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MINISTRY OF EDUCATION AND TRAINING NATIONAL UNIVERSITY OF CIVIL ENGINEERING Tran Viet Tam STUDY ON PUNCHING SHEAR CAPACITY OF PRESTRESS FLAT SLABS Major: Construction Engineering Code: 9580201 SUMMARY OF DOCTORAL DISSERTATION HA NOI –2019 The Dissertation was completed at the National University of Civil Engineering Academic advisors: Prof Dr Phan Quang Minh Assoc Prof Dr Nguyen Ngoc Phuong Examiner : Prof Dr Nguyen Tien Chương Examiner : Assoc Prof Dr Truong Hoai Chinh Examiner : Dr Nguyen Dai Minh The doctoral dissertation will be defensed at the level of the State Council of Dissertation Assessment’s meeting at the National University of Civil Engineering at hour day month year 2019 This Dissertation is available for reference at the Libraries as follows - National Library of Vietnam; - Library of National University of Civil Engineering PREFACE REASON FOR SELECTING THE TOPIC Nowadays, reinforced concrete and pre-stressed concrete flat slabs are used commonly in buildings in Vietnam and worldwide, since they have many advantages in architecture, structures and construction In design of flat slab structures, the design against punching shear has always been of special interest because this is a dangerous type of brittle damage Some buildings in the world were damaged by punching shear and they caused serious consequences such as: Sampoong Department Building in Korea (1995), causing 502 people killed and 1000 injured [86]; the Virgina State Skyline Plaza - USA (1971) caused casualties for more than 14 workers [56] The current design standard in Vietnam is "Concrete and reinforced concrete structures - TCVN 5574-2018 design code" [7] based on Russian standard SP 63.13330 [80] It introduces mainly the calculation principles of punching shear strength for normal reinforced concrete members In the formulae to predict the punching shear capacity of reinforced concrete and prestressed concrete flat slabs, influences of the factors such as reinforcement ratio in the tention zone, compressive stress in concrete due to prestress and the column dimensions have not been considered To increase economic efficiency in using the flat slabs, the study of punching shear capacity of reinforced concrete and prestress concrete flat slabs, then proposing new formulae validated with experimental results are needed RESEARCH PURPOSES a) Review extensively previous studies on punching shear capacity of reinforced concrete and prestressed concrete flat slabs b) Study punching shear capacity of flat slabs by the numerical method, to investigate the parameters affecting the punching shear capacity of flat slabs including tension reinforcement ratio, precompressive stress in concrete, slab thickness, column dimensions c) Propose a new formulae to predict the punching shear capacity of reinforced concrete and prestressed concrete flat slabs d) Setup an experimental model to verify the proposed formulae for the punching shear capacity of reinforced concrete and prestressed concrete flat slabs SCOPE OF WORK, SCIENTIFIC BASIS AND RESEARCH METHOLOGY Scope of work: Study on the punching shear capacity of reinforced concrete and prestressed concrete flat slabs with the concrete grade is not greater than B60, no shear stirrup, no slab openings, no consideration of influence of moment at column-slab connections, columns with circle or rectangular section only Scientific basis: Through the theoretical study, numerical simulation and experiment to predict the punching shear capacity of reinforced concrete and prestressed concrete flat slabs, clarification of stress state and deformation at column-slab connections Reinforced concrete and prestressed concrete flat slabs have been widely used in buildings Therefore, to study and to propose a suitable formulae with TCVN 5574-2018 are the practical meanings of this thesis The proposed formulae takes into account the influence of tensile reinforcement ratio and pre-compressive stress in concrete, the ratio between the column height and the slab effective depth It helps to predict the punching shear capacity of flat slabs more exactly and to give more reasonable results in design Research methodology: Theoretical, numerical simulation and experimental study NEW CONTRIBUTIONS OF THESIS a) Create numerical simulated models in ANSYS software written in ADPL