Planda düzensiz yapıların deprem yönetmeliğine göre incelenmesi
A Study of the irregular structures in plan according to national earthquake code
- Tez No: 83056
- Danışmanlar: DOÇ. DR. A. NECMETTİN GÜNDÜZ
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1999
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 103
Özet
PLANDA DÜZENSİZ YAPILARIN DEPREM YÖNETMELİĞİNE GÖRE İNCELENMESİ ÖZET Depremlerden sonra yapılan incelemelerde taşıyıcı sistem düzensizliklerinden dolayı pekçok hasarın meydana geldiği görülmüştür. Bu amaçla deprem riski olan ülkelerde deprem yönetmeliklerinde düzensiz yapılar için özel şartlar getirilmiştir. Bu şekilde depremin zararlı etkilerinden korunma amaçlanmıştır. Bu çalışmada, deprem yüklerinin yapılar üzerindeki etkisi hakkında bilgiler verilmiş, depreme dayanıldı yapı tasarımı, mimari ve taşıyıcı sistem düzenlenmesi başlıkları altında açıklanmıştır. Deprem yönetmeliğimizin taşıyıcı sistem düzensizliklerine bakışı ve düzensiz yapılara karşı getirdiği şartlar incelenmiştir. Bu düzensizlik durumlarından Afet Bölgelerinde Yapılacak Yapılar Hakkında Yönetmelik, 1997'de Al, A2 ve A3 türü düzensizlikler olarak tanımlanmış, planda düzensizlik içeren üç ayrı yapı örneği SAP90 Yapı Analizi Programı yardımı ile analiz edilmiştir. Burulma Düzensizliği (Al) içeren yapı örneği Yönetmeliğimizin öngördüğü şartlar analizlerde gözönüne alınarak Eşdeğer Deprem Yükü ve Mod Birleştirme yöntemlerine göre çözülmüş, böylece mevcut düzensizliğin hangi yöntemle daha iyi temsil edildiği araştırılmıştır. Diğer iki düzensizlik durumu ( A2 ve A3 ) için, ele alınan örnekler, yapıların döşemelerinin rijit diyafram davranışı yaptığı kabulüne ve yapmadığı duruma göre analiz edilip, her iki çözüm birbiriyle kıyaslanarak Yönetmelik'in öngördüğü hesap biçiminin gerekliliği incelenmiştir. viii
Özet (Çeviri)
A STUDY OF THE IRREGULAR STRUCTURES IN PLAN ACCORDING TO NATIONAL EARTHQUAKE CODE SUMMARY The heavy damage and even collapse of irregular buildings during earthquakes, take attention of the researchers to this subject. Those who have studied the performance of buildings in earthquakes generally agree that the buildings' form greatly influences its performance under ground motion. This is because the shapes of the building have a major effect on the distribution of earthquake forces. A simple and symmetrical building form allows the most balanced distribution of forces. Earthquakes repeatedly proved that the simplest structures have the greatest chance of survive. Only simplicity of form will not ensure low torsional effects. For example, even in simple rectangular buildings the location of the stiff stair, elevator cores and rigid structural elements that add mass to only one part of the building can result in different locations of the centre of mass and the centre of rigidity ( Figure 1 ). Such building irregularities are often responsible for major damage during a severe earthquake, as they may lead to detrimental torsional effects that were not considered during the design process. Figure 1 Torsional Irregularity IXTorsion occurs in a building subjected to dynamic loads when the centre of mass does not coincide with the centre of rigidity, causing the potential for rotation to occur about its centre of rigidity. It has been observed that this often leads to increasing the effect of lateral forces on structural components in direct proportion to their distances from the centre of rotation. When symmetry does not exist, a building tends to experience severe twisting as well as the usual rocking back and forth. The twisting action often has its greatest effects on the joints between elements of the bracing system. Through investigation and careful detailing of these joints for construction are necessary for a successful design. Analysis may show that in some buildings torsional effects may be negligible. However, as a result of normal variations in material properties and section geometry, and also due to the effects of torsional components of ground motion, torsion may arise also in theoretically perfectly symmetrical buildings. Hence codes require that allowance be made in all buildings for so-called 'accidental' torsional effects. This effect is considered by acting lateral storey forces at the ±5% of the building plan in the National Earthquake Code 1997 as so many codes. When analysing a building by using Equivalent Static Load Method that has torsional irregularity in plan, National Earthquake Code 1997 is required to increase the ±5% eccentricity by multiplying it with ' Di ' coefficient that is dependent to ' Torsional Irregularity Coefficient '. If the torsional irregularity coefficient arises 2.0 The Code doesn't permit to analyse by Equivalent Static Load Method. But using Superposition of Modes Method isn't limited. A common building form that presents seismic design problems is the 're-entrant corner'. The re-entrant corner is the common characteristic of overall building configurations that, in plan, assume the shape of an L, T, H, +, or a combination of these shapes. These building shapes permit large plan areas while still providing rooms with access easily to air and light. Because of these characteristics, they are commonly used. These forms of buildings cause two related problems. The first is that; these shapes produce variations of rigidity. The differential motions between different parts of the building result in a local stress concentration at the re-entrant corner. In Figure 2, if the ground motion is considered to be occurred with a north-south direction at the L shaped building shown, the wing oriented north-south will, purely for geometrical reasons, tend to be stiffer than the wing oriented east-west. The north-south wing, if it were a separate building, would tend to deflect less than the east-west wing. This will cause; the different parts of the building vibrate at different rates that results additional stresses. The second problem is that, the centre of mass and the centre of rigidity in this form can not geometrically coincide for all possible earthquake directions. This will resultin additional torsional forces. Unless the two wings are designed with the capacity to resist and dissipate these torsional effects adequately, the building system may absorb severe damage, particularly at the notch where the wings meet. In addition, the wings of a re-entrant corner building often are