problems of bridge bearings

7/31/2019 Problems of Bridge Bearings

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PROBLEMS OF BRIDGE BEARINGS ON SLOPING SPANS & AT ABUTMENTS

By Dr. C V KAND

SYNOPSIS

PTFE bearings are extensively provided on flyovers which havegenerally some spans in gradient of 1 in 25 or flatter. If the movement of bearings is not stopped during construction, the deck is likely to slidedown gradient; similarly Roller rocker bearings and Elastomericbearings are provided extensively on the bridges. In case of high Abutments these are likely to deflect due to earth pressure. This will affect not only the bearings on abutments but also on the adjoining pier. These movements can also cause cracks in the deck besidesdamaging the bearings. Some case studies highlighting these effectsand rectification measures carried out are discussed in this paper.Following five case studies are brought out in this paper:-

1. Narmada Bridge at Hoshangabad 2. Bairma Bridge3. Chambal Bridge4. Pipe Bridge at Wanchhoo Point near Indore with gradient 1/45. Flyover, gradient 1/25

Case 4 is reported in IRC paper published in IRC Journal in 1997. It isreproduced since similar phenomenon is repeated on sloping spans.

1. NARMADA BRIDGE AT HOSHANGABAD: -

The bridge consists of 30 spans of 24.4m and two end spans of 12m. The

bridge was opened to traffic in 1967. The piers are two circular piers connected by

diaphragm, cast steel roller rocker bearing are provided for a superstructure

consisting of prestressed girders with a cast in situ deck slab on it. The prestressed

girders were cast in the yard and launched on the piers by a steel truss launching

girder. The centering for the slab was fixed to the girder and removal of the

centering was done by a specially made structural frame (see figures 1, 2 & 3). The 2

end spans of 12 m have also girder section similar to prestressed girder but these are

RCC girders, precast & launched in position. The girders are provided with cut steel

circular roller bearing at one end and rocker bearing at the other end

Foundations were open type foundations on rock; the abutment is located on

higher bed level. On the abutment there are roller bearings, the abutment is of RCC

A shaped two frames one below each girder and of spill through type.


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The roller bearings at abutment were observed to have tilted since 1971 that

is 4 years after completion. By 1980 the expansion gap at abutment closed

completely. In 1981 the end RCC spans showed cracks.

Investigations showed that A Frame of the abutment gradually tilted since

this is founded on rock there is no possibility of any settlement. On closing of the

expansion gap the deck started moving towards the pier and the expansion gap on

the pier was also closed. Thus the deck started acting as a fixed beam and cracks

were developed in the girder as shown in the sketch and photograph. The expansion

joints consist of 2 angles and the mild steel plate above which can slide.

Fig.1.1. Cracks in the girder

1.1 Rehabilitation

The expansion joint at abutment was removed. The span was lifted by 50mm.

The roller bearings were removed and replaced by elastomeric bearings on the

pedestal, the girders were lowered. The inner faces of the dirt walls behind the

girders were chipped of to create an expansion gap of 50mm. the cracks in the

girders were injected by epoxy resins. At abutment a curtain wall upto bed level and

a apron were provided to prevent further tilting of the abutment. This work was

done about 40 years ago and it is behaving well.


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1.2 The lessons

In respect of high abutments, it is advisable to calculate the deflection due to

earth pressure and based on that provide adequate expansion gap. In the railway

bridge on Mumbai- Delhi railway line the height of abutment for Betwa Bridge is

about 18m, the expansion joint provided is more than 300mm that is why the

deflection of abutment due to earth pressure did not affect the bearings and the

adjoining spans. Cast in steel roller bearings are provided for this abutment. A sketch

of this abutment is also given.


