Behavior of Structures in Earthquakes
Discuss About The Earthquake Vibration Control Using Modified?
The earthquakes occurred in the past have either completely collapsed the buildings or left many of them and experience various kinds of damages, structures made by the reinforced concrete. There have been various kinds of investigations carried out, on the strength of the structures and buildings, against the capability to withstand to the normal to severe earthquake. Some of the structural deficiencies discovered with the reinforced concrete made buildings are poor quality concrete, weak columns, splice lengths that are inadequate, incomplete considerations of the design and short column behaviour. Eventually, as attempts to minimize the structural deficiencies, various building codes are introduced and developed. Lateral stiffness, required ductility and strength are lower compared to the ones, imposed by the modern codes of buildings developed. Since these structures possess low ductility, these structures are more prone to the demand of the large displacement, because of the deficiencies in the lateral strength and stiffness.
When the earthquake is occurred , vibrations of the earth surface generate strong waves and disturb the buildings and infrastructure present on the surface of the earth. The loss of life does not happen directly, because of the earthquake directly, but because of the disturbing structure that are built, with no knowledge and safety against the earthquake force. Hence, there is an increasing need of understanding of the earthquake behaviour and relative construction methods for earthquake resistant buildings.
Shear walls act as strong structural systems, offering strong and lateral stability, against the earthquake, wind loads and many other lateral loads, to the structures. Usually, construction of the structural systems is done by the timber or plywood unreinforced masonry, reinforced concrete and reinforced masonry. The reinforced masonry is further divided, into shear wall frames, coupled shear walls, staggered walls and shear panels. These shear walls are modified in various techniques and methods for varied applications. (Venkatasairamkumar, 2014)
The objective of using the shear walls is to provide more resistance to the most of the lateral loads portion, in the structures’ bottom portion and provides better support through the framed element of the top structures portion. The top structure portions and their strength is necessary for wider range, small story, medium to high rise buildings. Most of the basements present in the world are used, generally, for garages, offices or parking and top floors are usually, allocated for the purpose of residence.
Modified Shear Walls - Materials and Reinforcement
The behaviour of the shear walls depends ultimately, on the material used, thickness of the shear wall, length of the wall, positioning of the wall, in the frame of building and several factors, such as the reinforcement, based on the provisions of the code.
The study of the existing buildings strengthening, based on the seismic evaluation is conducted. The study has emphasized on the identification of the risky buildings most vulnerable components and retrofit of the same, so that the risk of complete or partial collapse can be worked out to decrease. Though this evaluation does not stop or control the earthquake, but the performance of the building cna be increased, by strengthening the buildings, when earthquakes are occurred. In technical aspects, the word strengthening refers to retrofitting and practically, seismic performance improvement (Durgesh & Rai, N.D.)
The gravity and lateral system of load resisting has reinforced concrete slabs and walls. When the shear walls are considered, the major difference is that it is a main vertical structural element having a resisting dual role, with the lateral loads and gravity loads. The thickness of the wall varies in between 14 to 50 cm and is based on the age of the building, total number of stories and requirements of thermal insulation. Most often, the shear walls are constructed as continuous, throughout the height of the building. But, there are some walls which are discontinued, at the basement level or street front, so that parking spaces or commercial spaces can be allowed. It is also usual that the layout of the wall is symmetrical, according to, minimum of one axis of symmetry, when the plan is considered. Most often, the floor slabs are cast-in-situ flat slabs and however, precast hollow-core slabs also do exist. The thickness of the slabs does vary in between 12 cm, as in previous USSR and to 22 cm, when hollow-core slabs are considered in Kyrgyzstan, etc. Support of buildings is done by mat foundations or concrete strip and the mat foundations are commonly used for the buildings having the basements. Usually, modifications of structures are less common, for this kind of construction.
The seismic forces are the basic code requirement to design and build the shear wall buildings. It depends on the country specific seismic design provisions and method of analysis. For instance, when a Chilean seismic code is considered, it prescribes a coefficient of base shear, for the buildings of shear wall of 5 to 6.7% and it depends over the seismic zone. According to Canadian seismic requirements and Chilean requirements, the maximum drift of lateral story would be limited to 0.0002, when the regular buildings are considered and it would be 0.0005, when the Colombian seismic code is considered.
Seismic Evaluation and Strengthening of Buildings
The requirements of reinforcement are according to the specific code requirements of each country. In most of the cases, the reinforcement of the wall has two distributed reinforcement layers, both vertical and horizontal, throughout the wall. And bars of vertical reinforcement are provided, close to the openings of window and door and at the barbells or as boundary elements, called as, end zones of the wall, as well.