of different heights so that the vertical discontinuity of a set back in elevation is combined with the horizontal discontinuity of the re-entrant comer in plan, resulting in an even more serious problem. When earthquake take place, where the set-backs occur, a notch is created that causes stress concentration. Figure 2 The L shaped building There are two basic alternative solutions to this problem: to separate the building structurally into two shapes, or to tie the building strongly at the lines of stress concentration and locate resistant elements to resist against torsion. When individual parts of the building is designing, there may be a potential problem; when moving at the same time, a problem for the separate parts becomes the actual dimension of the separation that must be provided to prevent them from bumping each other (called battering or hammering). Gaps separating adjacent structures must be large enough to ensure that even during a major seismic event, no hammering of adjacent structures will occur due to out-of-phase relative motions of the independent structures It is generally recognized that large discontinuities or sudden changes in the strength or stiffness of a building can cause huge seismic effects. This is particularly the case where there are sudden changes in the vertical arrangement of the structure that result in changes of strength from floor to floor. The most effective of the problems caused by such a discontinuity is the soft first story, a term applied to a ground level story that is more flexible than above. Although a soft story at any floor creates a problem, a stiffness discontinuity between XIthe first and second floors tends to result in the most serious condition because forces are generally greatest near the base of buildings. This discontinuity may occur because one floor, generally the first, is significantly taller than the others, results in decreased stiffness at this floor. Discontinuity also may occur when some vertical framing elements are not brought down to foundation and stopped at the second floor to increase the openness at the ground level. This condition creates a discontinuous load path resulting in a sudden change of strength and stiffness at the point of change. Finally, the soft story may be created by an open floor that supports heavy structural or nonstructural walls above. The basic problem with all these variations of the soft story is that most of the earthquake forces in the building, and any consequent structural deformity, tends to be concentrated in the weaker floor or at the point of discontinuity instead of being more uniformly distributed among all stories. This causes tremendous stress concentrations at the lower floor connections. Failure may occur at this points and results in the collapse or partial collapse of the upper floors. The earthquake loads at any level of a building will be distributed to the vertical structural elements through the roof and floor diaphragms. The roof/floor diaphragms respond to loads like a deep beam. Three factors are important in diaphragm design. First, the diaphragm must be adequate to transfer the forces and must be tied together to act as one unit. Secondly, the collectors (members or reinforcing) must transfer the loads from the diaphragm into the shear wall. And the last of all, openings or re entrant corners in the diaphragm must be properly placed and adequately reinforced. Inappropriate location or excessive size of openings (elevator or stair cores, skylights) in the diaphragm create problems. This reduces the natural ability of transferring the forces and may cause failure in the diaphragm. In Figure 3 there are examples of this type of irregularity according to National Earthquake Code 1997. D ? O O Ab1 fAb2 " p a a Ab/A > 1/3 A^: Sum of Opening Areas A : Total Floor Area Ab=Ab1 +Ab2 Figure 3 Irregularities in Diaphragms Cross- section A- AUnder seismic actions floor slabs play the important role of transmitting the inertial forces to the lateral-force-resisting system, and of tying together the elements of the latter into a three-dimension (3D) entity. To perform these roles slabs should be monolithically connected with their supporting beams, walls and columns, and should have adequate strength to remain elastic under the design seismic action. The in-plane stiffness of floor slabs should be properly recognised and included in the model for the seismic analysis of the structure. Slabs are commonly assumed as absolutely rigid diaphragms and modelled as such. The most convenient way of achieving this is by introducing at each floor level an additional node termed 'master' node, with only three degree of freedoms: two translations in the plane of floor and rotation about the normal to the plane of the floor. The master node should be placed at the centre of rigidity. The corresponding three degree of freedoms of all floor nodes are related to those of the master node through a 3x3 transfer matrix expressing the rigid-body- motion kinematic constraint. In National Earthquake Code 1997, the buildings that have irregularities in diaphragms as in Figure 3, are required to consider their floors do not act as rigid-diaphragm. And there should be considered enough nodes to add the elastic deformation of diaphragms into calculations. The thesis deals with the irregularities in structural systems where a special attention is paid to the irregularities in plan. The definition of irregular structures is given by considering National Earthquake Code 1997. After a detailed discussion of the behaviour of irregular structures under earthquake loads, three typical examples are presented. The first example that has torsional irregularity in plan is analysed by using the SAP90 program. The analysis is carried out by applying Equivalent Static Load Method and Superposition of Modes Method according to National Earthquake Code 1997. By comparing two methods Equivalent Static Load Method get bigger total shear force and internal forces than the other method. The second one is a building that has a large opening in its diaphragm. The third one is an L-shaped building. In the last two examples the diaphragms of the structures both considered as rigid- diaphragms and not. Additionally, stresses that occur in the diaphragms because of earthquake forces are checked.
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