28989377 Design of Reinforecment in Piles by J P Tyson

Design of reinforcement in piles

byJPTyson (Trafalgar House Technology Limited)

~ •

All exacutlve aGilitY 01 THE DEPARTMENT

4f• OF TRANSPORT

TRL Report 144

I I

The Transport Research Laboratory is the largest and most comprehensive centre for the study of road transport in the United Kingdom. For more than 60 years it has provided information that has helped frame transport policy, set standards and save lives. TRL provides research-based technical help which enables its Government Customers to set standards for highway and vehicle design, formulate policies on road safety, transport and the environment, and encourage good traffic engineering practice. As a national research laboratory TRL has developed close working links with many other international transport centres. It also sells its services to other customers in the UK and overseas, providing fundamental and applied

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Transport Research Foundation Group of Companies Transport Research Foundation (a company limited by guarantee) trading as Transport Research Laboratory. Registered in England, Number 3011746. TRL Limited. Registered in England, Number 3142272. Registered Office: Old Wokingham Road, Crowthorne, Berkshire. RG45 6AU

TRANSPORT RESEARCH LABORATORY An Executive Agency of the Department of Transport

TRL REPORT 144

DESIGN OF REINFORCEMENT IN PILES

by J P Tyson (Trafalgar House Technology Limited)

This report describes work commissioned by the Bridges Engineering Division of the Highways Agency under E553C/BG, Reinforcement in Piles (Desk Study)

Crown Copyright 1995. The contents of this report are the responsibility of the authors and the ChiefExecutive oflRL. They do not necessarily represent the views or policies of the Department of Transport.

Transport Research Laboratory Old Wokingham Road Crowthome, Berkshire, RG45 6AU 1995 ISSN 0968-4107

Highways Agency St Christopher House Southwark Street, London SE1 OTE

CONTENTS PAGE

1.0

EXECUTIVE SUMMARY

1

ABSTRACT

2

INTRODUCTION

2

1.1 1.2 2.0

3

DATA COLLATION 2.1 2.2

3.0

2 3

Scope of Study Research Strategy

General Data Sources

3 4

CODE REQUIREMENTS

4

3.1 3.2 3.3

Current UK Codes Historic/Superceded Standards/Codes Non UK Standards/Codes

4 10 12

4.0

DESIGN OF REINFORCEMENT IN Pll..ES

13

5.0

DISCUSSION

16

5.1 5.2

16 19 19 20 20 21 22 22 23 25 26

Changing Design & Construction Practice Pile Reinforcement Design 5. 2.1 Concrete Strength & Stiffness 5. 2. 2 Steel Reinforcement Strength 5. 2. 3 Design for Bending 5.2.4 Design for Shear 5.2.5 Design for Buckling 5.2.6 Early Thermal Cracking 5.2.7 Corrosion and Durability 5.2.8 Nominal Reinforcement 5.2.9 Curtailment of Reinforcement

i

CONTENTS (cont'd) PAGE

5.3 5.4

6.0

Design Relating to Free Standing Lengths of Piles

26

Design Relating to Piled Retaining Walls 5.4.1 Concrete Strength and Stiffness 50402 Steel Reinforcement Strength 504.3 Design for Bending 50 4 04 Design of Shear 5.405 Design for Buckling 5 .40 6 Thermal Cracking 5.407 Corrosion and Durability 5.408 Nominal Reinforcement 5.4.9 Curtailment of Steel

27 27 27 27 28 28 28 28 29 29

SUMMARY AND RECOMMENDATIONS

6.1

6.2 6.3

Summary 6 01.1 Fully Embedded Piles 6.1.2 Free Standing Lengths of Piles & Pile Retaining Walls Recommendations Areas for Further Study

7.0

ACKNOWLEDGEMENTS

8.0

REFERENCES

30 30 30

31 31 34

FIGURES

APPENDICES Appendix 1 - Data Sources for Research Study Appendix 2 - Buckling Resistance of Fully Embedded Piles Appendix 3 - Example Method of Calculating Spacing of Links to Prevent Local Buckling of Embedded Pile Appendix 4 - Calculation of Depth to Pile Fixity of Free Standing Length of Pile

ii

EXECUTIVE SUMMARY Feedback obtained from two construction sites in the UK has suggested that current design practices for pile reinforcement may be overconservative. This report investigates the development of the design of reinforcement in piles and assesses the applicability of current design codes to pile design. It also gives recommendations for amendments to the Standard BD 32/88 (DMRB 2.1) for piled foundations and suggestions for clarifying existing British Standard requirements. Areas for further research are highlighted. It is shown that developments in the understanding of concrete, steel and structural design, together with the development of geotechnics have led to an increase in the use of vertical piles to resist lateral loads. This in tum has resulted in a parallel requirement for increased steel quantities to resist lateral forces. Computer design techniques have also developed rapidly, allowing the effects of temporary loadings and deflections to be incorporated into the design. Higher design loads, and hence increased reinforcement, are invariably the result.

