We investigated the issue and found that deflection check as per AISC 360, IS 800, S16-14 codes are always reporting last load case in the serviceability envelope as critical load case for deflection check. The design however is correct and reports the correct critical ratio. It is just the reporting of the critical load case which is incorrect and that too when deflection governs. So for any design where deflection is not the governing criteria, this reporting error does not have any effect. This error is found to exist in version 20.07.11.70 and will affect versions 20.07.11.50 and 20.07.11.33 too. It will be addressed in STAAD.Pro SS6 SP3 version scheduled for release in Q2 of 2016. The issue is now reported with id# 438681 at
communities.bentley.com/.../22866.known-issues-in-staad-pro-ss6
communities.bentley.com/.../22866.known-issues-in-staad-pro-ss6