What it checks
The module designs the allowable soil bearing capacity for strip, square, circular, or rectangular footings. It calculates the ultimate bearing capacity using the Terzaghi-Meyerhof general bearing equation, incorporating shape and depth factors derived from the soil's friction angle, cohesion, and unit weight.
The design method follows Allowable Stress Design (ASD) principles, applying a specified factor of safety to the ultimate capacity. The module also evaluates IBC 2021 Table 1806.2 presumptive load values when provided. The sequence of checks involves computing the ultimate capacity, dividing by the safety factor to obtain the allowable capacity, and determining the governing allowable value by comparing it against the presumptive limit.
Inputs
- Geometry - Foundation type ("strip", "square", "circular", "rectangular"), footing width B (ft), footing length L (ft), and embedment depth Df (ft).
- Soil Properties - Undrained cohesion c (ksf), effective friction angle φ' (degrees), and soil unit weight γ (kcf).
- Water Table - Depth to water table (ft), where a value of 999 indicates a deep water table with no effect.
- Loads and Safety - Applied gross bearing pressure q_applied (ksf) and the ASD factor of safety (typically 3.0).
- Presumptive Limits - IBC Table 1806.2 presumptive bearing (ksf), where a value of 0 indicates the use of Terzaghi-Meyerhof results only.
What it reports
- Bearing Factors - Meyerhof bearing factors for cohesion (Nc), surcharge (Nq), and self-weight (Nγ) derived from the friction angle.
- Shape Factors - The set of shape factors [sc, sq, sγ] calculated from the footing dimensions and friction angle.
- Ultimate Capacity - The ultimate bearing capacity qu (ksf) calculated via the Terzaghi-Meyerhof equation.
- Allowable Capacity - The allowable bearing capacity qa (ksf), defined as the ultimate capacity divided by the safety factor.
- Governing Allowable - The governing allowable bearing capacity (ksf), which is the minimum of the calculated qa and the IBC presumptive value when the latter is provided.
- Net Applied Pressure - The net applied pressure (ksf), calculated as the gross applied pressure minus the product of unit weight and embedment depth.
Code references
The engine cites IBC 2021 §1806 / Table 1806.2 for presumptive bearing values and ASCE 7-22 §2.4 for ASD load combinations for bearing checks. It also references the Terzaghi (1943) and Meyerhof (1963) general bearing capacity equation.
Limits
The module assumes a deep water table when the input is 999, indicating no water table effect. If the IBC presumptive bearing input is 0, the module uses the Terzaghi-Meyerhof calculation only and ignores the presumptive limit. Results are engineering aids; the engineer of record is responsible for the design.