Thread: How far to span a slab?

If its well seasoned it will span 2900, if not well seasoned, RHS (SHS) in the middle

Its a few years old...... so if i put one support at say..... 900, so i have a cantilever of 900 and a span of 1100 in the middle, that should be fine?

Originally Posted by Spelunx

Its a few years old...... so if i put one support at say..... 900, so i have a cantilever of 900 and a span of 1100 in the middle, that should be fine?

I reckon 900 might be a bit too much, I'd go for for something like 700/1500/700 or 725/1450/725.
Probably best to draw them up to see what they look like.

Originally Posted by Spelunx

Its a few years old...... so if i put one support at say..... 900, so i have a cantilever of 900 and a span of 1100 in the middle, that should be fine?

It would work, but it'll look super weird, even 700 I think is pushing it. Here's a quick drawing, assuming a 50mm thick top and 100mm wide legs; top one is set in 600mm from the ends, bottom one is 700mm

Clipboard03.jpg

The slab in it’s own may cantilever 900 but if one of your heavy mates leans or sits on that end you could get a big enough deflection to spill the drinks !

Originally Posted by Sam

The slab in it’s own may cantilever 900 but if one of your heavy mates leans or sits on that end you could get a big enough deflection to spill the drinks !

Hmmmmm.... i hadnt thought of that.... the closer to the outside the better then.

Originally Posted by Spelunx

Hmmmmm.... i hadnt thought of that.... the closer to the outside the better then.

It depends if you want to be able to stand against the ends of the bar, if yes then some overhang will make that easier.

Looking at EJs drawing I agree that even 700 is too much, 600 is better but now I'm thinking 450?

As for mates leaning and sitting on it, I don't know what height you are planning, but 450 mm wide and 2900 long means it has a very narrow base so is more likely to tip over sideways than cantilever along its length. Is it going to be free standing? If so it may need weight added to the base.

The plan is to weld up trapezoidal legs from 100 x 50 RHS. Similar to the one attached, except this is flat bar. It looks like i can get away with having two legs.... would it be best to have a small cantilever, say 300, to reduce the span to 2300?

Also, i tossing up ways of attaching the legs underneath. What is the best option? I am also going to run some rhs down the middle as a footrest, so this will make things more rigid. I am planning on it being 1050 high..... on braking castors for ease of movement.

The overhang/span is really down to what you think looks best, the top won't sag regardless of what you choose. Some options:

Clipboard03.jpg

I'd be attaching them with threaded inserts in the top and some of these Demon Bolts Oversize holes in the legs will allow for seasonal movement of the timber while still being covered by the large head on the bolt

1050 mm high is not that different to a regular table so it's less likely to tip sideways.

I like 400mm for an overhang but 300mm is fine as well.

Regular table is 700 high
I suggest 450 to 600max in from each end

A well balanced table has ends half the height and leg span four to six times the length of end. Therefore assuming a standard table height of 720mm
the end overhang would be 360mm and leg span would be 1440mm to 2160mm. The shorter the leg span the chunkier the table look.

Originally Posted by rustynail

Therefore assuming a standard table height of 720mm

From post 9

Originally Posted by Spelunx

I am planning on it being 1050 high

Common guys, can't you run the slab through Sagulator https://www.woodbin.com/calcs/sagulator/

For a Jarrah slab, 450 wide and 55 thick, I'm getting acceptable spans of 2.5 METRES with a 200 KG load at mid span.
at 1050 high it's very unlikely that the slab will be used as a seat, but even if it is I doubt that deflection will be an issue.

Heck, even with two very fat blokes STANDING at mid span, a 2.0 METRE span is OK. (there wouldn't be room for a third fat bloke.)

I suggest you go for a leg spacing that looks "about right".

The bigger risk is that the leg to slab connection -- a few screws into a steel strap? -- will be the weakest part of the construction by at least an order of magnitude.
and this risk is present regardless of what span you adopt.