Challenge 2

 

Question 1:-

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

 

Anwser:-

A bridge is a complex structure built to span physical obstacles such as bodies of water, valleys, or roads, and provide passage over them. The main components of a bridge are the foundation, substructure, and superstructure.

These components are :-

1. Foundation:

The foundation is the part of the bridge that rests on the ground and transfers the weight of the bridge to the underlying soil or rock. It is provided sufficiently deep so that it is not affected by the scouring.

The types of foundations used in bridge construction include:-

• Spread Foundation    : Suitable for bridges of average height built on firm and dry ground.
• Raft Foundation         : Used where the allowable bearing capacity of the soil is low and the bed of the watercourse contains soft clay and silt.
• Pile Foundation          : Used in conditions where there is very soft soil and hard strata are not available at a reasonable depth.
• Well Foundation        : Used when the bed has sandy soil and hard soil is available at 3-4 m below the level of the watercourse.
• Caisson Foundation   : Used where there is a hard stratum near the river bed and there is excessive water depth.

2. Substructure:

The substructure is the part of the bridge that supports the superstructure and distributes the load to the bridge footings and foundation. It includes:

• Abutments        : These are vertical supports at the ends of a bridge span that offer vertical and lateral support. They also act as retaining walls to resist the lateral movement of the earthen fill of the bridge approach.
• Piers                  : These provide intermediate support between two bridge spans. They mainly support the bridge superstructure element and transfer the load to the foundation.
• Wing Walls        : These are earth-retaining structures in the bridge located adjacent to the abutments and act as retaining walls.

3. Superstructure:

The superstructure is the part of the bridge that bears the load passing over it.

It includes:

• Deck Slab              : This is the portion that carries all the traffic.
• Girders/Beams     : These support the roadway and prevent bending. Girders have composed of I-shaped cross-sections with two load-bearing flanges and a web for stabilization.
• Bearings                : These devices support the parts of the superstructure and transfer loads and movements from the deck to the substructure and foundation.

Each of these components plays a crucial role in the overall stability and functionality of the bridge. The design and construction of these components depend on various factors such as the type of bridge, the materials used, the geographical location, and the specific engineering requirements of the project. It's always important to consult with a structural engineer or use appropriate design software for precise calculations.

Question 2:-

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

 

Answer:-

Superimposed Dead Load (SIDL) refers to the non-moveable (static) loads that are added to the structure after the construction is completed. These loads are not part of the structural components of the building or bridge. They include utilities, pavement, sidewalks, parapets, and other elements that are added onto the structure after the initial construction.

For a metro viaduct bridge, the following loads need to be considered as Superimposed Dead Load:

• Track and track bed   : The weight of the track and the materials used for the track bed are considered as SIDL.
• Utilities                       : Any utilities that are added to the bridge, such as water pipes, electrical lines, etc., are considered as SIDL.
• Pavement                    : If the bridge has a roadway, the weight of the pavement is considered as SIDL.
• Sidewalks                   : If the bridge has sidewalks, the weight of the sidewalks is considered as SIDL.
• Parapets and railings : The weight of any parapets or railings that are added to the bridge are considered as SIDL.
• Station structures      : If there are any station structures on the bridge, their weight is considered as SIDL.
• Other elements: Any other elements that are added to the bridge after the initial construction, such as signage, lighting, etc., are considered as SIDL.

 

Question 3:

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

 

Answer:-

There are several types of girders that can be used for a metro viaduct.

Here are some of them:

• Pre-cast post tensioned segmental box girder: These are pre-cast in factories and then assembled on-site. They are post-tensioned to ensure structural integrity.
• Pre-cast post tensioned segmental U-girder Superstructure: Similar to box girders, these are U-shaped and are also pre-cast and post-tensioned.
• Pre-cast prestressed I-girder: These are I-shaped girders that are pre-cast and then prestressed. They are commonly used for short and medium span bridges.
• Pre-cast pre tensioned Double U-girder/I-Girder: These are similar to U-girders and I-girders but are double in size.
• Steel Composite Girders: These are made from a combination of steel and concrete to utilize the benefits of both materials.
• Open Web Girders: These girders have an open web design which can reduce their weight and material cost.
• Continuous spans such as CLC etc.: These are girders that are designed to span continuously over multiple supports.

The choice of girder depends on various factors such as the span length, load requirements, local conditions, and design constraints.

 Question 4

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

Answer:

Bearings in a metro viaduct are mechanical components that support and reduce the friction between moving parts. They are crucial in the construction of metro viaducts as they allow for the necessary movements of the structures due to various factors such as temperature changes, traffic loads, and wind forces, while also ensuring the overall stability of the structure.

There are several types of bearings used in metro viaducts:

1. Elastomeric Bearings: These are the most common type of bearings used in bridge structures. They are made of rubber and are designed to deform under load to accommodate movements and rotations. 

   Advantages: They are simple, reliable, and require little to no maintenance. They are also relatively cheap and easy to install.

