Unit - 5

Bridge Engineering

5.1 Bridge Engineering

• A bridge is a shape constructed to span the bodily boundaries without final the manner underneath, along with a frame of water, valley, or road, for the cause of offering the passage over the obstacle. Bridge engineering is an engineering area branching from civil engineering that includes the planning, layout, construction, operation, and renovation of bridges to make sure secure and powerful transportation of vehicles, human beings and goods.
• Bridge Engineering consists of the primary subjects and the fundamental standards of bridge engineering and presents the whole scope of contemporary facts important for powerful and cost-aware present day bridge.
• It displays new engineering and constructing traits, the maximum contemporary layout methods, and the ultra-modern enterprise requirements and policies. It presents a complete evaluation of the great traits for bridge engineering. It highlights the latest advancements, requirements, improvements, and info of the ultra-modern strategies within side the international market. It carries a set of the ultra-modern studies traits at the bridge engineering.
• It comprehensively covers the fundamental idea and exercise in enough intensity to offer a strong grounding to bridge engineers. It enables readers to maximise effectiveness in all sides of bridge engineering.
• Reputable supply and a treasured reference may be very relevant and beneficial for all professors, researchers, engineers, training professionals, trainee practitioners, college students and others who're interested by the bridge projects.

Key takeaways:

• Bridge Engineering consists of the primary subjects and the fundamental standards of bridge engineering and presents the whole scope of contemporary facts important for powerful and cost-aware present day bridge.

5.2 Classification

Bridges can be classified into various types on the basis of following criteria:

1.     According to Form or Type of Superstructures

1.     Slab Bridge: It is a bridge of a reinforced-concrete slab resting on abutments and having short span.

2.     Beam Bridge: It is the simplest & oldest type of bridge consisting of horizontal beams and vertical piers. Beam bridges are long-span bridges while the distance between adjacent piers is small.

3.     Truss Bridge: It is a load-bearing superstructure composed of elements forming triangular units.

4.     Arch Bridge: It is a bridge having abutments at each end and is shaped as a curved/ semi-circular arch. The weight of the bridge and its loads are transferred onto the abutments on either side.

5.     Cable-stayed or Suspended Bridge: This type of bridge has one or more towers, from which cables support the bridge deck, forming a fan-like pattern or a series of parallel lines.

2.      According to Material of Construction of Superstructure

1.     Timber Bridge

2.     Concrete Bridge

3.     Stone  Bridge

4.     R.C.C Bridge

5.     Steel Bridge

6.     P.C.C Bridge

7.     Composite Bridge

8.     Aluminum Bridge

3.     According to Inter-span Relationship

1.     Simply-supported Bridge: The bridge span is supported by an abutment or pier at each end.

2.     Cantilever Bridge: A cantilever bridge is a horizontal structure supported only on one end.

3.     Continuous Bridge: It is a truss bridge extending without joints across three or more supports.

4.     According to the Position of the Bridge Floor relative to Superstructures

1.     Through Bridge: It is also known as a half-through arch bridge and is made from steel or reinforced concrete. In this structure, the base of an arch structure is below the deck but the top rises above it. Therefore, the deck slab is within the arch, and the cables and beams suspend the central part of the deck from the arch.

2.     Suspension Bridge: In this type of bridge, the deck is hung below suspension cables with the help of vertical suspenders.

5.     According to Method of Connection of Different Part of Superstructures

1.     Pinned-connection Bridge

2.     Riveted-connection Bridge

3.     Welded-connection Bridge

6.     According to Length of Bridge

1.     Culvert Bridge: These bridges have span less than 6 m.

2.     Minor Bridge: These bridges have span in the range of 6-60m.

3.     Major Bridge: These bridges have span greater than 60 m.

4.     Long-span Bridge: These bridges have span greater than 120 m.

7.     According to Function

1.     Aqueduct Bridge: It is a canal over a river.

2.     Viaduct: It is a road or railway bridge over a valley or a river.

3.     Pedestrian Bridge

4.     Highway Bridge

5.     Railway Bridge

6.     Road-cum-Rail or Pipeline Bridge

Key takeaways:

Beam Bridge: It is the simplest & oldest type of bridge consisting of horizontal beams and vertical piers. Beam bridges are long-span bridges while the distance between adjacent piers is small.

