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Seismic Analysis and Design Methods for Bridges and Retaining Walls Based on IS 1893 Part 3 2002



Introduction




Earthquakes are natural phenomena that can cause severe damage and loss of life to structures and infrastructure. Therefore, it is essential to design structures that can resist earthquake forces and effects without collapsing or losing their functionality. In India, where earthquakes occur frequently in many regions, there is a need for a comprehensive and consistent standard that can provide guidelines and criteria for earthquake resistant design of structures.




Is 1893 Part 3 2002.pdf



IS 1893 Part 3 2002.pdf is a document that contains such a standard. It is part of the Indian Standard IS 1893: Criteria for Earthquake Resistant Design of Structures, which has five parts covering different aspects and types of structures. Part 3 specifically deals with bridges and retaining walls, which are important components of transportation systems and civil engineering projects.


The main features and objectives of IS 1893 Part 3 2002 are:



  • To provide a uniform basis for seismic design of bridges and retaining walls in India



  • To account for the regional variations in seismic hazard and soil conditions in India



  • To adopt a performance-based approach for seismic design and evaluation of structures



  • To incorporate the latest developments and research findings in earthquake engineering



  • To harmonize with other parts of IS 1893 and other relevant codes and standards



Scope and Applicability of IS 1893 Part 3 2002




IS 1893 Part 3 2002 applies to all types of bridges and retaining walls that are designed for vehicular or pedestrian traffic or both. It covers bridges with different spans, materials, configurations, foundations, bearings, joints, etc. It also covers retaining walls with different heights, shapes, materials, backfills, etc.


The standard assumes that the structures are designed for normal loads and service conditions, and that the seismic forces and effects are additional to those. It also assumes that the structures are located on firm or medium soil, and that the effects of liquefaction, landslides, fault movements, etc. are negligible. For structures that do not satisfy these assumptions, special studies and considerations are required.


The standard is intended to be used in conjunction with other parts of IS 1893 and other codes and standards that are relevant for the design of bridges and retaining walls. For example, IS 456 for concrete structures, IS 800 for steel structures, IS 883 for timber structures, IS 1343 for prestressed concrete structures, etc. The standard also refers to other codes and standards for specific topics, such as IS 1893 Part 1 for general provisions and buildings, IS 1893 Part 4 for dams and embankments, IS 13920 for ductile detailing of reinforced concrete structures, etc.


Seismic Zoning and Design Basis Earthquake




IS 1893 Part 3 2002 divides India into four seismic zones based on the expected intensity and frequency of earthquakes in each region. The zones are:



ZoneDescriptionZone Factor (Z)


IILow damage risk zone0.10


IIIModerate damage risk zone0.16


IVSevere damage risk zone0.24


VVery severe damage risk zone0.36


The zone factor (Z) represents the basic seismic coefficient that is used to calculate the design seismic force for a structure. The higher the zone factor, the higher the seismic force.


The design basis earthquake (DBE) is defined as the earthquake that has a reasonable probability of occurrence during the design life of a structure. The DBE is characterized by two parameters: the design horizontal acceleration spectrum (Sa/g) and the design horizontal acceleration coefficient (Ah).


The design horizontal acceleration spectrum (Sa/g) is a plot of the spectral acceleration (Sa) versus the natural period (T) of a structure. The spectral acceleration (Sa) is the maximum acceleration that a structure with a given natural period (T) would experience during an earthquake. The design horizontal acceleration spectrum (Sa/g) depends on the seismic zone, the soil type, and the damping ratio of the structure.


The design horizontal acceleration coefficient (Ah) is a single value that represents the peak ground acceleration (PGA) multiplied by a factor that accounts for the soil type and the importance of the structure. The PGA is the maximum acceleration that the ground would experience during an earthquake. The importance factor (I) is a value that reflects the relative importance of a structure in terms of its function, occupancy, post-earthquake operability, etc. The design horizontal acceleration coefficient (Ah) is given by:


Ah = ZISa/g


where Z is the zone factor, I is the importance factor, and Sa/g is the average spectral acceleration for 5 percent damping ratio.


Seismic Analysis and Design Methods




IS 1893 Part 3 2002 provides three methods of seismic analysis for bridges and retaining walls: static analysis, dynamic analysis, and pushover analysis.


Static analysis is a simplified method that assumes that the seismic forces and effects are proportional to the mass and stiffness of the structure. Static analysis can be used for regular structures with simple geometry, symmetry, and uniformity. Static analysis involves applying equivalent static lateral forces to the structure based on its mass distribution and fundamental natural period.


Dynamic analysis is a more accurate method that accounts for the dynamic response of the structure to the earthquake ground motion. Dynamic analysis can be used for irregular structures with complex geometry, asymmetry, or non-uniformity. Dynamic analysis involves solving the equations of motion of the structure using either modal analysis or time history analysis.


