DESIGN OF BRIDGES

• BRIDGE: STRUCTURE WHICH PROVIDES PASSAGE OVER AN OBSTACLE WITHOUT CLOSING THE WAY BENEATH

REQUIRED PASSAGE MAY BE FOR A ROAD, A RAILWAY,PEDESTRIANS, A CANAL OR A PIPE LINE

OBSTACLE TO BE CROSSED-RIVER, A ROAD, RAILWAY OR A VALLEY

IMPORTANCE OF BRIDGES

INVENTIONS WHICH ABRIDGE DISTANCE HAVE DONE THE MOST FOR THE CIVILISATION OF THE SOCIETY

BRIDGES FIGURED PROMINENTLY IN HUMAN HISTORY

CITIES HAVE BEEN DEVELOPEDAT A BRIDGE HEAD OR WHERE AT FIRST RIVER COULD BE FORDED AT ANY TIME OF THE YEAR

EXAMPLE-LONDON,CAMBRIDGE,OXFORD ETC

GREAT BATTLES HAVE BEEN FOUGHT FOR CITIES AND BRIDGES

MOBILITY OF AN ARMY DURING WAR IS OFTEN AFFECTED BY THE AVAILABILITY OR OTHERWISE OF BRIDGES TO CROSS RIVERS.

THAT’S WHY MILITARY TRAINING PUTS SPECIAL EMPHASIS ON LEARNING HOW TO DESTROY EXISTING BRIDGES WHILE RETREATING AND HOW TO BUILD NEW ONES QUICKLY WHILE ADVANCING

FACTORS TO BE CONSIDERED BEFORE THE CONSTRUCITON OF A BRIDGE

1. THE NEED FOR THE STRUCTURE AND ITS LOCATION

2. THE TRAFFIC THAT WILL PASS OVER THE STRUCTURE ON COMPLETION AND IN FUTURE(IN RESPECT OF THE VOLUME OF THE TRAFFIC AS WELL AS THE AXLE/WHEEL LOAD)

3. CHARACTERSTICS OF THE STREAM/ RIVER/ CANAL

4. SUB SOIL CONDITIONS

5. ALTERNATIVE SITES EITHER BY LONGITUDINAL SHIFT OF THE PROPOSED STRUCTURE OR BY LATERAL SHIFT OF THE ALIGNMENT ITSELF

6. CONSTRUCTION PROBLEMS THAT ARE LIKELY TO ARISE AND

7. COST AND ECONOMICS

COMPONENTS OF A BRIDGEA. DECKING, CONSISTING OF DECK SLAB,

GIRDERS, TRUSSES, ETCB. BEARINGS FOR DECKINGC. ABUTMENTS AND PIERSD. FOUNDATIONS FOR THE ABUTMENTS AND

PIERSE. RIVER TRAINING WORKS, LIKE REVETMENT

FOR SLOPES AT ABUTMENTS, APRONS AT THE BED LEVEL, ETC

F. APPROACHES TO THE BRIDE TO CONNECT THE BRIDGE PROPER TO THE ROADS ON EITHER SIDE AND

G. HAND RAILS AND GUARD STONES ETC

THE COMPONENTS ABOVE THE LEVEL OF BEARINGS ARE GROUPED AS SUPER STRUCTURE, WHILE THE PARTS BELOW THE BEARING LEVEL ARE CLASSED AS SUB STRUCTURE. THE PORTION BELOW THE BED LEVEL OF THE RIVER IS CALLED THE FOUNDATION. THE COMPONENTS BELOW THE BEARING AND ABOVE THE FOUNDATION ARE OFTEN REFERRED AS SUB STRUCTURE.

CLASSIFICATIONBRIDGES MAY BE CLASSIFIED IN MANY WAYS

a. ACCORDING TO FUNCTION AS AQUADUCT (CANAL OVER A RIVER), VIADUCT (ROAD OR RAILWAY OVER A VALLEY), PEDASTRIAN, HIGHWAY, RAILWAY, ROAD-CUM-RAIL OR A PIPELINE BRIDGE

b. ACCORDING TO THE MATERIAL OF CONSTRUCTION OF SUPER STRUCTURE AS TIMBER, MASONRY, IRON, STEEL, REINFORCED CONCRETE, PRESTRESSED CONCRETE, COMPOSITE OR ALUMINIUM BRIDGE

c. ACCORDING TO THE FORM OR TYPE OF SUPER STRUCTURE AS SLAB, BEAM, TRUSS, ARCH, CABLE STAYED OR SUSPENSION BRIDGE

d. ACCORDING TO THE INTERSPAN RELATIONS AS SIMPLE, CONTINUOUS OR CANTILEVER BRIDGE

e. ACCORDING TO THE POSITION OF THE BRIDGE FLOOR RELATIVE TO THE SUPER STRUCTURE AS DECK THROUGH, HALF THROUGH OR SUSPENDED BRIDGE

f. ACCORDING TO THE METHOD OF CONNECTIONS OF THE DIFFERENT PARTS OF THE SUPERSTRUCTURE, PARTICULARLY FOR STELL CONSTRUCTION, AS PIN-CONNECTED, REVETED OR WELDED BRIDGE

g. ACCORDING TO THE LOAD LEVEL RELATIVE TO THE HIGHEST FLOOD LEVEL OF THE RIVER BELOW, PARTICULARLY FOR A HIGHWAY BRIDGE, AS HIGH-LEVEL OR SUBMERSIBLE BEIDGE.

