JCSS Workshop on Semiprobabilistic FEM Calculations, Delft, 1&2 December 2009

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1 Delft JCSS Workshop on Semiprobabilistic FEM Calculations, Delft, 1&2 December 2009 Nonlinear Finite Element Analysis of Concrete Structures given the deterministic parameters of loads, geometry and material behavior Max A.N. Hendriks, Civil Engineering and Geosciences, TU Delft 1 December 2009 University of Technology Background Nonlinear Finite Element Analysis (NL-FEA) can provide a tool to assess safety using realistic descriptions of the material behavior with actual material properties. In this way, a realistic estimation of the existing safety can be obtained utilizing hidden capacities by using true material properties. Guidelines for Nonlinear Finite Element Analysis 2 16

2 Question Is this true for Nonlinear Finite Element ULS Analysis of Concrete Structures? Guidelines for Nonlinear Finite Element Analysis 3 16 NL-FEA: decisions & decisions Typically an analyst is confronted with many many options and other decisions on how to setup the Nonlinear Finite Element Analysis Many of these decisions can have great influence on the results of the FE Analysis Illustrative Examples: Round-robin exercises of blind predictions of test results Guidelines for Nonlinear Finite Element Analysis 4 16

3 Way out Guidelines for Nonlinear Finite Element ULS Analysis of Concrete Structures Guidelines for Nonlinear Finite Element Analysis 5 16 Objectives for Guidelines Limit the user factor and model factor in the results of NL-FEA Limit the work for the analysts: By (partly) unburden them from providing proofs that their assumptions and model choices are adequate. Limit the work for the reviewers: By introducing guidelines for reporting NL-FEA results Guidelines for Nonlinear Finite Element Analysis 6 16

4 Guidelines available from literature? fib Practitioners guide to finite element modelling of reinforced concrete structures (June 2008) aci jci Guidelines for Nonlinear Finite Element Analysis 7 16 Approach (1 out of 3) Version 1 of the Guidelines based on: scientific research, general consensus among peers, long-term experience with nonlinear analysis of concrete structures workability of the options availability of the options Guidelines for Nonlinear Finite Element Analysis 8 16

5 Approach (2 out of 3) Version 1 of the Guidelines based on: making (sometimes arbitrary) decisions. Example: smeared crack models Rotating crack models Fixed crack models Partly fixed crack models Micro-plane crack models Isotropic damage models Guidelines for Nonlinear Finite Element Analysis 9 16 Guidelines for Non-linear FEA of Concrete Structures (version 1) Guidelines for Nonlinear Finite Element Analysis 10 16

6 Approach (3 out of 3) Benchmarking the Guidelines based on case studies: Reinforced Beams (3x) Prestressed Beams (3x) Slabs (3x) Guidelines for Nonlinear Finite Element Analysis Benchmarking Load P (KN) Experimental NLFEA step Deflection (mm) Guidelines for Nonlinear Finite Element Analysis 12 16

7 Benchmarking: comparing & explaining Ultimate shear loads in FE analyses for average material strength and fine mesh. FE Code Element size [mm] Bond model Rotating crack model Ultimate shear load in [kn] Partly fixed crack model Fixed crack model DIANA 25 Perfect 86.8 NA 1) ATENA 30 Perfect ) (83) 4) ATENA 30 UG/UP 2) ) (90) 96.4 (87) 97.5 (95) 1) NA-not available 2) UG/UP unconfined good (bottom bars) and unconfined poor (top bars) bond condition 3) Reinforcement is yielding 4) Value between brackets reflects the full development of a flexural shear crack Guidelines for Nonlinear Finite Element Analysis Benchmarking: comparing & explaining Displacement control Load P [kn] Displacement control Force control Deflection at right loading point [mm] 3 Force control Belletti, Hendriks, Den Uijl, Damoni, 2010, fib, Washington DC Guidelines for Nonlinear Finite Element Analysis 14 16

8 Preliminary conclusions Guidelines indeed limit the user factor and model factor in the results of NL-FEA Guidelines do not replace the need for advanced FE modeling skills and proper interpretation of results Guidelines for Nonlinear Finite Element Analysis Missing No Sound safety format Currently (pragmatically): Based on characteristic values of material parameters benchmarking against case studies Guidelines for Nonlinear Finite Element Analysis 16 16