Slab on Grade - Point loadings near edge

[Wardzy]

Hi All,

I've been asked to design a slab on grade for a sand washing plant and I'm a little stuck on the loadings and their influence and the best way to design for them as this isn't my forte and all my notes on the subject have given loadings for the design.

The plant itself weighs approx 12 tonne and the structure is held up by four legs (3.8m square) that sit on 2 skids if it ever needs to be moved. I am unsure whether to design for point loads on the slab where the legs will sit or to distribute the force through the skids. The worst case live loading will be 60t of wet sand if the plant fails.

If the point loads are applied they will occur at the edge of the slab where i was planning to have a beam support around the perimeter of the slab. Do i need to analyze the edges as beams or just design as a two way slab?

Ive attached a sketch of the proposed layout and any help with the design process would be greatly appreciated.

 

[BAretired]

If the skids are adequate to spread the load when the plant is loaded with sand, you may be entitled to assume that it does. If not, you have to design a slab, possibly stiffened on each edge to spread the load over the length of slab.

Whether the skids are adequate or not, I think it is preferable to design the slab to handle point loads and disregard any benefit you might obtain from the skids. In that way, if the skids are ever in need of replacement, your slab will work on its own.

BA

 

[Wardzy]

Thanks BA.

Should I design it as two beams with point loads applied with a slab in between or as a whole unit? I've been trying to work out the depth required and it seems to be too large.

I'm not sure if I'm applying the loads right as I'm considering the plant load as pressure over a 0.5m x 0.5m area as the skid would create a plate action at the interface.

Should I be considering compressive reinforcement at the top of the slab along the beams as well?

 

[BAretired]

You could design it as an inverted flat plate or as an inverted one way slab spanning between edge beams. As a flat plate, you would have most of the steel in the column strip, but some steel would be required in the middle strip for temperature and shrinkage. Punching shear would need to be checked.

It may be more expedient to deepen the edges to gain more depth for the edge beams. As the beams are continuous, I would expect to end up with top and bottom steel running throughout the entire length to handle positive and negative moments. It is unlikely that compression steel would be required, but it is easily available if required. Beam shear would be resisted by stirrups in accordance with the concrete code.



BA

 

[dhengr]

Wardzy:
I believe that the skids are intended to be the first layer of the foundation/support for the four legs of the bin. They are probably stronger than you think and the legs should be braced too, so that the whole bin and skids acts as one unit. Thus, the skids will bend and distribute the bin leg loads over a number of feet of beam length, depending on the stiffness of the skids. I would call the bin manuf’er. and discuss this issue with them. BA’s second post is right on the money. In the area of the bin legs, I think you essentially have a one way slab btwn. the two side beams, under the skids. And you’ll have tension (and rebar) primarily in the top of the slab. I would put the down turned beams all around the slab as you will likely have heavy equip. running up onto the slab. Form the outside edge, compact the interior soil and base; just dig the inside edge of the beam, if the soil will stand fairly vertical.