Challenge 10

Aim:-

Que (1):- Explain the design codal provisions for pile cap of metro viaducts in detail.

Answer:-

A reinforced concrete slab or block which interconnects a group of piles and acts as a medium to transmit the load from wall or column to the Piles is called a Pile Cap. The Pile cap should normally be rigid so as to distribute the forces equally on the piles of a group. In general it is designed like a footing on soil but with the difference that instead of uniform reaction from the soil, the reactions in this case are concentrated either point loads or distributed.

As per IS 2911 (Part I/ Sec 3) -2010, the pile cap may be designed by assuming that the load from column is dispersed at 45˚ from the top of the cap up to the mid depth of the pile cap from the base of the column or pedestal. The reaction from piles may also be taken to be distributed at 45˚ from the edge of the pile, up to the mid depth of the pile cap. On this basis the maximum bending moment and shear forces should be worked out at critical sections.

DESIGN PARAMETERS OF PILE CAPS:-
(i) Shape of pile cap.
(ii) Depth of pile cap.
(iii) Amount of steel to be provided.
(iv) Arrangement of reinforcement.

• Shape of pile cap

Whittle and Beattie have developed through computer program the relationship between dimension of pile cap and the size of the pile. The minimum spacing of piles permitted from soil mechanics depends on the type and end conditions. CP 2004 requires a minimum centre- to –centre spacing of twice the diameter of the piles for end bearing and three times the diameter for friction piles. IS 2911 part1, sections 1 and 2 recommended a minimum spacing of two and half times the diameter of the pile for both driven cast in situ and bored cast in situ piles. For accommodating deviations in driving of piles, the size of the pile cap is made 300 mm more than the outer- to outer distance of the exterior piles. (150 mm on either side). The plan dimension of the pile cap is based on the fact that the actual final position of piles can be in construction up to 100 mm out of line from the theoretical centre lines. Pile caps should be made very large to accommodate these deviations. In practice, pile caps are extended as much as 150 mm beyond the outer face of the piles.

Standard Pile Caps:
s-spacing of pile = F x hp where
hp = diameter of pile in mm
F = spacing factor= centre to centre spacing
Pile diameter

• Depth of pile cap

The thickness of the Pile Cap is fixed such that it is adequate to resist shear without shear reinforcement and the bars projecting from the piles and the dowel bars for the column can be provided adequate bond length. As per IS 456- 2000, the minimum thickness on top of piles should not be less than 300 mm. Pile cap depth should be kept on the high side to effect economy in the consumption of steel and also to provide adequate rigidity to pile cap. Generally, pile cap thickness should not be less than 500 mm which may be reduced to 300 mm at the free edges. For pile caps to be rigid, pile cap has to be quite deep with 600 mm as the minimum depth. As a guide line the formula given in Reinforced concrete by Reynolds may be followed.
For Pile dia > 550 mm,
Pile cap depth (h) = (2 hp + 100)mm
For Pile dia ≥ 550 mm,
h= ⅓ x( 8 hp + 600) mm

• Amount of steel to be provide

The Pile Cap has to be designed either truss theory or beam theory. Although, the pile caps are assumed to act as a simply supported beam and are designed for the usual condition of bending and shear, their tendency is to fail by bursting due to high principal tension and they will therefore always require a cage of reinforcement in three dimensions to resist this tendency. The main reinforcement is usually bend (full bend) and extended for full depth of pile cap to fulfill the check for development length. Though IS 456-2000 is silent on specifying the minimum reinforcement, a minimum reinforcement of 0.15 % BD for main reinforcement and 0.12 % BD for secondary reinforcement may be provided as per clause 3 .11.4.1 and 2 of CP 110 code). For bursting (horizontal binders) it is suggested that 25 % of the main reinforcement (usually 12 Φ RTS at 150 mm c/c) shall be used. Cover :- A cover of 75 mm is usually provided for the pile cap surfaces in contact with earth and 60 mm against blinding concrete of 75 to 100 mm thick. In marine situations the cover should be increased to a minimum of 80 mm.

Practical Aspects on Pile cap Design:

The structural design of a pile cap is similar to the design of spread footing. The load acting on the pile cap from the superstructure and piles are resisted by the developments of bending moment and shear force in the pile cap. Codal provisions made in IS 2911(Part 1/sec3)-2010 :
1. The size of the pile cap is fixed in such way that it has clear overhang beyond the outermost pile not less than 100mm, but preferably 150mm.
2. It should be deep enough to allow the necessary overlap of reinforcements from column and piles.
3. The clear cover to the main reinforcement should not be less than 40mm.
4. The span to thickness ratio of the cap should not be more than 5 so that pile cap is rigid enough to distribute the load uniformly to the piles.
5. Generally, its thickness should not be less than 500mm which may be reduced to 300mm at the free edges.
6. The piles should atleast 50mm into the pile cap.
7. A leveling course of not less 75mm thick concrete should be provided under the pile cap.

