Challenge 6
1. What is elastomeric bearings? And what is made of and the uses of them.
Elastomeric bearings are a type of structural support element used in various engineering and construction applications. They are designed to provide flexibility and absorb dynamic loads, vibrations, and movements between different parts of a structure, such as bridges, buildings, pipelines, and industrial machinery. These bearings are commonly used to accommodate thermal expansion, contraction, seismic activity, and other types of movement.
Elastomeric bearings are typically made from layers of elastomeric material, which is a type of synthetic or natural rubber with high elasticity and resilience. The layers are often reinforced with steel plates or fabric inserts to provide additional strength and load-bearing capacity. The elastomeric material allows the bearings to deform and absorb energy under load, while the reinforcement prevents excessive stretching and tearing.
Uses of elastomeric bearings:
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Bridges: Elastomeric bearings are commonly used in bridge construction to allow for movement caused by temperature changes, seismic activity, and traffic loads. They provide flexibility and reduce stress on the bridge structure.
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Buildings: In buildings, elastomeric bearings are used to isolate vibrations and absorb movements caused by wind, earthquakes, and settlement. This helps protect the building's structural integrity and enhances occupant comfort.
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Industrial Equipment: Elastomeric bearings are used to support heavy industrial equipment, such as generators, turbines, and pumps. They help dampen vibrations and prevent excessive wear and tear on the equipment and the surrounding infrastructure.
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Pipelines: In the oil and gas industry, elastomeric bearings are used to support pipelines and accommodate ground movement, thermal expansion, and contraction. This prevents stress and damage to the pipelines.
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Seismic Protection: Elastomeric bearings play a crucial role in seismic isolation systems, which are designed to minimize the impact of earthquakes on structures. These bearings allow buildings and other structures to move independently of the ground motion, reducing the transmission of seismic forces.
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Expansion Joints: Elastomeric bearings are often used in expansion joint systems to allow movement between adjacent parts of a structure, such as between different sections of a bridge.
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Railway Tracks: They can be used in railway track systems to dampen vibrations and absorb dynamic forces caused by passing trains.
Elastomeric bearings are chosen for their durability, cost-effectiveness, and ease of installation. They are designed to withstand various environmental conditions and provide long-lasting performance. However, their effectiveness and lifespan depend on proper design, installation, and maintenance.
2. What are the importance of elastomeric bearing in the metro viaduct?
Elastomeric bearings play a crucial role in metro viaduct construction for several important reasons:
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Vibration and Impact Absorption: Metro systems involve the movement of heavy trains at high speeds along elevated tracks. Elastomeric bearings absorb and dampen the vibrations generated by these moving trains, preventing them from being transmitted to the surrounding structures and minimizing the impact on nearby buildings and the environment.
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Dynamic Load Distribution: Elastomeric bearings distribute the dynamic loads caused by the moving trains and passengers across the viaduct's supporting piers and columns. This prevents localized stress concentrations that could lead to premature deterioration of the structure.
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Thermal Expansion and Contraction: Metro viaducts are exposed to temperature variations due to changes in weather and sunlight. Elastomeric bearings allow the viaduct to expand and contract with temperature fluctuations, preventing the build-up of stress that could lead to structural damage.
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Seismic Protection: In areas prone to earthquakes, elastomeric bearings provide a degree of seismic isolation by allowing controlled movement of the viaduct during an earthquake. This reduces the transfer of seismic forces to the viaduct and helps protect it from damage.
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Settlement and Differential Movement: Metro viaducts are often constructed over different types of terrain, which can lead to settling and differential movement. Elastomeric bearings accommodate these movements without causing damage to the viaduct's structural integrity.
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Longevity and Reduced Maintenance: By absorbing and accommodating various types of movement and loads, elastomeric bearings help extend the lifespan of the metro viaduct. This reduces the need for frequent maintenance and repairs, leading to cost savings over time.
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Passenger Comfort: The use of elastomeric bearings contributes to a smoother and more comfortable ride for passengers. Vibration and noise transmitted to the train cars are minimized, enhancing the overall experience for commuters.
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Protection of Infrastructure: Elastomeric bearings help protect the viaduct's components, such as piers, columns, and joints, from excessive wear and tear caused by repeated movement and loading. This extends the life of these components and ensures the long-term stability of the viaduct.
