The devastating effects of earthquakes highlight the critical need for robust building designs that can withstand seismic forces. Reinforced concrete structures, although common in modern construction, often require additional strengthening to resist the unpredictable lateral forces generated during an earthquake. This article explores the most effective methods and techniques for enhancing the earthquake resilience of reinforced concrete buildings, ensuring both the safety of occupants and the longevity of infrastructure in seismically active regions.
1. Introduction to Earthquake Safety and Structural Strengthening Strategies
Earthquakes have been one of the destructive natural forces throughout history. These earth-shaking events, unlike other natural forces, are unpredictable, and failure of these incidents can be catastrophic. Buildings and structures that are poorly designed to resist earthquakes result in loss of human lives along with loss in infrastructure development. Earthquake events are harsh, rapid oscillations of the ground which induce forces on buildings and structures. These forces act on different parts of a building at different time intervals. The top portion of a building experiences the ground accelerations later than the ground floor and therefore is exposed to a different set of inertia forces. These differential forces induce lateral forces on building structures. Reinforced concrete building structures are generally strong in the vertical direction due to the presence of concrete, but they are weak in lateral directions due to the nature of the materials involved in the construction. This results in excessive lateral displacement, shear forces, and torsion, resulting in failure.
Reinforced concrete building structures can be divided into 2 and 3 directions. Each of these systems has its own strengths and weaknesses in resisting earthquakes. Most of the strengthened systems believe the myth that systems with more reinforced concrete walls are better at resisting earthquakes; however, this is far from the truth. On the contrary, there should be an optimum number of reinforced concrete walls in a strengthened system. Due to poor architectural configuration, some strengthened systems perform poorly during earthquakes. It is beneficial to have multiple types of systems participating in earthquake resistance. This takes care of system redundancy and ensures greater safety of human lives, especially in developing countries where periodic underestimated earthquakes are common but rare records of them being catastrophic.
In this era of technological advancement, proper resources for earthquake investigation are lacking; this also adds to the risk. Fortunately or unfortunately, the means of technological advancement are used by architects, builders, and designers in defaulting earthquake-resistant parameters of building structures. Therefore, either no or low priority is generally given to these systems. A few limited elements are used, but they are used very inefficiently. This should not be the case. Thorough analysis of the performance of these structures must be carried out to take precautionary measures against the actions of nature vengefully. However, this particularly points towards the necessity of thorough, preferably mechanical analysis of both bare frame and strengthened structures to assess their capability or non-capability of resisting such forces in the future, should the appropriately scientific design parameters of a building structure under these actions be done to consider it earthquake resistant.
2. Understanding Earthquakes
As long as people have lived in buildings, all around the world, people have felt their homes shaking and rattling, sometimes so violently that they fear for their very lives. Those sudden shocks have now been shown to be caused by movements within the Earth itself, perhaps thousands of miles away. Each of these rare but great movements can generate a disastrous shock wave, spreading out through the Earth much like that caused by an exploding bomb. For hundreds of years, earthquakes have inspired fear in people because they cause more loss of life than any other natural disaster.
The Earth consists of a solid outer crust or shell, a dense liquid interior, and a solid iron core. Both the outer shell and inner liquid layers are thought to consist of a number of rigid plates that can shift or move across the surface of the Earth. These moving plates can suddenly get stuck as a result of friction. The tectonic forces trying to push the plates together are constantly forcing them to bend near their edges. This flexing builds up a tremendous amount of energy, like twisting a stiff wire around and around until it finally snaps. A great shock is released, and this is what we recognize as an earthquake. Most of the energies released can travel as waves through the Earth's crust.
Two types of seismic waves can be recognized: those that travel on the surface of the Earth and those that travel inside. The first type is generally much more destructive. Earthquake waves can have one of two forms: fluid waves or elastic waves. Because the crust and upper layers of the Earth are considered to behave as elastic solids when subjected to stress, interest has centered on the study of elastic waves, which can be caused by sudden mechanical displacements in the Earth. There are compressional or P-waves, which travel through solids and liquids, and shear waves or S-waves, which can be transmitted only through solids.
Rocks subjected to compressive stress are deformed elastically as long as the applied stress does not exceed a certain limit. Brittle rocks are deformed elastically until that limit is reached. Once stress exceeds the limit, a sudden rupture occurs. An earthquake rupture, however, is not completed instantaneously. The rupture starts at a point and propagates across the surface of the Earth. Elastic waves generated by the rupture cause the ground to shake.
2.1. Causes of Earthquakes
Earthquakes are the sudden shaking of the land, and they have various effects on people, buildings, and nature. Everyone is curious about the causes of earthquakes. Just like people get angry, the Earth gets angry too, and earthquakes are a result of this anger. There is no system or device that can prevent earthquakes, but it is possible to develop structures that can resist the tremors created by an earthquake.
