lVI1NVWNDISaa3DQI)IH

1996

ISSUED UNDER THE AUTHORITY OF

THE GENERAL MANAGER

ROAD DEVELOPMENT AUTHORITY

SRI LANKA

9

FOREWORD

ThisManu~1 is intended essentially to introduce basic bridge design concepts and to present

guide lines in the technique or bridge design (or highway bridges.,

This manual has been studied and approved by the following committee.

01. Mr. P.B.L Cooray General Manager [Chairman of theCommittee]

02. Dr. G.LA.J. De Silva

03. Mr.Lionel Rajapakse

04. Mr.IlVV.Fernando

05. Mr.S. VVecrathunge

06. Mrs. H.Y. Fernando

Director (ES) [Committee Member]

Director (MM&C) [Committee Member]

Director (P&PM) [Committee Member]

Director (T) [Committee Member]

Dy. Director (BD) [Secretary of theCommittee]

07. Mr. Asoka \iVeeraratne Dy. Director (CM) [Committee Member]

08. Mr. T.L Chandrasiri Dy. Director (P&PM) [CommitteeMcmbcr]

09. Mr. D.IlR. Swarna Senior Engineer (BD) [CommitteeMember]

10. Mr. R.A.D.S.1l Ranathunge- Executive Engineer (MM&C) [CommitteeMember]

11. Mr. M. Chandrasena Bridge Consultant (MS.Chandrasena & Partners)[Committee Member]

12. Mr:J. Zavesky Bridge Desi.gn Expert (MS. RenardetConsulting Engineers) [CommitteeMember]

This manual has been drafted by the following members.

01. Mrs. H.Y. Fernando

02. Mr. D.IlR. Swarna

03. Mr. VV.E.S.1l 1i'ernando

04. Mrs. VV.B.S.H.Fernando

05. Mr. P.S. Sadadcharan

06. Mr. C.C.VV.Jayasuriya

1.0

2.02.12.22.2.12.2.22.2.32.2.42.2.52.2.62.2.72.2.8

3.03.13.1.13.1.23.1.33.23.3

4.0

5.0~.15.2

-' 5.2.15.2.25.2.35.35.3.15.45.5

6.06.16.1.16.26.2.16.2.26.2.36.2.46.2.56.2.66.36.3.1

INDEX

SCOPE AND GENERAL .. 01

DESIGN CO;')EGeneralLoads ..GeneralDead Loads ..Live LoadsBreaking and TractionHorizontal Forces due to Water CU1Tent, Debris, & Log ImpactWind Loads ..Temperature Siress in Concrete Bridge DecksCreep and Shrinkage

0202020203030303050508

..,

INVESTIGA T.~ONGeological InvestigationTopographical :.~wveyHydrological Survey _Technical Surv-y & Details of the Existing BridgeGeotechnical Lrvcstigation ..Waterway and Length of Bridge

090910101114

ALIGNMENT AND GEOlVIETRICAL CONSIDERATION 16

SELECTION OF BRIDGE TYPES & DESIGN CONSIDERATIONFoundationSubstructure ..AbutmentsWing WallsPiersSuper StructureDesign of Super StructureBridge BearingOther Features of Super Structures

181919202121222323

DESIGN OF SUBl\1ERSIBLE BRIDGESScopeIntroductionBridge Location, Proportioning & OrientationLocationProportioning Bridge & ApproachesDeck Level & TrafficabilityVertical AlignmentHorizontal AlignmentDeck CrossfallAnalysisUplift and Instalility

2425252525252626262626

; -- -- --- __ 1 ~ _~ __ __ ill

6.3.26.46.4.16.4.26.4.36.4.46.4.5

01.1

1.2

2.02.12.22.32.42.52.6

Critical Flood Levels & VelocitiesSuitability of Alrernative StructuresKerbs and Ban iers ..Super StructureBearings and Hold Down RestraintsSubstructure ..Batter Protection

27272727282828

GENERALScopeIntroduction

3030

BRIDGE LOCATION, PROPORTIONING & ORIENTATIONLocationProportioning Bridge and ApproachesDeck Level and TrfficabilityVertical AlignmentHorizontal AlignmentDeck Crossfall

303131313232

3.0 ANALYSIS3.1 Uplift. .and Instability3.2 Critical Flood Level and Velocities

4.04.14.24.3'~~.4

- -If-.5

/3233

SUITABILITY OF ALTERNATIVE STRUCTURESKerbs and BaIT--;--rSuper StructuresDearing and Hold Down RestraintsSubstructureBatter Protection

3434353536

1.0 SCOPE & GENERAL: :

1

Availability of construction materials & equipment, less maintenance and long life span arethe main factors in choosing concrete bridges abundantly in Sri Lanka As the other typessuch as steel bridges, arch bridges & timber bridges are limited in number, this note mostlycovers the design aspects for concrete bridges.

Bridge Design Manual is to supplement the Bridge Design Code adopted by the RoadDevelopment Authority, the British Standard 5400, for the loadings and effects where thelocal conditions require different provisions than those included in the British Standard.These include but are net limited to the provisions related to design live loading and to thelocal climatic conditions.

This is to provide a guidance to the designer in the interpretation of some of the provisionof the standard and in calculation of the effects prescribed by the standard and tosummarize and to advise the designer on the design practices adopted by the RoadDevelopment Authority in terms of selection of substructure and superstructure types. Itis recommended that there guide lines are used by other authorities for design of highway1__ = .l .. __

22.0 DESIGN CODE:

2.1 General -

Design of Bridges and ether related structures is carried out in accordance with the B.S.5400 with certain modifications to suit local conditions as stipulated herein.Permissible stresses to be adopted are to be in conformity with Part 4 of BS 5400.

However in mass concrete substructure the following criteria could be adopted.

Where overturning effects are considered in substructures, at any level, always Factor ofSafety should be greater than 1.00

Where F.O.S. ~=- Stability MomentOverturning Moment

When 1.0 < F.O.S. < 1.5 permissible tensile stress = 0.24 Nzmnr'When F.O.S. > :.5 permissible tensile stress = ~6 N/mm2

NOTE: But it i~.a good practice to have the F.O.S. of 1.3 always to cater forconstructional deficiencies.

Capping beams are designed for bending moments and shear forces due to loads acting onthem. Ballast wall in ab..tment capping beam is designed to take up horizontal pressurecreated by wheel load hohind the capping beam.

Ref - Reynolds Hand Hook

A 40 nun thick bearing seat is provided for the bearing pad. Sufficient reinforcement isprovided under the seat to resist the splitting forces .

2.2 Loads-

2.2.1 General -

Bridges in Sri Lanka de not need to be designed for effects due to earthquakes as SriLanka is not in a zone affected by earthquakes.

Generally the loading is to conform and applied according to BS 5400 part 2. Bridgesshould be able to resist tle effects of the loads & actions as listed below.(1) Dead Loads(2) Earth Pressure t(3) Live loads(4) Braking & Tractl,m of vehicle(5) Water current(6) Floating debris 8: Impact(7) Wind(8) Temperature(9) Shrinkage

32.2.2 Dead Loads -

I'll the case of precast slabs and beams, adverse stresses during handling, transporting andstacking should be considered.

In the case of submersible bridges, the effect of horizontal forces due to water and impactof debris and buoyancy should be considered.

Dead Load includes self weight, kerbs, sidewalks, handrails, uprights, wearing surface andweight of water mains and lamp posts when applicable.

2.2.3 Live Loads -

The following loads given in part 2 ofBS 5400 are used for design of bridges in the localhighway network.

(a) All bridges should be d~'~.;ignedto resist the effect ofHA loading specified in the relevantcode.

(b) Bridges should be able ;:0 resist the effect of 30 units of Hls loading for A & B class ofroads.

However the following condition is to be applied to suit local conditions.

(i) Always the Hli vehicle is to straddle two national lane widths.

2.2.4 Braking and traction-

The following factors a.e to be applied to the full tractive force decided according to thecode in designing subs.ructures for simply supported bridges to suit local conditions.

For Abutments

0.6 X Tractive force,applied at bearing level.

0.8 X Tractive force,applied at bearing level.

For Riers

The bridge is to be designed for HA Loading with HA tractive force and checked foradequacy to carry the nllocated HB Loading. In checking for HB, it is permissible todecrease the HB Tractive force by 25% to allow for an permissible overstress. Howeverthe live load surcharge should be limited to 10 kN/m2.

2.2.5 Horizontal Forces duv to 'Vater Current & debris and Log impact-

(a) Horizontal Forces due to Water Current -

Any part of a bridge structure which may be submerged in running water shouldbe designed to sustain safety the horizontal pressure due to the force of the current

4On piers parallel to the direction of the water current, the intensity of pressure isgiven by;

0.660.5 to 0.91.25

P = KW (v:'2/2g)P - intensity of Pressure in kg/m/\2 due to the water currentW - unit weight of water in kg/rn/\3V - velocity of current in rn/sec. at the point where the pressure

.ntensity is being calculatedg - acceleration due to gravity in m/sec.ozK - ;1 constant depending on the shape of pier as follows

with the normei values for W & g equation reduces to P = 52 Kv/\2

square ended pierCircular piers or semi circular cutwatersTriangular cutwatersTrestle type piers

Type c;~'Pier I k II1.5

The velocity V :s assumed to vary linearly from Zero at the point of deepest scourto a maximum at the free surface. The maximum velocity at surface for thepurpose is to be taken ';2 times the maximum mean velocity of the current.