h. ACCORDING TO THE METHOD OF CLEARANCE FOR NAVIGATION AS HIGH LEVEL, MOVABLE-BASCULE, MOVABLE SWING OR TRANSPORTER BRIDGE

i. ACCORDING TO THE SPAN LENGTH AS CULVERT (LESS THAN 8M) MINOR BRIDGE (8 TO 30M) MAJOR BRIDGE(ABOVE 30M) OR LONG SPAN BRIDGE(ABOVE 120M)

j. ACCORDING TO THE DEGREE OF REDUNDANCY AS DETERMINATE OR INDETERMINATE BRIDGE

k. ACCORDING TO ANTICIPATED TYPE OF SERVICE AND DURATION OF USE AS PERMANENT, TEMPORARY MILITARY(PONTOON, BAILEY) BRIDGE

STANDARD SPECIFICATIONS FOR ROAD BRIDGES

THE INDIAN ROAD CONGRESS (IRC) HAS FORMULATED STANDARD SPECIFICATIONS AND CODES OF PRACTICE FOR ROAD BRIDGES WITH A VIEW TO ESTABLISH A COMMON PROCEDURE FOR THE DESIGN AND CONSTRUCTION OF ROAD BRIDGES IN INDIA.

THE IRC BRIDGE CODE AS AVAILABLE NOW CONSISTS OF EIGHT SECTIONS AS BELOW

1. SECTION I - GENERAL FEATURES OF DESIGN2. SECTION II - LOADS AND STRESSES3. SECTION III - CEMENT CONCRETE (PLAIN AND

REINFORCED)4. SECTION IV - BRICK, STONE AND BLOCK MASONRY5. SECTION V - STEEL ROAD BRIDGES6. SECTION VI - COMPOSITE CONSTRUCTION7. SECTION VII -FOUNDATIONS AND SUBSTRUCTURE8. SECTION VIII - BEARINGS.

SECTION I GIVES THE SPECIFICATIONS FOR THE PRELIMINARY DATA TO BE COLLECTED, DETERMINATION OF DESIGN DISCHARGE, CLEARANCES, FOUNDATIONS ETC.

SECTION II SPECIFIES THE LOADINGS FOR WHICH THE BRIDGES HAVE TO BE DESIGNED.

THE OTHER SECTION GIVES RULES FOR GUIDANCE IN DESIGN OF THE BRIDGE SUPERSTRUCTURE IN MASONRY, REINFORCED CONCRETE, STEEL AND COMPOSITE CONSTRUCTION, FOUNDATIONS AND BEARINGS. GENERAL GUIDELINES FOR THE DESIGN OF PRESTRESSED CONCRETE BRIDGES ARE GIVEN IN A SEPARATE PUBLICATION.

WIDTH OF CARRIAGEWAY THE WIDTH OF CARRIAGEWAY REQUIRED WILL

DEPEND ON THE INTENSITY AND VOLUME OF TRAFFIC ANTICIPATED TO USE THE BRIDGE. THE WIDTH OF CARRIAGEWAY IS EXPRESSED IN TERMS OF TRAFFIC LANES, EACH LANE MEASURING THE WIDTH REQUIRED TO ACCOMMODATE ONE TRAIN OF CLASS A VEHICLES.

EXCEPT ON MINOR VILLAGE ROADS, ALL BRIDGES MUST PROVIDE FOR AT LEAST TWO-LANE WIDTH. THE MINIMUM WIDTH OF CARRIAGE WAY IS 4.25M FOR ONE LANE BRIDGE AND 7.5M FOR A TWO-LANE BRIDGE. FOR EVERY ADDITIONAL LANE, A MINIMUM OF 3.5M MUST BE ALLOWED. BRIDGE MUST HAVE CARRIAGEWAYS OF TWO OR

FOUR LANES OR MULTIPLES OF TWO LANES; THREE LANE BRIDGES SHOULD NOT BE CONSTRUCTED, AS THESE WILL BE CONDUCIVE TO THE OCCURRENCE OF ACCIDENTS. IN CASE OF A WIDE BRIDGE, IT IS DESIRABLE TO PROVIDE A CENTRAL VERGE OF AT LEAST 1.2M WIDTHS IN ORDER TO SEPARATE THE TWO OPPOSING LINES OF TRAFFIC, IN SUCH A CASE THE INDIVIDUAL CARRIAGE ON EITHER SIDE OF THE VERGE SHOULD PROVIDE FOR A MINIMUM OF TWO LANES OF TRAFFIC. IF THE BRIDGE IS TO CARRY A TRAMWAY OR RAILWAY IN ADDITION, THE WIDTH OF THE BRIDGE SHOULD BE INCREASED SUITABLY.

FROM CONSIDERATION OF SAFETY AND EFFECTIVE UTILIZATION OF CARRIAGEWAY, IT IS DESIRABLE TO PROVIDE FOOTPATH OF AT LEAST 1.5M WIDTHS ON EITHER SIDE OF THE CARRIAGEWAY FOR ALL BRIDGES. IN URBAN AREAS, IT MAY BE NECESSARY ALSO TO PROVIDE FOR SEPARATE CYCLE TRACKS BESIDES THE CARRIAGEWAY.

LOADS TO BE CONSIDERED

WHILE DESIGNING ROAD BRIDGES AND CULVERTS, THE FOLLOWING LOADS, FORCES AND STRESSES SHOULD BE CONSIDERED, WHERE APPLICABLE:

A. DEAD LOADB.LIVE LOADC.SNOW LOADD. IMPACT OR DYNAMIC EFFECT DUE

TO VEHICLESE.IMPACT DUE TO FLOATING BODIES OR

VESSELSF. WIND LOAD

G. LONGITUDINAL FORCES CAUSED BY THE TRACTIVE EFFORT OF VEHICLES OR BY BRAKING OF VEHICLES

H. LONGITUDINAL FORCES DUE TO FRICTIONAL RESISTANCE OF EXPANSION BEARINGS

I. CENTRIFUGAL FORCES DUE TO CURVATUREJ. HORIZONTAL FORCES DUE TO WATER CURRENTSK. BUOYANCYL. EARTH PRESSURE, INCLUDING LIVE LOAD

SURCHARGEM. TEMPERATURE EFFECTSN. DEFORMATION EFFECTSO. SECONDARY EFFECTSP. ERECTION STRESSQ. FORCES AND EFFECTS DUE TO EARTH QUAKER. WAVE PRESSURE

ALL MEMBERS SHOULD BE DESIGNED TO SUSTAIN ANY COMBINATION OF THE ABOVE FORCES THAT CAN COEXIST

TYPICAL COMBINATION OF LOADS AND FORCES TO BE CONSIDERED IN DESIGN AND ALLOWABLE INCREASE IN PERMISSIBLE STRESSES FOR CERTAIN COMBINATIONS ARE GIVEN IN THE CODE.

