RDA Bridge Design Manual
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1996 ISSUED UNDER THE AUTHORITY OF THE GENERAL MANAGER ROAD DEVELOPMENT SRI LANKA
This Manu~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.
Mr. P.B.L Cooray
General Manager Committee]
Dr. G.LA.J. De Silva
Director (ES) [Committee
Director (MM&C) [Committee Member]
Director (P&PM) [Committee
Director (T) [Committee
Mrs. H.Y. Fernando
Dy. Director (BD) Committee]
Mr. Asoka \iVeeraratne
Dy. Director (CM)
Mr. T.L Chandrasiri
Dy. Director (P&PM) Mcmbcr]
Mr. D.IlR. Swarna
Senior Engineer (BD) Member]
Mr. R.A.D.S.1l Ranathunge-
Executive Engineer (MM&C) Member]
Mr. M. Chandrasena
Bridge Consultant Chandrasena & [Committee Member]
Bridge Desi.gn Expert (MS. Renardet Consulting Engineers) [Committee Member]
This manual has been drafted by the following members. 01.
Mrs. H. Y. Fernando
Mr. D.IlR. Swarna
Mr. VV.E.S.1l 1i'ernando
Mr. P.S. Sadadcharan
Member] [Secretary of the
SCOPE AND GENERAL ..
2.0 2.1 2.2
02 02 02
2.2.1 2.2.2 2.2.3
2.2.6 2.2.7 2.2.8 3.0
General Loads .. General Dead Loads .. .. , Live Loads Breaking and Traction Horizontal Forces due to Water CU1Tent, Debris, & Log Impact Wind Loads .. Temperature Siress in Concrete Bridge Decks Creep and Shrinkage
INVESTIGA T.~ON Geological Investigation
3.1 3.1.1 3.1.2 3.1.3 3.2 3.3
Topographical :.~wvey Hydrological Survey _ Technical Surv-y & Details of the Existing Bridge Geotechnical Lrvcstigation .. Waterway and Length of Bridge
AND GEOlVIETRICAL CONSIDERATION
DESIGN OF SUBl\1ERSIBLE BRIDGES
05 05 08
09 09 10 10
11 14 16
OF BRIDGE TYPES & DESIGN CONSIDERATION 18
Foundation Substructure .. 5.2 -' 5.2.1 Abutments 5.2.2 Wing Walls 5.2.3 Piers 5.3· Super Structure 5.3.1 Design of Super Structure 5.4 Bridge Bearing Other Features of Super Structures 5.5 ~.1
02 03 03 03
Bridge Location, Proportioning & Orientation 6.2.1 Location 6.2.2 Proportioning Bridge & Approaches 6.2.3 Deck Level & Trafficability 6.2.4 Vertical Alignment 6.2.5 Horizontal Alignment 6.2.6 Deck Crossfall 6.3 Analysis 6.3.1 Uplift and Instalility
20 21 21
22 23 23
24 25 25
25 26 26
26 26 26
6.3.2 6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.4.5
Critical Flood Levels & Velocities Suitability of Alrernative Structures Kerbs and Ban iers .. Super Structure Bearings and Hold Down Restraints Substructure .. Batter Protection
GENERAL Scope Introduction
2.0 2.1 2.2 2.3 2.4 2.5 2.6
27 27 27
27 28 28 28
BRIDGE LOCATION, PROPORTIONING Location Proportioning Bridge and Approaches Deck Level and Trfficability Vertical Alignment Horizontal Alignment Deck Crossfall
Uplift. .and Instability Critical Flood Level and Velocities
4.1 4.2 4.3
Kerbs and BaIT--;--r Super Structures Dearing and Hold Down Restraints '~~.4 Substructure - -If-.5 Batter Protection
_~ __ __
30 31 31 31 32 32 /
STRUCTURES 34 34
35 35 36
SCOPE & GENERAL: : Availability of construction materials & equipment, less maintenance and long life span are the main factors in choosing concrete bridges abundantly in Sri Lanka As the other types such as steel bridges, arch bridges & timber bridges are limited in number, this note mostly covers the design aspects for concrete bridges. Bridge Design Manual is to supplement the Bridge Design Code adopted by the Road Development Authority, the British Standard 5400, for the loadings and effects where the local 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 the local climatic conditions. This is to provide a guidance to the designer in the interpretation of some of the provision of the standard and in calculation of the effects prescribed by the standard and to summarize and to advise the designer on the design practices adopted by the Road Development Authority in terms of selection of substructure and superstructure types. It is recommended that there guide lines are used by other authorities for design of highway 1__ = .l .. __
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 of Safety should be greater than 1.00 Where F.O.S. ~=-
Stability Moment Overturning 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
But it i~.a good practice to have the F.O.S. of 1.3 always to cater for constructional deficiencies.