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2. BAIRMA BRIDGE

2.1 Structural Details:

The bridge consist of continues span units of 20-25-20 m. The end span is

15m simply supported with 3m cantilever on one side and a suspended span of 1.5m

over abutment and tip of cantilever. The deck is a 4 girder system RCC deck on mild

steel roller & rocker bearings. The piers are of mass concrete 1:3:6 with top width of

1.2m. The abutment was of RCC frame of 4 twin columns each 225 X 375mm as a

fully buried abutment, not designed for earth pressure and founded on soil at a level

6.5m above the foundation of pier P10 which is provided with rocker bearings. At P2

roller bearings were provided for span 1 and 2. The rocker bearing has only one

rocker pin in centre.The sketch shows the extent of excavation pit for pier P1. The abutment

frame was founded on filled up soil. With a view to allow traffic on the bridge, the

earth work around abutment and end pier was completed before monsoon.

Protection work of earthen bank could not be done before monsoon.

2.2 Deformations:

Several deformations were observed in this work:

i. The abutment frame settled by 127mm during monsoon (observed in

November). By next January the settlement was 165mm. Floating span (1.5m)

was lifted to maintain road level.

ii. Expansion gap on pier P2 closed and the same at articulation increased.

iii. Earth work on river side of P1 was washed off during floods. On the other

side (bank) it was 6m higher.

iv. Top and bottom plate of rocker bearings on pier P1 displaced, maximum

displacement was 51mm. displacements of four bearings were not uniform.


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Fig.2.1. Tilting of end pier due to Earth Pressure

Detailed study of the bridge and these deformations were carried out. Firstly

the abutment frame on filled up soil settled. The pier P1 being subjected to

differential earth pressure of 6m height and surcharge due to slope, deflected

towards P2. Expansion gap on pier P2 was closed. On account of thermal movement

of the span and continued deflection of pier, and since there was no space for

further movement of span towards P2, this movement was reflected at P1 by

shearing of rocker pin and displacement of its top plate from the bottom one.

Simultaneously span 2 was also undergone thermal movement which pushed span 1

towards bank, rocker pin having been sheared, no resistance was offered at P1.


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Pic#2.1. Deflection of pier due to settlement of abutment frame

Pic#2.2. Displacement of bottom plate from top and shearing of

pin rocker bearing


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2.3 Rectifications:

Since pier P1 deflected on account of differential earth pressure, reverse

conditions were created by digging a deep trench on bank side adjoining the pier

and the earth was placed on the river side. When the trench was 7.5m deep, the pier

re-deflected by 20mm. This trench was maintained for 2 months. Expansion gap at

P2 was opened up to 43mm. however, there was no rectification of top and bottom

plate of rocker bearing, indicating that the pin was sheared off and it acted as a

sliding bearing. The earth work around the pier was made in proper slopes, pitched

and toe wall were provided.

It was also observed that the RCC abutment frame buckled and damaged.

Crib type abutment and retaining block with RCC cribs frames filled by boulders wasprovided. This was expected if settled additional layer of crib will be added and the

road formation level will be maintained. This work was done in 1968. During

seventeen years, the crib wall was observed to have settled by 150mm & thereafter

it is reported to be stable. The rocker bearings are rectified as shown in the sketch.

2.4 Indications:

This case study has brought out following important points:

2.4.1 Slopes of earth work around abutment and its protection work must be designed

as carefully as the design of main structure and completed before opening the

bridge to traffic.

2.4.2 Abutment structure, even if it is buried, must be designed for earth pressure

from bank side since the earth on river side is vulnerable for being washed during

floods. In important bridges, abutments are designed for all around scour

conditions. Latest IRC-78 stipulates this provision.

2.4.3 The foundation of non-load bearing abutment may be kept at level higher than

the foundation level of pier but the level should be such that the founding soil of

abutment is not disturbed due to excavation for pier foundations.

2.4.4 Based on these observations two design criteria have been evolved:


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a.) If the piers are provided on well foundations, the foundations level of non-

load bearing abutment shall not be higher than the level of the bottom of

well cap of adjoining pier. Open excavation for wells shall be limited to the

bottom of well cap.

b.) In open foundations, the difference of foundations level of the abutment and

the adjoining pier shall not be more than

h = 1 L for every hard soil like murrum.

