Pile caps guidance.pdf

TECHNICAL REFERENCE MANUAL Civil and Structural

DESIGN OF PILE CAPS

TRM 73 Rev 10 Date 10/08 Page 1 of 7

DESIGN PRINCIPLES

1

The pile cap design procedures in this TRM have been prepared in accordance with BS 8110-1 and BS 8004. After some research and consultation, the recommended shear enhancement has reverted to BS 8110-1 rules in preference to BS5400-4. Design is based on bending theory rather than truss analogy for all but two pile caps. The reasons for adopting these design principles are explained in Appendix A.

2

Concrete grade for pile caps (to BS 8110) 2

This is generally taken as f cu = 28/35 N/mm , but may need to be increased if the column strength is greater than 40/50, see TRM 171 High strength concrete columns intersected by lower strength slabs. Where aggressive soil conditions are present refer to BRE Digest 363 for guidance on concrete grade and mix design. 3

Plan shape A compact arrangement of piles gives the most economic size of cap. The minimum spacing of piles is controlled by the soil conditions. Generally this will be three times the pile diameter as the majority of piles carry their load by a combination of shaft friction and end bearing. Pile caps should extend at least 150 mm beyond the theoretical circumference of the piles.

4

Depth of cap Determine the initial depth of the pile cap as equal to the horizontal distance from the centreline of the column to the centreline of the pile furthest away from the column. It may be necessary to increase the depth after carrying out the shear checks in section 7. The depth of the cap must also be sufficient to meet anchorage-bond length requirements for starter bars and beam shear and punching shear requirements. The minimum cover to the main reinforcement = 50 mm.

5

Loading The design axial load for a pile cap should be the sum of the capacities of the piles in the group. This gives an important additional capacity for future upgrading, and only in exceptional circumstances should a lower value be used. To determine the ultimate capacity of a pile clause 7.4.4.3.1 of BS 8004 states: “The average compressive stress under working load should not exceed 25% of the specified works cube strength at 28 days calculated on the total cross sectional area of the pile”. This is 0.4 f cu (approx) at the ultimate limit state. Hence the ultimate capacity of a pile = 1.5 x SWL (but no greater than 0.4 f cu (π/4) φ2) where SWL = safe working load of pile (controlled by soil conditions) = 28 day cube strength of concrete in pile f cu φ = diameter of pile.

WSP Group

TRM 73

TECHNICAL REFERENCE MANUAL

Rev 10 Date 10/08

Civil and Structural

Page 2 of 7

DESIGN OF PILE CAPS Table 1

Design data - truss analogy TENSILE FORCES ACROSS PILE CAP

PILE GROUP x

A

NEGLECTING COLUMN SIZE

B Ft ( AB ) =

N 2d eff

2

A 2

x

Ft ( AB ) = Ft ( BC ) = Ft ( CA) =

y

C

2N 9d eff

B 2

A

B x

Ft ( AB ) = Ft ( BC ) = Ft ( CD ) = Ft ( DA) =

y

2

Ft (total )( long . & trans.) =

D

C

N 2d eff

2

D

A x

E

2

2 N deff x

Ft ( AB ) = Ft ( BC ) = Ft ( CD ) = Ft ( DA) =

y

D

C

Note:

N 4d eff

Ft (total )( long . & trans.) =

0.8 N 4d eff

0.8N 2d eff

2

= = = =

distance between centre of piles ultimate axial load (refer to section 5) effective depth y (with a square column)

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TECHNICAL REFERENCE MANUAL Civil and Structural

Rev 10 Date 10/08 Page 3 of 7

DESIGN OF PILE CAPS 6

TRM 73

Design method On plan, columns must be located at the centroid of the pile group. The axial load N is taken as equally shared by all piles in the group. Truss analogy is used for the design procedure for two pile caps and bending theory for all other pile arrangements. Both methods are recognised by BS 8110 but test results suggest that bending theory gives more accurate results when the bars are uniformly distributed across the cap. This is explained in more detail in Appendix A. Both design procedures neglect the size of the column when determining the tensile force to be resisted by the reinforcement. Formulae for calculating tensile force using truss analogy are provided for reference in Table 1. The design procedures are based on pile caps resisting axial load only from the column above. The procedures may also be used when the cap is to resist moments in addition to axial load provided all piles stay in compression and nP is less than the ultimate capacity of the pile cap; P is the maximum load on any pile in the group and n is the number of piles in the group. The design procedures are not appropriate where moments cause uplift and where there are substantial horizontal forces. If columns are designed to transmit moments into the pile cap, then the pile caps (and piles) will need to be designed accordingly.

