Section 2E Foundations of plain concrete

Foundations of plain concrete

  1. Conditions relating to the ground
    1. Design provisions
      1. Minimum width of strip foundations
        1. Minimum depth of strip foundations

          Conditions relating to the ground

          2E1. There should not be:

          • A: Non-engineered fill (as described in BRE Digest 427) or wide variation in ground conditions within the loaded area; nor
          • B: Weaker or more compressible ground at such a depth below the foundation as could impair the stability of the structure.
           

          Design provisions

          2E2. The following design provisions relate to foundations:

          • A: The foundations should be situated centrally under the wall;
          • B: For foundations in chemically aggressive soil conditions guidance in BS 8500-1 and BRE Special Digest 1 should be followed. In non-aggressive soils, concrete should be composed of Portland cement to BS EN 197-1 and -2 and fine and coarse aggregate conforming to BS EN 12620 and the mix should comply with one of the following recommendations:
            • i: In proportion of 50kg of Portland cement to not more than 200kg (0.1 cubic metres) of fine aggregate and 400kg (0.2 cubic metres) of coarse aggregate; or
            • ii: Grade ST2 or grade GEN I concrete to BS 8500-2;
          • C: Minimum thickness T of concrete foundation should be 150mm or P, whichever is the greater where P is derived using Table 10 and Diagram 23. Trench fill foundations may be used as an acceptable alternative to strip foundations;
          • D: Foundations stepped on elevation should overlap by twice the height of the step, by the thickness of the foundation, or 300mm, whichever is greater (see Diagram 21).For trench fill foundations the overlap should be twice the height of the step or 1m, whichever is greater;
          • E: Steps in foundations should not be of greater height than the thickness of the foundation (see Diagram 21);
          • F: Foundations for piers, buttresses and chimneys should project as indicated in Diagram 22 and the projection X should never be less than the value of P where there is no local thickening of the wall.
           

          Minimum width of strip foundations

          2E3. The recommended minimum widths of foundations given in Table 10 may be used. *****Image Diagram 21 Elevation of stepped foundation *****Image Diagram 22 Piers and Chimneys *****Image Diagram 23 Foundation dimensions *****Image Table 10 Minimum width of strip footings The table is applicable only within the strict terms of the criteria described within it.

          Minimum depth of strip foundations

          2E4. Except where strip foundations are founded on rock, the strip foundations should have a minimum depth of 0.45m to their underside to avoid the action of frost. This depth, however, will commonly need to be increased in areas subject to long periods of frost or in order to transfer the loading onto satisfactory ground. In clay soils subject to volume change on drying (‘shrinkable clays’, with Modified Plasticity Index greater than or equal to 10%), strip foundations should be taken to a depth where anticipated ground movements will not impairthe stability of any part of the building taking due consideration of the influence of vegetation and trees on the ground. The depth to the underside of foundations on clay soils should not be less than 0.75m on low shrinkage clay soils, 0.9m on medium shrinkage clay soils and 1.0m on high shrinkage clay soils, although these depths may need to be increased in order to transfer the loading onto satisfactory ground, or where there are trees nearby

          • 1 Size and proportion of residential buildings of not more than three storeys Diagram 1 Size and proportion of residential buildings of not more than three storeys
          • 2 Size and proportion of non-residential buildings and annexes Diagram 2 Size and proportion of non-residential buildings and annexes
          • 3 Determination of wall thickness Diagram 3 Determination of wall thickness
          • 4 Parapet walls - height Diagram 4 Parapet walls - height
          • 5 Maximum floor area enclosed by structural walls Diagram 5 Maximum floor area enclosed by structural walls
          • 6 Fig 1-2 - Map showing wind speeds in m per s for maximum height of buildings Diagram 6 Fig 1-2 - Map showing wind speeds in m per s for maximum height of buildings
          • 6 Fig 3 - Map showing wind speeds in m per s for maximum height of buildings Diagram 6 Fig 3 - Map showing wind speeds in m per s for maximum height of buildings
          • 7 Max height of buildings Diagram 7 Max height of buildings
          • 8 Measuring storey and wall heights Diagram 8 Measuring storey and wall heights
          • 9 Declared compressive strenght of masonry units Diagram 9 Declared compressive strength of masonry units
          • 10 Maximum span of floors Diagram 10 Maximum span of floors
          • 11 Differences in ground levels Diagram 11 Differences in ground levels
          • 12 Openings in a buttressing wall Diagram 12 Openings in a buttressing wall
          • 13 Buttressing Diagram 13 Buttressing
          • 14 Sizes of openings and recesses Diagram 14 Sizes of openings and recesses
          • 15 Lateral support by floors Diagram 15 Lateral support by floors
          • 16 Lateral support at roof level Diagram 16 Lateral support at roof level
          • 17 Size and location of openings Diagram 17 Size and location of openings
          • 18 Wall thinckness Diagram 18 Wall thinckness
          • 19 Lateral restraint at roof level Diagram 19 Lateral restraint at roof level
          • 20 Proportions for masonty chimneys Diagram 20 Proportions for masonry chimneys
          • 21 Elevation of stepped foundation Diagram 21 Elevation of stepped foundation
          • 22 Piers and chimneys Diagram 22 Piers and chimneys
          • 23 Foundation dimensions Diagram 23 Foundation dimensions
          • 24 Area at risk of collapse in the event of an accident Diagram 24 Area at risk of collapse in the event of an accident
          • Table 1 Areas at risk from house longhorn beetle Table 1 Areas at risk from house longhorn beetle
          • Table 2 Wall types considered in this section Table 2 Wall types considered in this section
          • Table 3 Minimum thickness of certain external walls, compartment Table 3 Minimum thickness of certain external walls, compartment
          • Table 4 Imposed loads Table 4 Imposed loads
          • Table 5 Cavity wall ties Table 5 Cavity wall ties
          • Table 6 Declared compressive strength of masonry units complying with BS EN 771-1 TO -5 [N PER nn SQ] Table 6 Declared compressive strength of masonry units complying with BS EN 771-1 TO -5 (N/mm SQ)
          • Table 7 Normalised compressive strength of masonry units of clay and calcium silicate blocks complying with BS EN 771-1 and 2 [Nn/mm SQ] Table 7 Normalised compressive strength of masonry units of clay and calcium silicate blocks complying with BS EN 771-1 and 2 (N/mm SQ)
          • Table 8 Value of Factor X - see Dia 14 Table 8 Value of Factor 'X' (see Dia 14)
          • Table 9 Lateral support for walls Table 9 Lateral support for walls
          • Table 10 Minimum width of strip footings Table 10 Minimum width of strip footings
          • Table 11 Building consequence Table 11 Building consequence