The general procedure used in the Strip Footing Design tool agrees with the "Design Guide on the ACI 318 Building Code Requirements for Structural Concrete" of the Concrete Reinforcing Steel Institute CRSI (2020). Some equations developed there are referenced to this guide in places where the Code has not a specific reference. The tool considers also the stability checks for soil bearing, maximum eccentricity, overturning, and sliding checks. It is always desirable to avoid concentration of stresses in the soil and thus the program has a warning status when the resultant is outside the middle third of the footing. In such cases, the user is advised to enlarge the footing to have a better distribution of stresses.

The calculator allows the use of lateral loads (shear and moment) and it can deal with three load cases (dead load, live load, and wind or any other lateral load case). The user should check the used load combination factors of the table and add or modify them accordingly to the adopted load code. This tool works with imperial units. By default, the tool works with net soil stresses and does not consider the footing's own weight and overburden soil pressure. It offers, however, the option to consider the overburdened soil and footing weight and in this case, it will be working with gross stresses.

The ACI Strip Footing calculator can deal with the eccentricity of the wall about the footing base. However, for large eccentricities special measures may be needed like the implementation of negative reinforcement and the consideration of the overburden pressure in the strength design. These special measures are out of the scope of the tool.

The wall footing tool works with a try-and-error philosophy. The user can modify the input data until all checks will pass. Normally when there are failures, the solution involves enlarging the footing or incrementing the reinforcement. In any case, the tool also checks minimum and maximum conditions that help avoid excessive reinforcement. It is suggested to enlarge the height for shear failures, enlarge the width for stability failures, and increment the reinforcement area for bending failures when the footing height and shear checks are OK.

• What foundation-soil friction angle should be used?
  • This angle is normally between one-half and two-thirds of the soil friction angle. ("Principles of Foundation Engineering" from Braja M. Das)
• Which reduction factors are used in the strength verifications?
  • The ACI strip footing calculator uses φ = 0.75 for shear, φ = 0.90 for bending (for reinforced concrete assuming a tension-controlled condition involving a reinforcement area that is less the maximum limit for this condition), φ = 0.60 for plain concrete bending, and φ = 0.65 for bearing.
• What value is used for concrete unit weight ?
  • The default value used is 150 lb/ft3 as suggested by the standard for normal-weight concrete.
• What value of soil unit weight can be used?
  • The common values are between 90 to 130 lb/ft3. It is advised to use the humid value suggested in the geotechnical report of the project.
• Is the wall reinforcement used for in calculations?
  • It is not used, only for drawing purposes. The wall dowels, however, are used in the load forces transfer check.
• Why does my footing have a small maximum spacing?
  • The ACI 318-19 Section 24.3.2 specifies quite low values considering the values used for the concrete cover (normally around 3 inches). Some references (Avoiding Problematic Uses of Slabs on Ground, Jan, 2021 Structure Magazine By Alexander Newman, P.E., F.) mention that the ACI should consider exempting these provisions for footings and slabs on ground, but, as of now, they still apply, and therefore they are included in the program.

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