Earthquakes can be caused by various factors. One of the primary causes is the sudden rupture of rocks, leading to the occurrence of tension waves and seismic waves, which shake the crust of the Earth. The Pacific Plate moves towards the north relative to the American Plate, causing stress and tension in the rocks. When the resultant stress exceeds the elastic limit of the rocks, the rocks suddenly rupture, releasing energy in the form of seismic waves, leading to shaking on the surface and an earthquake. Other types of earthquakes, known as volcanic earthquakes, occur due to the collapse of underground magma reservoirs. The circum-Pacific belt is abundant in earthquake activity with many volcanic systems and deep ocean trenches due to plate tectonics.
In addition to tectonic earthquakes, earthquakes can also occur due to anthropogenic activities such as mining, quarrying, or nuclear explosions. These explosions generate tension waves that may develop surface waves as the velocity of these waves diminishes, creating a shaking effect. The energy contained in the primary wave and surface wave determines the intensity of the shaking, and this energy is dissipated in the crust of the planet, leading to a decay in tremors and the eventual ceasing of shaking.
2.2. Effects on Structures
Buildings and large structures are often subjected to destructive forces from earthquakes, feeling the effects of vibrations in the ground, ranging from mild to catastrophic. For a large structure, each aspect of the effect is magnified, and thus, more attention should be paid to protect it against disastrous effects from an earthquake. These vibrations generated due to the ground shaking travel in a wave-like fashion parallel to the surface of the ground, called the seismic wave. The velocity of these waves depends on the type of layer, i.e., surface wave velocity in the soil, and also the properties of materials. The properties include density, stiffness, and viscosity, and these properties define the ability of the material to resist deformability and strength. Building or structure design failure will likely occur without understanding the properties or effects of earthquakes on buildings. As a design engineer in the field of building construction, the knowledge of these aspects is essential for the safety and economy of a project. Various design methods have been proposed for the economic and feasible design of structures under the effect of devastating earthquake vibrations.
The effect of an earthquake is the transfer of the ground shaking vibrations to the mass, stiffness, and configuration of the structure. The design of the structure determines its degree of vulnerability to the vibrations. The magnitude of the effect depends on the mass of the building, the number of floors, height, and stiffness. Low-rise, high-shear stiffness, laterally braced buildings are the most brittle. For buildings with a larger number of floors, the relative deformation of the floors is large compared to the height of the building. These buildings have smaller shear stiffness and are best when designed as moment-resisting frames. The fundamental time period is an important parameter to investigate its vulnerability against seismic forces. The fundamental time period is the time taken to complete one cycle of free vibration, and per its proper nailing, the stiffness should be provided to depict their oscillational behavior. Periods lesser than the natural time period of the ground shall magnify the lateral forces.
Earthquakes have a magnitude determined on the Richter scale, from just a minute 1-magnitude earthquake that is seldom felt, to more than 7-magnitude earthquakes. The direct shaking of the ground may cause buildings to sway and is the effect felt by the majority of people with any kind of seismic sensing. Each kind of building has a fundamental mode of vibration period, and if the period of the ground shaking matches or is close to the building’s period, the amplifying effect magnifies building sway. This amplifying effect determines the need for additional lateral force-resisting systems, such as shear walls or moment-resisting frames. The amount of sway in a building is also directly dependent on the height and the amount of weight, as well as how much resistance there is to movement, which moreover affects the swaying behavior of the building. This is understood by the stiffness-to-weight ratio: a heavier structure will shed more lateral forces than a lighter structure per volume and material used.
3. Importance of Earthquake Resistance in Reinforced Concrete Structures
Earthquakes are a natural calamity that leads to large-scale devastation if proper safety measures are not taken into account. Earthquakes mainly have the potential to cause loss of life, loss of property, malfunction of infrastructure, and other unpredictable conditions. In order to decrease the effects of an earthquake, buildings, particularly structures made of concrete, have to be designed to be earthquake-resistant.
The effect of an earthquake on a structure varies according to the geophysics of the land a structure is built on. The factors that affect the magnitude of destruction caused by an earthquake include the topography of the land, geology of the land, the distance from the center, energy concentration, waves of vibration, frequency, and height of the structure. Reinforced concrete structures are rigid and heavy, so the damage they experience is mostly due to deformation. As a result, as far as construction safety is concerned, all structures made of reinforced concrete have to be designed to resist earthquake forces.
Reinforced concrete structures are mainly beams, columns, slabs, walls, and other elements. Whether reinforced concrete construction is held at an acceptable level or not is decided in the design stage itself. As a requirement during the construction design, either design charts or software is to be used. Design charts are our guidance for designs. They must be practiced and/or verified by some software packages for specific building configurations at site conditions. Well-proven building design packages that comply with codes are specific design packages for unique members of structures. All the above-mentioned packages have to take into consideration lateral loading in a required manner.
