Concrete Shear Walls - Assignment Example

Concrete Shear Walls History of Shear Walls In various regions and countries around the globe that experience regular earthquakes, buildings with cast-in-situ reinforced shear walls are common. This building technique has been used since late 1950s and early 1960s in urban areas to construct tall buildings. Prior to the introduction of concrete shear walls, masonry structures were designed using empirical procedures based on the past performance of similar structures. Because shear wall technique was not being used, early concrete structures used to be massive so as to effectively withstand vertical as well as lateral loads. Even though this rational or empirical technique is still being employed on a limited basis, earthquakes and strong winds have shown that a more defined and logical technique is required to design concrete structures that can effectively withstand disastrous forces caused by strong winds and earthquakes. In early 1950s, elastic working stress design techniques were introduced to be used to design reinforced concrete structures. Using this new elastic design technique, builders started reinforcing concrete structures with steel so that the steel could withstand tensile forces while concrete took care of compressive forces. By mid 1950s, the Uniform Building Coded included working stress design techniques for masonry which allowed builders to size masonry members by making sure that expected service was not above the permitted design stress (Reitherman, 2012). With this working stress design technique, builders were able to design concrete structures throughout much of the 20th century until concrete shear walls were introduced in late 1950s. From then, the building engineering progressed slowly until early 1980s when the issues regarding energy dissipation and seismic performance became paramount. These issues led to the introduction of reinforced concrete shear walls. A dual system of ductile concrete moment framed coupled with confined concrete shear walls was recognized as the best approach to achieving the required strength to withstand seismic forces. This approach worked perfectly with a perimeter moment frame and an interior shear wall core, or the other way through the use of a perimeter ductile wall and an interior ductile moment frame (Reitherman, 2012). In late 1980s and early 1990s, concrete shear walls coupled with yielding shear links were developed after a successful assessment conducted in 1977. This approach became one of the creative researches that are used to build a less expensive shear wall that can dissipate seismic energy effectively (Reitherman, 2012). During the period between 1950 and 1960, plywood walls were recommended as alternates to diagonally braced wall sections. The basis of preference of plywood for this purpose was their capacity to meet certain requirements outlined by the relevant authorities. However, owing to the advancement in technology and emergence of new building techniques, other prefabricated alternatives have made is possible to incorporate shear assemblies into narrow walls that fall at either side of the opening. Examples of new alternatives that have replaced plywood in shear walls are steel-backed and sheet steel shear panels that have been proven to offer stronger seismic resistance than the plywood. Benefits of concrete shear walls The benefits of concrete shear walls in the structural design of highrise buildings have long been acknowledged. According to Harne (2014), concrete shear walls are very beneficial because they make buildings strong and resistant to seismic forces caused by earthquakes and strong winds. In most of the tall buildings located in regions prone to earthquakes, concrete shear walls serve as the major lateral load-resisting element. This is why Buildings with reinforced concrete shear walls are widespread in many Earthquake-prone countries and regions, such as Canada, Chile, Romania, Turkey, Colombia, and other areas. According to Harne (2014), shear walls are efficient, both in terms of construction cost and of effectiveness in minimizing earthquake damage in structural and non-structural elements like glass windows and building contents. Properly designed and detailed buildings with shear walls have shown very good performance in past earthquakes. Research found in the past 30 years of the record service history of tall building which containing shear wall element, none of them has collapsed during strong winds and earthquakes (Harne, 2014). According to Marzban, Banazadeh, and Azarbakht (2012), shear walls are the main vertical structural elements with a role of resisting both the gravity and lateral loads such as wind load and earthquake load. It provide sufficient strength and stiffness to buildings in the direction of their orientation, which significantly reduces lateral sway of the building and thereby reduces damage to structure and its contents (Marzban, Banazadeh, & Azarbakht, 2012). In other words, the shear walls act as cantilevers in withstanding lateral loads because they are usually subjected to axial forces and moments. These shear walls either can be closed section, open sections or planar typically located around elevator and stair cores or located at the ends of