Presentation on theme: "Issues to be considered :foundations Professor Chris Gorse and Ian Dickinson These slides should be read in conjunction with Emmitt, S. and Gorse, C. (2010)"— Presentation transcript:
Issues to be considered :foundations Professor Chris Gorse and Ian Dickinson These slides should be read in conjunction with Emmitt, S. and Gorse, C. (2010) Barry’s Advanced Construction of Buildings. Oxford, Blackwell Publishing
Advanced Construction Technology By Professor Chris Gorse & Ian Dickinson – licensed under the Creative Commons Attribution – Non-Commercial – Share Alike License http://creativecommons.org/licenses/by-nc-sa/2.5/
Position of loads on foundations Loads should be positions centrally on foundations ensuring that loads are distributed evenly across the whole foundation area
Walls should be positioned centrally on the strip foundation Loads should be evenly distributed to avoid uneven or differential settlement Uneven distribution of loads may cause the foundation to crack If the foundation exerts different loads on the ground under the wall, the foundation will settle into the ground to different extents and the foundation and wall may crack.
Differential settlement due to different foundation systems If different foundations systems are used within the same building, the buildings may need to be separated to prevent cracking. Where different foundation systems are used buildings will move and settle differently. Where multiple foundation systems are used buildings should be treated as separate buildings and have movement joints.
Strip foundation and foundation design The following chart provides an extract from the Building Regulations. Look at the different types of soil. Stronger soils can take higher loads (rock, sand, gravel and stiff clay). Where soils are weaker (sandy soft clays) the width of the foundation has to be increased.
Minimum width of strip foundations – (Source, Approved Document A, 2000)
Distribution of compressive loads Un-reinforced concrete foundations have good compressive loads but poor tensile strength. Compressive loads are distributed through the concrete at an angle of approximately 45 o.
Avoiding shear failure Strip foundation P T P should be less than depth T to avoid shear Load distribution - angle 45 o
Forces outside the compressive zone As the compressive force can only be contained within a 45 degree angle, un-reinforced foundations that are wider than that accommodated within the compressive zones will suffer shear failure. If the width of the foundation is too wide and not sufficiently deep tensile forces will be exerted on the foundation. Concrete is poor in tension and will crack.
Shear failure Strip foundation Shear failure angle 45 o P T If P is greater than T then the foundation may shear at 45 o reducing the width of the foundation and bearing area. Following the shear failure the load is concentrated on a smaller area, the ground may consolidate under the increased load. The foundation fails where tension is exerted on the concrete
Stepping foundations To ensure that the thickness of a foundation is not reduced when it is stepped up or down a hill guidance is given by the Building Regulations. A minimum overlap of 300mm of the foundations thickness is required (see the next slide).
Minimum thickness of stepped strip foundations Minimum overlap L = twice height of step or 300mm whichever is greater L T S The step (S) should not be greater than the foundation depth (T).
Stepping foundations to tie in with brick or block courses When stepping foundations it is useful to consider how brick and block courses will be accommodated on the foundations. (see the following guidance)
Increasing the depth of foundations It is quite common to experience problems with high level sub-strata. Often the upper levels of strata are inconsistent, soft, weak or have been built-up over time (made-up ground). To ensure good load-bearing strata is used as the founding material the depth of the foundation is increased. Where foundations are deep they may be made from mass fill concrete or the difference in levels can be accommodated by blockwork (see the next slide).
Foundation depths - seasonal variation Foundations need to constructed sufficiently deep so that they are not affected by moisture movement. Clay soils expand and shrink due to changes in moisture content. Foundations must be constructed on ground which is below the level that is subject to changes in moisture content due to changes in the weather. Apart from extreme conditions the moisture content in the ground is relatively consistent below 750mm (even in summer and winter).
Foundation depths The expansion of frozen water within the soil can cause foundations and buildings to lift and crack. Foundations must be constructed at a depth where the water is not subject to freezing. Apart from extreme climates water within the ground will not freeze below 450mm.
Foundation depths The depth of the foundation will depend on the type of soil, distance from the tree and water demand of the tree. Water demand is dependant on the height and type of tree. E.g. for a 20m high Oak tree in high shrinkage soil, 10m from the face of the foundation, the foundation should be at least 2.50m deep To avoid frost heave foundations should be constructed at least 450mm below the surface. Shrinkable soils (Clay) should be at least 750mm deep for clays with a low potential shrinkage and 1m where there is high potential shrinkage. Min. 450 mm Min. 750 mm High shrinkage soils close to mature tree 1m – 3.43m deep
Soil movement Where soil is susceptible to movement, floors and the sides of foundations may need to be protected so that the soil does not exert pressure on the floors and foundations. If lifting pressure is exerted on floors it may cause them to crack. Pressure on the side of foundations may cause them to shift out of line. Compressible material can be used to protect foundations and floor slabs.
Precautions against heave Where the ground is susceptible to heave precautions should be taken. Voids should be left below floor slabs and compressible material should provide a barrier between the soil and foundations Compressible material or void former to the inside face and underside of the ground beam, this prevents the beam lifting or moving when the clay expands Void 125 –300mm Backfill Slip liner can be used around pile
Precaution against heave Where the ground is susceptible to heave precautions should be taken. Voids should be left below floor slabs and compressible material should provide a barrier between the soil and foundations Where trench fill is greater than 1.5m deep a compressible material or void former should be used against the inside face of the foundation, positioned in accordance with manufactures instructions Prevents the foundation being pushed outwards Void 125 –300mm Backfill 500mm
High ground floor levels Where the ground floor of a building is higher than the ground level the material contained within the building will impose a load on the internal face of the wall. The Building regulations suggest that the height of a ground floor should be limited to 4 times the thickness of the external wall (see the next slide). Where the wall is higher than this design calculations would need to be provided.
Force of hardcore on external walls Maximum combined dead and imposed load should not exceed 70 kN/m at the base of the wall (Building Regulations 2000, A1/2) H must not be greater than 4 times the thickness of the wall. Where there is no concrete fill in the cavity the thickness should not included the cavity T Max H = 4 x T 70 kN/m T Max H = 4 x T 70 kN/m Structural floor level Compacted fill
Altering foundations to suit cohesive and non cohesive soils Different ground conditions deal with loads differently. Due to the way soils move and displace the contact pressure and loads are dealt with differently. In non cohesive soils extra pressure should be exerted towards the edge of the structure In cohesive soils extra pressure should be concentrated on the centre of the structure
Further information supporting these slides can be found in the following publication and websites. Emmitt, S. and Gorse, C. (2010) Barry’s Introduction to Construction of Buildings. Oxford, Blackwell Publishing Emmitt, S. and Gorse, C. (2010) Barry’s Advanced Construction of Buildings. Oxford, Blackwell Publishing Virtual Site (2010) Virtual Site at Leeds Met University http://www.leedsmet.ac.uk/teaching/vsite http://www.leedsmet.ac.uk/teaching/vsite Virtual Site Gallery (2010) Virtual Site Gallery at Leeds Met University http://www.leedsmet.ac.uk/teaching/vsite/gallery http://www.leedsmet.ac.uk/teaching/vsite/gallery