Deprem kuvvetlerine karşı betonarme perdelerin davranışı ve boyutlandırılması
The Behaviour and design of reinforced concrete structural walls for earthquake resistance
- Tez No: 66655
- Danışmanlar: PROF. DR. ZEKAİ CELEP
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 154
Özet
ÖZET Bu çalışmada, yatay kuvvetleri taşımada perdelerin önemi üzerinde durulmuştur. Perdeli sistemler, perdelerin boyutlanması ile ilgili ortaya çıkan problemlerin ayrımını kolaylaştırmak için çeşitli sınıflandırmalar yapılarak incelenmiştir. Bununla birlikte perdelerin davranışında sadece yatay kuvvetlerin etkili olmadığı gösterilmiştir. Deprem kuvvetlerine karşı perdeleri modellemek için önemli kriterler, yapılan kabuller ve ayrıca boyutlama yapan proje mühendisinin sağlamayı amaç edineceği üç kriter rijitlik, dayanım, süneklik incelenmiştir. Perdelerin plastik davranışının, çeşitli özelliklerini incelemek ve boyutlama için gerçekçi bir yöntem geliştirmek için yapılan kabuller verilmiştir. Dayanım ve süneklik için perde elemanların nasıl boyutlanması gerektiği; perdelerde göçme şekilleri, eğilme dayanımı, süneklik ve stabilite bozukluğu, konstrüktif kurallar ve detaylandırmanın önemi gibi konular düşünülerek işlenmiştir. Yeni Deprem Yönetmeliği 1996 da perdelerle ilgili kayıtlar incelenmiş ve diğer yönetmeliklerle karşılaştırılmıştır. Temel prensiplerin ve boyutlama esaslarının daha iyi anlaşılması, özellikle Deprem Yönetmeliği 1996 da perdelerle ilgili kayıtların sonuçlara etkisini göstermek için örnekler çözülmüştür. Kanatlı perdelerin, deprem etkisi altında, taşıyıcı sistem içindeki davranışını incelemek ve kullanılan çözümleme tekniklerini karşılaştırmak amacıyla bir taşıyıcı sistem üzerinde, değişik perde kanat boylan seçerek yapılan bir dizi çözüm incelenmiştir. Dayanım ve süneklik için perde elemanlarının boyutlandırılması ile ilgili tavsiyeler ve dikkat edilmesi gereken koşullar verilmiştir. Son olarak da deprem etkisi altında betonarme elemanların plastik davranış ilkelerinin perdeler için de uygun olduğu sonucuna varılmıştır. XIII
Özet (Çeviri)
SUMMARY THE BEHAVIOR AND DESIGN OF REINFORCED CONCRETE STRUCTURAL WALLS FOR EARTHQUAKE RESISTANCE In the design of reinforced concrete multistory buildings, in which lateral load resistance has been assigned to structural walls, the emphasis should be on a rational strategy in the positioning of walls and the establishment of a hierarchy in the development of strengths to ensure that in the event of a very large earthquake brittle failure will not occur. It is required that the preferred mode of energy dissipation of structural walls should be flexure in a predictable region. Therefore, failures due to diagonal tension or compression, crushing of concrete in compression, sliding along construction joints, instability of wall elements or reinforcing bars and breakdown of anchorage's should be considered in the deterministic design philosophy and dimensioning of potentially plastic regions of walls. When walls are situated in advantageous positions in a building, they can form an efficient lateral-force-resisting system, while simultaneously fulfilling other functional requirements. For buildings up to 20 stories the use of structural walls is often a matter of choice. For buildings over 30 stories, structural walls may become imperative from the point of view of economy and control of lateral deflection. In studying various features of inelastic response of structural walls and subsequently in developing a rational procedure for their design, a number of fundamental assumptions are made: 1. In all cases studied in this study, structural walls will be assumed to posses adequate foundations that can transmit actions from the superstructure into the ground without allowing the walls to rock. Elastic and inelastic deformations that may occur in the foundation structure or the supporting ground will not be considered in this study. 2. The foundation of one of several interacting structural walls does not affect its own stiffness relative to the other walls. 3. Inertia forces at each floor are introduced to structural walls by diaphragm action of the floor system and by adequate connections to the diaphragm. In terms of in- plane forces, floor systems (diaphragm) are assumed to remain elastic at all times. 4. The entire lateral force is resisted by structural walls. 5. Walls considered here are generally deemed to offer resistance independently with respect to the two major axes of the section only. It is to be recognized, however,that under skew earthquake attack, wall sections with flanges will be subjected to biaxial bending. Suitable analysis programs to evaluate the strength of articulated wall sections subjected to biaxial bending and axial force, are available. They should be employed whenever parts of articulated wall sections under biaxial seismic attack may be subjected to significantly larger compression strains than during independent orthogonal actions. The basic criteria that the designer will aim to satisfy are those discussed in this study (i.e., stiffness, strength, and ductility). Structural walls provide a nearly optimum means of achieving these objectives. Buildings braced by structural walls are invariably suffer than framed structures, reducing the possibility of excessive deformations under small earthquakes. It will thus often be unnecessary to separate the possibility of excessive deformations under small earthquakes. It will thus often be unnecessary to separate the nonstructural components from the lateral-force- resisting structural system. The necessary strength to avoid structural damage under moderate earthquakes can be achieved by properly detailed longitudinal and transverse reinforcement, and provided that special detailing measures are adopted, dependable ductile response can be achieved under major earthquakes. The view that structural walls are inherently brittle is still held in many countries as a consequence of shear failure in