Play slide design

[Korto]

Anyone here qualified to judge this?
I'm making a play gym out the back yard as part of my kid's OT, and given it's a custom design, I wasn't able to find a manufactured slide that met specs (at least, nothing where the cost of then shipping it interstate wasn't double the cost of the slide), so I came up with a slide design myself.

Up top, the slide bolts onto a bearer, 6x2 pine.
The slide itself is a left and right rail sandwiching a sheet of 1mm galvanised steel. It's got a drop of 1.2m and is 2.3m long plus 0.3m in the flat bit.
Each rail is two parts, a top part of 4x2 pine, and a bottom part of 2x2 pine, sandwiching the sheet steel inbetween, and screwed together along the length every 300mm.
At the bottom is a cross-piece of 4x2 pine.
The steel sheet at the top is rolled over the bearer and screwed into the other side (underneath the level of the slide platform). At the bottom it is rolled over the end and screwed into the bottom of the cross-piece.
The slide is supported at the bottom by another 6x2 pine bearer, which has 25mm recesses cut in for the bottom rails to sit in.
The slide is a total of 500mm wide, 400mm wide inside the rails (there is some thought about 600/500, but space is at a premium).



At the top, the inside top rail is bolted into a 4x4 post it rests against, the outside top rail in screwed into a baluster.

Points of interest
1) The joint is not directly supported. Weight going on the joint will cause a downward thrust, compressing the top 4x2 pine rails butting against each other, and stretching the 1mm sheet steel. This is meant to bear the load. The load I wish it to bear is 100kg safely. (My kids don't weigh 100kg, of course, but I don't believe I need to explain generous safety margins). Upward thrust is to resisted by the bottom rail and sheet steel in the same manner, but should be much less.
2) There will be an inward force, pulling the two rails slightly inwards when weight in is the slide, but I don't feel this will be any problem, more a 'feature'.

Thoughts?

[madd0ct0r]

you can vastly, and I mean vastly improve the strength of that bottom joint by including a small steel plate screwed onto the bottom of the bottom rails. That will carry the tension across the join and means you are using the full depth of the beam.
Without the ends of the top rail will get slowly mashed in, making the slide looser and bouncier as time goes by.

You also want to consider the horizontal thrust force acting at the base of the slide. It's pushing outwards - is there a risk the bearer will buckle? Should another cross piece behind the bearer be included?

From the 100kg person, the load is 981N, which translates into a force of 511N down the direction of the slide (this creates the horizontal loads at the ends) and also a load of 836.88N perpendicular to the sides.

So you can test the composite 6x2 rail by having it span 2.3m on the ground and loading it with 838.88N (about 85kg). I say test rather then just calculating it because 1) we both know that size wood should take it and 2) wood is a pain because it is all about the location of knots and other discontinuities, so it's worth checking with the specific wood you have.

The wood will loose strength very quickly outdoors, long before visible signs of rot are there so give it a proper treatement ect.

The contribution of the steel sheet to the slide's bearing capacity is minimal (since it is located very close to the central axis). The bow in the slide under load will be large, but a couple of straps across the bottom rails would be enough there.

[LaCroix]

If I read your sketch correctly, the bottom rails are only attached to the top plate by bolts. These bolts would need to bear the weight of the user and whatever he comes up with, and if even one of those connection fail (rust, getting loose by repeated jumping, etc...), the sheet would only be supported by the screws holding the fold. It'd rip out, almost immediately. (Depending on sheet dimensions either the sheet will break at the holes or ripp the screw out of the old wood.)
I'd not trust this kind of setup - better to move it so that the bottom rails actually sit on top of the top plate, and the screws only guard against slipping.

Also, all-metal slides have raised walls to provide stability - yours does not have this, it's all hinging on screws that sandwich the sheet between two pieces of wood. It will hold up for some time, but it will fail somewhere down the road, once the elements had their fun with it. Which will cause the contents of the slide at this paticular time to spill over the failing side of the slide.

Put a sheet of plywood underneath the steel to support the weight it has to carry. The steel should only be a lining that facilitates injury-free sliding, not what supports your weight. It will also remove the problem of the slide buckling over time, no matter how long the kids jump up and down on it (which they will do).

[Korto]

you can vastly, and I mean vastly improve the strength of that bottom joint by including a small steel plate screwed onto the bottom of the bottom rails. That will carry the tension across the join and means you are using the full depth of the beam.
Without the ends of the top rail will get slowly mashed in, making the slide looser and bouncier as time goes by.