UNDER ANY COMBINATIONS MAXIMUM STRESS IN STEEL MEMBERS SHOULD NOT EXCEED THE YIELD STRENGTH OF STEEL.

IRC STANDARD LIVE LOADS

LIVE LOADS ARE THOSE CAUSED BY VEHICLES, WHICH PASS OVER THE BRIDGE AND ARE TRANSIENT IN NATURE. THESE LOADS CANNOT BE ESTIMATED PRECISELY, AND THE DESIGNER HAS VERY LITTLE CONTROL OVER THEM ONCE THE BRIDGE IS OPENED TO TRAFFIC. HOWEVER HYPOTHETICAL LOADINGS, WHICH ARE REASONABLY REALISTIC NEED TO BE EVOLVED AND SPECIFIED TO SERVE AS DESIGN CRITERIA.

THERE ARE FOUR TYPES OF STANDARD LOADINGS FOR WHICH ROAD BRIDGES ARE DESIGNED.

IRC CLASS AA LOADING

IRC CLASS AA LOADING (REFER TO IRC LOADING DIAGRAMS)

THIS LOADING CONSISTS OF EITHER A TRACKED VEHICLE OF 700 KN OR A WHEELED VEHICLE OF 400 KN WITH DIMENSTIONS AS SHOWN IN FIG. THE TRACKED VEHICLE SIMULATES A COMBAT TANK USED BY THE ARMY. THE GROUND CONTACT LENGTH OF THE TRACK IS 3.6M AND THE NOSE TO TAIL LENGTH OF THE VEHICLE IS 7.2M. THE NOSE TO TAIL SPACING BETWEEN TWO SUCCESSIVE VEHICLES SHALL NOT BE LESS THAN 90M. FOR MULTI-LANE BRIDGES AND CULVERTS,

ONE TRAIN OF CLASS AA TRACKED OR WHEELED VEHICLES WHICHEVER CREATES SEVERER CONDITIONS SHALL BE CONSIDERED FOR EVERY TWO-LANE WIDTH. NO OTHER LIVE LOAD SHALL BE CONSIDERED ON ANY PART OF THE ABOVE TWO-LANE CARRIAGEWAY WHEN THE CLASS AA TRAIN OF VEHICLES IS ON THE BRIDGE. THE CLASS AA LOADING IS TO BE ADOPTED FOR BRIDGES LOCATED WITHIN CERTAIN MUNICIPAL LOCALITIES AND ALONG SPECIFIED HIGHWAYS. NORMALLY, STRUCTURES ON NATIONAL HIGHWAYS AND STATE HIGHWAYS ARE PROVIDED FOR THESE LOADINGS. STRUCTURES DESIGNED FOR CLASS AA LOADING SHOULD ALSO BE CHECKED FOR CLASS A LOADING, SINCE UNDER CERTAIN CONDITIONS, SEVER STRESSES MAY BE OBTAINED UNDER CLASS A LOADING.

IRC CLASS 70R LOADING

IRC CLASS 70 R LOADING (REFER TO IRC LOADING DIAGRAMS)

IN RECENT YEARS, THERE IS AN INCREASING TENDENCY TO SPECIFY THIS LOADING IN PLACE OF CLASS AA LOADING. THIS LOADING CONSISTS OF A TRACKED VEHICLE OF 700KN OR A WHEELED VEHICLE OF TOTAL LOAD OF 1000KN. THE TRACKED VEHICLE IS SIMILAR TO THAT OF CLASS AA EXCEPT THAT THE CONTACT LENGTH OF THE TRACK IS 4.57M, THE NOSE TO TAIL LENGTH OF THE VEHICLE IS 7.92M AND THE SPECIFIED MINIMUM SPACING BETWEEN SUCCESSIVE VEHICLES IS 30M.

THE WHEELED VEHICLE IS 15.22M LONG AND HAS SEVEN AXLES WITH LOADS TOTALING TO 1000KN. IN ADDITION, THE EFFECTS ON THE BRIDGE COMPONENTS DUE TO A BOGIE LOADING OF 400KN ARE ALSO TO BE CHECKED. THE DIMENSIONS OF THE CLASS 70R LOADING VEHICLES ARE SHOWN IN FIG. THE SPECIFIED SPACING BETWEEN VEHICLES IS MEASURED FROM THE REAR MOST POINT OF GROUND CONTACT OF THE LEADING VEHICLE TO THE FORWARD MOST POINT OF GROUND CONTACT OF THE FOLLOWING VEHICLE IN CASE OF TRACKED VEHICLES; FOR WHEELED VEHICLES IT IS MEASURED FROM THE CENTER OF THE REAR MOST WHEEL OF THE LEADING VEHICLE TO THE CENTER OF THE FIRST AXLE OF THE FOLLOWING VEHICLE.

