FOR CASE STUDIES USING SAP2000 – SAFE NODAL METHOD AND SAFE INTERNAL METHOD
(WOOD-ARMER) FOR THIN AND THICK PLATESUnjustified 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-DIRECTIONIf 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
X Moment on SAP2000 Model
X Moment on SAFE Model
Mxy Moment on SAP2000 Model
Mxy Moment on SAFE ModelAssuming 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 ModelParticularly, 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:
- 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.
X Strips Reinforcement From SAFE Model Using Nodal MethodConsidering the
reinforcement computed by SAFE nodal method above, the following results were
found:1. 102.9 cm2 of reinforcement.
3. 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.