Construction of tall buildings began in the 19th century. At first, tall buildings referred to buildings that were higher than others. But in recent years, the term refers to buildings that are taller than 100m. Especially, many tall buildings have been built in Asia during the last 10 years. Over t...
Construction of tall buildings began in the 19th century. At first, tall buildings referred to buildings that were higher than others. But in recent years, the term refers to buildings that are taller than 100m. Especially, many tall buildings have been built in Asia during the last 10 years. Over the past decade, tall buildings have been designed to not only account for height but also a free-form shape. This trend has already been discussed in the Council on Tall Buildings and Urban Habitat(CTBUH) 2006 World Conference in Chicago. The major topic of the conference was “Thinking outside the Box: Tapered, Twisted, Tilted Towers”. 3T(Tapered, Twisted, Tilted) designs have been suggested for free-form shaped buildings. So, free-form shaped tall buildings have become a landmark in cities and many clients want a free-form building.
For this reason, new structural systems have been developed for free-form structures. The diagrid structural system is the most famous free-form structure.
A free-form shape building is required a review of seismic performance. When earthquake occurred, a free-form shape building can be collapsed by eccentric mass form tapered, tilted and twisted. Therefore, seismic performance evaluation of diagrid structure is one of the important elements in structural design. The diagrid structural system is a very efficient system for both cases. Thus, the diagrid structural system has a double advantage for tall buildings; good resistance to loads and free-form shapes.
The diagrid is a mixed word composed of diagonal and grid. The diagrid structural system consists of triangular modules without vertical columns. It is more difficult than X-brace and K-brace structural systems. The triangular module is made of two braces and one beam. This system is resistant to lateral load due to the braces. When a lateral load occurs, the braces resist the associated tensile and compressive forces. The beam restrains the braces. A change in the diagonal brace length and angle can result in a differently shaped triangle module. The diagrid structural system can create a free-form shaped building through the combination of different triangular module shapes. Therefore, the diagrid structural system has various node shapes. Each node shape and angle is different; Mary Axe uses circular steel tubes, CCTV uses rectangle sections, Capital Gate Tower uses boxed sections, and Macquarie Bank uses H-section. Many researchers have studied the shape and angle of diagrid nodes.
Moon et al. (2007) reported that the optimum angle of diagrid nodes was 60 degrees. Their results were limited to analysis of the 2D-model and did not include actual tests of the diagrid node (Moon and Ali, 2007; Moon et al, 2007).
Structural performance analysis of the diagrid node is required for inelastic analysis of the seismic design, because the diagrid node has a very complex behavior due to the number of components(2 beam, 4 brace) connected at one node.
From the other research, they have been evaluated the structural performance of diagrid node. The evaluation of node was done through experiment and FEM analysis. Based on these researches, it was conducted a safety evaluations of the diagrid structure. And, the evaluation of diagrid structure is following the FEMA method. FEMA(Federal Emergency Management Agency,) propose to carried out seismic performance evaluation through the IDA(Incremental Dynamic Analysis) for structures such as the diagrid structure. IDA is a parametric analysis method that has recently emerged in several different forms to estimate more thoroughly structural performance under seismic loads. In the process, IDA is using 22 earthquake data for analysis. These data are corresponding to the earthquake ground motions in high seismicity regions. However, in the case of Korea, the effective ground acceleration(s) is 0.22g. According to the effective ground acceleration, the Korea have region represented in a low-to-moderate seismicity region. Therefore, the seismic performance evaluation from FEMA method is excessive design.
In this paper, the seismic performance of the diagrid structures was evaluated by modification IDA method. The modification IDA method was used to a 22 earthquake data in a low-to-moderate seismicity region. The course of study was conducted the diagrid node & frame test, FEM analysis, Push-over analysis of frame and IDA. The structural performance of diagrid was used the results through experiments. Through such conduct, the evaluation of diagrid could be a more accurate.
(1) Ground Motion in Korea
An international perspective, the seismicity region of Korea is low. But they have 0.22g of EPA. This value is high position in the low seismicity region. And one or more earthquake of magnitude 5.0 are occurs every year. From this result, the environment of Korea is a low-to-moderate seismicity region. In this paper introduced the seismological observatory systems in the Korea. And analysed the magnitude, acceleration spectrum and characterisity of earthquakes since 1987. In the seismic characteristics of Korea, the acceleration spectrum value is large in the short period region and the earthquake energy is exhausted after 0.5 second period. This characterisitics are more similar to AS 1170.4(Australian Standard part.4) than ASCE 7-10(American Society of Civil Engineers).
(2) The suggestion of design spectrum in the low-to-moderate region
Through the regression analysis of 18 seismic data, the design spectrum of the low-to-moderate seismicity region was suggested. The design response spectra ware based on the proposed Newmark & Hall equation and AS 1170.4. As a result, in the short-period region, it has a higher value than the KBC standard, and the attenuation of the spectrum occurs early.
(3) Response Modification Factor of the Diagrid Structural System
FEMA P795 is suggests a methodology for the seismic performance of the diagrid structural system. FEMA P795 method used 22seismic earthquake data in the building to collapse. This earthquake data is based on the high-seismicity region. In the low-to-moderate seismicity region area, these 22 earthquake data is not occur. According to the FEMA method, it could be an excessive seismic design. The Korea peninsula is generally conceived to be a low-to-moderate seismicity region. So, in this paper, the seismic performance of diagrid structures was evaluated used to a 22 earthquake data in a low-to-moderate seismicity region. From the evaluation of seismic performance, the response modification factor R of the diagrid structural system is 6.0. This value is the same as the concentrically braced steel frame based on the KBC.
