Bridges are heavy structures that are built when there is a necessity to cross over a natural obstacle, railway lines, urban infrastructure or water channels.
Bridges are varied in type, but most of them have almost the same components that complete it. These members can be structural or non-structural. The selection of the bridge type relies primarily on the design requirements of the physical nature of the problem it seeks to solve.
Sometimes, aesthetic requirements of a certain nature may guide the selection process. Other factors include:
· Feasible Method of construction
· Cost constraints
All bridges can be summarized into three main parts:
1. The foundation
2. Sub structure
3. Super structure
Substructure: as the diagram indicates, this category includes the crash barriers, piers, piercaps, pedestals, bearings, etc.
Foundation: This part lies beneath the ground-namely the open or pile type of foundation, the pile cap, etc.
Superstructure: the entire viaduct is the superstructure, the rails, rail pedestals, parapets, blisters, etc.
The FOUNDATION
Open Foundations
Open Bridge footings are similar to the shallow footings in buildings. These foundations are members with a shallow width in proportion to its height. It is
It is ensured that in its serviceability life, the footings effectively resist punching shear and direct shear transferred from piers above.this has to be mainly ensured by the depth of the footing and increasing the width based on the governing moment acting on it.
Piles
Piles are usually provided to give support to a bridge and make up the initial foundation. The piles help the weight and stresses applied by the bridge to be transmitted evenly through the ground making it stable and strong.
The material and design of a pile depend on several factors such as soil type, ground instability and load bearing capacity constraint. For bridges on rivers, scouring is also considered before the bridge is designed.
Piles are designed as a group. A single pile cap for a pier can have 3, 4 or more number of piles per pile cap. For heavy duty piers, a single pile cap may host as many as 12 or more piles.
Pile Caps
A pile cap is a thick REINFORCED concrete slab that is constructed on top of a group of concrete foundation piles as a part of a deep foundation of a bridge
Caps provide additional load transferring capacity to the piles. They are also known as pile caps as they are placed right on top of the pile foundation. Caps are often made of very heavy concrete to give maximum strength to the upper part of the bridge.
The SUBSTRUCTURE
Piers
Bridge Pier support the spans of the bridge and transfer the loads from superstructure to the foundation. Piers should be strong enough to take the both vertical and horizontal load. Its main function is to transfer the load from the bridge superstructure foundation below it.
They are subjected to huge axial loads and bi-axial moments and shear forces in transverse and longitudinal direction. Said that, pier are primarily the compression elements of a bridge and render an intermediate support link between two bridge spans.
Piers cross sections are as varied as per the aesthetic or structural requirement.
They can be circular, rectangular, elongated or be provided in more in a group for a single long piercap.
Piercap
Pier caps are flanged members of the substructure piers that host the girders via the bearing pedestals.
They usually host the bearing pedestals, seismic arrestors, launcher sleeves, drain holes, etc upon their arrangement.
Their role, primarily, is to transfer the loads from the superstructure to the piers. They do this by holding the bridge girders on bearing pedestals to disperse the loads from the bearings pads to the piers and thence to the footings.
Their design is based upon the type of pier cap that is designed i.e. RCC, PSC, etc.
Usually classified as bending members, they may have to be designed as corbels in place of a bending member, If the flange width is insignificant to the depth.
Please read the article on corbels to know more about corbel design.
Bearing pedestals
The structural member responsible for hosting the bearing is a bearing pedestal. Pedestals are rectangular members integrated as a part of the main piercap at the top. A small socket of about 100mm in depth is provided for the bearing to be properly fixed in place. Their dimensions are dependent upon the dimensions of the bearings.
The reinforcement is monolithically cast along with the pie and an additional bursting reinforcement is provided to prevent bursting of concrete due to incident loads.
In special cases, Pedestals are designed as transverse or longitudinal arrestors when the girder resting on them is an I section type girder. Hence, they double as arrestor for which special design considerations must be made.
For more on arrestors, read the article on Arrestors.
Bearings
Bearings are structural members capable of transferring loads from the deck to the substructure. These displace stresses and load to the piers through the girders to allow movement between parts of a bridge. The movement can be linear as well as torsional. Bearings provide allowance between these parts.
A bearing is an external component of a bridge that typically provides a contact interface to the bridge piers and the bridge girders.
The functions of a bearing is primarily to enable controlled movement to the entire viaduct span and dissipate the girder loads below to the pier. They have to be periodically be replaced by lifting the bridge deck in order to maintain the integrity of the bridge structure.
Deciding the appropriate bearing type for the span is of vital importance. This selection is based on a number of factors including loading conditions, length of bridge span, axis of restraint/freedom.
Elastomeric bridge bearing are perhaps the most common form of modern bridge bearing followed by neoprene bearings.
Read the article on bridge bearings to know more about bridge bearings.
Seismic arrestors
A seismic arrestor is characterised by its function to bear the impact of seismic forces on a bridge structure and the direction of forces it must bear.
For box girders, there is an opening within the soffit in the end segment of the box girder to accommodate this member. Arrestors are categorised as both transverse or as longitudinal arrestors in special conditions and both. The notch at the soffit of the box girder in the end segment adjusts itself to the arrestor dimensions with a minimum clearance of 50mm up to 150mm. The soffit being thick at this end span segment successfully dissipates the forces to the arrestor.