IRC CLASS A AND IRC CLASS B LOADING

IRC CLASS A LOADING: (REFER TO IRC LOADING DIAGRAMS)

CLASS A LOADING CONSISTS OF A WHEEL LOAD TRAIN COMPOSED OF A DRIVING VEHICLE AND TRAILERS OF SPECIFIED AXLE SPACING AND LOADS, AS SHOWN IN FIGS

THE NOSE TO TAIL SPACING BETWEEN TWO SUCCESSIVE TRAINS SHALL NOT BE LESS THAN 18.5 M. NO OTHER LIVE LOAD SHALL COVER ANY PART OF THE CARRIAGEWAY WHEN A TRAIN OF VEHICLES IS ON THE BRIDGE. THE GROUND CONTACT AREA FOR THE DIFFERENT WHEELS AND THE MINIMUM SPECIFIED CLEARANCES ARE INDICATED IN THE FIGURES. CLASS A LOADING IS TO BE NORMALLY ADOPTED ON ALL ROADS ON WHICH PERMANENT BRIDGES AND CULVERTS ARE CONSTRUCTED.

IRC CLASS B LOADING (REFER TO IRC LOADING DIAGRAMS)

CLASS B LOADING COMPRISES A WHEEL LOAD TRAIN SIMILAR TO THAT OF CLASS A LOADING BUT WITH SMALLER AXLE LOADS AS SHOWN IN FIG. THIS LOADING IS INTENDED TO BE ADOPTED FOR TEMPORARY STRUCTURES; TIMBER BRIDGES AND BRIDGES IN SPECIFIED AREAS.

THE STANDARD LOADS ARE TO BE ARRANGED IN SUCH A MANNER AS TO PRODUCE THE SEVEREST BENDING MOMENT OR SHEAR AT ANY SECTION CONSIDERED. THE LOADING VEHICLES ARE TO BE ALIGNED SO AS TO TRAVEL PARALLEL TO THE LENGTH OF THE BRIDGE. WHEN THESE VEHICLES ARE ON THE SPAN, NO OTHER LIVE LOAD NEED BE CONSIDERED AS ACTING OVER THE UNOCCUPIED AREA. VEHICLES IN ADJACENT LANES ARE TO BE ASSUMED TO BE MOVING IN A DIRECTION PRODUCING MAXIMUM STRESSES.

IMPACT EFFECT

LIVE LOAD TRAIN PRODUCE HIGHER STRESSES THAN THOSE WHICH WOULD BE CAUSED IF THE LOADING VEHICLES WERE STATIONARY. IN ORDER TO TAKE INTO ACCOUNT THE INCREASE IN STRESSES DUE TO DYNAMIC ACTION AND STILL PROCEED WITH SIMPLER STATISTICAL ANALYSIS, AN IMPACT ALLOWANCE IS MADE. FOR FOOT BRIDGES, NO ALLOWANCE NEED BE MACE FOR IMPACT. THE IMPACT ALLOWANCE IS EXPRESSED AS A FRACTION OR PERCENTAGE OF THE APPLIED LIVE LOAD, AND IS COMPUTED AS BELOW;

A. FOR I.R.C CLASS A OR B LOADING I = A/(B+L)

WHERE I = IMPACT FACTOR FRACTIONA = CONSTANT OF VALUE 4.5 FOR R.C. BRIDGE AND

9.0 FOR STEEL BRIDGESB = CONSTANT OF VALUE 6.0 FOR REINFORCED

CONCRETE BRIDGESL = SPAN IN METERS.

FOR SPANS LESS THAN 3 METERS, IMPACT FACTOR IS 0.5 FOR R.C. BRIDGES AND 0.545 FOR STEEL BRIDGES WHEN SPAN EXCEEDS 45 METERS, THE IMPACT FACTOR IS TAKEN AS 0.154 FOR STEEL BRIDGES AND 0.088 FOR R.C BRIDGES. ALTERNATIVELY, THE IMPACT FACTOR FRACTION MAY BE DETERMINED FROM THE CURVES GIVEN IN FIG

B. FOR IRC CLASS AA OR 70 R LOADING:A. FOR SPANS LESS THAN 9M

I. FOR TRACKED VEHICLE 25% FOR SPANS UPTO 5M LINEARLY REDUCING TO 10% FOR SPANS OF 9M

II. FOR WHEELED VEHICLE 25%

B. FOR SPANS OF 9M AND MORE

C. FOR TRACKED VEHICLE - FOR R.C. BRIDGES, 10% UPTO SPAN OF 40M AND IN ACCORDANCE WITH FIG 3.6 FOR SPANS EXCEEDING 40M. FOR STEEL BRIDGES, 10% FOR ALL SPANS

D. FOR WHEELED VEHICLE - FOR R.C BRIDGES, 25% FOR SPANS UPTO 12M AND IN ACCORDANCE WITH FIG FOR SPANS EXCEEDING 12M. FOR STEEL BRIDGES, 25% FOR SPANS UPTO 23M, AND AS IN FIG. FOR SPANS EXCEEDING 23M.

THE SPAN LENGTH TO BE CONSIDERED IN THE ABOVE COMPUTATIONS IS DETERMINED AS BELOW:

I. SIMPLY SUPPORTED, CONTINUOUS OR ARCH SPANS THE EFFECTIVE SPAN ON WHICH THE LOAD IS PLACED

II. BRIDGES HAVING CANTILEVER ARM WITHOUT SUSPENDED SPAN - 0.75 OF EFFECTIVE CANTILEVER ARM FOR LOADS ON THE CANTILEVER ARM AND THE EFFECTIVE SPAN BETWEEN SUPPORTS FOR LOADS ON THE MAIN SPAN.

WHEN THERE IS A FILLING OF NOT LESS THAN 0.6M INCLUDING THE ROAD CRUST AS IN SPANDREL FILLED ARCHES, THE IMPACT ALLOWANCE MAY BE TAKEN AS HALF THAT COMPUTED BY THE ABOVE PROCEDURE.