4. Assessment of Structural Vulnerability to Earthquakes
Earthquakes bring an inevitable risk to structures. The vulnerability of structures needs to be assessed as part of a seismic rehabilitation design. The evaluation of earthquakes requires representation of ground motion in the state-space domain. The foundation of the equations of motion development is considered the coordinates of a system with displacement in its mass center. To represent seismic disturbances, an earthquake ground motion in the vicinity of the mass center of a structure is considered. Formulated differential equations can be represented in state-space form. These equations of motion describe the translational motion of each mass in the model. The estimation of mass distribution conditions of structures takes into account dynamic parameters of structures, their lateral vibration frequencies, and values of the mass centers of each story. The width and height of structures and the arrangement of the mass center of a structure are considered. On the basis of approximated equations of motion determination, the state-space representation is utilized to evaluate the accessibility of four global earthquakes. Time intervals are computed by utilizing ordinates of the occurrence of global earthquakes accessible from the beginning of the period. The method of finite differences is applied to create time paradigms. Ground accelerations at these locations to be utilized for the examination of structure vulnerability are emphasized. Relative displacements of the mass center stories are depicted to examine the most deformed story of structures. Examination operatives describe the maximum lateral displacements and vibration form of the fifth story by considering analytical sequences of displacements of the mass center story. Deformed positions of structures were depicted by applying every second story as a rectangular scheme. The deformed scheme of structures can determine design dispositions of building structure lateral supports.
5. Strengthening Techniques for Reinforced Concrete Structures
Strengthening techniques are employed to increase the load-carrying capacity of reinforced concrete (RC) members. Concrete members may experience deterioration due to corrosion of reinforcement, cracks, spalling, and numerous other phenomena that lead to reduced load-carrying capacity. In structures where the load is increased after construction, strengthening methods may be necessary to allow for the increase in load-carrying capacity. Earthquakes can also lead to structural damage, which is another scenario where strengthening methods may be necessary. Strengthening methods can be classified as internal or external methods. Internal methods require the placement of additional materials in the concrete before the setting of the concrete mixture. These methods are suitable for reinforced concrete members that must be strengthened during the construction stage. External strengthening methods can be further categorized as passive, active, or hybrid, depending on whether the materials used in the strengthening method protect, augment, or balance the actions on the structure. External methods can also be classified based on the materials used for strengthening, such as steel, polymers, fiber-reinforced polymers, near-surface mounted reinforcement, and other composite materials. External strengthening methods involve applying external materials to the structure. Two common methods are the addition of external fiber-reinforced polymers systems and steel plate bonding. Strengthening with fiber-reinforced polymers systems is an innovative and efficient technique to upgrade the capacity of reinforced concrete structures. Fiber-reinforced polymers systems are characterized by their high strength-to-weight ratio, ease of installation, resistance to chemical attack and corrosion, and excellent fatigue behavior. The fiber-reinforced polymers system consists of the resin, fibers, and the material used in the form of sheets or laminated sections. In steel plate bonding, steel plates are affixed to the tension or compression faces of reinforced concrete flexural members by adhesive bonding. This technique, however, may affect the vulnerability of structures to corrosion and also requires careful placement. Internal strengthening techniques may involve demolishing concrete to allow for the placement of additional reinforcement. For members experiencing tension stresses, sections of the concrete member may be removed, and additional reinforcement bars may be placed or external tendons may be post-tensioned. Existing reinforcement may also be replaced with tendons of a higher strength. These methods are expensive and difficult to execute and are essential to the construction of the structure. Shear strengthening techniques may involve the bonding of steel reinforcement plates or bars to the surface of the concrete section with epoxy. In the case of shear tie replacement, the selection of the shear tie must be made when the structure to be demolished is in service.
5.1. External Strengthening Methods
External strengthening is definitely the most widely used technique. It involves attaching external materials to an existing structure, thereby enhancing load-carrying capacity, ductility, and defect repair. Although many types of materials have been tried in designs, fiber reinforced polymer composites have proven to be the most effective.
Fiber Reinforced Polymer Composites
Compared to steel, fiberglass reinforced plastics are less rigid, passive, and light. In fact, their density is roughly a third of that of steel. Once installed, they do not weigh anything during the life of the service. Also, they can be bonded to steel properly as an external strengthening technique. Several parametric studies on concrete structures showed that this new system is viable, efficient, and even revolutionary regarding cost benefits when compared to the conventional. Basically, FRP composites are a grouping of materials made of three different elements: a fiber, a polymeric resin, and some fillers. This resulting composite behaves like a new material in space. Since the fabric of fibers gives a positive tensile weight to the composite, it has high tensile strength and low ductility. Besides that, also influencing the behavior of the composite, the matrix is the polymeric resin, acting as a glue, protecting fibers against external agents, and increasing friction. Fibers can be combined with polymerics, epoxies, acrylics, thermoplastics, or phenolics.
