IStructE Exam question

[Bridgeengineer2025]

I am currently practicing for my IStructE exam but I am struggling with the past paper (July 2024) and was wondering if any experienced bridge engineers could point me in the right direction.

The brief asks for a bridge with two decks staggered at different heights https://www.istructe.org/resources/exam-preparation/chartered-membership-exam-july-2024-past-paper/

My question: Is it possible to cantilever an RSJ beam from a main girder as per my attached diagram? Is this something that would work? (The RSJ is bolted into the web of the plate girder) I have concerns that there is no back span to the cantilever and it may be difficult to keep deflections within acceptable limits. The width and loading on both decks is 2.5 metres and 5 kN/m2 respectively. Any advice would be much appreciated.

 

[jhnblgr]

You’d have to design a flush plate moment connection. Then check the connecting member it torsion. The lower bracing would need to be modified to something more rigid.

 

[Bridgeengineer2025]

Thank you for your response that’s very helpful. Yes I agree that more torsional rigidity needs to be provided. I have definitely seen decks cantilevered this way before but the span was much short only about 0.5m or so but cantilevered from the top flange