Seismic retrofitting involves modifying existing structures to enhance their ability to withstand seismic activity, ground motion, or soil failure resulting from earthquakes. Each structure must possess two essential load-resisting systems: a vertical load-resisting system for bearing vertical loads and a horizontal load-resisting system for transferring horizontal loads to the vertical system. The horizontal framing system must effectively collect seismic forces and transfer them into the vertical lateral resisting systems. Any disruptions or irregularities in this load path or transfer mechanism can significantly contribute to structural damage during intense earthquakes. Hence, seismic retrofitting techniques are imperative for reinforcing existing reinforced concrete buildings.
Factors Influencing Selection of Seismic Retrofitting Techniques for Existing RC Buildings
The choice of seismic retrofitting techniques for existing RC buildings relies on the characteristics of both the horizontal and vertical load-resisting systems, as well as the materials employed in the original construction. Insight into failure modes, structural behavior, and identification of weak and strong design elements, often gleaned from earthquake damage surveys, greatly informs the retrofitting method selection process. Seismic retrofitting entails implementing structural enhancements aimed at mitigating damage to buildings and safeguarding occupants during seismic events. Retrofit measures may encompass various strategies tailored to address specific vulnerabilities present in concrete masonry, unreinforced masonry, soft story, and concrete tilt-up constructions.
In the seismic evaluation process, the primary objective is to assess whether a structure meets predefined target levels of structural performance. This assessment is crucial during earthquakes, aiming to prevent building collapse and minimize the risk of injury or fatality to occupants. Additionally, the evaluation seeks to ascertain the vulnerabilities and weak points of a building in anticipation of seismic events.
Seismic rehabilitation techniques play a pivotal role in improving the seismic performance of buildings and rectifying identified deficiencies. These techniques involve enhancing the strength, stiffness, or deformation capacity of structural elements and improving connections to bolster the building’s resilience against seismic forces.
Retrofitting Strategies for Reinforced Concrete Buildings in Seismic Zones
Concrete Shear Walls
Shear walls are essential components in building structures, possessing sufficient lateral stiffness to mitigate interstory distortions caused by earthquake-induced motions. These structural members are specifically designed to withstand lateral forces, offering resistance through their ability to counteract loads via cantilever action, particularly in slender walls prone to flexural deformation. Essentially, shear walls serve as vertical elements within the horizontal force resisting system.
In concrete structures, the design typically integrates consistent shear walls to bear both lateral seismic loads and gravity loads. Such configurations often forego traditional beams and columns, relying solely on the robustness of concrete shear walls to withstand seismic forces. This reliance underscores the significance of shear walls as a primary seismic resistance mechanism in medium- to high-rise buildings.
The shear wall system stands as one of the most prevalent and effective lateral load resisting systems, widely deployed in structures of varying heights. Its efficacy lies in providing ample strength and stiffness crucial for countering wind and earthquake loads. However, successful implementation hinges on meticulous design considerations that prioritize both the strength and ductility of the shear walls.
Steel Braced Frames
In buildings, they serve as prevalent lateral-load resisting systems. Unlike standard bridge or roof trusses, which bear vertical gravity loads and span horizontally, braced frames primarily endure horizontal seismic inertia loads, functioning akin to vertical cantilevers. The stability of trusses stems from their fundamental triangular units, which effectively resist structural loads through the development of axial forces within their members. When these forces align with the centerlines of pinned joints, pure truss action is achieved. Concentrically braced frames (CBFs) integrate beams, columns, and braces at shared work points, enhancing structural integrity.
Moment-Resisting Steel Frames
A moment-resisting frame comprises beams and columns arranged in a rectilinear formation, with beams firmly connected to columns. Its ability to resist lateral forces relies chiefly on rigid frame action, characterized by the generation of bending moments and shear forces within frame members and joints. Due to the rigid connections between beams and columns, lateral displacement in a moment frame necessitates bending either the beams or columns, contingent upon the connection’s geometry. Thus, the bending rigidity and strength of frame members serve as the principal contributors to the overall lateral stiffness and strength of the frame.
Concrete Diaphragm wall
Diaphragms play a crucial role in enabling walls or frames to collaborate effectively in withstanding lateral forces. Without diaphragms, the vertical structural elements responsible for resisting lateral forces would resemble a disorganized crowd attempting to function independently. As walls, moment-resisting frames, and braced frames typically have limited out-of-plane resistance capacities, diaphragms serve as the mechanism through which vertical elements can transmit their stabilizing effect to other components. Therefore, diaphragms are integral components of the seismic force-resisting system, facilitating coordinated action among structural elements.
Columns Jacketing
Jacketing is a method utilized to enhance the strength of existing structural elements such as columns and beams by enveloping them with an additional layer of material, termed a “jacket.” Columns, originally designed to bear loads from top to bottom, may undergo increased loading due to factors like prolonged aging, uncalculated excessive loads, or changes in function over time. In order to accommodate these additional loads effectively, the dimensions of columns often need to be augmented. This process involves adding a supplementary layer of reinforcement and concrete to the existing column, referred to as column jacketing. Typically applied to buildings post-earthquakes or in aging structures, jacketing aims to prolong the structural integrity and lifespan of the building.
Beam Jacketing
The application of jacketing to beams is advised for various reasons, including providing continuity to columns and enhancing the overall strength and stiffness of the structure. However, when jacketing a beam, careful consideration must be given to its flexural resistance to prevent the formation of a strong beam-weak column system. In retrofitted structures, the introduction of column jacketing may lead to a change in the mode of failure and the redistribution of forces, potentially resulting in beam hinging. Therefore, thorough analysis and design considerations are essential to ensure the effectiveness and safety of the retrofitting process.
FRP composites
In recent years, seismic retrofitting using Fiber Reinforced Polymer (FRP) materials has become increasingly popular. This method is widely accepted due to its effectiveness and cost-efficiency in repairing structures. FRP composites are utilized to enhance the structural capacity of members against shear, flexural, and compressive forces, as well as for blast mitigation and seismic load upgrades. Additionally, FRP systems play a crucial role in controlling the propagation of existing cracks. Common applications of FRP systems include fiber wrapping, near-surface reinforcement, and bonding pultruded FRP plates.
Seismic Base Isolation
Seismic base isolation, or simply base isolation, stands out as a widely adopted method for safeguarding structures against earthquake forces. This approach involves a combination of structural elements designed to effectively disconnect a superstructure from its substructure, which rests upon the shaking ground. By doing so, base isolation aims to preserve the integrity of buildings or other structures during seismic events. Considered a cornerstone of earthquake engineering, base isolation falls under the category of passive structural vibration control technologies.
Various techniques are employed to achieve isolation, including rubber bearings, friction bearings, ball bearings, spring systems, and other methods. The primary objective is to ensure that a building or structure can withstand the potentially destructive impacts of earthquakes, either through initial design considerations or subsequent modifications. Importantly, the implementation of base isolation has been shown to significantly enhance both the seismic performance and sustainability of structures in certain cases.
Conclusion
Seismic base isolation emerges as a pivotal strategy in earthquake engineering, offering a robust defense mechanism against seismic forces. Through a combination of structural elements, base isolation effectively decouples the superstructure from the shaking ground, thereby preserving the integrity of buildings and structures. This passive vibration control technology, encompassing various techniques such as rubber bearings and spring systems, plays a crucial role in mitigating the potentially devastating impacts of earthquakes. By enabling structures to withstand seismic events and enhancing their overall sustainability, base isolation stands as a cornerstone in the realm of seismic resilience and safety. As research and technology continue to advance, base isolation remains a vital tool in safeguarding communities and infrastructure from the unpredictable forces of nature.