I was considering a couple of pieces of flat-bar, bent to fit the joint and screwed underneath. I was leaving it off to keep the design simple, unless it seemed necessary, but if a piece of plate would prevent the butt ends of the top rails from being slowly crushed, then that's another story.
I'm very reluctant to put anything exposed on the top rails; screws, plates, bars, etc; as I'm concerned that anything like that could catch on someone using the slide.

You also want to consider the horizontal thrust force acting at the base of the slide. It's pushing outwards - is there a risk the bearer will buckle? Should another cross piece behind the bearer be included?

I can see there would be some horizontal force, but I wouldn't have thought the amount would be significant.
The bearers (top and bottom) are each screwed into the side of two 4x4 posts (2m apart), which are sunk 600mm into concrete. The posts and bearers are actually part of a climbing-ropes box frame, and the bearers extend out one side 500mm so the slide can be tacked on the side.
I still can't imagine the timber buckling, but if you believe there's a concern there, it could be further strengthened. My first thought would be a 4x2, laid horizontal and screwed into the bearer and post underneath the slide. Not ideal, as it's the screws taking a pulling force, but seems adequate.

The wood will loose strength very quickly outdoors, long before visible signs of rot are there so give it a proper treatement ect.

The timber being used is H4 grade ACQ treated pine (or at least should be. I was quite specific talking to the builder I'm using for the structural parts), and should last 25 years plus. Apparently. It also aggravates rust, but hot-dip galvanised is supposed to be acceptable. Apparently.

The bow in the slide under load will be large, but a couple of straps across the bottom rails would be enough there.

You'd expect more bowing than I thought, huh? Oh well, a steel sheet over the joint, and a few cross pieces along the slide.

PS. LaCroix, just saw your post, but it's late. Gotta go to bed.

[Elheru Aran]

Like Lacroix said, definitely put some plywood under there. Just steel won't do the trick-- it might hold up for a few years, but even galvanized will rust out eventually and the treated wood won't help it at all.

How I would do it: Use something a little wider like a 2x8. Cut a thin groove in the two side parts for the steel sheet rather than sandwiching the wood. Cut a dado immediately below the groove to fit the plywood in. A few cross beams between the two to help tie the sides together (good knot-free 1-by material should be sufficient here if you can get treated).

Another consideration: Make the horizontal part at the bottom a little longer, perhaps, and curve it off a tad more. The way you've drawn it looks a bit angular.

To groove/dado it, I would suggest either a table saw or a router with spiral cut bit for the groove. Although to do a curve at the bottom if you go that way you'd need a router (or some careful chiseling) so perhaps just go with the router. Use an edge guide for the long straight bit.

To help keep the slide slippery (not to mention cleaner) I might consider putting a layer of varnish on it. This will wear off eventually thanks to posteriors being repeatedly run down it, but it'll take a while if you do a thick enough layer. Alternatively (or with the varnish) you can occasionally spray some car wash solution with wax on it.

[madd0ct0r]

actually - go with Lacroix's solution of plywood under the steel. It's just easier.

[SCRawl]

About the slide itself -- wouldn't stainless be better than galv? Even a 400-series stainless should hold up better against corrosion.

[Elheru Aran]

About the slide itself -- wouldn't stainless be better than galv? Even a 400-series stainless should hold up better against corrosion.

I'm not Korto but I'm guessing the issue may be cost there. Galvanized is pretty cheap; stainless not so much. Availability is another point as well. Large sheets of stainless may be harder to find. Galvanized, well, any hardware store should have large pieces of steel roofing material...

[SCRawl]

About the slide itself -- wouldn't stainless be better than galv? Even a 400-series stainless should hold up better against corrosion.

I'm not Korto but I'm guessing the issue may be cost there. Galvanized is pretty cheap; stainless not so much. Availability is another point as well. Large sheets of stainless may be harder to find. Galvanized, well, any hardware store should have large pieces of steel roofing material...

These are good points. I used to work for a fabricator which used stainless all the time, so that's my go-to material of choice. If there's a service centre anywhere nearby, he could always try there, and ask for the front/back of a coil (if dents aren't a problem) next time they're de-coiling material of the right thickness. The width might be a problem, too, come to think of it, since that stuff (at least in this part of the world) tends to come in 48" wide or wider, about double what Korto needs.

