STRUCTURES DESIGN MANUALfor Highways and Railways

HIGHWAYS DEPARTMENTGovernment of the Hong Kong Special Administrative Region

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Government of the Hong Kong Special Administrative Region

First edition, August 1993. Second edition, November 1997.

Prepared by : Structures Division, Highways Department, 4th Floor, Ho Man Tin Government Offices, 88 Chung Hau Street, Homantin, Kowloon, Hong Kong.

This publication is available from: Government Publications Centre, General Post Office Building, Ground Floor, Connaught Place, Hong Kong. Overseas orders should be placed with: Publications (Sales) Office, Information Services Department, 28th Floor Siu On Centre, 188 Lockhart Road, Wanchai, Hong Kong.

Price in Hong Kong : HK$ 56.00 Price overseas : US$ 9.50 (including surface postage)

4 Cheques, bank drafts or money orders must be made payable to The Government of the Hong Kong Special Administrative Region

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FOREWORD

The Structures Design Manual for Highways and Railways sets out standards and provides guidance for the design of highway and railway structures in Hong Kong. The Manual was first published in August 1993, and has since been widely used as a reference for highway and railway structural works in the territory. The second edition incorporates some minor revisions as a result of new research information, refinements in design methods and feedbacks from the use of the previous edition. Practitioners are encouraged to comment at any time to the Structures Division of this Department on the contents of this Manual, so that further improvements can be made to future editions.

( K.S. Leung ) Director of Highways

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CONTENTSPage No.

TITLE PAGE FOREWORD CONTENTS 1 INTRODUCTION1.1 1.2 1.3 1.4 DEFINITION LIMIT STATE DESIGN RAILWAY BRIDGES APPROVED SUPPLIERS OF MATERIALS AND SPECIALIST CONTRACTORS FOR PUBLIC WORKS

1

35

15 15 15 16 16

2.

LOADS2.1 GENERAL 2.1.1 Carriageway 2.1.2 Notional Lanes COMBINATION OF FORCES WIND LOAD 2.3.1 Bridges with Spans Less than 100 m 2.3.2 Bridges with Any Span Greater than 100 m 2.3.3 Covered Footbridges TEMPERATURE EFFECTS 2.4.1 General 2.4.2 Effective Bridge Temperatures 2.4.3 Temperature Difference 2.4.4 Coefficient of Thermal Expansion

17 17 17 17 18 18 19 19 21 23 23 23 24 25 25

2.2 2.3

2.4

2.5

EFFECTS OF SHRINKAGE AND CREEP

8 Page No. 2.6 EARTHQUAKE FORCES 2.7 2.7.1 COLLISION LOADS Bridge Superstructures 2.7.2 Highway Overbridges 2.7.3 Railway Overbridges 2.7.4 Bridges over Navigation Channels PARAPET LOADING LOADS ON RAILWAY OVERBRIDGES FROM ELECTRICAL SUPPLY EQUIPMENT LIVE LOADING 2.10.1 Nominal Uniformly Distributed Load (UDL) 2.10.2 HA Lane Factors 2.10.3 Types HA and HB Loading Combined FOOTBRIDGE AND SUBWAY COVERS 25 26 26 27 28 28 28 28

2.8 2.9

2.10

28 29 30 30 31 31 31 31 31 32

2.11 2.12

DYNAMIC EFFECTS 2.12.1 Aerodynamic Effects 2.12.2 Highway Bridges 2.12.3 Footbridges 2.13 DEAD LOAD AND SUPERIMPOSED DEAD LOAD

3.

DESIGN OF STEEL BRIDGES3.1 3.2 3.3 3.4 3.5 3.6 GENERAL HOT FORMED STRUCTURAL HOLLOW SECTIONS FABRICATION BLAST CLEANING NON-DESTRUCTIVE TESTING OF WELDS HOT DIP GALVANIZING

33 33 33 33 34 34 34

9 Page No.

4.

DESIGN OF CONCRETE BRIDGES4.1 GENERAL 4.1.1 Design Standards 4.1.2 Cracking 4.1.3 Concrete Cover to Reinforcement MATERIAL PROPERTIES 4.2.1 Differences between British and Hong Kong Concretes 4.2.2 Elastic Modulus of Concrete 4.2.3 Strength of Concrete 4.2.4 Shrinkage of Concrete 4.2.5 Creep of Concrete 4.2.6 Coefficient of Thermal Expansion of Concrete 4.2.7 Strength of Steel Reinforcement EARLY THERMAL MOVEMENT PRESTRESSING 4.4.1 Grade of Concrete for Prestressing Work 4.4.2 Post-tensioning Systems 4.4.3 Specialist Prestressing Contractors 4.4.4 Secondary Moments and Shear

35 35 35 35 35 35 35 36 36 36 38 39 39 39 42 42 42 43 43

4.2

4.3 4.4

5. 6.

DESIGN OF COMPOSITE BRIDGES SPECIFICATION FOR MATERIALS AND WORKMANSHIP, STEEL SPECIFICATION FOR MATERIALS AND WORKMANSHIP, CONCRETE, REINFORCEMENT AND PRESTRESSING TENDONS RECOMMENDATIONS FOR MATERIALS AND WORKMANSHIP, CONCRETE, REINFORCEMENT AND PRESTRESSING TENDONS

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49

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9.

BEARINGS9.1 9.2 9.3 9.4 9.5 9.6 GENERAL

53 53 53 53 53 54 54

CLASSIFICATION OF BEARINGS SCHEDULE OF BEARINGS SUPPLY AND INSTALLATION OF BEARINGS TESTING COMPRESSIVE STIFFNESS OF ELASTOMERIC LAMINATED BEARINGS DESIGN OF FIXINGS FOR BRIDGE BEARINGS OPERATIONAL REQUIREMENTS

9.7 9.8

54 55

10. 11.

FATIGUE MOVEMENT JOINTS11.1 GENERAL 11.1.1 Movements 11.1.2 Selection of Joint Type PROPRIETARY MOVEMENT JOINTS TRAFFIC LOADING ON MOVEMENT JOINTS LOADING OF STRUCTURE BY STRAINING OF MOVEMENT JOINTS WATERTIGHTNESS OF MOVEMENT JOINTS FUNCTIONAL REQUIREMENTS OF PROPRIETARY MOVEMENT JOINTS 11.6.1 Requirements 11.6.2 Specification SUPPLY AND INSTALLATION OF MOVEMENT JOINTS

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59 59 59 59 60 60 61

11.2 11.3 11.4

11.5 11.6

61 62 62 63 63

11.7

11 Page No.

12 Page No. 11.8 DETAILING FOR PROPER INSTALLATION OF MOVEMENT JOINTS OPERATIONAL REQUIREMENTS 63

11.9

64

12.

FOOTBRIDGES AND SUBWAYS12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 GENERAL COVERS STAIRWAYS RAMPS LANDINGS CHANGES IN DIRECTION DIMENSIONS PARAPETS AND HANDRAILS DRAINAGE

65 65 65 65 66 67 67 67 67 68 69 69 69 70 70 71 71 72 72 72 73

12.10 LIGHTING 12.11 ESCALATORS 12.11.1 Provision of Escalators 12.11.2 General Requirements 12.11.3 External Applications 12.11.4 Inspection and Surveillance 12.12 FINISHES 12.13 WATERPROOFING 12.13.1 Covers for Footbridges, Covered Walkways and Pedestrian Subways 12.13.2 Pedestrian and Bicycle Subway Barrels 12.14 OTHER ROOF MATERIALS

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FOUNDATIONS AND SUBSTRUCTURES13.1 13.2 13.3 13.4 13.5 GENERAL SPECIALIST PILING CONTRACTORS PILING DOWNDRAG RAILWAY BRIDGE SUBSTRUCTURES HYDRAULIC EFFECTS 13.5.1 Effects to Be Considered 13.5.2 Backwater Effects 13.5.3 Effects of Waterborne Traffic RUN-ON-SLABS

75 75 75 75 75 76 76 77 77 77

13.6

14.

HEADROOM14.1 14.2 14.3 14.4 14.5 14.6 14.7 GENERAL REQUIREMENTS MEASUREMENT OF HEADROOM COMPENSATION FOR VERTICAL CURVATURE COMPENSATION FOR DEFLECTION OF STRUCTURE COMPENSATION FOR SIGNAL AND OTHER INSTALLATIONS TRAMWAY AND LIGHT RAIL TRANSIT OVERBRIDGES RAILWAY OVERBRIDGES

79 79 79 79 79 80 80 80

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PARAPETS15.1 15.2 15.3 15.4 GENERAL PARAPET GROUPS PARAPET HEIGHTS DESIGN DETAILS

81 81 81 81 81

14 Page No. 15.4.1 15.4.2 15.4.3 15.5 Materials Projections and Depressions Structures Not Exclusively Used by Highway Vehicles 81 82 82 82 82 83 84 84 84 84 85 85 85 85 86 86 87 87 87 87 87

METAL PARAPETS AND TOP RAILS 15.5.1 Design Requirements 15.5.2 Corrosion 15.5.3 Plinth 15.5.4 Bedding REINFORCED CONCRETE PARAPETS 15.6.1 Design Requirements 15.6.2 Longitudinal Effects P.1 AND P.2 PARAPETS P.3 PARAPETS P.4 PARAPETS

15.6

15.7 15.8

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15.9

15.10 P.5 PARAPETS 15.11 PARAPETS FOR HIGHWAY STRUCTURES 15.12 SIGHT DISTANCES 15.13 RAILWAY OVERBRIDGE AND UNDERBRIDGE WALKWAYS AND PARAPETS 15.13.1 High Containment Parapets 15.13.2 Overbridge Parapets 15.13.3 Underbridge Walkways and Parapets

16.

