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Bridge Engineering

1 Culvert is a cross drainage work whose length (total length between the inner faces of dirtwalls) is less than 6.0 m.

2 In any highway or railway project, the majority of cross drainage works fall under Culvert.

3 (a) Reinforced concrete slab culvert

(b) Pipe culvert

(c) Box culvert

(d) Stone arch culvert

(e) Steel girder culvert for railways

4 for culverts and minor bridges of total length less than 60 m

5 Kerbs of minimum width 500 to 600 mm and height 300 mm above top of deck slab

6 The waterway required for the culvert is designed in such a manner that the

estimated discharge can be passed with a low velocity, e.g. 1.5 metre per second, and the HFL is adequately below the bottom of deck

7 Raised footpaths need not be provided for culverts and minor bridges having total length of less than 30 m.

8 For bridges with footpaths, footpath load of 5 kN/m2 is to be considered.

9 Submersible bridges usually have reinforced concrete slabs for decking and adopt multiple spans up to 8 m each

10 Reinforced concrete slab culverts are economical for spans up to about 8 m, though the slab bridge type can be used for spans up to about 10 m. The thickness of the slab and hence the dead load are quite considerable as the span increases. 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.

11 The components of a culvert with R.C. deck slab are the following:

(a) Deck slab

(b) Abutments, wing walls and approach slabs

(c) Foundations

(d) Kerbs and railings.

12 Reinforced concrete is well suited for the construction of highway bridges in the small and medium span range. The usual types of reinforced concrete bridges are:

Slab bridges;

Girder and slab (T-beam) bridges;

Hollow girder bridges;

Balanced cantilever bridges;

Rigid frame bridges;

Arch bridges; and

Bow string girder bridges.

13 The width of the kerb may vary from 475 mm to 600 mm

14 If the width of the bridge is adopted as 12.0 m , at least four main girders will be necessary

15 The lateral spacing of the longitudinal girders will affect the cost of the bridge. Hence in any particular design16 with closer spacing, the number of girders will be increased, but the thickness of deck slab will be decreased. Usually this may result in smaller cost of materials. But the cost of formwork

will increase due to larger number of girder forms, as also the cost of vertical supports and bearings. Relative economy of two arrangements with different girder spacings depends upon the relation between the unit cost of materials and the unit cost of formwork. The aim of the design should be to adopt a system which will call for the minimum total cost. For the conditions obtaining in India, a three-girder system is usually more economical than a fourgirder system

for a bridge width of 8.7 m catering to two-lane carriageway.

17 Cross beams are provided mainly to stiffen the girders and to reduce torsion in the exterior girders. These are essential over the supports to prevent lateral spread of the girders at the bearings. Another function of the cross beams is to equalize the deflections of the girders carrying heavy loading with those of the girders with less loading. This is particularly important when the design loading consists of concentrated wheel loads, such as Class 70 R or Class AA wheeled vehicles, to be placed in the most unfavourable position

18 When thespacing of cross beams is less than about 1.8 times that of longitudinal girders, the deck slab can be designed as a two-way slab.

19 The thickness of the cross beam should not be less than the minimum thickness of the webs of the longitudinal girders

20 The depth of the end cross girders should be such as to permit access for inspection of bearings and to facilitate positioning of jacks for lifting of superstructure for replacement of bearings.

21 The cantilever portion usually carries the kerb, handrails, footpath if provided and a part of the carriageway. The critical section for bending moment is the vertical section at the junction of the cantilever portion and the end longitudinal girder

22 Based on the method of construction, prestressed concrete bridges may be classified into four categories: (a) cast-in-situ bridges; (b) bridges with precast girders; (c) bridgeswith segmental cantilever construction; and (d) incrementally launched bridges.

23 Scour denotes the washing away of the bed material in a stream due to the passage of a flood. The pattern of scour occurring at a bridge across a river depends on many factors including discharge, bed slope, bed material, direction of flow, and alignment of piers, their shape and their size. Hence the prediction of scour depth is difficult.

24 A shallow foundation is sometimes defined as one whose depth is smaller than its width.

25 a shallow foundation is taken as one which can be prepared by open excavation, and a deep foundation would refer to one which cannot be prepared by open excavation.

26 Footings and raft foundations are examples of shallow foundations.

27 In the case of a deep foundation, the load transfer is partly by point bearing at the bottom of foundation and partly by skin friction with the soil around the foundation along its embedment in the soil.