language, from which the important parameters affecting the punching shear capacity of flat slabs are easily investigated: tension reinforcement ratio, compressive stress in concrete due to prestress, the ratio between height and width of the rectangular column section b) Propose a new formulae to predict the punching shear capacity of reinforced concrete flat slabs suitable for TCVN 55742018 c) Propose a new formulae to evaluate the punching shear capacity of prestress concrete flat slabs suitable for TCVN 55742018 d) Set up an experimental model capable to verify the punching shear capacity of reinforced concrete and prestressed concrete flat slabs The collected experimental data are not only to verify the two proposed formulae of the thesis, but also for further studies on the punching shear capacity of flat slabs CONTENT OF THESIS The thesis includes preface, chapters, conclusions and recommendations, published works by the author and references CHAPTER RESEARCH OVERVIEW ON PUNCHING SHEAR CAPACITY OF FLAT SLABS 1.1 DEFINITION OF PUNCHING SHEAR CAPACITY OF REINFORCED CONCRETE FLAT SLABS Punching shear failure is a local failure caused by shear force in two directions at the positions of columnflat slab connection The failure area has a truncated cone around the column (Fig 1.1) Punching shear capacity of Fig 1 Typical punching shear reinforced flat slabs depends on failure in reinforced flat slabs many factors such as: concrete quality, tensile reinforcement (ratio, distribution of reinforcement), column size, column position, size factor, prestressing force in concrete, and boundary conditions 1.2 MODELS TO DETERMINE PUNCHING SHEAR CAPACITY OF REINFORCED CONCRETE FLAT SLABS Many theoretical and experimental models are summarized as follows: 1.2.1 Mechanical models based on equilibrium conditions  Shehata (1985) and Regan (1989) [79]  Brom’s model [28] 1.2.2 Truss model  Truss and tie model of Marzouk and Tiller [66] 1.2.3 Tension failure models  Truss and tie model of Alexander -Simmonds [25]  Truss and tie model of Georgopolous [74]  Truss and tie model of Menétrey [67] 1.2.4 Flexural approach 1.2.5 Critical shear crack theory (2008) Fig 1.2 Procedure to specify punching shear strength of slabs according to Critical Shear Crack Theory (Muttoni 2008) 1.3 MODELS TO DETERMINE THE PUNCHING SHEAR CAPACITY OF PRESTRESS CONCRETE FLAT SLABS 1.3.1 Principal tensile stress approach 1.3.2 Equivalent reinforcement ratio approach 1.3.3 Decompression stress approach 1.4 PREVIOUS EXPERIMENTAL STUDIES ON PUNCHING SHEAR CAPACITY OF REIFORCED CONCRETE FLAT SLAB The thesis has collected 270 published experimental specimens of punching shear studies, in order to verify the numerical models and the proposed formulae Details of these experiments can be found in Appendix A 1.5 SIMULATION ON PUNCHING SHEAR CAPACITY OF REINFORCED CONCRETE FLAT SLABS A new research method to determine the punching shear capacity of reinforced concrete flat slabs is the application of finite element method (FEM) and RC simulation software The numerical method takes into account of nonlinear behaviour of concrete, allowing the observation and evaluation before conducting the experimental research It is a low-cost and easy-to-build tool, and allows to change the Fig 1.13 Simulation model of parameters during the Aikaterini Genikomsou (2015) simulation 1.6 PRACTICE CODES AND STANDARDS 1.6.1 US Codes ACI-318-2014 [19] 1.6.2 European Codes EC2 (2004) [36] 1.6.3 Australian Codes AS3600 (2018) [18] 1.6.4 Canadian Codes CSA A23.3-14 [30] 1.6.5 Chinese Codes GBJ 50010-2010 [47] 1.6.6 British Codes BS 8110-1997 [29] 1.6.7 German Code DIN 1045-2008 [34] 1.6.8 FIB - Modal Code 2010 [37] 1.6.9 Vietnamese Code TCVN 5574-2018 [7] 1.7 PREVIOUS STUDIES ON PUNCHING SHEAR CAPACITY OF REINFORCED CONCRETE FLAT SLABS IN VIETNAM Associate Professor, Dr L T Huan, in the study "Effect of prestress in column-slab connections of flat slabs" [1], conducted specimens with an average compressive stress of MPa to verify the formulae: Fb = (1+n) Rbt um ho P N Vuong (2018) [3] in the study "Analysis of punching shear capacity of reinforced concrete flat slabs taking into account influences of boundary conditions by ANSYS software" simulated the column-slab connections and use this model to investigate the stiffness of edge beams that affects the punching shear capacity of flat slabs 