Slabs are subject to both in-plane and out-of-plane forces.

In-plane forces stretch and shear the slab, but do not cause it to deviate from the plane defined by the slab centroid. For horizontal slabs (like those in RAM Concept ), in-plane forces cause stretching, compressing and shearing of the centroid plane in plan view only.

Out-of-plane forces cause the slab to bend and twist, moving it perpendicular to the plane defined by the slab centroid. For horizontal slabs (like those in RAM Concept ), out-of-plane forces cause the slab to deflect vertically from the original centroid plane.

In a horizontal slab that has one continuous centroid elevation, the equilibrium equations of in-plane and out-of-plane forces are totally separate. However, if there is a shift in the centroid, the two sets of forces become interrelated due to equilibrium considerations and must be solved for simultaneously; RAM Concept handles this interrelation automatically.

For slabs that are not made of a linear-elastic material, the strains due to the in-plane and out-of-plane forces can no longer be linearly superimposed, so the equilibrium equations of the two force systems become indirectly related through their strains.

This interrelation of the two force systems’ strains for non-linear elastic materials can be seen in the simple example of a flat concrete slab that is subject to transverse loads that cause out-of-plane forces and deflections. If a uniform in-plane compression force is applied to the same slab, the slab will have less cracking, smaller out-of-plane displacements and a somewhat different out-of-plane force pattern.

RAM Concept ’s global analysis of structures assumes that the concrete behaves like a linear-elastic material. However, the following discussion of the in-plane and out-of-plane forces is based purely on equilibrium considerations, and therefore is valid for any material.