strong, lightweight slab material needed

[hj]

Hi. I am looking to create a wall-hung bench seat in an alcove. The dimensions are 2060mm wide, 400mm deep. It will sit directly above a wall-hung unit which isn't strong enough to sit on. This seat can be invisibly supported at each end, and am thinking of a metal angle along the whole back into which the slab sits. I can't figure out a way to have a strong full depth support from the mid back to the front, due to the need for this slab to sit almost on top of the bench to make it look as one unit. If that makes sense?

Thickness can be anything from thin (20mm?) to 50/60mm.

I am looking for appropriate sheet material to create that bench seat, with minimal deflection/sag. It can be something with a nice wood finish, or something I can use and cover it with the same wood I am using for the floor.

Any ideas?

 

[cdbe]

Kitchen worktops can make nice benches. I would want a support, either rectangular steel box section under the front or an angled support bracket formed from heavy-duty box section bolted to the wall at mid span, or a simple support leg at mid span if it can be worked in.

 

[scbk]

You could use a nice big slab of timber. Try ebay/gumtree or a small local sawmill

 

[hj]

Thanks for the suggestions. Support leg can't be worked in, unfortunately. Thinking of creating box-section support that I can bolt to the walls. Or make a wood frame although I would end up with a much deeper 'bench' to hide the frame.

I contacted local sawmill for a slab of douglas fir, to match the floor. Calculated the sag factor on that, which worked out ok. They wanted just under £500 for a piece to fit - which is way out of my budget.

So still thinking, looking - will figure it eventually.

 

[Deleted member 174758]

2m metres unsupported, and without sag? I think you are expecting far too much from any material. You perhaps need to rethink your approach a little. What is the structure of the wall behind the unit? Stud and plasterboard or solid masonry? If it is stud I think you are going to need to install a fairly substantial 18mm plywood pattress to carry the load. I'd the start thinking in terms of heavy support studs like the sort used for floating shelves, either fixed to the patresses through over sized base plates, or heavy threaded rod resin anchored into the wall if masonry. Done right you should be able to carry 250 to 300kg that way, or in other words 3 adults. A torsion box could then be built to fit onto the bars at about 60 to 80mm thickness. But the question has to be, "What is your budget?"

 

[foxhole]

Hi. I am looking to create a wall-hung bench seat in an alcove. The dimensions are 2060mm wide, 400mm deep. It will sit directly above a wall-hung unit which isn't strong enough to sit on. This seat can be invisibly supported at each end, and am thinking of a metal angle along the whole back into which the slab sits. I can't figure out a way to have a strong full depth support from the mid back to the front, due to the need for this slab to sit almost on top of the bench to make it look as one unit. If that makes sense?

Thickness can be anything from thin (20mm?) to 50/60mm.

I am looking for appropriate sheet material to create that bench seat, with minimal deflection/sag. It can be something with a nice wood finish, or something I can use and cover it with the same wood I am using for the floor.

Any ideas?

Strengthen the wall hung unit so it’s strong enough to sit on .
A pic would help .

 

[hj]

So, the slab would be supported at each end and across the whole length at the back. So only the front. unsupported.

The walls are plasterboarded masonry, so I was planning on using resin bolts for firm anchoring.

The wall-hung unit is lightweight, and challenging to strengthen for the wall hanging. If I could floor mount it, it would be ok, but I want the floating look.

Just checked out torsion box frames and I think that's the answer. Thanks for pointing me in that direction. There's a video showing what I'm looking at but on a smaller scale (and with a wall-hung unit underneath). Shouldn't be too difficult to build.

 

[scbk]

Thanks for the suggestions. Support leg can't be worked in, unfortunately. Thinking of creating box-section support that I can bolt to the walls. Or make a wood frame although I would end up with a much deeper 'bench' to hide the frame.

I contacted local sawmill for a slab of douglas fir, to match the floor. Calculated the sag factor on that, which worked out ok. They wanted just under £500 for a piece to fit - which is way out of my budget.