To provide for the possible variation of the direction of the current from thedirection assumed in the design allowance should be made in the design of thepiers for an ex.ra variation in the current direction of 20 degrees. In this casevelocity is resolved into two directions, parallel and normal to the pier with kassumed as l.5 for all except circular piers.

Ref. : Essential. of Bridge Engineering - D.S. Victor

(b) Horizontal Forces due to floating Debris and Impact-

(i) Debris-

Where debris i:, likely, allowance shall be made for the force exerted by aminimum depth of 1.2 m debris. The length of the debris applied to anyone piershall be one half of the sum of the adjacent spans with a maximum of 22.0 mwhere the deck is not submerged.

For debris the formula for water current shall be used the value of the constant Kbeing 1.0.

(ii) Log Impact

When there is a i'JOssibilityfor driftwood and other drifting items to collide with abridge, collisio.: force shall be calculated from equation.

~

5F = 0.1 W.v

Where P = Collision force (t)

\V = Weight of drifting item (t)(2 t log is assumed)

v = Surface velocity of water (m/s)

Ref: Specification for Highway Bridges Part I - Common Specifications byJapan Read Association

2.2.6 Wind Loads-

The mean hourly wind s;:-eedis determined for the location of the bridge, from the WindLoading zone map for ~::riLanka given in Fig. 2.1.

This mean hourly wind speed, to be used when calculating wind pressures using BS 5400 -Part 2, is found from th following table.

;==: I~' IIZON',3 11EAN HOURLY WIND SPEED

1 75 m.p.h. (33.0 mls)2 65 m.p.h, (28.9 m/s)

I' 3 I 50 m.. h. ;22.2 mis, II

2.2.7 Temperature Stress in Concrete Bridge Decks -

There are three causes resulting temperature stresses in concrete bridge decks.

(a) Effect of change (rise or fall) in the 'Mean Temperature of the body of the deck.11

For the purpose cr'this effect, it is assumed that the temperature of the entire bodyof the deck has one 'mean' value at any instant of time and that this 'body meantemperature' rises or falls over a long period of time, thereby wanting the structureto 'heave'. If the structure is free to permit this 'heave' ie, is free to expand orcontract (e.g. simply supported beam or a continuous beam), this causes nothermal stress. However, if the structure is unable to permit such a heave (e.g.arch, frame. fixed beam) ie, offers constraint to its desire to heave, moments etc.,are then caused; which create stresses (thermal stress type 1). These moments canbe evaluated by the usual methods of theory of elasticity.

For all bridges, extremes of shade air temperature for the location of the bridgeshall be obtainec from the maps of isotherms given in figure Nos. 2.2 & 2.3.These values have been obtained from extracts from Department of Me tea logy.

(b) Temperature Gradient-

Minimum and Maximum shade air temperatures-

6Adjustment for height above mean sea level -

The values of shade air temperature shall be adjusted for heights above 300 mabove sea level by subtracting 0.5 C per 100 m height.

Effective bridge temperatures -

The effect.ve bridge temperatures for different types of construction shall bederived fr.-rn the shade air temperatures by reference to table No. 2.1. Thedifferent types of construction are as shown in figure No. 2.4.

Table No. 2.1 Ef~.:~ctiveBridge Temperature

-

Shade Air f.---. Type of SuperstructuresTemperature

Gt::UD 1 Group 2

08 19 1609 19 17

"10 20 1811 21 1812 22 1913 23 2014 23 2015 24 2116 25 2217 26 2218 27 2319 27 2420 28 2521 29 2522 30 2623 30 2724 31 27

I 25 32 28I

26 33 29II 27 34 29

I 28 34 3029 35 31!. ,30 36 3131 37 3232 38 3333 38 3334 39 3435 40 35..

7(c) The effect of Non linear Distribution of temperature across the Deck-Depth.

If the top surface of the concrete deck is hotter than it's soffit surface, the ordinateof the thermal ~~radientat any intermediate depth follow a nonlinear variation.

Considering be build up of the total thermal gradient, it's uniform part at theinstant of consideration, is akin to the 'body mean temperature', the effect ofchange in which over a long period of time, is already taken case of in case (a).However, the .:ariable part, better called the 'differential thermal gradient' wouldheat each fibre r-..0 a different degree, the variation being non linear. If the fiberswere free of each other (i.e. unrestrained) then they could accept thecorresponding non linear thermal strains xi (x being the coefficient ofexpansion/contraction). But since their deformations must follow a linear law(plane sections must remain plane), they will not accept these non linearly relatedstrains, and the difference between the final 'linear' strain gradient and the'unrestrained' s.rain gradient will represent the uneven 'internal disturbance'. It'sstrain effect m.ry be called the 'Eigenstrain' and its stress effect may be called theEigenstress, beth of which would be zero if only the thermal gradient were linear(which is not). This Eigenstrcss and the Eigenstrain, as can be seen, is purely aninternal entity, not associated with any support reactions.

Eigenstrcs, or: its own, may be small or significant, depending on

(i) depth of section(ii) thickness & colour of pavement(iii) wind speed(iv) orientation of bridge and incidence of sun rays.(v) ambient temperature(vi) material properties

thermal conditionalspecific heatthermal diffusinirycoefficient of thermal expansion and contractioncoefficient of absorptivitycoefficient of surface - heat transfer

(vii) surface temperature(viii) shape of thermal gradient

The distribu'on of Eigenstres, not being linear, when added to the thermal'continuity' stress [see under (C)] may show significant stress not only at extremefibers but als. at intermediate fibers (e.g. mid height portion of webs) which areheavily loaded under shear. This can produce longitudinal cracks in webs.

(d) Effect of Intc.mediate - support resuaint on the Free Hogging (or Sagging) Desireofthe structure caused by unequal Extreme Fibre Temperatures - 'The continuityeffect'.

In 8 beam-type deck, the difference oftempemture between the extreme surfacescauses hoggiag (or sagging) of the beam.

If the beam.is simply supported, it merely hogs (or sags) as its supports do not

8prevent rotation, This free deformation is not a 'moment induced' deformation, butmerely a 'Strain induced' deformation, and no moment is caused.

However if the beam is continuous, its aforementioned desire to freely hog (orfreely sag) wiil be 'constrained' at the intermediate supports (presence of dead loadreactions wilt prevent it from lifting up and presence of supports will prevent itfrom going down at their supports. This 'continuity' effect sets up moments thatcause additional stresses called 'continuity stresses'.

Ref: Concrete Bridge Practice by Dr. V.K. Raina

Stress due to emperature should be calculated as per BS 5400 cl. 5.4. The shadeair temperature referred to in the clause should be taken from the tables given fordifferent districts in Sri Lanka.

For minimum effective bridge temperature same pattern is assumed as per table I IofBS 5400.

2.2.8 Creep and Shrinkage -

Creep and Shrinkage .mly have to be taken in to account when they are considered to beimportant Obvious srruations are where deflections are important and in the design of thearticulation for a bridge.

Loss of prestress due to creep & shrinkage can be calculated using BS 5400 : Part 4.Shrinkage per unit length is obtained for normal exposure of 70% relative humidity.

Stress due to shrinkage in reinforced concrete can be calculated using following method.

(a) Shrinkage restrained by the reinforcement;

Stress in reinforcement(compression;

= f", = Ecs' E.

1+ Ue. (Aj~)

Stress in concrete(tension)

= fel = A. . f'lC

Ac

Where; EroS free shrinkage strain refer fig. 2.5

Es modulus of elasticity of steel

As area of tension reinforcement

Ac area of concrete

~"e modulur ratio

9(b) Shrinkage fully restrained;

Stress in concrete(tension)

= fel = tcs . E,

Where ~,.,.-'-'0 Static secant modulus of elasticity of concrete

NOTE,' The value of C3 to be obtained either from BS 5400 :Part 4 : AppendixCor BS 8110,' Part for 80% relative humidity, (Fig. 2,5)

3.0 INYESTIGA TIONS :

3.1 Geographical Investigation -

"

A detail survey should be carried out at the proposed location to cover topographicalhydrological and technical details.

3.1.1Topographical Survey -

(a) A minimum length of 150 m on both ends of the bridge or the selected location ofthe bridge should be considered for detailed survey (i.e. Chain Survey. includingall the permanent & temporary features and levelling) unless there is a curveencountered in e .e close proximity of the bridge beyond this length. If there is acurve the Engineer has to justify the situation and survey should be extended.

(b) Chain survey need not be a close traverse unless it is a very important location butthe levelling should be a close survey.