7

Shear Both beam shear and punching shear should be checked irrespective of the method used to determine the tension reinforcement. The size of the column should be taken into account when considering the effects of shear. A small column size will give the worst conditions. Shear links are not required in pile caps provided v ≤ vc. If this is not the case increase the depth of the pile cap. The allowable shear stress vc may be obtained from table 3.8 of BS 8110-1. interpolation is required or it can be calculated directly from: vc = 0.79 {(f cu/25) 100 As/(b deff)}

1/3

(400/deff)

1/4

Some

/ γm

where γm = 1.25. This reduces to vc = 0.707 {100 As/(b deff)} 2

1/3

(400/deff)

1/4

when f cu = 28/35 N/mm . The limit in table 3.8 of (400/deff) triggered when deff 2000 mm.

1/4

not less than 0.67 is

The critical sections for the design shear strength of a pile cap are: i

Along a vertical section extending across the full width of the cap. This section is located 20% of the diameter inside the face of the pile as shown in figure 1.

ii

Punching shear should be checked on a perimeter located 20% of the pile diameter inside the faces of the piles as shown in figure 1.

Iii Around the perimeter of the column where the design punching shear stress 2 should not exceed the lesser of 0.8 f cu or 5 N/mm .

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TECHNICAL REFERENCE MANUAL Civil and Structural

DESIGN OF PILE CAPS Perimeter for punching φ/5 shear check (if necessary)

φ/5

TRM 73 Rev 10 Date 10/08 Page 4 of 7

φ

av

Critical section for shear check Figure 1. Critical section for shear check in a pile cap For (i) the shear resistance vc may be increased by a factor 2deff/av where av is the distance from the face of the column to the critical section. Note that BS 8110-1 allows this shear enhancement over the full width of the pile cap (providing the spacing of the piles does not exceed 3 x pile diameter) whereas BS 5400-4 cl 5.4.4.1 restricts the enhancement to the width of the pile(s). This revision of the TRM uses the BS 8110-1 rule. The conclusions of reference 1 have been considered but do not provide adequate grounds to justify adopting the more conservative BS 5400-4 approach. This is explained in more detail in Appendix A. For (ii) the allowable shear resistance vc may be increased by a factor 1.5 deff/av when checking punching resistance. It should be noted that the maximum enhanced value of shear stress should not exceed 2 the lesser of 0.8 f cu or 5 N/mm . 8

Anchorage A full anchorage is required beyond the pile centres when designed using truss analogy whereas for caps designed using bending theory an anchorage equal to the effective depth beyond the critical section for shear is all that is required. Clause 3.11.4.4 (c) of BS 8110-1 requires that tension reinforcement should be provided with a full anchorage. Bars in the sides of the cap enhance the shear capacity by acting as shear links. It is therefore preferable to continue the bars up the sides of the cap for the full depth (less the cover). The use of standard radii in the bar bends may cause the bearing stress to rise above the limits in the code. The containment of the concrete and reinforcement provided at right angles to the reinforcement under consideration allows the allowable bearing stress to be increased.

9

Pile anchorage into pile cap All pile reinforcement must be properly anchored into the pile caps. Trimmed pile heads must be cast into the pile cap by 75 mm.

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TECHNICAL REFERENCE MANUAL Civil and Structural

DESIGN OF PILE CAPS

TRM 73 Rev 10 Date 10/08 Page 5 of 7

Pile reinforcement anchored into pile cap 75 mm Pile head cast into pile cap

Figure 2: Pile anchorage into pile cap 10

Lacers Horizontal bars around the sides of the cap should not be less than H12 bars at a vertical spacing not more than 250 mm.

