SO SÁNH PHÂN TÍCH SÀN GIỮA SAFE VÀ SAP2000

So sánh giữ phần mềm phân tích kết cấu SAFE và SAP2000 trong phân tích nội lực của cấu kiện sàn , móng.

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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

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

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

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

Fig 3: X Moment on SAP2000 Model

Fig 4: X Moment on SAFE Model

Fig 5: MXY Moment on SAP2000 Model

Fig 6: MXY Moment on SAFE Model

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

At these particular locations, the approximate averaging of Mxy on the considered strips is:

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)

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

Fig 10: Y Moment on SAP2000 Model

Fig 11: Y Moment on SAFE Model

Fig 12: MXY Moment on SAP2000 Model (Fig.5)

Fig 13: MXY Moment on SAFE Model (Fig.6)

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

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

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

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

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

Fig 19: X Moment on SAP2000 Model

Fig 20: X Moment on SAFE Model

Fig 21: MXY Moment on SAP2000 Model

Fig 22: MXY Moment on SAFE Model

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:

Fig 24: X Strips Reinforcement From SAFE Model Using Nodal Method

Considering the reinforcement computed by the SAFE internal moment method (Fig.25) the following results were found:

a) 26.758 cm2 of reinforcement

b) 55.927 cm2 of reinforcement

Fig 25: X 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 the hand calculations using SAP2000 values The SAFE nodal method in this case will give reinforcement taking into account Mxx only

III-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 margin as the X Direction, which can also be neglected

Fig 26: Y Moment on SAP2000 Model

Fig 27: Y Moment on SAFE Model

Fig 28: MXY Moment on SAP2000 Model (Fig.21)

Fig 29: MXY Moment on SAFE Model (Fig.22)

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 30: Y Strips Moment From SAFE Model

Considering the reinforcement computed by the SAFE nodal method (Fig.31) and the SAFE internal method (Wood-Armer) (Fig.32), the following results were achieved:

Fig 31: Y Strips Reinforcement From SAFE Model Using Nodal Method

Fig 32: 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 SAP2000 values, but the difference between the two methods is due to additional Mxy only

IV CONCLUSION

Considering the results found for both thick and thin shell elements using SAFE and SAP2000, and referring to the response made by CSI Technical support team (Appendix A), it is

recommended to adopt the following analysis methods for each case described below:

a 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)

b 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

c 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

APPENDIX A

From: CSI Technical Support [mailto:support@csiberkeley.com]

Sent: Thursday, December 13, 2007 7:47 PM

To: Charbel Ghanem; CSI Technical Support

Subject: RE: - (DarRef: GENERAL.1/ Bey-07-129910 EML)

Dear Charbel,

SAFE version 6.46 and later versions allows users to choose between the nodal moments method of design as documented in the SAFE manual and the Wood-Armer method of design as documented in Eurocode 2 1992, Section A2.8(3) The Wood-Armer method explicitly accounts for Mxy moments and some jurisdictions require that The results are normally close for the two methods except when

concentrated loads are present and the thick plate element has been used In this case the nodal

moment method gives overly conservative results The user chooses the method to use in the Preference Dialog box

Our testing shows that the nodal method and the internal moments method (Wood-Armer) produce similar reinforcements when the reinforcement is being provided in essentially the principal directions In this case the Wood-Armer method gives slightly higher reinforcements as it involves some absolute terms On the other hand when the twist governs the Wood-Armer method is more reliable Whether the nodal method gives more or less reinforcement is dependent upon how we treat moments of different signs at the two nodes of the same element, what element (thin or thick) is used, the aspect ratio of the

mesh, etc Actually, the main reason we put the Wood-Armer method in SAFE, besides that some

codes now require it, is that we were getting very large reinforcements in mats under the columns when the nodal method was used with thick shell elements

Regards,

Faisal

From: Charbel Ghanem [mailto:Charbel.Ghanem@dargroup.com]

Sent: Thursday, December 13, 2007 6:11 AM

To: CSI Technical Support

Subject: (DarRef: Not Referenced)

“thick plate” should be used for shells where shear deformation is important ex rafts and transfer slabs…, and is it normal to obtain that much increase in reinforcement under core wall, knowing that the design ultimate shears and moments are not compatible with the reinforcement results

Waiting for your reply, please accept my sincere salutations

Best Regards

Charbel Ghanem

APPENDIX B

SAFE v8 - CSiDETAILER

ANALYSIS, DESIGN and DRAFTING of SLAB SYSTEMS

SAFE is a special purpose program that automates the analysis and design of simple to complex concrete flat plates and foundation systems using powerful object based modeling The program can analyze and design slabs or mats of arbitrary shapes and varying thickness, drop panels, openings, edge beams and discontinuities Foundations can be combinations of Mats, Strip Footings

or Isolated Spread Footings.

Introduction

SAFE is a sophisticated, yet easy to use, special purpose analysis and design program developed specifically for concrete Slab/Beam, Basemat/Foundation systems SAFE couples powerful object-based modeling tools with an intuitive graphical interface, allowing the user to quickly and efficiently model slabs of regular or arbitrary geometry with openings, drop panels, ribs, edge beams, and slip joints supported by columns, walls or soil Design is seamlessly integrated with modeling and

analysis, and provides comprehensive reporting of the required reinforcing calculated by the program based on the user’s choice of design code And with the optional CSiDETAILER program, detail

drawings may be produced almost effortlessly for the slabs and beams designed using SAFE

The analysis is based upon the Finite Element method in a theoretically consistent fashion that properly accounts for the effects of twisting moments Meshing is Automated based upon User Specified

Parameters Foundations are modeled as thick plates on Elastic Foundations, where the

Compression Only Soil Springs are automatically discretized based upon a modulus of subgrade reaction that is specified for each Foundation Object

The Software produces reinforcing layouts and evaluates the effects of punching shear around column supports Options are available for including cracked properties in the finite element model based upon the slab reinforcing that is provided

Also a comprehensive export option from ETABS is available that will automatically create complete SAFE models of any ETABS floor or foundation for immediate design by SAFE

Based on the finite element method, this program offers accuracy and flexibility that cannot be

matched by traditional hand calculations or equivalent frame computer programs Traditional methods for the analysis of simple slab systems are tedious and time consuming, and are often inapplicable for complex geometries or loadings General purpose finite element programs are capable of handling much more complex models, but are often cumbersome and difficult to use and also produce results that are not directly usable by the structural engineer