Bridges

Definition of Bridge – It is a structure constructed spanning road, river, valley or

any other structure with a purpose to have through passage for communication. The bridges are constructed for Roadways as well as Railways.

Classification of Bridges-

1. Important Bridges- The bridges having total waterway of 1000 sq.m or total linear waterway of 300 m or more and the bridges classified as important by Chief Engineer / Chief Bridge Engineer, depending upon consideration such as depth of water way, extent of river training works and maintenance problems. 

2. Major Bridges- Major bridge is one of which has a total linear waterway of 18 m or more for multiple span and total linear water way of 12 m or more for single span. 

3. Minor Bridges- Major bridge is one of which has a total linear waterway less

than 18 m for multiple span and total linear water way less than 12 m for single

span.

4. Culvert – Bridge having linear water way lea than 6.00 m called as culvert.

Definition –

Afflux – It is the rise in the flood level of the river up stream of the bridge as a result of obstruction to natural flow caused by the construction of the bridge.

Clearance– This is the vertical distance between the high flood level inclusive of

afflux and the formation level.

Free Board - This is the vertical distance between the high flood level inclusive of afflux and the bottom most part of bridge super structure like slab girder etc.

Via duct – When the bridge is constructed to cross a valley for railway instead of

filling it the bridge as called via duct.

Skew Bridge – When the bridge is not right angle to the axis of river or other such opening it is called a skew bridge.

Cause Way – A bridge constructed to pass flood water over the railway track is called cause way.

Clear Span – It is the clear distance between any two bridge supports like abutments piers etc.

Effective span – It is the distance between centers of two adjacent bridge supports.

Over all Span – It is the over all length of girder.

Scour – It is the rise in the distance from rail level and bottom of rivers.

Water Way – The area of the opening of a bridge blow super structure through which water flows is called water way.

Economic Span of a Bridge. –

The span for which the total cost of bridge will be minimum is known as the economic span.

The ratio of Cost of super structure and Cost of Sub structure = 1

Economical Span = Ö` P / a ,

Where - a = p / l2 , p = Cost of pier ( Super structure ) , l = Span,

P = Cost of Pier ( Sub Structure ) , a = Constant of variation.

Depth of Scour-

The maximum scour depth in a stream can be ascertained when ever possible by actual sounding at or near the site proposed for the bridge during or immediately after a flood. Before the scour holes have had time to silt up due to allowance should be made in the observed depth for increase in scour resulting from.

i)The designed discharge being greater than the flood discharge during which the scour was observed.

ii. The increase in velocity due to the obstruction in flow caused by construction of the bridge.

Where the above practical method of determining scour is not possible the following theoretical method may be used as a guide when dealing with natural streams is alluvial beds.

 D = 0.473 [ Q / F ]⅓

Where D = Depth of scour below HFL for regime conditions in a stable channel

in meters.

Q = Design discharge in cumecs.

F= Silt factor = 1.76 √ m , m is the mean diameter of particles in mm.

Afflux in a river –

Afflux is the rise in the flood level of the river, up stream of a bridge, as a result of the obstruction to natural flow caused by the construction of the bridge. Afflux is normally measured as difference in water level between up stream and down stream of the bridge.

Afflux is calculated by following formula –

Molesworth’s formula –

                                   V2

                         ha = ------ + 0.015 [ ( A/a )2 - 1 ]

                                  17.9

Where – ha = Afflux in meter. V = Velocity of approach in meter per second. A =

Natural water way area at the site. a = Contracted area in square meters.

Danger level –

It is not possible to lay down any ad-hoc levels for fixing danger levels. It has to be decided on the spot for each and every individual bridge depending upon the site conditions. Local conditions such as nature of bed, condition of approach bank, depth of foundation & free  board to be allowed should influence the decision. 

The marking of danger level would imply that is would be safe for the flood water to reach up to the danger level with out affecting the safety of the bridge structure or of its foundations or approaches. Danger level for a particular bridge

should be fixed after site inspection, examination of records of past history, local

enquiries about flood, afflux and velocity of water under bridge.

