EC3 DD design in STAAD.Pro considers members that are primarily in bending and/or shear as beams and performs cross section and member capacity checks in accordance with the code. The main requirement for a beam is to have sufficient cross-section resistance to the applied bending moment and shear force. The possibility of lateral-torsional buckling is also taken into consideration when the full length of the member has not been laterally restrained.

The bending capacity is primarily a function of the section type and the material yield strength and is determined according to Cl. 5.4.5 of the code. The shear capacity and the corresponding shear checks are done as per section 5.4.6 of the code.

There are four classes of cross-sections defined in EC3. Class 1 and 2 sections can both attain full capacity with the exception that the class 2 sections cannot sustain sufficient rotation required for plastic analysis of the model. Hence the full plastic section modulus is used in the design calculations. Class 3 sections, due to local buckling, cannot develop plastic moment capacity and the yield stress is limited to the extreme compression fibre of the section. The elastic section modulus is used to determine the moment capacity for class 3 sections. Class 4 sections do suffer from local buckling and explicit allowance must be made for the reduction in section properties before the moment capacity can be determined. Further, because of interaction between shear force and bending moment, the moment resistance of the cross-section may be reduced. This, however, does not occur unless the value of applied shear forces exceeds 50% of the plastic shear capacity of the section. In such cases the web is assumed to resist the applied shear force as well as contributing towards the moment resistance of the cross-section.

As mentioned in the previous section, the design of class 4 sections is limited to WIDE FLANGE, TEE, SINGLE CHANNEL, SINGLE ANGLE, and RECTANGULAR HOLLOW SECTIONS. The effective section properties are worked out as described in Cl. 5.3.5 of the code.

Beams are also checked for lateral-torsional buckling according to section 5.5.2 of the code. The buckling capacity is dependent on the section type as well as the unrestrained length, restraint conditions and type of applied loading. The lateral torsional buckling checks involves the calculation of the "Elastic critical moment", Mcr, which is calculated in STAAD as per the method given in Annex F of the code.

In the presence of a shear force, beams are also checked for shear as per section 5.4.6 of the code. In cases where the members are subject to combined bending and shear, the combined bending and shear checks are done in STAAD as per clause 5.4.7 of the code.