Tower Crane Base Design: Loads, Foundations and BS 5975

A tower crane base is one of the highest-consequence temporary structures on any project. It carries every lift for the life of the crane, often two years or more, and if it moves, everyone notices at the top.

It is also the point where two organisations have to meet in the middle, and frequently do not. The crane supplier sends a table of foundation loads. The contractor has to design a base that puts those loads into ground the supplier has never seen. Most of the difficulty in tower crane base design lives in that handover.

This article covers how to approach it: where the loads come from and what they include, the foundation options and how to choose between them, how deep the ground investigation needs to go, and the procedural controls that apply under BS 5975.

Start With the Crane

The foundation problem is set by the crane, so the sequence matters. Choose the crane type first, then design the base — not the other way round.

Three broad types turn up on UK sites:

• Self-erecting cranes rig themselves up and down, often truck or trailer mounted, and are used for shorter or lighter duties. Their foundation demands are usually modest.
• Saddle (horizontal) jib cranes run a trolley out along a fixed horizontal jib to change radius. The load geometry is different from a luffer, and so is the foundation reaction.
• Luffing jib cranes raise and lower the whole jib. They are used where the crane cannot oversail neighbouring land — a common constraint on tight urban sites — and they apply a distinctly different, often more onerous, out-of-service loading because the jib is brought up steep when parked.

Each type loads its base differently. So the decision to use a luffer instead of a saddle jib, made on planning and oversailing grounds, is also a foundation decision. Get that sequence backwards and the base design is chasing a machine somebody already ordered.

The Loads Are More Than Self-Weight

This is where errors creep in, because the instinctive picture — a heavy tower pressing straight down — is the least important part of the load case.

The crane supplier provides foundation loads for the specific machine, and they typically include:

• Vertical load from the self-weight of the tower, jib and any load being lifted
• Horizontal load from wind and from crane operation
• Overturning moment, which is usually the governing action

That overturning moment is the point. A tower crane is a tall cantilever with an asymmetric jib, and the moment at the base from wind and from an offset load dwarfs the significance of the vertical weight. The base is not designed to resist being pushed down; it is designed to resist being tipped over.

In-Service and Out-of-Service

Two cases have to be considered, and the worst one is not always obvious.

In-service is the crane working — lifting, slewing, subject to the maximum wind speed at which it is permitted to operate.

Out-of-service is the crane parked, weathervaning freely, subject to the full storm wind it must survive without operating. For many cranes, particularly luffers, the out-of-service storm case governs the foundation design. The crane is doing nothing, and that is its most demanding moment for the base.

Design for the case that governs, which means checking both rather than assuming the working case is worst.

Foundation Options

There are three common ways to found a tower crane, and the choice is driven by the ground, the loads, the programme and the cost.

• Pad (mass) foundation — a large reinforced concrete pad bearing directly on competent ground. Simple where the ground can take it, but the pads are large, and in poor ground the size becomes impractical.
• Piled base — a pile cap on piles, taking the load down to a competent stratum. Used where near-surface ground cannot carry a pad, or where settlement under a pad would be unacceptable.
• Ballasted (kentledge) base — a base that resists overturning by weight rather than by fixity, often using cruciform arrangements with ballast. Useful where a cast foundation is undesirable, but it consumes space and material.

These bases can be very large, which is exactly why economic viability enters the conversation early. The cheapest base that still works is a correct answer, and on a tight programme the fastest adequate base may beat the theoretically most efficient one.

How Deep Must the Ground Investigation Go?

Deeper than people expect, and deeper than the foundation itself.

Soils information has to extend below the deepest part of the foundation, and "deepest" means the foundation you might actually build, not the one on your first sketch. If the pad might become a piled base, the boreholes need to go below the anticipated pile toes, not to the underside of a pad that may never be built.

A borehole that stops at the base of a pad tells you nothing about what piles would sit in if the ground comes back weaker than assumed. Investigating only for your preferred solution is not investigating — it is confirming a hope.

On variable or brownfield ground, that is precisely how a pad design becomes a piled design mid-project with no data to support it.

Tying Into the Permanent Works

Sometimes the most economical base is one that already exists.

The permanent core foundations or piles are going in anyway. Tying the crane base into them can save an entire separate structure — a real saving given the size of these bases. But it comes with two conditions that are easy to overlook.

First, the permanent works designer has to be involved before the decision, not informed after it. The permanent structure will now carry a temporary works load case it was never originally designed for, and only its designer can confirm it can.

Second, the design needs load cases both with and without the permanent structure in place. The crane arrives long before the building does, so the base has to work when it stands alone and when it is tied in.

Very tall cranes take this further and tie the mast back to the rising structure with collars and ties. The same principle applies, and more sharply: the permanent frame is being asked to carry crane loads, and someone has to confirm the frame is willing to lend that strength.

