Mevcut betonarme binaların deprem etkisindeki davranışının değerlendirilmesi

Abstract

Deprem dinamik bir etki olup yapılarda hasar oluşmasına yol açmakta ve buna bağlı olarak da can kayıplarına sebep olmaktadır. Deprem mühendisliğinin en önemli görevi bu etkinin belirlenerek sebep olabileceği kayıpların önlenmesine çalışmaktır. Depremlerin sebep olacağı can ve mal kayıplarının önlenebilmesi amacıyla ülkemizdeki yapıların deprem güvenliklerinin sistemli bir çalışmayla gözden geçirilmesi gereklidir. Yeni yapılacak yapılarda olumlu bir deprem davranışı sağlamak amacıyla Taslak Yönetmelik koşullarının uyulmasına özen gösterilmelidir. Daha önce inşa edilmiş mevcut binalarda ise yeni gelişmeler ve düşüncelerle ortaya çıkan modern bir deprem yönetmeliği esaslarının ne derece sağlandığı kontrol edilmelidir. Bu çalışmada bu tür mevcut binaların deprem davranışının belirlenmesinde kullanılacak olan ve birbirinden ayrıntıya girme yönünden farkeden üç yöntem incelenmiştir. Bu yöntemlerden ilki ATC-21 [2] de verilen hızlı davranış değerlendirme yöntemidir. Bu yöntemin uygulanması nisbeten az masraflı ve kolay olduğundan ülkemizde yapılacak genel bir deprem güvenliği çalışmasında hemen göze çarpan ve depreme karşı açık bir şekilde yetersiz olan binalar bu yöntemle belirlenebilir. Yani bu yöntem ayrıntılı incelemeler öncesinde uygulanan bir ön çalışma niteliğindedir. Bu hızlı değerlendirme sonrasında yapılacak daha ayrıntılı bir inceleme de kullanılacak bir diğer yöntem Bölüm 3 de verilmiştir. Bu yöntem yapılacak ayrıntılı bir incelemenin tüm binada dikkate alınması yerine olumsuzluğun görüldüğü elemanlarda olmasmı öngörmektedir [3]. Bu yöntem bir anlamda yapının modern deprem yönetmeliğindeki koşullan sağlamayan bölümlerinin tesbiti olarak da görülebilir. Bu şekilde elemanların deprem davranışı açısından değerlendirilmesiyle yapının deprem davranışım olumsuz olarak etkileyen yapı bölümleri tesbit edilebilir. Uygulanacak bir güçlendirme programında dikkatin hangi noktalara yoğunlaşması gerektiği de yöntemdeki sonuçlardan elde edilebilir. Bu iki gözlem esasına dayalı yöntemin haricinde "Standard For Evaluation of Seismic Capacity of Existing Reinforced Concrete Building [4] de verilen deprem davranışı için indeksleme yöntemi incelenmiştir. Bu yöntemde elemanların sünekliğinden hareketle dayanımları hesaplanarak deprem davranışını temsil etmek üzere bir indeks tanımlanır. Daha sonra bu indeks, gözönüne alman inceleme seviyesindeki deprem davranış karşılaştırma indeksi ile kıyaslanarak yapının muhtemel bir depremdeki davranışı tahmin edilmeye çalışılır. Bu yöntemler bir yapının muhtemel bir depremde sergileyeceği davranışın ayrıntılı bir çerçeve analizine gidilmeden tesbitine imkan vermektedir. Şayet yapıda deprem açısından önemli eksiklikler görülüyorsa yapılacak tam bir çerçeve analiziyle güçlendirmeye karar verilebilir.