28 Deep foundations are further classified as:

(a) Pile foundations, and

(b) Caisson foundations

29 A pile is defined as a column-support type of foundation which may be precast or formed at site. Caisson (well) foundation is a structure with a hollow portion, which is generally built in parts and sunk through the ground to the prescribed depth and which subsequently becomes an integral part of the permanent foundation 30 Caisson foundations are of two types:

(a) Open caissons (also known as well foundations in India) ; and

(b) Pneumatic caissons

31 An open caisson is one that has no top or bottom cover during its sinking. It is more popularly known as well foundation. A pneumatic caisson is a caisson with a permanent or temporary roof near the bottom so arranged that men can work in the compressed air trapped under it. Pneumatic caisson can be used for a depth of about 30 m below water level, beyond which pile foundations would have to be resorted to 32 For larger depths and for work under water, it would be necessary to use shoring with sheet piles or to resort to the provision of cofferdams. The purpose of shoring and cofferdams is to permit excavation with minimum extra width over the foundation width and to facilitate working on the foundation "in the dry", using suitable water pumping arrangements. In case of shoring, sheathing with timber planks supported by wales and struts is provided as the excavation proceeds

33 The size of the excavation at the bottom should be sufficiently large to permit adequate space for fixing formwork around the footing and to leave a working space of about 300 mm all around. The limiting depth of cofferdams is normally about 10 m. When the excavation reaches the foundation level, the exposed

area of the bottom of the pit is leveled and compacted by ramming.

34 In case pumping of water is necessary, a sump is provided to drain the water. A leveling course of about 150 mm thickness with lean concrete (1:3:6 or 1:4:8 by volume) is laid

35 Pile foundations may be considered appropriate for bridges in the following situations:

(a) when the founding strata underlies deep standing water and soft soil;

(b) when the foundation level is more than 30 m below the water level, so that pneumatic sinking of wells is difficult;

(c) when suitable founding strata is available below a deep layer of soft soil; and

(d) in conditions where pile foundations are more economical than wells.

36 Pile foundations may be divided into two groups:

(i) Foundations with friction piles, and

(ii) Foundations with point bearing piles.

37 The minimum spacing of piles should be 2.5 to 3.0 times the diameter of the larger pile

38 Piles can be of timber, steel, reinforced concrete or prestressed concrete. Timber piles are not used for bridges nowadays, firstly due to lack of suitable logs of long length, and secondly due to susceptibility to damage by rot, borers, etc.

39 H-section steel piles can be used, but they are not common in India due to shortage of steel. Reinforced concrete piles are in general use. Prestressed concrete tubular piles of diameter 1.5 to 6.0 m have been used in USA and Japan, in view of better ductility and high axial compressive capacity

40 Concrete piles can be precast or cast-in-situ. Each type has advantages as well as disadvantages. Since bridge structures involve foundations under water or in soil with a high water table, precast piles are often preferred. Precast piles can be made to high quality with respect to dimensions, reinforcement disposition and concrete strength, and hence the structural capacity of these piles can be relied upon. However, the length of pile is limited to about 20 m depending on the driving equipment.

41 The main advantage of an in-situ pile is that there is no wastage of concrete and no chance of damage to pile during driving exists. Also, the soil occurring at the foundation level is seen prior to placing the concrete for the pile. A serious disadvantage is the possibility of inadequate curing due to chemical attack in

aggressive subsoil water, which condition is likely to go undetected during construction. Further, the occurrence of necking of concrete during removal of the forming shell and mixing of the concrete with the surrounding soil should be carefully guarded against.

42 A hybrid procedure is sometimes used combining a few features of precast and castin-situ piles. In this case.

43 A rigid pile cap in reinforced concrete should be provided to transfer the load from the pier to the piles as uniformly as possible under normal vertical loads. The plan dimensions of the pile cap should be such that there is a minimum offset of 150 mm beyond the outer faces of the outermost pile in the group.

44 The cap thickness is usually kept at least one-half of the pile spacing

45 The reinforcement of the pile cap should consider the shape of the cap

and the disposition of the piles. The top of the piles should be stripped to an adequate length and the exposed longitudinal reinforcement of the piles should be fully embedded into the pile cap. The top of the pile cap is generally specified to be at the bed level in case of rivers with seasonal flow or at the low water level (LWL) in case of perennial rivers. In some cases, the bottom of the pile cap is kept at about 150 mm above the LWL. While the former approach leads to better aesthetics but with difficulties in construction, the latter method facilitates

faster construction and inspection of the underside of the pile cap during service.