Design Check Category and Procedure

Tower crane bases sit within BS 5975 temporary works procedures like any other significant temporary works, and they commonly attract a higher design check category — frequently requiring an independent check by someone outside the design team, and sometimes by an independent organisation, depending on the specifics. Given that the base carries every lift for the crane's life, that scrutiny is proportionate. How the categories, the brief and the coordinator's permits work in general is set out in The Temporary Works Coordinator and BS 5975.

What is specific to a crane base is the end of that chain. Foundation construction and inspection is where the design finally meets the ground: reinforcement, cover, concrete quality and setting-out all have to be verified against a drawing that assumed everything would be correct, and a permit to load is issued before the crane is erected and put to work.

Common Mistakes

Designing before the crane is confirmed. The base design chases a machine chosen on other grounds, and the loads arrive after the foundation concept is fixed.

Missing the out-of-service case. Designing for the crane working and overlooking the storm case that actually governs — particularly for luffing jibs.

Treating vertical load as the problem. The overturning moment governs. A design focused on bearing pressure alone has missed the point.

Investigating too shallow. Boreholes stopping at the base of a pad, leaving no data if piling becomes necessary.

Tying into permanent works without the permanent designer. Loading the permanent structure with a crane case its designer never saw.

Underestimating the wind data. Using generic wind loads rather than the values appropriate to the site, its exposure and the crane's height.

Weak permit discipline. Erecting and loading the crane before the base inspection is complete and signed off.

Best Practice

1. Confirm the crane first — type, height, configuration — then design the base.
2. Use the supplier's foundation loads, and check both in-service and out-of-service cases to find the governing one.
3. Design against overturning as the primary action, not vertical bearing alone.
4. Investigate below the deepest credible foundation, allowing for the possibility of piling.
5. Choose the foundation type on ground, loads, programme and cost together, not on habit.
6. Involve the permanent works designer early if tying into permanent foundations or the structure.
7. Assign the design check category deliberately and obtain an independent check appropriate to the consequence.
8. Inspect the constructed base and issue a permit to load before the crane is erected.

Frequently Asked Questions

Who designs the tower crane base?

The crane supplier provides the foundation loads for the specific machine, and the contractor's temporary works designer designs the base to put those loads safely into the ground. The two are separate responsibilities, which is why clear transfer of the load data and the ground conditions matters so much.

What loads does a tower crane foundation carry?

Vertical load from self-weight and the lifted load, horizontal load from wind and operation, and an overturning moment — which is usually the governing action. The base is designed principally to resist overturning rather than vertical bearing, because the crane is a tall cantilever with an offset jib.

What is the difference between the in-service and out-of-service case?

In-service is the crane working, limited to the maximum wind speed at which it may operate. Out-of-service is the crane parked and weathervaning in a full storm wind. For many cranes, particularly luffing jibs, the out-of-service storm case governs the foundation design even though the crane is doing no work.

Can a tower crane base be part of the permanent works?

Yes, and it is often economical to tie into permanent core foundations or piles. But the permanent works designer must be involved beforehand, because the permanent structure will carry a temporary works load case it was not originally designed for. Load cases are needed both with and without the permanent structure present.

How deep should the ground investigation be for a tower crane base?

Below the deepest part of the foundation that might be built — which means below the anticipated pile toes if piling is a possibility, not merely to the underside of a proposed pad. Investigating only to the depth of the preferred solution leaves no data if the ground proves weaker than assumed.

What design check category applies to a tower crane base?

It is assigned under BS 5975 according to complexity and consequence, and tower crane bases commonly fall into a category requiring an independent check, given that the base carries every lift for the life of the crane. The categorisation should be made deliberately at brief stage.

What foundation types are used for tower cranes?

Pad (mass) foundations bearing on competent ground, piled bases taking load to a deeper stratum, and ballasted (kentledge) bases resisting overturning by weight. The choice depends on ground conditions, the crane loads, the programme and cost, and these bases are frequently large enough that economy drives the decision.

Does the wind load really govern the design?

Very often, yes — through the overturning moment. Wind on a tall crane, particularly the out-of-service storm case, generates the moment that sizes the base. Using appropriate site-specific wind data rather than a generic allowance is important, because the crane's height and exposure both matter.

Conclusion

A tower crane base is a foundation defined by a moment, not a weight, transferring loads that one organisation calculated into ground that a different organisation has to understand.

Get the sequence right — crane first, then base. Design against overturning, and check the out-of-service storm case rather than assuming the working case is worst. Investigate deep enough to cover the foundation you might end up building. Bring the permanent works designer in before tying into their structure. And apply a design check category that reflects the fact that this single element carries every lift for years.

The base is under the ground and out of sight within days of the crane arriving. That is exactly why the scrutiny has to happen before the concrete is poured, not after something moves.