Earthquakes result from motion betweeen plates comprising the earth's crust. These plates are driven by the convective motion of the material in the earth's mantle, which in turn is driven by heat generated at the earth's core. Heat from the earth's core causes material to rise to the earth's surface. Forces between the rising material and the earth's crust cause the plates to move. The resulting motions of the plates relative to one another generate earthquakes. These large pieces of the earth's surface, termed tectonic plates, move very slowly and irregularly. Forces may build up for decades or centuries at the interface between plates, until a large movement occurs all at once. These sudden, violent motions produce the shaking that is felt as an earthquake. The shaking can cause direct damage to buildings, roads, bridges and other man-made structures as well as trigger fires, landslides, tidal waves and other damaging phenomena. Four major factors can affect the severity of ground shaking and thus potential damage at a site. These are the size of the earthquake, the type of earthquake, the distance from the source of the earthquake to the site and the types of soil at the site. Larger earthquakes will shake longer and harder, and thus cause more damage. Generally, the farther from the source of an earthquake, the less severe the motion. The rate at which motion decreases with distance is a function of the regional geology and inherent characteristics of the earthquake and its source. The underlying geology of the site can also have a significant effect on the amplitude of the ground motion. Many different types of damage can occur in buildings. Damage can be divided into two categories: Structural damage and non-structural damage, both of which can be hazardous to building occupants, Structural damage means degradation of the building's structural support systems (i.e., vertical and lateral force resisting systems), such as the buildings frames and walls. Non-structural damage refers to any damage that does not affect the integrity of the structural support system. Examples of non structural damage are a chimney collapsing, windows breaking or ceilings falling. vni The type of damage to be expected is a complex issue that depends on the structural type and age of the building, its configuration, the proximity of the building to neighboring buildings and the type of non-structural elements. Damage can be due to structural members (beams and columns) being overloaded and/or differential movements between different parts of the structure. If the structure is sufficiently strong to resist these forces or differential movements, little damage will result. If the structure cannot resist these forces or differential movements, structural members will be damaged and collapse may occur. Building damage is related to the duration and the severity of the ground motion. Longer earthquakes tend to shake longer and harder and therefore cause more damage to structures. In addition to damage caused by ground shaking, damage can be caused by building's pounding against one another, ground failure that causes the degradation of the building foundation, landslides, fires and tidal waves. The level of damage that results from a major earthquake depends on how well a building has been designed and constructed. Buildings can experience horizontal distortion when subjected to earthquake motion. When these distortions get large, the damage can be catastrophic. Horizontal distortion comes into being when the center of rigidity and the center of mass of floors do not coincide. Therefore, Most buildings are designed with lateral force resisting systems (LFRS), to resist the effects of earthquake forces. In many cases LFRS make a building suffer and thus minimize the amount of lateral movement and consequently the damage. LFRS are usually capable of resisting only forces that result from ground motions parallel to them. Basically, LFRS consist of axial (tension and/or compression), shear and/or bending-resistant elements. In order to provide a tool to evaluate the danger of building collapse due to earthquakes, in this research three evaluation method different one to another in details are given. These evaluation methods are Rapid Screening Procedure (ATC- 21), Seismic Evaluation Procedure (ATC-22) and Evaluation of Seismic Capacity using Seismic Index (Japanese Standard). 1-The Rapid Screening Procedure (ATC-22) : RSP method consists of inspecting a building from the exterior (termed a "sidewalk survey" ) in order to determine quickly if the building is probably adequate for the earthquake forces it is likely to experience or whether there may be reasonable doubts as to the building's seismic performance. The result of the RSP method is a finding as to whether the building should or should not be subjected to more detailed investigation with respect to its seismic adequacy. A basic concept of the RSP is to identify, for the building under review, which of several typical building types it correspond to. Based on this building type, a Basic Structural Hazard score can be assigned to a typical building in each category, depending on the earthquake forces it is likely to experience. These scores range ix from 1 to 8.5, depending on the structural type and seismic zone. The values have been determined so that a seismically good building has a high value and a potentially weak or hazardous building has a low value. There are significant factors, such as irregularities in the structural system, deterioration of structural materials, adverse soil conditions or excessive wall openings that can negatively affect a building's seismic performance or adequacy. In order to account for these factors, a series of Performance Modification Factors (PMFs) have been determined, which, when subtracted from the Basic Structural Hazard Score, result in the final Structural Score (S) for the building under review. The Structural Score S is the basic measure of the degree of adequacy of the building. It can be related to the probability of major damage. A high S score is good, and a low S denotes probable poor seismic performance, and that the building should be reviewed in detail by a professional engineer experienced in seismic design. Generally, if a building's Structural Score S is less than about 2, then the seismic performance of that building may not meet modern seismic criteria and the building should be investigated further. It should be obvious that no rapid visual examination can provide highly reliable estimates of seismic performance, and the RSP method is simply intended to identify those buildings where reasonable doubts may exist. In some cases the RSP may miss buildings that in reality are seismically weak. 2- Seismic Evaluation Procedure (ATC-22) : This method is a technical manual that offers guidance for engineers in the seismic evaluation of existing buildings. A building does not meet the life-safety objective of this method if in an erthquake one or more of the following event occurs: 1- The entire building collapses 2- Portions of the building collapse 3- Components of the building fail and fall 4- Exit and entry- routes are blocked, preventing the evacuation and rescue of the occupants The identification of life-safety hazards in an existing building consists of determining whether any of these events could potentially happen for that building during an earthquake that could be expected to occur during its lifetime. The objective of this method is to identify typical structural flaws that have been observed in past earthquakes to lead to failure and falling of structural components and to partial or total collapse, with an attendant loss of life. Hence, a major portion of this method is dedicated to directing the evaluating engineer on how to determine if there are any weak links in the structure that could precipitate structural or component failure. x For investigation of existing buildings, some building codes specify a force lower than that new code for new buildings. In this condition, we can tolerate less