gravity loads reaction in ETABS

[cenju]

Hello folks,

I have a situation here:

I was checking the foundation reaction of a 9-story concrete flat plate structure. I was only interested at the gravity loading (DL and LL)

The reaction from a column load take-down (manually calculated) appears to be (20% to 75%) less than the reaction from ETABS output. I checked this on a basic 1x1 bay 4-storey structure in ETABS and turns out the reaction is 10% more than the traditional tributatry load-take-down. It appears as if the program internally applies a load factor.

Has anyone ever encountered this situation or am i missing something? ....I used to consider myself a comfortable user of ETABS but this is kind of frustrating!!

Your response is greatly appreciated.

 

[drile007]

I think, that the answer could be in how did you mesh the floors.

Drile007

 

[johnjeed]

If you model your floors as membranes you will find that the distribution of reactions in the columns matches that if you performed the calculation manualy using tributary areas. The difference in reactions is due to the in plane stiffness of the floor when modelling using shell elements.