i have modeled RC footing system for 5 floors reinforced concrete structure using CSI safe, after exporting the reactions of base from CSI ETABS, noting that i have taken the restraints of structure as fixed supports so they would transform moments too!
the problem is with the massive amounts of steel under columns and walls directly, i mean just for small areas the amounts of steel are really huge in these areas compared what is common for such types of buildings, but in other areas the amount drop significantly
any advice to redistribute steel amounts?
i have attached some photos and the model below.
Please check soil properties beneath your foundations.
Hi
Before I answer to your question, I have a question about how to model of sub-foundation springs? It seems that you have used Mat foundation for this structure. Please note that you should use of Modified-Winker Model for modeling of springs. Otherwise, your modeling wont be correct. For getting more information please refer to Coduto's Book or Bowles's Book.
Sincerely,
I think the boundary of the structure should be as beam on elastic foundation. Then the results will be acceptable.
Dear Yasmeen Ghassan
please check following factors in your model:
1. the compression strength of concrete in safe.
2. the yield and ultimate strength of steel in safe.
3. the unit of ETABS and SAFE when you export and import the model (the unit in ETABS and safe shall be the same while exporting and importing).
4. the thickness of the foundation.
Hope that you can address the problem using above-mentioned factors.
Best regards
hello, thank you all for your recommendations, actually i have checked everything you mentioned and it is okay!
i have reduced mesh size of etabs walls and re exported again to safe, i have also reduced the mesh size in safe and what i have got is reduced area of huge steel amounts, actually it became significantly smaller!
but it still exist!
does it have any relation with the definition of column stiff area?
Yasmeen Ghassan
Hi
Did you model Rigid Areas for columns and wall? How did you model them? Notice that using Stiff element alone is not enough. You'd better model the columns and walls with stiff element together and in this case you should use 3D Analysis and Diaphragm above through Advanced Option in SAFE Software.
I think that you use low bearing capacity, so the subgrade reaction will be less value than the actual value this condition makes the reinforcement higher than the actual value
Hi, I have the experience of detailing the reinforcment from the finite element analysis. Guess that you are using the plate element and you connect the columns and wall over a single node or a line of nodes.The FE results give you a high stress concentration at the contact point between slab and the columns/wall. The finner the mesh the higher the stress concentration because of the singularity at point load or single line load. This problem can be solved by asking the computer program not to output the stress at the node but at the center of the element, i.e. taking the average. Of course, your element at this area should not be too large, I think limit to the dimension 1/2 of the thickness of the slab would be fine. Since concrete is not a perfect elastic material it would cracks and crushs then redistributes. Remember that the best way to design the reinforcement is from a output in force and moment, not in stresses since the concrete is cracked in tension the stress contour can only be used in the SLS limit check when the concrete is less likely to be cracked, whereas the ULS limit RC design should be based on moment and shear force resullts so that the normal assumption that concrete has no tension strength be valid. - a little share of my experience.
Carlos Wong
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