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What are the basic differences between ACI 318 and Eurocode 2?

Structural codes were created to guide the design process to ensure that the structure is safe under most conditions. ACI 318 is an American norm and EC2 is a European norm. As you can imagine, the epistemological approach is very different for historic reasons. I have used both reinforced concrete codes. ACI is basically an empirical code with traditional formulas that resemble rules of thumb. Eurocode 2 is based on a sound underlying theory of concrete mechanics (see Fédération Internationale du Béton and the Model Code 90) that then is adjusted to experimental results by means of coefficients. ACI is a pleasant piece to read, truth be said while teading the EC2 is like reading any bureaucratic EU law or any arid materials science paper.

For me the main resulting diferences are:

  • How the safety factors are applied. EC2 applies constant safety factors to the materials. Safety factors in ACI are not applied directly to the materials but to the reinforcement situation (tension, compression, shear or flexure). That is why interaction diagram in Europe is more like an oval but the American interaction diagram looks like a squid.

ACI interaction diagram

EC2 interaction diagram
  • Punching and shear approaches are very different in ACI 318 and in EC2. Both neglect variables that may be important for some beams and slabs, and both have scatter of results when compared to empirical tests. Results may be unconservative or overconservative in some cases, so I recommend to use and compare both codes in case of doubt.
  • Cracking estimation does not exist in ACI 318. Ok, there is a rule of thumb for the separation of reinforcement bars for 0.41 mm average crack. Calculation of crack widths under quasi-permanent loads is mandatory under EC2. This shows a more modern approach.
  • In my opinion, ACI underestimates the influence of creeping.
  • Minimum reinfocement of columns according to ACI is 1%, everywhere and in any case. Minimum reinfocement of columns in non-seismic areas according to EC2 is just 0.4%.

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