Expert: Arshad Khan - 12/26/2009

Question
QUESTION: hello sir,
i want to model ground supported industrial slabs using sap2000 software and then want prepare a parametric study.for that i want some standard formulas to check the results of software,like wise max.b.m,deflection,stress.if possible plz guide me how can i model the slab using sap2000.

ANSWER: Hi Yogesh,

In order to do this properly, you need to do full site investigations, including a plate-bearing tests at the appropriate depth so that you can get the load/settlement criteria from which you can determine the soil modulus. From this you can work out spring stiffness and model them within SAP2000. Then you should do a sensitivity analysis to determine upperbound and lowerbound load/deflection results from which you can design the appropriate rebar.

The following websites should also assist:

www.army.mil/usapa/eng/DR_pubs/dr_a/pdf/tm5_809_12.pdf

http://www.pdhonline.org/courses/s132/s132.htm

http://www.constructionknowledge.net/concrete/concrete_slabs_on_grade.php

www.ladbs.org/.../IB-P-BC%202008-100%20Design%20&%20Analysis%204%20Slabs-On-Grade.pdf



---------- FOLLOW-UP ----------

QUESTION: hello sir,
thank you very much for quick reply.
as i mentioned earlier i want to develop some analysis charts which will going to be help full for design purpose.for that i am going to model my slabs on ground in sap2000.and i will collect the results for central,edge and corner load.as far as the value of sub grade modulus is concern i am dividing the soil in good,medium,loose and poor.
now for modeling in sap.
1)how can i decide the meshing?
2)how can i compare the results?
3)plate,shell,or membrane which model will provide accurate results?
i think i am asking too many things but plz provide me guidance.  

Answer
Hi Yogesh,

1. Meshing is dictated by overall slab proportions and any concentrated loading positions. There should be at least 6-10 elements between any loading positions, and at least 10-12 for any length/breadth of a slab. Also, as a rule of thumb, the thickness to size ratio should be at least 1:5, if you are using thin elements. e.g. for a 200mm thick slab, make the meshes maximum 1.0m size.

2. You can compare the results by keeping one item constant and varying the other parameters- i.e. loading, mesh sizing, thickness, spring stiffness.

3. Best to use Plate elements for thin slabs, and thick/solid elements if your thickness/size ratio goes beyond 1:2 to 1:2.5 approximately.