Slab supported on 3 edges.

[Bagman2524]

Anyone have a recommendation for design method for reinforced concrete slab with support along 3 edges and a 3:1 length to width ratio. One of the long sides is unsupported. Thanks

 

[Lion06]

Look in Roark for elastic moments (for a plate with support conditions that match yours) and design for that.

 

[civeng80]

Couldn't agree more.
Roark is great for this sort of thing.

 

[BarryEng]

Do a google (top line of this page - within the web site) for rectangular slabs.

On one of these threads, look up the reference for the USBR manual for rectangular slab analysis (Moody tables). It is about 6 Meg (I think) but is a very useful reference - many proportions of slab, support conditions, loading, etc & gives support loads, & moments in both directions.

For a three to one ratio, most of the load will be resisted by cantilever action from the other long edge. There will be minimal support from the short edges except for a bit near the corners.

I usually look up the 'Moody' tables & (very quickly) find the max moment (either at the edge of the beam action span, or in the centre of the long wall for cantilever moment). This will give you the thickness of the section & the reinforcement req'd.

I then check out the min reinforcement (req'd by code) & try to arrange a dia to match the spacing of the main reinf.

I am usually very surprised on how great an area will be only min reinf.

I did a WTP recently (a series of rect tanks in one structure, for sea water desal - 120 m long, 26 m wide, & 12 m deep). Once you do the above calcs, most of the time the max moment is the cantilever moment (bottom of the long wall) & because of the cubic relationship for moment with depth, the moment decreases rapidly to min reinf.

After you do the calcs, compare the moments for 'fixed on three sides, free on fourth' with 'fixed on three sides, supported on fourth'. Then check what you require to provide the fourth support (eg a longitudinal beam), & compare the designs.

All of the above depends on the size of your project. For a small slab - what the heck! No need for the above details.

 

[himat42]

bagman2524

Please check ACI Journal. you will find your case
(I think the paper is on Rectangular Tank). I don't at this moment the issue. I will post tomorrow if you can't find.

HTS

 

[hokie66]

Barry's advice may be appropriate if you can indeed cantilever the slab off the wall, but you didn't say what the support condition was. If all sides are pinned, then it may be better to provide a beam or thickening along the unsupported long side.

 

[BarryEng]

hokie66 - I agree. I did assume that the support was fixed (or continuous).

A 3 to 1 slab has very little support in the long direction. It usually fades away at about 1.5 to 1.

My answer above really applied to rectangular tank walls (as an example of a rectangular slab). This would mean a triangular load compared to a uniform load. The USBR (Moody charts) gives a large number of support conditions & loadings.

 

[steve1]

You can do a hand calculation to determine the amount of load being being carried by the cantilever strip and the simply supported strip by equating the deflection at the tip of the cantilever to the center deflection of the simply supported strip.

 

[csd72]

Or you can do a yield line analysis and base your reinforcement on that.

 

[Bagman2524]

Thanks to all for your assistance

 

[COEngineeer]

I would just buy PCA rectangular tank design book. It has a moment and edge shear chart for 3 to 1 pin pin free wall, which can also be used for floor like you are talking about.

OR, just model it on a 3d software and see the moments.

Never, but never question engineer's judgement