Isolated Footing design as per ACI 318-08 involves calculating the dimensions, reinforcement, and bearing capacity to ensure stability, prevent excessive settlement, and meet the structural requirements. Here’s a step-by-step approach based on the ACI code:

1. Determine Loads

• Dead Loads (D): Self-weight of the footing, columns, and other permanent loads.
• Live Loads (L): Variable loads such as furniture, occupants, or equipment.
• Wind or Earthquake Loads (W or E): Applicable lateral loads if the structure is in a high seismic or wind-prone area.

2. Size the Footing

• Area Calculation: Use the bearing capacity equation:A=Pu/A ​​where:
  • A = Area of footing,
  • Pu​ = Factored load (accounting for load factors from ACI 318, typically $1.2D+1.6L$),
  • qallow​ = Allowable soil bearing capacity.
• Footing Dimensions: For a square footing, B=sqrt{A}. For rectangular footings, adjust dimensions to suit the column layout and architectural requirements.

3. Thickness of Footing (h)

• Calculate based on shear requirements:
  • One-way shear (beam action): Vc=ϕ×2fc′×b×d
  • Two-way (punching) shear around column perimeter: Vc=ϕ×4fc′×bo×d
  •  where:
    • ϕ= Strength reduction factor (usually 0.75 for shear),
    • fc′ = Concrete compressive strength,
    • bo Perimeter at critical section (at distance d/2 around the column).

4. Reinforcement Design

• Calculate the reinforcement using bending moments at the critical sections.
  • Moment (M): Determine based on soil pressure and load eccentricity.
    • Bending Strength Check: ϕMn≥Mu
    • where Mu is the factored bending moment, and Mn is the nominal moment capacity.
    • Reinforcement ratio limits: Based on ACI requirements for minimum and maximum reinforcement (typically 0.0018 for fy=60).

5. Development Length (ld)

• Ensure bars have adequate development length per ACI standards for bond stress​​

6. Check for Bearing and Settlement

• Bearing Check: Ensure footing bearing stress on soil does not exceed allowable bearing pressure.
• Settlement Check: Conducted for service loads to verify footing dimensions are within acceptable limits for settlement.

7. Additional Checks for Seismic or Wind Zones

• Shear reinforcement if necessary,
• Extra reinforcement for uplift or lateral loads based on ACI Chapter 21 for earthquake-resistant design.

Each design step should satisfy ACI 318-08 requirements for safety, stability, and constructability, including minimum thickness, reinforcement, and compliance with serviceability and strength limits.