46 Precast concrete piles can be round, square or octagonal in section. A circular pile is difficult to make due to difficult formwork, whereas a square pile is easy to cast; and octagonal pile is a compromise between the two.

47 The maximum length of a driven pile is usually about 36 m from transportation consideration. Field splicing, though possible, involves delay and additional cost. The heavy weight of the pile is a disadvantage.

The bearing capacity of the individual pile normally ranges from 300 to 1500 kN

48 Cast-in-place concrete piles are constructed in their permanent position by filling with concrete the holes which have been formed in the ground in various ways for the purpose.

There are two types: (a) the shell pile, in which a steel shell is first driven with a mandrel and concrete is placed, leaving the shell in place, and (b) the shell-less pile, in which the pipe and mandrel used for making the hole are removed as the concrete is filled in. Reinforcement is provided for the entire length of the pile, the minimum area being 0.4 per cent of the gross cross-sectional area of the pile.

49 Well foundation is preferable to pile foundation when the foundation has to resist large lateral forces, the river bed is prone to heavy scour, heavy floating debris are expected during floods and when boulders are embedded in the substrata.50 The shape of wells may be circular, double-D, square, rectangular, dumb-bell, etc. The circular well has the merit of simplicity for construction and sinking. Double-D, rectangular and dumbbell shapes are used when the bridge has multi-lane carriageway.

51 The minimum size of a dredge hole is 2.5 m

52 From practical considerations, the maximum size of a single circular well should be limited to about 12 m diameter for a concrete steining and 6 m

diameter for a brick masonry steining.

53 The steining is normally of reinforced concrete. The concrete used for steining should be M15 for normal exposure. In areas of marine or adverse exposure, the concrete for steining should be at least M20 with cement content not less than 310 kg/m3 of concrete with water/cement ratio not more than 0.45.

54 A bottom plug is essential to transfer the load from the well steining to the base soil

55 Cofferdam refers to construction in a water body to provide an enclosed site, which can be dewatered to facilitate construction of a bridge pier in almost dry condition. Medium sized cofferdams of 10 to 50 m diameter have been used for construction of bridge piers in water up to about 20 m depth.

56 The cofferdams must be designed to resist the lateral force due to water currents, waves and unbalanced soil loads. A concrete seal is usually provided at the bottom of the cofferdam by tremie method to facilitate dewatering. This seal should have emergency relief pipes to prevent structural failure of the seal in case of failure of the dewatering system.

57 The box caisson is usually a structural shell of steel or concrete section, often with cellular cross section, with sizes ranging from hundred to many thousands of tonnes. The box caisson is fabricated on shore.

58 Bearings are provided in bridges to transmit the load from the superstructure to the substructure in such a manner that the bearing stresses induced in the substructure are within permissible limits.

59 On certain major bridges, the cost of bearings accounts for as much as 10 to 15% of the total cost of the bridge.

60 Since metallic bearings are expensive in first cost and maintenance, the recent trend is to favour elastomeric bearings8. An elastomeric bearing accommodates both rotation and translation through deformation of the elastomer. These bearings are easy to install, low in first cost and require practically no maintenance. They do not freeze, corrode or deteriorate. Barring an earthquake, the only probable causes for failure of an elastomeric bearing are inferior materials, incorrect design or improper installation. Elastomeric bearings are 'forgiving' in that they can tolerate loads and movements exceeding the design values.

61 A wearing course (sometimes referred as wearing coat) is provided over concrete bridge decks to protect the structural concrete from the direct wearing effects of traffic and also to provide the cross camber required for surface drainage. The wearing course may be of asphaltic concrete or cement concrete. Asphaltic concrete wearing course is currently the preferred option as this permits the use of buried expansion joint for short spans facilitating a smooth transition between the bridge and the approaches for the riding surface.

62 A reinforced concrete approach slab is usually provided on either side of a concrete bridge to function as a smooth transition between the paved roadway and the riding surface of the bridge. The slab serves to minimize bumps to traffic and the resulting impact to abutment due to potential differential settlement between the approach embankment and the abutment.

63 Approach slab should cover the full width of the roadway and should extend for a length of not less than 3.5 m into the approach. The top of the approach slab should conform to the cross profile of the top of the deck slab. The slab has a minimum thickness of 300 mm at the ends with the maximum thickness adjusted to suit the cross camber

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