   Disadvantages: They have limited capacity for rotational movement and are not suitable for structures with high vertical loads or large horizontal movements.

2. Pot Bearings: These bearings consist of a confined elastomer disc, steel piston, and a pot. The elastomer disc allows for rotational movement while the pot and piston carry the vertical load.

   Advantages: They can accommodate larger rotations and are suitable for structures with high vertical loads.

   Disadvantages: They are more complex and expensive than elastomeric bearings. They also require regular maintenance and inspection.

3. Spherical Bearings: These bearings consist of a convex surface (calotte) and a matching concave surface (slide plate) that allow for rotational movement in any direction.

   Advantages: They can accommodate large rotations in any direction and are suitable for structures with complex load and movement conditions.

   Disadvantages: They are the most complex and expensive type of bearings. They require regular maintenance and inspection, and their installation requires high precision.

4. Sliding Bearings: These bearings allow for horizontal movement by sliding of one surface over another.

   Advantages: They can accommodate large horizontal movements.

   Disadvantages: They have limited capacity for rotational movement and are not suitable for structures with high vertical loads.

In conclusion, the choice of bearing type depends on the specific requirements of the metro viaduct, including the expected loads, movements, and rotations, as well as considerations of cost, maintenance, and durability.

 

 

Question 5:

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?

 

Answer:

Designing the height of metro viaducts across water passages requires careful consideration of various topographical factors. Here is a step-by-step checklist:

 

1.   Water Level  : The first step is to measure the average water level of the water passage. This will help in determining the minimum height of the viaduct.

 

2.   Flood Level  : It's also important to consider the maximum flood level that the area has experienced. The viaduct should be designed to withstand such conditions to prevent any damage or disruption in service.

 

3.   Ship Navigation  : If the water passage is used for ship navigation, the height of the tallest ship that passes through needs to be considered. The viaduct should be high enough to allow these ships to pass underneath without any hindrance.

 

4.   Topography of the Land  : The land on either side of the water passage should be studied. The height of the viaduct may need to be adjusted based on the elevation or depression of the land.

 

5.   Geological Survey  : A geological survey of the area should be conducted to understand the soil composition and rock structures. This can affect the stability of the viaduct and may influence its height and design.

 

6.   Environmental Impact  : The potential environmental impact of the viaduct should be assessed. This includes studying the local flora and fauna, and ensuring the viaduct does not disrupt their habitats.

 

7.   Future Projections  : Future projections for changes in water levels due to climate change or other factors should be considered. The viaduct should be designed to accommodate these potential changes.

 

8.   Legal Requirements  : Finally, any legal requirements or restrictions regarding the height of structures over water passages in the area should be taken into account.

 

Each of these steps plays a crucial role in ensuring the safety, functionality, and longevity of the viaduct.

 

Question 6:

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.

 

Answer:-

 

In metro viaduct construction, there are several types of girders that can be used, each with its own unique characteristics and applications. Here are some of the most common types:

 

1. Pre-stressed Concrete Girder: This type of girder is made by casting concrete around steel cables that are under tension. Once the concrete hardens, the tension is released, which compresses the concrete and helps it resist tensile forces. Pre-stressed concrete girders are strong and durable, making them ideal for heavy loads and long spans.

 

2. Steel Girder: Steel girders are made from high-strength steel and are often used in large, complex structures. They are strong, durable, and flexible, allowing them to withstand heavy loads and dynamic forces. However, they require regular maintenance to prevent corrosion.

 

3. Composite Girder: Composite girders are made from a combination of steel and concrete, taking advantage of the strengths of both materials. The steel component provides tensile strength, while the concrete component provides compressive strength. Composite girders are often used in bridges and other structures that require high strength and durability.

 

4. Box Girder: Box girders are a type of girder that have a hollow, box-like shape. They are typically made from steel or concrete and are used in large structures like bridges. Box girders are strong and rigid, making them ideal for long spans and heavy loads.

 

When choosing a girder type for a metro viaduct design, a designer must consider several factors. These include the load the structure will need to support, the span of the structure, the environmental conditions the structure will be exposed to, and the cost and availability of materials. The designer must also consider the construction methods and equipment available, as some girder types may require specialized construction techniques or equipment.

 

Question 7:

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

Answer:

1. Precast U girders are often chosen by metro designers in India due to several reasons. Firstly, they are manufactured in a controlled environment, which ensures high quality and durability. Secondly, they can be produced in large quantities and transported to the construction site, which speeds up the construction process. Lastly, they are cost-effective as they reduce the need for on-site labor and materials.

2. An open web structure is a type of structure that has a web-like design with multiple interconnected members. This design allows for the distribution of forces and stresses throughout the structure, making it more stable and resistant to loads.

3. The load flow behavior in an open web girder is such that the load is distributed evenly across the structure. The web members of the girder carry the shear forces while the top and bottom flanges carry the bending moments. This distribution of load helps in maintaining the stability of the structure.