Rail and rail joints on Bridges –

(1) Longitudinal Profile of Rails – In standard plate girders no camber is provided. Open web girders of span 30.5 m. and above are provided with camber. Track on these bridges are laid correctly following the camber of the girder. While retimbering is done it should be ensured that the longitudinal level of rails follows the camber of girders.

(2) Rail Cant – On Bridges the rail should be laid with an inward cant of 1 in 20 by continuing the same cant as on the approaches.

(3) Rail joints over the Bridge – In the case of small bridge openings less than 6.1 m. rail joints should be avoided. For other spans, the preferred position of the rail joint is at 1/3 the span from either end.

(4) SWR on Bridges – SWR may be continued over girder bridges with unballasted decks upto 13.3 m. opening if the length of SWR is symmetrical to the centre line of bridge and upto 6.1 m. opening if the length of SWR is unsymmetrical to the centre line of the bridge. No fish-plated joint should be located on the girder or within six metres from either abutment. In all such cases rail free fastenings, such as rail screws, dog spikes or rail free clips shall be used, so that relative movement between rail and sleepers may take place.

(5) LWR/CWR on Bridges – In the case of laying LWR/CWR, provisions of LWR Manual should be followed. 

(a) In the case of girder bridges (unballasted deck) LWR can be continued over bridges where overall length is not more than 20 m. In case of bridges where the overall length is between 20 and 43 m., LWR can be continued on B.G., in case the track is laid with 52 kgs. and 90-R rails under certain conditions.

(b) LWR with rail free fastenings can be provided from pier to pier with S.E.Js. On each pier over the free end in the case of rollers on one side and rockers on other side. Box anchoring for a few sleepers should be done at the fixed end. In case of roller bearings on both sides, the central portion of the welded rails shall be box anchored on a few sleepers.

(c) LWR can be continued over a girder bridge with the provision of S.E.J. at the far end of the approach of the bridge using rail free fastenings over girder bridge. The length of the girder bridge will be restricted as per the table given in LWR

Manual.

(6) Precautions for arresting Creep – Track on girder bridges with un-ballasted deck is always laid with rail free fastenings in all cases. Track on girder bridges laid with standard single rails and fish-plated joints be isolated from the SWR if

existing, on approaches on either side by providing at least two well anchored

Standard rail lengths. Similarly the track on the girder bridges not laid with

LWR/CWR shall be isolated from LWR/CWR by a minimum length of 36 metres

of well anchored SWR on either side.

Bridge Timbers (C.S.No. 102)

Note:- B.G:- For all works like rebuilding, registering, TSR clear distance between consecutive sleepers not to exceed 450mm. (CS.No.102)

N.G.:_Clear distance between consecutive sleepers not to exceed 152mm in case of 610mm gauge and 254mm in case of 762mm gauge in place of 125mm as mentioned above. (CS.No.102)

All details are for timbers directly resting on longitudinal girders. (CS.No.102) 

Clear distance between joint sleepers: – The clear distance between joint sleepers

should not exceed 200 mm. both for B.G. and M.G.

Use of Rail Free Fastening in Girder Bridges –

Rail free fastenings such as canted mild steel bearing plates with four rail screws may be used on wooden sleepers. No anti-creep bearing plates should be used. If channel sleepers are used, suitably designed canted bearing plates with rubber pads and rail free clip and bolt type of fastening should be used.

Provision of Guard Rails on Bridges –

(1) Location- Guard rail should be provided on all girder bridges (including restressed Concrete girder bridges without deck slab) whether major or minor. Guard rails should also be provided on all major and important ballasted bridges and also on such other minor bridges where derailment may cause serious damages. On all flat top, arch and prestressed concrete girder bridges with deck slab, where guard rails are not provided the whole width of the bridge between the parapet walls shall be filled with ballast upto the top of sleeper level.