poorly detailed walls. For this reason some codes require buildings with structural walls to be designed for lower ductility factors than frames. A major aim of this study is to show that the principles of the inelastic seismic behavior of reinforced concrete components developed for frames are generally also applicable to structural walls and that it is relatively easy to dissipate seismic energy in a stable manner. Naturally, because of the significant differences in geometric configurations in structural walls, both in elevation and sections, some modifications in the detailing of the reinforcement will be required. Individual walls may be subjected to axial, translational, and torsional displacements. The extent to which a wall will contribute to the resistance of overturning moments, story shear forces, and story torsion depends on its geometric configuration, orientation, and location within the plane of the building. In choosing suitable locations for lateral-force-resisting structural walls, three additional aspects should be considered: 1. For the best torsional resistance, as many of the walls as possible should be located at the periphery of the building. The walls on each side may be individual cantilevers or they may be coupled to each other. 2. The more gravity load can be routed to the foundations via a structural wall, the less will be the demand for flexural reinforcement in that wall and the more readily can foundations be provided to absorb the overturning moments generated İn that wall. 3. In multistory buildings situated in high-seismic-risk areas, a concentration of the total lateral force resistance in only one or two structural walls is likely to introduce very large forces to the foundation structure, so that special enlarged foundations may be required. XVIf structural walls have boundary elements, boundary elements are often present to allow effective anchorage of transverse beams. Even without beams, they are often provided to accommodate the principal flexural reinforcement, to provide stability against lateral buckling of a thin-walled section and, if necessary, to enable more effective confinement of the compressed concrete in potential plastic hinges. Most cantilever walls without openings can be treated as ordinary reinforced concrete beam-columns. Lateral forces are introduced by means of a series of point loads through the floors acting as diaphragms. The floor slab will also stabilize the wall against lateral buckling, and this allows relatively thin wall sections to be used. In such walls it is relatively easy to ensure that when required, a plastic hinge at the base can develop with adequate plastic rotational capacity. In low-rise buildings or in the lower stories of medium- to high-rise buildings, squat walls may be used. These are characterized by a small height-to-length ratio, hw / lw. The potential flexural strength of such walls may be very large in comparison with the lateral forces, even when code-specified minimum amounts of vertical reinforcement are used. Because of the small height, relatively large shearing forces must be generated to develop the flexural strength at the base. Therefore, the inelastic behavior of such walls is often strongly affected by affects of shear. It is possible to ensure inelastic flexural response of such walls. Energy dissipation, however, may be diminished by effects of shear. Therefore, it is advisable to design such squat walls for larger lateral force resistance in order to reduce ductility demands. In many structural walls a regular pattern of openings will be required to accommodate windows or doors or both. When arranging openings, it is essential to ensure that a rational structure results, the behavior of which can be predicted by bare inspection. The designer must ensure that the integrity of the structure in terms of flexural strength is not jeopardized by gross reduction of wall area near the extreme fibers of the section. Similarly, the shear strength of the wall, in both the horizontal and vertical directions, should remain feasible and adequate to ensure that its flexural strength can be fully developed. Windows in stairwells are sometimes arranged in such a way that an extremely weak shear fiber results where inner edges of the openings line up. It is difficult to make such connections sufficiently ductile and to avoid early damage in earthquakes, and hence it is preferable to avoid this arrangement. From the point of view of seismic resistance, an undesirable structural system may occur in medium to high-rise buildings when openings are arranged in such a way that the connecting beams are stronger than the walls. Story mechanism is likely to develop in such a system because a series of piers in a particular story may be overloaded, while none of the deep beams would become inelastic. Because of the squatness of such conventionally reinforced piers, shear failure with restricted ductility and poor energy dissipation will characterize the response to large earthquakes. Even if a capacity design approach ensures that the shear strength of the piers exceeded their flexural strength, a soft-story sway mechanism would result, with excessive ductility demands on the hinging piers. XVTHaving examined the features of behavior, analyses and detailing of cantilever walls relevant to ductile seismic performance, in the following paragraphs the main conclusions are summarized while, step by step, the application of the capacity design philosophy is reviewed. Step 1: First of all, the layout of cantilever wall systems and strategies in the location of structural walls is reviewed for capacity design of structural wall systems. The positioning of individual walls to satisfy architectural requirements is to be examined from a structural engineering point of view. In this respect the following aspects are of particular importance: 1. Regularity and preferably, symmetry in the positioning of walls within the building to reduce adverse torsional effects 2. Efficiency of force transfer from diaphragms to walls where large openings exist in the floors 3. Checking of the configuration of walls in elevation to ensure that feasible shear resistance and flexural strength with adequate ductility capacity can readily be achieved. 