Seismic Retrofitting Techniques for Concrete Structures:
The vulnerability of concrete structures to seismic forces has become a significant concern, especially in light of the frequent occurrence of moderate to severe earthquakes worldwide over the past three decades. The resulting damage and failures of these structures have prompted the need for seismic retrofitting techniques. The focus of this practice is to evaluate the seismic vulnerability of existing reinforced concrete buildings and retrofit them with innovative techniques such as base isolation and mass reduction to improve their earthquake resistance. Seismic retrofitting is particularly crucial for historic monuments, areas prone to severe earthquakes, and tall or expensive structures. Retrofitting techniques such as jacketing can be employed to enhance the earthquake resistance of these structures.
1. Introduction to Seismic Retrofitting Techniques:
Earthquakes can cause significant destruction, resulting in the loss of life, financial losses, and structural failures. Therefore, it is crucial to prioritize the upgrading of building systems to increase their resistance to seismic activity, particularly for existing structures that may be at risk of earthquake damage or vulnerability.
There are two primary categories of buildings concerning earthquakes: those that have already suffered damage from previous seismic events, and those that are at risk of damage due to their vulnerability. Retrofitting is a cost-effective and practical solution for these buildings, as it can provide immediate shelter and address the problems without requiring a full replacement of the structure.
Retrofitting involves modifying existing structures to improve their earthquake resistance, such as by strengthening the foundation or adding support elements. This approach not only enhances the building’s ability to withstand future earthquakes, but it also prolongs the lifespan of the structure.
Compared to the replacement of the entire building, retrofitting is a more feasible and economical option, particularly in cases where the building’s historical, cultural, or sentimental value is significant. In addition to the financial benefits, retrofitting also provides an immediate solution to the problem, allowing people to continue using the building while ensuring their safety during future seismic events.
1.1 Seismic Retrofitting of Concrete Structures:
Retrofitting refers to the process of modifying structures that already exist in order to make them better equipped to withstand seismic activity, ground motion, or soil failure caused by earthquakes. This approach to construction is also useful for mitigating the impact of other types of natural disasters, such as tropical cyclones, tornadoes, and severe winds from thunderstorms.
By retrofitting existing buildings, engineers and architects can improve their ability to resist the powerful forces exerted by natural disasters. This can involve adding reinforcements to key structural elements, such as foundations, walls, and roofs, or installing new features like dampers or base isolators to absorb or redirect the energy of an earthquake or other event. Retrofitting may also involve improving a building’s ability to withstand lateral loads or improving its ability to resist soil liquefaction.
Overall, retrofitting is an important tool for ensuring that existing structures are able to withstand the powerful forces of natural disasters. By making targeted modifications to key elements of a building’s design and construction, engineers can help to protect people and property from harm, reduce the risk of catastrophic damage, and ensure that our communities are able to recover more quickly after a disaster strikes.
1.2 Need for Seismic Retrofitting:
To ensure the safety and security of a building, it is essential to take measures to reduce hazards and losses from non-structural elements. This includes focusing on structural improvements to reduce the risk of damage and loss in the event of an earthquake.
One of the primary concerns when it comes to building safety is seismic hazard. Therefore, it is important to prioritize structural improvements that can help to mitigate this risk. This may include reinforcing walls, floors, and other structural elements, as well as upgrading building systems and equipment to make them more earthquake-resistant.
In particular, it is crucial to prioritize the strengthening of buildings that provide essential services in the aftermath of an earthquake. Hospitals, for example, are critical facilities that need to remain functional even in the wake of a seismic event. By strengthening these buildings, we can help to ensure that they are able to continue providing important services to those in need, even in the face of disaster.
1.3 Problems faced by Structural Engineers are:
The effectiveness of retrofitting methods varies greatly due to the lack of standards for these methods. The effectiveness of a given method can be influenced by several factors, such as the type of structure being retrofitted, the condition of the materials used, and the extent of damage to the structure. These parameters can greatly impact the overall success of a retrofitting project, and therefore it is crucial that standard guidelines be established for retrofitting methods in order to improve their effectiveness and ensure consistency across different projects. Without such standards, it can be difficult to determine which method is best suited for a particular structure, leading to potential safety issues and unnecessary costs. Therefore, establishing clear standards for retrofitting methods is essential for ensuring the safety and long-term stability of structures.
1.4 Basic Concept of Retrofitting:
The given context states that the aim is to improve the lateral strength of the structure, which refers to the ability of a structure to resist forces acting in a direction perpendicular to its length. The objective is to increase the capacity of the structure to withstand lateral loads and ensure its stability.
