Building vertical braced bay eccentric to wind load

[novembertango88]

Hi all,
Please can you help?

I'm working thru a critiqued past paper of the IStructE exam, see attached. (ref https://www.istructe.org/resources/exam-preparation/critiqued-exam-answer-script-february-2023/)
pg 26 shows the calculation of the bracing loads.

The proposed solution for lateral support to the building is by a core of 3No braced bays in each direction (this is to allow for lift/stair access to the core).
The candidate finds the load on each braced bay proportionally but the examiner comments that eccentricity has not been accounted for.

I have tried to account for this by using a beam analogy but am finding that all of the wind load is applied to the central bay.

My working is attached.

Thanks

 

[human909]

Your calculation is correct and is evident by inspection given there is equal load on each side of A

However your beam analogy approach is too simplistic to give a final answer. Neither A nor B are fully rigid. So all the load on A is also incorrect. Deflection of A will cause some of the wind load to come onto B. The result will be torsional in plan view as the bracing in plan is eccentric to the wind load.

I can't really help your further as I don't know what the examination is really after. It probably is just after a recognition that the braced bay is eccentric.

 

[Smoulder]

Statically determinate structures theoretically unaffected by support stiffness or settlement. What beam analysis misses is flexibility of beam. If all reaction is at A then beam has to be infinitely stiff not to load B.

Maybe you've ignored floor diaphragm in your analysis? Core will move laterally and rotate due to eccentricity of load from centre of stiffness. That causes stress in transverse bracing. When braces contribute to both X and Z direction resistance then you need to check 45 degree case. That was the problem with that New York building that had to be retrofitted in secret.