Loading on Transfer Slab

[aenger]

Hello Everyone, I have a 6 storey apartment that has a transfer Slab on the first floor.
The Transfer Slab is post tension concrete slab. So to work out the point load/line load that need to be transfer on this level, what do you guys normally do? I build the whole model including the transfer slab in Etabs and then read the pier/frame axial force to get the loading that apply on the transfer slab.

However, Some people would use traditional way to work out the loading, i.e. Calculate the structure above the transfer slab separately (either by hand calculation or establish to model of the structure above the transfer floor (second floor to roof) using 3D RC software like Etabs) and then get the reaction of each column and wall and apply to the Transfer Slab. These two different way will lead to two very different results. I tend to build the whole structure as calculating the structure above separately will ignore the stiffness of the structure below. The supports that we assume in method two are actually elastic springs. And the loading distribution will be affected by the structure layout (stiffness condition below).

So what do you guys normally do? Do I need to envelope these two different method (considering the results differ greatly)?

Thank you.

 

[SteynvW]

It is briefly discussed in this thread.

https://www.eng-tips.com/viewthread.cfm?qid=449204

 

[rapt]

For a start Etabs will assume the transfer slab deflects elastically with the full structure applied t the same time.

None of this is correct.

The slab will not be elastic.

The PT will affect deflections.

An envelope would be best.

 

[aenger]

Thanks SteynvW. Sorry I didn't know it was discussed. Just had a look in that thread.

Thanks for your reply Rapt. I do agree the slab will not be elastic and the PT will affect the deflection. But using tributary area method is even worse as we treat the slab as infinite stiff by doing that. It will give a way too conservative result for transferred columns/walls and unsafe results for other columns/walls. The slab is gonna work as spring instead of support that has no displacement. When taking the long term creep, shrinkage and cracking of the slab into considertaion, the stiffness of the spring will be reduced, which means the axial force in transferred columns/walls will be actually further reduced and other supports increased. So I think using traditional tributary area method and static analysis will not envelope the loading on transfer floor if you wants to take long term creep and cracking into consideration. The construction sequence and the PT effect will do increase the transferred columns/walls loading but I tend to incread the slab stiffness by say 2-5 times instead of using tributary area method. Actually I haven't seen people using hand calculation like this for high rise projects in Asia. It will be very tedious and you can hardly work out the correct tributary area for each support even if you ignore the stiffness of the transfer slab issue.