1.8 SALIENT REMARKS FROM LITERATURE REVIEW The punching shear capacity of reinforced flat slabs depends on the concrete strength, ratio of tension reinforcement, stirrup, slab-column size parameters, pre-compressive stress in concrete The formulae predicting the punching shear capacity of reinforced concrete and pre-stressed concrete flat slabs according to the design standards were based on theoretical and experimental models, but there is still discrepancy For example, using the two most commonly used Codes, ACI-318-14 and EC2-2004, to verify with 270 published specimens, the average discrepancy between EC2-2004 and ACI-318-2014 is 15.66% (ACI-318 gives greater results in prediction) However, in many cases the discrepancy can be over 30% Using TCVN 5574-2018 to predict the punching shear capacity of reinforced concrete and pre-stressed concrete flat slabs with 270 published specimens as shown in Figure 1.22, the ratio of Putest / PuTCVN code is 1.45 It should be noted that the predictions not consider the influence of tensile reinforcement and compressive stress due to prestress in concrete In some cases, when the slab thickness or the ratio between the height and width of the rectangular columns is large, the prediction PuTCVN code is not in the safe side (Appendix A, No 236 slab samples, 240, 250, 251, 252, 255) Nowadays, with development of the numerical methods and RC simulation software especially Ansys, it is possible to setup a simulation model to analyse the punching shear capacity of reinforced concrete and pre-stressed concrete flat slabs The advantages of the numerical models are easy to change the parameters to be investigated such as floor and column sizes, materials, reinforcement layout, pre-compressive stress in concrete This is a new trend to solve the problem when the boundary conditions, the shape of the column-slab connections are complicated, and cost-effective The above remarks are the continuous research orientation of the thesis The thesis uses the numerical method to investigate the parameters affecting the punching shear capacity of reinforced concrete and pre-stressed concrete flat slabs From the numerical results, it proposes the formulae which can predict the punching shear capacity of reinforced concrete and pre-stressed concrete flat slabs, taking into account the effect of reinforcement ratio, compressive stress in concrete, parameters of slab and column sizes The proposed formulae are suitable with TCVN 5574-2018 and are validated by the experimental results of the thesis CHAPTER RESEARCH OF PUNCHING SHEAR CAPACITY OF RC FLAT SLABS BY NUMERICAL SIMULATION METHOD 2.1 INTRODUCTION Nowadays, numerical simulation has become one of the reliable and effective methods to study the punching shear capacity of reinforced concrete and prestressed concrete flat slabs Using this method, it is possible not only to determine the value of punching shear force, but also to consider other influential parameters such as tensile reinforcement ratio, effect of column size, slab thickness, boundary conditions, pre-compressive stress in concrete Many prediction formulae have not taken into account these factors ANSYS [12] [26] is a powerful structural software based on FEM method that can simulate and analyze reinforced concrete structures The advantages of this software are that it uses nonlinear material models and template modules available in the software The users also can integrate the material models suitable with the problems to be studied, or it is possible to write a model with APDL parametric design language for each problem ADPL parametric design language (ANSYS Parametric Design Language) [19] is FORTRAN programming language, providing full functions to create variables, constants, functions, vectors, matrices, iterations to model the problems with complex boundary conditions, when you need to solve iterations and create common modules The model is built in ADPL language as a file that contains written source code, allowing to change parameters of input data such as model size, reinforcement grid, tendon grid, model and material strength, load This chapter presents the study on punching shear capacity of reinforced concrete and prestressed concrete flat slabs using the numerical simulated models in ANSYS Mechanical V.15.0 software written in APDL language Using these models, the parameters