Fixings for MF to block and beam ceiling

[saveasteading]

For a self tapping concrete screw, all the advice is good. I would just emphasise the importance of the hole being perfect, ie exactly the right diameter and cut cleanly.

The instructions will say.

Also is an SDS going to roughen the side too much?

 

And, it is normal to discount the first 20mm of concrete in calculating fixing length. this is because it can be all of fine stuff. AND because the pullout failure mode is conical, so strength increases dramatically with depth.

Hence the screws are a bit long.

 

I don't know what the sharp point is for either. Maybe that is just how screws are made to get the tapering effect when the cutting starts.

OR because people complain that there is no point.

[Thorfun]

2 hours ago, saveasteading said:

And, it is normal to discount the first 20mm of concrete in calculating fixing length. this is because it can be all of fine stuff. AND because the pullout failure mode is conical, so strength increases dramatically with depth.

I didn’t know that! So 50mm minimum then which will give 30mm of strong fitting. 

[Thorfun]

3 hours ago, Oz07 said:

How do blocks take to being sds drilled when used in transverse on b and b floor. I'd be a little concerned it would crack. Sure I'm worrying about nothing, you'd soon get a feel for it

I honestly have not got a clue. But I can’t drill in to the beams so it has to be the blocks. 
 

I presume it’ll be fine otherwise @nod and all the other builders would’ve found out by now, right?

[Nickfromwales]

6 hours ago, Gone West said:

Last year I used concrete screws to fix some timber to a concrete block wall. They are very sensitive to the hole being exactly the right size.

Concrete screws want a 6mm pilot hole, but with concrete blocks you can pull them loose by over-tightening a short screw. 50mm or 60mm would be best.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

If you don't just save shitloads of wasted time and effort and gas nail them with a Spit gun.........

 

4 hours ago, Oz07 said:

How do blocks take to being sds drilled when used in transverse on b and b floor. I'd be a little concerned it would crack. Sure I'm worrying about nothing, you'd soon get a feel for it

Shirley, you can't be serious?

 

This is getting out of hand folks.

 

These are concrete blocks, and can be drilled multiple times into the same block without breaking a sweat.

 

3 hours ago, saveasteading said:

And, it is normal to discount the first 20mm of concrete in calculating fixing length. this is because it can be all of fine stuff. AND because the pullout failure mode is conical, so strength increases dramatically with depth.

Hence the screws are a bit long.

What he said. 👌

[saveasteading]

Don't worry.

Cracked blocks occur in beam and block floors a lot. It doesn't matter because any crack is a very ragged interface and it is all still touching and jammed in place.

 

if you try to take out  a cracked block to replace it , then it is nigh impossible. So don't worry, and summarise the very good advice above....then do it.

 

One last thing. This floor/ceiling already exists and is floored over I am assuming.

 

Next time (for you or anyone) , there are proprietary hooks that can be hung down between the blocks.

[Gone West]

15 hours ago, Thorfun said:

i guess i'm completely misunderstanding the meaning of "self-tapping". i thought it means you don't have to drill a hole but that's not true is it? you drill the hole and then the self-tapping screw creates the thread that holds it in to the hole, right?

Yup, if you were using self tapping screws into metal you would drill a hole first, unless you were using Tek screws when you wouldn't have to drill a hole.

[dpmiller]

self-tapping and self-drilling are two different things...

[Thorfun]

4 hours ago, dpmiller said:

self-tapping and self-drilling are two different things...

and that's where my confusion lay. I'm am now educated. 🙂