(c) The chainage marked should be always in the direction of the road, (i.e. InColombo - Kaney Road chainage 00+00 m should be started in the Colombo endof the bridge) The 00+00 m chainage should be tied.

(d) Longitudinal sections along the centreline of the road and cross sections should berecorded systematically with the chainages and the distances from the centre line.

(e) At lease 05 cross sections should be taken at intervals of 05 m close to the bridgeon both ends of tle bridge and the balance should be at 10 m intervals and 15 mintervals.

(f) On a curve of the mad also the cross sections should be taken at intervals of 05 m.

(g) The levels & chaiaages of every expansion joint of the bridge at the L.HS .centreand R.H.S. should be taken. Also the invert levels of the waterway should betaken.

(h) Cross sections should be taken to a distance at least 15 m from the centre line ofthe road on either side unless there are considerable changes in the levels. In caseif there is a possibie deviation of the existing road is involved, the cross sectionshould be taken as necessary.

10

If considerable level differences are encountered cross section should be extendedas necessary.

(i) The site survey should include the river banks to a distance of30 m. If there is achange in the direction of the stream the length should be extended as necessary.

G) The reduced level of the M.S.L. also should be taken if it is marked in the closeproximity of the oridge by other organisations such as the Survey Department,Irrigation Department etc ..

T.B.M. must be en a permanent structure in close proximity of the bridge.

(k) The direction of ~,orth should be marked.

(I) If there are services crossing the river or carried by the bridge the necessary detailssuch as size of the pipe, the distance from the bridge to the pipe line, type &number of supports etc. should be taken.

(m) High tension power lines or any other structures closer to the bridge which can beaffected during ccnstruction should be noted down.

The possibility of cetouring and accommodating traffic during construction shouldbe found out. SUI'. ey & levelling should cover the detour area.

Possible alternative locations for the bridge apart from the existing bridge) to beconsidered and thcr merits/demerits noted.

_.1.2 Hydrological Survey-

(a) The flow directioi. of the waterway over which the bridge is to be constructedshould be clearly marked. The banks of the waterway also should be marked.

(b) Bed level and cross sections of river on up stream and down stream sides shouldbe taken, to a distance of 30 m approximately .

(c) The lowest water level, the duration of the same and high flood level and

frequency of floods should be gathered from flood gauges and the natives. Theflood marks on the: existing structure should be noted where ever possible.

(d) Scouring of river be.' & river meandering patterns should be checked & any localscour patterns documented.

(e) The approximate SI;-_ of the floating debris if there are any should be inquired &noted.

3.l.3 Technical Survey & Detacs of the Existing Bridge ~

fa) Type of bed material, rock out crop/boulders etc. should be noted down.

Ib) Environmental condition. sal inc/rnari nc atmosphere windy condition etc. shoul-i I)"~taken

11

(c) Any visible settlement of the existing structure should be marked. In doing soparticular attention to be given for alignment of parapetslhandrails, kerbs etc.

(d) Sketches of the ~!ridgefoundations, substructure and superstructure must be givenwith all dimensons. Where ever possible existing bridge foundation type shouldbe indicated th-ough inspection or from data collected by the neighbours.

Conditions of existing structures nearby to be noted if any.

(e) Bearing points (1.1 the existing capping beam of the bridge should be marked clearlywith dimension..

(f) Details of existing bridge should be taken in the form of photographs.

3.2 Geotechnical Investigation -

(a) Subsurface Invr stigation -

Detailed sub sur.ace investigations are carried out in the form of bore holes usingrotary core percussion drilling machines. In certain cases where good soilconditions or bed rock are expected at shallow depths, soil investigations may becarried out by digging test pits,

Bore holes shou: ,ibe carried out at suitable intervals in the form of a grid coveringthe entire area. The spacing of the grid is decided on the nature of the structureand the variation of soil conditions at the site.

The Geotechnical Repoi t prepared by the Geotechnical Consultant at thecompletion of t.e geotechnical investigation should include:

Description of i'ie geotechnical investigation undertaken ..

Dctai led assessment of stratigraphy and subsurface condition .

Site plan and longitudinal profile/profiles of stratigraphy.

Datum for bore ;'Ioles and co-ordinates of the location of boreholes.

It is desirable to sink c.l the bore holes to bed rock in order to obtain ail necessaryinformation unless bed reck is at a large depth and bridge could be founded at a shallowdepth.

Additional boreholes ma be required at sites where the bores indicate variability of sub-surface conditions.

The site investigation should include:

In situ field test: 'which may include standard penetration tests or static conepenetrometer so .ndings.

Definition of be.irock properties, where applicable.

12

Colour photographs of cores.

Laboratory classification of main soil types.

The following soil conditions should be determined as appropriate.

Stratigraphy

Physical description and area distribution of each stratum.

Thickness and devation at various locations of top and bottom of each stratum.

For each Stratum of C.

13

Sources of inflow to each aquifer, where determinable.

Temperature.

Bedrock

Depth over enti re site.

Type of rock and physical properties of intact rock.

Extent and character of weathering.

Joints including distribution, spacing, and whether open or closed.

Faults.

Solution effect in limestone or other soluble rocks.

Rock quality designation,

Spycial considerations

Chemistry of so.l or ground water as it would affect buried structures, e.g. sulphateattack on concrete, or acids as encountered in industrial areas.

Dynamic soil parameters, if required.

Ambient vibration levels, if such could be a source of distress to the proposedstructure or to t5e public.

Problem soils 0\ conditions.

The underground investigation that have been obtained by the Engineer should be madeavailable to the Geotechnical Consultant.

Notwithstanding the above, the responsibility for the location of underground utilities lieswith the geotechnical Consultant who should make all appropriate arrangements to ensurethat underground utilities will not be damaged in the course of geotechnical investigation.

(i) Standard Penetration Test.

Due to the extreme difficulty in obtaining undisturbed samples from granularsoils, their strenghs are determined by taking disturbed samples and carrying outstandard penetration tests. Penetration tests should be done not less than every 1.5m and at least one in each of the different soil strata Once the bedrock is reached,to ensure the rock formation the bore hole is carried to a depth of another 3.0 m.

14

(ii) RQD, Core recovery & fracture index

Once the bed rock is reached RQD, core recovery and fracture index values areobtained to classify the quality of rack.

(b) Estimation 0;'Allowable Bearing Pressure

Foundation and other recommendations should include the following asappropriate:

Assessment cf alternative foundation systems.

Allowable bearing capacities and the appropriate levels of the foundation system.

Estimates of settlements and lateral stability.

Relevant sou-rock design parameters.

Comment on relative cost of alternatives.Assessment of possible construction problems (e.g. dewatering, usery/permanentcasing, preboring, excavation stability).

Empirical Charts arc referred to obtain shear strength parameters (C,

15

afflux created sl.ould not have any harmful effects in the region prepared for thebridge. The effect of the additional scour or the stability of the foundation shouldalso be checker'.

In determining i~lewaterway requirements of bridges, the streams/rivers should bedivided into three groups.

(i) Stream/river those banks and bed with hard and inerodible.(ii) Stream/river with inerodible banks but with erodible beds.(iii) Stream/river with erodible banks and bed.

As the extra waterway required during the floods may create partly by rising waterlevel and partly by the flood water causing scour in the bed and banks, it isnecessary to study the nature and the action of the scour in river/stream banks andbed. Depth of scour should be determined, and if the bridge is founded above thescour depth a SI) itable protection for bridge piers/abutments should be done.

While designing the water way through bridges. "afflux" has to be calculated incase of bridges where there is a reduction in the overall width ofthe waterway overthe natural width. The afflux should be kept minimum and limited as far aspossible to 150 mm, otherwise when there is a higher affiux, the more will be thevelocity produced through the obstruction. Hence an estimation of the afflux isnecessary in de.ermining the soffit of the deck.

Ref: Considerations in the Design &. Sinking of Well Foundations, for BridgePiers by B. Balwant Rao & C. Muthuswamy.

(b) Determination Of Design Maximum Discharge-

A number of empirical formulae are available for estimating the maximumdischarge at a point on a river or any other water course. These formulae cannotbe applied indiscriminately as they have been derived for specified condition.Hence for a particular bridge site the formula applicable to specific conditions ofthe catchment hes to be selected and used. This is very important as otherwisespurious result may be obtained. It is also necessary to check the validity of theresults thus obtained by the following commonly used methods for estimation ofmaximum flood discharge.

(i) Area Velocity Method.

(ii) Using Rational formulae involving the rainfall and other Characteristics.

Using recorded d:ta on existing structures on the same waterway in the vicinity orby collecting dat.. through inspection and investigation.

(i) Area Velocity Method:

The cross sectional area of the river is measured in a straight reach and thisis rnultipl.ed by the velocity calculated from the Maning's Formula.

16

(ii) Rational Formula :

The catchment is the area upstream of a point in the river from which allrain water falling in that area will tend to flow to that point. This iscomputed with the help of Topography sheets.