[Korto]

The builder's messed up and cut the top bearer too short. To fix it, he's screwed an extension on. I could rest the slide on top of this extension piece, and still wrap the sheet steel around the bearer.
Using ply is an idea. I'd have difficulty cutting a dado; I don't have a router; and so I'd be inclined to screw it on from underneath.
For the screws, I'm thinking of getting some stainless steel ones. a small bag of 75mm wont set me back too much, although Elheru Aran is right that stainless steel for the slide got ruled out on cost and availability.
The bottom wasn't meant to be that angular. I'm thinking I'll be knocking the corners off where ever the metal rolls over, to make it all 45 degree angles. I was also thinking of car waxing the slide.

On Tuesday, I'm going to try and get back to the TAFE where I used to work. If any of the teachers there still remember me in the Sheet Metal and Carpentry sections, I might be able to get some help from them.
Maybe they'll even know a better supplier for stainless sheet.

[Elheru Aran]

Using a dado was mainly so you would have a mechanical back-up in case of screw failure. Otherwise I think you're pretty much set. Photos when you're done?

[Spoonist]

The bottom wasn't meant to be that angular. I'm thinking I'll be knocking the corners off where ever the metal rolls over, to make it all 45 degree angles.

A small point which I think you have most probably thought of, so more of a just in case thing.
Rain.
I hope I misunderstand the quote above because the joint between the slide and bottom is at +45 degrees or 135 from source, which after some shoes hit the bottom part of the joint will result in a bump which if the angle of the bottom isnt steeper will collect rainwater and decrease the lifetime of the slide. lots of commercial slides prevent this by having the bottom slightly rounded where shoes and ass hits the bottom. Maybe placing the bottom jint closer to 140+ from source would help?
Then dont you get real bad rains on that side of australia? If so you need to think carefully where you are aiming that water cannon you are building. I've seen basements flooded due to badly aimed slides. (Small plastic party type ones, yours seem much larger).
Also consider the mud puddle effect of most private slides, which reduce overall fun and lifetime of support beams.

Also if the end of the slide is too close to the ground it decreases the fun. I dont know the

I was also thinking of car waxing the slide.

After sliding a while most kids will try climbing the slide "the wrong way", waxing it will then risk leading to faceplants which you usually get when kids try to reverse climb water slides. Its why regulated slides has to have a specified amount of friction.
Its better to have a "science class" of decreasing friction not on the slide but from what you slide on, ie clothes and other stuff like plastic bags. Make the kids feel-think-and-guess first. Its more fun

[Korto]

Elheru Aran, I knew what the dado was for, but no practical method for me to make one leaps to mind. I don't believe the builder could, either, as he's more a 'circular saw and nail gun' sort of guy. Not that it now matters.

Went to the TAFE, to ask their advice, see if they could lend a little assistance. Bend a bit of reinforcing steel perhaps, or cut a dado into a piece of 6x2 with their bench router. Worked there for some years a few years ago, right next to Sheet metal, so I had good reason to be optimistic.
Anyway, I explained what I was doing, and why, to one of the teachers there, and I drew a sketch of what I was thinking. The wooden rails, the plywood and sheet metal, the joint, etc. I finish describing it, and the teacher looks down at my drawing, takes it out of my hand and he speaks.
"No. This is what we're doing."
Two lengths of round pipe, bent to shape on their pipe-bender, with sheet metal welded on underneath in a continuous line using some method I can't remember what he called it. Fold the ends of the sheet back. All done. I was told to go and get him the precise measurements, and that's end of story.
Ummm... OK.
You know, I kind of would have liked to have a go at the slide myself, see how well my design worked, but this is being made with more suitable materials, by a qualified professional, and I'm not an idiot. Anyway, I've got heaps of other work to do on the rest of it. I dropped him in the measurements today.

Spoonist, to nitpick, the angle at that joint is actually 30 degrees (45 degrees is too steep). That's just a nitpick, though, because if the bottom is made flat you're right, there's a good chance of pooling water. Which would take longer to dry after rain, reducing play time and increasing rust.
I'll mention it to the Tafe teacher (coincidentally named John), and suggest the flat is actually a slight, but definite, incline.
As regards to basements, however, I suppose there may be some houses in Australia that actually have basements, but I've never seen one. Only the half-buried downstairs from a house built on a steep slope. We live in the bottom of a small basin (we could get feet of water in the backyard until the council fixed the stormwater drainage, now we just get inches), on short piers.
But we don't get bad rain down here. You're probably thinking of up north. Queensland. We personally only had trouble with the yard flooding as we're in a basin, surrounded by uphill, and with the only drainage being blocked by a rather stupidly designed block of flats. But it's fixed now.