Span between joists

[Matty F]

If the flooring is 50mm x 240mm timber and a car weighing 2000kg will be driving on it, how far apart can the joists be? Is 600mm OK?

[Roger Mills]

How are you defining 'ok'? Is it simply ok if it doesn't break, or mustn't it delect by more than a specified amount - 'cos it might pass one test but fail the other?!

[Matty F]

I just don't want it to break. Actually there will be 12mm plywood on both sides of the timber as well, 300mm vertical.

[Matty F]

OK, here's a picture of somebody else's idea.

another horribly dangerous-looking thing that will work like a dream as usual!

[Tony Bryer]

Domestic floor joists can be spaced at 600mm with 22mm chipboard decking and 12.5mm plasterboard under. The issue here might well be the decking on which the car is driven. From a quick check assuming an axle load of one tonne on one joist, if it's a 50x240 C24 it will fail in shear regardless of span. With designed decking that is adequate for wheel loads between joists and is stiff enough to spread the wheel load across two or more joists there's no reason why this cannot work, but it does need to be properly designed.

[Nightjar]

If you use an RSJ with the web horizontal for the top, it won't break and the flanges will act as wheel guides.

Colin Bignell

[John Williamson]

It would help if you clarified whether you're using 50 x 240 boards or

50 x 240 joists.

If the latter, then for a 3 metre span, they would fail, no matter how close together they were. The maximum uniformly distributed load for the size you give is about half a ton. Point loads need much stronger structures.

For what you seem to be planning, then steel is probably needed.

[Cash]

Matty,

Are you saying that the span of the joist is only 240mm (91/4 inches) or do you mean 2.400 metres (941/2 inches)?

If its the first measurement, then you may well have difficulty in fitting the car on it - if it's the latter, then the joist size would seem sufficient, but I would suggest spacing the joists at 400mm centres, supported (if possible [1]) by 230mm x 75mm purlin in the centre of the floor - or as suggested elsewhere an RSJ of suitable size.

[1] If not possible, then can you increase the width of the joist to perhaps 300mm x 50mm - or try reducing the joist spacing to 300mm

All guesswork of course on this side of the world to yours, but it may be of help.

Cash

[Nightjar]

Using steel would be my first thought for a job like that anyway, but I suppose it depends upon each person's particular skills, available materials and tool kit. As I said elsewhere in the thread, using an RSJ laid sideways for the top would give built-in wheel guides. I would also make the support structure quite a bit wider at the base than at the top, for stability when driving on and off, which would be fairly easy with a welded steel framework.

Colin Bignell

[Matty F]

I need to be able to lift each piece of this device by myself, and stack it at the side of the garage when not in use. That means that it cant't be too heavy. I was intending to extend the plywood up past the timber to act as a guide. I have all the materials lying around, and I am starting in about 3 hours! OK, I'll use 400mm spacing for the "Joists", We usually use 600mm joists for flooring in NZ. Maybe our timber is stronger!

[John Rumm]

Reading between the lines here, and looking at the picture...

I am assuming you are talking about using the board in the style of a floorboard - i.e. the 50mm vertical. Thus need vertical struts every so often to support the board. leaving a box section:

W r W ============================= # # # # # # #< d ># # # # # # # # =============================

So effective span is d, wheel loads at W and a bit of cantilever restraint from r.

If you plate up the sides with ply, then you will get a substantial extra amount of support from that - assuming the deck is fixed to it. Your main risk would be the timber deck splitting at the fixings, but that ought to be mitigated with using lots of smaller fixings (i.e. nail through the ply into the edge of the deck on regular intervals)

If that sounds like what you had in mind, and based on a quick play with Tony's excellent bit of software, can't see any problem with it myself if you use 400mm spacings. (You have the added benefit of having a photograph of a similar design working!)

[Nightjar]

I suspect what you plan is going to be fairly heavy for one person anyway. I would fit jacking points (or handles if light enough to lift one end at a time) and slide skates underneath to move it. Many are the heavy things I have moved around by myself using only a couple of triangular bits of thick ply, fitted with 100mm castor wheels at each corner.

That does rather limit the options :-)

Or they bounce more than ours.

Colin Bignell

[The Natural Philosopher]

I've lifted a whole car front on a 6x4 spanning about 8 feet when the engine didn't want to come out..

deflection was several inches, but it didn't break..the 'two ton' pulley block bent a lot tho..two tonnes to break doesn't mean it can take a ton and not bend..

In short you will be surprised how little wood it takes to support a couple of tonnes. A typical pellet can take that sort of weight.

Especially if decent wood is used..rough sawn quality pine is good..I think a lot of heavy structural work used to be done with fir as well.

[Gazz]

If you wanted to make a good height car ramp out of steel, i'd choose the type of design they use for putting cars on at sales forecourts,

seesaw ramps they are called,

[Matty F]

That may not be proof enough. A guy has his Porsche on these horrible things:

will screw the plywood all along the timber deck and screw the ramps to sheets of ply on the floor. The floor is compacted scoria and the wheels of my trolley are better on plywood!

[Dave Liquorice]

Interesting things from the piccies. My thoughts but probably too late now. B-)

Use the box sides to support the edges of the boards along their full length. Say 18mm ply fitted underneath them, not up the sides and screwed in. 50 x 50 used to hold the sides in place under the top.

Also include vertical "joists" from the same timber as the running board, will need to be ripped down by 2 x the side thickness. Sides screwed into those as well. 400 to 500 centers "looks right".

That sort of box construction shouldn't need diagonal braces to stop it collapsing along the length. Big screws, 3" No. 10 at least and plenty of them, every 6" or maybe closer.

What really worries me is what stops them falling over sideways with the car on top? There you are heaving away on a recalcitrant bolt with lots of sideways force and it all ends up on top of you... I think you need at least two sets of two diagonal cross braces joined at the center with one set at each end.

[Gazz]

i dunno what scoria is, but if it implies the floor is not solid as in a concrete slab, then can you not get the similar end result (i.e access under the car to work on it) but digging a shallow-ish pit?

[The Natural Philosopher]

volcamic ash or pumice IIRC.

but if it implies the floor is not solid as in a

Depends wehther its solid granite under the ash or not :-)

[Matty F]

Yes I certainly could dig a pit. There's a foot of scoria which I put there. That's a type of porous volcanic rock. Under that is a few feet of clay. The pit would fill with water, and there's no easy way to drain the water as the drains are higher than that. In addition I'd have to line the walls of the pit with concrete or similar, otherwise the ground would collapse when the car is driven near it. I'd probably need a Council permit to dig a hole. So that's all getting to be a major exercise.

[Dave Liquorice]

Not even a "Big One"?

Think I'd go for three or four bolts in snug holes with large penny washers under the heads and nuts. It shouldn't move, but if it does there will be a hefty mechanical advantage from 2 tonnes of car the best part of a 18" away...

Wise, stops 'em splaying in or out when you are trying to drive up.