The rainfall records in the catchment are obtained from the Irrigation Departmentor the Meteorological Department.

Ref: 01.02.03.

Bridge Engineering by S. PonnuswamyEssentials of Bridge Engineering by D.l VictorDesign ofIrrigation head works for small catchments by - A.lP.Ponrajah

(c) Spacing and Location of Piers & Ahutments :

The positioning [;;ld spacing of the piers and abutments are finally decided takinginto consideration the requirements of the waterway outlined above and the resultof the bore hole investigations and available standard beam lengths afterconsidering the e. onomic aspects for alternate proposals for different type of superstructure as well as substructures.

4.0 AUGNMENT AND G~:QMETRICAL CONSIDERATION:

The superstructure is the visible feature of the bridge. By selecting the correct shape forme superstructure aesthetic appearance of the bridge is enhanced. Superstructure consistsof the deck, kerbs. hand rails, uprights and lamp posts. Service ducts are also provided inthe superstructure as a means for carrying service mains across the river.

A bridge may be right or ;:kew. Skew angle is defined as the inclination of the abutmentto the perpendicular to its rree edge. A bridge with a skew angle of zero degrees is a rightbridge. In simply suppor ed bridges the effect of skew in general is neglected up to 20degreesand if the skew angle is more than that. bridge deck should be designed to resistthe effects.

The bridge deck may be e.rher of reinforced or prestressed concrete. Factors that affectthe choice of deck are the spans. foundation condition, aesthetics etc ..

Alongthe Road Network ~,IIbridge decks should provide for carriageway widths. to suitthe traffic requirements as given below. A minimum width of7.4 m for Class A roads andaminimum width of6.8 171 for Class Broads should be maintained with adequately widefoot walks on either side. ';"'heminimum foot walk width adopted is 1.2 m which variesto a larger width depending on the specific location with respect to the pedestrian volume .

i ADT PCU/day 4000v72000

- 25000 - 18000 - 300 -

17

For bridges over highway and railway a minimum vertical clearance of 5.25 m should beprovided.

The approach road layout along with the bridge centre line should be designed inaccordance with the ; Iighway Designs Manual which includes design of horizontal andvertical alignments. !.ongitudinal camber also depends on the aesthetic requirements andtype of construction. Cross camber is so designed (to a slope of 1:60 for concrete bridges)to lead the water to t:le lower kerb which selves as a surface drain leading water throughthe rainwater outlets.

5.0 SELECTION OF 1SRIDGE TY!)ES ...\ND OEHGN CONSIDERATION:

Types of bridges are , lassificd depending on the material used and the type of constructionadopted'. The common types of bridges are :-

1) Concrete Bri.lges2) Steel Bridge=3) Stone or bride masonry arch Bridges4) Concrete arcl. Bridges5) Timber Bridges6) Box culverts

Concrete bridges are used in most of the places, because oflong life span and speed ofconstruction. The materials for construction of concrete bridges are readily available. Ithas also been found tl.u the maintenance of concrete bridges is less costly than for othertypes of bridges.

On account of the shortage of steel involvement of foreign exchange and non availabilityof rolled sections, it is preferable to avoid use of steel bridges as far as possible. However,where steel trusses in ::ood condition from dismantled bridges are available, these can beused on certain class ,:f roads. A disadvantage in using steel bridges is the high cost ofmaintenance .

For small culverts and ondges of moderate span, where the available headway is adequate.stone or brick masonry arches can be used with advantage when bricks or stones are locallyavailable. Services of skilled workmen me required for this type of construction.

Concrete arch bridges are generally used in places to fit into the aesthetic appearance ofthe area. In hilly areas. where the velocity of flowing water is such that it is not possibleto construct any intermediate piers, a concrete arch bridge is advantageous and convenient.

Use of timber bridges i,: limited to areas \.,hen timber logs are found in plenty. As timbergets easily deteriorated under normal weather conditions, such bridges are generally builtfor temporary construction and for light loadings.

At places where the flood spread is large, since providing a all weather bridge isuneconomical, a submersible bridge is acceptable. It effects great economy ofconstruction. However the formation level of the structure should be fixed depending onthe period of inundatic " of the structure.

In addition, the type of ~'ridge, to be provided at a site is generally decided on economic

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analysis, availability o: materials and CIlSC' of construction. Use of precast prerensionedbeam in the bridge decx hac; the advantage of case of construction due to the f~ryffiadeproduct as well as the low thickness of deck in the case of design of the vertical profile forlow level approaches. However the diDicu:ties that may have to be encountered in thetransportation of prer ast beam with respect to the location of the bridge should beconsidered. Before decision is taken to adopt a post tensioned beam deck for the bridge,the possibility of pro" ding a beam casting yard close to the bridge location should belooked into. Standard nrccast prctcnsioned beams are available from 4.3 m upto 16.23 m.A combination of pretcnsioned/post tensioned beam with precast, 16.23 m unit is availablefor spans of 19 m.

The commonly used bridge types and components arc described here in detail. It is thedecision of the desig.ier to adopt or to deviate from the types indicated herein asappropriate to the circumstances

5.1 Foundations:

Types of foundations .xnnrnonly used ar

(a) Shallow foundationsSpread footing iounded on rock or 0n suitable soil strata.

(b) Deep Foundations(l) Pile Fotndations

(i) Cw;t Insitu or Bored f'tb(ii) Precast driven Pi!.:;s(iii) Ti-nber Piles

(2) Caisson Foundations(i) Cii cular(ii) Rc 'tangular

9

The choice of the type of foundation depends primarily on following;

(a) Nature of Soil Sl rata(b) Magnitude of tl. loads to be carried(c) Site conditions(d) Economy(e) Availability of Construction Techmques,(I) Maximum likely scour depth(g) Minimum grip length required In the case of deep foundations

Foundation types can be- classified as shallow & deep. Spread footing & side by sidecassons can be consider: .d as shallow ff'und.'l:iom and spread footing can be provided~Jcre a suitable soil strar.m can be found ;:11 a shallow depth within about 3-4 m belowgroundlbcd level. Where .he founding layer is b(twcen 4.0 - 6.0 In side by side caissons,an improvise method of ~::read foundation could be adopted. Where water table is high,the problems of cofferdai.. ing and de-watering should be considered when adopting thesetypes of foundations,

19Pilefoundations may be adopted when suitable bearing strata are found deeper than 6.0m from ground level. Bored piles should be adopted when driven piles are liable todamageexisting structures.

Pre-castpiles or cased c.ist in situ piles are preferred where peat over layers are found orwhen foundations have to be constructed in water.

Timberpiles are used where there is no risk of decay of timber and loads to be transmittedfromthe structure are not excessive.

In the design of driven ;Iiles in particular, two different capacities should be taken intoaccount.

(a) The capacity of the pile as a structural member

(b) The capacity of the pile to transmit loads to the foundation material.

III the case of piles on good rock the capacity is governed by condition (a). Wheresignificant horizontal forces are present, raked piles could be used.

Caissonfoundations am~large diameter bored piles are used when heavy loads have to betransmitted and when tue foundations have (0 be carried very deep.

5.2 Substructure -

Thesubstructure mainly consists of three components, abutments and wingwalls and piers.Abutments,wingwalls arid piers must be so proportioned so as to satisfy both the practicalas well as theoretical consideration S. Selection of the type of substructure should becarriedout to suit the par: icular site conditions. Alternative proposals should be consideredfor economic feasibiliry. The overall dimensions are first determined from practicalconsiderations and components are designed to resist the various forces acting on them.

The height of abutment and/or piers should be selected to give a sufficient clearancebetweenthe highest flood level and the bearing level) unless designed as a submersible,.bridge. This free board t.';, usually taken as 1.0 ill, or a minimum of 0.6 m due to restrictedconditions. Partial or ~jl! flooding is not acceptable on Class A & B roads unless aconsiderablesavings on construcrlon may be achieved on bridges oflesser classificationh!' providing reduced wa.erway area and accepting short duration flooding by floods withshorterthan the full Design Recurrence Interval. The selection depends on the number ofparameters, including availabil ity of alternative routes, short term inconvenience connectedwithshort duration floodng weighed against [he cost saving. The width and length of thesubstructures are governed by the loads to be carried (both vertical and horizontal) selfweight(necessary to redi.ce loads on foundation), economy in construction, use of localmaterial to a maximum ;"Indpossible obstructions of waterway in the case of piers.

5.2.1 Abutments-

Different types of abutments commonly used are;

(a)(b)

Mass concreteReinforced conCi~te - 1. Reinforced concrete wall

2. Reinforced concrete column (openabutments)

20Assessment of Loads:

Openabutments are more economical if there IS no risk of earth fill being washed awayas in the case of flyover or bridges with protected banks.

(1) Vertical Loads (i) Dead Load reactions -From Superstructure

(ri) Live Load reactions -From Superstructure

(di) Self Weight(iv) Buoyancy

(2) Horizontal Load s (i) Earth Pressure(ii) Pressure due to Surcharge(iii) Tractive Force(iv) Temperature effects(v) Shrinkage effects.

