Q Bank for Civil Engineer LDCE Exam Write the short notes (9)

Q.No.27 Write the short notes of the followings:- 

(i) Causes of rehabilitation of bridges 

(ii) Rail Clusters 

(iii) Sleeper Cribs 

(iv) Railway Affecting Works 

(v) Types of bearings in bridge.

i. Causes of Rehabilitation A bridge may require rehabilitation on account of the various reasons some of the reasons may be as under:

(i) Physical distress 

(ii) Vulnerability on hydrological consideration 

(iii) Use of obsolete/non standard materials such as 

a. Early steel girders 

b. Laterite stones 

c. Cast iron screw piles 

d. Corrugated steel pipes

e. Earthen ware pipes In case, due to introduction of new rolling stock, existing bridge is not safe, both its strengthening is to be done and, if not possible, rebuilding is to be done. 

(b) Rail Clusters Rail clusters are used for temporarily supporting the track for spans upto 3.66m(12ft).These are designed for M.L standard loading with 10% impact.90 Ibs.&75 Ibs. rails are normally used for the rail clusters. Number of rails required for 90Ibs & 75 Ibs section. Temporary arrangements with rail cluster on sleeper cribs 16.17

(a) Details of Rails for Rail clusters

Sleeper Cribs:-

Sleeper Cribs are required for supporting the rail clusters or Service spans at a particular height. Sleeper cribs are designed for a base loading of 1 Ton/Sq ft. If it is considered that soil is not fit for this loading, boulders must first be laid in such a manner as to spread the load until a safe bearing pressure is reached. The top surface of boulders is leveled with smaller stones to form an even surface for the first layer of sleepers. Single Cribs of wooden sleepers are made for height up to 3.05 meters (10ft),whereas double cribs are made for greater heights. Following points require special attention during construction of cribs. 

(i) Size of crib depends upon the type& span of temporary girder and nature of rail. 

(ii) Cribs should be constructed of good sound sleepers of size 2750x250x130mm(9’x10’x5”).

(iii) Sizes of sleeper cribs ranging from 1.525m to 7.625m(5ft to 25 for height and above along with speed restriction generally being followed for each, are given below:

(iv) Rails in sleeper cribs should be introduced at more than 1.83m(6’) interval in height. 

(v) A layer of kacha plaster 75mm(3inches) thick should be laid on all exposed surfaces o the crib after the track has been laid to prevent danger from fire. 

(vi) Should the height of the crib exceed 1.5m(5’),the cribs should be suitably tied with through bolts. 

(vii) A watchman, equipped with fire fighting appliances such as fire buckets etc. should be employed to deal with any fire that may break out on the cribs.

(viii) Cribs forming abutments have to take the side thrust due to earth pressure of the embankment, apart from the load of the Engine& the train, and therefore such cribs require special attention. Normally an auxiliary crib is used behind the main crib to lower the plane at which the live load is applied and reduce the horizontal earth pressure on the main crib. 

(ix) The arrangement of sleepers directly below the girder seating should be so done that the concentrated load gets spread over the length & breadth of cribs 16.26. 

Railway Affecting Works : Railway Affecting Works are those, the construction, design, maintenance of operation of which may cause serious damage or breaches or flooding of Railway line or bridges. Some of the examples are: 

(i) Irrigation and Water supply tanks or reservoirs 

(ii) Canals in embankment and river bunds 

(iii) Road embankment with inadequate waterway for drainage situated upstream or downstream of railway line. 

(iv) Temporary channels cut for irrigation or other purpose from beds of active rivers (v) Other works, which alter or impede the natural course of flood or cause increase of flow such as new irrigation projects, new townships, Large scale deforestation etc.

Types of bearing “

1. Shoe-plates: Shoe plates are attached to the underside girder and these rest directly on masonry. This is an unsatisfactory arrangement, as due to figh action between masonry &steel, expansion is resisted and heavy longitudinal and forces are imparted to the masonry, these type of bearings are gradually being replaced by centralized bearings. 

2. Centralized bearings or flat bearings: These are provided for plate girders generally up to a span of 24.4(80ft).In this arrangement, a curved or chamfered bearing plate is fixed to the bottom flange of the plate girder just below the bearing stiffener and the same rests on a bed plate fixed to the heel block. The bed plate at the free end has to be periodically greased to reduce the co-efficient of action. Greasing also prevents rusting of steel surface. This works as preventive maintenance of bearing as well as of bed plates and prolongs their service life. The rocking ‘of the girder is facilitated by the curved shape of the bearing plate. 

 3. Phosphorous bronze bearings: In this type of bearings the bed plate has a phosphor bronze surface. This is useful for long spans, particularly for spans above 10.5m (100ft.) because of the lower co-efficient of friction of steel on bronze surface(0.15),than that on steel surface(0.25).It has however to be ensured that the hearings are very clean, without any dirt. Phosphor bronze bearings need not be greased as these are corrosion resistant and retain the smooth surface. 

4. Rocker and Roller bearings : In this type of the bearings at the fixed end, the girder end is fixed in a position but rocking due to deflection under load is permitted by the curved knuckle pin. At the free end, the arrangement for rocking is similar but the knuckle assembly is mounted on a set of rollers to take linear expansion. It has to be ensured during inspection that the knuckle pin and the rollers are well lubricated. 

5. Elastomeric bearings or Neoprene bearings : These are provided for bridges having RCC or prestressed girders and can take deflection as well as movement . Normally, these bearings are provided for Road bridges, but recently Railway bridge are also being provided with this type of bearings.

(i) Elastomeric bearings are made of natural or synthetic rubber if shore hardness of approximately 50 to 70. They are very stiff in resisting volume change but are very flexible when subjected to shear or pure uni-azial tension. They are generally reinforced with steel or concrete girder these permit moderate longitudinal movements ad small rotations at the ends.

(ii) These bearings require periodic cleaning . they may require replacement in service depending on the condition and usage. 

6. Oil bath bearings :- this type of bearings is encased in oil baths and does not need periodic oiling and greasing. 

7. P.T.F.E. bearings :- this type of bearings consists of a self lubricant ply-tetra- fuoroethylene surface across a smooth stainless steel plate , fixed on a mild-steel plate which slides on it, it makes a non-rotational free end by vulcanizing the P.T.F.E. sheet on an elastomeric bearing. This also gives a very low coefficient of friction in its sliding action. These bearings also require periodic cleaning of the bearing surface. The interface should be protected from dust. Lubricating the mating surface by silicon grease reduces the coefficient of friction and is desirable. 

8. Segmental bearings : these are provided in special situations where piers are too slender, as in case of RCC piers.