Tower crane base: chassis or foundation section?

In brief

• A static tower crane is usually set up in one of two ways: on a chassis with counterweights, or on a foundation section (also called anchor legs) cast into a concrete foundation.
• As a rule, a mast on a foundation section takes up a cross-section of about 1.6×1.6 to 2×2 metres at surface level, although the foundation itself is much larger, whereas a chassis generally takes up between about 4.5×4.5 metres and about 8×8 metres, depending on the model and the crane’s height.
• The choice depends on the free area, the type of ground, the height of the crane and the project schedule.
• The foundation is designed by a structural engineer from the loads in the manufacturer’s data sheet.

A tower crane base is the part that transfers all of the crane’s loads to the ground: the weight of the mast and jib, the load, the counterweights and the wind. The choice of base type affects the area the crane will occupy on site, the foundation costs and the schedule. In this article we explain the difference between a chassis and a foundation section, and when each method suits.

Two methods for founding a tower crane

Static tower cranes, such as the Potain MD and MDT cranes in RMA’s rental fleet, are usually set up in one of two ways:

• A chassis or cross base: a steel frame on which concrete counterweights are placed. The frame sits on a slab or on spot foundations, and the counterweights provide stability.
• A foundation section, or anchor legs: a special mast section, or anchoring legs, embedded in a reinforced-concrete foundation. The foundation itself provides stability, and the mast rises directly from it.

The Safety at Work (Tower Cranes) Regulations, 1966, generally require a static crane to stand on a stable foundation, with its stability secured by anchoring or another suitable method, and the counterweights to be positioned and secured properly. When the crane is placed on a building, the building must be strong enough for the crane, its operating range and the safe working load (regulation 35(b)). The engineering details are determined in the specific design for each site.

When to choose a chassis

A chassis is particularly suitable when there is enough free area around the crane’s position on site. The main advantages:

• There is no need to cast a section into the foundation, so preparation is simpler in most cases.
• At the end of the project the chassis and the counterweights are dismantled, and no cast steel component is left on site.
• It suits relatively short projects, or sites where the area has to be cleared quickly at the end.

Counterweight logistics

The chassis counterweights arrive on site by truck and are put in place by a mobile crane. So access and working space for the crane and the trucks must be planned around the base too, and not only at the crane’s position itself. The number and weight of the counterweights are determined from the manufacturer’s data sheet for the chosen model and height.

The main drawback is the area: a chassis and its counterweights take up a significant area, and on dense urban sites that may be area needed for excavation, storage or vehicle movement.

When to choose a foundation section

A foundation section suits when space is limited, when the crane is placed inside the building footprint or in the excavation pit, or when there is no room for a chassis and its counterweights. The advantages:

• A very small footprint at surface level, provided that the concrete foundation, whose dimensions are much larger than the mast cross-section, is below surface level or integrated into the building’s foundations.
• The option of placing the crane inside the building, for example in a lift shaft or in an opening that will be completed later.
• The option of integrating the crane foundation into the building’s foundations, in full coordination with the structural design.

On the other hand, a foundation section requires early planning, casting, and curing time for the concrete before erection. The anchor legs or the foundation section must be original manufacturer components for the specific mast, positioned with a location template and to the tolerances in the manufacturer’s instructions, and components that have already been cast into a foundation must not be reused. At the end of the project some components usually remain inside the foundation, and it must be planned how to close the opening in the building after the crane is dismantled.

Area and dimensions: how much space the base takes

The following figures are general ranges. The exact dimensions are determined by the crane model, its height and the manufacturer’s data sheet:

• Mast on a foundation section: as a rule, the mast cross-section takes up about 1.6 to 2 metres.
• Chassis: as a rule, from about 4.5×4.5 metres up to about 8×8 metres, depending on the model and the crane’s height.

Beyond the base itself, allow for working clearance around it for erection and dismantling, and access for a mobile crane and trucks.

Coordination with the structural engineer

In either method, the foundation is designed by a structural engineer on behalf of the project. For the design to be accurate, the structural engineer needs the loads the crane transfers to the foundation, from the manufacturer’s data sheet for the chosen model, mast height and jib length. Points to coordinate:

1. The final crane model, mast height and jib length.
2. The ground data from the soil survey, and whether piles are needed.
3. The exact position relative to the walls, slabs and openings in the building.
4. If ties to the building are expected, where and at which floors.
5. How the crane will be dismantled at the end of the project and what will remain in the foundation.

Common mistakes in choosing a base

• Deciding on a foundation section too late, after a raft or slab has already been cast at the intended position.
• Planning a chassis without checking that the area stays free throughout the project, including the excavation and development stages.
• Designing the foundation for the starting height only, without allowing for the expected raisings and ties.
• Not considering how the crane will be dismantled at the end, when the building is already built around it.

RMA’s consulting team works with the structural engineer and provides the data needed for design. More details are on the consulting and planning of crane placement on a building site page, and in the article Planning crane placement on the building site.

Comparison: chassis versus foundation section

RMA operates from a logistics yard in Lod. For information on base components see the mast sections, parts and chassis for cranes page. To coordinate the design, call 08-9202882 or send a WhatsApp message.

Frequently asked questions

What is the difference between a chassis and a foundation section?

A chassis is a steel frame on which concrete counterweights are placed to provide stability. A foundation section is a mast section or anchoring legs cast into a concrete foundation, so the foundation itself provides the stability.

How much area does a tower crane chassis take up?

As a rule, a chassis takes up an area of about 4.5×4.5 to about 8×8 metres. The exact dimension is determined by the model and the crane’s height.

How much area does a crane on a foundation section take up?

As a rule, the mast cross-section takes up about 1.6 to 2 metres, so a foundation section suits dense sites or placement inside the building.

Who designs the crane foundation?

A structural engineer on behalf of the project, based on the loads in the manufacturer’s data sheet for the chosen model, mast height and jib length.

What happens to the foundation at the end of the project?

With a chassis, the frame and the counterweights are usually dismantled. With a foundation section, some components usually remain inside the foundation, and the closing of the opening in the building must be planned in advance.

Can a tower crane be placed inside the building?

Yes, usually with a foundation section in the foundation, for example in a shaft or in an opening that will be completed later. This requires full coordination with the structural engineer.

The information in this article is general only and is not engineering, safety or legal advice. Before any decision, follow the manufacturer’s instructions and the applicable law, and consult certified professionals.