This subclause covers the design of pad footings and pile caps.
4.10.2 Moments and forces in bases
4.10.2.1 Except where the reactions to the applied design ultimate loads and moments are derived by more accurate methods, e.g. an elastic analysis of a pile group or the application of established principles of soil mechanics, make the following assumptions:
a) when the base is axially loaded, assume the reactions to ultimate design loads to be uniformly distributed per unit area or per pile; and
b) when the base is eccentrically loaded, assume the reactions to vary linearly across the base or across the pile system.
4.10.2.2 The critical section for bending moment in the design of an isolated base may be taken at the face of the column or wall.
4.10.2.3 The design moment on a vertical section passing completely across a base should be taken as the moment due to reactions to all design ultimate loads on one side of this section. No redistribution of moments should be made.
4.10.2.4 When the flexural and shear strengths of sections are being calculated, account should be taken of pockets for precast members unless they are to be subsequently grouted with a cement mortar of compressive strength at least equal to that of the concrete in the base.
4.10.2.5 When the resistance to bending is being calculated, bases may be regarded as beams or solid slabs, as appropriate.
4.10.3 Design of pad footings 4.10.3.1 Design moments and forces See 4.10.2.
4.10.3.2 Distribution of reinforcement
The reinforcement considered in this subclause is that at right angles to the concrete section. The reinforcement required in the shorter cross-section of a rectangular base should be placed evenly across the section. If any reinforcement is required in the longer section of a rectangular base in order to resist the bending moment, it should be distributed as follows:
a) the amount equal to As 2 of reinforcement should be spread over a band centred on the ß1 % 1
column or support and of width equal to the dimension of the short side of the base;
As is the total area of reinforcement required and β1 is the ratio of the longer to the shorter side.
b) the remaining reinforcement should be spread evenly over the outer parts of the section.
Where there are two or more columns and lc is the greater of half the spacing between them or the distance to the edge of the pad, then the following should be considered:
When lc exceeds (3c/4 + 9d/4), where c is the column width and d is the effective depth of a pad footing, two-thirds of the required reinforcement should be concentrated within a zone from the centre-line of the column to a distance 1,5d from the face of the column; otherwise the reinforcement should be uniformly distributed over lc.
4.10.3.3 Shear
4.10.3.3.1 The design shear force is the algebraic sum of all the ultimate vertical loads and reactions acting on one side or outside the periphery of the critical section.
4.10.3.3.2 The shear strength of bases in the vicinity of concentrated loads or reactions is governed by the more severe of the following two conditions:
a) shear along a vertical section that extends across the full width of the base (for pad footings, this section may be considered at 1,5 times the effective depth from the face of the loaded area and the provisions given in 4.3.4.1 will apply); and
b) punching shear around the loaded area, where the provisions given in 4.4.5.2 will apply.
4.10.3.4 Bond and anchorage
The provisions given in 4.11.6 apply to reinforcement in bases.
The critical sections for local bond stress are a) the critical sections described in 4.11.6, and
b) sections at which the depth changes or any reinforcement ends.
4.10.3.5 Limit state of deflection This limit state may be ignored for bases.
4.10.3.6 Crack control in bases
The provisions given in 4.11.8.2 concerning the maximum distance between bars in tension apply to bases, but reinforcement need not be provided in the side of bases to control cracking.
4.10.4 Design of pile caps
4.10.4.1 General
Pile caps are designed either by the bending theory or by truss analogy; if the latter is used, the truss should be of triangulated form, with a node at the centre of the loaded area.
The lower nodes of the truss lie at the intersections of the centre-lines of the piles with the tensile reinforcement.
4.10.4.2 Shear forces
The design shear strength of a pile cap is normally determined by the shear along a vertical cross-section of the full width of the cap. Critical sections for the shear should be assumed to be located at 20 % of the diameter of the pile inside the face of the pile, as indicated in figure 23. The whole of the force from the piles with centres lying outside this line should be considered to be applied outside this line.
4.10.4.3 Design shear resistance
The design shear resistance of pile caps may be determined in accordance with 4.4.5, subject to the |
limitations given below. Amdt 1, Apr. 1994 |
4.10.4.3.1 Where the spacing of the piles is less than or equal to 3 pile diameters, the enhancement of the shear strength may be applied over the whole of the critical section.
Where the spacing is greater, the enhancement may only be applied to strips of width equal to 3 pile diameters, centred on each pile. Minimum stirrups are not required in pile caps where v < vc (enhanced if appropriate).
4.10.4.3.2 The tension reinforcement should be provided with a full anchorage, in accordance with 4.11.6.
Figure 23 — Critical section of shear check in a pile cap
4.10.4.4 Punching shear
The design shear stress calculated at the perimeter of the column should not exceed the maximum value of shear stress (see 4.3.4.1).
In addition, if the spacing of the piles exceeds 3 pile diameters, punching shear should be checked in accordance with 4.6.2 on a perimeter as indicated in figure 23.
The maximum shear capacity may also be limited by the provisions of 4.4.5.2.6. Amdt 1, Apr. 1994
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