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Consider Upper Columns in Slab Design

Consider Upper Columns in Slab Design

Consider Upper Columns in Slab Design

I saw some older discussion in thread803-403840: COLUMN TOP RESTRAIN - CsI SAFE of modeling the columns above, for slab design. I'm interested to hear other opinions about this for two-way flat plate design: My understanding has been that since the forms are released to allow the slab to carry its self-weight before the next floor is completed, the slab will carry that self-weight load (which may be around 75% of the total load in some cases) while only the columns below are in place. Only after the floor above is completed, then the columns above will contribute their stiffness in resisting the live load. It seems that you either need to assume the columns above are there for all the load (which overestimates the column stiffness for the self-weight, which is a majority of the total load), or design for all load with only the columns below (which underestimates the column stiffness for the live load).
If anyone can provide some insight for the justification of one assumption or the other, I would appreciate your thoughts.

RE: Consider Upper Columns in Slab Design

Traditional flat plate slab design (check any book) for example equivalent frame method , or ACI 318 Handbook. Always, accounts for columns above and below, the rotational stiffness of the node is different, which can affect punching shear checks. This makes sense, since usually form work are striped after in some case 2 or 3 floors above are poured so effectively the column above will be there once the forms of that floor are removed.



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