Capping beams are designed for bending moments and shear forces due to loads acting on them. Ballast wall in ab..tment capping beam is designed to take up horizontal pressure created 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 is provided under the seat to resist the splitting forces .
2.2.1 General Bridges in Sri Lanka de not need to be designed for effects due to earthquakes as Sri Lanka is not in a zone affected by earthquakes. Generally the loading is to conform and applied according to BS 5400 part 2. Bridges should 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
2.2.2 Dead Loads the case of precast slabs and beams, adverse stresses during handling, transporting and stacking should be considered.
In the case of submersible bridges, the effect of horizontal forces due to water and impact of debris and buoyancy should be considered. Dead Load includes self weight, kerbs, sidewalks, handrails, uprights, wearing surface and weight 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 local highway network.
All bridges should be d~'~.;ignedto resist the effect ofHA loading specified in the relevant code.
Bridges should be able ;:0 resist the effect of 30 units of Hls loading for A & B class of roads. 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 tractionThe following factors a.e to be applied to the full tractive force decided according to the code in designing subs.ructures for simply supported bridges to suit local conditions. For Abutments ••
0.6 X Tractive applied 0.8 X Tractive applied
force, at bearing level. force, at bearing level.
The bridge is to be designed for HA Loading with HA tractive force and checked for adequacy to carry the nllocated HB Loading. In checking for HB, it is permissible to decrease the HB Tractive force by 25% to allow for an permissible overstress. However the live load surcharge should be limited to 10 kN/m2. 2.2.5
Forces duv to 'Vater Current
& debris and Log impact-
Horizontal Forces due to Water Current Any part of a bridge structure which may be submerged in running water should be designed to sustain safety the horizontal pressure due to the force of the current
4 On piers parallel to the direction of the water current, the intensity of pressure is given by;
intensity of Pressure in kg/m/\2 due to the water current unit weight of water in kg/rn/\3 velocity of current in rn/sec. at the point where the pressure .ntensity is being calculated acceleration due to gravity in m/sec.oz ;1 constant depending on the shape of pier as follows
with the normei values for W & g equation reduces to P = 52 Kv/\2
Type c;~'Pier square ended pier Circular piers or semi circular cutwaters Triangular cutwaters Trestle type piers
0.66 0.5 to 0.9 1.25
The velocity V :s assumed to vary linearly from Zero at the point of deepest scour to a maximum at the free surface. The maximum velocity at surface for the purpose 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 the direction assumed in the design allowance should be made in the design of the piers for an ex.ra variation in the current direction of 20 degrees. In this case velocity is resolved into two directions, parallel and normal to the pier with k assumed 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-
DebrisWhere debris i:, likely, allowance shall be made for the force exerted by a minimum depth of 1.2 m debris. The length of the debris applied to anyone pier shall be one half of the sum of the adjacent spans with a maximum of 22.0 m where the deck is not submerged. For debris the formula for water current shall be used the value of the constant K being 1.0.
Log Impact When there is a i'JOssibilityfor driftwood and other drifting items to collide with a bridge, collisio.: force shall be calculated from equation. ~
Collision force (t)
Weight of drifting item (t) (2 t log is assumed)
Surface velocity of water (m/s)
Specification for Highway Bridges Part I - Common Specifications by Japan Read Association
2.2.6 Wind LoadsThe mean hourly wind s;:-eedis determined for the location of the bridge, from the Wind Loading 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.
11EAN HOURLY WIND SPEED
75 m.p.h. (33.0 mls) 65 m.p.h, (28.9 m/s) 50 m.. h. ;22.2 mis,
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 body of the deck has one 'mean' value at any instant of time and that this 'body mean temperature' rises or falls over a long period of time, thereby wanting the structure to 'heave'. If the structure is free to permit this 'heave' ie, is free to expand or contract (e.g. simply supported beam or a continuous beam), this causes no thermal 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 can be evaluated by the usual methods of theory of elasticity. (b)
Temperature GradientMinimum and Maximum shade air temperaturesFor all bridges, extremes of shade air temperature for the location of the bridge shall 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.
