Earthquake Resistant Systems-Books Pdf

Earthquake Resistant Systems
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Fig 3 1 Building Frame System, 3 Moment resisting frame systems shown in Figure 3 2. provide a complete space frame throughout the building to. carry vertical loads and they use some of those same frame. elements to resist lateral forces,Fig 3 2 Moment Resisting Frame System. 4 A dual system is a structural system in which an. essentially complete frame provides support for gravity. loads and resistance to lateral loads is provided by a. specially detailed moment resisting frame and shear walls. or braced frames The moment resisting frame must be. capable of resisting at least 25 percent of the base shear. and the two systems must be designed to resist the total. lateral load in proportion to their relative rigidities. This system which provides good redundancy is suitable. for medium to high rise buildings where perimeter frames. are used in conjunction with central shear wall core. Concrete intermediate frames cannot be used in seismic. zones 3 or 4,3 2 Lateral Force Resisting Elements, Lateral force resisting elements must be provided in every. structure to brace it against wind and seismic forces The. three principal types of resisting elements are shear walls. braced frames and moment resisting frames,3 2 1 Shear Walls. A shear wall is a vertical structural element that resists. lateral forces in the plane of the wall through shear and. bending Such a wall acts as a beam cantilevered out of the. foundation and just as with a beam part of its strength. derives from its depth Fig 3 3 shows two examples of a. shear wall one in a simple one story building and another. in a multistory building,ATC SEAOC Joint Ve nture Training Curriculum.
Fig 3 3 Shear Walls, In Fig 3 3 a the shear walls are oriented in one direction. so only lateral forces in this direction can be resisted The. roof serves as the horizontal diaphragm and must also be. designed to resist the lateral loads and transfer them to the. shear walls, Fig 3 3 a also shows an important aspect of shear walls in. particular and vertical elements in general This is the. aspect of symmetry that has a bearing on whether torsional. effects will be produced The shear walls in Fig 3 3 a. show the shear walls symmetrical in the plane of loading. Fig 3 3 b illustrates a common use of shear walls at the. interior of a multistory building Because walls enclosing. stairways elevator shafts and mechanical shafts are mostly. solid and run the entire height of the building they are. often used for shear walls Although not as efficient from a. strictly structural point of view interior shear walls do. leave the exterior of the building open for windows. Notice that in Fig 3 3 b there are shear walls in both. directions which is a more realistic situation because both. wind and earthquake forces need to be resisted in both. directions In this diagram the two shear walls are. symmetrical in one direction but the single shear wall. produces a nonsymmetrical condition in the other since it is. off center Shear walls do not need to be symmetrical in a. building but symmetry is preferred to avoid torsional. Shear walls when used a lone are suitable for medium rise. buildings up to 20 stories high,Shear walls may have openings in them but the. calculations are more difficult and their ability to resist. lateral loads is reduced depending on the percentage of. What is a Shear Wall Building, Reinforced concrete buildings often have vertical plate like. RC walls called Shear Walls Fig 3 4 in addition to slabs. beams and columns These walls generally start at, foundation level and are continuous throughout the.
building height Their thickness can be as low as 150mm. or as high as 400mm in high rise buildings Shear walls are. usually provided along both length and width of buildings. Fig 3 4 Shear walls are like vertically oriented wide. beams that carry earthquake loads downwards to the. foundation,Fig 3 4 Reinforced Concrete Shear Wall,Advantages and Disadvantages of Shear Walls in. Reinforced Concrete Buildings, Properly designed and detailed buildings with shear walls. have shown very good performance in past earthquakes. Shear walls in high seismic regions require special. detailing However in past earthquakes even buildings. with sufficient amount of walls that were not specially. detailed for seismic performance but had enough well. distributed reinforcement were saved from collapse Shear. wall buildings are a popular choice in many earthquake. prone countries like Chile New Zealand and USA Shear. walls are easy to construct because reinforcement detailing. of walls is relatively straightforward and therefore easily. implemented at site Shear walls are efficient both in terms. of construction cost and effectiveness in minimizing. earthquake damage in structural and nonstructural elements. like glass windows and building contents, On the other hand shear walls present barriers which may. interfere with architectural and services requirement. Added to this lateral load resistance in shear wall buildings. is usually concentrated on a few walls rather than on large. number of columns,Architectural Aspects of Shear Walls. Most RC buildings with shear walls also have columns. these columns primarily carry gravity loads i e those due. to self weight and contents of building Shear walls. provide large 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, Since shear walls carry large horizontal earthquake forces.