Concrete Stiffening Repairs with Embedded FRP
This system requires adding layers of concrete, inside of which there are some reinforced polymers. This reinforcement is placed in tension after the concrete curing, causing this new volume to work in compression. Because the laminated polymers take tension and the concrete takes compression, both materials work jointly enabling an excellent structural solution. Combined with passive reinforcement in the lower regions of the beams, load-carrying capacity increases dramatically. An advantage of this system is that it can be used with the structure in use. Often, the circumscription of the new repairing volume is delimited with expanded metal and these repair areas are injected with a self-leveling mix for the embedding option. With the external option without demolishing any volume, a double sheet of external rebars is considered in flexure elements.
Bonded Steel Plate Reinforcement
Plates made of high-resistance steel are attached to the face of columns or beams, completely or in part. The bonding between plate and concrete is obtained using epoxy resin. These systems have been intensively studied in the last decade and many systems have been developed. The best way of implementation must take into account aspects of constructive circulation, the weight of the plates, activities on the work, as well as loading types. Up to now, despite some conflicting results pointed out after completed analysis, in terms of cost/benefits, the best type and location of intervention option is still under discussion.
Composite Materials and Repair with CFRP-FRP
Reinforced concrete school buildings of the 60s, with inappropriate structural fragility, were analyzed using the patchwork method of composite materials during the first reinforcement season. Composites constituted of a glass fiber-reinforced polymer and a concrete matrix were applied for reduced instrumentation. After that, changes in the loads acting on the structure and their consequences must be evaluated, using monitoring, failure mechanisms models, and computer methods of installation stress field computation for composite bonding.
5.2. Internal Strengthening Methods
This paper presents the study of building strengthening techniques and the most effective methods in reinforced concrete structures for earthquake resistance, as earthquakes are one of the most dangerous and damaging natural calamities. There are various methods to prevent buildings and structures from collapsing during such seismic activities, of which external and internal strengthening techniques are discussed. Internal strengthening methods are further sub-categorized into the use of steel plates, the addition of reinforcement, and special concrete shaping techniques like hollow lifting, hollow sections, and shear walls. Steel plates provide transverse and longitudinal enhancement of strength and stiffness, while the addition of separately designed reinforcement increases the ductility of the members, and the hollow lifting and section techniques improve the overall resistance to bending and shearing forces.
Internal Strengthening Methods
- Use of Steel Plates
Reinforced concrete sections are often inadequate in strength and stiffness under combined bending and shearing due to added loads or poor structural design. Many of these sections can be strengthened by adhering steel plates to their surfaces with adhesive resins. Modification types include the addition of either longitudinal or transversely bonded plates. Longitudinal bonding reinforces the moment capacity, while transverse bonding improves stiffness with a greater increase in strength. An assessment of the external bonded steel plates in reinforced concrete beams showed a beneficial effect on the overall strength and load capacities after experimentation.
- Addition of Reinforcement
Reinforcement in compressive reinforcement frames is often added to columns, with the intention of increasing the members’ ductility through carefully designed reinforcement for tension and compression. Extra ductility can also be achieved by advanced steel shapes bent with chains for free hinge connections, preventing the initiation of plastic hinges in sections at beam-column joints. Beams near column faces often need to be increased in strength and ductility to avoid early local buckling in under-reinforced beams.
Extra reinforcement can also be added in tensioned reinforced slabs to increase capacity against punching shear for flat slabs and slabs with column connections.
- Shape Modification Techniques
Specially shaped concrete beams and girders in bridge and rooftop construction are pre-intended to bear large bending forces and moments, reducing the material’s consumption while maintaining extensive resistance against negative moments at spans through incorrect and poor reinforcement designs. Such beam sections can be strengthened by either hollow sections or shear walls inside the hollow lifting section, preventing internal structures from failure and maintaining an adequate response against seismic activities.
