vR4 y

v1.6 y

Rsk Rbk (857 A

Design resistance is Rcd

Ys Yb

Verification of compression resistance v509 y kN

Ys Yb

Verification of compression resistance

© Correlation factors are applied to the total derived pile resistance to identify whether the minimum or the mean gives the most critical result. In this case the minimum value governs.

© Once it has been established that the minimum is critical, the base and shaft components need to be identified so that the different shaft and base partial factors for CFA piles may be applied.

© For DA1, DA1-2 governs and suggests that the piles may be slightly over-designed. DA2 and DA3 yield similar levels of utilization.

© DA2 provides small partial factors to both the shaft and the base at the same time as applying partial factors to the actions. The net result is to produce a slightly more onerous condition than DA1-1.

© The UK National Annex (NA) provides more onerous correlation factors than Annex A of EN 1997-1. The minimum derived value of resistance still governs the design.

© The UK NA values for partial resistance factors for CFA piles are larger than those given in Annex A of EN 1997-1.

® Based on the UK NA, the design just exceeds the requirements of BS EN 1997-1 but at 101% utilization would probably be considered satisfactory.

13.14.5 Designing to a set with a pile driving formula

Example 13.5 looks at how driven concrete piles may be designed according to Eurocode 7 to a specified 'set' with a commonly-used pile driving formula.

A new five-storey teaching block is needed to improve facilities at an existing school. Details of the design of driven piles for an existing school building (including the results of static load tests) are available. It is proposed to found the new teaching block on similar driven piles. Measurements of pile set will be used to demonstrate that the piles have adequate resistance and settlement performance.

The Hiley formula20 was used for the previous construction and suitably calibrated against the pile tests.21 In this example, 300mm-square x12m-long concrete piles will form a 16-pile group. Design Approaches 1 and 2 have been considered (Design Approach 3 is not suitable for use with driving formulae). The effects of using the revised correlation factors given in the UK National Annex to BS EN 1997-1 are also investigated.

Example 13.5 Designing to a set with a pile driving formula Verification of strength (limit state GEO)

Design situation

Consider the design of piles for a new 5-storey teaching block at a school. Ground conditions have been established from a ground investigation and are similar to those under the existing school buildings. These buildings are founded on driven concrete piles which were originally designed using the Hiley pile driving formula, and whose capacity was confirmed by static load tests.

The new building will be founded on B = 300mm square piles, L = 12m long. A total of N = 16 piles will be used to carry characteristic permanent and variable actions Fgk = 600kN and Fqk = 200kN.

A drop hammer of weight W = 50kN, drop height h = 500mm, and efficiency e = 90% will be used to drive the concrete piles to a maximum set of S = 6mm per blow. The total temporary compression of the pile and ground is estimated to be C = 12 mm. ©

Design Approach 1

Actions and effects Ignore the self-weight of pile

Characteristic total action Fck = Fgk + FQk = 800 kN

Partial factors from Sets vA2 j

Design total action per pile is Fcd = Yg Fgk + Yq FQk =

V860 j kN

Measured resistance

The energy transferred from the drop hammer to each pile is:

E = Wx h x e = 22500 kN mm© Using Hiley's formula to calculate the resistance of the pile:

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