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So sánh giữ phần mềm phân tích kết cấu SAFE và SAP2000 trong phân tích nội lực của cấu kiện sàn , móng.
` 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 II THICK PLATE EXAMPLE II-1- MODEL DESCRIPTION II-2- MOMENTS IN X DIRECTION 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 Page 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 Page 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 Page 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 Page Fig 3: X Moment on SAP2000 Model Page Fig 4: X Moment on SAFE Model Page Fig 5: MXY Moment on SAP2000 Model Page Fig 6: MXY Moment on SAFE Model Page 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 Page 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 Page 19 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 Page 20 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 about 0.3% (14.496t.m for SAP2000 and 14.537t.m for SAFE) The above differences are minimal and can be neglected Page 21 Fig 19: X Moment on SAP2000 Model Fig 20: X Moment on SAFE Model Page 22 Fig 21: MXY Moment on SAP2000 Model Fig 22: MXY Moment on SAFE Model Page 23 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 Page 24 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) Page 25 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 Page 26 Fig 27: Y Moment on SAFE Model Fig 28: MXY Moment on SAP2000 Model (Fig.21) Page 27 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: Page 28 Fig 31: Y Strips Reinforcement From SAFE Model Using Nodal Method Fig 32: Y Strips Reinforcement From SAFE Model Using Internal Method (Wood-Armer) Page 29 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 underdesign of structures Page 30 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 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) Dear Mr or Mm, I have an important question regarding the SAFE software, and I will appreciate it if I get an answer as soon as possible I am doing a SAFE model for a raft foundation on soil support; using a thick plate shell, I am obtaining reinforcement under a core wall that is more than three times the reinforcement obtained from thin plate analysis So please let me know when to use thick plate analysis, knowing that from my experience, the “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 Page 31 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 Page 32 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 objectbased 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 Page 33 ... (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... plate using SAP2000 and using SAFE V8.0.8 Fig 17: Slab Model on SAFE Page 20 Fig 18: Slab Model on SAP2000 III-2- MOMENTS IN X DIRECTION Comparing the moments in X direction between the SAP2000 model... X Moment on SAFE Model Page 22 Fig 21: MXY Moment on SAP2000 Model Fig 22: MXY Moment on SAFE Model Page 23 Assuming that the moments differences between the SAP2000 model and the SAFE model are