5.3 Identification and site selection

• A bad bridge region makes it vulnerable to harm and a bunch of different problems. Therefore, the region for a bridge is as crucial because the traits of the bridge itself. Selecting an awesome bridge page includes numerous elements like environmental and geological concerns, hydrology and hydraulics, initial engineering and roadway alignment.
• Generally, the area for a roadway crossing is chosen in the course of the making plans and area section of a motorway project. The very last area have to be decided on handiest after acquiring certain survey records and finishing a initial HEC-RAS hydraulic study. Although hydraulic standards aren't the only attention in bridge area and design, those troubles have to get hold of main interest withinside the preliminary making plans of the motorway.
• The area and alignment of the motorway can both enlarge or remove hydraulic issues on the crossing. Adverse situations have to be diagnosed withinside the early tiers of area choice in order that capacity issues get hold of ok overview and attention. If the price of the desired systems is prohibitive, remember rerouting the motorway.
• Bridges which can be built in a bad vicinity or which can be the incorrect length are extra at risk of failure. Therefore, it’s essential to get the layout and vicinity proper the primary time.
• Good bridge siting entails many disciplines. A strong and powerful evaluation calls for cautious interest to initial engineering, hydrology and hydraulics, move alignment, and environmental and geomorphic concerns. The precise hydraulic necessities are protected below: The bridge ought to be focused on the principle channel of the complete floodplain.
• This might also additionally imply an eccentricity withinside the vicinity with appreciate to the complete move go section, however this method permits for a extra powerful lodging of the everyday low flows of the move. The bridge waterway beginning ought to be designed to offer a float region sufficiently big to preserve the through-bridge pace for the layout discharge no more than the allowable through-bridge pace.
• The headers and indoors bents ought to be orientated to comply to the streamlines at flood stage. Standard skew values of 15°, 30°, and 45° ought to be used in which feasible. The piers and the toe of slope of the header ought to be placed farfar from deep channels, cuts, and excessive pace regions to keep away from scour troubles or interference with move low flows. Consider which include both comfort openings or manual banks if the intrusion of both or each roadway headers into the move floodplains is extra than approximately 800 feet. Existing flora ought to be included into the general bridge plan.
• Where practicable, bushes and shrubs ought to be left intact even in the proper-of-way. Vegetation that stays intact additionally has a tendency to manipulate turbulence of the float into, through and out of the bridge. For a few configurations, roadway methods might also additionally want to deal with overflow. Such overflow methods permit floods that exceed the layout float to overtop the roadway, thereby lowering the danger to the bridge shape itself. Protection of the methods from overflow harm ought to be considered.

Key takeaways:

• The area and alignment of the motorway can both enlarge or remove hydraulic issues on the crossing. Adverse situations have to be diagnosed withinside the early tiers of area choice in order that capacity issues get hold of ok overview and attention. If the price of the desired systems is prohibitive, remember rerouting the motorway.

