Seismic Retrofitting of Structures

The basics of seismic retrofitting are based on the evaluation and improvement of the level of building performance. To this end, the purpose of the improvement is defined and, based on the desired performance levels of the building, the objective of the seismic retrofitting of the building is chosen. The objective of retrofitting and improving the seismicity of the building, in order to achieve the desired performance level, should be determined based on the principles of assessment and improvement of the building. Technical characteristics of the building must be examined by the designer and in coordination with the employer, before any seismic retrofitting action. These characteristics may include: the characteristics of structural components of the buildings, the level of earthquake hazard in the building, the initial results of seismic resistance assessment, history and future of exploitation of the building, the assessment of the needs and wishes of the operator, economic considerations, and the rules and regulations.

 Conventional Strengthening Methods

Conventional retrofitting and upgrading of steel structures and reinforced concrete can include the external reinforcement of a member of the structural steel with ordinary or prestressed steel, an increase in the cross-section of concrete members by concrete and shotcrete, and the fitting of prefabricated pieces and bonding of steel sheets. Strengthening concrete members by using concrete or steel jacketing increases the compressive strength, shear and also the enclosure to increase the stiffness or bonding of concrete and reinforcement. With this method of retrofitting, the shear capacity, compression and enclosure of columns, walls, slabs and beams can be provided, and thus their bearing capacity increases against earthquake and vertical gravity and lateral loads. Pre-tension is to create a constant and consistent tension in a concrete member such that, due to this stress, some of the tensions caused by dead and live loads in this member will be neutralized and as a result, the load bearing capacity of the member will increase. Pre-tension is one of the most commonly used retrofit methods used to strengthen reinforced concrete structures. The elasticity of the cables increases the active bearing capacity of the member immediately and strengthens the present structure.

Structural Health Monitoring

Nowadays, the methods of detecting and monitoring damage and cracking in structures is the subject of a wide range of ongoing researches due to its importance in structural failures. Instantly, the cause of many structural failures is the rupture of their material. The initiation of these fractures has been accompanied by cracking which, with its expansion, is considered as a serious threat to the behavior of the structure. Accordingly, methods of detecting and monitoring cracks and defects are the subject of numerous studies that have been done so far and are still ongoing. Structural health monitoring is a process for obtaining accurate momentary information from structural conditions and performance. In structural monitoring, damage is defined as changes occurring during the operation of the structure and identifies the damage to all methods and techniques that identify the failure and express the position and severity of the failure . In recent years, with advances in the field of science, remote monitoring methods are used with lasers, fiber optic sensors, remote sensing techniques and image processing techniques. One of the methods for failure detection is dynamic signal processing. During the operation of a structure such as large-sized frames, high-rise buildings and bridges, the local and internal cracks in the structure may increase steadily and eventually lead to collapse of the entire structure. The effect of cracking on the response of the structure is a deviation in local stiffness, which has a significant effect on the dynamic response of the structure. This can be interfered with by changing natural frequencies, and the analysis of these changes makes it possible to detect cracking. Today, building owners need a sense of confidence in their civilian assets. The owners of the towers and buildings want to know if their building is damaged after a vibrational event such as an earthquake or severe wind.

Nondestructive Assessment (NDT) Health evaluates the status or lack of health of structures. This assessment usually takes into account three main objectives:

1 – Structural health monitoring as the provider of information in the restoration and reinforcement of structures.

2-Structural health monitoring  is a tool for estimating the remaining useful life of the structure.

3- Structural health monitoring  is a tool for announcing the warning before the failure of the structure, as well as determining the escape routes and safe points of the structure in critical situations such as war, earthquake, severe winds, etc.

Seismic Retrofitting Techniques for Concrete Structures

Many reinforced concrete or so-called RC structures need to be rehabilitated, retrofitted, strengthened and renewed for various reasons, such as design or construction errors, structural changes in use, and the loss of part of the structural strength due to corrosion of steel bars. Finding a suitable solution for reinforcing concrete structures and improving the bearing capacity of such structures has always been a concern for designers and constructors of RC structures. Concrete structures, as a large part of the structures, are designed and constructed in accordance with accurate calculations and ductility requirements, but construction quality in some structures is very poor for various reasons. It is obvious that every retrofitting program eventually result in strengthening of a component or the whole of structure.

Strengthening Techniques for Concrete Structures

There are several strengthening methods for concrete structures that are very common and almost most of civil engineers are familiar with following methods:

• strengthening with steel or concrete jacketing
• strengthening with steel shear wall
• strengthening with steel bracing
• strengthening with concrete or masonry walls
• strengthening by adding a new moment frame

Strengthening  Concrete Structures with FRP

In retrofitting of concrete structures, there is a need for materials that can provide good durability for concrete against adverse environmental conditions, in addition to appropriate increase in the structure’s capacity. The development of new materials technologies and polymers with different mechanical properties has pushed the engineering community to use the various capabilities of polymer and composite products and to replace traditional materials with them. By arrival of FRPs in the building and construction industry, as one of the most interesting and promising technologies, many problems were addressed by building and building rehabilitation activists and new ways to reinforce and repair concrete structures appeared. In these methods, various forms of FRP materials such as fibers, sheets and reinforcement are used to improve the bearing capacity, repair, strengthening and rehabilitation of concrete structures.

Seismic Retrofitting Techniques for Steel Structures

Steel buildings are very suitable for design and execution, with precision, strength and ductility, but due to the lack of use of specialist construction forces and wrong assumptions in design phase about the construction of steel buildings, there are disadvantages and defects that need to be retrofitted. Steel buildings are often damaged due to local buckling under seismic loading and do not function properly or due to corrosion, need to be retrofitted.

Strengthening Techniques for Steel Structures

There are several strengthening methods for steel structures that are very common and almost most of civil engineers are familiar with following methods:

• strengthening with concrete jacketing
• increasing stiffness in
• strengthening the joints with steel plates
• adding new shear wall
• adding new bracing
• adding new secondary beam

Strengthening  Steel Structures with FRP

In the method of using FRP composites for retrofitting steel beams, FRP composites are placed on the surface of the steel beams and lead to bracing them against local buckling. Depending on the type of cross section of the member, the support conditions, and the dimensional characteristics of the wing and the cross section, different layouts can be selected for placing on the . Of course, the thickness and dimensions of the FRP composite used to reinforce the beam, due to various factors such as computational conditions and economic considerations, can be chosen to cover part of the section or to be used to cover the entire cross-section.