the building. Ambrose and Vergun (1997) note that buildings built using concrete shear walls are very strong because of a number of aspects. First, these concrete shear walls have strong anchorage that contributes to the strength of the building. According to Ambrose and Vergun (1997), the anchorage of a concrete shear wall consists of the attachment of the shear wall to the foundation to withstand the overturning and sliding effects caused by the lateral loads on the wall. This entails an extensive range of probable situations, depending on the construction of the wall and the amount of forces. Ambrose and Vergun (1997) also note that the strength of a shear wall system is determined by a number of factors such as the strength of the sheathing system; the size, spacing, and type of the fasteners; the panel aspect ratio, described as the ratio of long to short dimension of shear panel; and the strength of the studs. Due to these parameters, the design strength of shear walls is normally based on tests of full height specimens (Marzban, Banazadeh, & Azarbakht, 2012). The second aspect that make concrete shear wall beneficial is that overturning effect is taken into consideration when designing them to ensure that the building is stable even in situations of earthquakes or strong winds. According to Ambrose and Vergun (1997), the overturning effect is taken into consideration by carrying out the usual assessment for the overturning moment due to the lateral loads and determination of the safety aspect resulting from the resistance provided by the dead loads and passive pressure from the soil. Thirdly, concrete shear walls also contribute to the resistance of the building to horizontal sliding. According to Ambrose and Vergun (1997), horizontal sliding is the direct, horizontal force resistance in opposition to the lateral loads, and it is generated by some combination of passive soil pressure and soil friction or may be transferred to other parts of the building structure. When designing the shear wall, the form and amount of distribution of the vertical soil pressure on the foundation caused by the combination of load and moment are usually compared with the standard design limits to ensure that acceptable limits are achieved (Lao, & Han, 2011). Why Buildings should have Shear walls All tall buildings especially those in areas that experience regular earthquakes should have the shear walls because of a number of reasons. Firstly, shear walls offer the lateral strength to buildings in order to withstand horizontal earthquake forces. According to Harne (2014), shear walls have sufficient strength to transfer the horizontal earthquake waves to the adjacent component in the load conduit beneath them. These other elements in the conduit of the load could be slabs, a floor, footings, other shear walls, or even the foundation walls. In addition, shear walls have been proven to provide tangential firmness to prevent the floor or the roof from swaying sideways excessively. Harne (2014) notes that sufficiently strong shear walls will stop roof and floor framing components from sliding off their supporting components. In addition, structures with sufficient rigidity will normally experience minimal nonstructural damage (Harne, 2014). The second reason why buildings especially those in areas that experience regular earthquakes should have the shear walls is that shear walls have strong anchorage that contributes to the strength of the building. According to Ambrose and Vergun (1997), the anchorage of a concrete shear wall consists of the attachment of the shear wall to the foundation to withstand the overturning and sliding effects caused by the lateral loads on the wall. In addition, overturning effect is taken into consideration when designing them to ensure that the building is stable even in situations of earthquakes or strong winds. The other reason why buildings should have shear walls is that they serve as the major lateral load-resisting element especially in situations of strong earthquakes. When shear walls are located in advantageous locations in a building, they can be quite effective in withstanding lateral loads that are caused by earthquakes or strong winds. Reinforced concrete framed buildings are sufficient for withstanding both horizontal and vertical loads that act on them. It is also important to note that construction of shear walls is very easy because reinforcement detailing of the walls is rather simple and hence can be implemented at site with ease. References Ambrose, J. & Vergun, D. 1997. Simplified Building Design for Wind and Earthquake Forces. New York: John Wiley & Sons. August, Marzban, S., Banazadeh, M., & Azarbakht, A. 2012, ‘Seismic performance of reinforced concrete shear wall frames considering soil–foundation–structure interaction’ DOI: 10.1002/tal.1048 Harne, R. 2014. Comparative Study of Strength of RC Shear Wall at Different Location on Multi-storied Residential Building. International Journal of Civil Engineering Research. Volume 5, Number 4 pp. 391-400. Lao, X., & Han, X. 2011, ‘Performance index limits of high reinforced concrete shear wall components’ DOI:10.1007/s11771-011-0829-9 Reitherman, R. 2012. Earthquakes and Engineers: An International History. Reston, VA: ASCE Press. Read More