In summary, elastomeric bearings are essential components in metro viaduct construction because they provide flexibility, absorb dynamic loads, dampen vibrations, and accommodate movement. Their use enhances the safety, longevity, and performance of metro viaducts while contributing to passenger comfort and the protection of surrounding structures and the environment.
3. What are the design input parameters need for the designing of the elastomeric bearings? Explain the inputs that shall be provided by the vendor/material supplier?
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Load Capacity and Load Distribution: The load capacity of the elastomeric bearing needs to be specified to ensure that it can safely support the anticipated loads from the structure, such as dead loads, live loads, and dynamic loads from traffic or machinery. The distribution of these loads across the bearing's surface also needs to be accounted for in the design.
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Vertical and Horizontal Movement: The maximum vertical and horizontal movements that the bearing will need to accommodate should be provided. This includes movements due to thermal expansion/contraction, settlement, seismic events, and other factors that might affect the structure.
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Environmental Conditions: Information about the environmental conditions at the installation site is important. This includes factors such as temperature ranges, exposure to sunlight, humidity, and potential exposure to chemicals or other corrosive substances.
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Seismic Design Parameters: If the structure is located in a seismic zone, the relevant seismic design parameters must be provided. This includes the seismic zone category, design ground motion parameters, and any specific requirements for seismic isolation.
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Structure Type and Geometry: Details about the structure that the elastomeric bearing will support are necessary. This includes the type of structure (bridge, building, etc.), the dimensions, geometry, and configuration of the bearing's location within the structure.
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Bearing Layout and Arrangement: The layout and arrangement of elastomeric bearings within the structure need to be specified. This includes the number of bearings, their positioning, and any variations in bearing properties across the structure.
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Temperature Tolerance: The expected temperature range that the elastomeric bearings will be subjected to should be provided. This helps in selecting elastomeric materials that can withstand the temperature variations without compromising performance.
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Durability Requirements: The desired lifespan and durability of the elastomeric bearings should be communicated. This will influence the selection of appropriate materials and construction methods to ensure the bearings' longevity.
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Load-Deformation Characteristics: The load-deformation behavior of the elastomeric material, including its stiffness, compression and shear properties, should be provided. This information helps in designing the bearing's response to various loads.
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Safety Factors and Design Codes: The required safety factors and relevant design codes or standards that the elastomeric bearings must comply with should be shared. This ensures that the bearings are designed to meet safety and regulatory requirements.
Inputs provided by the vendor/material supplier:
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Material Specifications: The vendor should provide detailed specifications of the elastomeric material being used, including its mechanical properties, chemical composition, and physical characteristics.
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Manufacturing Process: Information about how the elastomeric bearings are manufactured, including any quality control measures and testing procedures, should be provided by the supplier.
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Installation Guidelines: Clear guidelines on how to properly install and maintain the elastomeric bearings should be included. This ensures that the bearings are installed correctly to achieve their intended performance.
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Technical Support: Vendors should offer technical support and expertise to assist in the design process, answer any questions, and address concerns related to the elastomeric bearings.
By providing accurate and comprehensive information, both the designer and the vendor can work collaboratively to ensure that the selected elastomeric bearings are suitable for the specific application and will perform reliably over the intended lifespan.
When designing bearings, especially elastomeric bearings used in structural applications, several types of design loads need to be considered to ensure the stability, safety, and performance of the structure. These loads can vary based on the type of structure and its intended use. Here are some common design loads that are considered:
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Dead Load: Dead load refers to the static weight of the structure itself and any permanent fixtures, such as walls, floors, beams, and columns. It includes the weight of construction materials, finishes, and other permanent elements.
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Live Load: Live load is the variable load that the structure is designed to support during its use. For example, in buildings, it includes the weight of occupants, furniture, equipment, and any other movable loads. In bridges, it includes the weight of vehicles and pedestrians.
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Dynamic Load: Dynamic loads are forces that change rapidly or periodically, such as those generated by moving vehicles, machinery, or wind. These loads can induce vibrations and impact on the structure and its bearings.
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Seismic Load: In regions prone to earthquakes, seismic loads must be considered. These loads are caused by ground motion during an earthquake and can vary based on the seismic hazard level of the area.
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Wind Load: Wind loads account for the force exerted by wind on the structure. The magnitude of wind load depends on factors such as wind speed, building shape, and local topography.