= L / 1.5 for Medium type of soils.

= L / 2 for soft soil.

Where, h = difference of level between the foundation levels of abutmentand adjoining pier.

L = clear horizontal distance between the footing of pier &

abutment.

2.4.5 Wider expansion gap should be provided on abutments which are likely to settle

and deflect due to earth pressure. 50mm or larger gap in place of usual 25mm

gap is recommended. It is also advisable to provide earthwork behind abutment

to a length of 30m before casting end span supported on abutment. This

stipulation is also given in tender conditions. With this provision, a large part of

deflection of abutment will take place before laying the span.

2.4.6 It is desirable to provide roller bearings at abutment or end pier.

2.4.7 Each rocker bearing must be provided with minimum 2 pins.

2.4.8 Before opening a bridge to traffic, it must be observed that all expansion gaps

are free and the bearings are free to slide or permit angular movement of deck.

3. CHAMBAL BRIDGE:

3.1 Details of the Bridge and Accident:

The bridge is provided with continues RCC trough slab with span arrangement

15 18.75 15 m. Depth of slab is 1.1m. RCC roller bearings are provided on end

piers and one intermediate pier and RCC rocker at one pier in the unit.


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Concreting of the deck was being done in layers continuously. First layer of

45cm was laid in entire length of 48.75m in 24 hrs. Second layer of 45cm was done in

one end span and 1.5m length of central span. At this stage the carpenters attending

to centering heard a cracking sound and they jumped out from the location.

Suddenly a loud cracking sound was heard and the whole unfinished structure came

down. The end span which was concreted to 90cm depth, collapsed and the green

concrete was reduced to pieces. Other end spans was pulled out from the abutment

support and it came down. The middle span sagged down, the forms were broken

and the green concrete collapsed. The reinforcement cage of central span was

hanging in the catanary shape between P1-P2 over crushed timber centering; which

however, did not collapse fully.

3.2 Details of construction:

The figure giving details of centering will show that the bed level in the centre

of river is nearly 12m below the bed level at abutment. These levels were disturbed

during open foundation for piers and the abutment. The bed levels were made up by

providing embankment in steps between piers and centering was erected on filled

up soil between spans 1, 2 & 3 respectively. The width of bank along river was 10m

and side slopes of 1.5 to 1. It was reported to have been properly consolidated. The

centering was of cut timber sections 10 X 15cm and these were laid on wooden

sleepers.

3.3 Causes of Failures :

Investigations showed that the earthen embankment, supporting staging

settled. The pier which was subjected to differential earth pressure with surcharge

of centering load appears to have tilted. Normally bearings are clamped during

concreting of deck. These bearings were not clamped at abutment & piers.

Due to settlement of props in span 3, the shuttering with steel cages and

concrete in side slipped off from the unclamped roller bearings over P3 and it came


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down forcibly due to heavy load of 90cm deep green concrete. The centering of span

3 damaged almost completely, and the reinforcement cage hung along the face of

Fig.3.1. Failure of centering by not clamping bearings during concreting.

P2 and was pulled in towards span 2 over the unclamped roller bearing of P2.

The collapse of span 3 induced horizontal forces on continuous decking. Span 2 tried

to move towards P1. Since rocker bearing is provided on P1, span 2 sagged down.

Upper stage of centering in span 2 crumbled. The sag is nearly 3m. The pull damaged

rocker bearing on P1. The anchor bars in the bearing were pulled out from RCC pier

cap. Span 1 was pulled towards P1. The end of span 1 at abutment slipped and came

down, forcibly, damaging the centering and the shuttering.

3.4 Indications :

This case study has brought out the following points:-

a.) Earthen banks should not be allowed for erection of staging of bridges. If the river

bed is slopping, it should be cut in steps and props erected on cut portion. It is


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advisable that wooden piles are provided below props, if the bed contains soil or

sand.

b.) Piers of bridges should not be subjected to one side earth pressure if these are not

designed for such a condition.

c.) Bearings must be clamped before concreting of deck.

d.) Centering of continuous deck must be absolutely free from settlement.

