prestressed/ precast lift slab capacity

[maechh]

I am trying to determine the capacity of an existing (1968) precast/prestressed lift slab (24'x24' column spacing). It seems the original engineer did not include some significant equipment loads and my client would like to add some heavier equipment. If there is a simplified method or software out there please let me know how I can acquire it. Thank you!!

 

[ishvaaag]

The warning by other answering this post in other channel about punching shear is the most serious one for lift slab. Lift slab -and steel columns, even with collars- are insets in the floors, and very sensitive to vertical dynamic shakeout. That the static capacity may be astounding as many tests may show does not diminish what above stated. Upon shakeout the connections are to suffer severily. This a child that has pierced a paper with a stick very well knows. It is like the failed joint in the sunk floating offshore norse structure, if the failure is what Collins and Mitchell indicated, heck, to seam parts 1,5 m thick one cannot use 20 cm insets in each side, whatever the calculations may say about the length of necessary embedment.

 

[Ingenuity]

maechh,

I hope that you have access to the original structural drawings.

If the slab was post-tensioned then you can use any current P-T software. I use RAPT for 2D analysis and SCS-FLOOR for more complex situations where FEA is required. I would think that a 2D analysis based upon equivalent frame would be sufficient and adequately able to model the structure.

BUT you will need to review the original details of the column/floor connection. The shearhead collar connections were often proprietry, with channel, angles and steel adjustment wedges welded permanently after the floor slab elevation was set.

You will need to check ACI 318 (if the project is in the US) where the code required a minimum of 2 tendons in each direction through the critical shear section over the columns.

I am sure that you are aware that several LIFT SLAB projects collaped during lifting, in both the US and UK.

Also, consider that for a P-T floor system built in 1968 with unbonded (?) P-T, depending on the exposure and environment, corrosion may be present in the tendons with possible strand breakages. You may need to do a bit of investigation to see the "real" condition of the structure.

HTH