STORMWATER DRAINAGE16.1 16.2 GENERAL PIPES AND PIPE LAYOUT 16.2.1 Minimum Diameter 16.2.2 Material 16.2.3 Layout MOVEMENT JOINTS

89 89 89 89 89 90 90

16.3

15 Page No. 16.4 16.5 INTAKES OUTLETS 91 91

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AESTHETICS17.1 17.2 17.3 17.4 17.5 17.6 17.7 17.8 17.9 GENERAL PRINCIPLES FORM PROPORTION HARMONY SCALE EXPRESSION OF FUNCTION VISUAL STABILITY RHYTHM AND RHYME

93 93 93 94 94 95 96 96 97 97 98 99

17.10 ILLUSION 17.11 LIGHT AND SHADE 17.12 TEXTURE 99 17.13 COLOUR 17.14 LONG TERM APPEARANCE 17.15 THE ADVISORY COMMITTEE ON THE APPEARANCE OF BRIDGES AND ASSOCIATED STRUCTURES (ACABAS)

100 101 102

18.

OPERATIONAL CONSIDERATIONS18.1 SERVICE 18.1.1 18.1.2 18.1.3 LIFE Access for Inspection and Maintenance Maintenance Accommodation Spare Parts

103 103 103 103 104 104

18.2

SAFETY CIRCUITS FOR BRIDGES OVER NAVIGABLE CHANNELS

17 Page No. 18.3 LIMITING ENVELOPE FOR STRUCTURAL ELEMENTS AND INSTALLATIONS PAINTING OF GALVANIZED STEELWORKS INCORPORATION OF UTILITY INSTALLATIONS IN HIGHWAY STRUCTURES NOISE MITIGATION MEASURES ON HIGHWAY STRUCTURES MATERIALS FOR HOLDING DOWN AND FIXING ARRANGEMENTS ON HIGHWAY STRUCTURES 104

18.4 18.5

104 106

18.6

107

18.7

108

REFERENCES TABLESLIST OF TABLES TABLES

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113 115 117

FIGURESLIST OF FIGURES FIGURES

139 141 143

PLATESLIST OF PLATES PLATES

173 175 177

APPENDICESAPPENDIX A : APPENDIX B : NATURAL FREQUENCY AND ACCELERATION DESIGN OF HIGH CONTAINMENT PARAPETS

195 197 205

18 Page No. FOR RAILWAY OVERBRIDGES

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1.1.1 DEFINITION

INTRODUCTION

A highway structure is a structure intended to carry highway vehicles, and/or bicycles and pedestrians over, under or through a physical obstruction or hazard, and may be a bridge (which may be in the form of a culvert exceeding 2 m in diameter or span), a flyover, a viaduct, an underpass, a subway or a covered walkway. A covered walkway is an at-grade structure in the form of a roof cover intended to provide shade and shelter from the sun and rain for pedestrians. A railway bridge may be an underbridge or an overbridge. A railway underbridge is a structure intended to carry railway track and the locomotives and rolling stock using it over or through a physical obstruction or hazard. A railway overbridge is a structure intended to carry vehicles, pedestrians or services over one or more railway tracks. A railway overbridge may be a highway structure if its primary intention is not for carrying service installations. A culvert is a drainage structure designed as a closed conduit for conveying stormwater from one side of a highway or railway track to the other. A culvert exceeding 2 m in span or diameter corresponds to a small bridge, and shall be treated as a highway structure or railway bridge. A drainage conduit or nullah forming part of a more extensive drainage system which incidentally passes under a highway or railway track at a point or points along its route is a drainage structure, and for the purposes of this document is regarded as neither a highway structure nor a railway bridge. A wall designed to hold soil or rock in position is an earth-retaining structure. A wall designed to act as an abutment to a highway structure or railway bridge, or to support an approach to a highway structure or railway bridge, although in itself an earth-retaining structure, shall be treated as part of a highway structure or railway bridge.

1.2

LIMIT STATE DESIGN

Highway structures and railway bridges shall be designed on the basis of the limit state philosophy contained in BS 5400 - Steel, Concrete and Composite Bridges. The two limit states to be adopted are the ultimate limit state and the serviceability limit state as defined in Clause 3 of BS 5400 : Part 1. All undated references to BS 5400 refer to the current edition except BS 5400 : Part 2 which shall be as published in Appendix A to the United Kingdom Department of Transport Departmental Standard BD 37/88.

21 However the provisions of some parts of BS 5400 are not relevant to Hong Kong conditions, and for these, the recommendations of this document, or other appropriate criteria approved for adoption by the Chief Highway Engineer/Structures, shall be substituted. 1.3 RAILWAY BRIDGES Before the design of any highway structure crossing a railway track, or of any railway underbridge, is commenced, the requirements of the appropriate railway authority shall be ascertained. Preliminary and detailed drawings, with calculations if required, shall be referred to the appropriate railway authority for comments. The approval of the appropriate railway authority shall be obtained before any work is undertaken. In the absence of specific comment, the contents of this document shall be deemed to apply to railway overbridges and railway underbridges as well as to highway structures.

1.4

APPROVED SUPPLIERS OF MATERIALS AND SPECIALIST CONTRACTORS FOR PUBLIC WORKS

Main contractors engaged on projects involving the supply of special materials or specialist works on highway structures shall either themselves be registered as approved suppliers or specialist contractors in the appropriate category of the List of Approved Suppliers of Materials and Specialist Contractors for Public Works, or shall be required to engage one of the approved suppliers or specialist contractors registered in the category to supply the special materials or to carry out the specialist works on highway structures.

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2.2.1 GENERAL

LOADS

Highway and railway structures shall be designed for the loads and forces, and combinations of loads and forces, specified in BS 5400 : Part 2, published as Appendix A to the United Kingdom Department of Transport Departmental Standard BD 37/88, except where modified by this document. For superimposed dead load, the following values of fL shall be substituted for the values recommended in BS 5400 : Part 2 Clause 5.2.2 : ULS deck surfacing other loads 1.75 1.50 SLS 1.20 1.00

The value of fL for dead load imposed by deck surfacing may be reduced to 1.5 for the ULS if it is absolutely certain that the thickness (and hence the weight) of the surfacing will not be increased during the life of the bridge, e.g. where open texture friction course is always milled off before re-treatment. Further reduction of these values will not be permitted. The recommendations of BS 5400 : Part 2 Clauses 3.2.9.1 and 3.2.9.3.1 shall be replaced by the following Clauses 2.1.1 and 2.1.2 respectively to suit carriageways in Hong Kong.

2.1.1

Carriageway

For the purposes of this document, a carriageway is that part of the running surface which includes all traffic lanes, hard shoulders and marginal strips. The carriageway width is the width between raised kerbs. In the absence of raised kerbs, it is the width between concrete barriers and/or metal parapets, less the amount of set-back required for these barriers. This set-back measured from the traffic face (at running surface level) of each barrier shall be taken as 0.5 m on the off-side fast lane. On the near-side slow lane, the set back shall be taken as 0.5 m or the width of the marginal strip whichever is the greater subject to a maximum set-back of 1.0 m. The carriageway width shall be measured in a direction at right angles to the line of the raised kerbs, lane marks and edge markings (e.g. for a typical dual 3 lane Urban trunk road with median barriers having a traffic lane width of 11.0 m, 1.0 m marginal strip on near-side and 0.5 m marginal strip on off-side, the carriageway width for the purpose of Clause 3.2.9 of BS 5400 : Part 2 will be 11.0 m (11.0 + 1.0 + 0.5 - 1.0 - 0.5), assuming no raised kerbs).

2.1.2

Notional Lanes

For carriageway widths of 5.00 m or more, the width of notional lanes shall be taken to be not less than 2.50 m. Where the number of notional lanes exceeds two, their individual widths shall

23 be not more than 3.67 m. The carriageway shall be divided into an integral number of notional lanes having equal widths as follows : Number of Notional Lanes 2 3 4 5 6

Carriageway Width 5.00m 7.50m 11.01m 14.68m 18.35m up to and including up to and including up to and including up to and including up to and including 7.50m 11.01m 14.68m 18.35m 22.02m

above above above above

2.2

COMBINATION OF FORCES The combination of forces specified in BS 5400 : Part 2 shall be considered.

To allow for the possibility of earthquakes, an additional combination shall also be considered comprising the permanent loads, the seismic force described in Clause 2.6 and the live load utilised in deriving the seismic force. The partial load factors of combination 2 shall be used for the permanent loads and the live load. The partial load factors given in Clause 2.6 shall be used for seismic force.

2.3

WIND LOAD

The provisions for wind load in BS 5400 : Part 2 are based on wind gust speeds derived from British records. The recommendations of Clause 5.3.2 of BS 5400 : Part 2 regarding wind gust speeds consequently cannot be used, and must be replaced by the following which are based on Hong Kong conditions. Table 1 gives details supplied by the Hong Kong Observatory of maximum hourly wind and gust velocities for Waglan Island, which is exposed to south-easterly winds with a long fetch over open sea, and for the Hong Kong Observatory itself. The Hong Kong Observatory figures are for the period before the surrounding area became built-up, and are representative of an exposed urban location. The maximum gust velocity is related to the dynamic pressure head by the expression q where q = = 613 x 10-6vc

dynamic pressure head (kN/m) maximum gust velocity (m/s).

vc =

24 By interpolating from the values for Waglan Island in Table 1, the above expression gives a value of 3.8 kN/m for the dynamic pressure head corresponding to the maximum gust velocity of 79 m/s for a 120-year return period at an exposed location. For bridges with any spans greater than 100 m the provisions of Clause 2.3.2 shall be followed. In all other cases the simpler requirements of Clause 2.3.1 are deemed adequate. A designer experiencing difficulty in deciding on an appropriate degree of exposure for a particular site shall consult the Chief Highway Engineer/Structures for advice.