Earth pressure and pressure due to surcharge are determined from Rankine formula. Fortractive force and live loads reference should be made to the notes given under loads.

Inaddition, abutment should be checked for vertical and horizontal forces acting duringconstruction stage.

Proppedabutment type ~:~TUcturesoften provide an economical solution for single shortspan bridges, provided that complete scouring of the till behind abutments can beeliminated. Significant part of the horizontal forces is transferred through the deckbetweenthe abutments wid only the unbalanced horizontal forces need to be resisted bythe foundations.

5.2.2 Wingwalls-

Different types of wingv.alls commonly used are;

(a) Wingwalls cantilevered from abutments(b) Mass concrete wngwalls(c) Reinforced conc.ete wingwalls.(d) Sheet pile wingwalls.

Forshort wingwalls of m-sdi urn heights and where there is no risk of scour cantilever typemaybe used.

Wingwalls are to be designed as earth retaining structures subjected to active earthpressure. The mass concrete stepped section is to be designed as sloped back retainingwallon the stepped side. However to be conservative the vertical component of the earthpressure may be ignored and the full pressure assumed to act horizontally.

Sheet pile wingwalls are designed according to standard design practice.

~\~f ~evrO\dsTi~~Boo"

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Weep holes are provided in abutments and wingwalls to reduce the build up of porewaterpressure in the earth rill behind. They are usually spaced at 1.5m centres horizontally andvertically as appropnate. The lowest row of weep holes is provided at 0.3 m above thenormal water level.

When abutments anu wingwalls are founded at different levels on soils of different bearingcapacities, a slip joint between the abutment and the wingwall from top to bottom isprovided.

5.2.3 Piers -

Different types of ;:iers commonly used are;

(a) Mass concrete stems(b) Reinforced concrete walls(c) Reinforced concrete colwnns

Piers should be checked for the following loads.

(1) (i) Dead load reaction(ii) Self weight(iii) Live load reaction

(a) Both spans loaded(b) One span loaded

(iv) Buoyancy

Vertical Leads -

(2) Horizontal "Forces -Longitudinal Direction (i)

(ii)(iii)(iv)(v)

Tractive forceForce due to water currentForce due to floating debris & impact.Temperature effectsShrinkage effects

Transverse Direction (i) (ii)

(iii)

Force due to water currentForce due to floating debris & impactWind

The design criteria adopted for mass concrete piers is given under stresses in the note.

5.3 Superstructure-

When selecting tne type of bridge superstructures, span, length, location of bridge,maximum deck tlickness that could be accommodated etc. should be considered.

For short spans, up to about 6 m Rfc slabs or pre-tensioned rectangular units can beprovided and for spans of 6m - 19m m decks with pre-tensioned P.S.c. beams placed sideby side with insitu infiller concrete deck and for spans more than 19 m post tensionedP.S.C. beams with post tensioned or reinforced concrete deck slab can be provided

When deciding en the length of precast beams consideration should be given to thetransportation capability and if post tensioned beams are used the space required for post

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tensioning bed and its proximity to the bridge site should be considered.

The common types of structural arrangement of bridge decks are;

(a) Plain Slab Deck, R.C. or precast P.S.C. units

(b) Beam & Slab Decks.

(c) Steel concrete composite deck

(a) Plain Slab Dr.cks :

Mostly used for short spans. They are also used for larger spanswhere limitationson construction depth and economy are governing factors of design.

The different '~pes that have been adopted are ;

(1) Plain concrete slab - Precast or Insitu

(2) Composite construction of precast prestressed concrete beams with orwithout insitu infiller concrete to form the deck slab. Type precastprestressed beams of standard lengths are available for this type ofconstruction.

(b) Beam & Slab Decks:

The deck comprises of several longitudinalbeams and transverse diaphragmswitha concrete deck slab. The beams may be either ofT,! or Box Section.

The different types that are being used are;

(1) . Reinforced concrete beam and slab

(2) Prestressed concrete beam with end and intermediate diaphragms with

(i) Prestressed concrete deck slab or(ii) R.einforcedconcrete deck slab

(c) Steel concrete.composite deck

The deck comprises of several longitudinal steel beam with concrete slab on top.

5.3.1Designof Superstructure -

Analysisof superstructure is carried out using the following methods.

1. 'Elastic Analysis ego Load DistributionTheories

2302. Plastic Analysis : eg. Yield line theory

Permissible stresses to be adopted are to be in conformity with Part 4 ofBS 5400. Inprestressedconcrete decks in general, permissible stresses to be in accordance with ClassII requirements ofBS 5400.

(a) Slab Decks -

Slab decks are designed as one way spanning, either simply supported orcontinuous.

(b) Beam and Slab Decks-

Reinforced concrete deck slab is designed in the same way as the slab bridge.

InthePost tensioned slab prestressing steel is designed using empirical method proposedbyGuyon in "Prestressed Concrete", However nominal reinforcement of 0.1% is alsoprovidedin the direction of prestress. In the perpendicular direction steel is provided toresistthe Bending Moments and Shear forces in that direction.

Compositeaction of the slao may be taken into account in the design of the longitudinalbeams.Beam is designed hi cater for the portion of lane load depending on the spacing.

Thediaphragms are placed. at supports, mid span and quarter span points.

Transversemoments and lcngitudinal moments due to HB loading is worked out usingdeck analysis.

Ref. : Concrete Bridge Design by R.E. Rowe

SA Bridge Bearings - ~

Loadsimposed by the vehicle on the superstructure are transmitted to substructure throughthf; bearing.

Bearingsmay be of steel concrete or rubber.

Bearingsshould designed in accordance with BS 5400 Part 9.1.

S.5 Other Features of Superstructure -

Handrails, Uprights & Parapets -

Parapets and Handrails may be of masonry or concrete. Shape of hand rail andupright or parapet is an architectural feature. The minimum height of the railingor parapet of a high' ..vaybridge should be 1.0 m.

Parapets are not normally designed for collision loads and may be designed forpedestrian loads. Where the situation demands crash barriers should be used.

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End Pilasters -

End pilasters may be of masonry or reinforced concrete. The size and shape isdesigned to give a good appearance to the bridge.

Kerbs -Kerbs are provided at the edge of carriageway to deflect the vehicle back. Kerbsshould be a solid section not less than 225 mm wide at base and not less than 225mm high above the adjacent road surface.

Service ducts -Service ducts are provided under the sidewalks to carry cables and water mains.

Lamp Posts -Spacing of lamp posts, height of lamp posts, etc. are designed according to theillumination requirements. Provision shall be made in the deck or the sub-structureto accommodate the lamp post.

Expansion Joints -

In the case of simply supported spans there is a complete separation betweenabutting spans wuich permits change in length of superstructure due to temperaturevariation. The gap in-between should be sufficient to accommodate the expansionof the deck within temperature range expected. Expansionjoints should extendover the entire width of the deck. It should not allow penetration of water tocapping beams. As it is very difficult to achieve full water proofing of theexpansion joints -md particularly to maintain through out the life of the structure,it may be acceptable on some bridges and much more economical and easier toallow the surface water to penetrate the joint and to make appropriate provisionsfor its drainage.

Rain water outlets -

Rain water outlets are usually kept at 4.5 to 6 m centres. Rainwater outlets shouldproject out side the deck sufficiently to prevent water dripping on any part of deckor substructure.

6.0 DESIGN OF SUBME1~SmLE BRIDGES

6.1 Scope

TheseGuidelines are intended to be used for low level bridges subjected to submergenceby floods frequently.

TheseGuidelines are not .ntended for use as a code of practice or a design code. It is theresponsibility of the designer to consider and evaluate all aspects relevant to the bridgeunderconsideration. The design process may need to include seeking the opinion of alltheusers of the bridge. o~~questions such as the level of service. location and choice ofbarriers.

2S

6.1 Introduction

BridgeType T Flooding Frequency-rI Frequently submerged1 100 year

IHigh-levelbridge.l 'I

Low-levelbridge

Bridgessubjected to sui.mergence are usually adopted for reasons of economy often wherethe difference between ~')rrna1water level and the flood level is large but the floods are ofrelativelyshort duration, or where it is impractical to raise the bridge and approaches abovefloodlevel because of the resulting backwater effects. This type of crossing may also bechosen where usage is limited and flood free alternatives exist.

6.2 BRIDGELOCATION, PROPORTIONING & ORIENTATION

6.2.i Location

Submersiblebridges are suitable for flat and arid areas inland where large floods occurinfrequentlyor in remote forested hilly areas where flash floods could be frequent but lastonlya short time. Submersible bridges are also suitable on large flood plains as flow overtheapproach roads is often acceptable and velocities are generally not high.

6.2.2 Proportioning Bridge & Approaches

Theentire bridge structure should be designed to minimize catching debris or at least toalloweasy removal of any debris caught. Bridges with short individual spans tend to catchmoredebris than those w~::nlonger individual spans. Where debris loading is significantspanlengths less than 10 metres should be avoided. The span lengths chosen should atleastexceed the expected lengths of debris to be passed.

