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tab, and are as follows:Pile Cap Design Methodologies
The pile cap design module provides two methods for the structural design of the pile cap: ‘beam theory’ and the ‘strut and tie’ method. The designer can specify which method is used directly but can also set options to select either the minimum or maximum design results from each method.
The design method can be set under the Design Method tab. The method set will be reflected in the design calculations.
In Beam Theory, the pile cap is treated as a standard concrete beam for the purposes of the design for both shear and bending and designed in accordance with Sections 6.1, 6.2 and 6.4 of BS EN 1992-1-1:2004. However, beam theory is based on the assumptions implicit in Euler-Bernoulli beam theory; in particular, the assumption that plane sections remain plane. This requires that the stress distribution due to bending at any cross section is linearly varying.
The assumption of a linearly varying stress distribution is valid when the depth of the beam is small compared to the span, but as the span/depth ratio decreases, the effect of shear deformation becomes increasingly significant, and the assumption of a linearly varying stress distribution becomes increasingly inaccurate. For span/depth ratios between 3 and 5, the use of the beam theory is considered to be somewhat questionable. For span to depth ratios less than this, the published design guidance advises the use of the strut and tie method.
The strut and tie method is based on the idealisation of the structure into a truss form, whereby the zones of compressive and tensile forces in the concrete are idealised as prismatic members, similar to the members of a truss. The general idea is to consider the path or flow of the stresses within the reinforced concrete structural element and model strut or tie members in these zones.
While structures can be conceptualised as consisting of Bernoulli (B) regions and Discontinuous (D) regions, where the strut and tie method is used for D-regions while traditional beam design used for the B-regions, in the case of a pile cap, given the overall dimensions and lengths of the struts involved, the strut design assumes a prismatic element of constant cross section only and so designs the strut and ties as D regions throughout.
The components of the strut and tie design are:
Struts: struts are members that represent the zones of compressive stress within the pile cap. The strength of the struts is determined by the effective compressive strength of the concrete. Struts are taken to be unreinforced for the purposes of determining their axial capacity. In the pile cap module, compression struts are taken to be the same cross section as the node dimensions, that is, the strut cross section is based on the area of the pile (which determines the area of the node). In the pile cap module, struts are taken to be of constant width – bottle or fan shaped struts are not used.
Ties: ties are tension only members and their capacity is based on the tensile capacity of reinforcement. The reinforcement is required to have a suitable anchorage to ensure that it can develop the required tensile capacity. In the pile cap module, the ties are formed by the bottom reinforcement bars within the pile cap.
Nodes: nodes are points where either struts change direction, or where struts and ties connect. In the pile cap design, the arrangement used for the struts means struts do not change direction, and so the only nodes considered occur at the junction of struts and ties.
Where the span/depth ratio is less than 3, the pile cap can be conceived of as a beam and the standard design method for bending applied, in accordance with the required design code. The design bending moments are determined based on the pile reactions and the lever arm to the face of the column in two orthogonal directions to determine the maximum Mxx and Myy bending moments. The pile cap is then designed in each direction as a rectangular cross section in bending, with the bottom bars in tension.
The shear capacity verification on the pile cap checks the punching shear at the face of the column, and a punching shear perimeter located at 2*deff (where deff is the average effective depth for the reinforcement in both the x- and y-axis directions) from the column face, limited to a perimeter taken to lie 20% of the pile diameter from the inside ‘face’ of the piles. Vertical ‘beam’ shear is also checked for the critical section located at 20% of the pile diameter inside the inner pile face. For both the beam shear and punching perimeter checks, shear enhancement is considered, where applicable.
Punching and beam shear checks are used for both the beam and strut and tie design methods.
The pile cap module provides a number of options to control the design method used for the design of the pile cap. These options are found under the tab, and are as follows:
Beam Theory – the pile cap is designed as a rectangular concrete beam for bending.
Strut and Tie method – the pile cap is designed using the truss analogy using tension and compression elements.
Min of Beam & Truss – the pile cap design is carried out using both the Strut and Tie method and the beam method and the smaller reinforcement area required from each method is used.
Max of Beam & Truss – the pile cap design is carried out using both the Strut and Tie method and the beam method and the larger reinforcement area required from each method is used.
There are also three further options to control the design:-
Design Bending for Net ULS loads – the pile loads and bending moments in the pile cap are based on the applied column loadings only, no adjustment is made for the self-weight of the pile cap. This option applies to the beam theory only.
Design truss compression zone – for the strut and tie design to BS 8110 and SABS 0100, the design will use the check for a CCT node taken from the EC2 design code.
Deep Beam Theory – for use with the beam theory method. The design will treat the pile cap as a deep beam, using the method presented in the Concrete Designers’ Manual, Reynolds & Steedman, 10th Edition. This provides an alternative design method for deep beams as an alternative to the strut and tie method.
A design brief represents the design for a specific pile cap. Since a project will likely require multiple pile cap layouts, multiple design briefs can be added to a design file. Since a specific design methodology may not apply to all pile caps, the design method can be selected for each design brief independently.
The strut and tie design requires the pile cap to be idealised into a series of strut and tie members and the forces in these members to be determined. To achieve this, formulae have been developed that will calculate the necessary forces in the strut and tie elements, based on the geometry of the pile cap. However, this places the following constraints on the design:
1.Compression only piles – tension piles would require alternative strut and tie arrangements to account for distribution of the compression and tension forces within the pile cap.
2.Standard pile arrangements – the pile arrangements need to be symmetrical. Non-symmetrical pile arrangements would require a more detailed analysis of the stresses in the concrete to determine an appropriate strut and tie arrangement.