5.4 Flood discharge

• Discharge is the extent of water that passes via a given pass phase in keeping with unit time, commonly measured in cubic toes in keeping with second (cfs) or cubic meters in keeping with second (cms). Stage is the extent of the water floor over a datum (frequently sea level).
• As discharge increases, level increases, but the courting isn't always linear. Flood level is the level at which overbank flows are of enough significance to reason sizable inundation of land and roads and/or notably threaten lifestyles and property. Students frequently confuse this with overbank glide that is while water overtops the channel and has geomorphic impacts. Crest (or peak) is the very best level reached in the course of a hydrologic event (consisting of a flood).
• The making of a score curve is difficult, however is critical to taking measurements (like degree) and turning them into the a great deal extra beneficial discharge information. Ideally, I could take college students out to a movement more than one instances and feature them degree discharge and degree so that they get an concept of those measurements (see Discharge and Sediment Transport withinside the Field for an example).
• That is generally now no longer possible, so I lead an open ended dialogue of ways we will degree the release. I start off with the definition of discharge after which have them provide you with the strategies they could use. I then have them reflect onconsideration on how, in the event that they made that dimension more than one instances over a few years they may increase a score curve.
• Rating curves are an fantastic subject matter for sophistication dialogue due to the fact they frequently appear easy to college students before everything glance, however are a great deal extra complex. I frequently ask matters like "what occurs to a river whilst the release increases?" after which have college students discover the instead complex answers.
• For example, score curves can also additionally extrade with time and there's frequently sizeable scatter withinside the data, which may be used as a launching factor for discussing regressions and error.

Key takeaways:

That is generally now no longer possible, so I lead an open ended dialogue of ways we will degree the release. I start off with the definition of discharge after which have them provide you with the strategies they could use.

5.5 Waterways

• A waterway is any navigable frame of water. Broad differences are beneficial to keep away from ambiguity, and disambiguation could be of various significance relying at the nuance of the equal phrase in different languages. A first difference is essential among maritime delivery routes and waterways utilized by inland water craft. Maritime delivery routes pass oceans and seas, and a few lakes, wherein navigability is assumed, and no engineering is required, besides to offer the draft for deep-sea delivery to technique seaports (channels), or to offer a quick reduce throughout an isthmus; that is the characteristic of deliver canals.
• Dredged channels withinside the sea aren't normally defined as waterways. There is an exception to this preliminary difference, basically for prison purposes, see beneathneath global waters. Where seaports are positioned inland, they're approached through a waterway that might be termed "inland" however in exercise is typically known as a "maritime waterway" (examples Seine Maritime, Loire Maritime).
• The term "inland waterway" refers to navigable rivers and canals designed to be utilized by inland waterway craft only, implicitly of lots smaller dimensions than seagoing ships. In order for a waterway to be navigable, it have to meet numerous criteria: it have to be deep sufficient to deal with vessels loading to the layout draft; it have to be huge sufficient to permit passage of the vessels with the layout width or beam; it have to be freed from limitations to navigation along with waterfalls and rapids, or provide a manner round them (along with canal locks or boat lifts); its present day have to be moderate sufficient to permit vessels to make headway upstream without undue difficulty; the wave height (on lakes) have to now no longer exceed the fee for which the magnificence of vessel is designed.
• Vessels the usage of waterways range from small animal-drawn barges to large ocean tankers and ocean liners, along with cruise ships.

Key takeaways:

Dredged channels withinside the sea aren't normally defined as waterways. There is an exception to this preliminary difference, basically for prison purposes, see beneathneath global waters.

5.6 Scour depth

• It is defined as the removal of sediments such as gravel and sand around or under the bridge abutments and piers. It is caused by the swift movement of water, resulting in scour holes thereby, compromising the stability of the bridge structure.
• Scour depth is important to determine the depth of the foundation.

5.7 Economic span

• The span of a bridge can be so decided that the entire price of the bridge is a minimum. The span to fulfill this circumstance is referred to as the monetary span. The general price of the bridge includes the price of the substructure and that of the superstructure.
• Very regularly it's miles visible that the price of the substructure bureaucracy almost 50 consistent with cent of the entire price of the bridge.
• The price of pier will now no longer alternate drastically for small alternate in span. Even the price of the floor-manner isn't always affected a great deal for small version in span. But it's miles visible that the price of the trusses and bracings is at once proportional to the span of the bridge.

Hence, the cost of the whole bridge is a minimum when the cost of one pier and the cost of trusses and bracings corresponding to one span are equal.

Key takeaways:

• Scour depth is important to determine the depth of the foundation.

