The maximum utilization ratio displayed in the BS5950:2000 code check doesn’t match the maximum utilization ratio stated in the ‘Critical loads for each clause check’ table at the bottom.
For members with axial compression and moment, two principal interaction formulae must be satisfied – Cross Section Capacity check (4.8.3.2) and the Member Buckling Resistance check (4.8.3.3). Three types of approach for the member buckling resistance check have been outlined in BS5950:2000 - the simplified approach (4.8.3.3.1), the more exact approach (4.8.3.3.2) and Annex I.1 for stocky members (note that the Annex I.1 check will be done only if the sections are plastic or compact). These checks are controlled by the design parameter ESTIFF.
If You would look to the chapter British Codes - Steel Design Per BS5950:2000 in the International Design Codes manual, You will find the exact description of the ESTIFF parameter. Here is written:
0.0 = Fail ratio uses MIN of 4.8.3.3.1, 4.8.3.3.2. and Annex I1 checks.
1.0 = Fail ratio uses MAX of 4.8.3.3.1, 4.8.3.3.2. and Annex I1 checks.
STAAD.Pro by default uses value 0.0 and this means, that only the minimum ratio of those 3 clauses will be used.
In example, let’s say that the minimum ratio from all those 3 checks for a member is from Annex I1. As the ESTIFF parameter is 0.0 (by default), other two ratios (for clauses 4.8.3.3.1 and 4.8.3.3.2) are being neglected. If a value of 1.0 would be assigned for the ESTIFF design parameter for the same member, the maximum value of those 3 checks would be used in finding the most critical ratio.
Sometimes the more exact approach (clause 4.8.3.3.2) may be more conservative than the simplified approach (clause 4.8.3.3.1). It has been found, however, that this is not always the case and STAAD.Pro therefore performs both checks, comparing the results in order that the more appropriate criteria can be used. So it is up to the user to decide if he wants to use a more conservative approach or not by using ESTIFF parameter.
Answer Verified By: Modestas