Stiffness attract Forces.

Forces here means Reactions. Forces just sounds way cooler. The reactions are the responses by the structure to a load. It can be Axial Forces, Shear, Moment and Torsion. 

Now what is Stiffness? It is the ability of a member to resist deformation under bending. It highly depends on the Moment of Inertia around the axis of Bending. Please be reminded that it's the Area Moment of Inertia and not Mass Moment.

For a Solid Rectangular Cross Section, Moment of Inertia = b*h^3/12.




Onto the deliveries, below is an ETABS model of the same plan with two different thickness profile. 

A. Raft of equal thickness.
 

B. Raft of different thickness.
B has a raft of 400 mm with a thickening of 1500 mm near the edges. This thickened portion will support the wall from above. 

Now we analyze the model for the same loads and loading combination over the same process. I have taken the M22 case for comparison, but any other reaction would have been fine.

A.

B.


As we can see, the reactions are more prominent at the edges where there is a thickening. The max M22 values remain almost similar: 973.383 kN-m/m for A and 972.986 kN-m/m for B. 

The higher stiffness area attracts the forces.




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