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Temperature Effects: Temperature changes can cause thermal expansion and contraction of the structure, inducing forces and movements. Bearings need to accommodate these temperature-related effects.
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Accidental Loads: Accidental loads refer to unexpected events that might impact the structure, such as vehicle collision, explosions, or impact from falling objects. Bearings may need to be designed to absorb or distribute such loads.
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Fluid Loads: In structures like bridges, fluid loads from sources like water flow or hydrostatic pressure need to be considered, especially if the structure is over or near water bodies.
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Snow Load: In regions with snowfall, the weight of accumulated snow on the structure must be taken into account. This load varies depending on factors such as snow density and local regulations.
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Construction Loads: During the construction phase, temporary loads due to construction equipment, materials, and workers need to be considered. These loads can be significant and affect bearing design.
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Soil Settlement and Foundation Movements: The bearing's response to soil settlement, differential settlement, and other foundation movements must be accounted for to ensure that the structure remains stable.
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Prestressing or Post-Tensioning Loads: In structures that use prestressing or post-tensioning, the bearing design needs to consider the additional forces introduced by these methods.
The combination of these various loads and their potential interactions can influence the design and selection of bearings, as well as other structural components. It's essential to accurately assess and account for these loads to ensure the safety, durability, and functionality of the structure and its associated bearings.
5.Design an elastomeric bearing considering following design parameters and design data:
| Design Parameter | ||||
| Span Length | L | = | 27 | m |
| Bearing to Bearing Length | Lo | = | 25 | m |
| Young's modulus of concrete for M60 | E | = | 36000 | Mpa |
| Static Shear modulas of elastomer | G | = | 0.9 | Mpa |
| Dynamic Shear modulus of elastomer | Gs | 1.8 | Mpa | |
| Inertia os section ( I Beam only ) - At mid span | I | = | 0.27835 | m4 |
| Inertia os section ( I Beam+Slab ) - At mid span | I | = | 0.62468 | m4 |
| Max reaction due to total dead load ( From Grillage Model) | PDL | = | 51 | T |
| Max reaction due SIDL ( From Grillage Model) | PSIDL | = | 39 | T |
| Max reaction due Live Load + Impact ( From Grillage Model) | PLL | = | 33 | T |
| Maximum rotation due to live load ( From Grillage Model) | αLL | = | 0.002 | rad |
| Rotation due to ( DL + Slab + X-girder) ( From Grillage Model ). | αDL | = | 0.004 | rad |
| Rotation due to ( SIDL ) ( From Grillage Model ). | αSIDL | = | 0.003 | rad |
| Creep Coefficient | ecr | = | 4.30E-05 | /Mpa |
| Translation due to Creep | ucr | = | 7.28 | mm |
| Shrinkage coefficient | eSH | = | 3.00E-04 | |
| Translation due to shrinkage | USH | = | 4.05 | mm |
| Coefficient of thermal expansion | eT | = | 1.17E-05 | |
| Translation due to thermal expansion | UT | = | 5.53 | mm |
| Translation due to BR/TR= F/K ( K = Stiffness of bearing = G*A/T) | UBR/TR | = | 11.33 | mm |
| Max translation (+) = UT + UBR/TR | Umax (+) | = | 16.86 | mm |
| Max translation (+) = Ucr + USH + UT + UBR/TR | Umax (-) | = | 28.19 | mm |
| Design Data | ||||
| Dead Load | PDL | = | 440782 | N |
| SIDL | PSIDL | = | 144762 | N |
| Total Slow Vertical Load | PC | = | 585544 | N |
| Rapid Max Vertical Load | Psmax | = | 0.00 | N |
| Rotation in Longitudinal direction. due to slow loads | αc | = | 0.01 | rad |
| Rotation in Longitudinal direction. due to rapid loads | αs | = | 0.00 | rad |
| Rotational due to position tolerance | αo | = | 0.00 | rad |
| Translation in Longitudinal direction. due to slow loads | UC | = | 16.86 | mm |
| Translation in the Longitudinal direction. due to rapid loads | US | = | 0.00 | mm |
| Total Translation in Longitudinal dir. | U | = | 16.86 | mm |
| Dimensioning of Bearing | ||||
| Overall width of bearing in longitudinal direction | a | = | 420 | mm |
| Overall width of bearing in Translation direction | b | = | 420 | mm |