Design of Guard rails – The typical arrangement of a guard rail, with the important dimensions for B.G., M.G. and N.G. are shown in the sketch and table as shown below 

Fixing of Guard rails-

The ends of guard rails should be bent vertically and buried and a block of timber

fixed at the end to prevent entanglement of hanging loose couplings. To ensure that guard rails are effective, they should be spiked down systematically to every sleeper with two spikes towards the centre of the track and one spike on the opposite site. Notching of the rail foot for spikes fixing the guard rails should be done on every (Cs.No102) sleeper. Sleepers should be tied at both ends by MS Flats/tie bars fixed through holding down bolts. 

The fixing of guard rail on concrete sleepers shall be done as shown in drawing No. RDSO/T- 4088 to 4097 by proper tightening of rail screws. Provision of MS flats/tie bars for tying PRC sleepers together at ends is not required.

Provision of walkways –

Over all girder bridges, footways (walkways) should be provided in the centre of track over sleepers to help the Engineering staff for inspection. The footways  walkways should be made of chequered plates with holes. (CS.No.102)

Inspection and Maintenance of Bridge

Inspection and maintenance of Track on Approaches of Bridges :–

(a) For all Bridges-

(1) On the bridge approaches, sleepers with arrangement for fixing guard rails should be provided for provision of guard.

(2) Full complement of track fittings at bridge approaches up to 100 metres should be provided to maintain required track geometry and effort should be made to immediately recoup deficiency noticed, if any.

(3) Rail level of track at approaches of bridges should be maintained as per designed L-section and dips in rail level immediately after the abutments should be avoided. The alignment and super elevation in case of curved track should also be maintained (CS No.105)

(4) Rail joints should be avoided within three metres of a bridge abutment.

(5) In case of LWR track, full ballast section as specified in LWR Manual should be provided up to 100 metres from the abutment.

(6) Switch expansion joints should be provided at the bridge approaches in LWR/CWR track as per provisions of LWR manual.

(b) For important and major bridges – In addition to Para (a) above, following should also be provided.

(1) In case of CST-9 or wooden sleeper track, concrete/steel trough sleepers with

elastic fastenings should be provided up to 100m/upto full breathing length  wherever LWR is provided in approach of bridge.

(2) On the bridge approaches, for a length of abut 100 metres, width of cess should be 90 cm clear of full ballast section to maintain ballast profile. For maintaining ballast section, suitable ballast retaining arrangement should also be provided.

Inspection and Maintenance of Track on Bridge proper –

(1) Condition of track: – It should be ascertained whether it is central on the rail bearers and the main girders and in good line and level. Departure from line is caused by incorrect seating of girders, shifting of girders laterally or lengthwise, incorrect seating of sleepers on girders or rails on sleepers, varying gauge or creep. Departure from level is caused by errors in level of bed blocks or careless

sleepering. The adequacy of clearances of running rails over ballast walls or ballast girders at the abutments and condition of timbers and fastening on the run

off and skew spans should be inspected.

(2) Sleepers - The condition of sleepers and fastenings should be checked. The spacing of sleepers should not exceed the limits laid down. Squareness of sleepers shall be ensured. Sleepers requiring renewals should be marked with paint, and renewals carried out. End bolts should be provided on sleepers which

have developed end splits.

(3) Hook Bolts – Hook bolts should be checked for their firm grip. Position of arrows on top of the bolts should be at right angles to the rails pointing towards the rail. Hook bolts should be oiled periodically to prevent rusting.

(4) Creep and joint gaps should be checked and rails pulled back wherever necessary. Rail fastenings should be tight. Defective rails should be replaced. Where switch expansion joints are provided on the girder bridge, it should be ensured that free movement of the switch is not hindered.

(5) Guard rails – Adequacy of guard rail arrangements should be checked. Correct distance between the running rail and guard rail should be maintained as per the prescribed dimensions.

(6) Camber packing where provided should be in sound condition.

(7) On girder bridges adequacy of path ways for inspection should be checked. 

(8) Sand bins which are provided for putting out fires should be filled with dry and loose sand.