giant floating bookshelves

[Gus Potter]

1 hour ago, SteamyTea said:

That is the bit I had forgotten how to calculate

But I know you know, you've just have had better things to do!

 

Yes you get the direct shear and the bending force also creates additional forces that cause localised failure in the masonry.

 

The maths bit I know you know. But it's how you apply the maths to this particular problem of a fixing that is not something that most folk wake up in the morning thinking about unless you are @pocster. We have the direct shear and the bending moment that I have decoupled so we have an approximation of the shearing force just at the point where the fixing enters the wall.

 

The assumtion in this model is that we allow the masonry to crush under the fixing rod very slightly just at the point of entry to the wall. We know the compressive strength of the masonry so we can say.. how much area of masonry do we need under the fixing to resist the load. My guess was 20mm. Now we assume the force acts at the mid point so say 10mm in from the face of the wall. At the embeded end (the force is in an upwards direction here) we assume the same and that give us our lever arm. But for us to calculate the actual capacity of the fixing we need the quadratic function / solution because as the bearing length changes so does the lever arm and thus the forces.

 

What I outlined above is just a very simplistic model. In reality the rod is bending and subject to curvature, the masonry is elastic to some extent and not rigid thus the bearing preassure over my assumed length will not be unifiorm. The masonry is not a uniform material.

 

You could get closer mathematically by using a finite element model.

 

The fag packet calcs above would get you on your way provided you leave say 40% margin of safety at preliminary design stage.

 

In practice Hilti /Fischer say just put their fixings into things, test them physically and produce capacity tables. That is where the problem occurs on BH as folk are using fixings that are into old brick etc and they have not been tested for this.

 

 

 

 

 

 

[Pocster]

11 hours ago, Gus Potter said:

The maths bit I know you know. But it's how you apply the maths to this particular problem of a fixing that is not something that most folk wake up in the morning thinking about unless you are @pocster.

Yep . I can be awake 2am thinking about time fitting / wiring / “ did that wrong “ ….

[joe90]

I once had to fix a basin to a wall, no pedestal, the wall was so damp/crap I ended up putting stainless threaded rod right through it (9” solid) with a plate and nut on the outside then repaired the render over it. Might be over engineered but I but it’s still there.

[Adsibob]

Right, slowly making progress. Worked out there are three studs in my wall, which are each 100mm thick. The brackets which @Hobbiniho posted a link for can have 70mm inbedded in the wall. Shelves will be about 380 deep, so hopefully the 320mm brackets will work (250mm in the shelf plus 70mm in the plasterboard and wall).

 

So a few questions:

 

1) With 236cm wide shelves and only three studs to anchor to, will I need additional support in the form of visible brackets, say one at either end of the shelves, so that instead of 3 anchor points I have 5?  If so, what would be the best way to conceal these as much as possible? Would a 15cm bracket be enough, so that it sits quite far back on a 38cm deep shelf?

 

2) The chap that’s going to put these up for me has recommended 50mm thick MDF. I’m worried this will be quite heavy. Ikea’s floating shelves are also that thickness, but they are made up of two separate sheets of chipboard with paper based filling in between. Is there a lighter material that I can use that paints well? Maybe something like Ikea’s construction, but in bespoke sizes. Or can I get away with a thinner sheet of MDF. The rebar brackets are 12mm thick, so needs to be thick enough to take that hole and still have enough thickness to support itself.

 

3) some MDF suppliers advertise a product described as “ready to paint”. Is this just to save me having to prime it with my own primer, or is there something more special about this finish?

[markocosic]

M16 grade 8.8 threaded rod every 600 mm has always worked for me.

 

Resin fix into brick (single; drilled full depth and relying on resin for airtightness) or screwed into timber (195*45 studs though!).

 

Fixed into 45 mm thick softwood shelves or slight thinner scaff boards.

 

Limit load case is the wife checking the tops of kitchen units and hanging off then whilst they're completely filled with wine.

 

M12 will bend imo. Especially hollow eBay grade 2.cheese material. The less these form the more they spread your load onto 100 mm of support not 50 mm.

 

If the depth into the (solid wood) shelf and the wall are equal it's fine IMO.

 

380 mm is deep though. 250 mm more normal for books etc.