It is shown that, although conventional structural analyses can be applied to pile reinforcement design, consideration must be given to factors unique to the piling situation. In particular, the supporting effect of the surrounding ground and protection provided against corrosion are significant factors in determining reinforcement requirements. For fully embedded piles nominal requirements for links, minimum numbers of bars and crack control steel can be ignored. Crack control steel need only be applied to the control of early thermal cracking and then only if this is required to ensure the serviceability of the pile. Some evidence suggests that crack control steel may not be effective in reducing long term corrosion of steel. Dense concrete, resistant to carbonation, should be used with external sleeving or steel coatings provided in extreme corrosion environments to achieve a durable pile. Free standing lengths of piles and the upper portions of pile retaining walls should, however, be designed as columns in air but only down to a point of fixity below ground level. A method for determining the point of fixity is suggested.

1

DESIGN OF REINFORCEMENT IN PILES

ABSTRACT The quantity of reinforcement installed in concrete piles appears to have increased significantly over the years. Recent case histories have suggested that overly conservative designs may be generated when current design standards are applied. This report considers the historical development of the design of reinforcement in piles and reviews the reasons behind the increase in pile reinforcement quantities. The report also researches the concepts underlying current design requirements and their applicability to pile design. Advice is given on the implementation of existing British Standards and recommendations given for amendments to the Standard for piled foundations BD 32/88 (DMRB 2.1). 1.0

INTRODUCTION

1.1

Scope of Study

In July 1994, the Transport Research Laboratory (TRL) commissioned Trafalgar House Technology to undertake a desk study into the design of reinforcement in piles. The specific requirements were to identify reasons for the increase in pile reinforcement in recent years and to establish whether the present high levels of reinforcement are justified. The catalyst for this work is feedback from two completed projects. The first was an unpublished study, commissioned by the DOT, into the design of the Holmesdale and Bell Common Tunnel retaining walls. This reviewed various methods of deriving the lateral forces applied to the walls and considered the implications for quantities of reinforcement. For the diaphragm walls of Holmesdale tunnel, one of the findings was that the application of crack control criteria significantly increased the steel reinforcement requirements. The second project was work being undertaken for the Medway Crossing. Here, a number of piles were exposed adjacent to a marine environment and, despite the relatively light reinforcement, all appeared to be in good condition. This study researches the current and historic methods of the design of the reinforcement in piles necessary to resist the calculated design forces. It covers fully embedded piles, piles exposed along part of their length and those acting as retaining walls. The study deals principally with reinforcement provided to resist the forces applied to the pile and to provide for a durable pile. The derivation of such forces, however, is not included within this study. Pre-cast concrete piles are excluded as the reinforcement for these is generally controlled by the handling and insertion forces and not the in-service forces. 2

1. 2

Research Strategy

To consider as many aspects of pile reinforcement design as possible in the time available, a research strategy was devised which incorporated a review of current and superseded design codes, published literature and consultation with external organisations. A flow chart indicating the design strategy is presented in Fig. 1.

2.0

DATA COLLATION

2.1

General

Reference material from a variety of sources including UK and non-UK Standards, published literature, private correspondence and internal company case histories has been gathered and collated. This material has been analysed and the key issues influencing pile reinforcement design identified. These are listed below and discussed in detail in Section 5.2 of this report. o o o o o o o o o o

Changing design and construction practice Concrete strength and E values Steel reinforcement strength and E values Design for bending Design for shear Design for buckling Thermal cracking Corrosion and durability Nominal reinforcement Curtailment of steel at depth

The literature search was supplemented by a consultation process instigated to gather the experience of a cross section of external organisations. Three consultants and three contractors were chosen to ensure a broad cross section of experience. External consultees were as follows:Consultants

Contractors

Mott MacDonald Group Ove Arup and Partners Rendel Palmer and Tritton Ltd

Bachy Limited Cementation Piling & Foundations Ltd Keller Foundations

3

The consultation was in two stages. Firstly the companies were approached for their willingness to participate and sent an initial questionnaire canvassing their views. Their responses were collated and analysed and subsequently re-circulated to the respondents for further comment. Results of the consultation are considered in Section 4.