5.8 IRC classification of Loads

Loads on Bridges:

The various loads, forces and stresses to be considered for the design of bridges are the following:

(i) Dead Load:

Dead load is the weight of the floor slab, track stringers, ballast, guard rails, bracing system, rails sleepers etc.

The dead load of the various components may be taken at the following values:

For a single track,

Weight of rails, guide rails and fastening – 3000 N/m

Weight of concrete – 25000 N/m3

Weight of ballast – 1000 N/m3

Wooden tier – 8000 N/m3

The dead load of a bridge depends on various factors like depth of girder or truss, span, number of panels, width of bridge etc.

The dead load of trusses may be estimated by the following methods:

(a) Hudson Formula:

Weight per metre of trusses and bracings = 0.785 A Newton/metre.

Where, A = Maximum net area of the tension chord

(b) Fuller’s Formula:

Weight per metre of truss bridges = (150 L + 5500) Newton/metre

(spans 30 m to 90 m)

Weight per metre of plate girders = (200 L + 1000) Newton/metre

(spans 10 m to 30 m)

Where L = span of the bridge in metres

(c) For a Single Track Railway:

Weight of both the girders including bracings system, per metre = (52 to 53) L √w N/w

Where, L = span in metres,

w = Heaviest axle load of the engine in kN

(ii) Live Load:

Live Load on Highway Bridges:

Live Loud on Footways (Foot Bridges):

The live load on foot bridges shall be taken as follows:

For all parts of bridge floors accessible to pedestrians and animals and for all foot ways, the loading shall be 4000 N/m1.

Where crowd loads are likely to occur, such as a foot bridge located near towns which are either centres of pilgrimage or where congregational fairs are held seasonally, the intensity of the footway loading shall be increased from 4000 N/m2 to 5000 N/m2.

Kerbs 0.6 m or more in width shall be designed for the above loads. If the kerb width is less than 0.6 m no live load shall be considered.

The main girder, trusses, arches or other members supporting the footways shall be designed for the following live loads per square metre of the footway area, the loaded length of the footway taken in each case being such as to produce the worst effects on the member under consideration.

Key takeaways:

Weight per metre of trusses and bracings = 0.785 A Newton/metre.

5.9 Forces

Longitudinal Forces:

• In all street bridges, provision will be made for longitudinal forces bobbing up from anybody or greater of the subsequent causes:

(a) Tractive efforts triggered through acceleration of the using wheels

(b) Braking impact attributable to the software of brakes to braked wheels, and

(c) Frictional resistance supplied to the motion or loose bearings because of extrade of temperature or some other cause.

Centrifugal Forces:

• In case a road bridge is situated on a curve, all portions of the structure affected by the centrifugal action of moving vehicles are to be proportioned to carry safely the stress induced by this action in addition to all other stresses to which they may be subjected.
• The centrifugal forces shall be determined from the following equations:

C= Centrifugal forces appearing generally to the traffic

(1) on the factor of movement of the wheel hundreds or

(2) uniformly disbursed over each metre duration on which a uniformly disbursed load acts.

W = Live load

(1) in TV in case of wheel hundreds, every wheel load being taken into consideration as appearing over the respective floor touch duration and

(2) in N/m in line with metre in case of a uniformly disbursed stay load.

V = The layout velocity of motors the use of the bridge in kilometres in line with hour and

R = The radius of curvature in metres

• The centrifugal pressure will be taken into consideration to behave at a top of 1.2 m above the extent of the carriageway. No boom for effect impact will be made at the stresses because of centrifugal movement. The overturning impact of the centrifugal pressure at the shape as an entire shall additionally be duly taken into consideration.

