Design example on a pile foundation

In this section e shall outline the design of a pile foundation for a typical column of the building for which the seismic designs were carried out in earlier chapters. f course in reality the design of pile foundations ill be carried out for individual columns with the associated reductions in the pile lengths and/or pile diameters to suit the design load on the column. Here we shall only consider one typical column along the D line on the plan of the building.

Another premise that is made here is the requirement of the pile foundations. It is assumed that the building will be located at 'Site A for economic and operational reasons.

9.7.1 Configuration of the problem

In Chapter 8, the EC 8 Part 5 provisions were used to determine the liquefaction potential of 'Site A. The soil profile at this site as determined from borehole data is presented in Figure 9.15. Based on this it was determined that this site has:

• A non-liquefiable clay crust of 2 m thickness close to ground.

• Liquefaction potential analysis confirms that a 10 m thick layer of loose sand underlying the clay layer is 'liquefiable' during the design earthquake event.

Borehole Data For Pile Foundation
Figure 9.15 Borehole data from Site A

Table 9.5 Loading on the foundation from the columns

Column C Column D Axial load 5978 kN 862 kN Shear load 826 kN 826 kN

Moment load 2405 kNm 2088 kNm

The above ground conditions at this site would necessitate the requirement of pile foundations. he pile foundations ould be required to pass through the loose sand layer and end bearing fully into the dense sand layer.

9.7.2 Structural loading on piles

In Chapter 3 the structural analysis of the building frame is considered. Here we use the loading obtained from those analyses (using q factor of 3.9 and choosing the concrete frame building that has the more severe loading case). These loads are obtained with due consideration to the capacity design aspects and are shown in Table 9.5. Please note that the worst loading occurs on columns along the lines C and D, each line reaching a maximum load hile the other is at a iniu.

herefore the loading on the pile group is: Design vertical load NEd = 5978 kN Design moment load MEd = 2505kNm Design horizontal, shear load VEd = 826 kN

Based on the above requirements, the following will be assumed regarding the pile foundations. Choose:

• 2 X 2 pile group for columns along the D line

• steel tubular driven pile

• pile diameter 800 mm; pile wall thickness 20 mm

• pile group efficiency n = 70 per cent (conservatively).

Various other pile types can be considered for this application, such as concrete bored piles, precast concrete driven piles or steel H-piles for example.

9.7.3 Static pile design

The piles are required to be designed according the provisions of EC7. Here the UK National Annex provisions are also taken into consideration.

9.7.3.1 Assumptions and simplifications

Assume pile density is equal to soil density.

Assume moment on group is carried by couple in piles.

Individual axial pile load, QA, is given by:

Ignore shaft friction from upper clay layer.

Assume pile is plugged and can develop full end bearing capacity.

9.7.4 Axial pile design

Use BS EN 1997 Design Approach DA-1.

Two combinations must be considered. In Combination 1, partial factors are applied to the pile loading. In Combination 2, partial factors are applied to components of the pile resistance. Note: refer to the UK National Annex for appropriate partial factors for pile design.

9.7.4.1 Combination 1

Partial factor sets A1 + Ml + R1 apply.

From A1 adopt factor gG = 1.35. (Note: this is a simplification. Separate factors apply to permanent and transient loads.)

For M1 all material factors gM = 1.

For R1 all resistance factors gR = 1.

Note: a model factor, Mp, is also required. From the UK National Annex the model factor is 1.4 if the pile has been designed from soil test data alone. If the pile capacity has been verified using a maintained load test the model factor is 1.2.

BS EN 1997 is not prescriptive concerning the method of calculating the pile capacity, only requiring that the ethod should be one that is verified against pile load test data.

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  • Filmon
    How to design pile capacity when seismic conditions?
    28 days ago

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