What We Can Do
Let beamcalc.co.uk help you get your project out of the ground with our comprehensive foundation design service. We are experts in designing all types of foundations for domestic construction projects across the UK. Whether you need simple strip foundations designing or a bespoke raft design were the first port of call for foundation calculations.
Our comprehensive foundation design calculation packages contain the necessary calculations, specifications and drawings which are suitable for submission and approval by your Building Control provider and perfect for your contractor to price from, order building materials and build from.
Strip Foundation Design
Strip foundations are one of the most common types of shallow foundations used in construction. They are covered under Approved Document A – Structure of the Building Regulations 2010 (as amended) and are particularly suitable for low-rise buildings such as dwelling houses and extensions to dwellings. Consideration must be paid at the design stage with regard to trees in close proximity of the proposed structure and underground drainage or sewers affecting the site as the shallow strip foundations would need to become deep strip foundations.
Definition:
A strip foundation consists of a continuous strip of concrete that supports linear structures like load-bearing external and internal walls. It distributes the weight of the building over a wider area of soil, helping to prevent excessive settlement. Most strip foundations are 600mm in width.
Key Characteristics:
- Typically used where the soil has adequate bearing capacity at shallow depths usually accepted for design to be 100kN/m2 @ 1000mm below ground level. (BGL)
- The width of the strip foundation depends on the load it carries and the strength of the subsoil. Usually for standard construction in a good cohesive stratum the accepted width of a strip foundation is 600mm.
- The depth should protect against frost (typically a minimum of 450–750mm) and reach below topsoil and any vegetation, roots and filled ground or construction backfill and drains/sewers.
- If trees are within a zone of influence of the proposed structure the foundations would normally be expected to be deeper than 1000mm BGL and special heave protection would be required.
Approved Document A Requirements:
Approved Document A (Structure):
- Strip foundations must be designed to avoid settlement and ensure structural stability.
- Minimum widths and depths are suggested, with adjustments based on local ground conditions.
- Table 10 in the document provides minimum foundation widths based on standard wall loads and soil type.
- Additional provisions apply for areas with clay soils, trees, or nearby drains due to potential ground movement risks.
Design References:
- BS 8004: Code of practice for foundations.
- Eurocode 7 (EN 1997): Geotechnical design standards.
Raft Foundation Design
A raft foundation, also known as a mat foundation, is a large reinforced concrete slab that spreads the load from the structure over a wide area, often the entire footprint of the building. It is especially useful in poor ground conditions or where loads are heavy and differential settlement must be minimised.
Definition & Application:
- A raft foundation supports multiple columns and/or walls by distributing the load across the whole area, not just beneath individual elements.
- Commonly used for:
- Low-rise buildings on soft or loose soils.
- Sites with variable ground conditions.
- Areas with a high-water table or where strip foundations would require excessive excavation meaning a higher cost.
Construction Features:
- Typically consists of a reinforced concrete slab, possibly with thickened areas (ribs or ground beams) beneath load-bearing walls or columns.
- May include integral edge beams with or without a toe for stiffness and containment.
Regulatory Requirements (Approved Document A):
Approved Document A does not prescribe specific design dimensions for raft foundations, However, Part A requires that:
- The foundation must provide adequate support for the building.
- The bearing capacity of the ground must be established.
- The foundation must prevent excessive or uneven settlement.
Design should be in accordance with:
- Eurocode 7 (EN 1997) – Geotechnical design.
- BS EN 1992-1-1 – Design of concrete structures.
- BS 8004 – Foundations code of practice.
When to Use Raft Foundations:
Use raft foundations when:
- Soil bearing capacity is low and a strip foundation would be economically too wide.
- Loads are closely spaced or heavy (e.g., in commercial or heavy buildings).
- There is a risk of differential settlement.
