Finite element analysis (FEA), especially nonlinear FEA, has become a trusted method in both research and practice, forming the basis for many of the design equations and code provisions used today. Questions often arise about whether IDEA StatiCa Detail complies with ACI 318 for the design of D-regions and anchorage zones — and the answer is: yes, it does.
US codes and standards, including ACI, AISC, and others include clauses that allow the use of nonlinear finite element analysis for structural elements and connections that are not covered by the codes, which are typically complex. If the codes allow the use of this method for complex problems, then it can solve even simple problems that have assumptions that simplify the solution. Research showed limit states of members and connections design can be captured by finite element analysis.
Additionally, nonlinear FE analysis is explicitly permitted in ACI 318 (Ch. 6.8 and 6.9).
- STM is allowed but not exclusive (Ch. 23).
- The stress-field / MCFT (Modified Compression-Field Theory) framework underlying CSFM is recognized by ACI 445R.
- Computer-based tools for D-regions are explicitly acknowledged (ACI PRC-445.2-21).
IDEA StatiCa Detail uses the Compatible Stress Field Method (CSFM), which is a nonlinear stress-field approach explicitly aligned with the design principles recognized in ACI 318, the ACI 318 Commentary, and ACI 445R. ACI also allows nonlinear FE analysis as a valid alternative to Strut-and-Tie, as long as strength checks and detailing rules are satisfied.
FEA and Nonlinear FEA References and Validations
Here are some of the references and validations concerning FEA and nonlinear FEA listed in various sections of the ACI guidelines:
- ACI 318-19, Sec. 6.8 “Inelastic analysis” + Sec. 6.9 “Acceptability of finite element analysis”
The Code explicitly permits inelastic (nonlinear) analysis and finite element analysis for determining internal actions, provided the analysis procedure is validated and the model is appropriate for the intended use (6.8.1.2, 6.9.2–6.9.4). These provisions apply to all regions, including D-regions. Therefore, a nonlinear model such as CSFM/NLFEA is permitted to determine design forces in discontinuity regions. - ACI 445R-99, Sec. 2.2–2.6 and Sec. 4.4.6 “Modified Compression Field Theory”
ACI 445R presents MCFT and stress-field theory as valid design bases and notes that MCFT can be implemented from simplified hand methods up to fully nonlinear FE models. Since CSFM is a stress-field / MCFT-based nonlinear formulation, this provides direct theoretical recognition of CSFM-type nonlinear analysis as a legitimate alternative to STM. - ACI 445R-99, Chapter 6 “Design with Strut-and-Tie Models”
STM is presented as one member of the broader family of stress-field / plasticity-based models. Therefore, a more refined stress-field method implemented in nonlinear FE (CSFM) is inherently consistent with the foundational theory behind STM, and can safely replace STM when properly validated.
Summary
Structural engineers and connection designers can use a set of tools to get their job done - to safely, accurately, and quickly design steel connections as per ACI. IDEA StatiCa, with its unique and tested FEA solution, is part of this toolkit. Read more about the method in our Theoretical background and extensive set of verifications.