Foundation Calculations

What We Can Do

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 and are particularly suitable for low-rise buildings such as dwelling houses and extensions to dwellings.

Definition:

A strip foundation consists of a continuous strip of concrete that supports linear structures like load-bearing walls. It distributes the weight of the building over a wider area of soil, helping to prevent excessive settlement.

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.
• 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 if 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.

Approved Document A Requirements:

According to 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 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.

A raft foundation, also known as a mat foundation, is a large concrete slab that spreads the load from a 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 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.
  • Structures where basements are included.
  • Areas with a high-water table or where strip foundations would require excessive excavation.

Construction Features:

• Typically consists of a reinforced concrete slab, possibly with thickened areas (ribs or beams) beneath load-bearing walls or columns.
• May include integral edge beams for stiffness and containment.

Regulatory Requirements (Approved Document A):

While Approved Document A does not prescribe specific design dimensions for raft foundations, 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 must comply 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 too wide.
• Loads are closely spaced or heavy (e.g., in commercial or heavy buildings).
• There is a basement planned or a risk of differential settlement.

Want to find out more about our foundation design calculation services? Get in contact with us today for a free and competitive quote via our easy-to-use online application form, by calling us on 0208 243 8618 or by email at [email protected].

We understand the importance of getting accurate structural calculations when working on a domestic building project.