FULL IMPACT ALLOWANCE SHOULD BE MADE FOR DESIGN OF BEARINGS. BUT FOR COMPUTING THE PRESSURE AT DIFFERENT LEVELS OF THE SUBSTRUCTURE, A REDUCED IMPACT ALLOWANCE IS MADE BY MULTIPLYING THE APPROPRIATE IMPACT FRACTION BY FACTOR AS BELOW:

I. AT THE BOTTOM OF BED BLOCK - 0.5

II. FOR TOP 3M OF THE SUB-STRUCTURE BELOW THE BE BLOCK , 0.5 DECREASING UNIFORMLY TO ZERO

III. FOR PORTION OF THE SUBSTRUCTURE MORE THAN 3M BELOW THE BED BLOCK - 0.0

APPLICATION OF LIVE LOADS ON DECK SLABS:

ANY RATIONAL METHOD MAY BE USED FOR CALCULATING THE EFFECT OF CONCENTRATED LOADS ON DECK SLABS. THE DISPOSITION OF THE LOADING SHOULD BE SO ARRANGED AS TO PRODUCE THE MAXIMUM BENDING MOMENT OR SHEAR FOR THE DECK SLAB.

IN CASE OF DECK SLABS SPANNING IN ONE DIRECTION OR CANTILEVER SLABS, THE BENDING MOMENT PER UNIT WIDTH OF SLAB CAUSED BY CONCENTRATED LOADS CAN BE CALCULATED BY ESTIMATING THE WIDTH OF SLAB THAT MAY BE TAKEN AS EFFECTIVE IN RESISTING THE BENDING MOMENT DUE TO THE CONCENTRATED LOADS. FOR PRECAST SLABS, THE ACTUAL WIDTH OF EACH PRECAST UNIT SHOULD BE TAKEN AS THE WIDTH OF SLAB. SLABS DESIGNED ON THIS BASIS NEED NOT BE CHECKED FOR SHEAR.

SLABS SUPPORTED ON TWO OPPOSITE SIDES

The maximum bending moment caused by a wheel load may be assumed to be resisted by an effective width of slab-measured parallel to the supporting edges.The effective width of a single concentrated load is computed from equationbe = k x (1-x/l) + bw --------(1)

 Where be = the effective width of slab on which the load acts.

L = the effective span in the case of simply supported slabs and equal to the clear span in the case of continuous slabs.X= the distance of the center of gravity of the concentrated load from the near support.        bw = the breadth of the concentration area of the load, i.e., the

dimension of the tyre or track contact area over the road surface of the slab in a direction parallel to the supporting edge of the cantilever plus twice the thickness of the wearing coat over the structural slab.The effective width should be limited to one-third the length of the cantilever slab measured parallel to the support. Further, when the concentrated load is placed near one of the two extreme ends of the length of the cantilever slab in the direction parallel to the support, the effective width should not exceed the above value, nor should it exceed half the above value plus the distance of the concentrated load from the nearer extreme end, measured in the direction parallel to the fixed edge when two or more loads act on the slab, and when effective width for one load overlaps the effective width of the adjacent load, the resultant effective width should be taken as the sum of the respective effective widths for each load minus the width of overlap. DISPERSION OF LIVE LOAD THROUGH DECK SLAB                               The effective length of slab on which a wheel load or track load acts shall be taken as equal to the dimension of the tyre contact area over the wearing surface of slab in the direction of the span plus twice the overall thickness of the slab inclusive of the thickness of the wearing surface. DISTRIBUTION REINFORCEMENTThe distribution reinforcement in slabs spanning in one direction shall be provided at right angles to the main reinforcement. This reinforcement is provided to produce a resisting moment equal to 0.3 times the live load moment and 0.2 times the dead load moment, in cantilever slabs, the distribution steel is computed to resist a moment equal to 0.3 times the live load moment and 0.2 times the dead load moment, and the steel is provided half at the top and half at the bottom of the slab. The pitch of the distribution bars shall not exceed three times the effective depth of the slab or 300mm whichever is less.            SLABS SPANNING IN TWO DIRECTIONS                   For slabs spanning in two directions, the moments in the two directions can be obtained by any rational method. The use of curves given by M.Pigeaud is recommended.Pigeaud’s method is applicable to rectangular slabs supported freely on all four sides and subjected to a symmetrically placed load.Let L and B be the span lengths in the long and short span directions‘a’ and ‘b’ be the dimensions of the tyre contact area in long and short span directionsU and V be the dimensions of the load spread after allowing for dispersion through the deck slab.K be the ratio of short span to long spanM1 and M2 be the moments along the short and long spansm1 and m2 be the co-efficient for moments along the short and long spans.    Graph        ‘µ’ be the value of poisson’s ratio, generally taken as 0.15 for reinforced concrete.‘P’ be the load from the wheel under consideration the dispersion of load may be assumed to be at 45 degees through the wearing coat and deck slab. It is also sometimes assumed to be at 45 degrees through wearing coat but at a steeper angle through the deck slab. The latter assumption is used here.The values of moments along the shorter and longer directions would depend on the ratios K, u/B and v/L .Bending moment co-efficients m1 and m2 for different values of k are given in the form of curves, from which the values of m1 and m2 for the particular set of u/B and v/L values can be obtained. Them M1 and M2 are calculated from equations M1= (m1+µ m2)P-------(1)

  M2= (m2+µ m1)P-------(2)

 

  

bw = the breadth of the concentration area, of the load, i.e., the

dimenison of the tyre or track contact area over the road surface of the slab in a direction at right angles to the span plus twice the thickness of the wearing coat or surface finish above the structural slab.K = a constant having values as shown in table depending on the ratio LI/L where be is the width of the slab.

