Slab on grade (Moment)

[AKRAMCRANE]

I need to know if there is a book/reference on how to design slab on grade subjected to concentrated moment and tention. common case if a jib crane is installed in already buit factory. any idea how to chack if its ok or not?

 

[whyun]

Jib cranes usually require a pad footing but if it is already installed on slab on grade, using the thickness of the slab, find the required width and length (imaginary square pad footing) to resist the jib crane loading. Then check whether the existing reinforcing in the slab is adequate. Though dependent on the size of the jib abd capacity, it is unlikely that slab will work.

 

[AKRAMCRANE]

Any reference, I still not sure how big should i assume the pad...any one has an example. Thanks

 

[UcfSE]

What steel is in the slab? How do you know where the reinforcing in the sog is? Do you think the crew put the wwf or rebar where it needs to be, or anywhere other that right on the soil?

 

[whyun]

Any concrete textbook should have an example for designing a pad footing with axial and bending moment at a column. Determining the pad dimensions is a trial an error process.

 

[csd72]

treat it as an upside down flat slab and check for punching shear with moment.

 

[AKRAMCRANE]

I hared that I have to analyis it as slab on grade and figure the K value for the soil. most of the time I assume it as Un-reninforced conc. It would be nice if there is a software can do it or even a spreadshet

 

[csd72]

there are many FEM or beam on flexible foundation programs that can analyse this for you.

 

[AKRAMCRANE]

Could you please clarify how this can be done as beam on elastic foundation....the pad let say 4ft x4ft x6in slab

 

[tails142]

I had similar issue of having to 'design' a slab 'on grade'.

There is unfortunately little to it. I did come upon a book 'Ground Bearing Concrete Slabs' by John Knapton.

He goes through several methods; none of which I particularily liked. Based on loading his methods alter the depth of the slab; with pretty much the same steel remaining on all condition, standard 8 or 10mm structural mesh. It is worth a read though as I havent explained it very well.

I specced some minor top steel to control cracking and hopefully resist hogging moments at the corners and edges of the slab.

It is difficult when you're analysing the slab to take a point load, such as a fixed crane or a mobile crane outrigger, and determine where the load is spread out to. As if the slab is very stiff it could be spread far out from where the load is applied.

In the end I took my point load as acting on a 'pad' of the slab two square metres. Checked for bending and punching, was ok - so I moved on. Its not a correct method but it will give you an indication of whether your slab is adequate.

I would echo the original posters question; if you have a point load coming onto a ground bearing slab on grade; how do you determine the area that this load bears onto; how do you define its extents?

 

[shin25]

You first need to decide, if it is a rigid or flexible foundation. From the description, it seems like the slab on grade should be a flexible foundation. So, you have to consider the modulus of subgrade reaction 'k' to find out the extent of the contributing slab area.

There is a good example of flexible foundation in the foundation book by B.M DAS.

 

[AKRAMCRANE]

Could you please post example or even refere us to one. that will be appreciated