4. As per IRS CBC, Table 11 (Clause 10.2.2), the stress limitations of SLS (Serviceability Limit State) are defined. My understanding of this table is that it provides the maximum allowable stress values for different materials and conditions to ensure the safety and serviceability of the structure. These values are crucial in designing a viaduct for metro rail construction as they help in determining the appropriate materials and design parameters.

5. Crack widths refer to the width of cracks that may form in a structure due to various factors such as load, temperature changes, shrinkage, etc. According to IRS CBC, there are three different design crack widths: permissible, tolerable, and maximum. Permissible crack width is the smallest crack width that is considered acceptable. Tolerable crack width is larger than permissible but does not cause any significant harm to the structure. Maximum crack width is the largest crack width that the structure can withstand without failing. These crack widths are important considerations in the design and maintenance of structures.

 

Question 8:

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

Answer:

An open web structure refers to a system where information is freely available and accessible to everyone. It is a decentralized approach to digital content, where no single entity has control over the entire network. This structure is the foundation of the internet as we know it today.

Advantages of an open web structure include:

1. Accessibility: The open web structure allows anyone with an internet connection to access information. This promotes inclusivity and equal opportunity.

2. Freedom of Expression: Since no single entity controls the open web, it provides a platform for freedom of speech and expression. 

 

3. Innovation: The open web structure encourages innovation as developers and users can build upon existing content without any restrictions.

4. Collaboration: The open web promotes collaboration as it allows users to share and exchange information freely.

5. Transparency: The open web structure is transparent, which means that it is possible to see how information is created and shared.

6. Resilience: The decentralized nature of the open web makes it more resilient to failures or attacks. If one part of the web goes down, the rest of the network remains unaffected.

 

Question 9:

Explain the load flow behavior in a open web girder. 

Answer:

Open web girders, also known as truss girders, are a type of structural element used in construction and engineering. They are designed to withstand loads and distribute them evenly across the structure. Here's a step-by-step explanation of the load flow behavior in an open web girder:

1. Load Application: The load is applied to the girder. This could be a static load (like the weight of the building materials) or a dynamic load (like wind or traffic on a bridge).

2. Load Distribution: The girder distributes the load evenly across its length. This is due to the triangular web configuration in the girder, which helps in evenly spreading out the forces.

3. Load Transfer: The top chord of the girder, which is under compression, transfers the load to the web members. The web members, which are designed to handle both tension and compression, then transfer the load to the bottom chord.

4. Load Absorption: The bottom chord, which is under tension, absorbs the load and transfers it to the supports (like columns or walls) at the ends of the girder.

5. Load Dissipation: Finally, the supports dissipate the load into the foundation of the structure, which then disperses it into the ground.

This process ensures that the load is effectively managed, preventing structural failure and ensuring the stability and safety of the structure.

 

Question 10:

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.

Answer:

Table 11 in the Indian Railway Standard Code of Practice for the Design of Substructures and Foundations of Bridges (IRS CBC) provides stress limitations for the Serviceability Limit State (SLS) of a structure.

The SLS refers to the condition where the structure is still functioning properly and meets the required serviceability criteria.

The table specifies the maximum allowable stresses for different materials and conditions. These stress limitations are crucial in the design of a viaduct for metro rail construction as they ensure that the structure remains within safe limits and performs adequately during service.

When designing a viaduct, engineers refer to Table 11 to determine the maximum allowable stresses for the materials used in the construction. This helps in selecting appropriate materials and designing the structural elements, such as beams, columns, and foundations, to ensure they can withstand the expected loads and environmental conditions.

By adhering to the stress limitations specified in Table 11, designers can ensure that the viaduct will have the necessary strength, stability, and durability to safely support the metro rail system and provide a reliable transportation infrastructure.

 

Question 11:

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

Answer:

Crack widths refer to the extent of visible separation on the surface of a concrete structure due to tension caused by applied loads, shrinkage, or changes in temperature. They are an important consideration in structural engineering because excessive crack widths can lead to durability issues, such as water ingress and corrosion of reinforcement.

The Indian Railway Standard Concrete Bridge Code (IRS CBC) provides guidelines for the design of concrete structures, including the control of crack widths. While the IRS CBC does not explicitly define three different design crack widths, it does provide guidance on limiting crack widths based on the exposure condition of the concrete structure. 

 

1. Mild Exposure Conditions: For structures in sheltered or indoor environments with mild exposure conditions, the crack width can be allowed up to 0.2 mm.

2. Moderate Exposure Conditions: For structures in areas with moderate exposure conditions, such as urban environments with normal weather conditions, the crack width should be limited to 0.2 mm.

3. Severe Exposure Conditions: For structures exposed to aggressive atmospheric conditions, such as coastal areas with high salinity, the crack width should be limited to 0.1 mm.

These limits are set to ensure the durability of the structure, prevent the ingress of harmful substances, and maintain the aesthetic appearance of the structure. The actual design process involves calculating the expected crack widths under service loads and comparing them with these limits. If the expected crack widths exceed the limits, the design must be modified, for example by increasing the amount of reinforcement, changing the concrete mix, or altering the structural form.