 Obviously the effective width should no exceed the actual width of the slab. Further, when a concentrated load is close to the unsupported edge of a slab, the effective width shall not exceed the above value nor half the above plus the distance of the load from the unsupported edge.

For two or more concentrated loads in a line in the direction of the span, the bending moment per meter width of slab shall be calculated separately for each load according to its appropriate effective width of slab from eqn 1. For two or more concentrated loads in a direction perpendicular to the direction of the span, it may sometimes happen that the computed effective widths for two adjacent loads overlap. In such cases, the resultant effective width will be equal to the sum of individual widths minus the overlap

CANTILEVER SLAB

The effective width of dispersion measured parallel to the supported edge, for concentrated loads on a cantilever solid slab is to be obtained from eqn. be = 1.2x + bw ……. -------------------- 1

Where be =effective width

X = distance of the center of gravity of the concentrated load from the face of the cantilever support.bw = the breadth of the concentration area of the load, i.e., the

dimension of the tyre or track contact area over the road surface of the slab in a direction parallel to the supporting edge of the cantilever plus twice the thickness of the wearing coat over the structural slab.The effective width should be limited to one-third the length of the cantilever slab

measured parallel to the support. Further, when the concentrated load is placed near one of the two extreme ends of the length of the cantilever slab in the direction parallel to the support, the effective width should not exceed the above value, nor should it exceed half the above value plus the distance of the concentrated load from the nearer extreme end, measured in the direction parallel to the fixed edge when two or more loads act on the slab, and when effective width for one load overlaps the effective width of the adjacent load, the resultant effective width should be taken as the sum of the respective effective widths for each load minus the width of overlap. 

DISPERSION OF LIVE LOAD THROUGH DECK SLAB

The effective length of slab on which a wheel load or track load acts shall be taken as equal to the dimension of the tyre contact area over the wearing surface of slab in the direction of the span plus twice the overall thickness of the slab inclusive of the thickness of the wearing surface

DISTRIBUTION REINFORCEMENT

The distribution reinforcement in slabs spanning in one direction shall be provided at right angles to the main reinforcement. This reinforcement is provided to produce a resisting moment equal to 0.3 times the live load moment and 0.2 times the dead load moment, in cantilever slabs, the distribution steel is computed to resist a moment equal to 0.3 times the live load moment and 0.2 times the dead load moment, and the steel is provided half at the top and half at the bottom of the slab. The pitch of the distribution bars shall not exceed three times the effective depth of the slab or 300mm whichever is less.

SLABS SPANNING IN TWO DIRECTIONSFOR SLABS SPANNING IN TWO DIRECTIONS, THE MOMENTS IN THE TWO DIRECTIONS CAN BE OBTAINED BY ANY RATIONAL METHOD. THE USE OF CURVES GIVEN BY M.PIGEAUD IS RECOMMENDED.PIGEAUD'S METHOD IS APPLICABLE TO RECTANGULAR SLABS SUPPORTED FREELY ON ALL FOUR SIDES AND SUBJECTED TO A SYMMETRICALLY PLACED LOAD.LET L AND B BE THE SPAN LENGTHS IN THE LONG AND SHORT SPAN DIRECTIONS'A' AND 'B' BE THE DIMENSIONS OF THE TYRE CONTACT AREA IN LONG AND SHORT SPAN DIRECTIONSU AND V BE THE DIMENSIONS OF THE LOAD SPREAD AFTER ALLOWING FOR DISPERSION THROUGH THE DECK SLAB.K BE THE RATIO OF SHORT SPAN TO LONG SPANM1 AND M2 BE THE MOMENTS ALONG THE SHORT AND LONG SPANS

m1 AND m2 BE THE CO-EFFICIENT FOR MOMENTS ALONG THE SHORT AND LONG SPANS.'µ' BE THE VALUE OF POISSON'S RATIO, GENERALLY TAKEN AS 0.15 FOR REINFORCED CONCRETE.'P' BE THE LOAD FROM THE WHEEL UNDER CONSIDERATION

THE DISPERSION OF LOAD MAY BE ASSUMED TO BE AT 45 DEGEES THROUGH THE WEARING COAT AND DECK SLAB. IT IS ALSO SOMETIMES ASSUMED TO BE AT 45 DEGREES THROUGH WEARING COAT BUT AT A STEEPER ANGLE THROUGH THE DECK SLAB. THE LATTER ASSUMPTION IS USED HERE.THE VALUES OF MOMENTS ALONG THE SHORTER AND LONGER DIRECTIONS WOULD DEPEND ON THE RATIOS K, U/B AND V/L .BENDING MOMENT CO-EFFICIENTS m1 AND m2 FOR DIFFERENT VALUES OF K ARE GIVEN IN THE FORM OF CURVES, FROM WHICH THE VALUES OF M1 AND M2 FOR THE PARTICULAR SET OF u/B AND v/L VALUES CAN BE OBTAINED. THEM M1 AND M2 ARE CALCULATED FROM EQUATIONS

M1= (m1+µ m2)P-------(1)

M2= (m2 +µ m1)P-------(2)

WHEN THE SLABS ARE CONTINUOUS, THE SAME PROCEDURE AS ABOVE MAY BE ADOPTED AND THE MAXIMUM MIDSPAN MOMENTS MAY BE TAKEN AS 0.8 OF THOSE FOR THE CASE OF FREE SUPPORTS.PIGEAUD'S METHOD HAS THE FOLLOWING LIMITATIONS

I. ONLY LOADS PLACED AT THE CENTER CAN BE CONSIDERED. IN PRACTICE, A NUMBER OF WHEEL LOADS MAY OCCUR ON A

SINGLE SLAB PANEL, WHILE ONE LOAD CAN BE PLACED AT THE CENTER, OTHER LOADS WILL BE NON-CENTRAL. SOME APPROXIMATION WILL HAVE TO BE USED WHILE CONSIDERING THE NON-CENTRAL LOADS.II. WHEN v/L IS SMALL, THE READING OF VA.LUES OF m1 AND m2

FROM THE CURVES BECOMES LESS ACCURATEIII. THIS METHOD IS MOST USEFUL WHEN 'K' IS MORE THAN 0.55

CULVERTS

CULVERTS ARE CROSS DRAINAGE WORKS WITH CLEAR SPAN LESS THAN 8M. IN ANY HIGHWAY OR RAILWAY PROJECT, THE MAJORITY OF CROSS DRAINAGE WORKS FALL UNDER THIS CATEGORY. HENCE THIS STRUCTURES ARE COLLECTIVELY IMPORTANT IN ANY PROJECT, THOUGH THE COST OF INDIVIDUAL STRUCTURES MAY BE RELATIVELY SMALL.