2.2

Data Sources

Design codes and published literature were generally obtained using standard library database searches. Some unpublished data was obtained from TRL and external consultees. A complete list of data sources used is given in Appendix 1.

3.0

CODEREQUIREMENTS

3.1

Current UK Codes

At present, specific references in UK codes to the design of reinforcement in piles is limited to two documents, BS 8004:1986 "Foundations" and BD 32/88 "Piled Foundations" (DMRB 2.1). This latter document is mandatory on DOT jobs only. A summary of the requirements of these codes is given below: i)

BS 8004: 1986 "Foundations"

(a)

Vertical piles which are axially loaded need not be designed as structural columns unless part of the pile extends above ground level (Cl7.3.3.3). For this latter case, only the upper portion of the pile need be considered as a column down to a point of fixity. Para 2 Cl 7.3.3.3 states:"where part of the finished pile projects above ground, that length should be designed as a column in accordance with BS 8110, CP114 orBS 449. The effective length to be taken in the calculation is dependent on the lateral loading if any and on the degree_of fixity provided by the ground, by the structure which the pile supports and by any bracing. The depth below the ground surface to the point of contraflexure varies with the type of soil. In firm ground it may be taken as about lm below the ground surface; in weak ground, such as soft clay or silt, it may be as much as one half of the depth of -penetration into the stratum but not necessarily more than 3m. The degree of fixity, the position and inclination of the pile top and the restraint supplied by any bracing should be estimated as in normal structural calculations".

4

(b)

All forces acting on the pile are to be determined and the pile reinforced accordingly (Cl7.3.3.4, Cl 7.3.3.6, and Cl7.4.4.3.2). Some or all of the pile length may be unreinforced (Cl 7.4.4.3.2). Pre-cast concrete piles are to be designed to BS 8110 (or CP 116).

(c)

Where tensile forces are to be resisted by the pile, adequate reinforcement is required to resist the entire tension stresses. The reinforcement should be provided for the full length of the pile or where tensile forces are small, to a depth at which the tensile forces have been fully transmitted to the ground (Cl 7.3.3.7 and Cl 7.4.5.3.2).

(d)

Minimum spacing of links is given as 150mm (Cl 7.4.4.4.2).

(e)

For raking piles, loads may be considered as axial with an applied bending force at the top (Cl 7.3.3.5).

(f)

Durability and protection of reinforcement against corrosion is provided by dense impermeable concrete free from defects (Cl10.4.7). Nominal cover for various exposure conditions should be as BS 8110.

'ii)

Standard BD 32/88: 1988 "Piled Foundations" (DMRB 2.1).

(a)

This standard applies to both the design of driven and bored piles (Cl 2.1) and is mandatory on all DOT projects.

(b)

Pile caps are to be designed to BS 8004 but Cl 3 .1 states that the structural design of all concrete elements of the pile is to be to BS 5400 Pt 4.

BS 8110 states that embedded piles need not be designed as columns and piles carrying axial load only need not be reinforced. Some rules are provided regarding calculation of axial forces which may be accommodated without reinforcement but no guidance is given on calculation of shear capacity. Where reinforcement is required, BS 8110 (and CP116 and CP114) is mentioned for design but is not specifically invoked, except for pre-cast piles. No guidance is given for curtailment of longitudinal steel. BS 5400, the design code for bridges, is widely accepted as being a more stringent design standard than the general civil engineering concrete code BS 8110. Additional forces are imposed on a bridge structure such as impact and braking forces and abutment earth pressures. Also the often exposed and relatively long and flexible nature of bridges leads to high wind and thermal expansion forces. The difficulties in determining the magnitude of these forces and their effect on the structure has required a more conservative design approach which is reflected in the bridge code. The requirements of BS 8004, BS 5400 Pt 4 and BS 8110 are summarised in Tables 3.1.1 and 3.1.2. For piles used as earth retaining structures, (ie contiguous bored pile walls) the exposed portion of the pile may be designed either to BS 8002, code of practice for retaining walls or, if applicable, BD 30/87 (DMRB 2.1) for backfilled retaining walls. However, a new Standard, BD42/94 (DMRB 2.1) has just been released which deals 5