2.3.1

Bridges with Spans Less than 100 m

Table 2 gives values of dynamic pressure head to be used for design purposes in Hong Kong for bridges with maximum span less than 100 m. As the probability of much traffic being present on a bridge at gust velocities exceeding 44 m/s is low, the corresponding dynamic pressure head of 1.2 kN/m may be used for the loaded state at all locations. The values of dynamic pressure head to be used for the unloaded state at locations of intermediate exposure are to be interpolated, by the use of engineering judgement, between the extremes given for sheltered and exposed locations in Table 2. To aid designers in choosing suitable values, descriptions and examples of typical locations are given in Table 3. Values of dynamic pressure head derived from Table 2 shall be used to determine the nominal transverse, longitudinal and vertical wind loads described in Clauses 5.3.3, 5.3.4 and 5.3.5 of BS 5400 : Part 2. In all other respects, the provisions of Section 5.3 of BS 5400 : Part 2 regarding wind load shall be followed for structures with spans of 100 m and less.

2.3.2

Bridges with Any Span Greater than 100 m

For bridges with spans exceeding 100 m due account shall be taken of the loaded length under consideration and the height of the structure above ground. Due to the likelihood of wind loading governing the design of certain components higher load factors fL are required to account for the wind climate of Hong Kong. (1) Maximum Dynamic Pressure q for Sites in Exposed Terrain. For bridges with spans exceeding 100 m in exposed terrain the dynamic pressure head, q, shall be obtained from Table 4. (2) Minimum Dynamic Pressure q' on Relieving Areas of Bridges without Live Load. Where wind on any part of a bridge or element gives relief to the member under consideration, the effective coexistent value of minimum dynamic pressure on the parts affording relief shall be taken from Table 4 as the appropriate hourly wind speed dynamic pressure q'.

25 (3) Maximum Dynamic Pressure q on Bridges with Live Load. The maximum dynamic pressure, q, on those parts of the bridge or its elements on which the application of wind loading increases the effect being considered shall be taken as : (a) For highway and foot/cycle track bridges, q = 1.2 x qL/q20 kN/m but not less than q' where qL is the dynamic pressure obtained form Table 4 appropriate to the height of the bridge and the loaded length under consideration. q20 is the dynamic pressure obtained from Table 4 appropriate to the height of the bridge and a loaded length of 20 m. q' is given in Table 4 appropriate to the height of the bridge. (b) For railway bridges, q is the dynamic pressure obtained from Table 4 appropriate to the height of the bridge and the loaded length under consideration.

(4) Minimum Dynamic Pressure q' on Relieving Areas of Bridges with Live Load. Where wind on any part of a bridge or element gives relief to the member under consideration, the effective coexistent value of dynamic pressure q' on the parts affording relief shall be taken as 1.2 x q'/q kN/m where q' and q are obtained from Table 4 appropriate to the height of the bridge and the loaded length under consideration.

(5) Nominal Transverse Wind Load. The nominal transverse wind load Pt (in N) shall be taken as acting at the centroids of the appropriate areas and horizontally unless local conditions change the direction of the wind, and shall be derived from Pt = q A1 CD where q is the dynamic pressure head obtained from Sub-clauses (1) to (4) above.

A1 is the area defined in BS 5400 : Part 2 Clause 5.3.3.1. CD is the drag coefficient defined in BS 5400 : Part 2 Clauses 5.3.3.2 to 5.3.3.6 (6) Nominal Longitudinal Wind Load. The nominal longitudinal wind load PL (in N) shall be derived in accordance with BS 5400 : Part 2 Clause 5.3.4 using the appropriate value of q for superstructures with or without live load being adopted as obtained from Sub-clause (1) or (3) above. (7) Nominal Vertical Wind Load. The nominal vertical wind load PV (in N) shall be derived in accordance with BS 5400 : Part 2 Clause 5.3.5 using the appropriate value of q for

26 superstructures with or without live load being adopted as obtained from Sub-clause (1) or (3) above. (8) Load Combinations. The load combinations specified in BS 5400 : Part 2 Clause 5.3.6 shall be considered. (9) Design Loads. For design loads the factor fL shall be taken as follows : Ultimate Limit State 1.2 1.9 Serviceability Limit State 1.0 1.1

Wind Considered with erection dead load plus superimposed dead load only, and for members primarily resisting wind loads appropriate combination 2 loads relieving effects of wind

1.2 1.0

1.0 1.0

(10) Overturning Effects. Where overturning effects are being investigated the wind load shall also be considered in combination with vertical traffic live load. Where the vertical traffic live load has a relieving effect, this load shall be limited to one notional lane or to one track only, and shall have the following value : (a) (b) on highway bridges, not more than 6 kN/m of bridge; on railway bridges, not more than 12 kN/m of bridge.

(11) Load Factor for Relieving Vertical Live Load. For live load producing a relieving effect, fL for both ultimate limit state and serviceability limit state shall be taken as 1.0. (12) Aerodynamic Effects. Consideration shall be given to wind excited oscillations and the guidance provided in the Draft British Design Rules shall be followed. (13) Topographic Effects. To account for topographic effects factor S1 shall be determined in accordance with the provisions of CP3 Chapter V Part 2 Appendix D (1986 Amendment).

2.3.3

Covered Footbridges

Hong Kong Special Administrative Region Government policy requires footbridges either to be covered, or to be designed so that covers can be added subsequently. The provisions for wind loading in BS 5400 : Part 2, are only applicable to uncovered bridges. The following recommendations shall therefore be followed for covered footbridge.

27 Wind tunnel tests have been carried out on sections of decks and roofs commonly adopted for covered footbridges in Hong Kong as shown in Figure 1 to determine suitable wind load coefficients for design purposes. Details of the test designs, and results of the tests, are given in the reports entitled "Aerodynamic Loads on Covered Footbridges" by British Maritime Technology. The reports include values of drag and lift coefficients for decks with roof and for roof only, covering the full range of designs at angles of wind inclination () varying between 20 to the horizontal. Footbridges resembling the test designs as shown in Figure 1 shall be designed to resist wind loads derived from values of drag and lift coefficients taken from Tables 5 and 6. The coefficients given in the tables are the most unfavourable values between wind inclination of 5 because normal turbulence will cause wind inclination to vary between these angles. Where sidelong ground is concerned, the coefficients for angles of inclination corresponding to the fall of the ground shall be taken from Tables 7 and 8, and used as design values if greater than the coefficients for wind inclination varying between 5. For footbridges with shapes differing widely from the test designs, advice should be sought from aerodynamic specialists. A stairway model was also included among the wind tunnel tests. The stairway model test results indicate that the wind forces acting on a stairway may be greater than those acting on the adjacent main span. Values of CD = and 1.2 x main span value given in Tables 5 to 8,

CL = +1.7 or -1.1

shall accordingly be used for the design of stairways. The longitudinal wind load is also significant for stairways and shall be allowed for using a value of PL = q x Cs x A1, where PL = Cs = = Nominal longitudinal wind load acting horizontally, Coefficient of longitudinal load acting horizontally on stairway or ramp. 2.35, and

the definitions of q, CD, CL, and A1 are given in BS 5400 : Part 2. Ramps will similarly experience wind forces greater than those acting on the adjacent main span. The values recommended above for stairways shall also be used for ramps. For stairways and ramps, the area A as defined in Clause 5.3.5 of BS 5400 : Part 2 to 3 obtain the vertical wind load shall be the inclined area of the deck, and not the projected area of the deck in plan.

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29 2.4 TEMPERATURE EFFECTS

While the general recommendations of Section 5.4 of BS 5400 : Part 2 are valid for all highway structures and railway bridges, those recommendations which relate to particular environmental effects and material properties are specific to United Kingdom conditions. Recommendations given in Clauses 5.4.1 to 5.4.6 of BS 5400 : Part 2 including Figure 9, Tables 10, 11 & 12 and Appendix C consequently cannot be used and shall be replaced by the following recommendations formulated for Hong Kong conditions.

2.4.1

General

Daily and seasonal fluctuations in shade air temperature, solar radiation, re-radiation, etc. cause the following : (a) Changes in the effective temperature of a bridge superstructure which, in turn, govern its movement. The effective temperature is a theoretical temperature calculated by weighting and adding temperatures measured at various levels within the superstructure. The weighting is in the ratio of the area of cross-section at the various levels to the total area of cross-section of the superstructure. Over a period of time, there will be a minimum, a maximum, and a range of effective bridge temperature, resulting in loads and/or load effects within the superstructure due to : (i) restraint of associated expansion or contraction by the form of construction (e.g. portal frame, arch, flexible pier, elastomeric bearings) referred to as temperature restraint; and friction at roller or sliding bearings where the form of the structure permits associated expansion and contraction, referred to as frictional bearing restraint.

(ii)

(b)

Differences in temperature between the top surface and other levels in the superstructure. These are referred to as temperature differences and they result in loads and/or load effects within the superstructure.

2.4.2

Effective Bridge Temperatures

Values of basic effective bridge temperatures shall be obtained from Table 9 for superstructure Groups 1 to 4. Basic effective temperatures appropriate to a return period of 120 years shall be used except for the cases given below. Basic effective bridge temperatures appropriate to a return period of 50 years may be used for : (a) foot/cycle track bridges,

30 (b) carriageway joints and similar equipment likely to be replaced during the life of the structure, erection loading.

(c)

(1) Adjustment of Effective Bridge Temperature for Thickness of Surfacing. The effective bridge temperatures are dependent on the depth of surfacing on the bridge deck, and the values given in Table 9 assume surfacing depths of 40 mm for Groups 1 and 2 and 100 mm for Groups 3 and 4. Where the depth of surfacing differs from these values, the minimum and maximum effective bridge temperatures shall be adjusted by the amounts given in Table 10. (2) Adjustment for Height above Mean Sea Level. The values of effective temperature given in Table 9 shall be adjusted for height above mean sea level by subtracting 0.5C per 100 m height for minimum effective temperatures and 1.0C per 100 m height for maximum effective temperatures. (3) Range of Effective Bridge Temperature. In determining load effects due to temperature restraint, the effective bridge temperature at the time the structure is effectively restrained upon completion of construction shall be taken as the datum effective bridge temperature. The load effects shall be calculated for expansion up to the maximum effective bridge temperature and contraction down to the minimum effective bridge temperature from this datum temperature. For design purpose, this datum effective bridge temperature shall be assumed to be in the range of 10C to 30C.

