Yüksek Binalarda Taşıyıcı Sistem Etkinliği

Abstract

Yüksek binalarda taşıyıcı sistem etkinliği ve çekirdek alanının kat alanına oranını araştırma amacındaki bu çalışmada, Türkiye'de yapılmış ve yapılmakta olan yüksek binalar hakkında yapısal bir değerlendirme yer alacaktır. İncelenen binaların tümü betonarme iskelet sistemde inşa edilmiş olup, Türkiye'nin çeşitli deprem bölgeleri ve farklı kat sayılarından oluşacak şekilde seçilmiştir. Çalışmaya, dünyada 1890'larda ülkemizde 1960'larda başlayan yüksek binaların günümüze kadar olan gelişmeleri mimari ve statik açıdan incelenerek girilmiştir. Bunu izleyerek yüksek bir binanın kat alanında toplam taşıyıcı sistem ve çekirdek alanını değiştiren faktörler ile perde ve kolon alanları arasındaki ilişki araştırılacaktır. Çalışmanın ilk bölümünde yüksek bina tanımı, tarihçesi ve gelişimi, ardından bu çalışmanın ana kavramları olan plan yoğunluk göstergesi ile çekirdek alanı ve kat alanına oranı terimleri açıklanarak tezin amaç ve kapsamı belirtilmiştir. İkinci bölümde yüksek bina taşıyıcı sistemleri, çekirdek çeşitleri, deprem ilişkisi, yüksek binaya etkiyen yükler ve bu yükler karşısında binanın davranışı gibi genel tanımlar açıklanacaktır. Üçüncü bölümde Türkiye'de 1985 ve sonrası yapılmış veya yapılmakta olan seçkin yüksek binalar üzerinde bir değerlendirme yapılacaktır. Bu kısımda binalar ile ilgili genel açıklamalar, plan, kesit ve çalışmaya yön verecek sayısal değerler tablolar halinde yer alacaktır. Bu tablolar son değerlendirme bölümüne de kaynak oluşturacaktır. Dördüncü bölümde Türkiye'de yüksek binaların taşıyıcı sistem alan ve çekirdek oranlarının bina yüksekliği, kat adedi, deprem bölgesi, kullanım amacı, taşıyıcı sistem çeşidi ve yapım yıllarına göre bir değerlendirilmesi yer alacaktır.

The objective of this study is to examine the plan density index in high-rise buildings. Plan density index can be explained as the ratio of cross-sectional area of vertical load-bearing elements to gross area of normal floor and can be expressed as the percentage of area. The ratios of cross-sectional area of shear walls and area of the core to gross area of normal floor are also examined separately and, nearly 30 buildings, which was built or is being built in Turkey, is examined in this content. Plan density index is a value which depends on floor numbers, structural system, earthquake region, the function of the building and the construction year. When a comparison of plan density indices is made, although it is 100% in Egyptian pyramids, 50% in Tac Mahal and 25% in St. Peters, it is among 4% and 8% in buildings which was built in Turkey and reinforced concrete system was used and it is among 2% and 3% in buildings which were built in the world where and composite systems were used. Mostly in office and commercial buildings wide spaces and suitable situations for open plans are required but also long and wide shear walls are needed against horizontal loads in high-rise buildings. Because of this, cores made by connecting shear walls is used in these kinds of buildings. These cores can also contain vertical circulation systems and energy network systems. Although small core areas are needed for usage profitability, it generally covers 20-25% of whole area. The area of the core depends on the function of the building, structural system and the whole area of the building. High-rise building is a concept which is hard to explain because of its relative character. For example a five-storey building is a high-rise building when compared with a single-storey building or a six-storey building is not high-rise building in big cities of Europe but it can be in a village or in big cities like New York and Chicago, 70 or more storey buildings are called as high-rise buildings. Another difficulty in explaining the term arises because of the different explanations used in different professional areas. For example, according to one explanation, buildings which the height of the top floor exceeds 22 m., can be called as high-rise building, but according to another explanation used in USA buildings which are at least 12 floors higher than the neighbour buildings can only be accepted as high-rise buildings. When the history of high-rise buildings is examined, some of the reasons why the concept high-rise building was arise, were the population explosion and rapid urbanisation. After the industrial revolution, immigration to cities caused population explosion and disorderly growth of cities and these caused the area prices rise. As a solution, instead of horizontally growing buildings, vertically growth was preferred. Since the first era, humankind had wished to reach higher and higher. Also this aim can be accepted as one of the reasons of this arise. Babylon tower, Rhodes sculpture and Egyptian pyramids, which had reached to 146.70 m. height with is solid body, were the examples or their era and they would have been the motivation and pride of humankind. XIII In the first centuries, two systems were used in buildings; wooden and masonry. The lack of resistance to fire in wooden structures and the heavy dead load of masonry structures had limited the construction of higher buildings. The upper limit of masonry structures was the 17-storey Monadnock Building, which was built in Chicago in 1891. The thickness of the walls was 183 cm. and they covered 15% of whole area