Why the compression capacity of a single angle L90X8 (32 kN) is almost the same as of a double angle 2L90X80 (33.7 kN) when the design using EN 1993-1-1:2005 code is performed?Or
Why is the slenderness ratio reporting a much higher value than expected while using EN 1993-1-1:2005?
As stated in the STAAD.Pro Help section Design > D. Design Codes > D5. European Codes > D5.C. European Codes - Steel Design to Eurocode 3 [EN 1993-1-1:2005] > D5.C.5 Member Design > D5.C.5.1 Members Subject to Axial Loads:
"EN 1993-1-1:2005 does not specifically deal with single angle, double angles, double channels, or Tee sections and does not provide a method to evaluate the slenderness of such members. In these cases, the EC3 (EN 1993) 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 and Table 25 of BS 5950-1:2000 are used in the current version of the Eurocode 3 design module."
As we know, while calculating the compression capacity of a member, slenderness is one of the components having an impact to it (cl. 6.3.1 of EN 1993-1-1:2005). As EC3 does not explain how to calculate slenderness of single or double angles, the approach used in BS 5950-1:2000, cl. 4.7.10 had been implemented in STAAD.Pro. Table 25 from that clause is used to determine the slenderness of such members.
In STAAD.Pro, the design parameter LEG is used to select the specific case as per the Table 25 of BS 5950-1:2000. If no LEG parameter is assigned, the LEG 0 design parameter is used in the program by default, which means:
a) cl. 18.104.22.168(b) will be used for a single angle;b) cl. 22.214.171.124(d) will be used for a double angle.
As can be seen in the cl. 126.96.36.199(d), the calculation of slenderness for the double angles is a bit more complex than simple KL/r ratio:
In this case, the limiting slenderness is 1.4 x Lambda_C, where Lambda_C is the slenderness about the principal v-v axis of a single angle used to form that double angle section (NOTE 5 in the Table 25). This limit leads to a large slenderness of the double angle, because the slenderness of the single angle, which compounds that double angle, is considered. At the end, this leads to a final slenderness of the double angle quite similar to the slenderness of single angle, so the compression capacities are similar as well.
One can reduce this slenderness ratio, by using the LVV design parameter with some value smaller than the default one (which is equal to the actual length of the analytical mamber). Lvv is measured between interconnecting bolts for back-to-back struts or between end welds or end bolts of adjacent battens for battened angle struts.
As an example, setting the LVV design parameter to a value of 0.25m would reduce the slenderness significantly and the compression capacity of the double angle will be increased accordingly.
To answer the question in short, you are getting a small capacity of double angle, because the program finds a large slenderness of its component single angle as per cl. 188.8.131.52(d) in the Table 25 of BS 5950-1:2000. You can reduce this slenderness by using a small value for the design parameter LVV. This will reduce the slenderness of the double angle increase the compression capacity of it as expected.