Table 3.1.1

Summary of Code Requirements

Longitudinal Reinforcement Min Min. Max No. Dia. Spacing

Code/Standard (Date)

Cover to Reinforcement

BS 8110 Pt 1 (1985)

75mm (Concrete cast against ground)

4 (Rectangular) 12mm 6 (Circular)

BS 5400 Pt 1 (1990)

45mm (Buried C30 Concrete)

4 (Rectangular) 12mm 6 (Circular)

-

Min %Tension: 0.8%(mild (steel) 0.45%(high yield)

Transverse Reinforcement Max Min Diameter Spacing

Crack Control Max Crack Width

12 times smallest main bar size

~

times largest main bar size or 6mm

Only checked i f N < 0. 2fcu.A: Then max. width = 0. 3mm

12 times smallest main bar or 0.74 times effective depth

)( times largest main bar size

0.2Smm (Buried concrete) 0 .lmm i f groundwater pH< 4.5

0'1

BS 8004

As BS8110 but add 40mm for concrete cast against ground -------·

-~---------

1%- or 0.15 Nry 300mm

---- As BS 8110 -----------

------As BS8110---------------

j

j

I

Not mentioned

--------

Table 3.1.1 cont/d

Summary of Code Requirements

Code/Standard (Date)

Permissible Stresses Concrete Steel

BS 8110 Pt 1 (1985)

Compression 0. 67 feu/Ym

fy/ym (Tension & and compression)

Shear Ve

BS 5400

Maximum Axial Load Without Reinforcement With Reinforcement

0. 4

feu. Ac

0 . 4 . f cu . Ac + 0 . 7 5 . A.c . f

y

0.4

feu. b. de

0 . 4 . feu. b. de + fye. A

+ fsl. A.2

+ 0 . 6 . N. V. h Ac.M

ComPression

Compression

0. 67. fcu/Ym

fy/ (Ym+fy/2000)

Shear

Tension

0. 5. feu

fy/ym

Pt 4 (1990)

1

sl

-.l

(Triangular Stress Distribution) 0. 3.8. feu

(Uniform Stress Distrubtion) BS 8004 (1986)

Ac A'sl A.2

A..e

b

As BS 8110

As BS 8110

- area of concrete - area of compression - area of reinf. in other face - area of vertical reinf. - width of section

de fw fd fy h

-

As BS 8110

As BS 8110

depth of concrete in compression characteristic concrete cube strength stress in reinf. in other face characteristic strength of reinforcement overall depth of section

M N V ve Ym

-

applied design moment applied design axial load applied design shear design concrete shear stress partial safety factor for strengh of material

Table 3.1.2

Summary of Current UK Design Codes "

Global Reinforcement Design Requirements

BS8110

BS8004

Detailed design rules for structural design (See Table 3.1.1)

BD 32/88 (BS5400 Pt 4)

o Axial Compression

No guidance Refer to BS 8110

o Axial Tension

General guidance given on length of pile to be reinforced

"

o Flexure

No guidance Refer to BS 8110

"

"

o Shear

No guidance Refer to BS 8110

"

"

o Buckling

Check for Buckling if Cu < 20kN/m2

No checks required

Detailed design rules for structural design (See Table 3.1.1)

"

No checks required

Rules for: oMinimum Reinforcement 0

Links

o Calculation of Rebar

No minimum reinforcement required Minimum spacing 150mm Refer to BS8110. Only upper part of pile above ground level to be designed as a column

8

"

"

"

"

"

"

Table 3.1.2 cont/d Summary of Current UK Design Codes

Global Reinforcement Design Requirements

o Cover

BS8110

BS8004

75mm

As BS8110 plus 40mm

(Concrete cast against ground)

BD 32/88 (BS5400 Pt 4)

45mmmax +40mm in accordance with BS8004 cl 2.4.5

o Crack Control

Not mentioned

Only required for N 0.711' therefore Per

= (EIK)'h 2

= 118 MN which is equivalent to an applied stress of 600 MN/m which is well in excess of the 28 day concrete 2 characteristic strength of, say 40 MN/m 2.2

Axial loading of the pile can be considered by dividing 1. by (1 - PIPer),,

where P is the axial load.

In the above example, ifP =50 MN, Per= 118 MN and (1 - PIPer),, = 0. 76 1. = 1'/(2),,

= 6.5/(2)'11 = 4.6m

Adjusted 1. = 4.6/0.76 = 6.0m. but 0.71.1. = 4.26 which is still < L ( = 15m) so Per remains 118 MN.

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