Q.
a) Explain the factors to be considered in selecting a building foundation. (7 marks)
b) With the aid of a diagram, explain the characteristics of Strip and Pad foundation. (18 marks)
(25 marks, 2011 Q1)
A.
(a)
D06 2013 Q1
Earlier posts on this topic are referred.
The choice of the appropriate type of foundation is governed by some important factors such as
- The nature of the structure
- The loads exerted by the structure
- The subsoil characteristics
- The allotted cost of foundations
Therefore to decide about the type of foundation, subsoil exploration must be carried out. Then the soil characteristics within the affected zone below the building should be carefully evaluated. The allowable bearing capacity of the affected soil strata should then be estimated.
http://osp.mans.edu.eg/sfoundation/foundtion.htm
(b)
Please refer earlier post on Shallow Foundation - Strip, Pad and Raft foundation D06 2013 Q1 here, and D06 2012 Q1b here. (All type of foundation - see here: Explain the types of Building Foundations)
So, Strip and Pad foundations are shallow foundations.
Characteristics of Strip Foundation:
Strip foundations are used where the soil is of good bearing capacity. The key sizes of a strip foundation for concrete cavity wall construction and timber frame cavity wall construction are similar. The size and position of the strip is directly related to the overall width of the wall.
The principle design features of a strip foundation are based on the fact that the load is transmitted at 45 degrees from the base of the wall to the soil. The depth of a strip foundation must be equal to or greater than the overall width of the wall. The width of the foundation must be three times the width of the supported wall.
It is vital that the strip foundation is strengthened through the inclusion of steel reinforcement.
A compacted hardcore base of minimum 150mm is installed to form a platform for the subfloor and the subsequent loads of the dwelling.
The 150mm concrete subfloor is poured on the hardcore in order to provide a strong, smooth platform for the insulation.
A radon barrier is installed to form a continuous seal on the entire footprint of the house.
A Damp Proof Course is installed in order to repel any rising moisture. It is vital that the DPC is carried up into the blockwork to form a water tight seal over the entire floor area.
The DPC must run through the blockwork at a minimum of 150mm above finished ground level.
100mm of rigid insulation is installed below the finished floor to ensure that there is no heat lost through the foundation. 75mm concrete screed then provides the finished floor.
Ref:
http://constructionstudiesq1.weebly.com/strip-foundation.html
Characteristics of Pad Foundation:
Pad foundations are used to support individual or multiple columns, spreading the load to the ground below. They are generally square or rectangular in plan, with the plan area being determined by the permissible bearing pressure of the soil. The shape in plan will be dictated by the arrangement of the columns and the load to be transferred into the soil.
The thickness of the slab must be sufficient to ensure distribution of the load. The top of the pad may be sloping (i.e. the pad is thicker in the centre than it is at the edge). This is an economic solution, although there may be construction problems involved with casting the slope.
In simple cases the pad may be constructed from mass concrete. In general, some reinforcement (either welded steel fabric or reinforcing bars, depending on the loads involved) will be required in both directions. For design purposes, the pad is treated as if it were an inverted cantilever carrying the soil pressure and supported by the column.
Pad foundations are generally shallow foundations, but can be deep depending on the ground conditions. They are a form of spread foundation formed by rectangular, square, or sometimes circular concrete ‘pads’ that support localised single-point loads such as structural columns, groups of columns or framed structures. This load is then spread by the pad to the bearing layer of soil or rock below. Pad foundations can also be used to support ground beams.
They are generally of a uniform thickness, but sometimes the upper face may be sloped or stepped. Their plan shape will depend on the nature of the applied load and the allowable bearing capacity of the layers below. Their thickness must be sufficient to distribute the load across the plan shape. They are generally reinforced on all but the smallest structures, with the reinforcement allowing higher loads to be imposed and the construction of shallower pads which require less excavation and use less concrete.
The arrangement of pad foundations will vary depending on the nature of the structure they are supporting, the loads imposed, the allowable bearing capacity of the layers below and the space available on site. They may be:
- A series of discrete, well-separated pads.
- Balanced base pads that support more than one point load.
- Continuous pads, where there are a number of point loads close together.
- Pad and beam, where a series of pads support a continuous beam.
Pad foundations can be selected as they do not require much excavation, and are generally suitable where the bearing capacity of ground is sufficient at relatively low depths. However, they can be large in plan shape and may not be effective against differential settlement, uplift forces or wind forces.
Ref:
http://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=347
http://www.designingbuildings.co.uk/wiki/Pad_foundation