4. A review of foundation conditions to ensure that overturning moments, can be transmitted to the soil. Implications for the foundation structure and the possible rocking of walls should be studied. Step 2: After estimating the likely sizes of all structural components and the contribution of the total building content, as well as the code-specified live loads: Design dead and live loads and their combinations are derived for each wall of the cantilever system. From the total gravity loads over the entire plan of the building, the participating weights Wi (masses) at all floors are quantified. Thus earthquake design forces, the total design base shear Vb, the component forces Fi to be assigned to each level are estimated. Step 3: With the evaluation of section properties for all walls, the actions due to lateral forces can be distributed in proportion to wall stiffness. In this evaluation the design eccentricities of story shear forces are to be considered. To determine the critical condition for each wall in each direction of seismic attack, different limits for torsion effects must be examined. Step 4: For each wall the appropriate combination of gravity load and lateral force effects are determined, using appropriate load factors, and critical design quantities with respect to each possible direction of earthquake attack are found. Spot checks may be made to ascertain that the chosen wall dimensions will be adequate. A redistribution of design actions should also be considered. Step 5: Each wall of the structural wall system is designed. The design of each wall for flexural strength involves: XVII1. Determination of the amount and arrangement of vertical flexural reinforcement at the base by using approximations or otherwise. 2. Checking that limits of reinforcement content, bar sizes, and spacing are not exceeded and that chosen wall dimensions satisfy stability criteria. 3. Using enhanced yield strength of the steel (Xo fyk), the determination of flexural overstrength of the base section Mo»- As bar arrangements are finalized, the analysis of the section, including the finding of the depth of compression c, can be carried out. 4. The checking of ductility capacity (c < cc) and the need for confining a part of the compressed regions of the wall section over the height of the plastic hinge. 5. Consideration of section at higher levels and curtailment of the vertical reinforcement. Step 6: Magnified design wall shear forces Vwaii and corresponding shear stresses are determined. The latter is compared with maximum allowable values in both the potential plastic hinge and the elastic regions. For each of these two regions the contribution of the concrete to shear strength Vc is found, and hence the necessary- amount of horizontal shear reinforcement is evaluated. Sliding shear resistance is checked. Step 7: The final stage of wall design involves the detailing of transverse reinforcement: 1. The determination of transverse hoop or tie reinforcement in the end regions of wall sections to satisfy confining requirements for compressed vertical bars or possibly for the compressed concrete with due regard to limitations on tie spacing. 2. Determination of the spacing and anchorage of horizontal wall shear reinforcement are determined. Deprem Yönetmeliği 1996 provisions for seismic design of reinforced concrete structural walls are reviewed and compared with the other codes. Furthermore, numerical examples, illustrating the execution of these provisions, are presented in this study. For better allocation of space or for visual effects, individual structural walls of a group may have different sections. Walls meeting each other at right angles will give rise to flanged sections. In order to study the behavior of such walls within the structural system and to compare the analysis procedures that are used in practice, twelve numerical examples with different flange dimensions were examined. The principal conclusions obtained from the numerical examples are as follows; 1. The solutions indicate that the total base shear force has decreased by 30 percent, because of the increase of the fundamental period of vibration due to reduction of flange dimensions. Therefore the fundamental period of vibration is important to XVIIIcalculate the total base shear force so while the fundamental period of vibration is calculated, it is not advisable to neglect the flanges of the structural wall. 2. Another results shows that 10 to 15 percent of the total base shear force is carried by the columns and 85 to 90 percent of the total base shear force is carried by the structural wall. This is why the assumption that the structural walls carry the total base shear force is on the safe side. Nevertheless, the structure can carry more earthquake loads. Furthermore, 1 to 2 percent of the total base shear force is carried by the flanges of the structural wall. Hence, in the general calculation, the lateral seismic forces carried by the flanges can be neglected. 3. When the code-specified static analysis according to Deprem Yönetmeliği 1996 is compared with the dynamic analysis, the proportion of the total base shear force with respect to the dynamic analysis to the total base shear force with respect to the dynamic analysis is changed between 76 percent and 80 percent. Therefore the code- specified static analysis is safer than the dynamic analysis. 4. The results show that while the flanges of the structural wall have a pronounced effect in resisting to the overturning moment, they do not have much significant contribution for resisting to the base shear force. XIX
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