In the case of an arrangement of I-section girder of either steel or PSC, two seismic arrestors, one for each direction, are placed upon the piercap. They wither behave as a Corbel or a bending member.
Vertical bearings
Vertical bearings are bearings that are placed vertically as an interface between the girder and the seismic arrestors to avoid rupture of concrete. In case of box girders, it is placed in the end segment of the box girder viaduct.
A sub-structure that primarily resists lateral forces on either sides of bridge and serves as the support structure for the girders. The launch apron ends at the abutments. It serves as a retaining wall for the backfill from the launch apron.
These are the structural link between the road and the bridge structure. They support the launch apron or one edge of an approach slab and on the other side support the bridge girders.
All Single-span bridges have abutments at each of their extreme ends. These provide vertical and lateral support for the span, as well as acting as retaining walls to resist lateral movement of towards the bridge launch approach.
Wing Walls
The walls that flare out of the bridge diagonal or perpendicular to the main span of the bridge are wing walls. They are categorized as backfill retaining structures. Their purpose is to retain the earth fill at the approach embankment and direct the stream through the piers downstream.
In a bridge structure, the adjacency of the wing walls to the abutments enhances their role as retaining walls. Usually provided on both the sides, in both upstream and downstream directions, they are generally constructed of the same material as those of abutments. The wing walls can either be attached to the abutment or be independent of it. Their design depends upon the nature of the embankment.
Crash barriers
A Crash barrier is a structural strengthening at the bottom of urban piers.
Vehicles impact the piers in case of an accident, this will impart a sudden impact force on the bottom of the pier. Thus, an extra provision for bracing against such a force must be made. Crash barriers are aesthetically the increased thickness of the pier section at this part with additional reinforcement.
The SUPERSTRUCTURE
Girders
Girders are the most essential part of the bridge are the structural members that support the deck slab for the traffic. They can be likened to the beams constructed in buildings. Girders based on their type can be a single span simply supported or, or even multiple spans continuing for more than one span.
The material of construction is either of:
· PSC Concrete
· Pre-Fabricated Concrete
· Steel Plates
· Steel Truss
Based on the type of supporting condition of girders, bridges are either simply supported, cantilevered, fixed, etc. the orientation of the girders is along the longitudinal axis of the structure.
In some bridges, the main girders work together with the deck slab to resist loads. Bridges based on such a principle are known as ‘Composite girder bridges’, and bridge structures designed to resist loads with the main girders only are called non-composite girder bridges.
Types of girders
· I section girders
· Box girders
· U section girders
· Steel girders
· Truss girders
Crash Parapets
Crash parapets are the protective walls on wither sides of the bridge girder. They are positioned as a safety and protective measure and may vary in shape as per the aesthetic or streamlined requirements. Most of these members carry utility trays that carry fixtures, ropes, rails, fences, or other conduits.
Deck slab
The flat slab surface that allows vehicles and pedestrians to cross highways is a deck slab. It is an orthotropic slab member supported by the girders in composite bridges. For bridges with post-tensioned box girder viaducts, the top flange acts as the deck slab.
For steel bridges, this deck might be fabricated with plates and directly bear vehicular loads and to contribute to the bridge structure's overall load-bearing behavior.
The section of the deck slab has a cross slope that enables suitable drainage channeled to the hose.
Deck slabs are either of the following type based on the type of structure:
1. RCC deck slabs
2. Steel deck slabs
3. Composite deck slab
4. PSC deck slabs
Diaphragm
A diaphragm member is a structural concrete wall constructed at certain intervals laterally along a bridge girder system.
This is essentially the laterally supporting structure that resists lateral forces and transfers loads to girders
Their main function, like that of the bracings, is to augment the overall stiffness of the entire structure. Thus during earthquakes and construction, they render vital lateral stability to the entire structure.
They have to be separately designed and laterally analyzed. Their design principle is based on the respective Strut and tie model generated.
Diaphragms placed at the end of the girders are called as the ‘end diaphragms’.
In end diaphragms, an additional notch is provided at the soffit at one or more locations. This notch transfers lateral forces to the substructure via the seismic arrestor. This type of an arrangement is more prominent in the arrangement of girders with I section geometry either concrete or steel. They can be about 250mm in thickness or more depending upon their intervals and stiffness requirements.
Bracings
A bracing system is a transverse member that connects the girders and provides additional stiffness. It is also called as lateral bracing system and helps keep the top chords of the steel plate girders from bending or deforming in or out for steel bridges. Quintessentially, it is a stiffness member and thereby a vital element of the bridge superstructure.
They stabilize the main girders during construction, and contribute to the distribution of load effects.
“The stiffness of the bracing system is the stiffness of the entire bridge structure”
Main purpose of this member is to:
· Inhibit buckling of the main beams
· Distribute the loads Load distribution
· Provide stiffness to girder arrangement
For more details, read the article on bracings here
Rail plinth
The elongated precast concrete members that supports the rails and their attachment accessories are rail plinths.
In order to avoid rebuilding rail plinth frequently and having to disrupt the operational traffic, they have a design service life of about 100 years. A higher grade of concrete is ensured along with an attention to reinforcement detailing.
Expansion Joint
A bridge expansion joint is a gap/opening within the bridge deck which allows the concrete to expand and contract under temperature, load variations. It forms a break between the concrete and other parts of a structure to allow movement without causing stress, which can lead to cracking. They should be used in large concrete slabs such as foundations and concrete driveways.
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