Challenge 10

1. Explain the design codal provisions for pile cap of metro viaducts in detail.

Ans:

• Pile Cap
  • Pile cap, which is a thick, square structure made of concrete that rests at top of the pile , provides the system’s foundation with stability.
  • The Pile cap should normally be rigid so as to distribute the forces equally on the piles of a group.
  • The total weight of the superstructure is transferred from one to another pile cap, results a reliable bearing capacity.
• Importance of Pile cap
  • When the soil beneath the structure cannot support a spread footing, the load is transferred to deeper, firmer strata using piles. Piles may also be used to move the load below the level of such influence in areas where seasonal variations significantly impact the soil.
  • The capacity of a pile to support loads is equal to the product of its end-bearing capacity and its capacity to reduce skin friction with the earth around it.
  • A series of equal piles are typically utilised when the load that must be reinforced surpasses the bearing capability of an individual pile.
  • To disperse the load among the group piles, the group is capped by a spread footing or a cap.
• Note -: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 structure of pile cap
  • The pile cap looks like a mat that rests over the pile cluster. According to IS 2911-2010, the pile cap is kept in such a way that the column load is broadened out at a 45° angle from the apex of the cap.
  • The pile cap needs to be firmly fixed.
  • Even though piles are only slightly elastic columns, some deformations will still be present, and that must be measure properly.
  • To protect against the winding that can occur in a pile, pile heads should pivot along with the pile cap.
• Shape & Size of the Pile cap
  • The shape and plan dimensions of the pile cap depends on the number of piles in the group and the spacing between each pile.
  • These pile cap shapes minimize the plan area for a symmetrical arrangement of piles about the load. The pile cap should overhang the outer piles by at least 150mm but should not be excessive, generally not more than the diameter of the pile diameter.
• Depth of pile cap
  • There are various factors which governs for the selection of depth of pile cap. Such as
    • Shrinkage of clay
    • The pile cap’s shear capacity
    • Pile anchorage
    • A natural occurrence such as a frost attack
    • The water table of the ground surface.
  • Among them shear is the most critical criteria for selection of depth of pile cap. Generally, a pile cap is much deeper than a pad footing of similar dimensions, because it is subjected to much higher concentrated reactions, and consequently to much higher BM & SF. However, the increased depth gives the pile cap greater rigidity which is advantageous in spreading the load evenly to all piles.
  • As per general practice the depth of the pile cap is being provided as 1.5 times of the dia of the pile.
• Pile cap Design
  • 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.

2. Explain the design philosophies involved in the design of pile cap.

Ans:

• 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.

• 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.

  • Where the distance between the piles is kept less due to some contraint, 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. In this type of method shear links is not being provided.

  • 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.

• 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 the pile cap need to be design 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
  • As the span to depth ratio is more than 2, the structure is to be desgined for bending and shear.
  • In some cases, the design by beam theory may require less steel than that by truss theory. But this diffrence insteel quantity is not so much remarkable.
  • 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.