CULVERTS MAY BE CLASSIFIED ACCORDING TO FUNCTION AS HIGHWAY OR RAILWAY CULVERT. THE LOADINGS AND STRUCTURAL DETAILS OF THE SUPERSTRUCTURE WOULD BE DIFFERENT FOR THESE TWO CLASSES.

BASED ON THE CONSTRUCTION OF THE STRUCTURE , THEY CAN BE OF THE FOLLOWING TYPES.(A) REINFORCED CONCRETE SLAB CULVERT(B) PIPE CULVERT(C) BOX CULVERT(D) STONE ARCH CULVERT(E) STEEL GIRDER CULVERT FOR RAILWAYS

THE CLEAR ROADWAY BETWEEN KERBS IS KEPT AT 7.5M FOR IMPORTANT ROADS TO SUIT TWO LANE TRAFFIC. HOWEVER FOR CULVERTS ON NATIONAL HIGHWAYS, IT IS DESIRABLE TO KEEP THE DISTANCE B/W THE OUTER TO OUTER OF PARAPETS TO BE EQUAL TO THE FORMATION WIDTH(9.8M), ESPECIALLY WHEN THE CLEAR SPAN OF THE CULVERT IS LESS THAN 6M. KERBS OF WIDTH 600MM ANDHEIGHT 300MM ABOVE TOP OF DECK SLAB ARE DESIRABLE.

REINFORCED CONCRETE SLAB CULVERT

REINFORCED CONCRETE SLAB CULVERTS ARE ECONOMICAL FOR SPANS UPTO ABOUT 8M. THE THICKNESS OF THE SLAB AND HENCE THE DEAD LOAD ARE QUITE CONSIDERABLE. HOWEVER, THE CONSTRUCTION IS RELATIVELY SIMPLER DUE TO EASIER FABRICATION OF FORMWORK AND REINFORCEMENTS AND EASIER PLACING OF CONCRETE. THIS TYPE OF CULVERT CAN BE USED BOTH FOR HIGHWAY AND RAILWAY.

THE COMPONENTS OF A CULVERT WITH R.C DECK SLAB ARE THE FOLLOWING:(A) DECK SLAB(B) ABUTMENTS AND WING WALLS(C) FOUNDATIONS(D) PARAPETS, KERBS ETC

DECK SLAB

THE DECK SLAB SHOULD BE DESIGNED AS ONE WAY SLAB TO CARRY THE DEAD LOAD AND THE PRESCRIBED LIVE LOAD WITH IMPACT AND STILL TO HAVE STRESSES WITHIN THE PERMISSIBLE LIMITS. FOR A CULVERT ON A STATE HIGHWAY, THE CLEAR ROADWAY BETWEEN THE KERBS WILL BE 7.5M FOR TWO LANE TRAFFIC. THE DECK SLAB SHOULD BE DESIGNED FOR THE WORST EFFECT OF EITHER ONE LANE OF IRC CLASSAA TRACKED VEHICLE OR ONE LANE OF CLASS AA WHEELED LOADING OR TWO LANES OF CLASS A LOAD TRAINS. THUS ACCORDING TO THE PRESENT PRACTICE, IT IS NECESSARY TO COMPUTE THE MAXIMUM LIVE LOAD BENDING MOMENT FOR THREE CONDITIONS OF LOADING AND THEN ADOPT FOR THE DESIGN THE GREATEST OF THE THREE VALUES

FROM THE EXPERIENCE IT HAS SHOWN THAT, FOR THE COMPUTATION OF LIVE LOAD BENDING MOMENT, ONLY ONE LOADING CONDITION NEED BE CONSIDERED, NAMELY CLASS AA WHEELED VEHICLE FOR SPANS UPTO 4M AND CLASS AA TRACKED VEHICLE FOR SPANS EXCEEDING 4M. IF THE SHEAR IE DESIRED TO BE COMPUTED, CLASS AA WHEELED VEHICLE IS TO BE CONSIDERED FOR SPANS UPTO 6M AND TRACKED VEHICLE BEYOND 6M FOR SINGLE LANE BRIDGES. HOWEVER, FOR TWO LANE BRIDGES, THE SHEAR DUE TO CLASS AA TRACKED VEHICLE CONTROLS THE DESIGN FOR ALL SPANS FROM 1M TO 8M. THE DESIGN MOMENT FOR DISTRIBUTORS IS TAKEN AS 0.3M OF THE LIVE LOAD MOMENT PLUS 0.2 OF THE DEAD LOAD MOMENT.WEARING COAT OVER THE DECK SLAB FOR CULVERTS MAY BE OF CEMENT CONCRETE WITH AVERAGE THICKNESS OF 75MM OR OF ASPHALTIC CONCRETE WITH AVERAGE THICKNESS OF 80 MM.

