Challenge 2

Q1. What are the structural components of a bridge? Explain in detail about the foundation, sub structure and super structure components of the bridge.

Ans:

• The structural components of a bridge are:

1. Superstructure: Consists of Parapet/Railing and Girders.
2. Substructure: Consists of Bearing,Pier Cap,Pier and Pile Cap
3. Foundation: Consists of Piles

• Superstructure:
  • The portion which supports the deck slab and girder and connects one sub structure to the other. That means all the elements of the bridge attached to a supporting system can be categorized as superstructure.
  • The superstructure of the bridge structure consists of deck slab, girder, truss etc. These components vary based on the type of bridge (whether concrete or steel or composite). Superstructure of the bridge bears the load passing over it.
• Substructure:
  • The parts of the bridge which support the superstructure and transmit all the structural loads of the bridge to the foundations. For example, piers, abutments, etc.
  • The substructure of a bridge is the component that supports the structure and distributes the load to the bridge footings and foundation. They consist of piers, abutments, wing walls etc.
• Foundation:
  • Foundation is the portion which transmits loads to the bearing strata. Foundation is required to support the piers, bridge towers, portal frames. Generally, piles and well foundations such as H-pile, bore pile, pipe pile or precast concrete piles are adopted.

Q2. What is Super Imposed Dead Load means? What are loads that needs to be considered as SIDL for a metro viaduct bridge?

Ans:

• Superimposed dead loads are gravity loads that consist of other permanent items such as parapets and road surfacing and other non-structural and architectural attachments to the bridge. Such items are long term but might be changed during the lifetime of the structure. Similar to self-weight it is calculated as the product of volume and material density.
• SIDL Loads considered for a metro viaduct
  • Footpath loading
  • Permanent utility
  • Track loads

Q3. What are the different types of girders that can be used for a metro viaduct?

Ans:

• The bridge in which girders are used for supporting its deck is called the Girder bridge. This bridge functions by transferring the load coming from the deck to the piers and the abutments through the girders.

• Such girders can be either of the folllowings.

  • Rolled Steel Girder Bridge:

    • It is fabricated by rolling a blank cylinder of steel through a series of dies to create the desired shape.
    • 

  • Plate Girder Bridge:

    • A plate girder bridge is fabricated by welding plates together to create the desired shape. Usually, the large steel plates of desired thickness are cut into the flanges and webs from the plates in the desired length and shape.
    • 

  • Box Girder Bridge

    • This type of girder bridge offers high resistance to torsion and is mostly used when the bridge is likely to be subjected to torsion or toppling effects.
    • 
  • Feeder U girder
    • This type of girder specifically used for the case of metro constrcution. It is very ligth in weigth & economic also as compared to the conventional box girder.
    • 

Q4. What are bearings in a metro viaduct. Explain the types of bearings and analyses the types with respect to their advantages and disadvantages.

Ans:

• Bridge bearings are structural equipment or devices installed between bridge substructure and superstructure to transfer the applied various load.
• Different types of bearings are
  • Elastomeric Bearing- This bearing is also known as pot bearing which is commonly used for bridge construction. The purpose of the elastomeric bearings is to support a bridge or other heavy structure in a way that permits the load to shift slightly, in a horizontal direction, relative to the ground or foundation. Without such bearings, the bridge support might crack or fracture when it moves due to ground movements or thermal expansion and contraction. Elastomeric bearing pads compress on vertical load and accommodate both horizontal rotation and horizontal shear movement.
  • 
  • Pot-PTFE Bearing- POT PTFE BEARING is a bearing which carries vertical load by compression on an ealastomeric disc confined in a steel cylinder and which accomodates rotations by deformations of the disc .The elastomer under a triaxial pressure offers low resistance to deformation but high vertical stiffness.This bearing is more durable and more costlier. They can only be provided when elastomeric bearing cannot be provided.
  • Roller Rocker Bearing - This type of bearing consist of roller type of system near to the support. A rocker bearing is a support usually used in beams that are free to rotate or have no moment reaction and have reaction restraint on the horizontal and vertical axis.
  • 

Q5. During the construction of metro viaducts across water passages, explain the topographical check list that needs to be considered for the designing the height of the viaducts?