conservatism in an existing building because it can be strengthened only at substantial cost in money and disruption of use. Existing buildings that are upgraded are required to meet the force level of the previous code, which is about two-thirds of the force level of the current code. This is especially a problem in our country, where the seismic code is updated several times. If one investigates buildings very strictly, it may be difficult to find an existing building which has seismic safety defined in the current code. Therefore it is unavoidable to use some tolerance when one investigates seismic safety of an existing building. This method given in ATC-22 is centered on a set of questions -one set for each of fifteen model building types- that is designed to uncover the flaws and weakness of the building. The questions are in the form of positive Evaluation Statements that describe characteristics of the building type that essential in avoiding the failures that have been observed over and over again in past earthquakes. The engineer addresses each Statement and determines whether it is true or false. True statements identify conditions that are acceptable. If a building passes all applicable statements with true responses, it can be passed without further evaluation, i.e., it is deemed not to be a life-safety hazard. False statements identify issues or concerns that need further investigation. The method specifies a process for dealing with Statements that have been found to be false. If any such potential life-safety hazards are identified, an appropriate detailed analysis is recommended, with acceptance criteria suggested for each element of concern. At the conclusion of the analysis, the engineer should assemble the results of the analysis and the answers to the other concerns, review them, and establish a list of deficiencies. The evaluation will be enhanced by further investigation of the elements that do not meet the basic acceptance criteria. This Seismic Evaluation Procedure provides a methodology that can be applied nation wide to all existing buildings that are suspected of possing a potentially serious risk of loss of life and injury in case of a damaging earthquake. 3- Evaluation of Seismic Capacity Using Seismic Index (Japanese Standard) : This method may be applied to evaluate the seismic performance of an existing reinforced concrete building, except for high rise buildings. Three screening procedures with the different phases are available to estimate the seismic performance of a building and to evaluate the results; the first, second and third level screening. The result estimated on a building presents a serial Seismic Index. The preliminary inspection, which includes the building scale, the structural system and the building age should be conducted properly before applying this method. XI This method may be applied basically to the building whose number of stories less than six and whose structural system is the moment resisting frame with / without shear walls. The very old building over 30 years and with the severe deterioration, the fire experience, the extremely low material strength or unusual structural system should not be adaptable for this method. The seismic performance of a building is represented by the two indices; seismic index of structure, Is, and seismic index of non-structural elements, In. As for the indices estimated, it may be recognized that the higher value, the more excellent seismic performance. These indices are usually estimated independently but the structural performance, in particular the ductility of the frame, should be referred to estimate the seismic index of non-structural elements. A index of Is should be estimated by using the following equation: Is = Eo Sd T Eo : the basic structural performance Sd : the sub-index on the structural design of the building T : the sub-index on the time dependent deterioration of the building Either phase screening procedure among the first, second and the third may be available to estimate the index Is. However, basically the higher level procedure applied, the more reliable estimation obtained. The index Eo is calculated using mainly the Ultimate Strength Index, C, and the Ductility Index, F, of a structure. The result computed may be recognized as that the higher strength or the higher ductility the structure has the larger index Eo the building must have. The equations to compute the index Eo differ with the level of screening used. The influence of irregularity of a structure or stiffness and / or mass concentration to the seismic performance should be estimated by the sub-index Sd, taking account of the emprical or technological decision. The structural performance, such as the member strength, stiffness, ductility, etc., is computed assuming the structure without any cracks or deformations, because any reasonable and simple technique to take account of the influence of such deterioration has not been developed yet. The influence of deterioration may be macroscopically taken account by the sub-index T. xn The seismic performance of the possible falling objects, which are attached on the exterior walls on a building, is evaluated by the index In. The basic policy to evaluate the seismic performance of non-structural elements should be focussed on the potential how injurious to a human life the falling objects are. The seismic performance would be judged individually for the structure and the non structural elements. The concept of seismic judgement for a structure is generally represented following equation. Is > Iso Is < Iso Is ; is the seismic performance index for structure Iso ; is the seismic judgement index for structure The building which satisfies above first equation, may be assumed to be "safe", but the building which the seismic performance index is less that the seismic judgement index should be assumed to be uncertain for the seismic performance against an assumed earthquake ground motion. In the last part of the thesis these evaluation methods are applied to an existing structure in order to show the application process of the methods. The results are given in comparatively. Seismic Evaluation of existing reinforced concrete structures are very important for Istanbul as well as for other large cities, because: 1) There are a lot of buildings which are constructed without getting any civil engineering service neither on the design phase nor on the construction phase. 2) Although some buildings are designed by following design concepts given in the code, the structural systems are modified without consulting the design engineers. Sometimes structural systems is changed by removing some columns or beams. Sometimes due to restrictive architectural detailing a structural system having defective seismic performance comes into being. Unusually it is very difficult to strengthen that system often it is build. 3) Another important weakness of some buildings are due to low quality of concrete. Although the minimum compressive concrete strength is 140 kgf/cm2 for lowest quality concrete, and although much higher values are adopted in the proportioning of the structural elements, the average (not minimum) compressive strength is much lower than this. A detailed study carried out by the structural laboratory of the Technical University revealed that the average concrete strength of the buildings is about 100 kgf7 cm2. This type of weakness also in one reason to evaluate the seismic performance of existing buildings. xiii The methods given in the present thesis to investigate and to develop some kind of structural performance score or index. In the final part of the process this score or index is compared to a referance index and it is decided whether the performance is adequate or not. The author feels that the referance index needs to be investigated detaily. The adoption of the referance values used in USA and Japan may be not suitable for our conditions. Therefore before a wide application of these method has to be investigated and probably modified by considering our construction quality.