| Thickness of Individual layer of elastomer | ti | = | 12 | mm |
| Thickness of top/bottom layer of elastomer | te | = | 6 | mm |
| Total number of internal layer | n | = | 5 | |
| Thickness of steel laminate | ts | = | 4 | mm |
| Total plan area, A=a*b | A | = | 176400 | mm2 |
| Effective plan area, A' = (a-U) x b | A' | = | 169318.8 | mm2 |
| Total Thickness of Elastomer Bearing | T | = | 72 | The factor |
| Factor of sefserviceability | ||||
| Factor for servicibility state (SLS) | ||||
| Dead Load | = | 1 | ||
| SIDL | = | 1.2 | ||
| Live Load | = | 1 | ||
| Factors for fracture state (ULS) | ||||
| Dead Load | = | 1.2 | ||
| SIDL | = | 1.8 | ||
| Live Load | = | 1.3 | ||
| Partial safety factor of steel at Limite State | gs | = | Yield | |
| Yeild Strength of Steel | σa | = | 240 | Mpa |
| Max. Permissible compressive stress in bearing | σo max | = | 11 | Mpa |
| Min. Permissible compressive stress in bearing | σo min | = | 2 | Mpa |
Design of Bearing:
A.Mean Pressure Serviceability State (Clause 4.3.31)
σm = P/A' ≤ 11 Mpa = 4.35690 MPa
where,
P= PC + PS
CONDTION VERIFIED.
B.No Slip Serviceability State (Clause 4.3.3.2)
B1 σm mini = Pmini / A ≥ 2.0 Mpa = 4.314080 MPa
B2. H f=0.12+0.2/ σm For Slow loads: T>(G*A*Uc)/(0.12 Pc + 0.2A) T > 25.3606186 mm For Rapid Loads: T>G*A*(Uc + 2Us) / (0.12 Pc + 0.2A) T > 25.3606186 mm CONDTION VERIFIED. C.Limitation of Distortion - Serviceability State (Clause 4.3.3.3) tH/ G ≤ 0.7 T > 24.08571429 mm or T > (Uc + Us) / 0.7 CONDTION VERIFIED. D. No uplift at the edge with the least load- Fracture Load ( Clause 4.3.3.4) D.1 tanα < 6> For internal layer Si= ab/ 2ti (a+b) Si = 8.75 σmc = Pc/ A' σmc = 3.458233817 Mpa eic1= tiσmc / (4GSi2 + 3σmc) ∑eic1 = 0.14510087 mm For top & bottom layer Si = ab / 2te (a+b) Si = 17.5 eic2= tiσmc / (4GSi2 + 3σmc) ∑eic2 = 0.037289738 Total eic ( ∑eic1 + ∑eic2) ∑eic = 0.182390609 Therefore 6∑eic/a = 0.00260558 Tanαc = (αC + αO) Tanαc = 0.01 CONDITION VERIFIED. D.2 tan αc + 1.5 tan αs < 6> αm = ( Pc + Ps ) / A' σm= 3.4582338 Mpa For internal layer ei1 = tiσmc/(4GSi2 + 3σm) ∑ei1 = 0.145100871 mm For top & bottom layer ei2 = tiσmc/(4GSi2 + 3σm) ∑ei2 = 0.037289738 tanαc + 1.5 tanαs 6∑ei/a = 0.013027901 = 0.01 CONDITION VERIFIED. Shear Strength of Plate E. No buckling (Clause 4.3.3.5) a/10 ≤ T ≤ a/5 T= 72 mm a/10 = 42 mm a/5 = 84 mm CONDITION VERIFIED. F. Shear-Serviceability State (Clause 4.3.3.6) tp + th+ ta < 5G>
tp = 1.5 ( PC + 1.5 Ps) / (Si a b) tp= 0.569041788 Mpa tH = (GUc + GsUs) / T tH = 0.21075 Mpa ta = G a^2 ( tanαc +1.5tanαs) / (2 T ti ) ta = 0.91875 Mpa tp + th + ta = 1.70 Mpa 5G = 4.5 Mpa CONDITON VERIFIED. G. Strength of Plate- Fracture State (Clause 4.3.3.7) ts > a/Si. Sm/Sa. Gs a/Si = 48 mm Sm/Sa = 0.0144093 gs = 1.2 a/Si. Sm/Sa. Gs = 0.82997611 mm ts = 4 mm CONDITION VERIFIED. 6. What are the other types of bearings used in a design of metro viaduct? Explain the locations where these are used. Conditions where elastomeric bearings used? In the design of a metro viaduct, various types of bearings are used to accommodate different types of movements, loads, and structural configurations. Here are some of the common types of bearings used in metro viaduct designs, along with explanations of where they are typically used: 1. Elastomeric Bearings: Elastomeric bearings are made of layers of rubber-like material (elastomer) reinforced with steel plates. They provide flexibility to absorb movements such as rotation, translation, and deformation. Elastomeric bearings are often used in areas with moderate movements and where there is a need to reduce vibrations. They are commonly used in metro viaducts at locations where small rotations, translations, and lateral movements are expected, such as expansion joints and areas with moderate thermal effects. 2. Pot Bearings: Pot bearings consist of a concave pot-like structure that holds a spherical or cylindrical bearing pad. These bearings can accommodate rotations in multiple directions and provide vertical load-carrying capacity. Pot bearings are suitable for areas with larger rotations and translations. They are often used in curved sections of metro viaducts, where the structure requires flexibility to accommodate curvature-induced movements. 3. Sliding Bearings: Sliding bearings allow longitudinal movement along the bridge axis while restricting other movements. They consist of low-friction materials that facilitate sliding. Sliding bearings are typically employed in areas where the metro viaduct is expected to undergo significant longitudinal expansion and contraction due to temperature variations. 