are varied to investigate the factors affecting punching shear capacity of flat slabs, then a formulae is proposed to predict the punching shear capacity of flat slabseVietnamese design Code for reinforced concrete structures TCVN 5574-2018 2.2 MODELLING OF REINFORCEMENT IN CONCRETE According to the FEM method, there are three different models to model reinforcing rebars in concrete: smeared model, embedded model, discrete model [35] [38] In this study, stresses of concrete and reinforcement are required at every stage, so that the "discrete" model is chosen to simulate reinforcement element in concrete for the tested specimens 11 2.8.1 Yaser Mirzae’s specimens 2.8.2 Alam’s specimens 2.8.3 Franklin and Long’s specimens 2.8.4 Rahman’s specimens 2.8.5 Comments The validation results for punching shear force from Ansys simulation for published specimens with reinforced concrete slab and prestressed concrete flat slabs are close to each other On the other hand, because in the simulation models, bonding behavior between concrete and reinforcement is assumed to be perfect; the boundary conditions, crushing and cracking behavior in concrete are not exactly the same as in the experiment; and due to shear locking phenomenon in solid element, the deflection at the midpoint of the plate in simulation is usually smaller than one in the experiments 2.9 INVESTIGATION ON THE INFLUENCE OF REINFORCEMENT RATIO TO PUNCHING SHEAR CAPACITY OF FLAT SLABS Fig 2.32 Relationship between reinforcement ratio and punching shear capacity of flat slabs in group N1R 2.10 INVESTIGATION ON THE INFLUENCE OF PRE- COMPRESSIVE STRESS TO PUNCHING SHEAR CAPACITY OF PRE-STRESS CONCRETE FLAT SLABS 12 Hình 2.38 Relationship between effective stress and punching shear capacity of pre-stress flat slabs 2.11 INVESTIGATION ON THE INFLUENCE OF CONCRETE STRENGTH, SIZE EFFECT TO PUNCHING SHEAR CAPACITY OF FLAT SLABS 2.11.1 Influence of concrete strength 2.11.2 Influence of slab effective depth 2.11.3 Influence of the ratio between the depth and the width of rectanglar columns Fig 2.45 Relationship between c and punching shear capacity of reinforced concrete flat slabs 2.12 PROPOSED FORMULAE TO DETERMINE THE PUNCHING SHEAR CAPACITY OF REINFORCED CONCRETE FLAT SLABS 2.12.1 Principles for proposed formulae 13 2.12.2 Proposed formulae to determine the punching shear capacity of normal reinforced concrete flat slabs (2.33) F   k s k c u m h R bt where: - - - - - : Coefficient of concrete type, for heavy-weight concrete and 0.8 for light-weight concrete; Rbt: Concrete tensile strength; ho: Slab effective depth; ks : Coefficient taking into account of influence of tensile reinforcement ratio; 0.9s uc ks  (1  ) ; 1.0 ≤ ks ≤ 1.30 0.021  s ud s : Average reinforcement ratio distributed in the floor in x direction (sx) and y direction (sy), but not greater than 2% [36] : s = (sx + sy)/2 kc: Coefficient considering the influence of the ratio between the depth and the width of rectangular columns (c): if c >2 0.15 then kc   hc / h0  ; if c ≤ then kc=1; um: Control perimeter of the design section taking as h0/2 from the column edge, in case of rectangular sections then um = 2(bc+hc +2h0) ud: Bottom perimeter of the punching shear cone taking as h0 from the column edge, in case of rectangular sections then um = 2(bc+hc +4h0) 2.12.3 Proposed formulae to determine the punching shear capacity of pre-stresss concrete flat slabs (2.34) F   k s k c u m h ( Rbt  0.12 p ) Where: p: Effective compressive stress in pre-stress concrete, taking not greater than 3.5 MPa [19] 2.12.4 Evaluation of the proposed formulae with numerical 14 simulation results 2.12.5 Evaluation of the proposed formulae with the published test data Fig 2.49 Comparison between the proposed formulae 2.33, 2.34 with the published test data Figure 2.49 shows the comparison results of the proposed formulae 2.33 and 2.34 with 270 published specimens in the world for punching capacity of reinforced and pre-stress concrete flat slabs The details of the specimens can be found in Appendix A The average value of the ratio of Pcttest / Pctformulae is 1.27, the force deviation = 0.200, the variation coefficient  = 0.158 It can be concluded that equations 2.33 and 2.34 have a suitable safety factor when predicting the punching shear capacity of reinforced and pre-stress concrete flat slabs 2.13 REMARKS OF CHAPTER In Chapter 