Acompromise tor the calculated backwater, and drag effects must be made between theadvantagesof longer span: with fewer piers (but incurring a deeper superstructure) andshorterspans with more piers (but allowing a shallower superstructure),

6.2.3 Deck Level & Trafficability

Ifstaticplus velocity head at the crown or highest edge of a carriageway exceeds 300 mmovertoppingflood depths must always be indicated with gauge markers.

Thusovertopping of submersible bridges and their road approach embankments may betoleratedto provide traffical.ility for a river crossings subject to low serviceability floods.Alternatively,the bridge deck could be placed above low flood level (but for economicreasons,below larger flood levels while the approaches, set at a lower predetermined level,couldpermit overtopping by the low flood and yet still remain trafficable.

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6.2.4 Vertical Alignment

Alevelgrading should be provided for the full length of bridge so that the bridge acts asaweir when the rising upstream water surface just over-tops it. If only portions of thestructureare overtopped 6e pattern of flow in the stream could be severely disturbed. Adeckon a grade or vertical curve is also a hazard to traffic, because the water depth is notconstant Drivers negotiating a flooded crossing should not encounter an unexpectedincrease in depth of water.

6.2.5 Horizontal Alignment

Submersiblebridges should be on a straight alignment and located as square as possibletothe most common direction of flood flow.

6.2.6 DeckCrossfall

As the water subsides, debris and silt will tend to be left on the upstream side ofsubmersiblebridge with normal two-way crossfall. For this reason, the preferred decksectionwith one-way falling cross fall toward the downstream side is preferable.

6.3 Analysis

3.1 Uplift& Instability

The stability of submerged bridge should be assessed for both uplift and overturningeffectsfrom the following simultaneously occurring forces due to the stream flow, viz:

(a) Buoyancy uplift

(b) Hydrodynamic dra-; forces resulting from stream flow past the superstructureandlor on debris ceught against the upstream edge of the superstructure;

(c) Unbalanced hydrostatic pressures acting on the upstream sides of the bridge fromponding ~ux effect), aggravated by the collection of debris;

(d) Floating object impact forces

(e) Some superstructures may also trap debris, which if buoyant, will create furtheruplift forces.

It isessential that all parts or the structure are considered for reduction of dead load duetobuoyancy when the bricge is submerged. The possible use of concrete aggregateslighterin weight than the assumed design value causing over estimates of the stabilizingdeadload of the structure should also be considered. The possibility of air pockets beingtrappedunder the deck creating destabilising buoyancy should also be considered.

Calculationof the destabilisi vg forces listed above on bridges from stream flow effects areoftensomewhat uncertain due to lack of data on actual flood flow and debris as well asknowledgeof appropriate drag factors or hydraulic flow patterns to be used.

27To account for uncertainties, the bridge must have a very large factor of safety againstinstability under flood submergence. Unless the stabilizing restraint provided by gravityforce provides a factor of safety of at least 02 or more on unfactored loads thesuperstructure must be securely tied down to the substructure.

6.3.2 Critical Flood Levels & Velocities

It is suggested the following load case categories for submersible bridges be considered.

(a) Partial submergence of superstructure

(b) Overtopping o f superstructure

(c) Deep submergence of superstructure

Stream velocities as well as frequency, magnitude and duration of the respectivesubmergence calculated for various flood flows will determine the required scourprotection of the eml.ankrnents for the road approaches and abutments. Where roadapproach or bridge abutment embankment is overtopped by flood waters special protectionworks are required. Additional culvert openings may be required

6.4 Suitability of Alternative Structures

6.4.1 Kerbs & Barriers

This section deals with some aspects of the choice of appropriate kerbs and barriers. Therequirements are sometimes contradicting, e.g. a grill type pedestrian or combinedtraffic/pedestrian barrier is not suitable for sites with substantial amounts of debris, yet itmay be required if the !,ridge is located in town and there is heavy pedestrian usage. Thechoice of the appropriate kerb or barrier depends on circumstances. Where deep wateris present use os collapsible railing should be considered.

If it is considered that kerbs are warranted, they should either be castellated (short sectionsof kerb sepasated by full-depth gaps not exceeding 200 mm) or if continuous, providedwith slots to allow water to drain freely from the deck and to aid the manual removal ofdebris. Small diameter drainage holes and scupper pipes are prone to clocking with debris.

6.4.2 Superstructure

It is desirable to select a form of superstructure with a shallow depth, to minimizehydrodynamic force, be__ckwater and scour effect particularly at the critical flood heightwhen the bridge and approaches are about to be overtopped.

Closed cell structures si-ch as single and multicell steel or concrete box girders, steel trussesor composite steel or co.crete through girders, multicelled decks formed from linked broadflange concrete girders, voided concrete slabs, and other voided concrete girders aregenerally considered unsuitable.

All parts of concrete '",uperstructures subject to frequent flood submergence must beconsidered to be 'in contact with water' for calculation of required cover to prestressed andnon-stressed reinforcement. This may lead to slightly greater cover requirements for PSC

.~

28

girders, especially in rockets where silt may be trapped and remain damp aftersubmergence.

Ajudgement must be made whether siltation due to submergence will occur frequently oronly rarely in the life of 1he bridge.

6.4J Bearings & Hold-down "Restraints

It is usual to provide separate anchor bars. Typically 24 mm diameter galvanized steelholding down bolts are specified between each P.S.C. plank on composite slab structureswhilemore substantial angle brackets may be specified to straddle each P.S.C. girder oncomposite decked structures. Shear blocks or dowels connected between end diaphragmsandabutments or piers will prevent a submerged superstructure from lateral displacement.

Ifelastomeric bearings are used, the holding-down bolts should be suitably de-bonded andanair space left above the heads of the bolts, to allow the bearings to deform under liveload. Frictional restraint against creep of elastorneric bearings is diminished during floodsubmergence due to reduced bearing reactions from superstructure buoyancy. Under theinfluenceof rotation from hogging effects this creep usually moves the bearings towardsthecentre of the girder spans.

6.4.4 Substructure

Thefoundations for piers and abutments of all bridges crossing fast flowing waterwaysshouldeither be keyed on ti: rock or cast on piled footings. This avoids the possibility ofunderminingof the base material under the foundation footing pads by the formation ofadjacent local scour holes. Eddies also tend to wash out fill behind abutments. Localscourfrom turbulence around piers may be reduced by hydraulically shaping the pile capsandcolumns.

6.(5 Batter Protection

Protectionworks of embankments adjacent to abutments will be similar to those for highlevelbridge and may include rock fill, rip-rap enclose, concrete revetment mattresses orrigidreinforced concrete slabs as well as sheet pile toe walls. However abutment batterprotectionof submersible bridges not only must be secured against stream bed scour butmust be secured against SCOtt! effect during overtopping.

Grassingof batters may be adequate when the velocity of water flow over the embankmentislessthan 1.5 metres/sec. Generally this is the case when tailwater levels are not morethanaround 300 mm below tl.e downstream edge of the road formation when overtoppingfirstoccurs. Grass batters may not be suitable where frequent overtopping occurs for aperiodof more than two or three hours during floods. Grass batters are not suitable forshadedareas under the bridge superstructure.

Moreelaborate protection wo.ks against scour are detailed similarly to the requirementsforcauseways. It is essential ~;l the following protection works are either taken below theanticipatedstream bed general scour level or local scour hole depths using cut-offtrenches,orareprovided with level aprons extending into the waterway to accommodate scourerosion.In particular. batter protection works subject to flood submergence must also beanchoredalong their upper edges to resist scour erosion during overtopping.

29Nominalcover to all reinforcement (including links) to meet durability requirements - AdoptedfromBS 5400 for Sri Lankan Practice.

Environment Examples Nominal cover (in any caseshould notbe less than thed!a. of the bar) (mm)

, Concrete Grade,- i

(

, 25 30 40 50andover

Extreme 65 55Concretesurfaces expo red Parts of structure in contacttosbrasi ve action by sea with sea waterwater

- -Very Severe 50 40Concretesurfaces directly Concrete adjacent to the seaaffectedby sea water spray -.Severe 50 45 35 30IConcrete surfaces exposed \Valls and structure supportstodrivingrain romote form theor carriagewayalternativewetting and Bridge deck soffits Burieddryirig E:U1:s of structure

IModerate 50 40 30 25Concretesurface above Surface protected by bridgegroundlevel and fully deck water proofing or byshelteredagainst all of the . permanent form workfollowingrain sea water Interior surface ofspray pedestrian subways voided

superstructure or cellularabutments concretet..:!,rmanently under water --

3001. GENERAL

1.1 Scope

TheseGuidelines are intended to be used for low and intermediate-level bridges subjected tosubmergence by floods with Average Recurrence Intervals (ARl) less than 100 years.

This document is not intended for use as a code of practice or a design code. It is theresponsibilityof the designer to consider and evaluate all aspects relevant to the bridge underconsideration. The design process may need to include seeking the opinion of all the users of thebridge,on questions such as the level of service, location and choice of barriers.