2.4.3

Temperature Difference

Effects of temperature differences within the superstructure shall be derived from the data given in Figure 2. Positive temperature differences occur when conditions are such that solar radiation and other effects cause a gain in heat through the top surface of the superstructure. Conversely, reverse temperature differences occur when conditions are such that heat is lost from the top surface of the bridge deck as a result of re-radiation and other effects. (1) Application of Effective Bridge Temperatures. Maximum positive temperature differences shall be considered to coexist with effective bridge temperatures at above 25C for Groups 1 and 2 and 15C for Groups 3 and 4. Maximum reverse temperature differences shall be considered to coexist with effective bridge temperatures up to 8C below the maximum for Groups 1 and 2, up to 4C below the maximum for Group 3, and up to 2C below the maximum for Group 4. (2) Adjustment of Temperature Difference for Thickness of Surfacing. Temperature differences are sensitive to the thickness of surfacing, and the data given in Figure 2 assume depths of 40 mm for Groups 1 and 2 and 100 mm for Groups 3 and 4. For other depths of surfacing, the values given in Tables 11, 12, and 13 may be used as appropriate.

31 2.4.4 Coefficient of Thermal Expansion

For the purpose of calculating temperature effects, the coefficients of thermal expansion shall be taken as 12x10-6/C for structural steel and 9x10-6/C for concrete.

2.5

EFFECTS OF SHRINKAGE AND CREEP

Section 5.5 of BS 5400 : Part 2 refers to the need to take into account the effects of shrinkage or creep in concrete, and similar sources of strain. The recommendations of BS 5400 in respect of shrinkage and creep in concrete are not suitable for use in Hong Kong. Reference shall be made to Clauses 4.2.4 and 4.2.5 for amplification with regard to Hong Kong conditions.

2.6

EARTHQUAKE FORCES

Although the risk of a major earthquake occurring close to Hong Kong is small, seismicity records for southern Guangdong show a recurrence period of about 400 years for an earthquake of magnitude 6 or above. The possibility of such an earthquake occurring must accordingly be considered. The Guangdong records indicate that structures built in Hong Kong to withstand ground accelerations of 0.07 g would probably have survived all the earthquakes recorded in Guangdong since 288 A.D. Highway structures and railway bridges shall be designed to withstand seismic forces corresponding to accelerations of this magnitude. Allowance for seismic effects shall be made by means of the equation V = CW where V C W = = = nominal seismic force; seismic coefficient assumed to be 0.05; and total vertical load comprising : (a) the permanent vertical loads; and (b) (i) either : for highway structures, 1/3 type HA loading on one notional lane in each direction; or

(ii) for railway bridges, type RU loading on one railway track. The nominal seismic force shall be multiplied by partial load factors of 1.00 for the serviceability limit state and 1.40 for the ultimate limit state to obtain the design seismic forces. The

32 design seismic force for the ultimate limit state will thus correspond with the figure suggested by the Guangdong records. The ultimate limit state has more relevance to earthquakes than has the serviceability limit state. The design seismic force shall be applied successively longitudinally and transversely at footing level and to the superstructure, making four loading conditions to be considered in all. Statical treatment of seismic effects is adequate for short span structures, but may result in uneconomically large loadings for long span structures. Dynamic seismic analysis shall accordingly be considered for structures of more than 100 m span. Seismic forces occurring during major earthquakes often cause the superstructures of bridges to slip sideways off their supporting substructures. Consideration shall be given to this possibility, and if necessary physical restraints shall be provided to prevent any such sideways movement.

2.7 2.7.1

COLLISION LOADS Bridge Superstructures

The overall structural integrity of the bridge shall be maintained following an impact due to collision of heavy goods vehicles with bridge superstructures, but local damage to a part of the bridge deck can be accepted. In applying these requirements checks shall be made for overall stability, local effects and progressive failure after removing elements whose load bearing capacity would be directly impaired as appropriate such that : (a) (b) The bridge deck must not lift or slide off its bearings. In the case of bridge decks with a number of carrying members e.g. beam and slab type decks, the structure as a whole must not collapse with any one of the carrying members being assumed to have failed; alternatively individual members can be checked for failure as at (c). In the case of bridge decks with a single carrying member e.g. spine beams, local failure or damage of elements (e.g. webs or flanges) or of joints between elements may be allowed but the structure as a whole must not collapse.

(c)

For bridge decks with a small number of beams or girders, the designer may choose to include the reduced contribution of an individual damaged beam rather than assume it to be ineffective. This is also applicable to parts of voided slabs. All design checks are to be carried out at the ultimate limit state only and checks at the serviceability limit state are not necessary.

33 The applicability of the various checks to different types of bridge decks is described in Table 14. 2.7.2 Highway Overbridges Supports exposed to possible vehicle collisions shall be protected by metal or concrete barrier fences. Gantry supports shall be designed to resist a nominal load of 50 kN acting in the worst possible direction and at the worst height up to 3000 mm above the adjacent carriageway. (1) Nominal Load on Supports (Replaces Clause 6.8.1 of BS 5400 : Part 2). The nominal loads are given in Table 15 together with their direction and height of application, and shall be considered as acting horizontally on bridge supports. Supports shall be capable of resisting the main and residual load components acting simultaneously. Loads normal to the carriageway shall be considered separately from loads parallel to the carriageway. (2) Nominal Load on Superstructures (Replaces Clause 6.8.2 of BS 5400 : Part 2). The nominal loads are given in Table 16 together with their direction of application. The load normal to the carriageway shall be considered separately from the load parallel to the carriageway. The loads shall be considered to act as point loads on the bridge superstructure in any direction between the horizontal and vertical. The load shall be applied to the bridge soffit, thus precluding a downward vertical application. Given that the plane of the soffit may follow a superelevated or nonplanar form, the load can have an outward or inward application. For the design of lightweight structures, such as footbridges, the reduced nominal loads shown within brackets in Table 16 shall be used. (3) Associated Nominal Primary Live Load (Replaces Clause 6.8.3 of BS 5400 : Part 2). No primary live load is required to be considered on the bridge. (4) Load Combination (Replaces Clause 6.8.4 of BS 5400 : Part 2). Vehicle collision loads on supports and on superstructures shall be considered separately, in combination 4 only, and need not be taken as coexistent with other secondary live loads. (5) Design Load (Replaces Clause 6.8.5 of BS 5400 : Part 2). For all elements excepting elastomeric bearings, the effects due to vehicle collision loads on supports and on superstructures need only be considered at the ultimate limit state. The fL to be applied to the nominal loads shall have a value of 1.50. The design loads shall be applicable for global effects only i.e. local effects at the point of impact are to be ignored. For the design of lightweight structures, such as footbridges, the supports shall be designed to the reduced main load and residual load components shown within brackets in Table 15. For elastomeric bearings, the effects due to vehicle collision loads on supports and on superstructures shall be only considered at the serviceability limit state. The fL to be applied to the nominal loads shall have a value of 1.0.

34

(6) Vehicle Collision Loads for Foot/Cycle Track Bridge Supports and Superstructures (Replaces Clause 7.2 of BS 5400 : Part 2). The vehicle collision loads specified in Clause 6.8 of BS 5400 : Part 2 shall be considered in the design of foot/cycle track bridges.

2.7.3

Railway Overbridges

The potential collision loading on a bridge over a railway track is many times any of the loadings given in Clause 2.7.2. Effective protection of supports against derailment collisions is accordingly difficult. Clause 13.4 contains recommendations concerning not only the collision loadings for which allowance shall be made but also other means by which the severity of collision effects may be ameliorated.

2.7.4

Bridges Over Navigation Channels

Bridge piers situated in navigation channels may be subjected to ship collision loadings. The magnitude and form of such collision loadings depend so much on the location of the bridge and the nature of the shipping using the navigation channel that specific guidance cannot be given, but the possibility of ship collisions shall always be considered at the design stage and appropriate protection provided. Clauses 13.5.5 and 18.2 also deal with ship collisions.

2.8

PARAPET LOADING

Loads transmitted by vehicle collisions with parapets to structural elements supporting parapets shall be dealt with as described in Section 6.7 of BS 5400 : Part 2. Parapets shall be designed in accordance with the requirements of and to resist the loads described in Chapter 15 of this Manual.

2.9

LOADS ON RAILWAY OVERBRIDGES FROM ELECTRICAL SUPPLY EQUIPMENT

A bridge crossing a railway track may be required to carry overhead electrical supply equipment. Reference should be made to the appropriate railway authority for the extra loading to be carried.

2.10

LIVE LOADING

Highway structures and their elements shall be designed to resist type HA loading, or type HA loading combined with type HB loading, whichever is more severe in its effects. Generally 45 units of type HB loading shall be used, but for the serviceability limit state, 25 units of type HB

35 loading may be used when calculating crack widths in reinforced concrete, and when investigating limitations on flexural tensile stresses in prestressed concrete under load combination 1. Highway structures spanning less than 15 m situated on rural roads other than trunk or main roads shall be designed for type HA loading only. Where special considerations indicate that a lesser live load would be appropriate, the agreement of the Chief Highway Engineer/Structures to its use must first be obtained. Traffic flows, representative of those predicted to use heavily trafficked commercial routes in Hong Kong at around the year 2011, have been mathematically simulated and statistically analysed in order to determine characteristic (1 in 2400 chance of being exceeded in a year) live load effects which can arise on bridge structures. It has been found that it is justifiable to use BS 5400 : Part 2, published as Appendix A to the United Kingdom Department of Transport Departmental Standard BD 37/88 as the basis for Hong Kong bridge loading, with amendments to suit Hong Kong traffic types as necessary. The recommendations of BS 5400 : Part 2 Clause 6.2.1 including Table 13 and Figure 10 regarding nominal Uniformly Distributed Load (UDL) of the type HA loading and Clauses 6.4.1.1, 6.4.2 including Table 14 and Figure 13, regarding the application of type HA loading shall be replaced by the following requirements which are based on Hong Kong conditions.