Seismic Force:

• If a bridge is located in a vicinity situation to earthquake, allowance will be made withinside the layout for the seismic pressure and earthquake-resistance functions will be embodied withinside the structural information of the layout.
• The seismic pressure will be taken as a horizontal pressure identical to the approximate fraction (distinct below) of the load of lifeless and stay leads performing above the phase below attention (elements of the shape embedded in soil shall now no longer be taken into consideration to supply any horizontal pressure).
• Regions vulnerable to minor damage: 1/20 of Gravity Regions vulnerable to extreme damage: 1/10 of Gravity For bridges located in epicentral tracts wherein massive devastations have took place withinside the beyond because of earthquakes, the chances will be constant through the engineer answerable for the layout, with due regard to the neighborhood situations concerning the depth of earthquakes typically skilled in those regions. T
• He horizontal forces because of seismic pressure will be taken to behave via the centre of gravity of all of the masses below attention. The course of this pressure need to be such that the consequent stresses withinside the member below attention are maximum. Seismic and wind forces shall now no longer be taken into consideration to behave simulaneously.
• The significance of the seismic pressure shall now no longer be decreased because of discount in weight because of buoyancy received in a submerged mass.

5.10 Stresses

Secondary Stresses:

• These are the extra stresses delivered into play due both to the motion of helps or to the deformations withinside the geometrical form of the shape or its contributors on account of reasons consisting of pressure of quit connection or hundreds implemented at intermediate factors of trusses or restrictive shrinkage of concrete ground beams.
• All bridges will be designed and built in a way such that the secondary stresses are decreased to a minimum; and that they will be allowed for withinside the design.
• For reinforced concrete members the shrinkage coefficient for purposes of design may be taken as 2 × 10-4.

Erection Stress:

• Allowance will be made withinside the layout for stresses installation in any member in the course of erection; such stresses can be unique from the ones which the member can be subjected to in the course of real working.

Key takeaways:

• In case a road bridge is situated on a curve, all portions of the structure affected by the centrifugal action of moving vehicles are to be proportioned to carry safely the stress induced by this action in addition to all other stresses to which they may be subjected.

5.11 IRC Specification & code of practices

5.12 Critical combinations

• All systems need to be designed to assist their personal weight together with any superimposed forces, consisting of the useless masses from different materials, stay masses, wind pressures, seismic forces, snow and ice masses, and earth pressures (if buried underground).
• Because diverse masses might also additionally act on a shape simultaneously, load combos need to be evaluated to decide the maximum excessive situations for layout (worst case scenario). These load combos range from one record to every other, relying upon the jurisdiction. There are a fixed of combos for the allowable pressure layout and every other set that consists of load elements for electricity layout.

Basic load combinations of IBC Section 1605.3.1 (allowable stress design):

1. Dead load plus lateral fluid pressures, [D + F]
2. Dead load plus hydrostatic lateral soil plus lateral fluid pressures plus live load, [D + H + F + L]
3. Dead load plus hydrostatic lateral soil plus lateral fluid pressures plus either roof live load, or snow load, or rain load, [D + H + F + (Lr or Sor R)]
4. Dead load plus hydrostatic lateral soil plus lateral fluid pressures plus 0.75 times live load plus 0.75 times either roof live load, or snow load, or rain load [D + H + F +0.75(L) + 0.75(Lr or Sor R)],
5. Dead load plus hydrostatic lateral soil plus lateral fluid pressures plus (0.6 times wind load or 0.7 times earthquake load) [D + H + F +(0.6Wor 0.7E)],
6. Dead load plus hydrostatic lateral soil plus lateral fluid pressures plus 0.75 times (0.6 times wind load or 0.7 times earthquake load) plus 0.75 times live load plus 0.75 times (either roof live load or snow load, or rain load) [D + H + F +0.75(0.6W or 0.7E) + 0.75L+ 0.75(Lr or Sor R)],
7. 0.6 times dead load plus 0.6 times wind load plus hydrostatic lateral soil [0.6D + 0.6W + H],
8. 0.6 times (dead load plus lateral fluid pressures) plus 0.7 times earthquake load plus hydrostatic lateral soil [0.6(D+ F) + 0.7E + H]

