Moment Transfer Between Flat Slab and Columns when Columns Resist Lateral Loads

[2kwiggle]

We've followed a pretty standard method at designing flat slab building columns - Pin all columns and the cores take all the lateral loads. Unbalanced moments are only considered for punching shear design.

I have a different case - a three story building with no cores. The columns are to be designed to resist lateral loads.

FEM analysis with all columns fixed creates moments in the columns as predicted. However, there are two contributors to the moments:

1. Lateral loading (this is expected)
2. Vertical loading - It seems like the dead and live loads create moments in the columns. I expected some moments but the moments transferred to the columns under vertical loading cases are much higher than the moments due to lateral loading. 30kNm from lateral loading and 150kNm from dead and live loads.

My question is - we usually ignore the moment from dead and live loads when we have a lateral restraining core. Can I do the same in this case and only design the columns for the moment from the lateral loads?

Note: This is a non seismic zone.

 

[slickdeals]

No! You should have small moments with equal spans on interior columns. At edge and corner columns, the moments should be considered. Pinning columns in analysis is a poor choice and should never be done for the gravity design. It's okay to pin columns when designing the overall lateral system, provided the columns are spaced far apart and slab-column frame action is minimal.

 

[atrizzy]

Adding to slickdeals' comment, often it is wise to use pinned column analysis for consideration of the slab design. You'll frequently (though not always) get larger deflections and positive moments in a slab when the columns are pinned. I typically design slabs for an envelope of pinned/fixed conditions. Column design and punching shear design should certainly be done with a fixed column analysis.

 

[GC_Hopi]

If you assume pin that means you are allowing pins to occur, elastic deformation when the high gravity load combination happen in your case. Then the question is with these pins how will your structure react to lateral loads, based on your description it will not be stable. You should design the structure to be stable when subjected to lateral loads and under the high gravity load combinations per design code so envelope the design as mentioned above.

 

[WARose]

My question is - we usually ignore the moment from dead and live loads when we have a lateral restraining core. Can I do the same in this case and only design the columns for the moment from the lateral loads?

I don't see how anyone could "ignore" moments in a two-way slab (be it from gravity loads or otherwise). One of the checks for such a slab is a pattern load case which will just about always result in some moment getting dumped into the column.

 

[hokie66]

Issy,

I worry about folks who use FEM analysis for design, accepting the results without really understanding how the structure performs. What struck me in your post was your statement that the program gave you moments in kN. kN is a unit of force, not moment.

 

[SteynvW]

Quick question, if your slab is not really deep <=250mm and the column rebar is just passing through the joint. How much moment can really be transferred to the column?