Another aim mentioned is to enhance the ductility of the structure. Ductility refers to the ability of a material to deform under stress without fracturing. In the context of structures, ductility is important as it allows them to absorb energy and deform during seismic events, reducing the chances of collapse. Therefore, improving the ductility of the structure is essential to ensure its resilience and safety.
Lastly, the context suggests that the overall goal is to increase both the strength and ductility of the structure. By enhancing the strength of the structure, it can withstand larger forces without failure, while improving the ductility allows it to absorb more energy during deformation without breaking. Together, these improvements can ensure that the structure can withstand various types of loads and remain stable and safe.
2. Classification of Retrofitting Techniques:
2.1 Adding New Shear Walls:
Retrofitting of non-ductile reinforced concrete frame buildings often involves the incorporation of additional elements, which can either be cast-in-place or precast concrete elements. It is recommended that these new elements be placed on the exterior of the building to avoid disrupting interior moldings. Consequently, adding new elements to the interior of the structure is not preferable. The objective of this approach is to enhance the structural integrity of the building, thereby improving its ability to withstand seismic activity.
2.2 Adding Steel Bracings
One potential solution for the need for large openings is to use a material with higher strength and stiffness. This can offer several advantages, including the ability to provide openings for natural light while minimizing the amount of work required. Additionally, this approach may reduce foundation costs and add less weight to the existing structure. Overall, using a material with these properties can be an effective way to achieve the desired outcome of creating large openings in a structure.
2.3 Jacketing (Local Retrofitting Technique):
The most widely used approach for enhancing the strength of building columns is being referred to in this statement. No additional information or details are provided about the method or any other related aspects. It simply acknowledges the popularity of this particular method in the field of building construction and renovation.
Types of Jacketing:
The following are different types of jackets used for structural reinforcement. The first type is a steel jacket, which is a layer of steel that is wrapped around a structure to provide additional support. Another type of jacket is a reinforced concrete jacket, which is a layer of reinforced concrete that is applied to a structure to improve its strength and durability. Finally, there is the Fibre Reinforced Polymer Composite (FRPC) jacket, which is made up of composite materials and is used to strengthen and protect a structure from external forces. These jackets are commonly used in the construction industry to enhance the performance and longevity of structures.
Purpose for jacketing:
There are three objectives that can be achieved by reinforcing concrete structures. The first objective is to increase concrete confinement, which refers to the confinement of the concrete within the reinforcing steel. This can be achieved by using stirrups or ties that surround the reinforcing steel and prevent the concrete from expanding under load. By increasing the confinement of the concrete, its strength and ductility can be improved.
The second objective is to increase shear strength. Shear strength is the ability of a material to resist lateral forces that are applied perpendicular to its axis. In reinforced concrete structures, this can be achieved by increasing the amount of shear reinforcement, such as stirrups, or by providing more concrete in the critical shear regions.
The third objective is to increase flexural strength. Flexural strength is the ability of a material to resist bending or deflection. In reinforced concrete structures, this can be achieved by increasing the amount of reinforcing steel or by using steel with a higher yield strength. Additionally, the use of prestressing can also increase the flexural strength of reinforced concrete structures.
2.4 Base Isolation (or Seismic Isolation):
Base isolation is a technique used to separate a structure’s superstructure from its foundation. This process is highly effective in controlling structural vibration through passive means. By isolating the superstructure from the foundation, the structure’s ability to absorb shock and vibrations is improved, thereby increasing its durability and longevity.
In practice, base isolation involves placing a flexible material or a series of bearings between the foundation and the superstructure. This layer of isolation allows the superstructure to move independently of the foundation during seismic or vibrational events, reducing the impact of these forces on the structure itself.
This technique is particularly useful in areas prone to earthquakes or other seismic events. By isolating the superstructure from the foundation, base isolation can greatly reduce the risk of structural damage or collapse during such events. It is a powerful tool in structural engineering, providing an effective and passive means of controlling vibrations and protecting buildings from seismic forces.
2.4.1 Advantages of Base Isolation
The technique of isolating a building from ground motion offers a range of benefits. By effectively separating the structure from the ground, the seismic loads are reduced, resulting in less damage to the building. As a result, there is minimal repair required for the superstructure. Additionally, the building can remain serviceable throughout the construction process. Furthermore, this technique does not require major intrusion into the existing superstructure, which helps to minimize any potential disruptions. Overall, building isolation is a highly effective approach for improving the resilience and longevity of structures, while also minimizing repair and construction costs.