When the Romania shear wall buildings are considered, they consist of the walls that are lightly reinforced, having one 12 mm diameter layer for vertical bars and 8 mm for the horizontal bars. The spacing of reinforcement for walls varies in between 25 cm to 15 cm, respectively, in transverse and longitudinal direction. There are boundary elements for the transverse shear walls, at the end of facade.
When Canada seismic requirements are considered, the classification of National Building Code is done for shear wall buildings, into the wall system of ductile and nominally ductile, having its factor of force modification, R to have 2 and 3.5 values, respectively. For all kinds of the shear walls, both vertical and horizontal distributed reinforcement would be required, with the ratio of 0.25%. When the ductile shear walls are considered, there is an area of 0.25% of the wall and at least 4 bars are needed, for each fo the end zone.
In the instance of the Chile, the reinforced concrete structures design has to be performed based on the code, ACI1318-95. The design of the shear wall does not require considering and following its specific clauses, referring to the boundary elements design of the structural walls. In coupling beams, confinement reinforcement is also used, but rarely, for the vertical bars, at the diagonal bars or at the ends of the wall. Here, a cover with reduced reinforcement is allowed.
The shear walls having perforated with the openings would be considered as coupling walls, and act as cantilevered walls that are isolated and connected with the beams of coupling and it is called lintels or spandrel beams, designed for shear effects and bending effects. The function of these beams is to act as fuses and have its applications in seismic energy dissipation, when it is designed as a ductile manner. Coupling beams are designed, having its diagonal reinforcement, for ensuring the strong and effective ductile seismic response. Joining of the reinforcement bars together is done, through lap splices or welding.
Coupling and Bracing Systems for Seismic Resistance
Cladding of exterior shear walls is done, in stucco, backed by the cold-farm masonry or steel framing veneer, panels of glazing or steel or precast.
Global structure strengthening is an effective and economical strategy of rehabilitation and where as seismic evaluation refers to the performance that is acceptable, because of the overall strengthened structure. This unacceptable performance can be identified, during the occurrence of global inelastic onset behaviour, at ground levels shaking, which are lesser substantially, compared to the design levels of the code. The ground motion threshold value can be raised possibly, by the introduction of additional strength to this resisting system of lateral force. Braced frames and shear walls are considered as the important elements, made use for the same purpose. However, sufficient stiffness has to be assured, compared to the building, supplementing. It demands the design to have all the lateral resistance of the structure. It is needed, when it is desired, since the existing members possess, inelastic strength at very little value. When a new structural system is added to the building that exists, would change the complete building’s dynamic behaviour automatically to a considerable extent, when earthquake is occurred. The choice of the size and number of the elements added and type of the element depends on the existing building’s vulnerability and also on the building’s functional layout. Shear walls can possibly provide the earthquake resistance, needed for the building structure, which is the most significant part, because of the lateral strength and greater stiffness.
There are close to six severe earthquakes of more than 8.0+ degree and have been eminent during the nineteenth century. These major and extreme loss earthquakes have taught significant lessons to the society and to the governments. Here, only a small number of buildings made with reinforced concrete are subjected and affected to these eminent earthquakes.
There has been an important study conducted for the epoxy grouted dowels guidelines in the projects of seismic strengthening. The existing concrete structures are to be used either to improve the seismic performance of the structures, towards stronger existence in the future or to rebuild the buildings with enough strength, in the earthquake regions, after the earthquakes are occurred. The strength of the buildings can be bettered, by adding a new steel member or adding a new concrete, to the structures existing. The best choice is the epoxy grouted dowels, because of the ease fo the epoxy resins installation and improved strength of the same. Creep concern is precluded, for the dowels short term loading, from the seismic loading. As the concrete mass is used to grout the dowels, enough insulation would be needed for the epoxy protection, from the sources of heat, like fire (Wylli, 1988).
Lessons from Historical Earthquakes
Another study has been conducted by National Research Council of Canada, with the objective of exploring and developing the building structures seismic upgrade guidelines. The fact discovered is that most of the buildings, in Canada, India and many other earthquake prone areas, are built with the older technology and were built with hardly any understanding and knowledge of resistance of earthquake. Eventually, these buildings are now found to be unsafe, when checked their strengths, against the present building codes. Hence, the code requirement can be justified, only for the new buildings. It is because, attempt to evaluate the existing buildings demand huge cost, for upgrade of the existing buildings and the heritage value would be easily destructed. Hence, a new set of procedures are developed as alternatives, by NRC and have published, in the Seismic Evaluation of Existing Building Guidelines. These guidelines are presently known and referred as ‘Guidelines for Seismic Evaluation’ (NRC, Canada, 1995).
