Unjustified reinforcement when using SAFE models for raft foundations supported on soil can be detected in case of irregular geometry of models, unsymmetrical grids of columns, shear walls near the edge of the raft, etc. (Wood-Armer) method is preferred in thick plate analysis in case of point loads (Raft foundations, Transfer slabs).
This is to compare between the analysis of SAP2000 and the analysis of SAFE using both the nodal method and the internal method (Wood-Armer).
THICK PLATE EXAMPLE
A raft foundation on soil, supporting two core walls near the edge and rectangular grids of walls and columns, was modeled as a thick plate on SAP2000 and on SAFE V8.
Assuming that the bending moments differences between SAP2000 model and SAFE model are insignificant all over the raft model, SAFE model will directly calculate the strips bending moments (column strips and middle strips), using an average value along the width of the strip:
Particularly, the two values specified above will be considered:
- 70.534 t.m on a 2.22m wide strip and a raft thickness of 1.0m => Reinf. should be 27.13 cm2
- 66.700 t.m on a 2.95m wide strip and a raft thickness of 1.0m => Reinf. should be 25.61 cm2
At these particular locations, the approximate averaging of Mxy on the considered strips is:
- 16 t.m + 70.53 = 86.53 t.m =>33.33 cm2 of reinforcement.
- 1 t.m + 66.70 = 68.7 t.m => 26.4 cm2 of reinforcement.
- 102.9 cm2 of reinforcement.
- 23.5 cm2 of reinforcement.
Considering the reinforcement computed by the SAFE internal method above, the following results were found:
- 33.3 cm2 of reinforcement.
- 26.6 cm2 of reinforcement.
It is recommended to adopt the following analysis methods for each case described below:
- For shell elements supporting point loads (ex.: raft foundations and transfer slabs); Model the shell as thick plate and use the internal moment method for the design (Wood-Armer).
- For shell elements supported on columns and walls, and having irregularities in column grids, slab limits, and geometry; Use the internal moment method to take into consideration the torsional moment Mxy that shall not be neglected.
- For slabs supported on columns and walls and having regular column grids, slab limits and geometry, there is no difference whether to use the nodal method or the internal moment method in the design, and this is due to the insignificant torsional moments Mxy in the slab; both methods should give approximately the same results.
Finally, it would be preferable to adopt the internal Wood-Armer method at all cases since it will always detect any torsional effects that might be neglected using nodal method, leading to under-design of structures.