The span ratio parameter is the primary geometric criterion used to decide whether an RCC slab is designed as a One-way slab or a Two-way slab.

Just to explain and show 'Span Ratio Decision Criterion', Let's consider the next slab span designation.
Lx = shorter span of the slab
Ly = longer span of the slab

The design of an RCC slab as one-way or two-way is governed by the span ratio:

Ly/Lx ≥ 2 → One-way slab
Ly/Lx < 2 → Two-way slab

Now, if we are considering One-Way Slab characteristics such as:
- Load transfer occurs mainly in the shorter direction.
- Main reinforcement is provided along the shorter span.
- Distribution reinforcement is provided along the longer span.

Ly/Lx ≥ 2 → One-way slab

We can list typical examples like corridors, narrow rooms, cantilever slabs and others as a One-Way RCC Slab structures.

And, if we are considering Two-Way Slab characteristics such as:
- Load is shared by both spans.
- Main reinforcement is required in both directions, and
- Bending moments are calculated using moment coefficients or plate theory.

Ly/Lx < 2 → Two-way slab

Typical examples: square or nearly square panels.

In addition to the above span-ratio design classification, the design of RCC slabs is also governed by two primary categories of characteristics:
1. Strength requirements and
2. Serviceability requirements.
Both must be satisfied for a safe and functional slab.
1. Strength (Safety) Characteristics:
These characteristics ensure that the slab does not fail under applied loads like:

(a) Flexural Strength
The slab must safely resist the maximum bending moments due to dead load, live load, and other applicable loads.
Main reinforcement is designed based on the limit state of collapse in flexure,

(b) Shear Strength
Although slabs rarely fail in shear, the design must check that nominal shear stress does not exceed permissible limits.
Usually, slabs are safe in shear without shear reinforcement due to their small depth, and

(c) Overall Structural Stability
Adequate anchorage, development length, and proper detailing are required to ensure safe force transfer and structural integrity.

2. Serviceability Characteristics
These characteristics ensure proper performance of the slab during its service life.
Thses can be:
(a) Deflection Control
Slab thickness and span-to-depth ratios are selected to limit excessive deflection under service loads,

(b) Crack Control
Limitation on bar diameter, spacing of reinforcement, and provision of minimum reinforcement help in controlling crack width,

(c) Durability and Comfort
Adequate concrete cover protects reinforcement from corrosion and ensures long-term durability.
Excessive vibrations and visible cracks are avoided for user comfort and appearance.