Question: Why do double angle sections fails in slenderness ratio when using the British Steel Code (BS 5950).
Answer:
Many a times it is seen that double angle sections fail in slenderness as per BS 5950 but passes when some other codes are used. The reason is explained as below:
Clause 4.7.8(g) of BS: 5950 states that: “The slenderness lambda_C of the main components (based on their minimum radius of gyration) between consecutive point where the lacing is attached should not exceed 50. If the overall slenderness of the member is less than 1.4 lambda_C the design should be based on a slenderness of 1.4 lambda_C . “
4.7.9(c) of BS:5950 states that : “The slenderness lambda_C of the main components (based on their minimum radius of gyration) between end welds or end bolts of adjacent battens should not exceed 50……”
Hence as per the above mentioned clauses , for double angle members, the slenderness lambda_C, which is the slenderness about the minor principal axis of the component single angle member, is checked against a limiting value of50 . Lambda_C is the value of Lvv/rv, where v-v is the minor axis.
While code checking ,the geometric check is performed first, that is , the program first checks for the slenderness ratio of all the members . If this check passes, then the program goes for the checking the entire load cases against the relevant codal provision . If the slenderness ratio exceeds the limit specified by the code, then the member is reported as “failed” by slenderness and no further code checking is done .
For double angle sections, the slenderness check is performed against the limiting value of 50 (as mentioned in 4.7.8(g) and 4.7.9(c))
The slenderness value is calculated as : Lvv/ rv.
Where Lvv is the member length and rv is the minimum radius of gyration.
For your detailed design report , kindly use the TRACK 2 design parameter . I have attached an example screenshot of the Staad Output file to demonstrate the calculation of slenderness ratio .
If you need to bypass the slenderness check, you have to specify a suitable Lvv parameter to restrict lambda_C within the specified limits. It is very important that you specify an appropriate “Lvv” and "Leg" parameter for a correct slenderness calculation to be done. Kindly go through article 2B.6 for details of these parameters.