giant floating bookshelves

[Adsibob]

I've been looking online at some bespoke floating shelves. I need some which are 258cm wide and 36cm deep. Found a few options, but they are all limited to 35kg to 50kg load bearing per shelf. For such a wide span, that doesn't seem very much. A children's encyclopaedia britannica for example (a relic of the past, i know) weighs 58kg and but for its weight would fit on a 258cm shelf with plenty of room to spare. I have both a children's encyclopaedia and the "adult" version (sorry @pocster, not that kind of "adult"). Are floating shelves this wide simply inappropriate for this kind of book display?

[SteamyTea]

I am currently making some shelves, for under the stairs.

They will only be 300mm deep, but part of them is going to be hinged so that I can hide stuff behind (scaffold tower and bicycle or two).

Not thought too hard about the mass on them, just going to rely on a bit of overengineering.

[Gus Potter]

I've made a few of these. The ones for taking heavy stuff are made out of 2 bits of oak faced MDF 18mm thick glued together to give a thickness of 36mm. I glued a solid oak edge trim on and it looks passable.

 

The main thing is the the fixings. Here I resin anchored two M12 rods into the masonry behind and tilted them upwards a fraction. Rods project about then offered up the shelves and gave them a "dunt" so the rods marked the back of the shelf. Then drilled them out to the rod depth and slide them over the rods.

 

I also made another lot (out of a recycled bit of oak furniture) that go onto a TF stud wall, they don't project so far though, ~ 275mm. Again I experimented and drilled the studs and used resin.

 

A year later disaster has not struck.. I will obviously come clean if it does later on.. promise.

 

Have attached photo of the lighter loaded shelves.. will try and take photos of the MDF shelves once I have tidied them up.

 

[Adsibob]

That looks very nice, but I don’t think it’s any different in terms of construction to the shelves I found online which are rated to a maximum of 50 kilos.

[MJNewton]

10 hours ago, Adsibob said:

That looks very nice, but I don’t think it’s any different in terms of construction to the shelves I found online which are rated to a maximum of 50 kilos.

 

With floating shelves I expect the weight limit is primarily related to limitations of the fixings used. In Gus's case the rods are (presumably) embedded in the wall whereas don't most off-the-shelf(!) offerings relay on some form of hidden bracket that is screwed into the wall?

[SteamyTea]

14 hours ago, Gus Potter said:

Here I resin anchored two M12 rods into the masonry behind and tilted them upwards a fraction

How deep into the wall?

 

There is a big difference between 20mm and 75mm.

 

https://www.engineeringtoolbox.com/amp/cantilever-beams-d_1848.html

[markc]

Embedment is one consideration but more important with cantilevers is the bending moment on the arms themselves.

a large load close to the wall is easy due to shear, but move that load away from the wall and the rods experience less shear but increased bending and tension (dependant on the point of rotation).

heavy duty floating shelves need box or circular section stubs coming out of the wall (which also needs to be able to resist the rotational moment).

make the shelves even deeper and the loads increase dramatically

[Hobbiniho]

i have fitted a few big shelves on timber stud walls and used these brackets https://www.ebay.co.uk/itm/114328853072?hash=item1a9e87da50:g:1eAAAOSwXKdeTY6s&amdata=enc%3AAQAHAAAA4ATIIWPI2wpBIExfpsusBsV0kc1Irjn6FmpJ8WARl0TeNd3hFS7EMfTvzX0W9n%2FZsr1yrT4r4RuIc5GhqMUnQS5LUX4NriFaEkcTlbS%2FSwZ%2Fq3efpqF1lY0%2FwYMiq2RxAgSihd1XE0Qy9yAKl1PKkWUFVh52CSFwxqRAHPoahO5kzXjSai53g%2B6mC%2BhxYu8YaRiAjvLuchWqZr4kRXZsa9JlIX78%2B%2F1u852aPQMFxqnS96MOwH%2FiAepI4M9iGnkBpnX1Su6Gazjmr19sXZKjKB5YWieZeYs5jZC62VZoWuFZ|tkp%3ABFBM4PaJwPVg

 

they work very well and i can easilly support my bodyweight on just 1 bracket, admittantly i sunk the bracket into the studs before plasterboard making the fixing plate completely hidden as can be seen in this photo

 

https://photos.app.goo.gl/ojAfDDAswKvb2NQw6

 

 

Edited October 6, 2022 by Hobbiniho

[markc]

Great find, not worth making them for that price

[Dreadnaught]

23 hours ago, Hobbiniho said:

i have fitted a few big shelves on timber stud walls and used these brackets

 

Nice solution. Slightly off topic, I am wondering how such a bracket could be mounted to a metal-framed internal wall (MF).