4. Roller Bearings: Roller bearings consist of cylindrical rollers that allow rotation and limited translation. They are used to accommodate rotations at fixed supports. Roller bearings are suitable for use in areas where the metro viaduct has piers or supports that need to allow for rotations. 5. Spherical Bearings: Spherical bearings allow both rotation and translation in multiple directions. They are particularly useful at locations where large rotations and complex movements need to be accommodated. Spherical bearings can be employed at the connections between different segments of a viaduct or where complex geometries are present. Conditions for Using Elastomeric Bearings: Elastomeric bearings are commonly used in metro viaduct designs under the following conditions: Moderate Movements: Elastomeric bearings are suitable for areas with moderate rotations, translations, and lateral movements. They provide flexibility to accommodate these movements without inducing excessive stresses. Vibration Isolation: Elastomeric bearings have damping properties that can help isolate vibrations caused by train movements and external forces. This makes them suitable for reducing vibrations in urban environments. Thermal Effects: Elastomeric bearings can absorb thermal expansion and contraction movements, making them ideal for areas with temperature variations. Expansion Joints: Elastomeric bearings are often used in expansion joints where the viaduct structure needs to accommodate movements caused by temperature changes and other factors. Simplified Maintenance: Elastomeric bearings are relatively easy to install and maintain, which can be advantageous in metro viaducts where maintenance access might be limited. In summary, elastomeric bearings are commonly used in metro viaducts to provide flexibility, absorb movements, and reduce vibrations in areas with moderate movements and conditions such as expansion joints and moderate thermal effects. Other types of bearings are chosen based on the specific structural requirements and movement patterns of the viaduct. 7. Why are PSC piers caps preferred over RCC pier caps in a metro viaduct? Pre-Stressed Concrete (PSC) pier caps are often preferred over Reinforced Concrete (RCC) pier caps in metro viaduct construction due to several advantages that PSC offers in terms of structural performance, durability, and cost-effectiveness. Here are some of the key reasons why PSC pier caps are preferred: 1. Higher Strength and Durability: PSC is a type of concrete that is pre-stressed with tensioned steel strands or wires before it is subjected to loads. This pre-stressing imparts compressive stresses to counteract the tensile stresses that develop when the structure is loaded. As a result, PSC pier caps exhibit higher strength and durability compared to conventionally reinforced RCC pier caps. 2. Reduced Material Consumption: PSC allows for higher design strengths, which means that smaller sections can be used to achieve the same load-bearing capacity as larger RCC sections. This results in reduced material consumption and lighter structures, which can be beneficial for elevated viaducts where minimizing dead load is important. 3. Longer Spans: PSC allows for longer spans between supports compared to RCC due to its higher strength-to-weight ratio. This can lead to fewer piers, resulting in reduced visual clutter, improved aesthetics, and reduced interference with the surroundings. 4. Crack Control: The pre-stressing in PSC pier caps helps control cracking, as the initial compression counteracts tensile stresses that could lead to cracking. This is especially important in environments like metro viaducts, where exposure to cyclic loading, temperature variations, and vibration is common. 5. Reduced Maintenance: PSC pier caps are less susceptible to environmental degradation, corrosion, and fatigue due to their higher durability. This leads to reduced maintenance needs over the lifespan of the structure, which is crucial in high-traffic and urban transit systems like metro viaducts. 6. Constructability: The use of pre-stressing in PSC pier caps allows for faster construction since the pre-stressing process can accelerate the curing of concrete. This can be advantageous in metro viaduct projects that need to minimize disruption to urban traffic and services. 7. Aesthetics: PSC pier caps can be designed with sleeker profiles due to their higher strength. This can enhance the visual appearance of the metro viaduct, contributing to a more modern and aesthetic urban landscape. 