Unit:5 Bridge Note (Highway Engineering -II)

  6.1 INTRODUCTION  

A structure constructed over an obstacle to provide the passage is known a bridge. If the passage is for the movement of traffic, it is known as Road Bridge. If the passage is for the movement of train, it is known as Railway Bridge. If the level of bridge is much more than general road it is known as over bridge. If the level of bridge is below the ground level and covered then, it is called sub-way.  

According to NRS 2045, the cross drainage structure whose span is more than 6m is termed as bridge and cross drainage structure having span less than 6m is known as culvert.  

  

 * Characteristics of Ideal Bridge:  

           i) The line of bridge should not have serious deviation from the line of approach road.   
ii) It should be in level 
iii) The width of bridge should be sufficient to cater future traffic.  
iv)Bridge should carry standard loading with reasonable factor of safety.   
v)Bridge should not produce undue obstruction of stream, hence provide adequate waterway width  
vi)Foundation should be kept on firm (strong) ground and they should be kept at sufficient depth to avoid damage by floods.  
vii) Bridge should fit into surrounding landscape.   
viii)Bridge should provide passage for services like water pipe, telephone etc. 
ix)Bridge surface should be similar to road surface. 
x)Bridge should be economical in terms of construction and maintenance. 
xi)Adequate provision should be made for the drainage of the road surface.   

#  Choice of Location of Bridge Site

The characteristics of location of bridge site are as follows: 

i)  A straight reach of river. 

ii)Steady river flow without serious whirls and cross culverts.  
iii) A narrow channel with firm banks   
iv)Suitable high banks above high flood level on each side. 
v)Rock or other hard erodible strata close to the river bed level  
vi) Absence of expensive river training works   
vii) Absence of sharp curves in approaches   
viii) Absence of excessive construction work underwater  
ix) Avoidance of long detours (deviation from direct)     

 # Choice of Bridge Type:  

The choice of a particular type of bridge should be made so that it is most suitable to carry the desired traffic, adequately strong to support the design loads, economical and aesthetically pleasing. Some of the factors influencing the choice of the bridge type are:  

1)  Economy in overall construction cost.  

2)  Large navigational clearances may dictate the use of particular type of bridge.

3) In order to reduce the no. of piers especially in a high level structure bridge it may be compel to select for special type of bridge either Cantilever bridge or a cable stayed bridge.  

4)  Availability of funds.  

5)  The type of traffic may restrict the choice of bridge type.  

6)  Personal preferences of the designer or specialization of the construction agency.  

 6.1.2 Classification based on span, loading, length, materials &Structures  

(A) According to the span (2045NRS)   

1)  Minor bridge (Span less than 20m but total length may be upto 20m)   
2)  Medium Bridge (Span less than 20m but total length may be more than 20m)   
3)  Major Bridge (Provide whose length & span greater than 20m)  

B) According to loading (NRS2045)  

1) Major bridges (HS20-44 or IRC class AA or any other equivalent loading or class 70 R additional loading)   
2)  Medium or minor bridges (HS15–44 or IRC A or any other equivalent loading)   
3) Temporary bridge (HS15-44 or IRC class B or any other equivalent loading or class 70 R additional loading)  

(C) According to structure  

1) RCC T-bridge or RCC simply supported bridge  

2)  Cantilever bridge (Single or double)   
3)  Arch bridge  
4)  Suspended or suspension bridge   
5)  Steel bridge   
6)  Cable stayed bridge  

D) According to materials 

1)  Timber bridge   
2)  RCC bridge   
3)  Masonry bridge   
4)  Steel bridge  

(E) Classification based on utility

1)  Highway bridge   
2)  Railway bridge   
3)  Compound bridge  
4)  Pedestrian bridge  

6.2.2) Detail of substructure & superstructure/ Choice of Bridge Type: 

The component of bridge are classified into two groups named substructure & super-structure. 