Design Aspects :

The reaction from the piles under the concentric axial load on the cap is assumed equal and is determined by,
Pp = Q/n ----- (1)
where Q = concentric axial load on the cap
n = Number of Piles

When the Pile cap is eccentrically loaded or subjected to a load and moments then the reactions from the Piles are determined as
Pp = Q/n +/- My x +/- Mx y ----- (2) where
∑x2 ∑y2
Mx, My = moments with respect to x and y axes.
X, y = distances from y and x axes to the Piles.

The critical section for bending moments and bond shall be calculated at the face of column or pedestal. The critical section for two way shear (Punching shear) will be at a distance d/2 from face of column or pedestal.
One way shear is checked at a distance of d/2 from the face of the column. The Clause 34.2.4.2 of IS 456 – 2000 states the following :- “In computing the external shear or any section through a footing supported on Piles, the entire reaction from any pile of diameter Dp whose centre is located Dp/2 or more outside the section shall be assumed as producing shear on the section; the reaction from any Pile whose centre is located Dp/2 or more inside the section shall be assumed as producing no shear on the section. For intermediate positions of the pile centre, the position of pile reaction to be assumed as producing shear on the section shall be based on straight line interpolation between full value at Dp/2 outside the section and zero value at Dp/2 inside the section.” In computing external shear on any section the entire (100%) reaction of the Pile shall be taken if the pile centre is located at 150 mm or more outside the section. The pile reaction will produce no shear (0%) if the pile centre is located at 150 mm or more inside the section. A linear interpolation shall be made for intermediate values of the pile centre. Let the centre of the pile be located at ‘x’ from the face of the column. Let ‘d’ be the effective depth of the pile cap. Then the critical section is located at d/2 from face of the column.
If pile centre is located at (d/2 –x) outside the critical section when x
< d/2.
If x > d/2, the expression (d/2-x) yields negative value indicating that
the pile centre is located at (x-d/2) inside the section. When
(d/2-x), outside is true for other case.
Let the fraction of pile reaction inducing shear be f R where R is the pile
reaction.
Rule for checking one way shear,
f = 150 +(x-d/2)
300
where x and d are in millimeters

Que (2):- Explain the design philosophies involved in the design of pile cap

Answer:-

The pile cap is designed by two methods first is the bending theory and the struct and tie theory

when the distance of pile is large then there is a possibility of bending of pile cap therefore pile cap behaves like a beam it needs to be designed for shear flexure affects the distance between the pile cap is kept as 2D to 4D

• Bending Theory

The pile can be designed by bending theory for shear & flexure effects if the distance between piles is large

If distance spacing between piles becomes 4D so we need to design the pile cap by bending theory because the load flow from the base can not flow at 45 degrees is the maximum angle from one point of the beam to the support

The action of the whole pile cap as a beam is easy to imagine. When the shear span/depth of (av/d) is 2 or more, the bending action is more predominant than the truss action. Thus, the tensile reinforcement at the bottom acts like the tension reinforcement in an ordinary beam. Then the pile caps are designed by the beam theory.

In some cases, the design by beam theory may require less steel than that by truss theory. It should be, however, be remembered that the realization of these different actions is not so much for the determination of the amount of steel and the savings that can be done in steel quantity as for the appreciation of the real behavior of the pile cap in the field and for the method of dealing of steel in pile caps. The necessity of anchoring the main steel at their ends should be fully appreciated when conditions are favorable for truss action. With truss action, the ends of the beams should be given full Anchorage by providing the full development length by suitable bending of the steel inside the pile cap.

It should also be remembered that, theoretically, one can assume higher allowable Anchorage bond stresses on top of the pile than specified for beams due to the effect of the compression present at the junction of the pile head and the pile cap.

• Strut and Tie Theory

Pile cap is designed by strut and tie theory if the distance between piles is small and force flows from the pier bottom to the pile top mainly through tension and compression of the pile cap For 1.5 D the load from the bottom directly flows through the compression in the pile cap

Consider the 4 Pile group where each pile is at the corner of the pile cap and the distance is kept as 2D, in that case, the load flow from the pier bottom to the pile by pure compression, and some tension is at the pile bottom this load flow is in tention and compression mechanism and also called as strut and tie theory in such case pile cap is design by strut and tie theory, for strut and tie theory we do not need to provide a shear link.

In strut and tie theory, it has usually been the practice to band the reinforcement along the lines joining the piles. The code now suggests that this method of banding is only necessary if the piles are spaced at more than 3 times the pile dia. For the more normal spacing of 3 times the pile dia the total reinforcement forming the tie force in one direction can be distributed uniformly across the cap with a three- pile cap designed on the truss theory, it is difficult to see how this can be done and it is suggested that the reinforcement is banded along the centre lines joining the piles.