Pushover analysis is a nonlinear method that evaluates the performance of the structure under increasing levels of seismic forces and effects. Pushover analysis can be used for structures with significant nonlinear behavior or ductility capacity. Pushover analysis involves applying incremental lateral forces to the structure until it reaches its ultimate capacity or target displacement.


The criteria for selecting the appropriate method of analysis for a structure depend on several factors, such as:



  • The seismic zone and soil type of the site



  • The type and complexity of the structure



  • The availability and reliability of the input data



  • The accuracy and efficiency of the analysis



  • The level of confidence and conservatism in the results



The steps involved in each method of analysis are:



Static analysis


  • Define the structural model and properties



  • Calculate the fundamental natural period of the structure



  • Determine the design horizontal acceleration spectrum (Sa/g) and coefficient (Ah)



  • Calculate the base shear force and its distribution along the height of the structure



  • Apply the equivalent static lateral forces to the structure and perform static analysis



  • Check the displacement and drift limits of the structure




Dynamic analysis


  • Define the structural model and properties



  • Perform modal analysis to obtain the natural periods, frequencies, and mode shapes of the structure



  • Determine the design horizontal acceleration spectrum (Sa/g) and coefficient (Ah)



  • Select an appropriate earthquake ground motion record or generate an artificial one



  • Scale the ground motion record to match the design horizontal acceleration spectrum (Sa/g)



  • Apply the scaled ground motion record to the structure and perform time history analysis



  • Check the displacement and drift limits of the structure




Pushover analysis


  • Define the structural model and properties, including nonlinear behavior and hinges



  • Select a lateral load pattern that represents the expected seismic force distribution



  • Apply incremental lateral forces to the structure and perform nonlinear static analysis



  • Plot the base shear force versus roof displacement curve (pushover curve)



  • Determine the target displacement of the structure based on its natural period and ductility factor



  • Evaluate the performance of the structure based on its capacity, demand, deformation, and damage




Seismic Forces and Effects




The seismic forces and effects are the internal forces and moments, displacements and rotations, stresses and strains, etc. that are induced in a structure due to earthquake ground motion. The seismic forces and effects depend on several parameters, such as:



  • The mass and stiffness of the structure



  • The damping and ductility of the structure



  • The natural periods and mode shapes of the structure



  • The design horizontal acceleration spectrum (Sa/g) and coefficient (Ah)



  • The earthquake ground motion characteristics, such as amplitude, frequency, duration, etc.



  • The soil-structure interaction effects



  • The geometric and material nonlinearities of the structure



The seismic forces and effects are calculated for a structure using either static analysis or dynamic analysis. In static analysis, the seismic forces are obtained by multiplying the mass of each element by its corresponding acceleration. The seismic effects are then obtained by applying these forces to the structure and performing static equilibrium equations. In dynamic analysis, the seismic forces are obtained by solving the equations of motion of each element using either modal analysis or time history analysis. The seismic effects are then obtained by integrating these forces over time.


The seismic forces and effects are distributed among the structural elements according to their stiffness, mass, location, orientation, etc. Generally, stiffer elements attract more seismic forces than flexible elements, heavier elements attract more seismic forces than lighter elements, elements closer to the base attract more seismic forces than elements farther from the base, and elements aligned with the direction of seismic force attract more seismic forces than elements perpendicular to it.


Seismic Design and Detailing Requirements




The seismic design and detailing requirements are the rules and specifications that govern the selection and arrangement of structural materials, elements, connections, etc. to ensure adequate strength, stiffness, stability, ductility, and durability of a structure under seismic forces and effects. The seismic design and detailing requirements depend on several factors, such as:



  • The seismic zone and soil type of the site



  • The type and complexity of the structure



  • The method of seismic analysis used for the structure



  • The performance level and acceptance criteria for the structure



  • The material properties and behavior of the structure



IS 1893 Part 3 2002 provides general principles of seismic design and detailing for bridges and retaining walls, as well as specific requirements for different types of structures. Some of the general principles are:



  • Use adequate lateral load resisting systems to provide sufficient stiffness and strength to the structure



  • Use capacity design approach to ensure that inelastic deformations are confined to predetermined locations and that brittle failures are avoided



  • Use ductile materials and details to enhance the energy dissipation and deformation capacity of the structure



  • Use adequate reinforcement and confinement to prevent buckling, spalling, cracking, or crushing of concrete



  • Use appropriate bearings, joints, isolators, dampers, etc. to accommodate relative displacements and reduce seismic forces



  • Use proper construction quality control and inspection to ensure compliance with design specifications



Some of the specific requirements for different types of structures are:



Bridges


  • Use continuous or integral superstructures to avoid unseating or pounding of girders



  • Use seat-type abutments with sufficient seat width and backwall height to prevent girder displacement or rotation



  • Use shear keys or restrainers to limit the relative movement between girders and abutments or piers



  • Use elastomeric bearings with adequate vertical stiffness and horizontal flexibility to reduce seismic forces and accommodate displacements



  • Use expansion joints with sufficient movement capacity and durability to prevent damage or leakage