Pic#3.1. Failure of centering.

4. REHABILITATION OF PIPE BRIDGE:

4.1 Details of the Bridge:

Narmada water supply scheme for Indore town was taken up in 1972. The

pipe line passes through Vindhyan Mountain ranges. Water of Narmada River is

lifted by about 242m. Five pumping stations are established to lift water by 242m.

Thereafter water is supplied by gravity to Indore and Mhow town. The pipe is

1200mm dia. steel pipe 8mm thick. A pipe bridge is provided in a valley with 3 spans

of 22m c/c and 2 spans of 20m c/c, two RCC girders, each span having 5 saddles on


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Fig.4.2. Wanchoo point bridge with pedestals

Fig.4.3. Wanchoo point bridge


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Fig.4.4. Wanchoo point bridge

b.) Span no. 2 with one end resting on pier P1 settled by 150mm and RCC bracket at

end of span crushed. The deck also slid by 200mm towards hill side. Drawing of

rocker bearing does not show any central pin. Thus at rocker end on P1, the

girders were free to slid. The bearings were distorted. The triangular concrete

brackets at inclined beams above bearings have cracked in all spans.

Investigations showed that there is no reinforcement in bracket. Thus due tocrushing of bracket at pier 1 the girders settled and in that gradual process deck

slid towards hill side. The pier P1 also tilted on that account. Since the pipe line is

continuous on all spans, uneven supports on saddles caused due to horizontal

and vertical moment of deck of span 2 appear to have caused some lateral sag in

pipe line and the bridge deck also.


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c.) Pier number one has tilted by 5mm towards hill side and this caused lateral

deflection of deck in gradient. The pipe line is also likely to have developed

similar effects.

d.) At saddles in span 2, large gaps developed between the socket and the pipe at

the one end. Thus pipe is partially supported on socket in span 2.

4.3 Rectification:

Following procedure and sequence has been adopted for rectification of defects.

a. Repair of concrete: The honeycombs in girders were rectified by grouting mixture

of cement mortar and 15% polymer.

b. Strengthening of piers and replacement of bearings: Piers were strengthened by

encasing these in RCC column. A pier cap with steel structural grillagereinforcement was laid as shown in sketch at a level where about 400mm

minimum vertical space is available to place hydraulic jacks. It was designed to

take point loads of jacks. The girders of span 3, 4, 5 were supported on jacks and

raised by 3mm. The existing bearings and part of old pier projecting above was

dismantled. Then steel plate pedestals were filled in concrete and elastomeric

bearing of size 250 X 250 X 52mm was placed on it. The soffit of existing girder

where brackets were provided was dismantled and new steel connectors were

welded with main steel in girders. Thus the triangular bracket was adequately

reinforced. A flat steel plate was fixed to bracket and it was concreted and

grouted. The pedestal with bearings was laid in position so that it touched the

steel plate of bracket. The jacks were lowered by 3mm and full contact was

established between bracket and bearings. Two 100 Ton capacity jacks were

used to lift deck.

c. Pier No. 1: At pier No.1 both vertical jack 100 Ton capacity and horizontal jack (10

Ton capacity) with transverse were provided. The arrangement is seen in

photographs. Lifting was done in five stages. Vertically 30mm and horizontally

40mm in each stage. This lifting presented lot of difficulties and obstructions.

These had to be removed. The pressure developed at vertical jack was 25 tons

before lifting and horizontal jack was 5 tons before sliding. When pressure was


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increased, it was noticed that this can happen due to some obstructions. When

obstructions were removed movement was smooth.