So still thinking, looking - will figure it eventually.

The price from the sawmill seems quite high, I would've said £50-£100 for a similar sized piece. Probably not much douglas fir growing in Suffolk? You might find it easier to get hardwood?

Have a look through ebay, some folks might offer delivery.

Also, the thicker scaffold boards are 63mm, and they are to span 2.5m centres

 

[opps]

So, the slab would be supported at each end and across the whole length at the back. So only the front. unsupported.

The walls are plasterboarded masonry, so I was planning on using resin bolts for firm anchoring.

The wall-hung unit is lightweight, and challenging to strengthen for the wall hanging. If I could floor mount it, it would be ok, but I want the floating look.

Just checked out torsion box frames and I think that's the answer. Thanks for pointing me in that direction. There's a video showing what I'm looking at but on a smaller scale (and with a wall-hung unit underneath). Shouldn't be too difficult to build.

Watching her using a wobbly table saw with a double blade and no blade guard is rather unnerving.

 

[Deleted member 174758]

Watching her using a wobbly table saw with a double blade and no blade guard is rather unnerving.

You could also add material insufficiently supported at the beginning (which risks a pinch kickback), dangerous design of rip fence on the table saw, running materials past the saw blade with neither guard nor riving knife with a bare hand not a push stick (yikes!), etc, etc. Typical American video.

More to the point, apart from the fact that they need a workshop full of static equipment to make something which can be done on site with hand tools (or quicker, with cordless power tools), the majority of the strength can be had by skipping the cross halving joints and just using butted joints, supported while they go off with thevjudicious placement of pins. It's really the glue and the amount of glueing area which makes a torsion box strong, not fancy joinery

 

[opps]

You could also add material insufficiently supported at the beginning (which risks a pinch kickback), dangerous design of rip fence on the table saw, running materials past the saw blade with neither guard nor riving knife with a bare hand not a push stick (yikes!), etc, etc. Typical American video.

More to the point, apart from the fact that they need a workshop full of static equipment to make something which can be done on site with hand tools (or quicker, with cordless power tools), the majority of the strength can be had by skipping the cross halving joints and just using butted joints, supported while they go off with thevjudicious placement of pins. It's really the glue and the amount of glueing area which makes a torsion box strong, not fancy joinery

Yeah, I noted that with each cut the MDF fell of the table/trestles but that was edited out of the video.

The first cut, on the trestles, it almost looks like the saw hit the metal trestles, resulting in the depth of cut being adjusted.

And why (at 16 seconds) was a spirit level used? - I watched it with the audio off- I am in the pub... if the trestles are not perfectly level, I don't see the point in using a spirit level.

I cannot comment on the importance of gluing (surface) areas, but I trust you enough to believe you.

If I need advice, I will come here and ask the likes of you rather than Americans that use tools that are not allowed in the EU/UK.

 

[Deleted member 174758]

I was amazed on my current job to have to explain to a site manager why I needed some timber to (i) make a pair of trestles and (ii) to make up some form of support (as it happens a 6 x 3ft cutting framework). Even without a proper bench you still need to support your materials - a sheet of plywood between two trestles just droops, so you can't ping a straight cutting line using a chalk line, you do risk a blade pinch, you also risk hitting the trestles with your saw bladea and you should forget about using any form of guide rail. BTW I got my timber - he got his lift car interior. Result!

 

[Deleted member 174758]

7And why (at 16 seconds) was a spirit level used? - I watched it with the audio off- I am in the pub... if the trestles are not perfectly level, I don't see the point in using a spirit level.

OK, so I watched it again, and then again (both times sound off). And you are right. It makes no sense at all given that there is no supporting frame or bench and that the prissy little level they used won't tell you if the surfaces are flat in any case (try a 6ft level, lady, on a solid flat work surface)

I cannot comment on the importance of gluing (surface) area...