2.10.1

Nominal Uniformly Distributed Load (UDL) (Replaces Clause 6.2.1 of BS 5400 : Part 2)

For loaded lengths up to and including 70 m, the UDL, expressed in kN per linear metre of notional lane, shall be derived from the equation W = 400 x (1/L)0.67 and for loaded lengths in excess of 70 m but less than 1400 m the UDL shall be derived from the equation W = 44 x (1/L)0.15 where L is the loaded length (in m) and W is the load per metre of notional lane (in kN). For loaded lengths above 1400 m, the UDL shall be 14.85 kN/m. Values of the load per linear metre of notional lane are given in Table 17 (replaces Table 13 of BS 5400 : Part 2) and the loading curve illustrated in Figure 3 (replaces Figure 10 of BS 5400 : Part 2).

36 2.10.2 HA Lane Factors (Replaces Clause 6.4.1.1 of BS 5400 : Part 2)

The HA UDL and KEL shall be multiplied by the appropriate factors from Table 18 (replaces Table 14 of BS 5400 : Part 2) before being applied to the notional lanes indicated. Where the carriageway has a single notional lane as specified in Clause 3.2.9.3.2 of BS 5400 : Part 2, the HA UDL and KEL shall be applied to a lane width of 2.5 m. The loading of the remainder of the carriageway width shall be taken as 5 kN/m.

2.10.3

Types HA and HB Loading Combined (Replaces Clause 6.4.2 of BS 5400 : Part 2) Types HA and HB loading shall be combined and applied as follows : (a) Type HA loading shall be applied to the notional lanes of the carriageway in accordance with Clause 6.4.1 of BS 5400 : Part 2 modified as given in (b) below. Type HB loading shall occupy any transverse position on the carriageway, either wholly within one notional lane or straddling two or more notional lanes.

(b)

Where the HB vehicle lies wholly within the notional lane (e.g. Figure 4(1)) or where the HB vehicle lies partially within a notional lane and the remaining width of the lane, measured from the side of the HB vehicle to the edge of the notional lane, is less than 2.5 m (e.g. Figure 4(2)(a)), type HB loading is assumed to displace part of the HA loading in the lane or straddled lanes it occupies. No other live loading shall be considered for 25 m in front of the leading axle to 25 m behind the rear axle of the HB vehicle. The remainder of the loaded length of the lane or lanes thus occupied by the HB vehicle shall be loaded with HA UDL only; HA KEL shall be omitted. The intensity of the HA UDL in these lanes shall be appropriate to the loaded length that includes the total length displaced by the type HB loading with the front and rear 25 m clear spaces. Where the HB vehicle lies partially within a notional lane and the remaining width of the lane, measured from the side of the HB vehicle to the far edge of the notional lane, is greater than or equal to 2.5 m (e.g. Figure 4(2)(b)), the HA UDL loading in the lane shall remain, the HA KEL shall be omitted. Only one HB vehicle shall be considered on any one superstructure or on any substructure supporting two or more superstructures. Figure 4 (replaces Figure 13 of BS 5400 : Part 2) illustrates typical configurations of type HA loading in combination with type HB loading.

37 2.11 FOOTBRIDGE AND SUBWAY COVERS

Covers shall be designed to withstand all the appropriate permanent, temporary and transient loads described in BS 5400 : Part 2. In addition, covers shall be designed to resist a live load of 0.5 kN/m2, which shall be considered as a secondary live load in conjunction with the other loads and partial load factors appropriate to combination 4. No other primary or secondary live loads need be considered.

2.12 2.12.1

DYNAMIC EFFECTS Aerodynamic Effects

The possibility of wind-excited oscillations occurring shall be considered, and due allowance made for their effects. If necessary, aerodynamic effects shall be investigated by testing. Flexible structures such as suspension bridges, cable-stayed bridges and sign gantries are particularly susceptible.

2.12.2

Highway Bridges

Dynamic effects on highway bridges are usually deemed to be covered by the allowance for impact included in live loadings. However, although such considerations may be sufficient structurally, the possibility of highway users being adversely affected shall also be considered. Complaints about the liveliness of highway structures have been made in Hong Kong as a result of the occupants of traffic stalled in one lane of a structure being subjected to oscillations caused by traffic moving in a neighbouring lane. Similar situations could recur at any time under the conditions prevailing in Hong Kong. Highway structures oscillate in sympathy with passing vehicles oscillating on their suspensions as a result of road surface irregularities. The worst oscillations occur when the natural frequency of a structure lies within the range of forcing frequencies imposed by passing traffic. Such forcing frequencies generally range between 2 Hz and 5 Hz. Highway structures shall accordingly be designed so that as far as possible their natural frequencies lie outside this range.

2.12.3

Footbridges

Pedestrians can be adversely affected by the dynamic behaviour of footbridges. To avoid unpleasant vibrations, the natural frequencies of footbridge superstructures and columns shall be not less than 5 Hz. If the natural frequency of a footbridge superstructure or column is less than 5 Hz, the maximum acceleration shall be limited to an acceptable value. The natural frequencies of footbridge columns shall in all cases exceed 2 Hz transversely and 1 Hz longitudinally. Appendix A gives

38 guidance on the calculation of natural frequencies and accelerations, and quotes acceptable values for accelerations. The possibility of a group of pedestrians deliberately causing a f otbridge to oscillate o resonantly shall be borne in mind. Footbridge bearings shall be designed to allow for this possibility, and prestressed concrete beams shall be provided with sufficient untensioned reinforcement to resist a reversal of 10% of the static live load bending moment. Guides shall be provided where necessary to prevent any tendency for a superstructure to bounce off its bearings.

2.13

DEAD LOAD AND SUPERIMPOSED DEAD LOAD

In assessing dead load, the weight of concrete shall be taken as not less than 24.5 kN/m3. If the structural concrete of the deck of a structure is to be used as the running surface, the assessment of dead load shall include allowance for a minimum extra thickness of 25 mm of concrete. If the running surface is to consist of asphalt, the assessment of superimposed dead load shall include allowance for a minimum thickness of 100 mm of asphaltic surfacing material. The values of dead load and superimposed dead load assumed for preliminary design purposes shall b carefully checked against the final values, when known, and if necessary, the e calculations shall be appropriately amended.

39

3.3.1 GENERAL

DESIGN OF STEEL BRIDGES

Steel highway structures and railway bridges shall be designed in accordance with the requirements of BS 5400 : Part 3 : Code of Practice for Design of Steel Bridges in so far as its recommendations are applicable to Hong Kong conditions. For conditions peculiar to Hong Kong, the recommendations of this manual shall be followed.

3.2

HOT FORMED STRUCTURAL HOLLOW SECTIONS

Hot formed hollow sections with steel properties in accordance with BS 4360 and section sizes conforming to BS 4848 : Part 2 shall be used for all structural steelworks. Designers shall check that the sections proposed will be available in the quantities required before finalising the design. The use of cold formed sections as an alternative shall not be permitted.

3.3

FABRICATION

Structural steelwork shall be fabricated and erected by specialist contractors in the "Structural Steelwork" category of the List of Approved Suppliers of Materials and Specialist Contractors for Public Works. All structural steelworks shall be detailed so that they can be hot dip galvanized after fabrication, and so that they can be erected without damaging the galvanizing and without on site welding. For long span trusses and structures too large for hot dip galvanizing after fabrication, consideration shall be given to the application of sprayed metal coating after fabrication. Where this is not possible, the agreement of the Chief Highway Engineer/Structures to the use of any proposed welding after the application of metal coatings must first be obtained. If non-ferrous components are used with steel fixings, insulation must be provided to prevent galvanic corrosion. Hot rolled steel sections shall be blast cleaned and protected with blast primers before fabrication and welding. This prevents the development of rust, which would be difficult to remove after fabrication. The use of steel that has rusted heavily during storage shall not be allowed for the same reason. When welding metal coated or zinc dust painted steel, the coating near the weld area shall first be removed, or the weld area be masked off before coating. After welding, scale and heat damaged coatings shall be removed by local blast cleaning and the areas renovated by re-applying the original coating. Damaged galvanized or metal sprayed surfaces shall be made good by : (a) (b) metal spraying on site; application of zinc rich paints to reinstate the original dry film thickness; or

40

(c)

application of low melting point zinc alloy heated by torch to a pasty condition with the fluxes contained therein removed.

The site welds on painted structures shall be blast cleaned before protection and repainting.

3.4

BLAST CLEANING

Blast cleaning of steelworks shall be carried out by specialist contractors in the "Class V : Hot dip galvanizing" of the "Specialized Operations for Highway Structure" category of the List of Approved Suppliers of Materials and Specialist Contractors for Public Works.

3.5

NON-DESTRUCTIVE TESTING OF WELDS

All structurally important welds of structural steelwork shall be subject to non-destructive testing in the form of radiographic or ultrasonic inspection and interpretation by specialist contractors in the "Class IV : Non-destructive testing of welds" of the "Specialized Operations for Highway Structure" category of the List of Approved Suppliers of Materials and Specialist Contractors for Public Works. The extent of testing is given in BS 5400 : Part 6, Clause 5.5.2. The designer shall specify the welds to be tested above this requirement.

3.6

HOT DIP GALVANIZING

All steel components shall be hot dip galvanized in accordance with BS 729 after fabrication by specialist contractors in the "Class V : Hot dip galvanizing" of the "Specialized Operations for Highway Structure" category of the List of Approved Suppliers of Materials and Specialist Contractors for Public Works. Steel hollow sections shall be sealed wherever this can be done without affecting the galvanizing process. If venting is necessary, the vents shall be carefully detailed and positioned so as to be inconspicuous, or be effectively sealed immediately after galvanizing.