  • Use circular or hollow rectangular piers with adequate transverse reinforcement and spiral confinement to provide ductility and strength



  • Use plastic hinge zones at the base of piers with sufficient longitudinal reinforcement and confinement to allow inelastic rotation



  • Use pile foundations with adequate lateral resistance and ductility to prevent failure or excessive settlement




Retaining walls


  • Use gravity walls with sufficient mass and base width to resist overturning and sliding



  • Use cantilever walls with sufficient depth and reinforcement to resist bending and shear



  • Use counterfort walls with sufficient spacing and connection to resist lateral earth pressure



  • Use reinforced soil walls with sufficient geosynthetic layers and frictional resistance to prevent rupture or pullout



  • Use backfill material with adequate drainage and compaction to reduce water pressure and settlement




Seismic Performance and Evaluation




The seismic performance and evaluation are the assessment of the actual or expected behavior of a structure under seismic forces and effects. The seismic performance and evaluation can be conducted for both new and existing structures, with different purposes and methods. The seismic performance and evaluation depend on several factors, such as:



  • The seismic zone and soil type of the site



  • The type and complexity of the structure



  • The method of seismic analysis used for the structure



  • The design basis earthquake (DBE) and other hazard levels for the structure



  • The material properties and behavior of the structure



  • The damage states and repair costs of the structure



IS 1893 Part 3 2002 provides two performance levels and corresponding acceptance criteria for bridges and retaining walls: serviceability limit state (SLS) and ultimate limit state (ULS). The serviceability limit state (SLS) corresponds to the design basis earthquake (DBE), which has a reasonable probability of occurrence during the design life of a structure. The acceptance criteria for SLS are based on limiting the displacements and drifts of the structure to prevent excessive damage or loss of functionality. The ultimate limit state (ULS) corresponds to the maximum considered earthquake (MCE), which has a low probability of occurrence but a high potential of causing severe damage or collapse. The acceptance criteria for ULS are based on ensuring adequate strength, ductility, and stability of the structure to prevent catastrophic failure or loss of life.


The methods of seismic performance and evaluation for bridges and retaining walls are:



For new structures


  • Use seismic design and detailing requirements to achieve the desired performance levels and acceptance criteria



  • Use seismic analysis methods to verify the compliance with the performance levels and acceptance criteria



  • Use sensitivity analysis to evaluate the effects of uncertainties in input data, analysis assumptions, etc.




For existing structures


  • Use visual inspection, material testing, structural modeling, etc. to assess the current condition and capacity of the structure



  • Use seismic analysis methods to estimate the expected performance levels and acceptance criteria under different hazard levels



  • Use seismic retrofitting techniques to improve the performance levels and acceptance criteria if needed




Conclusion




In this article, we have discussed the main aspects of IS 1893 Part 3 2002, which is a standard for seismic design and assessment of bridges and retaining walls in India. We have reviewed the scope and applicability, seismic zoning and design basis earthquake, seismic analysis and design methods, seismic forces and effects, seismic design and detailing requirements, and seismic performance and evaluation of this standard. We have also compared this standard with other parts of IS 1893 and other codes and standards that are relevant for bridge engineering.


We have learned that IS 1893 Part 3 2002 provides a comprehensive and consistent framework for achieving earthquake resistant design of bridges and retaining walls in India. It accounts for the regional variations in seismic hazard and soil conditions in India, adopts a performance-based approach for seismic design and evaluation of structures, incorporates the latest developments and research findings in earthquake engineering, and harmonizes with other parts of IS 1893 and other relevant codes and standards.


However, we have also identified some challenges and limitations of IS 1893 Part 3 2002, such as:



  • The applicability of static analysis for irregular structures with complex geometry, asymmetry, or non-uniformity



  • The availability and reliability of design horizontal acceleration spectra and coefficients for different seismic zones and soil types



  • The definition and quantification of damage states and repair costs for different types of structures



  • The incorporation of soil-structure interaction effects and nonlinear material behavior in seismic analysis and design



  • The assessment and retrofitting of existing structures that do not comply with the current standard



Therefore, we have provided some recommendations and suggestions for future research and development in this field, such as:



  • Developing more accurate and efficient methods of seismic analysis for bridges and retaining walls, such as nonlinear dynamic analysis, probabilistic seismic demand analysis, etc.



  • Updating and refining the design horizontal acceleration spectra and coefficients for different seismic zones and soil types based on more data and studies



  • Establishing more clear and consistent performance levels and acceptance criteria for different types of structures based on their function, occupancy, post-earthquake operability, etc.



  • Incorporating soil-structure interaction effects and nonlinear material behavior in seismic analysis and design using advanced models and tools



  • Developing more effective and economical methods of seismic evaluation and retrofitting for existing structures using new materials and technologies



FAQs




Here are some frequently asked questions and their answers related to this topic:



  • What is the difference between IS 1893 Part 3 2002 and IS 1893 Part 1 2016?



IS 1893 Part 3 2002 is a standard for seismic design and assessment of bridges and retaining walls, while IS


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