Construction of tall buildings began in the 19th century. At first, tall buildings referred to buildings that were higher than others. But in recent years, the term refers to buildings that are taller than 100m. Especially, many tall buildings have been built in Asia during the last 10 years. Over the past decade, tall buildings have been designed to not only account for height but also a free-form shape. This trend has already been discussed in the Council on Tall Buildings and Urban Habitat(CTBUH) 2006 World Conference in Chicago. The major topic of the conference was “Thinking outside the Box: Tapered, Twisted, Tilted Towers”. 3T(Tapered, Twisted, Tilted) designs have been suggested for free-form shaped buildings. So, free-form shaped tall buildings have become a landmark in cities and many clients want a free-form building.
For this reason, new structural systems have been developed for free-form structures. The diagrid structural system is the most famous free-form structure.
A free-form shape building is required a review of seismic performance. When earthquake occurred, a free-form shape building can be collapsed by eccentric mass form tapered, tilted and twisted. Therefore, seismic performance evaluation of diagrid structure is one of the important elements in structural design. The diagrid structural system is a very efficient system for both cases. Thus, the diagrid structural system has a double advantage for tall buildings; good resistance to loads and free-form shapes.
The diagrid is a mixed word composed of diagonal and grid. The diagrid structural system consists of triangular modules without vertical columns. It is more difficult than X-brace and K-brace structural systems. The triangular module is made of two braces and one beam. This system is resistant to lateral load due to the braces. When a lateral load occurs, the braces resist the associated tensile and compressive forces. The beam restrains the braces. A change in the diagonal brace length and angle can result in a differently shaped triangle module. The diagrid structural system can create a free-form shaped building through the combination of different triangular module shapes. Therefore, the diagrid structural system has various node shapes. Each node shape and angle is different; Mary Axe uses circular steel tubes, CCTV uses rectangle sections, Capital Gate Tower uses boxed sections, and Macquarie Bank uses H-section. Many researchers have studied the shape and angle of diagrid nodes.
Moon et al. (2007) reported that the optimum angle of diagrid nodes was 60 degrees. Their results were limited to analysis of the 2D-model and did not include actual tests of the diagrid node (Moon and Ali, 2007; Moon et al, 2007).
Structural performance analysis of the diagrid node is required for inelastic analysis of the seismic design, because the diagrid node has a very complex behavior due to the number of components(2 beam, 4 brace) connected at one node.
From the other research, they have been evaluated the structural performance of diagrid node. The evaluation of node was done through experiment and FEM analysis. Based on these researches, it was conducted a safety evaluations of the diagrid structure. And, the evaluation of diagrid structure is following the FEMA method. FEMA(Federal Emergency Management Agency,) propose to carried out seismic performance evaluation through the IDA(Incremental Dynamic Analysis) for structures such as the diagrid structure. IDA is a parametric analysis method that has recently emerged in several different forms to estimate more thoroughly structural performance under seismic loads. In the process, IDA is using 22 earthquake data for analysis. These data are corresponding to the earthquake ground motions in high seismicity regions. However, in the case of Korea, the effective ground acceleration(s) is 0.22g. According to the effective ground acceleration, the Korea have region represented in a low-to-moderate seismicity region. Therefore, the seismic performance evaluation from FEMA method is excessive design.
In this paper, the seismic performance of the diagrid structures was evaluated by modification IDA method. The modification IDA method was used to a 22 earthquake data in a low-to-moderate seismicity region. The course of study was conducted the diagrid node & frame test, FEM analysis, Push-over analysis of frame and IDA. The structural performance of diagrid was used the results through experiments. Through such conduct, the evaluation of diagrid could be a more accurate.
(1) Ground Motion in Korea
An international perspective, the seismicity region of Korea is low. But they have 0.22g of EPA. This value is high position in the low seismicity region. And one or more earthquake of magnitude 5.0 are occurs every year. From this result, the environment of Korea is a low-to-moderate seismicity region. In this paper introduced the seismological observatory systems in the Korea. And analysed the magnitude, acceleration spectrum and characterisity of earthquakes since 1987. In the seismic characteristics of Korea, the acceleration spectrum value is large in the short period region and the earthquake energy is exhausted after 0.5 second period. This characterisitics are more similar to AS 1170.4(Australian Standard part.4) than ASCE 7-10(American Society of Civil Engineers).
(2) The suggestion of design spectrum in the low-to-moderate region
Through the regression analysis of 18 seismic data, the design spectrum of the low-to-moderate seismicity region was suggested. The design response spectra ware based on the proposed Newmark & Hall equation and AS 1170.4. As a result, in the short-period region, it has a higher value than the KBC standard, and the attenuation of the spectrum occurs early.
(3) Response Modification Factor of the Diagrid Structural System
FEMA P795 is suggests a methodology for the seismic performance of the diagrid structural system. FEMA P795 method used 22seismic earthquake data in the building to collapse. This earthquake data is based on the high-seismicity region. In the low-to-moderate seismicity region area, these 22 earthquake data is not occur. According to the FEMA method, it could be an excessive seismic design. The Korea peninsula is generally conceived to be a low-to-moderate seismicity region. So, in this paper, the seismic performance of diagrid structures was evaluated used to a 22 earthquake data in a low-to-moderate seismicity region. From the evaluation of seismic performance, the response modification factor R of the diagrid structural system is 6.0. This value is the same as the concentrically braced steel frame based on the KBC.
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