41

4.4.1 4.1.1 GENERAL

DESIGN OF CONCRETE BRIDGES

Design Standards

Concrete highway structures and railway bridges shall be designed in accordance with the requirements of BS 5400 : Part 4 : Code of Practice for Design of Concrete Bridges in so far as these are applicable to Hong Kong. For conditions peculiar to Hong Kong, the recommendations of this manual shall be followed.

4.1.2

Cracking For the serviceability limit state of cracking : (a) reinforced concrete structures or structural elements shall be designed so that design crack widths do not exceed the values given in Table 20; and for the flexural tensile stress limitations in prestressed concrete as described in BS 5400 : Part 4 Clause 4.2.2(b), only Class 2 category will be permitted for load combinations 2 to 5.

(b)

4.1.3

Concrete Cover to Reinforcement

Concrete cover to reinforcement shall be provided in accordance with Table 20 for the envisaged conditions of exposure. In choosing the nominal cover for a structure, the possibility of the specified cover not being provided in practice shall be borne in mind. The reinforcement of Hong Kong bridges has on several occasions apparently suffered corrosion because of insufficient cover. In Britain, a survey found inadequate cover on a number of bridges; walls were found to have suffered more than soffits, with standard deviations of cover averaging 15.1 mm for the former and 6.4 mm for the latter. Allowance shall accordingly be made for imperfections of similar magnitudes in choosing values of nominal cover.

4.2

MATERIAL PROPERTIES

4.2.1

Differences between British and Hong Kong Concretes

The properties of concrete in Hong Kong differ from the properties given in BS 5400 : Part 4 for concrete in United Kingdom, probably because of the qualities of the local aggregate and the properties of the locally available cement.

42 Suitable values for concrete properties in Hong Kong are recommended later in this section. Other concrete properties mentioned in BS 5400 : Part 4 may also have different values in Hong Kong, and the possibility shall be borne in mind during design. The shape of the stress-strain curve given in Figure 1 of BS 5400 : Part 4 is generally applicable to Hong Kong, but the slope of the initial tangent shall be adjusted to 5.0 x ( fcu / m ) to allow for the difference in elastic modulus.

4.2.2

Elastic Modulus of Concrete

Values for the modulus of elasticity of concrete are significantly lower in Hong Kong than in Britain. The values used for design purposes shall be taken from Table 21 to replace the values for the short term elastic modulus given in BS 5400. The modulus of elasticity referred to in Table 21 is the static modulus described in BS 1881.

4.2.3

Strength of Concrete

Concrete gains strength at early ages more rapidly in Hong Kong than in Britain. After 28 days, the rate of gain of strength is lower in Hong Kong than in Britain. Table 5 of BS 5400 : Part 4 shall accordingly not be used in Hong Kong. Figure 5, giving ( fn / f28 ) (the ratio of strength at any time "n" to 28-day strength) against time, shall be used instead. The rate of strength gain indicated in the figure applies to OPC concrete only and not to concrete containing PFA or retarding/accelerating admixtures. Concrete for the carriageway and superstructures including concrete parapets shall be of Grade 40 or stronger.

4.2.4

Shrinkage of Concrete Shrinkage is the decrease in size that occurs over a long period as the water in concrete

dries out. Dimensional changes in concrete members resulting from shrinkage not only affect the stresses in statically indeterminate structures, but also contribute to loss of prestress in prestressed concrete structures. The design of such structures shall make allowance for the effects of shrinkage. Shrinkage has irreversible and reversible components. Irreversible shrinkage is caused by the concrete setting and drying out. Reversible volume changes occur when the moisture content of the concrete varies with the ambient relative humidity. Shrinkage is also affected by the composition of the concrete, the size of the member under consideration and the amount of longitudinal reinforcement in the member.

43 Details of the various factors affecting shrinkage are given in Appendix C of BS 5400 : Part 4. Experience has shown that the amount of shrinkage to be anticipated in Hong Kong is greater than the amount likely to occur in Britain, so the recommendation of BS 5400 shall be modified for Hong Kong use as follows : Appendix C of BS 5400 : Part 4 states that the shrinkage strain at any instant is given by cs = KL Kc Ke Kj in microstrains, where allowance is made by KL for relative humidity, Kc Ke Kj for concrete composition, for effective thickness, and for time.

Experiments by Chai (1980) conducted on 100 x 100 x 500 mm prisms of Grade 45 concrete at relative humidity of 70% to 100% indicated that, using the same expression for Hong Kong conditions, shrinkage shall be estimated from cs = Cs KL Kc Ke Kj in microstrains, where Cs = 4.0, and KL, Kc, Ke and Kj are as defined in Appendix C of BS 5400 : Part 4.

The shrinkage to be expected over an interval of time shall be taken as the difference between the shrinkages calculated for the beginning and the end of the interval. This is particularly important for prestressing applications, since prestress can only be transferred after some shrinkage has occurred. Shrinkage is greatly reduced by the presence of reinforcement. T values derived from he the foregoing expressions, which are for plain concrete, shall be multiplied by the reinforcement coefficient "K s" to obtain the corresponding values for reinforced concrete. The reinforcement coefficient is given by Ks = where 1 1 + e

= steel ratio = As / Ac

44

e

= modular ratio = Es / Ec

Figure 6 gives values of "K s" in graphical form for various concrete grades and steel ratios. Experience has shown that the allowance made for shrinkage in British codes was insufficient for Hong Kong conditions. Experiments were therefore carried out, in conjunction with creep testing, to determine suitable values of shrinkage strain for concrete in Hong Kong. These experiments have been reported by Chai (1980) and the extracts from the results are shown in Figure 7. Figure 7 indicates that the gross shrinkage strain to be anticipated in Hong Kong is about four times the gross shrinkage strain obtained using the recommendations given in Appendix C of BS 5400 : Part 4.

4.2.5

Creep of Concrete

Concrete under sustained loading deforms with time. The deformation which occurs is known as creep. Such deformations not only affect the stresses in statically indeterminate structures but also contribute to the loss of prestress in prestressed concrete structures. In addition, the horizontal movement of prestressed concrete members as a result of creep can significantly affect the design of bearings and joints. Allowance shall accordingly be made for the effects of creep. Details of the various factors affecting creep are given in Appendix C of BS 5400 : Part 4 and recommendations for the allowance to be made for creep deformation are given in BS 5400 : Part 4. These vary significantly from each other. Creep tests on Hong Kong concrete by Chai (1980) have shown that the recommendations given in Appendix C of BS 5400 : Part 4 are appropriate to Hong Kong conditions and shall be followed . The final creep deformation to be anticipated according to the theory of linear creep is given by the expression cc = fc . E28

where fc

= constant concrete stress,

E28 = 28 day value of concrete secant modulus, = creep factor.

The value of the creep factor is given by the expression = KL Km Kc Ke Kj

45

where the "K" coefficients are as defined in Appendix C of BS 5400 : Part 4. Creep is reduced by the presence of reinforcement in the same way as shrinkage. The values derived for cc from the foregoing are for plain concrete, and shall be multiplied by the reinforcement coefficient "K s" described in Clause 4.2.4 and Figure 6 to obtain the corresponding values for reinforced concrete. Creep tests were carried out to determine whether creep occurring in Hong Kong differed significantly from the recommendations of British codes and, if so, to recommend suitable values for use in Hong Kong. The tests have been reported by Chai (1980). Details of the specimens tested are given in Table 22, and extracts from the results are shown in Figures 8 to 10. Figures 8 to 10 indicate that the creep factor varies widely between concrete from different sources. Factors not described in BS 5400 such as the use of additives and the initial curing also significantly influence creep deformation. Nevertheless, the recommendations of Appendix C of BS 5400 : Part 4 give reasonable estimates of the creep deformation and prestress losses to be anticipated for Hong Kong concrete.

4.2.6

Coefficient of Thermal Expansion of Concrete

Values of the coefficient of thermal expansion are given in BS 5400 : Part 2 and Part 4 for concrete made from different aggregates. Tests have shown that a value of 9 x 10-6 per C is appropriate for use in Hong Kong and this value shall be used instead of the values given in BS 5400.

4.2.7

Strength of Steel Reinforcement

BS 5400 : Part 4 requires that only steel complying with relevant British Standards shall be used. In Hong Kong, Construction Standard CS2 specifies requirements for hot rolled steel bars for reinforcement of concrete. It covers plain round steel bars in grade 250 and deformed high yield steel bars in grade 460. Accordingly, designs shall be based on a characteristic strength of 250 N/mm2 for plain round steel bars and 460 N/mm2 for high yield steel bars. Welding of hot rolled high yield steel bars shall not be permitted.

4.3

EARLY THERMAL MOVEMENT

Immature concrete expands as a result of the heat released during hydration. Cracking can occur if any part of the immature concrete is restrained from moving when the heat of hydration dissipates, and cooling and contraction take place. Reinforcement shall be provided to control such cracking.