4.4 Further Problems:

It was expected that the large un-uniform gap between the pipeline and saddle will

be fully closed on restoring span and at pier 1 end. But this has not happened at two

sockets in span 2, three sockets in span 3, one socket in span 4 and two in span 5

(there are 5 sockets in each span). Thus out of 5 X 5 = 25 sockets, support at 8

sockets are not even. The gap is from 8mm to 30mm. Contact surface on circular

saddle is 1800mm. Out of this at present contact is only in half of length at abovepoints where uneven gap exists. The gaps are on downhill side in span 2, 3 & 4 and

uphill side in span 5. This could happen if the centre line of pipe and the same of

saddle i.e. the bridge are not in one vertical plane. It appears that in span 2, 3 & 4

the pipe line has sagged towards hill side in lateral plane upto expansion joint and

the span 5 i.e. beyond expansion joint the lateral sag is towards downhill side.

Movement due to thermal variations must be allowed in pipe line. Therefore, it is

proposed to provide Neoprene pads in the annular gap (one or two pads) and

provide Polysulphide in the remaining gap, so that proper contact of pipe line and

saddle is established in a circular contact line of 1.8m.

Pipe Bridge is not a road bridge. It is a pipe plus foot bridge. Yet the types of distress

which have occurred in this bridge have also occurred in many road bridges and

therefore this case is brought out in the paper. The thickness of pipe is 8mm and

design calculations show that it can safely stand a span upto 30m. Therefore, the

uneven partial support at saddles had not damaged the pipe line. Uneven supports

may develop strains in the pipe and therefore, restoration is necessary.


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5. SOLID SPAN OF A FLYOVER WITH SPAN IN GRADIENT :

5.1 Structural Details

Length 190 m

Width 10m

Span Arrangement: - 5 spans of 25m and 2 spans of 20 m.

Foundations: - Open foundations approximately 4.0m below ground level and laid in

rock.

Piers: - single circular RCC piers 2.5m dia. with cantilevered pier cap.

Deck Structures prestressed concrete box with cantilever deck slab on both sides.

7.5m wide 2 lane road way and 1.5m footpath on one side. The box is single cell with

inclined ribs. Deck is continuous over three spans or two spans. Central 2 spans of

20m has voided RCC slab over the existing road and is level. 25m spans are in

gradient of 1/25.

Bearings POT PTFE bearings.

Fig.1 gives General Arrangement Drawing and the cross section of the deck.

Fig.2 gives the layout of the bearings.

5.2 Mishap

PSC box girder between P3-P4 slid down in traffic direction. The bearing pins of the

top plate sheared off. There is no displacement of the box girder in transverse

direction. No cracks were observed in the box structure.

5.3 Causes of Distress

It appears that the deck would slide if the pins provided at the 4 corners of PTFE

bearing to prevent displacement of the top plate from the bottom plate during


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Fig.5.1 Bridge with sloping deck. Continuous spans

Fig.5.2. Bearing arrangement of a bridge in gradient.

construction are broken. It is customary to provide such pins to avoid movement of

deck during construction. In this case the movement could be along slope. The box

structure was designed as a simply supported structure with the deck slab. The


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continuity was to be given in the deck slab; however, on completing the box

structure continuity of the slab was not provided at P2 & P3 and that is why the deck

could slide. Sliding was 454mm in span P3-P4 towards P3

5.4 Rehabilitation

The centering of the spans P1 to P4 was not removed. A detailed procedure for

carrying out the rehabilitation consisted of the following operations:

5.4.1 To provide wooden blocks at the supports on P3 & P4 so that the span does not

slide further and ensure that the existing centering is strong enough.

5.4.2 To provide continuity of deck slab on P2 so that during the operation of re-sliding

the jack force will be on spans P1-P2 & P2-P3. This was done, however, the gap

on P3 is reduced by 454mm therefore it was not possible to complete this gap

slab.

5.4.3 To lift the deck at P4 & P3 by about 150mm by vertical jacks and retain it in that

position by timber blocks. The weight of the deck is about 450 Tons. Therefore, 4

jacks of 150Tons capacity are required to lift the deck.

