Determination of Effective Widths According to EN 1993-1-5, Annex E

Technical Article

Eurocode 3 provides Table 5.2 for the classification of cross-section parts supported on one or two sides and various load situations. Generally, the determination of effective widths is based on the limit stresses of the structural steel used. However, in DIN 18800, the b/t limits were determined on the basis of the actual stresses in the cross-section. Thus, unfavorable limits may arise, especially for stresses below the yield strength, according to Eurocode 3.

According to EN 1993-1-1 it is possible to adapt the actual stress in a cross-section by increasing the material factor ε. This adaptation is described in Clause 5.5.2 (9). However, this adaptation is not allowed in the case of stability checks of equivalent members according to 6.3 of EN 1993-1-1. Therefore, you can only increase the material factor ε in RF-/STEEL EC3 if the stability analysis option is inactive.

Figure 01 - Dimmed Option of Increasing Material Factor ε if Stability Analysis Active

In order to still enable the design based on the actual cross-section stress, the Annex E of EN 1993-1-5 is available. According to EN 1993-1-5, the plate slenderness can be improved using Expression (4.4) on the basis of the effective compression stress, which is determined according to the second-order analysis applying global imperfections:
$${\overline{\mathrm\lambda}}_{\mathrm p,\mathrm{red}}\;=\;\sqrt{\frac{{\mathrm\sigma}_{\mathrm{com},\mathrm{Ed}}}{\displaystyle\frac{{\mathrm f}_\mathrm y}{\mathrm\gamma'_{\mathrm M0}}}}$$

After determining the reduced plate slenderness, the effective widths can be determined directly according to Appendix E. This calculation option is also available in RF-/STEEL EC3.

Figure 02 - Option of Annex E in Details - Ultimate Limit State

Example

The following example compares the design of a hollow section column regardless of the actual stresses with the design according to Annex E.

Figure 03 - System and Loading

Figure 4 shows the results of the effective widths determination based on fy.

Figure 04 - Results Based on Yield Strength

However, the design using these cross-section values is not fulfilled.

Figure 05 - Design Not Fulfilled

After activating the Annex E option, the following cross-section properties are obtained. The plate slenderness can be significantly improved and thus the effective cross-section properties are also better.

Figure 06 - Favorable Cross-Section Properties According to Annex E

The design using the cross-section values according to Annex E is fulfilled.

Reference

[1]  Eurocode 3: Design of steel structures - Part 1-5: Plated structural elements; EN 1993-1-5:2006 + AC:2009

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