[markc]

57 minutes ago, Dreadnaught said:

 

Nice solution. Slightly off topic, I am wondering how such a bracket could be mounted to a metal-framed internal wall (MF).

I can’t see why not, if the wall isn’t boarded yet then additional noggins inserted between uprights close to where the brackets would be fitted will stop and twisting tendency. (C and E sections twist twist inwards with localised bending moments

[Adsibob]

1 hour ago, Dreadnaught said:

 

Nice solution. Slightly off topic, I am wondering how such a bracket could be mounted to a metal-framed internal wall (MF).

I have the same question, but my wall is very much boarded (and plastered and painted)!

[markc]

@Adsibob assuming you could find the studs, your biggest issue would be cracking of plaster and punching through the PB at the heal of the bracket. Cantilevers are great but the moments (loads) involved are many times the load on the arm itself. A vertical timber fixed to a wall with shelves fixed to that massively reduces the point loads changing them into shear which PB can withstand much better than pull-out and compression loads

[Adsibob]

1 hour ago, markc said:

@Adsibob assuming you could find the studs, your biggest issue would be cracking of plaster and punching through the PB at the heal of the bracket. Cantilevers are great but the moments (loads) involved are many times the load on the arm itself. A vertical timber fixed to a wall with shelves fixed to that massively reduces the point loads changing them into shear which PB can withstand much better than pull-out and compression loads

I'm pretty sure I will be able to find the studs with a magnet. Maybe I should just use regular exposed brackets. Would that be easier or do I have to fix timber to the wall and hang shelves off that?

[joe90]

If it were me I would put a timber on the wall in line (vertically) with the stud and bracket off that, no punching through the plasterboard and less rotational force in a small area 🤔.

[Carrerahill]

On 04/10/2022 at 17:31, Adsibob said:

I've been looking online at some bespoke floating shelves. I need some which are 258cm wide and 36cm deep. Found a few options, but they are all limited to 35kg to 50kg load bearing per shelf. For such a wide span, that doesn't seem very much. A children's encyclopaedia britannica for example (a relic of the past, i know) weighs 58kg and but for its weight would fit on a 258cm shelf with plenty of room to spare. I have both a children's encyclopaedia and the "adult" version (sorry @pocster, not that kind of "adult"). Are floating shelves this wide simply inappropriate for this kind of book display?

I made some. I built them in MDF and CLS. I build a fully screwed and glued CLS frame, that was fixed to the wall with chemical anchors, then I built slide over covers in MDF which I could prepare and spray paint in the garage, these simply slid over the frame and then I caulked them to the wall which is technically all that holds the painted MDF "cover" to the frame. I sat my brother and my son on them to test it - going no where. 

 

If I was doing it onto a TF wall I would probably remove a section of the plasterboard and fit a timber directly to the studs before a copy of the above. 

[Carrerahill]

On 04/10/2022 at 23:30, Adsibob said:

That looks very nice, but I don’t think it’s any different in terms of construction to the shelves I found online which are rated to a maximum of 50 kilos.

Construction in this case is far superior, if you knew Gus you would know this and the key here, is the fixing method I would say.

 

These shelves are probably designed to go onto all sorts of walls, including stud and PB - in which case they probably rate the shelf on weight bearing capacity of a PB fastener into a 12.5mm PB wall. I have some here that claim 17Kg a point. So they will then further de-rate it to eejit-proof it.

[Carrerahill]

On 05/10/2022 at 12:02, SteamyTea said:

How deep into the wall?

 

There is a big difference between 20mm and 75mm.

 

https://www.engineeringtoolbox.com/amp/cantilever-beams-d_1848.html

Given Gus is an SE... 

Edited October 7, 2022 by Carrerahill

[markc]

17 minutes ago, joe90 said:

If it were me I would put a timber on the wall in line (vertically) with the stud and bracket off that, no punching through the plasterboard and less rotational force in a small area 🤔.

@Adsibob if you can live with the vertical timber then absolutely yes! 

[SteamyTea]

25 minutes ago, Carrerahill said:

Given Gus is an SE... 

Why I asked.

[joe90]

If the verticals i suggested we’re oak or similar (or even steel)  you could bolt these to it with a metal plate on the reverse side.

https://www.ebay.co.uk/itm/124220875437?_

[Carrerahill]

21 minutes ago, SteamyTea said:

Why I asked.