2, numerical models have been built to analyze the punching shear capacity of reinforced and pre-stress concrete flat slabs The models have been simulated in ANSYS software, written in ADPL language The parameters can be changed, including slab and column sizes, reinforcement layout, materials, pre-compressive stress in concrete It is very convenient in numerical study and in design Using the simulation models, the parameters affecting the 15 punching shear capacity of reinforced and pre-stress concrete flat slabs have been investigated From the investigation results, the author proposes two formulae to predict the punching shear capacity of reinforced and pre-stress concrete flat slabs suitable with TCVN 5574-2018 Applying the formulae to predict over 230 published test data of reinforced concrete flat slabs and 40 specimens of pre-stressed concrete flat slabs, the ratio of Pcttest / Pctformulae is 1.27 as average The proposed formulae will be verified by the author’s experimental study in chapter CHAPTER EXPERIMENTAL STUDY ON PUNCHING SHEAR CAPACITY OF REINFORCED CONCRETE AND PRE-STRESS CONCRETE FLAT SLABS 3.1 EXPERIMENTAL OBJECTIVES AND PROGRAME 3.1.1 Experimental objectives a) To observe punching shear failure mode and measure the punching shear force of reinforced and pre-stress concrete flat slabs; b) To investigate the relationship between the load and : deflection at the middle point, slab rotation angle, concrete strain at the vicinity of column head, reinforcement strain c) To investigate the influence of tensile reinforcement ratio on punching shear capacity of flat slabs d) To investigate influence of pre-compressive stress in concrete on the punching shear capacity of pre-stress concrete flat slabs e) To verify the numerical simulated models and the formulae 2.33 and 2.34 proposed in Chapter 3.1.2 Research program 3.2 BASIS TO DESIGN SPECIMENS AND TO SET UP THE EXPERIMENTAL MODEL 3.2.1 Basis for designing specimens In this study, the author proposes a modal scale of 1/4 due to 16 the constraints of LAS-XD125 laboratory of the University of Civil Engineering, and also inherits some experimental results by Alam [23], Franklin and Long [40] and their partners 3.2.2 Setting up the experimental models The experimental specimens of the thesis can be seen in Figure 3.1 All reinforced concrete slabs are 1000mm long x 1000mm wide with the slab thickness of 60 mm The column section is 120x120 mm, provided 10 as longitudinal rebars and stirrup of 6 a100 Fig 3.1 Detail of tested specimens 3.3 SPECIMEN MATERIALS 3.3.1 Concrete Concrete grade of B30, using superplastic admixture for concrete to achieve the strength within 10-14 days 3.3.2 Reinforcement Vietnamese-Italian steel grade CB 240-T with diameter 6 The material test showed that reinforcement has a minimum yield strength of 367 MPa and a ultimate strength of 560 MPa 3.3.3 Tendon Tendon is the high strength steel type with a diameter of  = 7.1 mm According to the manufacturer's data (Phan Vu Investment JSC), the tendon has a yield strength of 1272 MPa and a ultimate strength of 1420 MPa 3.4 SPECIMEN DESIGN AND GROUPING 17 - Group (Non pre-stress) S0N1 includes specimens, namely S0N1-1, S0N1-2, S0N1-3 These are non-prestressed concrete specimens with a reinforcement ratio of 0.71% ((a100) In this group, the tendon layer 7.1 is still placed in the middle but will not be stressed - Group (Non pre-stress) S0N2 includes specimens, namely S0N2-1, S0N2-2, S0N2-3 These are non-prestressed concrete specimens with a reinforcement ratio of 1.35% ((a50) - Group (Non pre-stress) S0N3 includes specimens, marked as S0N3-1, S0N3-2, S0N3-3 These are non-prestressed specimens with a reinforcement ratio of 0.39% ((a200) - Group (Pre-stress) S1 includes specimens, marked as S1-1, S1-2, S1-3 These are pre-stressed concrete specimens with an effective stress in concrete of 1.50 MPa, normal reinforcement ratio of 0.71 % ((a100) - Group (Pre-stress) S2 includes specimens, namely S21, S2-2, S2-3 These are pre-stressed concrete specimen with an effective stress in concrete of 2.45 MPa, normal reinforcement ratio of 0.71 % ((a100) 3.4.1 Detailing of non-prestress group SON1, SON2, SON3 3.4.2 Detailing of prestress group S1, S2 3.5 LOADING SYSTEM 3.5.1 Vertical loading system The vertical loading system is designed to support the slab and the applied load from bottom to top Fig 3.2 