Which concrete anchors/bolts?

Lumen

So I want to attach a 550mm deep desk (20mm oak worktop) to a 100mm concrete wall using this bracket:

https://www.screwfix.eu/heavy-duty-brackets-black-500-x-300mm-pack-of-2.html

Each of the two brackets has 3 holes on the wall-facing strut of what seems like about 7mm diameter (M8 bolt doesn't fit, M6 bolt wiggles a bit). The holes are 30mm, 80mm and 270mm below the plane of the desk.

The wall is effectively a 100mm internal cavity, I think. At least part of the wall is a chimney converted for a stove, the flue pipe has some kind of an plastic liner. Another part is a simple 100mm solid block. Either way I have about 100mm to play with.

What sort of anchors should I use? I need lots of pull out strength. I would like, for instance, for a 80kg person to be able to stand on the outer edge of the desk without it pulling out of the wall.

A bolt going into a expanding threaded sleeve, ("wedge anchor"?)?

Same but bonded into the hole?

Or some kind of fixing with prongs that extends out the back face of the wall?

If the bracket was a simple pivot and I only had one bolt hole on each bracket 30mm from the pivot, with 40kg at 500mm, I think that would give 670kg pull out force. For the second pair of bolts, 250kg each. For the third, 74kg. I'm not sure how these loads will be spread, but if I divide by three I get worst case 223kg pull out force (almost 500lb).

I looked up the specs for some wedge anchors (Confast) and for 1/4" the pull out is 877lbs, which seems just about enough.

Or these M6 drop-in anchors:

https://www.ie.screwfix.com/easyfix-drop-in-anchors-m6-x-25mm-pack-of-100.html

...which don't have pull-out strength listed, but this site says >900lb for 1/4 inch.

https://www.concretefasteners.com/technical-specifications-for-drop-in-anchors

Not sure my maths is right though.

Help!

listermint

Widen the holes to suit he m8 or even M10 then use them I wouldn't use anything smaller.

Metal drill bit to widen

Lumen

I was passing a hardware store and found some brass (?) anchors in M8 and M10 in a loose parts bin.

The M10 ones were pleasingly longer, so I'll see whether I have enough material to safely drill the extra 2.something mm.

The hardware shop dude didn't know how much they should be torqued (I would have thought correct torque was vital with these anchors).

I'll try and find some with instructions.

listermint

Torque values

http://electronicfilters.tpub.com/TM-10-4330-237-13P/css/TM-10-4330-237-13P_105.htm


But personally I'd just feel hand tighten them with a socket and add an extra nip

...__...

That bracket looks rubbish to hold weight the cantilever would buckle if you put 80kg on it
do you have the space to make a frame either end?

Id go for (depending on the width of the counter) at least a bracket 80mm wide if counter was over 5 feet long one every 650mm with rawl bolts (you can get them in m6 m8 etc but id still be a fan of drilling bigger to fit)

even with all that I would be the one to wait till someone fatter than me jumps up on it to make sure its ok!!
cups and plates go on counters not people

Lumen

Turns out I measured the top holes, not the ones that go against the wall. The wall holes are good for M8, so I got some M8 anchor bolts from Chadwicks that look like this:

...__... wrote: »

do you have the space to make a frame either end?

Yes, but I can't imagine exactly what that would look like.

...__... wrote: »

That bracket looks rubbish to hold weight the cantilever would buckle if you put 80kg on it

Well Screwfix says they're 200kg per bracket, so...?

...__... wrote: »

even with all that I would be the one to wait till someone fatter than me jumps up on it to make sure its ok!! cups and plates go on counters not people

It's a study/computer desk for my 10 y/o daughter's bedroom. She probably weighs about 35kg but I'm assuming the worse case scenario where she and a friend stand on it at the same time.

So I will be the fatter person doing the testing. I'll post an update from A&E....

my3cents

Lumen wrote: »

...

Well Screwfix says they're 200kg per bracket, so...?

....

I would suspect that is 200kg evenly loaded across the bracket and not just 200kg at one end.

Lumen

Got it done, thanks for advice.

The bolts didn't torque up completely consistently, but I tested each bracket by balancing my 75kg on one foot at the end farthest from the wall, and they didn't budge.

Now for the drawers...