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Prof. Dr. Zahid A. Siddiqi, UET, Lahore. TALL BUILDINGS It is difficult to distinguish the characteristics of a building which categorize it as tall. The outward appearance of tallness is a relative matter. In a typical single-storey area, a five story building will appear tall. In large cities, a structure must pierce the sky around 70 to 100 stories if it is to appear tall in comparison with its immediate neighbors. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Elphinstone Place Atlanta20 Burj Dubai Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Taiwan_Taipei--101-Building Empire State Building Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Tall building cannot be defined in specific terms related to height or number of floors. There is no consensus on what constitutes a tall building or at what magic height, number of stories, or proportion a building can be called tall. Perhaps the dividing line should be drawn where the design of the structure moves from the field of statics into the field of structural dynamics. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. From the structural point of view, a building is considered as tall when its structural analyses and design are affected by the lateral loads, particularly sway caused by such loads. In contrast to vertical load, lateral load effects on buildings are quite variable and increase rapidly with increase in height. Under wind load the overturning moment at the base of a building varies in proportion to the square of the height of the building, and lateral deflection varies as the fourth power of the height of the building, others things being the equal. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Petronas Malayasia Spire_Chicago_Townhouses Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. There are three major factors to consider in the design of all structures: strength, rigidity, and stability. As height increases, the rigidity and stability requirements become more important, and they are often the dominant factors in the design. Either the size of the members may be increases beyond and above the strength requirements. The second and more elegant approach is to change the form of the structure into something more rigid and stable to confine the deformation and increase stability. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Due to the P-∆ effect, in which the eccentricity of the gravity load increases to such a magnitude that it brings about the collapse of the columns as a result of axial loads. Therefore, an important stability criterion is to assure that predicted wind loads will be below the load corresponding to the stability limit. The second consideration is to limit the lateral deflection to a level that will ensure that architectural finishes and partitions are not damaged. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Tallest in Shangai Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Tall Building In Japan Prof. Dr. Zahid A. Siddiqi, UET, Lahore. In general, for high-rise buildings the resistance to overturning moment and lateral deflection will almost always require additional material over and above that required for gravity load alone. The material required for floor framing is a function of the column-to-column span and not the building height. However, the material required for the vertical system, such as columns and walls, in high-rise structure is substantially more than that of a lowrise building. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. The material increases in the ratio (n + 1) / 2, where n is the number of floors, because the vertical components carrying the gravity loads will need to be strengthened for the full height of the building, requiring more vertical steel than a onestory structure having the same floor area. For example in a steel building using rigid frame action, the total weight of approximately 117 kg/m2 of structural steel is split evenly at about 39 kg/m2 for each of the three subsystems, namely, (1) floor framing, (2) gravity columns, and (3) wind bracing system. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. The material quantities needed with reinforced concrete buildings also increase as the number of stories increases. The increase in the material for gravity load is more than steel, whereas the additional material required for lateral load is not as high as for steel, since weight of additional gravity loads helps to resist the lateral deflection and overturning moment. The additional gravity load, on the other hand, can aggravate the problem of designing for earthquake forces. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. The structure usually accounts for 20 to 30 percent of the cost of a tall building. For building above 50 stories, the cost of a reasonable wind bracing system may work out, at most, to one-third of the structural cost. Therefore compared to the total cost of the building, wind bracing costs, which are in the range of 7 to 10 percent, represent far from an overwhelming portion of the total cost. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. It may appear from the foregoing that optimization of the structural cost alone may not be worth the effort. The goal, therefore, is to optimize the overall cost, a process that normally takes place in the early stages of the project development. The structural cost is almost invariably studied together with the impact of the structure on other items such as increased or decreased floor-to-floor height, leasability of the floors, etc. