Can someone help me out or tell me where to find information on how to structurally analyse a thickened slab foundation.....<br><br>thank you<br><br>Francisco
The Portland Cement Association (PCA) has detailed publications for designing slabs on grade. The publication(s) address considerations for truck, fork lift traffic and high stacking loads that are commonly found in warehouses. They are relatively inexpensive.
It was an indirect way to get someone to get more into the subject. But as I can see, I cant extend it any further.
Well when we design foundations manually, we go on finding moments and detail accordingly, and if it is a pile cap then shear stuff and deep beam action comes in. When you do that with software you always have to tell the software of your assumptions, like is it a thick or thin plate and in my office we always have to argue without actually having a feel for the differences between the two.
dear friends,
I guess you can classify "rigid" foundations as " thick" and " flexible" as " thin". And for sure you know what is a rigid foundaion.
regards
Your explanation makes a lot of sense and thanx , but what prompted me to ask for the boundary between the two has a lot more to do with software term "thick shell" and "thin shell". If you visit the Finite element forum where I found a thread related to this discussion, you will see what I mean-there simply are varying opinions, and surprisingly, no one mentions flexible or rigid, though most relate shear deformations to "thick shell" modelling. Excuse my ignorance and perhaps exxaggeration , but, is utilizing "thick shell" modelling in software the same as carrying out rigid foundation analysis?
A thickened slab is a portion of a slab that is deeper than the rest of the slab. This is usually done to increase capacity in a specfic area. In a warehouse for example, a slab is usually thickened at the truck bay door because this area sees more activity than the rest of the floor. If not for the thickened section, would be more likely to crack. Floor are also thickned to accommodate machine loads and vibrations, then reduced between the machines. Simarly suspended slabs may be thickened if the spacing between supporting joists must be increased in a particular area.
For SAP2000, I read in the manual, a thin shell or plate element is one in which you dismiss shear deformation (because you forecast out of the element dimensions and the structure itself shear effects will be negligible). Contrarily, for SAP2000 a thick plate is one in which you can't dismiss shear deformation...and this includes for example, quadrangular elements of slabs where the dimension to thickness ratio is under 2.5 or so. If you dismiss here shear deformation you will be forcing the solution be described in terms of bending, and this will be inaccurate because given your definition of the elements (them being thick) shear deformation would take, as it must, important part in the equilibrium. That in the end the differences be significant will depend anyway on the substructure we are talking about, I think.
To be even more clear, consider a flat long RC sleeper beam made of short quadrangular elements to which shear is relevant, say width of element to thickness ratio=2. For each and every element when solving for equilibrium if considered thin shell or plate element (without accounting shear effect), moment effects will be over-represented in each and everyone of them. If you take one, then the following towards one support, the effect happens again in it, moment will be over-represented since the shear deformation won't be there to diminish the bending energy intake required to equilibrate the external forces.
Hence, if you choose thin elements where shear needs be accounted element per element, as a result of the dimension to thickness ratio, you will get the moments bigger than they must (as it is seen when more exactly determined), and shear under-represented, i.e., less shear than required. To have more moment won't do harm, but having less shear than what to the case pertains risks brittle fractures that are better to avoid and so either you can rely in thin elements for your case or you better use thick ones to avoid this risk.
Practical Foundation Engineering Handbook
Robert Wade Brown editor
Mc Graw Hill
look at least section 3E but has more on it
There are various ways of addressing the thing. A slab stiff enough to tray the house atop is one solution. Cast concrete and (joint separated) perimetral pavements at almost the same time, this almost congeals the water content under the house and then the movement problems.
In any case one more favoured way of construction is use strong enough piles, and on them a set of parallel beams or a grid, waffle slab or slab, that for the most critical cases all but the piles must be neatly elevated from the ground, preferably creep space even registrable by a man.