6. Materials Used for Strengthening
The challenge of seismic strengthening of existing structures is investigated in terms of a unique approach defining the retrofitting strength needed by the current probabilistic and deterministic standards. Specific attention is devoted to the Indian codal provisions for the evaluation of seismic performance of structures at different levels of seismicity, and to the strengthening techniques currently being both researched and employed in India. The efficiency of different retrofitting systems is investigated by simplified analysis techniques to illustrate their potential in increasing the life of existing structures against earthquakes. Suggestions for further research are also made. It differs fundamentally from the design of new structures whereby the hazard is defined by the location of the project site. For an existing structure, unless there is evidence to the contrary, it must be assumed that it was constructed for a desired level of performance in the past, making the seismic analysis of existing buildings rather retrospective by nature. Existing structures can be seen as candidates for a number of unique problems, such as inappropriate hazard, inappropriate performance level, inadequate reliability, or inappropriate design code. All these problems can be assessed by retrofitting, resulting in increased safety of designed structures. Several techniques for strengthening or upgrading buildings to resist seismic actions are being used worldwide and can be classified as external mass and stiffness increases, foundation improvements, structural joint modifications, isolation systems, energy dissipation devices, external active control systems, and passive retrofitting techniques. Several of these have been employed in India. Of particular interest is the recent retrofitting project under strict safety and heritage preservation criteria. With the new project, improvements and further development of existing techniques and design applications are anticipated. Further development of passive retrofitting techniques is presently underway. In the case of masonry buildings, the use of steel plates or for the panel or installation of horizontal ties is likely with the aim of reducing eventual spurious torsional effects. The analysis can either be performed using a strain-based method with built-in geometric and material non-linearity.
6.1. Fiber-Reinforced Polymers (FRP)
Fiber-Reinforced Polymers (FRP) are innovative materials with a composite structure made up of a polymer matrix reinforced with fibers, like glass or carbon, to develop significant mechanical properties. In the 1980s, FRP composites were adapted for civil engineering applications to strengthen aging infrastructure. Nowadays, FRPs are widely employed in post-tensioned bridges, masonry arches, and numerous other structures, aiming to enhance their ductility using non-ductile materials. In reinforced concrete structures, FRPs are typically used externally in the form of laminates or sheets bonded to the concrete surface using an epoxy adhesive. Most applications involve externally bonded tension reinforcement with a concrete prism loaded in flexure. Being passive and non-corrosive, FRP composites are applied externally to enhance the flexural capacity of existing structures with insufficient rebar confinement. They are designed to control cracking and enhance either the flexural or shear strength of the structure. The tensile reinforcement ratio must be accounted for in the advance of service load cracking. The finite element method can analyze a number of frames with a similar typology and foundation under complex loading or simply supported beams. The quantity of compressive reinforcement, confined or unconfined concrete resisting axial action, and the degree of eccentricity can also be computed. The current level of experimental verification can be reduced to the computation of a single model on necessary experimental results. Additionally, innovative devices have been designed for assessing the effectiveness of the structural repair. It has been confirmed with analytical computations that the strength and ductility of the structure could be fully restored despite its damage. The significant contribution of strengthening techniques in enhancing the seismic performance of structures constructed with poor materials and technologies cannot be understated. Among several new and innovative materials, Fiber-Reinforced Polymers (FRP) composites are emerging as an incomparably more effective material for the strengthening and rehabilitation of existing reinforced concrete structures. To achieve the above goal, the necessity of composite development methodology based on structural reliability theory is outlined. It comprises the assessment of FRP material uncertainties and their influence on the most effective composite design. FRP material uncertainties will be emphasized and modeled as a first step. The development of design methodologies considering the uncertainty in these materials will be necessary to ensure their effectiveness and sustainability.
6.2. Steel Plates and Bars
Steel plate bonding is a kind of strengthening method in which a steel plate bonded to the tension face with a cement-based adhesive is used to reach the desired tensile strength of the girder. Cement-based adhesive can bond steel plates until reaching tension force because steel plates have much higher elastic modulus values than concrete.
There are three types of bonding methods: epoxy resin, sulfur cement, and cement mortar. The embedding length of the steel plate is influenced by concrete strength and the bond strength of the interface between the steel plate and concrete surface. However, environmental factors and other issues can affect the adhesive bonding effectiveness of the girder. Parameters affecting the performance and failure modes of steel plate bonding methods include concrete grade, the compressive load in concrete girders, support situation, bonded length of the steel plate, and thickness of concrete cover beneath the steel plate. Steel plates are rigid and bonded through an adhesive on the flat tension faces, mostly on the bottom surface of the girder. Each parameter must be considered together since multiple parameters exist in the construction phase of the girders. A simple and comprehensible method to determine the most influential parameter is essential, especially for construction sites where thorough analysis is limited. Any parameters like the bonded length of the steel plate must be elaborated since this can be easily adjusted in the construction of the girders. Moreover, finding a solution to rectify or limit the influences of a less critical parameter is also beneficial.
To come up with the parameter range, three typical concrete bridges were surveyed, including dry conditions and saturated conditions on various environmental temperatures. Steel plate bonding is expected to be a more popular methodology to strengthen the reinforced concrete structure because of its cost-effectiveness and construction advantages compared to other methods. Due to the various situations of the girders in the construction phase, prior evaluation is necessary to prevent possible issues during the construction stage.
