D5.B.5.3 Design of members with combined axial load and bending
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 EC3 DD design module in STAAD takes such a scenario into account and performs the necessary checks as per Cl. 5.4.8 of the code. Class 1 and class 2 sections are checked as per cl. 22.214.171.124 and Class 3 and Class 4 sections are checked as per clauses 126.96.36.199 and 188.8.131.52 respectively. The effective section properties for class 4 sections are worked out as given in Cl. 5.3.5 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. 5.5.3 of the code. In case of a combined axial compressive load and bending moment, the member will be checked as per the rules in section 5.5.4 of the code.
The presence of large shear force can also reduce the bending resistance of the section under consideration. If the shear load is large enough to cause a reduction in bending resistance, then the reduction due to shear has to be taken into account before calculating the effect of the axial load on the bending resistance of the section. If the member is subject to a combined shear, axial load and bending moment then the section capacity checks will be done as per Cl. 5.4.9 of the code.
As stated in the previous section, DD ENV 1993-1-1:1992 does not specifically deal with single angle, double angles, double channels or Tee sections and does give a method to work out the slenderness of such members. In these cases, the EC3 DD 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 DD design module. Please refer to the note in section 5B.5.2 for St and RA angle specifications.
Please note that laced or battened compression members are not dealt within the current version of EC3 DD design module in STAAD.Pro.