SO SÁNH PHÂN TÍCH SÀN GIỮA SAFE VÀ SAP2000

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

Jan. 2008


TABLE OF CONTENTS

I

REPORT SCOPE .................................................................................................................... 2

II

THICK PLATE EXAMPLE ................................................................................................. 3
II-1- MODEL DESCRIPTION ................................................................................................... 3
II-2- MOMENTS IN X DIRECTION ......................................................................................... 4
II-3- MOMENTS IN Y DIRECTION ....................................................................................... 12

III

THIN PLATE EXAMPLE ................................................................................................. 20

III-1- MODEL DESCRIPTION ................................................................................................ 20
III-2- MOMENTS IN X DIRECTION ..................................................................................... 21

III-3- MOMENTS IN Y DIRECTION ..................................................................................... 26
IV

CONCLUSION ................................................................................................................... 30

APPENDIX A .............................................................................................................................. 31
APPENDIX B............................................................................................................................... 32

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I REPORT SCOPE
We have been lately notifying unjustified reinforcement when using SAFE models for raft
foundations supported on soil. This situation was mainly detected in case of irregular geometry
of models, unsymmetrical grids of columns, shear walls near the edge of the raft, etc…
The above pushed us to contact the “CSI Technical Support” (refer to Appendix A) and we were
advised to use the internal method (Wood-Armer) along with thick plate analysis in case of point
loads (Raft foundations, Transfer slabs…refer to Appendix B).
In what follows, two shell examples (raft foundation and suspended slab) will be the subject of a
study to compare between the analysis made by SAP2000 and the analysis made by SAFE using
both the nodal method and the internal method (Wood-Armer). The results of this study will be
adopted as a base for all horizontal shell designs.

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II THICK PLATE EXAMPLE
II-1- MODEL DESCRIPTION
For this example, a raft foundation on soil, supporting two corewalls near the edge and
rectangular grids of walls and columns, was modeled as a thick plate on SAP2000 and on SAFE

V8.0.8.

Fig. 1: Raft Model Geometry on SAFE

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Fig. 2: Raft Model Geometry on SAP2000
II-2- MOMENTS IN X DIRECTION
If we are to compare the moments in X direction between the SAP2000 model and the SAFE
model, the difference under the right corewall is about 0.4% (59.5t.m for SAP2000 and 59.3t.m
for SAFE) and between the two corewalls 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.

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Fig. 3: X Moment on SAP2000 Model
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Fig. 4: X Moment on SAFE Model
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Fig. 5: MXY Moment on SAP2000 Model
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Fig. 6: MXY Moment on SAFE Model
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Assuming that the moments differences between the SAP2000 model and the SAFE model are
insignificant all over the raft model, the SAFE model will directly calculate the strips moments
(column strips and middle strips), using an average value along the width of the strip:

Fig. 7: X Strips Moment From SAFE Model
Particularly, the two values specified on Fig.7 will be considered:
a) 70.534t.m on a 2.22m wide strip and a raft thickness of 1.0m => Reinf. should be 27.13cm2
b) 66.70t.m on a 2.95m wide strip and a raft thickness of 1.0m => Reinf. should be 25.61cm2
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At these particular locations, the approximate averaging of Mxy on the considered strips is:
a) 16t.m + 70.53 = 86.53t.m =>33.33cm2 of reinforcement.
b) 1t.m + 66.70 = 68.7t.m => 26.4cm2 of reinforcement.
Considering the reinforcement computed by the SAFE nodal method (Fig.8), the following
results were found:
a) 102.9cm2 of reinforcement.
b) 23.5cm2 of reinforcement.

Fig. 8: X Strips Reinforcement From SAFE Model Using Nodal Method
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Considering the reinforcement computed by the SAFE internal method (Fig.9), the following
results were found:
a) 33.3cm2 of reinforcement.

b) 26.6cm2 of reinforcement.

Fig. 9: X Strips Reinforcement From SAFE Model Using Internal Method (Wood-Armer)
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Considering the above results, the use of the combination of 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 corewalls
(Refer to CSI technical support reply in Appendix A).
II-3- MOMENTS IN Y DIRECTION
Comparing the moments in the Y direction between the SAP2000 model and the SAFE model,
the difference will remain in the same margins as the X Direction.

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Fig. 10: Y Moment on SAP2000 Model

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Fig. 11: Y Moment on SAFE Model

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Fig. 12: MXY Moment on SAP2000 Model (Fig.5)
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Fig. 13: MXY Moment on SAFE Model (Fig.6)
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Assuming that the moments differences between the SAP2000 model and the SAFE model are
insignificant all over the raft model, the SAFE model will directly calculate the strips moments
(column strips and middle strips), using an average value along the width of the strip:

Fig. 14: Y Strips Moment From SAFE Model
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Considering the reinforcement computed by the SAFE nodal method (Fig.15) and the SAFE
internal method (Wood-Armer) (Fig.16), the following results were found:

Fig. 15: Y Strips Reinforcement From SAFE Model Using Nodal Method

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Fig. 16: Y Strips Reinforcement From SAFE Model Using Internal Method (Wood-Armer)
Considering the above results, the use of the combination of moments in Wood-Armer SAFE
method (Mxx + Mxy) is converging with hand calculations using the SAP2000 values, but the
difference between the two methods is due to additional Mxy only.
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III THIN PLATE EXAMPLE
III-1- MODEL DESCRIPTION

For this example, a solid slab, supported on walls and columns, was modeled as a thin plate
using SAP2000 and using SAFE V8.0.8.

Fig. 17: Slab Model on SAFE

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Fig. 18: Slab Model on SAP2000
III-2- MOMENTS IN X DIRECTION
Comparing the moments in X direction between the SAP2000 model and the SAFE model, the
difference at the column on axes B-3 is about 1.1% (32.4t.m for SAP2000 and 32.05t.m for
SAFE) and between the two columns C-3 and D-3 about 0.02% (6.478t.m for SAP2000 and
6.479t.m for SAFE) and at the tip of the wall on axis 4 about 0.3% (14.496t.m for SAP2000 and
14.537t.m for SAFE). The above differences are minimal and can be neglected.

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Fig. 19: X Moment on SAP2000 Model

Fig. 20: X Moment on SAFE Model
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Fig. 21: MXY Moment on SAP2000 Model

Fig. 22: MXY Moment on SAFE Model

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Assuming that the moments differences between the SAP2000 model and the SAFE model are
insignificant all over the raft model, the SAFE model will directly calculate the strips moments
(column strips and middle strips), using an average value along the width of the strip:

Fig. 23: X Strips Moment From SAFE Model
Particularly, the two values on Fig.23 will be considered:
a) 14.326t.m on a 3.0m wide strip and a slab thickness of 0.3m => Reinf. should be 19.96cm2
b) 45.768t.m on a 3.0m wide strip and a slab thickness of 0.3m => Reinf. should be 50.53cm2
At these particular locations, the approximate averaging of Mxy on the considered strips is:
a) 4.5t.m + 14.33 = 18.83t.m =>26.4cm2 of reinforcement.
b) 5t.m + 45.77 = 50.77t.m => 56.3cm2 of reinforcement.
Considering the reinforcement computed by the SAFE nodal method (Fig.24) the following
results were found:
a) 15.45 cm2 of reinforcement.
b) 50.534 cm2 of reinforcement.

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