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D5.C.5.5 Members Subject to Combined Forces

Members subject to Bending and Axial Force

When a member is subject to a combined axial load and a bending moment, the program evaluates a reduced moment capacity based on Cl. 6.2.9 of the code. For Class 1, 2, and 3 sections, the program evaluates the reduced moment from the equations given in Cl. of the code. For class 4 sections, the interaction equation given by equation 6.44 are checked.

In the case of members subject to axial load and biaxial bending, the program will consider the interaction equation 6.41 of the code.

Note: By default, the program will use the values of the constants "α" and ‘β’ as given in the code for the different sections types. However, you can override these values using the ALPHA and BETA design parameters (Refer to D5.C.6 Design Parameters).
Note: The program uses the parameter ELB (Refer to D5.C.6 Design Parameters) to override the Cl.6.2.9 checks for combined axial load and bending case. When specfied as 1, the program uses the more general equation 6.2 of EN 1993-1-1, instead.

Members subject to Bending, Shear, and Axial Force

When a member is subject to a combined axial load, shear force, and a bending moment, the program evaluates the reduced yield strength as given in Cl 6.2.10 (3) of the code. The reduction in the yield strength is done only when the applied shear force exceeds 50% of the design shear resistance Vpl,Rd. This reduced yield strength is then used to evaluate the reduced moment capacity of the section.

Members subject to Bending and Axial Compression

The bending resistance of members could be reduced by the presence of a co-existent axial load. This is then checked against the lateral-torsional buckling resistance of the section. The EN 1993 design module in STAAD takes such a scenario into account and performs the necessary checks as per Cl. 6.3.3 of the code.

Generally, EC3 requires checking cross-section resistance for local capacity and also checking the overall buckling capacity of the member. In the case of members subject to axial tension and bending, there is provision to take the stabilizing effect of the tension load into consideration. This is achieved by modifying the extreme compression fibre stress and calculating an effective applied moment for the section. The checks are done as per Cl. 6.2.9 of the code. In case of a combined axial compressive load and bending moment, the member is checked per the rules in section 6.3.3 of the code. The program checks to ensure that both the interaction equations 6.61 and 6.62 of the code are satisfied. The interaction factors kzz, kyy, kzy & kyz will be evaluated using Annex B of EN 1993-1-1 by default. Hence for the EN 1993-1-1 code in STAAD.Pro (without National Annexes), uses Annex B. The choice between using Annex A and Annex B will be based on the choice specified by a particular National Annex, if used. If the National Annex itself gives a choice between Annex A and Annex B, the program uses Annex B to evaluate the interaction factors.

Note: EN 1993-1-1:2005 does not specifically deal with single angle, double angles, double channels or Tee sections and does give a method to evaluate the slenderness of such members. In these cases, the Eurocode 3 (EN 1993-1-1) design module of STAAD.Pro uses the methods specified in BS 5950-1:2000 to calculate the slenderness of these members. Cl. 4.7.10 of BS 5950-1:2000 is used in the current version of the EC3 design module. See Single Angle Sections for ST and RA angle specifications.
Note: Laced or battened compression members are not dealt within the current version of EC3 (EN 1993) design module in STAAD.Pro.