Types of Substructure

There are a number of different substructure systems which can be used to support a building or structure.

The most suited to a project is selected by the design consultants, mainly the Structural Engineers.

Options available fall into 3 main categories:

Shallow Foundations

There are a number of options within Shallow foundations:

Concrete strip foundations

1. Strip Foundations - a continuous strip of concrete supporting load-bearing walls. It distributes the building loads to a wider area of ground.

Commonly used as foundations for brickwork garden walls, strip foundations often have steel mesh or light reinforcement steel cast in to provide strength to the concrete.

Unlikely to support brickwork walls more than two storeys tall.

2. Ground Beams - horizontal structural elements typically constructed of reinforced concrete that connect and support a series of isolated footings. This helps create a united foundation that can better distribute loads.

Often used in areas with weaker or variable soil conditions, where the use of continuous strip foundations might not be as practical.

Can be thought of as strip foundations but connected into other structural elements such as pile caps instead of continuous strips.

3. Individual/pad Foundations - stand alone footings placed under columns or other isolated points of load.

4. Raft Foundations - covers the entire building area under the structure. Distribute the load over a wider ground area so is useful for where ground has low bearing capacity.

Called a raft slab as it 'floats' on the ground as a raft floats on water.

Deep Foundations

Again, there are a number of options within Deep foundations:

1. Piled Foundations with pile caps - consist of vertical structural elements, called piles, which are driven or drilled into the ground to support a building's load. For more information see our posts on Piling.

Pile caps are horizontal concrete platforms constructed on top of piles to distribute the loads from columns or walls and provide a stable connection between the superstructure and the piles.

The pile caps effectively transfer the load from the building's structural elements to the piles, which in turn transmit the loads to deeper, more stable layers in the ground.

Secant Piled Wall

2. Secant Piled wall - series of overlapping piles used to retain the ground behind when excavating ground and provide a level of watertightness.

Mostly applicable to basement construction.

3. Load Bearing Wall - reinforced concrete walls constructed in the ground. If they are under the ground and take the weight from the structure they are an element within the substructure.

Combination

In many cases the final solution is a combination of shallow and deep foundations. For example, it is common to have ground beams used on top of pile caps to support a ground floor slab.

Structural Slabs

In terms of how loads are transferred, structural slabs can be in one of the following two categories:

1. Ground bearing Slabs - are in direct contact with the ground. They rely on the ground below to transfer load into.

2. Suspended Slabs - don't rely directly on the ground below. They are supported by beams, columns, walls or other structural elements.

There is normally an air void underneath the slab which makes it less susceptible to mould and dry rot.

Once the Structural engineer has decided what slab type would be best suited, the method of construction can be decided with the main contractors input. The following are options:

Reinforced concrete slab as concrete pour is happening

1. Reinforced Concrete (RC) In-situ Slabs - most common slab casting method. Concrete is poured over steel reinforced which has been laid.

Composite slab detail

2. Composite Slabs - when the buildings structure is steel, instead of reinforced concrete these are used. They comprise of steel decking and a concrete topping is poured on top - usually with a layer of mesh.

Pre-cast concrete planks

3. Pre-cast - manufactured in 'planks' off-site and lifted into place to form the slab. Usually they require a concrete topping to be cast above with mesh.

Definitions

Reinforced Concrete - concrete with embedded steel inside to increase strength and durability.

Footings - the lowest elements in the foundations of a building.

Bearing Capacity - maximum weight/loading that a soil can support without excessive settlement or failure

Superstructure - the part of a structure which is above its foundation/substrucure. Includes walls, floors, roofs, columns, above ground beams and other components which provide functional spaces and support for the building.

Posts to Follow

We plan to go into more details on Substructure and add sections on:

·       Logistics

·       Temporary Works

·       Build Sequence

·       Health, Safety & Environment

·       Quality

·       Programme

·       Commercial

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