Got it now. I misread the comment as being possible doubt of the rod holding, my mistake. 

[Gus Potter]

On 05/10/2022 at 12:02, SteamyTea said:

How deep into the wall?

 

On 05/10/2022 at 12:20, markc said:

Embedment is one consideration but more important with cantilevers is the bending moment on the arms themselves.

 

For ones into masonry have about 70 - 80mm embedment with really careful drilling so did not punch through the half brick thick wall.

 

The ones in the photo are drilled into timber studs. These are structural studs but only have a modest axial load and very little wind loading so not getting bent top to bottom.

 

Good point markc. Yes these kind of fixings are subject to a bending moment as well as shear. For non safety critical fixings like this you can calculate it out as a fag packet check. Have to say I did not do detailed calcs as it is my own house but if asked here is roughly how you prove it for a non safety critical fixing. 

 

The key is to let the shelf sit a fraction off the wall so it does not pry the fixing.. a tension force. Lets say you put 100 kg on the shelf and it is 2.0m long and your fixings are at 400mm centres so direct shear load per fixing = 100 * 0.4 / 2.0 = 20 kg per fixing. 20 kg is about 0.2 kN.. call this the working load.. an unfactored load. The 20 kg acts straight down the direct shear. A M10 /M12 threaded rod in a typical resin in soft brick will carry about 1.0kN  to 1.2 kN in direct shear... all looks good.

 

Until you see what @markc say about cantilevers.

 

Say your shelf is 300mm deep and it is loaded front to back with books. The centre of action of the books is 150mm out say. Now you have a leveraging (bending) effect also.. which is 0.2 kN * 0.15m = 0.03 kNm.. seems a very small number until you start to think.. hey this rod only goes a little into the wall .. 70mm , the bricks / timber are soft and just where the fixing goes into the wall you have a highly stressed area of brick / timber. Lets say here that your first 20mm of rod penetration is required to resist the compression under the fixing.. and you need about another 20mm at the far end of the rod where it wants to move upwards. So the effective lever arm is now 70 mm less (20 + 20) / 2 = 50mm. Now a bending moment can be what we call decoupled into an up and down force in this case. So we convert the bending moment into an up and down shear as follows. 0.03 kNm/ 0.05m (the effective lever arm) = 0.6 kN .. and this is the extra force that is generated as marc says.

 

Now we have to add the 0.6 kN force to the direct shear force = 0.6 kN + 0.2 kN = 0.8 kN. All of a sudden we have significantly increased the load on the masonry / timber although in this case it still looks ok.

 

One other check to do is to check the rod in case it bends or deflects too much. That is why I tilted the rods up at a bit of an angle so when the shelf is loaded it lies level. Remember that the shelf in the photo is just for a few pictures and stuff... no where near 100 kg.

 

For safety critical stuff we start with a fag packet sum to get a feel for things then we check carefully.  We use some theory from steel column base plate design and write a quadratic equation and or use excel to solve iteratively.. then we get close and do further checks and so on. You often see something like this on the side of a motorway or the base plate stantions for railway pylons.. the base plate sites proud of the concrete base.. your shelves are the same .. but hopefully cheaper!

 

There you go.. a bit of theory on flying shelves for a Saturday nights fun.

 

That said this can be applied to all sorts of cantilever structures that require fixings into masonry.

 

 

 

 

 

 

 

 

 

 

 

 

 

[joe90]

@Adsibob I think you need to measure the weight of the books you propose to put on each of these shelves first  😱 then boffins like @Gus Potter can give you more accurate info as to what’s required.

[SteamyTea]

59 minutes ago, Gus Potter said:

Lets say here that your first 20mm of rod penetration is required to resist the compression under the fixing.. and you need about another 20mm at the far end of the rod where it wants to move upwards. So the effective lever arm is now 70 mm less (20 + 20) / 2 = 50mm. Now a bending moment can be what we call decoupled into an up and down force in this case. So we convert the bending moment into an up and down shear as follows. 0.03 kNm/ 0.05m (the effective lever arm) = 0.6 kN .. and this is the extra force that is generated as marc says.

 

Now we have to add the 0.6 kN force to the direct shear force = 0.6 kN + 0.2 kN = 0.8 kN. All of a sudden we have significantly increased the load on the masonry / timber

That is the bit I had forgotten how to calculate, I knew it was there, just not looked at this sort of problem for 3 decades.

Thanks.