The steel members without continuous lateral support to the compression flange and subjected to uniaxial bending may produce erroneous results while designed as per the CSA S16-14 code. When such a member is designed for a LOAD LIST or a LOAD ENVELOPE, containing multiple load cases, the quantity ω2 defined in clause 13.6(a)(ii) of the code, is evaluated incorrectly which in turn leads to incorrect Moment of Resistance calculation.
As per clause 13.6(a)(ii) of CSA S16-14 specification, evaluation of the quantity ω2 depends on the maximum factored bending moment (Mmax) and factored bending moments at the quarter point (Ma), mid-point (Mb) and three-quarter point (Mc) of the unbraced segment of a member, as shown below.
The maximum moment value (Mmax) was incorrectly assigned/set by the program when used in a LOAD LIST or a LOAD ENVELOPE command. When designing the member in such a situation, instead of setting the maximum moment value per load case, the program erroneously determined the maximum moment considering all the load cases in the associated LOAD LIST or LOAD ENVELOPE.
As the factored moment capacity (Mr) is directly proportional to the Critical Elastic Moment (Mu) and Mu is directly proportional to ω2, the above-mentioned error resulted in overestimation of the value of Mr and produced an unconservative result.
Workaround
If the member is designed for a single load case, the program would calculate the correct value of ω2 for that load case. So for a LOAD LIST or a LOAD ENVELOPE consisting of a single load case, program would generate correct answer.
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Solution
The defect has been addressed in the current commercial release STAAD.Pro CONNECT Edition V22 Update 4 (Version 22.04.00.40). This will also be included in the next QA&R delivery.