5.4.4 To slide back the deck on P3 & P4 by horizontal jacks. 2 such jacks with a capacityof 40 Tons operated by a combined manifold were used. It took nearly half a day

in making preparations, actual pushing took about half an hour. For sliding back

long steel plate was specially manufactured and was kept on jack position below

the deck.

5.4.5 Before the pushing the top plate of PTFE bearing at both ends were removed.

Then the bottom plate was also removed. The bolts of the plate were unscrewed

from the pedestal. Holes of the bolt were filled up by epoxy mortar.

5.4.6 It was earlier proposed to provide PTFE bearing of earlier design at this location

but this would involve manufacturing of the bearing and fixing the top plate to

the deck and bottom plate to the pedestal. The manufacturers informed that

manufacturing of similar four bearing would require 8 weeks time. Considering

the risk in keeping the disturbed deck for a long time is likely to create more

disturbances such as slipping of temporary wooden blocks, disturbances of


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centering etc. therefore investigations were carried out to ascertain whether

Neoprene bearings can be provided, these investigations showed that if

Neoprene bearings are provided, the section of foundations may not be safe and

therefore it was decided to strengthen the section of footing by providing

additional concrete block of 1.2m over the existing footing so that reaction of

the foundation is reduced and the reinforcement provided in the foundation is

also safe. The reinforcement in the pier was also checked and found to be safe. A

sketch giving strengthening of footing is shown in Fig.5.7. Neoprene Bearings

manufacturer assured that he can manufacture & supply the bearings within one

week.

5.4.7

The designs of neoprene bearings were also checked. It was noticed that thebearings are not safe for minimum load condition. It was also noticed if

continuity of deck slab is to be given on pier P3 it will have effect on elastomeric

bearings on P3 and P4. This will induce movement of 20mm at P4 and 10mm at

P3. If movement of 20mm is considered the elastomeric bearings cannot be

designed for minimum load conditions. Therefore elastomeric bearings on P3 &

P4 have been designed by considering free support at P3 & p4. It is therefore not

possible to give continuity at P3, therefore expansion joint will be provided on

pier P3. Thus change of PTFE bearing to elastomeric bearing has caused 2

modifications a.) Strengthening of footing b.) No continuity at P3.

5.4.8 The bottom of the deck structure over the location of bearing has to be leveled.

If it is not level steel plate with different thicknesses at the ends have to be

provided so that the top will match with the bottom of sloping deck and the

bottom of the plate will be level on the bearing. It was observed that the top of

the pedestal is level. A sketch showing these plates is also enclosed.

5.4.9 The deck was slid and brought correctly over the locations of pedestals by

horizontal jacks. It was checked and rechecked to ascertain the correct location

longitudinally and laterally.

5.4.10 The entire deck was supported on jacks at both ends and the temporary timber

blocks were removed. The elastomeric bearings were placed at correct position


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with the bottom fix to the pedestal by epoxy. The end of the deck on P3 was

lowered down and centrally spaced over the bearings by placing the top steel

plate with different thicknesses at both ends and make the bottom of the plate

on elastomeric bearing level.

5.4.11 The same operation was carried out on P4. Top plate was extended on gap side

(on pier) to check the level and ensure that it is perfectly horizontal. On

completing this operation of lowering and placing the deck on the bearings and

ensuring that it is level, several checks were carried out to ensure the level. An

insignificant difference of 1mm was noticed.

5.4.12 A question was raised whether it is correct to provide different type of bearings

in the same bridge. This has been done in Hasdeo Bridge at Chappa which wasrehabilitated in 1970 since the foundations of the bridge gave way during floods.