46 The minimum amount of reinforcement to be provided is given by = fct / fy where = steel ratio = As / Ac fct = tensile strength of immature concrete = (fcu / 20)2/3 N/mm approximately fy As Ac = characteristic strength of reinforcement = area of reinforcement = gross area of concrete

The surface of immature concrete cools and contracts before the core. The proportion of reinforcement required shall accordingly be calculated using an "effective surface zone", assumed to be 250 mm thick, on each face for the area of concrete "A c". There are thus two cases to be considered : (a) members less than 500 mm thick, and (b) members equal to or more than 500 mm thick. For members less than 500 mm thick, the steel ratio "" shall be applied to the whole cross-sectional area to obtain the amount of steel required. This amount shall be provided equally divided between the two faces and shall be provided in each of the two directions. Considerations of crack widths and spacing generally mean that more reinforcement is required to control cracking than the minimum amount given by the above formula. The likely maximum spacing of cracks is given by s where = ( fct / fb ) x ( / 2 )

s = maximum crack spacing

fct / fb = ratio of tensile strength of immature concrete to average bond strength, which may be taken as = 1 for plain round bars = 2/3 for deformed bars

47 = bar size. For design purposes, the above relation may be more conveniently expressed as n ( fct / fb ) x ( 2bh / s ) where n = number of bars in section b = width of section h = depth of number. The maximum crack width which occurs during cooling from peak hydration temperature to ambient temperature may be taken as wmax = s . at .T1 / 2 where at = coefficient of thermal contraction of mature concrete T1 = fall in temperature between hydration peak and ambient

The permissible crack width "w" shall be that which is appropriate for the environmental conditions given in Table 20 less the crack width resulting from flexure. The effective thermal coefficient of immature concrete is taken as half the mature value given at Clause 4.2.6. Any further fall in temperature T2 due to seasonal variations will also contribute to cracking. Various factors due to ageing seem again to reduce the effect of thermal contraction by about half, so that the combined maximum crack width is wmax = s . at . ( T1 + T2 ) / 2 For members less than 15 m long, or with movement joints at 15 m centres or less, the effect of T may be neglected. T may also be neglected if the restraint is being provided by a 2 2 section subject to the same climatic exposure as that being restrained. The formulae given above may be used to determine the amount of reinforcement required to control cracking. Alternatively the amount of reinforcement to be provided may be taken from Figure 11, which has been prepared assuming values of 35C and 30C respectively for T1 and T2 as being representative of Hong Kong conditions. Reinforcement that is present in the section for other purposes may be included as part of the area of reinforcement necessary to satisfy the requirements for the control of early thermal cracking.

48 4.4 4.4.1 PRESTRESSING Grade of Concrete for Prestressing Work

In Hong Kong, the production of concrete with a strength consistently exceeding 50 N/mm2 may be difficult. Therefore, unless strict control can be relied upon, the strength of concrete for prestressed structures shall accordingly be limited to Grade 50. Strength of concrete for prestressing work shall not be less than Grade 45. 4.4.2 Post-tensioning Systems

Various proprietary post-tensioning systems are available in Hong Kong. To avoid any suggestion that the choice of a proprietary post-tensioning system might be influenced by other than engineering considerations, trade names shall not be included in specifications or drawings. Instead, general prestressing requirements shall be given in the contract documents, and the main contractor shall be required to submit detailed proposals to the Engineer for approval showing how one of the acceptable proprietary post-tensioning systems may be used to apply the required prestressing forces. Such general requirements may include, as appropriate, any or all of the following : (a) (b) (c) number, location and profile of prestressing tendons; number of wires, strands or bars per tendon; size and type of wire, strand or bar (standard, high-strength, compacted; normal or low relaxation); anchorage type (dead-end, coupling or stressing-end); order of applying prestressing force to tendons;

(d) (e)

(f) prestressing force; and (g) ducting and grouting requirements.

The contract documents shall make clear whether the value of prestressing force includes losses due to : (a) (b) (c) (d) relaxation of steel; elastic deformation of concrete; shrinkage and creep of concrete; friction and wobble;

49 (e) draw-in, where appropriate giving details of any assumption made, and also making clear whether allowance shall be made for anchorage and jack losses. Consideration must be given at the design stage to the practicability of fitting one or other of the acceptable proprietary post-tensioning systems into the work being designed, so that the posttensioning specialists are not set an impossible task. End-block reinforcement depends on the type of anchorage used, and so shall not be detailed, but, again, consideration shall be given at the design stage to likely requirements. The proposals submitted by the main contractor must accordingly include end-block reinforcement details, which shall be designed in accordance with the requirements of BS 5400 : Part 4.

4.4.3

Specialist Prestressing Contractors

All prestressed concrete works for highway structures shall be carried out by specialist contractors in the Prestressed Concrete for Highway Structures Category of the List of Approved Suppliers of Materials and Specialist Contractors for Public Works. The Prestressed Concrete for Highway Structures Category consists of two classes :Class I - Supply and Installation of Prestressing Systems; and Class II- Supply of Prestressed Concrete Units. The supply and installation of on-site prestressing work shall be carried out by a contractor in Class I. Precast prestressed units manufactured off-site shall be supplied by a contractor in Class II.

4.4.4

Secondary Moments and Shear

The redistribution of elastically derived moments for ultimate limit state analysis is permitted within certain limits by BS 5400 : Part 4. Secondary or parasitic moments in indeterminate structures, which are not lost under conditions of partial redistribution, shall be included in any such analysis with a partial load factor of 1.0. Secondary moments in indeterminate structures induce reactions at intermediate supports which are additional to those generated by dead and live loads. These reactions give rise to shear forces which must be taken into account when determining the total shear force at any point in such a structure.

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5.

DESIGN OF COMPOSITE BRIDGES

Composite highway structures shall be designed in accordance with the requirements of BS 5400 : Part 5 : Code of Practice for Design of Composite Bridges. Where Hong Kong specifications or conditions differ from the requirements or conditions described in BS 5400, adjustments appropriate to Hong Kong shall be made. In view of the large shrinkage of local concrete, due consideration shall be given to the effects of concrete shrinkage on composite structures early in the design stage.

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6.

SPECIFICATION FOR MATERIALS AND WORKMANSHIP, STEEL

Structural steelwork shall be fabricated in accordance with the requirements of BS 5400 : Part 6 : Specification for Materials and Workmanship, Steel in so far as its recommendations are appropriate to Hong Kong conditions. Where Hong Kong specifications or conditions differ from the requirements or conditions described in BS 5400, adjustments appropriate to Hong Kong shall be made.

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7. SPECIFICATION FOR MATERIALS AND WORKMANSHIP, CONCRETE, REINFORCEMENT AND PRESTRESSING TENDONSBS 5400 : Part 7 : Specification for Materials and Workmanship, Concrete, Reinforcement and Prestressing Tendons shall be followed in so far as they are applicable to Hong Kong. Where Hong Kong specifications or conditions differ from the requirements or conditions described in BS 5400, adjustments appropriate to Hong Kong shall be made.

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8. RECOMMENDATIONS FOR MATERIALS AND WORKMANSHIP, CONCRETE, REINFORCEMENT AND PRESTRESSING TENDONSBS 5400 : Part 8 : Recommendations for Materials and Workmanship, Concrete, Reinforcement and Prestressing Tendons shall be followed in so far as they are applicable to Hong Kong. Where Hong Kong specifications or conditions differ from the requirements or conditions described in BS 5400, adjustments appropriate to Hong Kong shall be made.

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9.9.1 GENERAL

BEARINGS

Highway structures and railway bridges flex, expand and contract. Bearings shall be provided at appropriate locations to enable such movements to take place freely and without damage to the structures. They shall be positioned to minimize the out of balance forces. The design and installation of bearings shall follow the recommendations of BS 5400 : Part 9 : Section 9.1 : Code of Practice for Design of Bridge Bearings and BS 5400 : Part 9 : Section 9.2 : Specification for Materials, Manufacture and Installation of Bridge Bearings respectively in so far as these recommendations are appropriate to Hong Kong conditions. Alternatively, the recommendations of other national standards may be followed subject to the prior approval of the Chief Highway Engineer/Structures. Bridge bearings shall not be subjected to uplift forces under any load combinations unless with the prior approval of Chief Highway Engineer/Structures.

9.2

CLASSIFICATION OF BEARINGS

Many proprietary brands of bearing are available commercially. However, trade names for proprietary bearings shall not be included in specifications or drawings to avoid any suggestion that the choice might be influenced by other than engineering considerations. Bearing requirements shall be given in general terms, using the classification given in Table 23 as an aid to specifying.

9.3

SCHEDULE OF BEARINGS

A schedule of bearings shall be prepared for all contracts covering highway structures and railway bridges for which bearings are required. Such a schedule shall detail the number and performance requirements for each class of bearing required for the contract. Concurrent vertical and horizontal loads shall be defined in the bridge bearing schedule. A specimen schedule is given in Table 24. If the maximum transverse and longitudinal loads are considered as acting with maximum vertical load, overdesign is likely in the majority of cases and the design is therefore not economical. Therefore, the designers shall specify different load combinations in the bridge bearing schedule.

9.4

SUPPLY AND INSTALLATION OF BEARINGS

The bearings shall be supplied and installed by specialist contractors in the "Bearings for Highway Structures" category of the List of Approved Suppliers of Materials and Specialist Contractors for Public Works. As such only those proprietary bearings already approved for supply and installation by these specialist contractors shall be used.

60 9.5 TESTING

The scope of testing and the test loads shall be specified in the Specification. The number and type of bearing tests must also be clearly stated in the Specification and itemized in the Bills of Quantities. Bearings designed and manufactured in accordance with the provisions of BS 5400 : Part 9 : Sections 9.1 and 9.2 will not normally require ultimate limit state testing.

9.6

COMPRESSIVE STIFFNESS OF ELASTOMERIC LAMINATED BEARINGS

BS 5400 : Part 9 : Section 9.1 adopts the expression "5GS 2" to evaluate the compressive stiffness of elastomeric laminated bearings. Apparently, the width to length ratio of the rectangular bearing is not taken into consideration in this expression. It has been pointed out in the "Malaysian Rubber Products Association, 1981, Code of Practice" (MRPA) that the coefficient of "5" is only sufficiently correct for a "long thin bearing with a width to length ratio of 0.25 but is more accurate for a ratio of 0.20 or less" (Reference - P.B. Lindley, Small-strain compression and rotation moduli of bonded rubber blocks, Plastics and Rubber Processing and Applications 1 (1981)). Since the width to length ratio and the compressive stiffness curve is not linear for rectangular bearings, the "MRPA" has recommended "CGS2" instead of "5GS2", and C = 4 + be ( 6 - 3.3 be ) le le where be = le = effective bearing width ) ) be < le effective bearing length )

It has been found that test results generally agree with the calculated values by using this "C" value, whereas the BS 5400 : Part 9 : Section 9.1 prediction is usually too soft. Hence the MRPA's recommendation shall be followed.

