Direct esign Method for Concrete Flat Plate Slab

[ajk1]

I am checking a flat plate slab designed in the mid-1960's to see if 25 psf more dead load can be added in one bay of a multispan slab. I believe that it was designed by working strength design (WSD),using the empirical method (the empirical method has evolved into the direct design method in current CSA (and ACI) code, but with a number if changes from the old empirical method. The drawings do not state the live load for which it was designed, but I suspect it may have been for 100 psf, although at the time the code required it to be designed only for 60 psf because of the fixed seats. I suspect 100 psf was used though because the same reinforcement and spans applied in the adjacent social hall area which did not have fixed seats and which the code (then and now) would have required a 100 psf live load. The design is unusual because it uses steel columns and steel bracket at the top of the column, to support the flat plate slab. Similar I suppose to lift slab construction but definitely not a lift slab. The column does not extend above the floor.

Question:[/b] Was it justified to use the empirical method (or current direct design method) when the slab is supported on steel columns with no dowels tying the column to the slab and no extension of the column through the floor? That method required a minimum column inertia, but if the column is not doweled to the floor with bending moment bars the way a concrete column would be, and does not extend thru the floor, then the floor rotation is not imposed on the column and the minimum column inertia would have no relevance. My opinion is that it was not correct to use the old empirical method, or the current direct design method to now check it for added load, in a case like this. What do you think?

 

[KootK]

I don't know about the older method but I think that one could argue that DDM still applies. Interior columns only really participate in moment resistance when there's unbalanced moment coming in from the slabs. And the stated limitations of the method should keep that in check. At exterior columns, there is no reliance on slab-column moment transfer if one uses the unrestrained edge option.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ajk1]

to Kootk - you are right on! I was at home yesterday looking at the old empirical method, and did not have the current Code, but today I am in the office and looked at the latest code and there are differences in the DDM from the old empirical, one of which is the DDM does not seem to require a minimum column inertia. I ran it as a frame using our in-house program and I also calculated the DDM moment, and the results were quite similar, so the whole issue is resolved. Thanks for your comment. Very helpful, as usual.