The guidelines emphasize the use of the shear walls. These shear walls usually are made up of reinforced masonry, reinforced concrete, steel or plywood on studs. The new system is guided to place wither in the interior of the building, as interior walls or also as exterior walls, or in the form of bracing. It is also seen as buttresses, outside the building. (NRC, Canada, 1995)
The study has been advanced with the recent retrofit of RC shear wall. The reinforced concrete shear walls are considered to provide better stiffness, lateral strength and better capacity of energy dissipation and so give better resistance against the lateral loads that are occurred from the wind loading or earthquakes. Hence, these RC shear walls have given better and wider applications in the buildings both in medium to high-rise, covering both residential and commercial purpose buildings.
The RC shear walls have gone through various modifications, in the aspect of their design, in the past decades. So, they are presently have the capability to adopt for both the new construction and strengthen the older constructions. The modern principles of capacity design and performance evaluation methods adopted are the major and significant advances, worth noting in the earthquake engineering advancement. So, an opportunity has been found to develop and enhance the seismic performance of the older buildings, by adopting the modern technology of adopting the RC shear walls. Hence, RC shear walls are found to match the need of the new seismic design techniques, based on the performance (Galal & El-Sokkary, 2008). The modern technology makes use of various retrofit techniques, with the help of various kinds of materials, like concrete, shape memory alloys, steel, fiber-reinforced polymers, all adopted, differently with different techniques of retrofit, towards upgrading the existing building seismic resistance.
A massive destructive earthquake in Turkey has taken lives of many residents and caused huge loss of infrastructure and buildings. The study conducted on the strength of the buildings, have revealed the fact that these buildings and reinforced concrete have no enough rigidity and strength, to withstand the earthquakes. Hence, the loss of the lives and property are recorded to be huge. The method of strengthening that could have been used in these kinds of RC buildings is the RC shear walls or infill walls application, in the buildings’ frame openings.
An attempt was made to retrofit these buildings, which are in unsafe conditions, since these buildings were in heavy risk conditions. However, people were not ready to rehabilitate easily, their buildings. So, easier methods were explored fro application, without vacating these buildings, to strengthen these structures. The application method for retrofit is the external shear wall application. These shear walls made of reinforced concrete and applied on the structures’ external surface.
The retrofit technique made use of total 7 test samples, in the geometrical scale of ½ and 1/3rd and the retrofitting technique performance is analyzed. The technique involved the shear wall, leaning over the frame, from the strengthening techniques and application of external shear wall building. The design includes some space, between the external shear wall and frame, by connecting the elements, through coupling beam (Kaltakci et al, 2010).
Another study has been conducted on the RC building seismic strength, through exterior shear wall. Seismic loadings were simulated, through the two stories framed model structures testing, under cyclic lateral sway, reversed and imposed. The study has reported the findings that the shear walls implemented to the system of structure has shown greater improvement to the bare frame capacity, as anticipated (Kaplan et al. 2011).
The type of building is the earthquake resistance consideration. Most of the reports show the good behaviour, during the past earthquakes. These buildings’ unfavourable earthquake performance is because of the construction quality that is inadequate and was in Moldova case. Other factors are inadequate detailing and amount in wall reinforcement and wall density in longitudinal direction and lateral confinement lack within the walls and the elements of boundary.
The possible deficiencies that could affect the seismic performance, adversely, for this kind of construction are soft-story mechanism, reduced density of wall and their effects.
Earthquake either damages the structures of the building or collapses completely and the result is based purely on the kind of construction and its strength. The important factor that influences the result is the reinforcement concrete. The performance of the building is also influenced by the severity of the earthquake. Shear walls are considered to be an important factor that can be controlled with the specification of the building code. The specification of the building code varies for each region, according to the seismic affect and possibility in the same region. The objective is to increase the lateral loads portion resistance for the structures. Seismic evaluation is done for each zone to develop the building code and according to it, structural features are defined and followed, in each region. Earthquake affects and influences can be better controlled with the modified framed shear walls, according to the possibility and severity of earthquake in each region. There have been many studies conducted for the shear wall design and presented.
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