Ans:

• The following things needs to be checked during the topographical survey for fixing the height of the bridge
  • The HFL of the stream to fix the FRL
  • Overhead electric cables which have to be placed on some certain cleareance.
  • If the stream is a waterway then the height of the boats needs to be considered for fixing the FRL.

Q6. What are the different types of girders that can be used in a metro viaduct construction? Explain the differences of the girders and elucidate how a designer has to choose his girder type whilst designing.

Ans:

• Precast Prestressed I-girder- This is the most important part applied in superstructure which is I-girder. A designer has to see the span length and depending upon the span length the girders are placed.
• Precast Prestressed Box Girder- They are the Closed Web Structures which behaves very well in torsion loading in which shear due to torsion covers the larger area as is it closed web.
• Small segment box girder- These are the box girders which are divided into small parts because at some place we cannot lift the large box girders as it can disturb the traffic and surrounding so these small segments which is 3m in length can be used upto to 40-45 m span length.
• Precast Pretensioned feeder U-girders- The most favourable girders which are used is U-girders they are better in quality, they are open section. Placing this girders can save our time and it can be placed in one go. The span length of this girders are 25-28m . The sides of this girders can be used as parapet/railing.

Q7. Explain why the choice of metro designers would be precast U girders in India.

Ans:

• A U-girder will form an entire span of a viaduct between two pillars in a single piece of around 25m length, which will be precast in a casting yard and assembled on site. Box girders have many small precast segments, each about 2.5m long, which are assembled together between two pillars and hence take more time for construction.
• This is light in weight as compared to the conventional box gider.
• This is also economical than other systems.

Q8. What is an open web structure? Brief the advantages of the system.

Ans:

• The structures which are open from top level and webs are not connected to top slab is knows as Open Web Structure.

Advantages

In this case the ntire span of a viaduct between two piers in a single piece, whereas box girders will have many segments between two piers.

• • It is very less time consuming according the concventional box gider construction.
  • As the metro construction is generally occuring within the city so it can be done with running traffic condition.
  • The weigth of the U girder also very less compared to box girder.
  • Rather than this this u girder is very economical also.

Q9. Explain the load flow behavior in a open web girder.

Ans:

• In a open web girder it is saucer type girder which can be bend in longitudinal direction and in the transverse direction which means perpendicular to the movement of the trains.
• So the structure needs to be design this open web girder in saucer like bending which means it has to be designed for longitudinal bending and transverse bending.
• In longitudinal bending it should be design for flexure and for shear And in transverse direction we have to design it for flexure because the shear effects in shorter span are not too high.

Q10. As per IRS CBC, Table 11(Clause 10.2.2) the stress limitations of SLS, Explain your understanding of the table and how it correlated while designing a viaduct for metro rail construction.

Ans:

• As per the Table 11 (Clause 10.2.2) of IRS code the limitations of tensile and compressive stresses are as below.
• 
• In case of concrete if there is triangular compressive stress due to bending for RCC type of construction its 50%fck value which is 28days cube strength of concrete and if the same value for Uniform or Near Uniform compressive stress which is P/A under this condition the permissible value is 38%fck.
• In case of PSC to avoid the brittleness or sudden failure the limit for PSC is lower.
• For Reinforcement:
  • When it is compression tension type of loading in RCC the value would be 0.75fy.
• For Prestressing:
  • Only tension loadind will be there which is done by tendons only.

Q11. Explain your understanding of crack widths. Brief the three different design crack width with respect to IRS CBC.

Ans:

• Crack Width
  • The crack width of a flexural member is obtained by multiplying the maximum crack spacing by the mean strain of the flexural steel reinforcement.
• Allowable Crack Width as per IRS CBC
• 
  
  • Moderate Condition - Concrete surface protected against weather or aggressive conditions. Concrete surface sheltered from severe rain or freezing whilst wet. Concrete exposed to condensation, concrete structure continuously under water. Concrete in contact with non-aggressive soil/ground water.
  • Severe Condition - Concrete surface exposed to severe rain, alternate wetting and drying or occasional freezing or severe condensation. Concrete exposed to aggressive sub-soil/ ground water or coastal environment.
  • Extreme Condition - Concrete surface exposed to sea water spray, corrosive fumes or severe freezing conditions whilst wet. Concrete structure surfaces exposed to abrasive action, surfaces of members in tidal zone. All other exposure conditions which are adverse to exposure conditions covered above.