11

Lateral forces/stability Lateral forces may occur from wind and overall stability considerations. These forces would be applied at the top of the pile cap. Such forces would not normally affect the design of the pile cap significantly. They could however alter the pile reactions, especially with deep pile caps (say pile cap depth h > 1.0 m). This effect must be assessed by the designer before adopting the standard design procedures.

12

Column eccentricity/setting out tolerance It has been assumed that the effects of column eccentricity due to setting out tolerances are resisted by ground beams and/or the piles. Two-pile caps should be braced by ground beams in the transverse direction. The ground beams should be designed to resist a moment arising from a column eccentricity of 75 mm. The effects on the larger multiple pile caps are usually insignificant.

13

Construction information Ensure that the following information is provided on pile cap detail sheets: • drawing number of general notes drawing • drawing number for location • concrete grade - and check whether SRC or partial cement replacement is necessary • location of holding down bolts for structural steelwork and reference to separate details • location of starter bars. Enter bar marks on pile cap schedules, or provide separate starter bar detail sheets. • provide setting out dimensions which can be related to the pile layout drawing • provide a level datum WSP Group

TECHNICAL REFERENCE MANUAL Civil and Structural

DESIGN OF PILE CAPS • • 14

TRM 73 Rev 10 Date 10/08 Page 6 of 7

provide the unique reference required to identify the pile cap type on the pile layout drawing enter the title block information.

Spreadsheet The spreadsheet has been re-written and is accessed as a separate file (TRM 73A).

REFERENCES

1

Bloodworth et al, Reinforced concrete pile caps: ICE Proceedings, Structures and Buildings 156 issue SB4, November 2003.

KEYWORDS

Calculations; concrete; foundations; groundworks; piles; pile caps; spreadsheet.

Author: Not known, rev 11 and spreadsheet rev 11 by Jeremy Wells, GTC Sponsor: Group Technical Centre Revision record: 11/05 Rev 6. BS 5400-4 method for shear introduced, calculations and spreadsheet updated. 5/06 Rev 7. Minor corrections. 10/06 Rev 8. Minor correction. 05/08 Rev 9. BS8110-1 shear enhancement rules reintroduced, change to bending theory design for 3, 4 and 5 pile caps, lacer requirement reduced, Appendix A added. 10/08 Rev 10. Spreadsheet updated. Partial safety factor for reinforcement increased to 1.15. Compression steel calculation and shear on 3-pile cap corrected. 03/09 Rev 11. Revisions to 3-pile cap only. Tensile reinforcement banded to align with IStructE SMDSC, corrections to shear calculation.

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TECHNICAL REFERENCE MANUAL Civil and Structural

DESIGN OF PILE CAPS

TRM 73 Rev 10 Date 10/08 Page 7 of 7

Appendix A Background to design approach for bending and shear 1

In 2002, a paper by Bloodworth, Jackson and Lee was published in ICE Structures and Buildings which took the form of a review of available experimental data on the strength of pile caps. It concluded that the BS8110-1 rules for shear enhancement were ‘unsafe’. On the basis of that paper, revisions 6 to 8 of this TRM were based on the more conservative BS5400-4 rules for shear enhancement. GTC has since reviewed the experimental data on which Bloodworth et al. based their conclusions. Of the 70 caps on which the 2002 study was based, the conclusion regarding shear enhancement was based on six tests by Blevot and Sabnis. Of these tests, one (Blevot 4N3b) was effectively a short column which could not conceivably have failed in longitudinal shear and probably suffered some form of crushing failure – the axial load at failure was 25N/mm2 compared with fcu of 31.3N/mm2. The results for the remaining five tests are summarized in table A1. Test

Blevot Q.1 Blevot Q.2 Blevot Q.2b Sabnis SS6 Sabnis SG2

BS8110 design values (kN)

Test Result (kN)

Notes

BS8110 result safe BS8110 would be unsafe if test result valid but recorded as “bond failure” BS8110 unsafe (9% high) by truss analogy but safe by bending theory BS8110 marginally unsafe (1% high) by truss analogy but safe by bending theory. BS8110 unsafe (23% high) by truss analogy but safe by bending theory. This was a ¼ scale model with only 3 bars on the bottom, one midway between piles and therefore unlikely to contribute much to truss action.

Truss analogy 360 871

Bending theory