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Tower Crane Foundation Check

Base pressure, eccentricity and overturning ratio in one pass.

Quick answer

With the default inputs: Vertical load Fv 500 kN · Horizontal load Fh 30 kN · Overturning moment M 1200 kN·m etc.Maximum base pressure 85.2 kPa

Edit the inputs below to recalculate.

Inputs

Length LDepth HWidth W
The side sets base area and the overturning lever arm, depth sets self-weight, and horizontal moment scales with depth.
kN
Maximum vertical load from the manual, including full load
kN
Wind and out-of-service horizontal load
kN·m
Maximum moment at foundation top, from the manual
m
Square pad foundation
m
m
Ground to base, used for the soil surcharge
kPa
From the ground investigation; the check uses 1.2×

Result

Maximum base pressure
85.2kPa
Foundation self-weight1260.0kN
Soil surcharge64.8kN
Total vertical load1824.8kN
Moment at base1242.0kN·m
Average pressure50.7kPa
Eccentricity e0.681m
Allowable e = B ÷ 61.000m
Maximum pressure85.2kPa
Minimum pressure16.2kPa
Capacity limit 1.2 fak216kPa
Overturning factor4.41
Concrete volume50.40m³

Both checks are mandatory. Bearing: pMax = N/A × (1 + 6e/B) must not exceed 1.2 fak, and eccentricity must stay within B/6 — beyond that the base goes into zero pressure (lifting off), the formula no longer applies and the pad must be enlarged. Overturning: K = N × B/2 ÷ M, normally at least 1.5. Remember to add the horizontal force couple Fh × h to the moment at the base; it is often forgotten. Assumes a square pad, uniform ground and catalogue maximum loads — an initial sizing only. The formal design must be a sealed calculation by a qualified engineer to the crane manual, the ground investigation and the tower crane foundation code.

Formula

Self-weightG = 25 × side² × depth
Soil surchargeGs = side² × (embedment − depth) × 18
Total vertical loadN = Fv + G + Gs
Moment at baseMk = M + Fh × depth
Eccentricitye = Mk ÷ N, require e ≤ B ÷ 6
Maximum pressurepMax = N ÷ A × (1 + 6e ÷ B)
Overturning factorK = N × B ÷ 2 ÷ Mk, require K ≥ 1.5
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.

FAQ

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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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.