# Tower Crane Foundation Check

Base pressure, eccentricity and overturning ratio in one pass.

- Page: https://jianzhujisuan.com/en/tower-crane-base
- Category: Temporary Works (https://jianzhujisuan.com/en/c/temporary-works)
- Chinese version: https://jianzhujisuan.com/zh/tower-crane-base.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Vertical load Fv 500 kN, Horizontal load Fh 30 kN, Overturning moment M 1200 kN·m, Foundation side 6 m, Foundation depth 1.4 m, Embedment depth 1.5 m, Allowable bearing fak 180 kPa): **Maximum base pressure 85.2 kPa**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Vertical load Fv | number | 500 | kN | Maximum vertical load from the manual, including full load |
| Horizontal load Fh | number | 30 | kN | Wind and out-of-service horizontal load |
| Overturning moment M | number | 1200 | kN·m | Maximum moment at foundation top, from the manual |
| Foundation side | number | 6 | m | Square pad foundation |
| Foundation depth | number | 1.4 | m |  |
| Embedment depth | number | 1.5 | m | Ground to base, used for the soil surcharge |
| Allowable bearing fak | number | 180 | kPa | From the ground investigation; the check uses 1.2× |

## Formula

- **Self-weight**: G = 25 × side² × depth
- **Soil surcharge**: Gs = side² × (embedment − depth) × 18
- **Total vertical load**: N = Fv + G + Gs
- **Moment at base**: Mk = M + Fh × depth
- **Eccentricity**: e = Mk ÷ N, require e ≤ B ÷ 6
- **Maximum pressure**: pMax = N ÷ A × (1 + 6e ÷ B)
- **Overturning factor**: K = N × B ÷ 2 ÷ Mk, require K ≥ 1.5

## Worked example

Fv 500 kN, Fh 30 kN, M 1200 kN·m on a 6×6×1.4 m pad embedded 1.5 m with fak 180 kPa: 1260 kN self-weight, 64.8 kN surcharge, 1824.8 kN total, 1242 kN·m at the base, 0.681 m eccentricity (under B/6 = 1.0), 50.7 kPa average, 85.2 kPa maximum (under 1.2 fak = 216), 16.2 kPa minimum and an overturning factor of 4.41.

## Questions

### Why must eccentricity stay within B/6?

Beyond B/6 part of the base lifts off the ground, so only one side carries load, the effective area shrinks, pressure spikes and the linear distribution formula no longer holds. Lift-off also tilts the pad, throws the mast out of plumb and can topple the machine. Code therefore makes e ≤ B/6 mandatory; if exceeded, enlarge the pad or increase its depth and ballast — extra reinforcement will not fix it.

### Why is the horizontal-force moment so easily missed?

Because the catalogue moment acts at the top of the foundation, while the check needs the pressure distribution at the base, and transferring it down adds horizontal force times lever arm (foundation depth). Fh looks small at a few tens of kN, but 30 kN over 1.4 m is 42 kN·m, 3.4% of 1242 kN·m. The deeper the pad the bigger it gets, so it cannot be ignored on deep foundations. Eccentricity of the vertical load adds further moment but is normally already inside the catalogue figure.

### What if the checks fail?

In order of effect: enlarge the pad — area grows with the square so pressure drops fast and the overturning lever arm improves too, which is the most efficient fix; deepen the pad, adding weight and lever arm but also horizontal moment; increase embedment to use soil surcharge; or improve the ground by replacement, compaction or a thicker blinding. If none work, change the form — a piled cap, or seat the crane on the raft of the main structure. Any change means re-running both checks, never just one.

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