8. Long-Term Performance: PSC pier caps provide consistent long-term performance over their lifespan due to their inherent durability and resistance to environmental factors, making them a reliable choice for metro viaducts that serve as critical transportation infrastructure. It is important to note that the decision to use PSC pier caps over RCC pier caps depends on various factors including project requirements, site conditions, design considerations, and cost implications. Both materials have their own merits, and the choice should be based on a comprehensive evaluation of these factors in the context of the specific metro viaduct project. Pier caps play a crucial role in the construction of bridges and elevated structures, including metro viaducts. They are an integral part of the support system for these structures and serve several important purposes. Pier caps are typically placed on top of bridge piers or columns and provide a stable platform for transferring loads from the superstructure (such as beams or girders) to the substructure (the piers or columns). Here is an explanation of the basic purpose of pier caps and the components that make up pier caps: Basic Purpose of Pier Caps: Load Distribution: One of the primary functions of pier caps is to evenly distribute the loads coming from the superstructure to the supporting piers or columns. This distribution helps prevent concentrated stresses that could lead to structural failures. Stability: Pier caps provide stability to the piers or columns by serving as a connecting element between them. They ensure that the individual piers work as a cohesive unit to support the superstructure above. Alignment and Leveling: Pier caps help in maintaining the alignment and leveling of the superstructure. They ensure that the superstructure elements such as beams or girders rest on a consistent and stable platform. Transfer of Forces: Pier caps transfer the vertical loads, lateral loads, and moments generated by the superstructure to the piers or columns below. This prevents excessive deflections and ensures proper load-carrying capacity. Components of Pier Caps and Their Uses: Pier caps consist of various components that work together to fulfill their purpose. The specific design and components of pier caps can vary based on the structural requirements and the design approach. Here are some common components and their uses: Top Slab: The top slab is the uppermost component of the pier cap and serves as the direct load-bearing platform for the superstructure. It transfers the loads to the supporting piers or columns. Diaphragms or Tie Beams: Diaphragms or tie beams are horizontal elements that connect the sides of the pier cap. They contribute to the lateral stability of the pier cap and help distribute loads to the piers. Transverse Beams: Transverse beams are perpendicular to the main beams and help in distributing loads evenly across the width of the pier cap. They enhance the structural integrity and load-carrying capacity. Main Beams or Girders: Main beams or girders are the primary load-bearing members of the pier cap. They support the superstructure elements and transfer the loads from the superstructure to the substructure. Reinforcement: Steel reinforcement is embedded within the concrete components of the pier cap to provide tensile strength and enhance the overall structural performance. Prestressing Elements (in PSC): In pre-stressed concrete (PSC) pier caps, pre-stressing tendons or cables are used to introduce pre-compression into the concrete. These elements help counteract tensile stresses and improve the pier cap's durability and load-carrying capacity. Expansion Joints: In some cases, expansion joints may be incorporated into the pier cap to accommodate thermal expansion and contraction, reducing the potential for cracking. Bearings: In some designs, bearings may be integrated into the pier cap to provide flexibility and movement between the superstructure and substructure. This is more common in cases where movements due to temperature variations or other factors are significant. Pier caps are integral to the overall structural integrity and functionality of elevated structures like metro viaducts. Their design and components are carefully considered to ensure that the loads are effectively transferred, stability is maintained, and the structure can perform safely and reliably over its intended lifespan.