Detail of supporting frame 18 3.5.2 Pre-stress loading system The pre-stressed loading frame is designed to generate prestress in tendons with a diameter of 7.1 in each direction Fig 3.7 Detail of prestress loading system 3.6 DIAGRAM OF MEASUREMENT EQUIPMENTS 3.6.1 Diagram of LVDT positions 3.6.2 Diagram of position of strain gauges 3.7 SPECIMEN FABRICATION 3.7.1 Casting 3.7.2 Stressing tendon sequence 3.7.3 Release anchoring sequence 3.8 MATERIAL TESTS 3.8.1 Compressive strength, tensile strength, elastic modulus of concrete 3.8.2 Reinforcement tensile strength 3.8.3 Tendon tensile strength 3.8.4 Stress losses in tendon 19 Fig 3.7 Pre-stress distribution in the specimens 3.9 EXPERIMENT OF PUNCHING SHEAR CAPACITY OF FLAT SLABS 3.10 EXPERIMENTAL RESULTS 3.10.1 Data and data analysis 3.10.2 Punching shear capacity of flat slabs Table 3.10 Result of punching shear capacity of reinforced concrete flat slabs S0N3 S0N1 S0N2 Group s 0.71 % 0.39 % 1.35 % Sample 3 Putest (i) 78.0 66.8 74.9 86.0 86.6 84.4 118.1 114.7 113.9 Putest aver 73.2 85.7 115.6 Table 3.11 Result of punching shear capacity of pre-stress concrete flat slabs Group S0N1 S1 S2 20 p 1.53 2.45 s 0.71 % 0.71 % 0.71 % Sample 3 Putest (i) 86.0 86.6 84.4 104.1 102.2 102.3 104.9 115.9 117.3 Putest aver 85.5 102.8 3.10.3 Maximum deflection at the slab midle point 3.10.4 Punching crack patterns 3.10.5 Load-deflection relationship with different reinforcement ratio 116.6 tensile Fig 3.38 Load-deflection relationship with different tensile reinforcement ratio Fig 3.41 Load-deflection relationship with different effective prestress 3.10.6 Load - concrete strain relationship 21 3.10.7 Load – reinforcement tensile stress relationship 3.10.8 Verification of numerical results with tested results 3.10.9 Verification of the proposed formulae with tested results Table 3.17 Comparison between the proposed formulae and the tested results with different reinforcement ratio Group S0N3 S0N1 S0N2 Reinforcement ratio 0.39 % 0.71 % 1.35 % ks 1.08 1.13 1.20 Putest 72.3 85.7 115.6 Pu proposed 65.3 68.3 75.5 Putest / Pu proposed 1.11 1.25 1.53 When the tensile reinforcement ratio in the slabs is less than 1%, Putest / Pu proposed is 1.11 and 1.25 The safety factor is suitable, since this is a common reinforcement ratio in flat slabs With a tensile reinforcement ratio greater than 1.35%, Putest / Pu proposed is 1.53 With the flat slabs having a large reinforcement ratio, the flat slabs may have a larger bending moment or a smaller thickness, thus it could require a higher safety factor Table 3.18 Comparison between the proposed formulae and the tested results with different concrete effective stress Group S0N1 S1 S2 Effective stress 1.53 2.45 Reinforcement ratio 0.71 % 0.71 % 0.71 % kp 1.12 1.12 1.12 22 Putest 85.7 102.8 116.6 Pu proposed 68.3 76.9 80.6 Putest / Pu proposed 1.25 1.34 1.45 When the effective stress in the concrete is less than 1.53 MPa, Pu / Pu proposed is 1.25 and 1.34 The safety factor is suitable because this is an effective stress normally designed in flat slabs With an effective stress greater than 2.45 MPa, Putest / Pu proposed is 1.45 When the effective stress in the concrete is too high, there will be some risks such as tendon break out, stress losses, thus it could require a higher safety factor test 3.11REMARKS OF CHAPTER Chapter of the thesis has set up an experimental model to determine the punching shear capacity of reinforced and pre-stress concrete flat slabs Through 15 specimens, the effect of tensile reinforcement ratio, pre-compressive stress in concrete was investigated, and the accuracy of the proposed formula of 2.33 and 2.34 was verified Experimental results on groups of reinforced concrete specimens show that the tensile reinforcement ratio will be significantly affected to punching shear capacity of reinforced flat slabs: when the tensile reinforcement ratio is increased from 0.39% to 0.71%, punching shear force increased by 1.17 times, when the tensile reinforcement ratio is increased from 0.39% to 1.35%, the punching shear force increased 1.57 times Formulae 2.33 for coefficients of Putest / Puproposed with the above survey groups are 1.11, 1.25 and 1.53 respectively Experimental results on groups of pre-stressed concrete specimens shows that the effect of pre-compressive stress in concrete increases the punching shear capacity of pre-stress