1.2 Introduction

A bridge with a superstructure which may be partially or fully submerged by any flood withrespectto Average Recurrence Interval CARl) is specified below. This return frequency wasgenerallyadopted as the design frequency for bridges and large culverts.

BridgeType r 'Flooding Frequency Typical ARILow-level bridge Frequently flooded < 20 *-

Intermediate-level bridge Rarely flooded 20>11to < 100

Hgh-level bridgeClearance above 100 year > 100flood

* The20 year ARI demarcation between frequently and rarely flooded bridges is indicative onlyanda reduced value may be more appropriate for some bridges that are intended to providelowerlevels of service.

A lowor intermediate-level type of crossing is usually adopted for reasons of economy oftenwherethe difference between normal water level and the flood level is large but the floods areofrelativelyshort dilration, or where it is impractical to raise the bridge and approaches abovefloodlevel because of the resulting backwater effects. This type of crossing may also be chosenwhereusage is limited and ficod free alternatives exist.

02. BHIDGE LOCATION, PROPORTIONING & ORIENTATION

2.1 Location

Low-levelbridges are suitable for flat and arid areas inland where large floods occur infrequentlyorin remote forested hilly area, where flash floods could be frequent but last only a short time.Large food plains are also su: table as flow over the approach roads is often acceptable andvelocitiesare generally not high.TIleideal low-level bridge site is where the stream bed is broad and shallow, with gently slopingbanks. The bridge should be located on a straight reach of the stream, not subject to scour orsiltation.Ifthebridge is located in a posit (on where the road approaches or bridge itself cannot be suitablyprotectedagainst a moving cha.inel, scour, debris or siltation, the economical advantage of thelow-levelcrossing may be outweighed by maintenance costs.

312.2 Proportioning Bridge & ,

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2.5 Horizontal Alignment

Low-levelbridges should be straight and located as square as possible to the most commondirection of flood flow. A skewed or horizontally curved bridge superstructure, whensubmerged, directs the flow sideways towards the downstream abutment. The resultingturbulencemay cause serious cour at this abutment. Moderate skews or very large horizontalradiimaybe tolerated provided that the downstream abutment and adjacent embankment battersare adequate protected against scour.

2.6 Deck Crossfall

Asthe water subsides, debris and silt will tend to be left on the upstream side of low-level bridgewith normal two-way crossfall. For this reason, the preferred deck section in the past hasadopted a slight one-way crossfall falling toward the downstream side. However such a super-elevateddesign of the bridge a.id matching adjacent road approach may incur greater damageto the upstream edge of road p.rvements from increase in flow velocity. Super-elevated bridgeswith soffits falling downstream may trap debris underneath the superstructure and be moresusceptibleto vertical 'lift' effects. Also, it is considered safer to drive across an overtoppedbridgewith crossfall falling, instead, towards upstream. Therefore, cross-sections with normaltwowaycrossfall are probably the best compromise although the choice of deck crossfall willdependon the type of bridge. structure proposed and local conditions for stream velocity anddebris.

03. ANALYSIS

3.1 Uplift & Instability

TIle stabilityof submerged bridge should be assessed for both uplift and overturning effects fromihe following simultaneously occurring forces due to the stream flow, viz:

(a) Buoyancy uplift

(b) Hydrodynamic drag forces resuJting from 'form' stream flow past the superstructure and/oron debris caught against the upstream edge of the superstructure;

(c) Unbalanced hydrostatic ;.:;ressuresacting on the upstream sides of the bridge from ponding(afflux effect), aggravated by the collection of debris;

(d) Floating object impact f.)rces

(e) Vertical Tift' forces actir-g under superstructures with soffits inclined to the stream flow.

(0 Some superstructures may also trap debris, which ifbuoyant, will create further upliftforces.

Notethat super-elevated bridg-es with superstructures soffits falling on a constant gradienttowardsdownstream \,.111 be pcrticularly susceptible to these latter two effects(e) and (f).

If sessential that all parts of ~he structure are considered for reduction of dead load due tobuoyancy when the bridge is s.ibrnerged. Attention is drawn to the possible use of concrete

33

aggregates lighter in weight than the assumed design value causing over estimates of thestabilizing dead load of the structure and also to the possibility of air pockets being trapped underthe deck creating destabilising buoyancy.

Calculation of the destabilisine forces listed above on bridges from stream flow effects are oftensomewhat uncertain due to lac': of data on actual flood flow and debris as well as knowledge ofappropriate drag factors or hydraulic flow patterns to be used.

For calculation purposes damage to piers from log impact is estimates at levels in othercountries. It is cautioned that the 2 tonne mass still specified for log impact in theAUSTROADS Bridge Design Code may not represent a realistically sized log and the stoppingdistance assumed in calculations significantly affects the forces generated. Damage to bridgepiers from log impact with an estimated 5 to 6 tonne mass has recently occurred in NSW.Current practice within many UK consultancy firms is to assume a 10 tonne mass for log impact.

Also a debris mat collected against the side of the bridge may offer a cushioning effect to impactfrom large floating objects, an uncushioned blow could displace a near buoyant. Buoyancy ofthe superstructure reduces the effectiveness of the lateral restraints.

Therefore, to account for these uncertainties, the bridge must have a very large factor of safetyagainst instability under flood cubmergence. Unless the stabilizing restraint provided by gravityforce provides a factor of safety of at least (say) 03 or more on unfactored loads thesuperstructure must be secur ely tied down to the substructure. This may be achieved by asuitable arrangement of bolts (,:'bars with positive end anchorages or the design may incorporatebearings with hold down restr.iinr. The small additional expense for provision of hold -downrestraints should be evaluated against the great expense of possible Joss of the whole bridgestruct.ure from instability. Naturally the substructure and bearings must be designed for theoverturning and uplift effects mentioned above.

3.2 Critical Flood Levels & Velocities

It is suggested the following load case categories for low level bridges be investigated:

(a) Partial submergence of substructure"

(b) Partial submergence of superstructure

(c: Overtopping of superstructure

(d) Deep submergence of suporstructure

Stream velocities as well as frejuency, magnitude and duration of the respective submergencecalculated for various flood flows will determine the required scour protection of theembankments for the road approaches and abutments, Where road approach or bridge abutmentembankment are overtopped by flood waters special protection works are required.

34

04. SillTABILITY OF ALTERNATIVE STRUCTURES

4.1 Kerbs & Barriers

This Clause deals with some aspects of the choice of appropriate kerbs and barriers. Therequirements are sometimes contradicting, e.g. a grill type pedestrian or combinedtraffic/pedestrian barrier is no; suitable for sites with substantial amounts of debris. yet it may berequired if the bridge is located in town and there is heavy pedestrian usage. The choice of theappropriate kerb or barrier depends on circumstances.

Ifit is considered that kerbs are warranted, they should either be castellated (short sections ofkerb separated by full-depth gaps not exceeding 200 mm) or if continuous. provided with slotsto allow water to drain free I', from the deck and to aid the manual removal of debris. Small

.'

diameter drainage holes and scupper pipes are prone to clocking with debris. Consideration maybe given to capping the caste'lations with steel channel fenders to provide a continuous tyrerubbing strip. In this case the 200 rom limit on gaps between castellations does not apply.

4.2 Superstructure

It is desirable to select a form of superstructure with a shallow depth. to minimize hydrodynamicforce. backwater and scour ei .ect particularly at the critical flood height when the bridge andapproaches are about to be overtopped.

9lm.P2ite decked bridges. ~:!singeither prestrxssed concrete or steel girders, have beensuc~s~f~Iy,_llSed for s!:!h.t!\$J1:'~d.~ti1~~prf~I>~Qi2r~9_(,'{)'J~~rders have utilized 'inverted~ and standard AtJSTRO\l)S .c UH ,J""~;\!I ":,;:,, "kd girders have utilized rolled orwelded plate sections. These ~(;ction" allow .loodwaters to rise between the girders.

Slender steel girders may requi- e additional midspan lateral bracing as strengthening to resist logimpacts against the bottom flange. If used, cross bracing should be orientated to avoid trappingdebris. For corrosion resistance, steel components must be detailed to avoid trapping ponds ofwater after submergence.

Closed cell structures such as s.ngle and multicell steel or concrete box girders, composite steelor concrete trough girder, multcelled decks formed from linked broad flange concrete girders.vo.ded concrete slabs. and other voided concrete girders are generally considered unsuitable andshould only be used with grea: caution when subject to submergence.

The greater buoyancy forces acting on these closed cell structures may require the use ofsignificant hold-down restrain-s.