Adjustment for height above mean sea level The values of shade air temperature shall be adjusted for heights above 300 m above 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 be derived fr.-rn the shade air temperatures by reference to table No. 2.1. The different types of construction are as shown in figure No. 2.4.
Table No. 2.1
Shade Air Temperature
13 14 15
17 18 19 20 21 22 23 24 25 26 27 28
Group 2 16 17 18 18 19 20 20 21 22
19 19 20 21 22 23 23 24 25 26 27 27 28 29 30 30
08 09 10 11 12
Type of Superstructures
24 25 25 26 27 27 28 29 29
33 34 34
38 39 40
32 33 33 34 35
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 ordinate of the thermal ~~radientat any intermediate depth follow a nonlinear variation. Considering be build up of the total thermal gradient, it's uniform part at the instant of consideration, is akin to the 'body mean temperature', the effect of change 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' would heat each fibre r-..0 a different degree, the variation being non linear. If the fibers were free of each other (i.e. unrestrained) then they could accept the corresponding non linear thermal strains xi (x being the coefficient of expansion/contraction). But since their deformations must follow a linear law (plane sections must remain plane), they will not accept these non linearly related strains, and the difference between the final 'linear' strain gradient and the 'unrestrained' s.rain gradient will represent the uneven 'internal disturbance'. It's strain effect m.ry be called the 'Eigenstrain' and its stress effect may be called the Eigenstress, 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 an internal entity, not associated with any support reactions. Eigenstrcs, or: its own, may be small or significant, depending on (i) (ii) (iii) (iv) (v) (vi)
depth of section thickness & colour of pavement wind speed orientation of bridge and incidence of sun rays. ambient temperature material properties thermal conditional specific heat thermal diffusiniry coefficient of thermal expansion and contraction coefficient of absorptivity coefficient of surface - heat transfer surface temperature 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 extreme fibers but als. at intermediate fibers (e.g. mid height portion of webs) which are heavily loaded under shear. This can produce longitudinal cracks in webs. (d)
Effect of Intc.mediate - support resuaint on the Free Hogging (or Sagging) Desire ofthe structure caused by unequal Extreme Fibre Temperatures - 'The continuity effect'. In 8 beam-type deck, the difference oftempemture between the extreme surfaces causes hoggiag (or sagging) of the beam. If the beam.is simply supported, it merely hogs (or sags) as its supports do not
8 prevent rotation, This free deformation is not a 'moment induced' deformation, but merely a 'Strain induced' deformation, and no moment is caused. However if the beam is continuous, its aforementioned desire to freely hog (or freely sag) wiil be 'constrained' at the intermediate supports (presence of dead load reactions wilt prevent it from lifting up and presence of supports will prevent it from going down at their supports. This 'continuity' effect sets up moments that cause additional stresses called 'continuity stresses'. Ref:
by Dr. V.K. Raina
Stress due to «emperature should be calculated as per BS 5400 cl. 5.4. The shade air temperature referred to in the clause should be taken from the tables given for different districts in Sri Lanka. For minimum effective bridge temperature ofBS 5400.
2.2.8 Creep and Shrinkage
same pattern is assumed as per table I I
Creep and Shrinkage .mly have to be taken in to account when they are considered to be important Obvious srruations are where deflections are important and in the design of the articulation 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.
Shrinkage restrained by the reinforcement; • Stress in reinforcement = f", = (compression;
1+ Ue. (Aj~) Stress in concrete (tension)
= A. .
free shrinkage strain refer fig. 2.5
modulus of elasticity of steel
area of tension reinforcement
area of concrete
Shrinkage fully restrained; Stress in concrete (tension) Where ~
= fel = tcs .
Static secant modulus of elasticity of concrete
The value of €C3 to be obtained either from BS 5400 : Part 4 : Appendix Cor BS 8110,' Part for 80% relative humidity, (Fig. 2,5)
3.1 Geographical Investigation A detail survey should be carried out at the proposed location to cover topographical hydrological and technical details.
3.1.1 Topographical Survey (a)
A minimum length of 150 m on both ends of the bridge or the selected location of the bridge should be considered for detailed survey (i.e. Chain Survey. including all the permanent & temporary features and levelling) unless there is a curve encountered in e .e close proximity of the bridge beyond this length. If there is a curve the Engineer has to justify the situation and survey should be extended.
