Design comparison between SAFE and SAP2000


FOR CASE STUDIES USING SAP2000 – SAFE NODAL METHOD AND SAFE INTERNAL METHOD (WOOD-ARMER)  FOR THIN AND THICK PLATES

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.

Raft Model Geometry on SAFE

Raft Model Geometry on SAP2000

 MOMENTS IN X-DIRECTION

If we compare the moments in X direction between SAP2000 model and SAFE model, the difference under the right core wall is about 0.4% (59.5t.m for SAP2000 and 59.3t.m for SAFE) and between the two core walls about 0.3% (33.96t.m for SAP2000 and 34.06t.m for SAFE) and directly under a column about 1.3% (148.48t.m for SAP2000 and 150.40t.m for SAFE). The above differences are truly minimal and can be neglected.

X Moment on SAP2000 Model

 X Moment on SAFE Model

 Mxy Moment on SAP2000 Model

  Mxy Moment on SAFE Model

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:

 X Strips Bending Moment From SAFE Model

Particularly, the two values specified above will be considered:

  1. 70.534 t.m on a 2.22m wide strip and a raft thickness of 1.0m => Reinf. should be 27.13 cm2
  2. 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:

  1. 16 t.m + 70.53 = 86.53 t.m =>33.33 cm2 of reinforcement.
  2. 1 t.m + 66.70 = 68.7 t.m => 26.4 cm2 of reinforcement.

X Strips Reinforcement From SAFE Model Using Nodal Method

Considering the reinforcement computed by SAFE nodal method above, the following results were found:

  1. 102.9 cm2 of reinforcement.
  2. 23.5 cm2 of reinforcement.

X Strips Reinforcement From SAFE Model Using Internal Method (Wood-Armer)

Considering the reinforcement computed by the SAFE internal method above, the following results were found:

  1. 33.3 cm2 of reinforcement.
  2. 26.6 cm2 of reinforcement.

Considering the above results, the use of the combination of bending moments in Wood-Armer SAFE method (Mxx + Mxy) is converging with the hand calculations using SAP2000 results. The nodal method in this case is giving “excessively conservative” reinforcement under the core walls.

Conclusion:

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.


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