7. Case Studies of Strengthening Projects
Strengthened structure where steel reinforcements are added to the framework in case 1. The case focused on beam-column joints as the critical part of the building structure. The brackets that hold the reinforced bars were connected to each other when adding the reinforcements. The main aim of this structure is to improve the connection and enhance the ductility of the structure under seismic effects. Effectively considering the behavior of beam-column joints in design and placing importance on them could improve ductility and stability. In 3D finite element modeling, the geometry and characteristics of the reinforced concrete were corrected. Applied loads on the structure have been properly allocated. The desired reinforced beam-column joint models were used in the initial structure in their relevant places. Overall, the automatic mesh generation was applied to the models. The joint rod was connected to the frame in the wide-face slabs. Supported shear bucket and Z-section models were used to transfer slab forces to the model. In the 3D finite element building models, considering temperature and shrinkage, a solitary expansion joint was allocated. The interaction of soil-foundation-structure was considered in the category of foundation soil stiffness. The time-history analysis was selected for the seismic effect. The structure experiences maximum displacements and rotations when subjected to striking inputs. The input has provoked damage either in the structural part or in rigidity. Based on damage evidence, one or multiple damage types could be included. It is worth mentioning that some types are predominant over others as they relate more to structural condition. The global damage factors for each structure depend on analysis. Improvements of various structures that needed a significant financial injection to assure performance attracted major consideration. The statement is expected as the result of previous soft openings and hard impacts. It is considered natural that structures having only one soft opening were built beforehand all events that have happened since. After strengthening, all structures experienced hard seismic input, and thus it was determined which further improvements should be executed. Additionally, worldwide events convergence is demanded as based on understanding the source of natural phenomena; developments could be anticipated. Essentially, this could happen only through knowledge pooling. The financial and political advisability of pooling would likely be negligible as permanent events worldwide could have fatal outcomes. Understanding and knowledge about the source of these events in advance would largely lessen misfortune. Considered typical case studies (before and after strengthening) show that the same structure types with different design principles, geography, usages, or any other factor could experience vastly different damage or even survive input being entirely unharmed. The completion of the strengthening projects on the considered buildings has enriched experience and methodologies to enhance reliability for building preservation worldwide within any construction type.
7.1. Before and After Comparison
The data obtained allows for a comparison of the structural capacity of a reinforced concrete frame 8-storey building before and after strengthening for each retrofitting arrangement. The most relevant parameters of interest are the values of interstorey drift, which will predict the structural response of both buildings in terms of the earthquake damage classification. The limits for interstorey drift, required to assess the damage of each building with reference to the earthquake intensity, represent a scale from slight to collapse damage of the structures under a maximum considered magnitude event (MCE). This interstorey drift is a primary factor in determining the yielding of MRF plastic hinges and the clearance of restraining bracing cables in BRB and PC MG bracing arrangements. Other relevant structural parameters after strengthening are the degree of damping of each arrangement, which results from the energy dissipation systems incorporated into the buildings along with the conditions that are set on their use.
A summary of the characteristics—interstorey drift and damage classification—of the frame structure 8-storey building before and after strengthening under the actions of the design earthquake event is presented below. There is significant damage estimation of the frame structure 8-storey building. If time history analysis is run with acceleration response spectrum, limit state damages occur for the frame structure building with a level of damage D4. If the level of severity is increased to MCE, ultimate limit state collapse occurs. Regarding the before and after strengthening comparison, shear bracing-retrofitted structural arrangement was implemented. Following results were obtained for the structural arrangement after strengthening: bracing clearances degree of limit state damages in bracing clearance, clearance of bracing cable, clearance of holding systems. With reference to the event with a return period of T=475 years, slight to moderate damage occurs under design peak acceleration of 0.25g ± 5%, which is expected for a non-retrofitted frame structure building. Regarding the before and after strengthening comparison, after strengthening arrangements showed effective guidance on retaining limit state damages at slight to moderate levels.
8. Experimental Testing of Strengthening Techniques
In order to verify and analyze the strengthening techniques most commonly used for structural and non-structural concrete members, a few typical experimental tests will be presented in this chapter. For simplicity, only a few tests are selected and modeled in detail, which aim at the most effective strengthening techniques and methods for concrete members. These basic tests can be extended with different parameters to investigate a wider spectrum of the subject.
All tests are modeled using the modeling software. This software provides a 3D modeling platform with advanced nonlinear materials suitable for the analysis of numerous types of construction structural and non-structural members. As a part of this chapter, a brief introduction to the FEM modeling software is given.
The basic analyses investigate an existing 7-story residential building undergoing an evaluation assessment for strengthening design. The structure is made of columns, beams, and slabs poured in place on site. Tests are run to analyze how the structure reacts under earthquake loading before and after the strengthening design intervention.
After the structure’s capacity is evaluated as insufficient, the design team is questioned about possible strengthening methods. To assess which method is the optimal solution, three options are modeled and analyzed: using steel braces, longitudinal steel plate bonding on the outside of columns, and outside wrapping of columns. For each option, a detailed description of the modeling assumptions and results will be stated.