Chain survey need not be a close traverse unless it is a very important location but the levelling should be a close survey.
The chainage marked should be always in the direction of the road, (i.e. In Colombo - Kaney Road chainage 00+00 m should be started in the Colombo end of the bridge) The 00+00 m chainage should be tied.
" Longitudinal sections along the centreline of the road and cross sections should be recorded systematically with the chainages and the distances from the centre line.
At lease 05 cross sections should be taken at intervals of 05 m close to the bridge on both ends of tle bridge and the balance should be at 10 m intervals and 15 m intervals.
On a curve of the mad also the cross sections should be taken at intervals of 05 m.
The levels & chaiaages of every expansion joint of the bridge at the L.HS .• centre and R.H.S. should be taken. Also the invert levels of the waterway should be taken.
Cross sections should be taken to a distance at least 15 m from the centre line of the road on either side unless there are considerable changes in the levels. In case if there is a possibie deviation of the existing road is involved, the cross section should be taken as necessary.
10 If considerable level differences are encountered cross section should be extended
as necessary. (i)
The site survey should include the river banks to a distance of30 m. If there is a change in the direction of the stream the length should be extended as necessary.
The reduced level of the M.S.L. also should be taken if it is marked in the close proximity 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.
The direction of ~,orth should be marked.
If there are services crossing the river or carried by the bridge the necessary details such as size of the pipe, the distance from the bridge to the pipe line, type & number of supports etc. should be taken.
High tension power lines or any other structures closer to the bridge which can be affected during ccnstruction should be noted down. The possibility of cetouring and accommodating traffic during construction should be found out. SUI'. ey & levelling should cover the detour area. Possible alternative locations for the bridge apart from the existing bridge) to be considered and thcr merits/demerits noted.
_.1.2 Hydrological Survey(a)
The flow directioi. of the waterway over which the bridge is to be constructed should be clearly marked. The banks of the waterway also should be marked.
Bed level and cross sections of river on up stream and down stream sides should be taken, to a distance of 30 m approximately .
• 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. The flood marks on the: existing structure should be noted where ever possible.
Scouring of river be.' & river meandering patterns should be checked & any local scour patterns documented.
The approximate noted.
of the floating debris if there are any should be inquired &
3.l.3 Technical Survey & Detacs of the Existing Bridge
Type of bed material, rock out crop/boulders etc. should be noted down.
Environmental condition. sal inc/rnari nc atmosphere windy condition etc. shoul- i I)"~ taken
Any visible settlement of the existing structure should be marked. In doing so particular attention to be given for alignment of parapetslhandrails, kerbs etc.
Sketches of the ~!ridgefoundations, substructure and superstructure must be given with all dimensons. Where ever possible existing bridge foundation type should be indicated th-ough inspection or from data collected by the neighbours. Conditions of existing structures nearby to be noted if any.
Bearing points (1.1 the existing capping beam of the bridge should be marked clearly with dimension..
Details of existing bridge should be taken in the form of photographs.
Geotechnical Investigation (a)
Subsurface Invr stigation Detailed sub sur.ace investigations are carried out in the form of bore holes using rotary core percussion drilling machines. In certain cases where good soil conditions or bed rock are expected at shallow depths, soil investigations may be carried out by digging test pits, Bore holes shou: ,i be carried out at suitable intervals in the form of a grid covering the entire area. The spacing of the grid is decided on the nature of the structure and the variation of soil conditions at the site. The Geotechnical Repoi t prepared by the Geotechnical Consultant completion 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 necessary information unless bed reck is at a large depth and bridge could be founded at a shallow depth. Additional boreholes ma be required at sites where the bores indicate variability of subsurface conditions. The site investigation should include: In situ field test: 'which may include standard penetration tests or static cone penetrometer 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. Strati graphy 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. " "
- ·-0 BANOfoR!} WELA
-- .. ,/ ~~_____
SRi LANK~. ISOTHERMS
MAHA tLLUPPALLAMA •
: .MATALE r
-:;:-=;.., _.- ....,
, ,, I
dif f e r e n c e
100 mrn sur tacinq ,If
-. . ,00 r-
___~oo mm surfllci:1g :;:>';F)~:::>'o:,