The structure below the bearings is known as substructure and the portion of bridge above the bearing is known as super structure. The different components/parts of the bridge are given below:  

           i)  Foundations for the abutments & piers of towers.   
ii)  Abutment & piers of towers (sub-structure)  
iii) Decking consisting of girders or trusses or cables & slab 
iv)Bearing for the girder or trusses   
v) Handrails, guard stones etc   
vi)  River training works necessary at U/S & D/S side   
vii) Approaches to the bridge to connect the road or railway to the bridge proper.  

  6.2    T-beam bridge

  

  

  

 

     6.3  Suspension bridge 

Usually main spans of suspension bridges  are longer than cables stay bridges, therfore,decks of the suspension bridges have less stiffness in comparison with cable stay bridges. As a result,suspension bridges have more vibration concerns. in addition design and construction of suspension bridges are more complicated rather than cable stay bridges,and that's the reason why most of the failures of the cables happened in suspension bridges.

                     

* Construction of various parts 

 A suspension bridge should consist of the components shown on the diagram, other elements are added for aesthetic purposes and design.

Deck : the deck on a suspension bridge is also referred to as a roadway,where vehicles are allowed to pass to and from points A&B. They can carry motorists, pedestrains<rail traffic etc. They are made out of steel reinforced concrete and each deck is of a large span.

Steel Cables : The decking or the roadway is suspended by steel cables. They can be as thick as a tall made human and are made up of many smaller steel cables, steel is used instead of iron because it is an alloy, which makes it superior in tension and compression and it is stronger. The smaller cables are fast need to one another formining one huge cable enough to hold up to 150000 tonnes.

Suspenders: the suspenders connect the decking to the steel cables and help shape the bridge.Without the suspenders the roadway would sway out of control; they help reinforce the decking even more as well as having steel cables.

Towers: The heavy weight of the steel cables are transferred onto the towers that help the bridge stay standing the weight that is now supported by the towers is focused onto the ground. reinforccing the tower feet into the ground and keeping the bridge up right.

Anchorage Block: these weight more than the amount of cables that is holding up the deck,this is because it has to withstand a huge proportion of the roadway.Not only this but it must be strong enough to endure the amount of road traffic and vechicles crossing the bridge at any time.They are often made out of concrete as it is extremely heavy and strong. They apper at both ends of the bridge and preserve the tension from the steel cables.

Foundation of Tower: because of the weight pushing down on the towers, they must have a secure foundation.The foundations are pushed far below the soil to keep the towars from tilting and to make sure that they are vertical and strong enough to withstand the weight from the cables. Depending on the softness of the soil,depends on how far down the foundation go,if the soil is soft ,then the foundation would be pushed further down.

Truss: The truss if found to be underneath the roadway/ decking to support it. not only this but it helps stiffen the decking which reduces the probability  of it swaying vertically just like it did in the Tacoma Narrows Bridge example.

Raw Materials 

Many of the components of a suspension bridge are made of steel. The girders used to make the deck rigid are one example.Steel is also used for the saddles or open channels on which the cables rest atop a suspension bridge's towers.

When steel is drawn (streched) into wires, its strength increases,consequently,a relatively flexible bundle of steel wires is stronger than a solid steel bar of the same diameter. This is the reason steel cable is used to support suspension bridges.For the Akashi kaikyo Bridge a new low-alloy steel strengthended with silicon was developed;its tensile strength ( resistance against pulling forces) is 12% greater than any previous steel wire formulation. on some suspension bridges, the steels wires forming the cables have been galvanized (coated with zinc).

The towers of most suspension bridges are made of steel,although a few have been built of steel -reinforced concrete. 

#  Comparison of Suspension & Suspended type trail bridge:- 

S.N	Suspension type	Suspended types
1	Tower taking load bearing in this type and cables are above the tower.	No tower in this types & walkway cables carrying load loading.
2	Walkway width is 106cm in it.	Walkway width is of two types i.e 70 aand 100cm in it.
3	Both the banks should be equal levelled in this type.	Both banks may or may not be levelled in this type.
4	It is too expansive (20-25% more than suspended bridge).	It is less expensive as compared to suspension type.
5	It is ancident one & was used in eighteenth century in Europe.	It is less ancident one & was used later one.