Pile Cap Design - what is the Pile One-Way Shear check and how is the pile effective perimeter distance PEPD calculated?

Environment

Not environment-specific

Pile one-way shear should not be confused with the ‘beam shear’ check which considers critical sections extending across the full width of the pile cap (see for example BS 8110-1 1997 Cl 3.11.4.3). The details of this check are given under the heading "Shear Capacity" in the Pile Cap design details.

The Pile one-way shear check is in addition to this and is always performed. It checks each individual pile and the shear arising from load spanning between the pile directly to the column. Only the US ACI 318 code makes specific mention of a need for this sort of check, and only in relation to corner piles. Neither British Standards nor the Eurocode explicitly mention it.

However, design codes and guides generally only consider very simple, symmetrical arrangments of piles, while in Tekla Structural Designer piles can be placed in practically any pattern. Hence it has been decided by our Structural Designer Development Engineers that it is prudent and logical that the one-way shear check be made for all design codes and all piles, not just corner piles.

The check requires an effective pile support point - termed P_ESP - at which the pile load is considered to act, and an effective perimeter distance - termed P_EPD. P_EPD is taken as the shortest distance from P_ESP (the pile effective support point) directly to the perimeter of the column which the pile cap supports.

For ACI 318 P_ESP is taken as the pile centre. For the British Standards and Eurocode reference has been made to Figure 3.23 of BS 8110-1 1997 which gives a critical section located 20% of the pile diameter inside the face of the pile. While the figure is not found in the Eurocode, it is given in the Concrete Centre Guide: “How to Design Concrete Structures using Eurocode 2” - CCIP-006 - see Figure 6, Piled Foundations pg 48. This figure is reproduced from the guide below.

Following this figure and its associated clauses, P_ESP is taken to be (0.5-0.2)φ = 0.3*φ from the pile centre in the direction of P_EPD (the pile side length is used for square piles in place of φ).

The check also requires an effective width - perpendicular to P_ESP - over which the pile load is considered to act. It is logical that this distance should be finite and have a limit, but this is not specifically addressed in all codes. A distance of 3*φ is used for this limit, principally following BS 8110-1 1997 Cl 3.11.4.4 and also considering what other codes suggest. This can be seen in the equation for the cross-section breadth given in the design details for the one-way check:

Again the pile side length is used for square piles in place of φ. The other terms in the equation for the cross-section breadth cater for the corner pile condition illustrated in the picture below.

An example of the calculation of P_ESP and P_EPD is given below for a pile cap with 4 number 400mmx400mm square piles supporting a 500mmx500mm square column in the arrangement shown. For this geometry P_EPD = 375.0 mm.