Expert: Jeanette A - 12/7/2008

Question
Dear Madam,
Would be very grateful if you could answer this (more theoretical) question: why do we have to apply 100 of load in both direction in design of flat slabs (two-way column-supported slabs)?
I have read some works regarding that subject but, still have some doubts.

Thank you for your time and consideration,
Oliver


Answer
Hello Oliver,

Apologies for my late response, was completing final exams. To answer your question think of it like this;

The middle strips in one direction carry the load to the column strips in the other direction.  These column strips then carry the load to the columns. If you use either the DD or EF Method, you use ACI 13.6 to divide the moment into a column strip portion and a middle strip portion.  And this explains why these slabs are designed this way. The column strip takes more of the moment (75% of the negative moment and 60% of the positive moment at interior spans without beams) because it is picking up load from the slab spanning in the perpendicular direction.  The middle strip takes less of the moment, because it is taking its load to the column strip in the perpendicular direction.

I hope that helps in some way.

Warm Regards,
Jeanette