The correct answer is: C. Rankine Grashoff formula.
The Rankine Grashoff formula is used to design a two way slab simply supported on edges and having no provision to prevent the corners from lifting. It is based on the assumption that the slab is a thin plate and that the loads are applied uniformly over the entire surface. The formula takes into account the following factors:
- The thickness of the slab
- The width of the slab
- The length of the slab
- The modulus of elasticity of the concrete
- The yield stress of the steel reinforcement
- The live load on the slab
- The dead load on the slab
The formula is given by the following equation:
$M = \frac{w l^2}{12} d$
where:
- $M$ is the bending moment at the center of the slab
- $w$ is the live load on the slab
- $l$ is the length of the slab
- $d$ is the effective depth of the slab
The effective depth of the slab is defined as the distance from the top of the slab to the centroid of the steel reinforcement.
The Rankine Grashoff formula is a conservative formula, which means that it will always overestimate the bending moment in the slab. This is because it does not take into account the effects of shear and torsion. However, it is a simple and easy-to-use formula that can be used to design a safe and economical slab.
The other options are not correct because they are not used to design a two way slab simply supported on edges and having no provision to prevent the corners from lifting.