In the rehabilitation work 5 different types of bearings were provided and the

case in reported in Paper No. 395 (1989). During last 40 years the structure is

behaving well. Therefore there is nothing wrong in adopting different types of

the bearings in the same bridge. Photo graphs and sketches of various operations

are enclosed. P3

Fig.5.3. Rehabilitation of sloping bridge Stage-1


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5.5 Following Equipments was employed at site to accomplish the rehabilitation:

a. Hydraulic Jacks to lift the deck 4 X 150 T = 600 Tb. Hydraulic Jacks to slide the deck 2 X 40 T = 80 Tc. Long sliding steel plate to slide deck.d. Two steel plates at each bearing with varying thickness to make contact level

on bearings.e. Manifold for operating two jacks together.

5.6 The entire work was started on 9 th April 2012 and completed on 12 th April 2012.



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Fig.5.7. Additional concrete block over the footing.


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Fig.5.8. A steel plate with varying thickness fixed to the deck at bearings to make levelsupport.

Pic#5.1 Dislodged Span


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Pic#5.2. Lifting Jack

Pic#5.3. Pushing Jack


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Pic#5.4. Placement of sliding assembly

Pic#5.5. Horizontal movement of deck


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Pic#5.6. Neoprene bearing with steel plate at top, at the bearing location

Pic#5.7. Steel plate and Neoprene bearing fixed with Epoxy Mortar.


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6. Lessons from the Mishaps

Lessons learned from case 1 to 4 above are reported in last Para of each case. The

provisions made in the specifications for roads and bridges of ministry of road

transport and highways contained in Para 2002. Similarly provisions made in the

standard specifications in the code of practice in section 9 of IRC-83 are seen. These

contain the following:-

6.2.1 The bottom of girder to be received on the bearings is plain at the location of

these bearings.

6.2.2 During concreting of girders the bearings shall be held in position securely by

providing temporary connection between the top and the bottom plates and

clamps at roller and plate bearings.6.2.3 For bridges in gradient the bearing plate shall be placed in horizontal plane.

6.2.4 On completing of the work clamps at bearings must be removed. If this is not

done the structure cracks. We have observed that in one Yamuna bridge the

clamps provided at the bearings during construction were not removed for more

than 10 years, causing several cracks in the cross girder. A case of not providing

clamps during concreting collapsed the structure during construction. This was

observed in Chambal Bridge (case-3) & Ambika Bridge in 1953.

6.2.5 The design of bearing is done by the manufacturer of bearings. He should record

important instructions as mentioned above in the drawing (about placing and

removal of clamps).

6.2.6 In case of sloping spans the pin between top and bottom plates should be strong

enough to prevent the movement of plates during construction. If the structure

is designed as continues only in slab the continuity must be established

immediately on completing the deck of individual span.

6.2.7 Inadequate clamping arrangements of the sliding bearings during construction is

the main cause of this mishap mentioned in Para 5.


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7. Conclusions

7.1 We have brought out actual field problems of mishaps of bearings laid on

deflectable abutments and laid on sloping spans. There appears to be need to

incorporate the findings in Para 6 in the specifications of bridges.

7.2 It is absolutely necessary ascertain the causes of distresses before taking up the job

of restoration.

7.3 In bridge works expansion joints are proprietary products which have some amount

of vulnerability since these are directly hit by the moving traffic. Expansion joint is

therefore called the pulse of the bridge and any trouble in foundations,

substructure, bearings and superstructure is reflected in the expansion joint.

Similarly bearings are also important units and any trouble in the units can causemishap as had happened in many bridges. It is advisable that bearings and expansion

joints are fixed by the manufacturers of these products.

7.4 The technology has advanced so much that structural distresses can be restored.

Such restorations have been done successfully by the author in several bridges,

building and water towers.

7.5 The rehabilitation project cannot be successful unless there is active involvement

and cooperation between the owner/client, the contractor and Rehabilitation

consultant

7.6 Although one case (No.4) is reported in IRC paper, it is repeated again since such

mishaps of bearings on sloping spans or mishaps in bearings at deflectable

abutments have occurred at many places, although not reported in lifetime. There is

need to modify the specifications for design and construction of bearings and also

insist upon the calculation deflection of abutment and make suitable provisions in

abutment cap size.

problems of bridge bearings

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