What should be the column spacing to have rigid foundation assumption?

In this post we will discuss about continuous foundation as it is concerned with column spacing in a continuous foundation. In the very before post we have discussed about relative stiffness and its influence on foundations to be assumed as either rigid or flexible.For convenience to understand this post we recall the Meyerhof formula to determine relative stiffness

Where

The explanation of each parameter of the equations are described in previous post “How to Determine A Foundation Whether to be Designed as Flexible or Rigid Body?” and not described as post becomes very long.

In this equation flexural stiffness of each member including columns and shear walls are considered to derive relative stiffness. The k_r indicates the rigidity or flexibility assumptions and a table is provided in that post. In this post we will try to make relation between column spacing and rigidity consideration.

The variation of soil pressure distributed below footing is dependent on the column spacing in a continuous footing. The conditions have to satisfy to derive soil pressure distribution from statics i.e. rigid foundation
assumption is summarized below:a. In a continuous strip average load of two adjacent spans does not vary greater than 20% of greater value but less than the value 1.75/λ.   b. In a continuous column spacing of two adjacent spans does not vary greater than 20% of greater value but less than the value 1.75/λ.
 

Now we have no idea about λ. What is λ?

λ is a factor that is used to evaluate footing stiffness. There have an expression for λ:

k_s =modulus of subgrade reaction in vertical direction.