The first experimental test shows how the structure reacts with a simple strengthening design to increase the stiffness of the columns by wrapping. Nonlinear modeling of concrete is performed with a concrete damaged plasticity material model. This model captures both the tension and compression states of the material. A detailed description of this model is given before the experimental test.
9. Challenges and Limitations in Strengthening Techniques
Strengthening or retrofitting of existing reinforced concrete structures against severe seismic events has become an important research area for many structural engineers. Several deficiencies in design practices adopted in past years have been established, and various strengthening approaches have been explored and investigated. However, there are still some unsolved problems regarding the upscaling of these approaches to wider applications in practical situations, and there are important feasibility issues that are still unexplored. Mankind has suffered several destructive earthquakes throughout history, with their impact on the built environment ranging from injuries to loss of life, economic losses, and disruption of lifelines. When constructing the urban landscape, it is widely known that historic local information about destructive earthquakes is available; states and societies' decisions have played a significant role in the urban landscape's vulnerability to seismic events, for instance, through false beliefs and practices that have persisted for several centuries. There is public acceptance of the validity of risk mitigation initiatives in light of the lack of experience with severe earthquakes.
Out of all structural materials, reinforced concrete has become the most broadly used one over the years. Even though it was essentially designed to improve elements' seismic performance, historic reinforced concrete structures have shown poor performance during severe seismic events due to deficiencies in design and construction practices. Some deficiencies in the seismic behavior of historic reinforced concrete structures designed and constructed according to past practices have been experimentally investigated, with detailing deficiencies being detected. Among these residual mechanisms, the restraint imposed by the non-damaged surrounding concrete has been found to significantly reduce the effectiveness of high-performance strengthening techniques such as the confinement provided by external fiber-reinforced polymer layers. It takes on even discrimination against widely used techniques like steel jacketing, where there is an evident need for further improvement. Up-to-date extensive numerical simulations have complemented the experimental studies by helping identify and understand the mechanisms leading to the distress and loss of effectiveness in the application of externally bonded plates or sheets of fiber-reinforced polymer.
In recent earthquakes, a significant amount of destruction to built environments and infrastructures has been observed. Reinforced concrete structures have been responsible for the loss of life and damages. The existing rural and urban structures in seismic zones should be properly evaluated and designed to meet the minimal strength and ductility demand in order to meet the modern code requirements. However, most of the existing structures are old, constructed before the implementation of the present code requirements. Thus, there is an urgent need to design or rehabilitate the structures to meet the present standards and be resistant to destructive seismic events. Recently, techniques for rehabilitation of existing structures and infrastructures using active or passive systems have been developed.
10. Innovations in Strengthening Techniques
Innovative techniques such as post-tensioning, spray-on materials, and polymer sheets have emerged to effectively strengthen and improve the earthquake-resisting capacity of existing reinforced concrete structures. Post-tensioning techniques can be applied to improve lateral resistance by increasing the lateral stiffness and lateral load capacity through tension or compression of members. New proprietary spray-on materials can be applied to vertically account for double curvature concrete shells and have helped in the development of large thin shell reinforcements. Thin polymer sheets bonded together can be used to strengthen hollow cross-section reinforced concrete poles that are common in transportation facilities and can break when the load is applied on the surface edge. This novel method showed the possibility of upgrading existing poles without replacing them and thus of high economic and social benefit. These novel techniques can be slope-improved for thin majorly prestressed concrete vertical shells. This technology allows very cheap modifications of concrete dome upper shells with strong safety improvements. It is simple and involves only the mounting of a constraining ring that calls for some construction work under the dome. It doesn’t influence the thermo-insulating and energy-saving properties of the shell, doesn’t damage the structure’s integrity, and only a small equipment stack is needed. Shells in hyperbaric structures, reservoirs, and gas holders can be modified relatively easily. Adding additional constraints is common for shells, which this invention is not in contradiction to. Innovations in strengthening techniques are being commercialized or at least published in peer-reviewed journals. Together with emerging innovations, these new or improved strengthening techniques will complement or otherwise expand the inventory of innovative or at least somewhat novel strengthening techniques that are expected to be available for the benefit of a wider range of users and gain acceptance both in research and design guidelines.
11. Cost-Benefit Analysis of Strengthening Techniques
So, you’ve got this building all ready for construction, yeah? You’ve already got the blueprints and everything. Just one issue – it’s in an earthquake-prone zone. But no worries! You have your hammer, nails, and the equipment to match. What to do? Well, there are three choices. One, willfully ignoring the problem; two, demolition of the building and starting from scratch; and three (the best choice), retrofitting it. Retrofitting involves the strengthening of the existing structure so it performs better in an earthquake. But wait a minute, haven’t you heard of the phrase “in this world, nothing is free?” Retrofitting costs money. Lots of it. Your deep pocket might want to consider not doing it if your structure is above average (carefully considering the risks). Or perhaps put more effort into using the best techniques there are if possible. So you think carefully about the costs and benefits of the different techniques you have at hand? Thus, all of the current aging buildings that need some TLC are retrofitted at the best cost possible.