of ground floor in this building. In 1885, William Le Baron Jenny proposed to use steel sections in floors by placing them in gridlike plan and this system was first used in 10-storey Home Insurance Building. Improvements in pig-iron and invention of elevator and hydrophore made possible to reduce the dead-load of the building and higher buildings could be built. In 1931, 60-storey Woolworth Building had reached to 242 m. in height and after the I. World War, 66-storey Wall Tower had reached to 293 m. and 77-storey Chrysler Building to 319 m. in height. Today one of the functions of high-rise buildings is advertisement. Empire States Building in New York with its 381 m. height, John Hancock Center in Chicago with its 449 m. height, World Trade Center in Chicago with its 442 m. height and Sears Tower in Chicago with its 442 m. height are the examples of this kind. Today, high-rise buildings can be seen not only in USA but also in other regions of the world, especially in Asia. For example Petronas Twin Towers, built in 1996 in Kuala Lumpur, the capital of Malaysia, exceed Sears Tower 7 meters with its 452 m. height and it is the tallest of the completed buildings in the world. Also in other Asian countries like China, Taiwan, Japan, South Korea, Hong Kong and Singapore, high-rise buildings are being built. For example the TORRE WFC building in Taiwan with its 460 m. height and the TORRE MILLENNIUM Building in Tokyo with its 840 m. height would exceed the record of Petronas Twin Towers when they would completed. Turks introduced with high-rise constructions with the minarets of the mosques. The minaret of Selimiye Mosque, built in 16th century, is 70 m. in height. The first examples of high-rise buildings, which were 20 m. in height, were built in Galata and Beyoğlu in 18th century. The first examples built in the time of the Turkish Republic were the public buildings in Ankara which were also 20 m. in height. Istanbul Hilton Hotel and Ankara Ulus Commercial building which were built in 1953 were also the first examples of higher one. The construction of buildings exceeding 20 storeys began with Harbiye Military Hotel, Istanbul and Hacı Ömer Sabancı Dormitory, Ankara. Today, the highest building in Turkey is 50-storey Mersin Merit building. This building is also the highest building in Europe which reinforced concrete system was used. The Head Office Building of İş Bankası which is being built would be the second highest of Turkey and the highest of Istanbul. The loads effecting high-rise buildings can be grouped as snow and rain loads, dead load, live loads, construction loads, wind loads, seismic loads, thermal expansion loads, impact loads, explosion loads and loads affecting together. The structural elements of high-rise buildings can be grouped into three as; one dimensional, two dimensional and three dimensional elements. One dimensional elements are columns and beams, two dimensional elements are walls and floors and three dimensional are core and whole facade. XIV While constructing a high-rise building, steel and reinforced concrete can be used as construction materials. These two have advantages but also disadvantages. But when these are used together, disadvantages get smaller in number and the weight and cost of the building reduces 30%. New construction systems can be formed by using combinations of the structural elements. Some examples are core systems, rigid framed systems, frame and shear wall systems, frame systems, suspended systems, tubular systems and systems with flat plate floors. In this study the effect of earthquake on high-rise buildings was widely examined for introducing it to architects, also some suitable forms and solutions against earthquake were given. Earthquake, the movement of ground, is a dynamic event and mass, mass distributions the geometry of the buildings and the materials used are the important factors which can be used against earthquake. According to the acceptation related with earthquake, in the occurrence of small scale earthquake, none of the load bearings systems and non-load bearing elements can be damaged, in occurrence of medium scale earthquake only non-load bearing elements can be damaged, in the occurrence of big scale earthquake load bearing elements can also be damaged but it must not collapse. Some reasons of earthquake damages in Turkey are unsuitable building geometry, insufficient load bearing capacity of the structural elements, less quality in constructions and lack of control. The important factor in designing on earthquake resistant buildings is to chose suitable forms and geometry. Once an unsuitable form is chosen, it is hard to have an earthquake resistant building but it can be modified by using joint-beds. In order to have an earthquake resistant building, vertical load bearing elements must be uniformly distributed and columns and shear wall must be joined with each other beams in both directions. In order to analysis the plan density index, buildings which was built since 1990, were chosen and most of them are in Istanbul. Data about floor numbers, earthquake region, structural systems, function of the building and construction year was placed into tables for each buildings. Also floor normal plans of each buildings and photographs of some buildings were given. The analyzed buildings are Türkiye İş Bankası building B