Notes on simple load aggregate of IBC Section 1605.3.1:

• Include lateral earth pressures withinside the layout in which they bring about a extra crucial aggregate. The IBC does now no longer require crane hook hundreds to be mixed with roof stay hundreds nor with extra than 3 fourths of the snow load or one-1/2 of of the wind load. For flat roof snow hundreds exceeding 30 psf, 20 percentage of the snow load will be mixed with the seismic hundreds.
• Flat roof snow hundreds of 30 psf or much less want now no longer be mixed with seismic hundreds. The ground stay load have to now no longer be covered if its inclusion might bring about decrease stresses for the shape or member being designed.
• Increases in allowable stresses shall now no longer be used with the weight combos given on this phase of the IBC. Multiply hydrostatic lateral soil pressures via way of means of 0.6 while they're everlasting and that they face up to the number one variable load effects.
• If the hydrostatic lateral soil pressures aren't everlasting, do now no longer take into account that they offer any resistance to different load effects. In Load Combination 8 (IBC load aggregate 16-16), the lifeless load thing can be extended to 0.nine for unique strengthened masonry shear walls.

Alternative basic load combinations of IBC Section 1605.3.2 (allowable stress design):

1. Dead load plus live load plus either roof live load, snow load, or rain load, [D + L +(Lr or Sor R)]
2. Dead load plus live load plus 0.6 times the coefficient ωtimes the wind load, [D + L +0.6 ωW]
3. Dead load plus live load plus 0.6 times the coefficient ωtimes the wind load plus onehalf times the snow load, [D + L+ 0.6 ωW+ S/2]
4. Dead load plus live load plus snow load plus 0.6 times one-half coefficient ωtimes the wind load, [D + L + S + 0.6 ωW/2]
5. Dead load plus live load plus snow load plus (1/1.4) times the earthquake load, [D + L + S + E/1.4]
6. 0.9 times the dead load plus (1/1.4) times the earthquake load, [0.9D + E/1.4]

Notes on opportunity simple load mixtures of IBC Section 1605.3.2:

• Include lateral earth pressures withinside the layout wherein they bring about a extra vital combination.
• The IBC does now no longer require crane hook hundreds to be mixed with roof stay hundreds nor with extra than three-fourths of the snow load or one-1/2 of of the wind load.
• For flat roof snow hundreds exceeding 30 psf, 20 percentage of the snow load will be mixed with the seismic hundreds. Flat roof snow hundreds of 30 psf or much less want now no longer be mixed with seismic hundreds. When the usage of those trade simple load mixtures that encompass wind or seismic hundreds, allowable stresses are authorized to be elevated or load mixtures reduced, wherein authorized with the aid of using the IBC or with the aid of using the referenced trendy of IBC, furnished that once wind hundreds are calculated with the aid of using Chapters 26 via 31 of ASCE 7, the coefficient ω withinside the above equations will be taken as 1.3.
• For different wind hundreds ωshall be taken as 1.0. If allowable stresses aren't elevated, the coefficient ω can be taken as 1. When those mixtures are used for foundations for hundreds together with seismic, the vertical seismic effect, Ev, in Equation 12.four-four of ASCE 7 is allowed to be taken as zero.
• When those mixtures are used to assess sliding, overturning, and soil bearing on the soil-shape interface, the discount of basis overturning from Section 12.13.four of ASCE 7 shall now no longer be used.
• For load mixtures that encompass counteracting outcomes of lifeless and wind hundreds, best two-thirds of the minimal lifeless load this is possibly to be in area at some point of the designed wind occasion will be used.

Key takeaways:

• These load combos range from one record to every other, relying upon the jurisdiction. There are a fixed of combos for the allowable pressure layout and every other set that consists of load elements for electricity layout.

References:

1. Pavement Design: Yoder and Witzak Wiley

2. Traffic Engineering: L.R.Kadiyali Khanna Publishers