concrete flat slabs: when the pre-compressive stress in concrete is 1.53 MPa, the punching shear force increases by 1.20 times 23 compared to the normal reinforced concrete, when the precompressive stress is 2.45 MPa, the punching shear force increases by 1.36 times compared to the normal reinforced concrete speciments Formulae 2.34 for the coefficients of Putest / Puproposed with the above two groups are 1.25 and 1.34 CONCLUSION CONCLUSION Based on the research results on the punching shear capacity of reinforced concrete and pre-stressed concrete flat slabs, the following conclusions can be drawn in the thesis: The proposed numerical simulation model in ANSYS software, written in ADPL language, ensures reliability With the numerical model, it is easy to change the slab parameters such as dimensions, materials, reinforcement layout, compressive stress in concrete due to pre-stress, position of pre-stressed tendon layout, boundary conditions to investigate the punching shear capacity of reinforced concrete and pre-stressed concrete flat slabs The thesis has proposed formulae 2.33 to predict the punching shear capacity of reinforced concrete flat slabs, considering influence of the tensile reinforcement ratio, the effect of the column size The formulae is quite suitable for Vietnamese Code TCVN 5574- 2018, and has been verified by specimens tested by the author and 230 published specimens in the world It is found that the formulae can be reliable and can be used in further research and in design practice The thesis has proposed formulae 2.34 to predict the punching shear capacity of pre-stressed concrete flat slabs suitable for Vietnamese Code TCVN 5574-2018 The formulae has been 24 verified by specimens tested by the author and 40 published specimens in the world It is found that the formulae can be reliable and can be used in further research and in design practice An experimental model is proposed and setup in this thesis, which is able to determine the punching shear capacity of reinforced concrete and pre-stressed concrete flat slabs Through 15 samples, the two formulae 2.33 and 2.34 have been verified The obtained test data will be an useful reference for further studies on the punching shear capacity of reinforced concrete and pre-stressed concrete flat slabs RECOMMENDATIONS In reinforced concrete and pre-stressed concrete flat slabs, the punching shear capacity of slab also depends on column position such as at corner, on edge or in middle, so it is necessary to develop the FEM models and the experimental models to propose a factor considering the column position Current studies often focus on solving problems of single slab-column connection, without considering the actual structure as a continuous space system Therefore, it is possible to study a continuous column-flat slab system numerically, from which the safety factor for single and continuous cases can be estimated and compared MOST PUBLISHED WORKS BY AUTHOR Tran Viet Tam, “Punching shear capacity of prestress flat slabs with un uniform spans”, Proccedings of the 17th conference on Reseaching and Development of Nuce , Construction Publishing House - Ministry of Construction, 2016 Tran Viet Tam, Pham Ngoc Vuong, “Influence of Reinforcement ratio on punching shear strength of concrete flat slabs”, Viet Nam Journal of Construction , Ministry of Construction, 07/2018, ISSN 0866-8762 Tran Viet Tam, “Influence of size effect on punching shear strength of concrete flat slabs”, Viet Nam Journal of Construction , Ministry of Construction, 01/2019, ISSN 0866-8762 Tran Viet Tam, “Influence of pre-compressive stress in concrete on punching shear strength of concrete flat slabs”, Viet Nam Journal of Construction , Ministry of Construction, 01/2019, ISSN 0866-8762 ... against punching shear has always been of special interest because this is a dangerous type of brittle damage Some buildings in the world were damaged by punching shear and they caused serious consequences... experimental study NEW CONTRIBUTIONS OF THESIS a) Create numerical simulated models in ANSYS software written in ADPL language, from which the important parameters affecting the punching shear capacity... Procedure to specify punching shear strength of slabs according to Critical Shear Crack Theory (Muttoni 2008) 1.3 MODELS TO DETERMINE THE PUNCHING SHEAR CAPACITY OF PRESTRESS CONCRETE FLAT SLABS