9.7

DESIGN OF FIXINGS FOR BRIDGE BEARINGS

Except for elastomeric bearings, bridge bearings, including bearings which are not required to provide horizontal restraint, shall be fixed to the superstructure and substructure with mechanical fixings fabricated from austenitic stainless steel. Materials used shall comply with the following : Wrought stainless steel Flat rolled stainless steel Stainless steel washers Stainless steel fasteners : : : : BS 970 Part 1, grade 316 S 33 BS 1449 Part 2, grade 316 S 33 BS 1449 Part 2, grade 316 S 33 BS 6105, grade A4-80

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The friction between the bearing and the superstructure or substructure may be used to resist part of the horizontal forces provided that a factor of safety of at least 2 is applied to the proven coefficient of friction and that the worst combination of vertical load and horizontal load is applied. At ultimate load under earthquake conditions, friction shall only be considered if the vertical reaction can be reasonably predicted.

9.8

OPERATIONAL REQUIREMENTS

The possibility that bearings may need to be replaced during the design lifetime of a bridge must be recognised. Provision shall therefore be made in the design for the removal and replacement of bearings should this become necessary. The jacking force and the jacking position for bearing replacement shall be indicated on the drawings. Where special procedures need to be followed for the replacement of bearings, a method statement shall be indicated on the drawings. Alternatively, such a statement shall be submitted to the maintenance authority at the time of handover of the completed structure. Where access to bearings would otherwise be difficult or impossible, special arrangements shall be included in the design to enable access to be obtained. Sufficient space shall be provided for bearings to be properly inspected and maintained. Bearings shall be detailed so that dirt and rubbish do not accumulate around them, and that they can easily be cleaned. They shall be detailed so that moisture cannot stand in their vicinity but will instead drain away elsewhere. In this connection, reference shall be made to Clause 11.5.

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10.

FATIGUE

The recommendations of BS 5400 : Part 10 : Code of Practice for Fatigue shall be followed in so far as they are appropriate to Hong Kong conditions. Where Hong Kong specifications or conditions differ from the requirements or conditions described in BS 5400, adjustments appropriate to Hong Kong shall be made. The fatigue loading spectrum of BS 5400 : Part 10 is onerous and full compliance with it may under some circumstances be uneconomical. In such cases, the Chief Highway Engineer/Structures shall be consulted regarding relaxation of the full requirements.

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11.11.1 11.1.1 GENERAL Movements

MOVEMENT JOINTS

Highway structures undergo dimensional changes as a result of temperature changes, shrinkage, creep and the application of the prestress. Live loads cause bearings to deflect and rotate, and bearings can also produce movements if carriageways are inclined. The resulting movements, which are illustrated in Figure 12, shall be determined at the design stage, and provision shall be made for such movements to take place without damage to the structures. Movement joints shall accordingly be included in the design of highway structures to accommodate anticipated movements. However, as movement joints are very difficult to repair or replace once the structures are opened to taffic, no more than the minimum number shall be r provided, even if a continuous or otherwise redundant structure has to be chosen for this reason. Transverse movement of the decks of curved or skew bridges can occur, causing damage to movement joints. Joints which can accommodate the transverse movement shall be used. Otherwise, guided bearings shall be provided to prevent any transverse movement. Longitudinal movement joints shall not be used unless unavoidable since such joints adversely affect the riding comfort and safety of vehicular traffic.

11.1.2

Selection of Joint Type

Selection of the type of joint to be provided is mainly determined by the total movement to be expected. Where movements up to 5 mm are expected, gap, filled or buried joints shall normally be used. Gap joints are, of course, the simplest and easiest, but are only suitable for motor traffic. They shall not be used where pedestrians or cycles are expected. Filled joints may consist of a gap filled with a compressible filler and sealed with a suitable sealant, or the top of the gap may be filled with a proprietary rubber or neoprene sealing strip inserted under compression and supported by rebates. Careful detailing of such joints is necessary to ensure that the filler does not fall out, leaving the sealant unsupported, when the joint opens; or that the compressive forces holding the sealing strip in place do not decrease when the joint opens enough for the sealing strip to jump out. The maximum gap width for such joints is 25 mm. Filled joints transmit a proportion of a horizontal force in the deck on one side of the joint to the adjacent deck, and this shall be borne in mind at the design stage.

66 Buried joints may be used on structures with asphaltic surfacing. In s joints, the uch asphaltic surfacing is separated from the gap by means of cover plates or other devices designed to spread out the movement over the length of the plates or devices across the joints, which shall be fabricated from corrosion resistant material, or a special flexible surfacing is used over the gap. Such joints must be designed and constructed with great care, as cracks tend to reflect up from the gap, causing the surfacing to deteriorate rapidly. The maximum gap width shall not exceed 20 mm. This kind of joint is not suitable for bridge decks sloping more than 1 in 30. Proprietary movement joints shall be used on all structures that carry vehicular traffic. The maximum size of continuous open gap which can be tolerated for motor vehicles is 65 mm. Where pedestrians and cyclists have access, all gaps shall be sealed and covered with nonslip cover plates fabricated from corrosion resistant material. If the gap is sealed with anything other than a hard, load bearing rubber, then so far as the riding quality is concerned, the joint shall be considered as an open gap.

11.2

PROPRIETARY MOVEMENT JOINTS

A large variety of proprietary movement joints is now available. Care shall be taken to ensure that : (a) (b) a movement joint inherently suitable for the required location is chosen; the design of the structure is capable of accommodating the movement joint selected (there will be no conflict between joint fixings and prestress anchorages/bursting steel or steel reinforcement etc.); and the installation is carried out so that the properties of the selected movement joint are fully exploited.

(c)

Experience has shown that correct installation of proprietary movement joints is essential for satisfactory performance. Overestimation of shrinkage and creep movements can result in the joint being constantly under compression and bowing upwards after installation thereby generating excessive noise during the passage of vehicular traffic. The movement to be expected shall accordingly be estimated with the greatest possible accuracy; in this particular application, over-estimation is not on the safe side. A slight downward tilt of the mountings, so that the joint sags under compression rather than hogs, may reduce this particular problem.

11.3

TRAFFIC LOADING ON MOVEMENT JOINTS

Movement joints shall be able to carry the same vehicular loads as the structures of which they are parts. For structures designed to carry the vehicular loads described in BS 5400 : Part 2,

67 movement joints and their holding down bolts shall be capable of withstanding the following loads, either separately or in combination : (a) vertically : two 112.5 kN wheel loads, 1000 mm apart, each spread over a contact area giving an average pressure of 1 N/mm, applied so as to give the worst possible effect; horizontally : a traction force of 75 kN per linear metre of movement joint acting at road level, combined with any forces that may result from straining the joint filler or seal.

(b)

The minimum diameter of holding down bolts shall be 16 mm. Holding down bolts and the component parts shall be fabricated from austenitic stainless steel. For prestressed holding down and fixing arrangements, the size of bolts could be reduced provided they have sufficient elastic working capacity.

11.4

LOADING OF STRUCTURE BY STRAINING OF MOVEMENT JOINTS

As movement joints open and close under the influence of temperature changes, shrinkage, creep and loadings, the proprietary components of such joints may be strained, depending on their design, and forces may be transmitted to the supporting structures. Allowance shall be made at the design stage of the structures for such forces. The force that a joint may exert on the supporting structure shall not be more than 5 kN/m, but for design purposes of the supporting structures a value of at least 20 kN/m shall be assumed.

11.5

WATERTIGHTNESS OF MOVEMENT JOINTS

Unsealed movement joints enable stormwater to penetrate onto the bearings, piers and abutments of highway structures and railway bridges. Such penetration is undesirable as it can cause corrosion of ferrous bearing components, staining of exposed surfaces and produces an undesirable appearance. Stormwater penetration through movement joints may be dealt with in three ways : (a) a proprietary movement joint designed so that the completed installation is watertight may be chosen (although in practice such joints are always liable to leakage and some means of drainage shall accordingly always be provided); a proprietary movement joint which allows the passage of stormwater may be used in conjunction with a drainage layer or channel added to catch stormwater and divert it to the drainage system; or

(b)

68 (c) the structure may be designed so that stormwater can pass freely through the movement joint to be collected on the piers and abutments and diverted to the drainage system without accumulating around bearings or staining exposed surfaces but such drainage system must be capable of being easily inspected and maintained. This is the most reliable method for structure with large movement joints.

A conscious decision shall be made at the design stage as to which of these alternatives is to be followed. Stormwater draining through track ballast onto a railway underbridge must be collected and led away. Not only shall joints be carefully sealed, but in addition a substantial heavy duty waterproofing membrane shall be applied to the bridge deck. The waterproofing membrane shall be continued across the deck ends and taken down behind the ballast walls, with drains to collect and remove water running down the membrane. The membrane shall be suitably protected against damage by track ballast.

11.6

FUNCTIONAL REQUIREMENTS OF PROPRIETARY MOVEMENT JOINTS Requirements

11.6.1

Proprietary movement joints selected for use on highway structures and railway bridges shall satisfy the following requirements : (a) it shall withstand traffic loads and accommodate movements of the bridge and shall not give rise to unacceptable stresses in the joint or other parts of the structure; it shall be easy to inspect and maintain, and parts liable to wear shall be easily replaceable; large metal surfaces exposed at road level shall have skid resistant surface treatments; it shall have good riding quality and shall not cause inconvenience to any road user (including cyclists and pedestrians where they have access); the joint shall not generate excessive noise or vibration during the passage of traffic; it shall either be sealed or have provision for carrying away water, silt and grit; joints with exposed rubber running surfaces shall not be used for new vehicular bridges; the holding down and fixing arrangements for the joints shall be effectively concealed at the carriageway level.

(b)

(c)

(d)

(e) (f) (g)

(h)