Experience has shown it is impossible to hermetically seal concrete closed cell structures againstthe ingress of water whether rhese structures are considered to be submerged or not. Theexception is those voided concrete structures using polystyrene void former which is leftpermanently in place. Otherwise noisture enters the air filled internal voids through cracksformed in the surrounding cone; ete by either temperature, shrinkage cracking or tearing of theplastic concrete during curing. Thus the cross-section must be detailed to facilitate draining ofnay water trapped in pockets, tycically with 25 mm diameter vertical drainage pipes placed at thegirder ends. Due to the provision of these drainage pipes in such structures the possibility alwaysexists that the internal voids wil. be filled completely during submergence. If the voids do notempty again at the rate of flood recession (perhaps due to siltation blocking the drainage holes)severe loads would be applied io the bridge from water trapped inside.

35

Even if the steel or concrete members of closed cell structures are modified with large openingsto permit rapid filling and emptying, when submergence occurs, it is anticipated significant siltand fine debris will be deposited inside when such submergence is frequent. This will have anadverse effect on the durability of the structure and may require significant future maintenance.Note that access would therefore be required inside the closed cells for maintenance inspectionand works. This cost could d-: tract from any economic advantage of this form of construction.

All parts of concrete superstn ....tures subject to frequent flood submergence must be consideredto be 'in contact with fresh '.'ater' for calculation of required cover to prestressed and non-stressed reinforcement. This may lead to slightly greater cover requirements for PSC girdersused in low-level bridges, especially in pockets where silt may be trapped and remain damp aftersubmergence.

A judgement must be made whether siltation due to submergence will occur frequently or onlyrarely in the li fe of the bridge. For example, submergence ever fifty years of an intermediate-levelbridge may be acceptable whereas submergence every second year of a low level bridge wouldbe totally unacceptable.

Steel trusses and suspended decks are considered unsuitable for low or intermediate level bridgesas these structures readily collect debris mats to a greater depth than solid section decks whiledebris or floating objects may damage slender bracing or support components. Suchsuperstructures should only b} use for high-level bridges.

4.3 Bearings & Hold-down Hestraints

Generally, the size of bridge ~:.1anschosen for typical low-level bridges will permit the use ofelastomeric bearings without v.arranting use of the higher load capacity of more sophisticatedspherical, rol.er or pot type ber.rings. However, pot type bearings may be sometimes appropriatefor intermediate-level bridges os these bearings can be economically provided with hold-downrestraints. This may prove advantageous where high lateral loads must be resisted by submergedbridges incurring reduced overturning stability.

It is usual to provide separate anchor bars. Typically 24 nun diameter galvanized holding downbolts are specified between each P.S.C. plank on composite slab structures while more substantialangle brackets may be specifie -,i to straddle each P.S.c. girder on composite decked structures.Shear blocks or dowels connected between end diaphragms and abutments or piers will preventa submerged superstructure from lateral displacement.

If elastomeric bearings are usee I the holding-down bolts should be suitably de-bonded and an airspace left above the heads oftP.~ bolts, to allow the bearings t deform under live load. Frictionalrestraint against creep of elastomeric bearings is diminished during flood submergence due toreduced bearing reactions fron. superstructure buoyancy: Under the influence of rotation fromhogging effects this creep usually moves the bearings towards the centre of the girder spans.

It4 Substructure

The foundations for piers and ~~'Iutmentsof all bridges crossing fast flowing waterways shouldeither be keyed on to rock or cr.' t on piled footings. This avoids the possibility of underminingof the base material under the f .undation footing pads by the formation of adjacent local scourholes. Eddies also tend to wasp. out fill behind abutments. Local scour from turbulence aroundpiers may be reduced by hydraulically shaping the pile caps and columns.

36In particular, the piers and abetment structures of low-level bridges will be subject to a greaterrisk ofundennining of the foundations by scour.

Where the substructure restrains the superstructure from overturning and uplift effects the pilesmay require design for tension and uplift resistance. If driven piles are used they should bedriven well below local scour level to a sufticient depth and sufficiently bard set to resist uplift.Where vertical piles are placed in closely spaced groups account must be taken of the soil actingas a solid block around the incividual piles. Spread footings or bored piles on rock should haveeither a sufficient socket length into rock or be provided with rock anchors.

Solid rather than framed piers may be chosen to provide an advantage of greater mass to resistoverturning effect. Framed piers also tend to collect more debris. Solid circular or elliptical piercolumns could be used to avoid the horizontal 'lift' force effects generated on blade type piersangied to the direction of stream flow.

Spill-through type abutments are generally preferred because of the smoother shaping effect ofthe front and side embankment batters in the stream flow. Portions of embankment washed outcan always be replaced after tile floods recede, whereas damage to the abutment structure itselfis to be avoided. These spill-through abutments must be adequately drained with weep-holes,geofabrics and gravel drainage layers, and as well, the embankment batters protected againstscour.

Adequate drainage works are especially critical for the stability of cantilever, cell (boxed) andReinforced Earth-type abutments subject to flood submergence. However the use of ReinforcedEarth type abutments is not recommended where stream velocity is such that scour couldundermine the base material or eddies could wash out the infill. Abutment details should includea sill drain behind the bearings if staining from water seepage over the front face of theabutments is of concern.

Abutment approach slabs subject frequent submergence should comprise reinforced concreteslabs laid on a free draining grovel base. Geofabrics should be used to prevent drainage layersclogging with silt. The embankments adjacent to the abutment must be properly drained andbatter slopes reduced where possible to avoid instability effect with rapid draw-down offloodwaters. Internal granular lenses may be incorporated to assist in drainage after floodsubmergence. Highly expansive clay fill must not be used under approach slabs to avoid flexingorthe slabs after periods of flood submergence.

4.5 Batter Protection

Overtopping flow oflong duration at frequent interval is likely to cause failure of pavements aswell as scouring of embankments, especially adjacent to the bridge abutments. Stream velocitiesover embankments adjacent to the abutments are increased where the road approachembankments direct flood waters towards the bridge opening. More substantial, but costly,embankment batter protection may be required at the abutments than that used along the roadapproaches, to protect the bridge structure from flood damage.

Protection works of embankments adjacent to abutments will be similar to those for high levelbridge and may include rock fill, rip-rap enclose din wire cages, concrete revetment mattressesor rigid reinforced concrete slabs as well as sheet pile toe walls or spur dykes. Howeverabutment batter protection of low-Ievel bridges not only must be secured against stream bedscour but must be secured aga.nst scour effect during overtopping,

37Selection of the form of embankment protection against scour is governed by :

(a) Whether flow across the embankment is free or submerged.

(b) Under free flow conditions, whether plunging or surface flow occurs on the downstreamembankment batter.

(c) The relative cost of protection works against the degree of protection required.

Grassing of batters may be adequate when the velocity of water flow over the embankment is lessthan 1.5 metres/sec. Generally this is the case when tailwater levels are not more than around300 mm below the downstream edge of the road formation when overtopping first occurs. Grassbatters may not be suitable where frequent overtopping occurs for a period of more than two orthree hours during floods. Grass batters are not suitable for shaded areas under the bridgesuperstructure.

More elaborate protection works against scour are detailed similarly to the requirements forcauseways. It is essential all th~ following protection works are either taken below the anticipatedstream bed general scour level or local scour hole depths using cut-off trenches, or are providedwith level aprons extending i-rto the waterway to accommodate scour erosion. In particular.batter protection works subject to flood submergence must also be anchored along their upperedges to resist scour erosion during overtopping. Geotextile filter cloths are required beneatheither flexible or rigid forms of protection to avoid leaching of fine material underlying theprotective layer by piping, jets or eddies.

Rock fill or hand packed rock placed on batters is the oldest type of embankment protection.uhough costly it provides a flexible treatment which is capable of deforming without loss of

integrity. Rock fill is now preferred as it is cheaper to place and accommodates embankmentdisplacement more easily than handpacked rock without sacrificing protection capacity againstscour. Rip-rap enclosed in wir..: cages also provides a flexible treatment and permits the use ofstone of small size. In highly corrosive conditions such as salt water, PVC coated wire is usedbut if the stream carries a bed load of boulders, which may damage the wire cages, this type ofprotectionmay not be suitable. Siltation within the cages and the growth of a protective coverassists in stabilizin& the rip-rap.

Rigid reinforced concrete slabs are suitable for extreme conditions such as a very low tail waterdepth at overtopping, and highly erosive material at the toe of the embankment. Care is requiredin their design and construe ..:ion to resist cracking induced by temperature changes andembankment deformations. N. well, their design must ensure that adequate open joints or weepholes are provided to relieve ~ydrostatic pressure and reduce uplift forces.

The selection of the type of protection to be used will depend on cost as well as the degree ofprotection required The depth of cut-off'trenches and the length of apron, choice of geo-textilesand drainage layers, whether more substantial protection is required behind or adjacent to theabutment must be determined {or each job.

Finally, design technique is sometimes used in flood plain crossings with long approaches. Thelevel of the bridge and the entire road approach embankment, is placed above (say) the 20 year(ARl) serviceability flood level hut a chosen section of the approach embankment is placed ata slightly lower level. When overtopped by rising floodwater, this section of the embankmentis designed to be scoured away, the breach thus acting as a 'fuse-plug' and preventingsubmergence of the adjacent bridge structure.

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