Compatibility of IDEA StatiCa Detail with ACI

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. 
• ACI 318-19, Sec. 23.1.2 (Scope of STM chapter) 
  The Code states that strut-and-tie models “shall be permitted” for design of D-regions. STM is therefore an allowed method, not the only prescribed method. Combined with the general allowance for nonlinear FE analysis in Chapter 6, this means that D-regions may also be designed using validated nonlinear analysis (e.g., CSFM), provided strength and detailing requirements are satisfied. 
• ACI 318-19, Commentary R23.2.1–R23.2.2 
  The Commentary explains that STM is based on lower-bound plasticity / stress-field theory and cites Schlaich, Collins & Mitchell, and ACI 445R. The key principle is equilibrium + compatibility + controlled stress levels, not the literal hand-drawn truss. CSFM is a compatible stress-field method, therefore it is consistent with the intent of Chapter 23 and can replace STM when applied conservatively. 
• ACI 318-19, Commentary R9.9.3.1 (minimum reinforcement for deep beams) 
  The Commentary states that minimum reinforcement requirements apply regardless of the design method used. This implies the Code anticipates multiple valid analysis methods, including nonlinear FE, not only STM, as long as required minimum reinforcement and detailing are respected. 
• ACI 318-19, Sec. 1.10 “Approval of special systems” + Sec. 4.4.3 (Alternative design approaches) 
  These sections allow use of alternative design systems if adequate structural performance is demonstrated. A nonlinear design methodology such as CSFM implemented in software can therefore be justified as a “special design system” under Sec. 1.10, providing a formal route to STM substitution. 
• ACI 318-19, Commentary R6.9.2–R6.9.3 (FE modeling guidance) 
  The Commentary states that FE models covered by Sec. 6.9 include elastic and inelastic (nonlinear) FE, with various element types, provided the model is appropriate and results are verified. This is explicit support for validated nonlinear FE design of D-regions, including stress-field methods such as CSFM. 
• ACI 318-19, Sec. 17.2.1 + Commentary R17.2.1 (anchors) 
  Sec. 17.2.1 allows plastic analysis where the governing failure is ductile steel. R17.2.1 explicitly references plasticity-based nonlinear analysis for anchor group behavior. This establishes a precedent for nonlinear analysis of concrete–steel force transfer regions, which extends directly to anchorage D-regions modeled with CSFM. 
• ACI PRC-445.2-21, Sec. 5.13 “Computer-based design aids” 
  This guide recognizes computer-based tools for STM and D-region design, requiring only that they enforce equilibrium, stress limits, and detailing rules. This directly supports the use of software such as IDEA StatiCa to design D-regions using refined numerical methods, including nonlinear CSFM. 
• ACI 318-19 Commentary examples (R22.9.4, R24.2.3.3, etc.) 
  Multiple commentary sections state that “other procedures may be used if shown to produce acceptable results.” This recurring principle reinforces that more advanced nonlinear analysis methods (such as CSFM/NLFEA) are acceptable substitutes for simplified formulas or STM, provided correspondence to experimental behavior and Code performance limits is demonstrated. 
• ACI 224R-01 Control of Cracking in Concrete structures
  ACI 318-19 does not explicitly limit crack width by a numerical value. Instead it addresses it indirectly through requirements on maximum bar spacing. Guidance for direct check of limit crack widths can be found in Theoretical Background and is in accordance with ACI 224R-01 where the table of reasonable crack widths can be found in Table 4.1.

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.