the restoring moment) which prevents the can falling away from the concrete face.

The problem as I see it is that we don't have an equal length of thread being held by the 12mm thick angle as we would have with an M24 nut. Therefore, I am unsure as to how to calculate the capacity of the bolted connection in direct tension.

I hope this explains the problem a little better. Andy Machon

 

[evelrod]

I don't know too much about tunnelling machines, but I do know about ledger angles. Tapping the 12mm outer leg on the rolled angle will not yield anywhere near the same strength as welding a M24 nut to the backside. Just mark and relieve the concrete area where the nuts will be, and maybe a bit more for bolt projection. You might also fabricate an angle with one leg thicker than 12mm., say 24mm?


Rod

 

[diamondjim]

I hope you have Machineries Handbook and check out the
strength of internal threads. I would assume since you
have a thread length of .5 times the diameter, that you would have to back off on the amount of torque than you
would apply to these bolts. Guess that you would use only
50 percent of the proof loads as a safe value. But you
can do the homework. You might have to consider doubling
the quantity of bolts and user smaller diameter bolts.

Again if the thick nuts were welded to the nearside, I doubt you could shear the weld at full strenth of the bolt.
You would have to have thru holes thru the flange to line
these up. If not thick nuts how about circular rings about 50mm od and threaded M24 thru and welded to the flanges? This would be very easy to make a sample and test all of these and use about a dozen samples to see where failure occurs. Please post your results or decision. Sound like an exciting endeavor.

 

[diamondjim]

Ginger,
Just a second thought. When you drill for the
m24 threads, consider drilling a smaller hole
thus maximizing the percent of thread. Under bolts
and nuts in Machineries Handbook, you will find the
formula needed. The old h28 handbook on threads may
have this formula as well.
If you need more specific info or want to see
the formula applied, please email me your fax
number and I will go thru the formula for you.
j.geisey@juno.com
I haved designed many bearing for Lovat Tunnel and
Robbins Tunnel Boring Eqipment, but have not gotten
into the poured concrete shells that encase the
equipment. Probably a different science altogether.
I know one of the guys at work has this in a basic
program and can probably do this for you as well.
He asked if you were a customer of ours and I could
not reply one way or the other. If you do e-mail,
let me know who you work for.