It's a bit like skirtings. Many people think that the nails or pins hold the skirting on the wall when in reality the nails or pins only hold the wood on the wall until the grip adhesive sets. With enough clamps you could make up a torsion box entirely without metal - pins nails and screws only hold the structure together whilst the glue sets (on a flat work surface, naturally). There was no need for the complexity of those cross halving joints, which if anything reduce the rigidity of the overall structure. If the OP wants to find out how to do the job properly I'd recommend he tries to find a copy of the original article published in Fine WoodWorking magazine in the early 1980s (I think). Kirby was an English cabinetmaker who introduced the concept of lightweight thin skinned (i.e. 1/4in or 6mm plywood) torsion boxes to a generation if American woodworkers. Seems, looking at the OPs video, that the concept has been corrupted by ignorance in the ensuing 40 years - the right idea, incredibly badly executed [/RANT_OFF]

Edit: There is a Kirby article downloadable as a PDF here which shows how he makes lightweight torsion boxes with Tricel (cardboard) these days

Edit 2: The original Ian Kirby's article was in issue #32 (Jan/Feb 1982) of Fine WoodWorking. There are references to it here which also refer to a (presumably more professional) video, which I haven't yet viewed. Some of the comments confirm the need for a dead flat surface on which to build a torsion box.

 

[Deleted member 174758]

Just a little indication of how strong and rigid a properly constructed torsion box can be:

Above: Lightweight torsion box made by Alan Schaffer

The following is a response I posted to a similar question asked elsewhere a few years ago :

That is a real torsion box and is not Photoshopped. It was made from 3/16in (4.8mm) thick hardboard for an article he published in American Wood Worker (magazine). It was 8 feet (2.4 metres) long, 14in (350mm) wide and 2in (50mm) thick with the ribs on something like 6in (150mm) centres. In the photograph it is shown loaded with over 300lbs (136kg) of bricks yet it is deflecting only 1/2in (13mm) in mid span. The first time I saw something like that demonstrated about 30 years back I was astonished. The first time I built a test piece I was equally impressed. Hopefully that illustrates the my comments effectively

Possibly time for a bit of research? For the OP, here's one of Ian Kirby's articles

 

[opps]

he got his lift car interior

I after my A/Levels, I spent about 9 months working as a lift engineers mate.

My cushiest job was two weeks at the old Daily Mail office (Fleet Street). Once a day, I had to put two wires together to move the lift car up or down (it was a hydraulic ramp) so that the plasterers could plaster the lift openings without falling in to the shaft.

I didn't see the point in sitting in a basement room for 8 hours a day, so I came to an "arrangement" with the site foreman. Once the nearest pub opened, he could find me there if I wasn't in the basement room. I wasn't swinging the lead, the lift engineering company had to have someone on site to move the car as required, but there was absolutely nothing else for me to do.

Over the two weeks, there was only one day when I had to move the car twice in one day.

Frankly, I didn't feel guilty about taking a wage for doing eff all. The previous job was UCHL on Gower Street. On my first day there the engineer was called off to an emergency job. The firm sent another engineer's mate to site, he turned up shrugged and left. We had a functioning lift with no external lift door on the top floor (most work is done by standing on the top of the car and stopping it as required. Obviously the car was in engineer mode (but that mode allows someone with access to the top of the car to move it), but there was ample space to walk on to the top of the car and fall in to the shaft. That day, I taught myself how to fit a new lift door and drill and tap the closing mechanism.

It took me 3 hours to do something that a trained engineer could do in less than an hour. I didn't get any overtime and they didn't seem to be "impressed" that a "mate" (off his own back) spent 11 hours on site and fitted a door mechanism to prevent someone, potentially, falling to their death. The other fitter's mate that tuned up and left got a full day's wages for a short day...

As an aside, when I watch movies about lift cars dropping 50 floors in 5 seconds, I know that it is Hollywoodification BS. They really are incredibly safe. If all of the cables were to snap (for whatever reason) the governor will kick in if they fall too quickly, and I have never seen a UK lift that has a hatch to the top of the car.