About

For heavy residential structures and commercial buildings situated on unstable upper soil layers, driven reinforced concrete piles are the standard solution. However, the cost of the foundation is not just the piles; the reinforced concrete grillage (pile cap/beam) often constitutes a major portion of the budget. This tool calculates the total concrete volume required for the grillage, deducting the volume of pile heads that must be embedded into the beam for structural integrity (typically 50-100mm).

It also provides an automatic estimation for reinforcement steel (rebar) based on standard engineering ratios for structural beams. Accuracy here prevents ordering excess ready-mix concrete or falling short during the pour, which can create dangerous cold joints in the foundation.

Formulas

The calculator determines the net volume of the grillage by subtracting the embedded pile heads.

Vnet = Lgrill × W × H − Npiles × Apile × hembed

Where hembed is the height of the pile inside the concrete beam. Rebar weight is estimated as Vnet × Ratio (typically 150 kg/m³).

Reference Data

Pile Section (mm)	Load Capacity (kN)	Grillage Min Width (mm)	Min Rebar Ratio (kg/m³)
150 × 150	150 - 250	300	120
200 × 200	300 - 450	400	135
300 × 300	600 - 900	500	150
350 × 350	800 - 1200	600	160
400 × 400	1100 - 1500	600	175

Frequently Asked Questions

A grillage (or grade beam/pile cap) is the reinforced concrete beam that sits on top of the driven piles. It connects all piles together to distribute the building's load evenly. It serves as the base for the walls.

When pouring concrete, the tops of the driven piles protrude into the formwork (usually by 5-10cm) to anchor the beam. This space is occupied by the pile, not fresh concrete. In large foundations with hundreds of piles, failing to subtract this volume can lead to significant over-ordering.

For heavy structural foundation beams, a density of 150kg to 180kg of steel per cubic meter of concrete is standard. This accounts for longitudinal bars (thick structural steel) and stirrups (thinner loops holding the shape).