the overturning effects on them are large Thus design of. their foundations requires special attention Shear walls. should be provided along preferably both length and width. However if they are provided along only one direction a. proper grid of beams and columns in the vertical plane. called a moment resistant frame must be provided along. the other direction to resist strong earthquake effects. Door or window openings can be provided in shear walls. but their size must be small to ensure least interruption to. force flow through walls Moreover openings should be. symmetrically located Special design checks are required. to ensure that the net cross sectional area of a wall at an. opening is sufficient to carry the horizontal earthquake. Shear walls in buildings must be symmetrically located in. plan to reduce ill effects of twist in buildings Fig 3 5. They could be placed symmetrically along one or both. directions in plan Shear walls are more effective when. located along exterior perimeter of the building such a. layout increases resistance of the building to twisting. Fig 3 5 Shear Wall Layout,Ductile Design of Shear Walls. Just like reinforced concrete beams and columns, reinforced concrete shear walls also perform much better if. designed to be ductile Overall geometric proportions of the. wall types and amount of reinforcement and connection. with remaining elements in the building help in improving. the ductility of walls,Overall Geometry of Walls, Shear walls are rectangular in cross section i e one. dimension of the cross section is much larger than the. other While rectangular cross section is common L and. U shaped sections are also used Fig 3 6 Thin walled. hollow reinforced concrete shafts around the elevator core. of buildings also act as shear walls and should be taken. advantage of to resist earthquake forces,Fig 3 6 Shear Wall Geometry. Braced Frames, A braced frame is a truss system of the concentric or.
eccentric type in which the lateral forces are resisted. through axial stresses in the members Just as with a truss. the braced frame depends on diagonal members to provide. a load path for lateral forces from each building element to. the foundation Fig 3 7 a shows a simple one story, braced frame At one end of the building two bays are. braced and at the other end only one bay is braced As with. Fig 3 7 a this building is only braced in one direction. and uses compression braces because the diagonal member. may be either in tension or compression depending on. which way the force is applied, Fig 3 7 b shows two methods of bracing a multistory. building A single diagonal compression member in one. bay can be used to brace against lateral loads coming from. either direction Alternately tension diagonals can be used. to accomplish the same result but they must be run both. ways to account for the load coming from either direction. Braced framing can be placed on the exterior or interior of. a building and may be placed in one structural bay or. several Obviously a braced frame can present design. problems for windows and doorways but it is a very. efficient and rigid lateral force resisting system. Fig 3 7 Braced Frames,3 2 3 Moment Resisting Frames. Moment resisting frames carry lateral loads primarily by. flexure in the members and joints Joints are designed and. constructed so they are theoretically completely rigid and. therefore any lateral deflection of the frame occurs from the. bending of columns and beams They are used in low to. medium rise buildings, The UBC differentiates between three types of moment. resisting frames, The first type is the special moment resisting frame that.
must be specifically detailed to provide ductile behavior. and comply with the provisions of the UBC, The second type is the intermediate moment resisting. frame which is a concrete frame with less restrictive. requirements than special moment resisting frames, However intermediate frames cannot be used in seismic. zones 3 or 4, The third type is the ordinary moment resisting frame This. concrete moment resisting frame does not meet the special. detailing requirements for ductile behavior, Ordinary concrete frames cannot be used in zones 3 or 4. Moment resisting frames are more flexible than shear wall. structures or braced frames the horizontal deflection or. drift is greater and thus non structural elements become. more problematic Adjacent buildings cannot be located too. close to each other and special attention must be paid to. the eccentricity developed in columns which increases the. column bending stresses, Two types of moment resisting frames are shown in Fig.
Fig 3 8 Moment Resisting Frames,Advantages, Provide a potentially high ductile system with a good. degree of redundancy which can allow freedom in, architectural planning of internal spaces and external. Their flexibility and associated long period may serve. to detune the structure from the forcing motions on. stiff soil or rock sites,Disadvantages,Poorly designed moment resisting frames have been. observed to fail catastrophically in earthquakes,mainly by formation of weak stories and failures. around beam column joints, Beam column joints represent an area of high stress.
concentration which needs considerable skill to,design successfully. Requires good fixing skills and concreting,3 2 4 Horizontal Elements Diaphragms. In all lateral force resisting systems there must be a way to. transmit lateral forces to the vertical resisting elements. This is done with several types of structures but the most. common way used is the diaphragm, A diaphragm acts as a horizontal beam resisting forces with. shear and bending action, There are two types of diaphragms flexible and rigid. Although no horizontal element is completely flexible or. rigid distinction is made between the two types because. the type affects the way in which lateral forces are. distributed, A flexible diaphragm is one that has a maximum lateral.
deformation more than two times the average story drift of. that story This deformation can be determined by,comparing the midpoint in plane deflection of the. diaphragm with the story drift of the adjoining vertical. resisting elements under equivalent tributary load The. lateral load is distributed according to tributary areas as. shown in Fig 3 9 a, With a rigid diaphragm the shear forces transmitted from. the diaphragm to the vertical elements will be in proportion. to the relative stiffness of the vertical elements assuming. there is no torsion as shown in Fig 3 9 b If the end. walls in the diagram are twice as stiff as the interior walls. then one third of the load is distributed to each end wall. and one third to the two interior walls which is equally.

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