The aim of this paper is to select cost-efficient techniques to strengthen reinforced concrete structures for earthquake-resistance purposes. With a computer program dealing with the mechanics of structures, models of RC-frame buildings will be generated. Models will be required to design the different elements of the frame, before and after retrofitting. The primary goal is to ensure that the existing frame fails principally in flexure prior to shear. Therefore, a software program has to be constructed in combination with this one to design different strengthening techniques. In addition, economic calculations need to be done using data for the different retrofit techniques. Consequently, the costs and benefits of using this technique have to be calculated in order to assess which one is the cheapest. There has been interest in funding to improve existing RC-frame buildings on account of their vulnerability to earthquakes. After this research is completed, they will compile and send a proposal regarding the financing of the different retrofit techniques. A proposal that will be favored more than others will also be presented.
12. Current Building Codes and Regulations for Earthquake Resistance
So, let’s talk about something pretty cool and important: how buildings can be made strong enough to withstand earthquakes. In most countries around the world, there are some rules that everyone has to follow when building structures like houses, schools, or offices. To keep everything safe and sound, especially in areas where earthquakes can happen, these rules are made to make buildings "earthquake-proof." Not really, but to make them as safe as possible! The seismic codes have become stricter since the 1980s, partly due to the big earthquakes that hit in 1989 and 1994. These earthquakes showed that some old buildings designed to the codes before those dates were seriously poorly designed.
In many countries, the minimum requirements for earthquake design and construction are laid down in national codes and standards. These codes and standards often take the form of written documents. In most countries, structures have to conform to small building codes and other codes depending on the size and use of the structures. For example, large hotels, theaters, pools, etc., and also on the use of the structures, e.g., schools, hospitals, etc. The building code does not concern itself with small buildings. Smaller towns or villages usually have building codes written by engineers of that area or engineers with experience in small buildings. The codes are published by relevant authorities. The first earthquake design standards were published in the 70s. In the view of designers, most of the standards failed catastrophically as large cities were devastated with high casualty rates during the earthquakes. In reaction to the failures of the standards, many countries re-evaluated their design codes and tried again. A new code was published for most of the regions in the mid-90s with lessons learned from the poorly designed buildings in the earlier earthquakes.
Basically, certain seismic design codes are adopted in many countries. A structure is designed according to the local codes as a basis for networks of civil codes. These codes, in turn, are usually amended by national codes. The performance of older buildings is often scrutinized with respect to the hindsight of the standards used. Even now, many countries do not have adequate codes, but on average, most codes do work fairly well.
13. Future Trends and Research Directions
With urbanization on the rise and some populations looking to expand into earthquake-prone regions, it’s vital that new RC structures adhere to strong seismic reinforcement codes. In an area with a 475-year design seismic return period, the probability of a structural system incurring significant post-earthquake damage must be less than 20%. Code-conforming structures must provide full collapse prevention. Moreover, when people return to the structure after an earthquake, it must not only be habitable, but they must also feel safe with the integrity of the structure. Thus, the probability of the structure being post-earthquake unusable should not be greater than 10%. Unfortunately, many countries depend on code prescriptions that are too often misapplied and lead to structures that are overly vulnerable to seismic events.
In light of these shortcomings, this research focused on RC building structures not conforming to current seismic design codes and proposing researched strengthening techniques that will be most effective in protecting against detrimental damage and aftershocks. The methodology followed in conducting this research provides structural engineers and designers certainty about the viability of existing RC structures as well as a general path forward for analysis and reinforcement. First, it is determined whether a structure is code-compliant. If not, a gap analysis of the deficiency is prepared, which outlines damage mechanisms should the structure experience a seismic excitation of a certain level. There are recommendations for further analysis, such as conducting a tiered evaluation process to study the vulnerability of the structure and the effectiveness of proposed repairs. Numerical modeling techniques adapted to the chosen finite element software used in the analysis often utilize past research regarding fragility and capacity. Reliable modeling with clearly defined and documented approaches is critical to the success of proposed repairs.
In analysis and modeling, the most critical aspect is the development of clear and reliable fragility curves that accurately define damage limits with a certain probability of exceedance, broken down by non-conforming structural and design parameters including the number of stories, structural system type, soil type, and level of repair. Given that certain input data is often not readily available, conducting a sensitivity analysis to study the importance of different input parameters on the vulnerability of the structure should be performed.