read the result
Average pressure makes the first assumption visible.
For a concentric vertical service load, the simplest screen divides the load by the rectangular footing area. The optional allowance keeps a small planning allowance visible instead of hiding it inside the input.
Governing equation
A = B × L. The screening load is Nscreen = N × (1 + allowance), and the average pressure is q = Nscreen / A.
Worked example using the starting values
For N = 300 kN, B = L = 1.5 m and a 10% allowance, the area is 2.25 m² and the screening load is 330 kN. Therefore q = 330/2.25 = 146.7 kPa. Compared with the entered 200 kPa allowable value, the arithmetic utilisation is 73.3%. That comparison is meaningful only if the allowable value is on the same gross/net and service-load basis as the calculated pressure.
Assumptions and common slips
This is a concentric vertical-load, uniform-contact, rectangular-area screen. Do not treat average pressure as the peak contact pressure when moments or eccentricity exist, or compare a gross calculated pressure with a net allowable value without reconciling the basis.
Do not stop at the average.
- Check eccentricity, moments, effective dimensions, contact loss, sliding, overturning, and uplift separately.
- Confirm the geotechnical basis for the allowable value, including groundwater, drainage, soil variability, settlement, and the applicable code or report.
- Check the reinforced-concrete footing for bending, one-way shear, punching shear, detailing, durability, and construction requirements.