EFFECTIVE WIDTH OF DISPERSION

THE BENDING MOMENT PER UNIT WIDTH SLAB CAUSED BY CONCENTRATED LOAD CAN BE CALCULATED BY ESTIMATING THE WIDTH OF SLAB THAT MAY BE TAKEN AS EFFECTIVE IN RESISTING THE BENDING MOMENT DUE TO THE CONCENTRATED LOAD.

ABUTMENTS AND WING WALLS

THE SECTIONS SHOULD BE SO DIMENSIONED THAT THE WALLS CAN SUSTAIN THE PRESCRIBED LEAD AND LIVE LOADS, BESIDES THE EARTH PRESSURES WITHOUT OVERTURNING OR SLIDING. FURTHER, THE PRESSURE ON THE SOIL SHOULD BE WITHIN THE ALLOWABLE PRESSURE. EQUIVALENT SURCHARGE FOR LIVE LOAD AS GIVEN IN CLAUSE 217 OF IRC BRIDGE CODE IE TO BE TAKEN INTO CONSIDERATION. IN FIG THE FRONT BATTER IS KEPT AT 1/12 OF THE HEIGHT OF THE WALL, WHILE THE REAR BATTER IS VARIED TO SUIT THE SOIL PRESSURE REQUIREMENTS. IN CASE OF WING WALLS, A SLIGHT MODIFICATION IS ALSO MADE IN THE FRONT OFFSET, DEPENDING ON THE

ALLOWABLE SOIL BEARING PRESSURE, TYPICAL VALUES FOR A,B AND C AS INDICATED IN FIG. ARE GIVEN IN TABLE1. FOR TWO CHOSEN VALUES OF ALLOWABLE SOIL PRESSURE, I.E., 200 AND 400 KN/M2. THESE DIMENSIONS MAY BE ADOPTED FOR CULVERTS AND IN SPECIAL CASES DETAILED CALCULATIONS MAY BE PERFORMED.

TABLE 1. DIMENSIONS OF ABUTMENTS AND WING WALLS FOR SLAB BRIDGES. 

 

 

    Value in meter for Height H in meters

1.5 2.0 2.5 3.0 3.5

  A 0.75 0.9 1.2 1.65 2.60

B 0.20 0.3 0.45 0.65 1.06

C 0.45 0.45 0.45 0.45 0.45

  A 0.3 0.45 0.55 0.75 0.95

B 0.2 0.3 0.45 0.65 0.85

C 0.15 0.15 0.15 0.15 0.15

THE FOUNDATIONS SHOULD BE TAKEN BELOW THE SCOUR DEPTH TO A DEPTH WHERE THE VARIATION IN THE MOISTURE CONTENT OF THE SUBSOIL WILL BE ABSENT. THE DEPTH OF FOUNDATION SHOWN SHOULD BE CONSIDERED THE MINIMUM, UNLESS ROCK IS ENCOUNTERED AT A HIGHER LEVEL. IN LATTER CASE, THE MASONRY SHOULD BE NEELED ON THE ROCK FOR A DEPTH OF ABOUT 200MM, OR CONNECTED BY DOWELS TO THE ROCK IF THE ROCK SURFACE IS SLOPING.

THE PURPOSE OF THE FOOTING IS TO SERVER AS A SEAL IN POOR SOIL OR AS A BEDDING COURSE IN WATER BEARING STRATA FOR THE MAIN MASONNY ABOVE. THE FOOTING IS NOT TAKEN AS EFFECTIVE IN DESIGNING THE ABUTMENT SECTION, WHEN THE CLEAN SPAN IS LESS THAN H/6 + 1.5M, THE BOTTOM FOOTINGS OF THE TWO ABUTMENTS OF A SINGLE SPAN CULVERT SHOULD BE COMBINED.

IF THE ENTIRE ABUTMENT IS OF MASS CONCRETE, NO SPECIAL TREATMENT EXCEPT LEVELING IS REQUIRED FOR THE BRIDGE SEAT. IF THE MAIN ABUTMENT IS OF BRICK OR STONE MASONRY, A BED BLOCK OF CONCRETE IS PROVIDED FOR A THICKNESS OF ABOUT 225MM. WING WALLS MAY BE EITHER SPLAYED AT AN ANGLE OF 45º OR TAKEN PERPENDICULAR TO THE ABUTMENT. IF SPLAYED, THE HEIGHT OF WALL WILL REDUCE AS IT EXTENDS AWAY FROM THE ABUTMENT. AT THE LOW END, THE HEIGHT OF WALL SHOULD NOT BE MORE THAN 1.2M. IF THE WALL IS AT RIGHT ANGLES TO THE ABUTMENT, SUITABLE REVETMENT SHOULD BE PROVIDED TO RETAIN THE EARTH OF EMBANKMENT.

THE EARTH FILLING BEHIND THE ABUTMENTS AND WINGS SHOULD BE SPECIALLY CONSOLIDATED IN ORDER TO AVOID AN EXCESSIVE DEPRESSION IMMEDIATELY CLEAR OF THE BRIDGE DECK. A POROUS BACKFILL OF ABOUT 500MM THICKNESS SHOULD BE PROVIDED IMMEDIATELY BEHIND THE ABUTMENT, WITH 150MM DIAMETER WEEP HOLES PLACED AT SUITABLE INTERVALS WITH A GENTLE SLOPE SO AS TO BE 150 MM ABOVE NORMAL WATER LEVEL ON THE VENT SIDE, TO ENSURE DRAINAGE FOR FILLING BEHIND THE ABUTMENTS.

HANDRAILS OR PARAPETS SHOULD BE PROVIDED OVER THE KERBS ON EITHER SIDE OF THE ROAD OVER THE DECK SLAB. FOR SMALL CULVERTS PARAPET WALLS OF HEIGHT 750MM WILL BE ADEQUATE. FOR SPANS ABOVE 6M, A COMBINATION OF R.C. POSTS AND RAILS MAY BE MORE PLEASING.