and D block, Esbank and Sınai Yatırım ve Kredi Bankası, Demirbank, Tatlıcı Plaza, Yapı Kredi Bankası D block, Toprak Center, Beybi and Spring Giz Plaza, Özsezen Business Center, Başak Sigorta, Ziyal office building, I. Levent Plaza, Üçgen İnşaat residential buildings, Hoşdere Towers, Hoşdere residential buildings, Yeşilkent residential buildings and Korkmaz Yiğit residential buildings. Taksim İş Merkezi and Mersin Mertim have the highest plan density index of 8%. The others are respectively Ankara Sheraton and İzmir Hilton Hotel 7.4% and Sabancı Center 6.6%. The plan density index of I. Levent Plaza is 2.4% and it is the lowest one in commercial buildings. Others are respectively Özsezen İş Merkezi 2.5%, Ziyal Büro Binası 2.8%. In residential buildings this ratio decrease this ratio decreases up to 1.8%. 42 storey İş Bankası B block building, which would be highest of İstanbul, have a plan density index of 5.6 is because of the system used. Although the floor number of İş Bankası building which tubular system is being used, is higher than the others, which XV reinforced concrete frame + core system were used, the plan density index of it is smaller. When 13 storey Demirbank building and 21 story Başak Sigorta building are compared although it is higher, the plan density index of Başak Sigorta building is lower. This ratio is 4.6% in Demirbank building and 3.6% in Başak Sigorta building. In Demirbank building structural elements are not uniformly distributed and this is a reason of this difference. Toprak Center building consists of two blocks of 12 and 13 storeys. In both blocks the storey height is 2.85 m and construction height of the beams is 22 cm. The plan density index is 3.2% and shear wall ratio to whole area normal floor is 1.2%. The plan density index of 25-storey Yapı Kredi Bankası building is 6.1%. When the construction is examined; the core is not placed in center, columns were connected with each other in only one direction and slenderness ratio of them is as high as 4.5. Because of these factors, plan density index is so high when compared with other buildings. When the relation between the function of the building and plan density index is examined; in residential buildings plan density index is smaller than others. Only Ankara Hoşdere Towers have the plan density index of 9.8%, but this is because tunnel formwork system was used. In public buildings generally plan density indices are high. When the reason is of this is examined; safety and economy are important factors for all structures, but inevitable conditions of the specifications are more important factors for public buildings when compared with private buildings. In the private sector, as the consultants are the experts of civil engineering, economy is provided better than the public sector. When the relation between plan density index and construction year is examined; plan density index reduces by the passing years. Today plan density indices are lower than the ones in 1980's. 25 storey İş Bankası building in Ankara have the plan density index of 6.7% but the new building of İş Bankası which is already being built would have a plan density index of 5.4% in D block and 32 storey Tatlıcı Plaza have the plan density index of 6.2%. Wide spread use of computer, the improvements in construction techniques and developments in construction materials can be told as the reasons of this reduce. The increase in floor numbers increases the plan density index. In 10-15 storey buildings plan density index is generally between 2.4% and 2.8% and in 20-25 storey buildings it is 3.4-5.4%. In buildings which the floor number exceeds 25 this index is between 4.7% and 6.2%. So it can be said that plan density index is directly related with floor number and place of the core. In the examined buildings, the ratio of shear walls to whole vertical structural elements is generally 70% and this ratio is directly related with floor numbers. 25 storey in Yapı Kredi Bankası building D block the ratio of shear walls to whole area of vertical load bearing elements is 95% and in 32 and 37 storey Sabancı Center buildings this ratio is nearly 86%. When the ratio of the core area is examined; generally it covers 20-25% of the whole area of floors and this ratio is directly related with the ratio of shear walls to whole area of normal floors. In 22 storey Spring Giz Plaza building the core ratio is XVI 20.9% and the ratio of shear walls to whole structural elements is 55.8%. In 32 storey Beybi Giz building the ratio of the core is 17.9% and the ratio of shear walls is 48.9%. Reasons of this difference in the ratio of core area would be because of the differences in the plan scheme and the differences in distribution of structural elements. In most of the buildings which were analyzed in this study, the construction system is reinforced concrete frame + core. In higher buildings tubular systems were preferred. It can be said that, In tubular systems the area of vertical load-bearing elements is smaller than the ones in frame systems. When the place of the core is examined, generally they were places in the center of the plan but different solutions can also be seen. When the core is not placed in the center, thicker columns and shear walls are needed. In slabs generally waffle slabs were preferred but flat plate floors and beamed floors can be seen also.