2.4.2 Disadvantages of Base Isolation
The retrofitting process can be quite expensive, and unlike other methods, it cannot be applied partially to structures. This presents a significant challenge in terms of implementation, as it is challenging to carry out the process in an efficient manner.
2.5 Mass Reduction Technique of Retrofitting:
The given context describes a scenario in which a building’s mass is reduced by removing one or more storeys. This removal of mass is expected to result in a decrease in the building’s period, which will subsequently lead to an increase in the required strength of the building.
The reduction in mass of a building can be achieved through the removal of one or more storeys. Such a change is likely to have an impact on the building’s dynamic response. Specifically, the period of the building – which refers to the time taken for one complete cycle of its natural oscillation – is expected to decrease.
As a result of the decrease in the building’s period, the required strength of the building will increase. This increase in required strength is necessary to ensure that the building can withstand external forces such as wind or seismic activity without collapsing. Therefore, while the removal of storeys may lead to a reduction in mass, it also necessitates a corresponding increase in strength to maintain the safety and stability of the building.
2.6 Wall Thickening Technique of Retrofitting:
The purpose of adding bricks, concrete, and steel reinforcement to the existing walls of a building is to increase their thickness and weight. This increase in weight allows the walls to bear more vertical and horizontal loads, making the building structurally stronger. The reinforcement materials are strategically placed to ensure that the transverse loads, such as wind or earthquake forces, do not cause sudden failure of the wall. This design ensures the safety and stability of the building, even under extreme conditions.
3. Indian Standard Codes for Earthquake Design of Structures:
The Indian Standards Institute has published several codes of practice and guidelines related to earthquake resistant design and construction of buildings and structures. These include IS 1893-2002 (part-1), which provides general provisions and criteria for earthquake resistant design of structures including buildings. IS 4326-1993 provides more specific guidelines for earthquake resistant design and construction of buildings.
In addition to these codes of practice, the institute has also published guidelines for specific types of structures and buildings. For example, IS 13920-1993 provides guidelines for ductile detailing of reinforced concrete structures that are subjected to seismic forces. IS 13935-1993 provides guidelines for the repair and seismic strengthening of buildings, while IS 13828-1993 provides guidelines for improving the earthquake resistance of low strength masonry buildings. Finally, IS 13827-1993 provides guidelines for improving the earthquake resistance of earthen buildings.
4. Conclusion – Seismic Retrofitting Techniques for concrete structures:
Seismic retrofitting has emerged as a viable technology for safeguarding a wide range of structures. Over the years, it has matured into a highly reliable method for mitigating the effects of seismic activity on buildings. However, the expertise required to execute seismic retrofitting at a basic level is lacking. Achieving a desired level of performance while keeping costs to a minimum is the main challenge in this field, which can be addressed through detailed nonlinear analysis. Optimization techniques are necessary to determine the most efficient method of retrofitting for a particular structure. Additionally, proper design codes need to be established and published as a code of practice for professionals working in this field.
5. References:
In the book “Earthquake Resistant Design of Structures,” written by Agarwal and Shrikhande and published by Prentice-Hall of India Private Limited in New Delhi in its 2nd edition in 2006, the authors discuss the principles and techniques of designing earthquake-resistant structures.
The article “Seismic Protection of Light Secondary Systems through Different Base Isolation Systems” by Cardone and Dolce, published in the Journal of Earthquake Engineering in 2003, delves into various base isolation systems for protecting light secondary systems during earthquakes.
The report “Fluid Viscous Dampers in Applications of Seismic Energy Dissipation and Seismic Isolation” by Constantinou, Symans, Tsopelas, and Taylor, published as ATC-17-1 by the Applied Technology Council in San Francisco in 1993, provides insights into the use of fluid viscous dampers for dissipating seismic energy and isolating structures from earthquake forces.
The publication “Lessons Learnt Over Time – Learning from Earthquakes Series: Volume II Innovative Recovery in India” by EERI in 1999, based in Oakland (CA), USA, highlights innovative recovery approaches adopted in India after earthquakes, as part of the Earthquake Engineering Research Institute’s ongoing efforts to learn from seismic events.
“IITK-BMTPC Earthquake Tip,” authored by Murty and published in New Delhi in 2004, offers practical guidance and tips related to earthquakes, specifically tailored for the Indian context, by the Indian Institute of Technology Kanpur and the Building Materials and Technology Promotion Council.