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Baron Jenney's First Leiter building, completed in 1879 in Chicago Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Historically, the unit weight of structural framing members in term of, say, average weight per unit floor area appears to be progressively decreasing over the years. For example, a survey of tall building built in the period 1950 – 1990 will verify that in this period it was possible to build a 100-story building with perhaps no more than 147 kg/m2 of steel as compared to the 205 kg/m2 of steel used for the Empire State Building in the 1930’s. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. The reasons for this gradual decrease are manifold, as can be seen from the following list. 1. Innovative design concepts. Structural engineers are continually seeking better and more efficient methods of resisting the lateral loads. Some of the common approaches are: i) Increase the effective width of subsystems to resist the overturning moment. ii) Design systems such that the components interact in the most efficient manner. iii) Use interior or exterior bracing for the full width of the building. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. iv) Arrange floor framing in such a way that all or most of the gravity loading is directly carried by the primary lateral-load-carrying components. v) Manipulate the dispersion of materials in composite construction consisting of concrete and structural steel in a manner such that materials are used to their best advantage. vi) Minimize the bending induced by wind loads in the primary components. vii) Employ truss action to eliminate bending in columns and spandrels. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. viii) Use rounded plan shapes to reduce the magnitude of the wind pressure. ix) Arrange closely spaced columns at the exterior to support most or all of the gravity loads and all the lateral loads. x) Suspend floors from a central core such that the total gravity load acting on the core will induce enough hold-down forces to counteract the overturning moment. xi) Use an interior braced core that interacts with exterior columns via belt and outrigger trusses. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. xii) Use exterior steel plate curtain walls to resist lateral forces. 2. Use of high-strength low-alloy steels. Today it is a common practice to use 345 MPa steel in most composite floor framing systems, gravity columns, and not too infrequently in lateral-load-resisting elements. 3. Increased use of welding as compared to bolting, which effects a saving in the range of 8 to 15 percent in the weight of steel. 4. Increased use of composite construction. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. 52 storey Woolworth Building (1913) Prof. Dr. Zahid A. Siddiqi, UET, Lahore. 5. Application of computers to both the design and the analytical processes. 6. Gradual increase in the allowable stresses in the materials based on research and successful past performance. 7. A reduction in the weight of other construction materials, like partition and curtain walls. In concrete construction, major factors responsible for reducing the reinforcement and concrete quantities are: Prof. Dr. Zahid A. Siddiqi, UET, Lahore. 1. New framing techniques, such as skip joist construction in which every other joist is eliminated, have caught on in high-rise construction with a consequent reduction in the weight of structural frame. 2. Increased use of mechanical couplers in reinforcement for transferring both compression and tensile forces. 3. Use of welded cage for column ties, beam stirrups, etc., which reduces the amount of reinforcement steel. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. 4. Use of high strength concrete; 40 MPa is quite common, and strengths up to 70MPa are being specified on vertical components of high-rises. 5. Use of lightweight aggregate typically reduces 50 to 100 kg/m2 in the dead load of the structure, resulting in savings of approximately 10 to 15 percent in the reinforcement requirement. 6. Most codes do not require as great a thickness of slabs when structural lightweight concrete is used. Typically a thickness of 12 mm of concrete can be taken off from floor slabs without reducing the fire rating. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. 7. Use of 520 MPa steel reinforcement. 8. Use of the state-of-the-art design methods. A comparative height list of various buildings is given in the following table: Prof. Dr. Zahid A. Siddiqi, UET, Lahore. m 1. Taipei 101, Taipei, Taiwan 2004 101 508 2. Petronas Tower 1, Kuala Lumpur, Malaysia 1998 88 452 3. Petronas Tower 2, Kuala Lumpur, Malaysia 1998 88 452 4. Sears Tower, Chicago 1974 110 442 5. Jin Mao Building, Shanghai 1999 88 421 6. Two International Finance Centre, Hong Kong 2003 88 415 7. CITIC Plaza, Guangzhou, China 1996 80 391 8. Shun Hing Square, Shenzhen, China 1996 69 384 9. Empire State Building, New York 1931 102 381 10. Central Plaza, Hong Kong 1992 78 374 11. Bank of China, Hong Kong 1989 70 367 12. Emirates Tower One, Dubai 1999 54 355 13. Tuntex Sky Tower, Kaohsiung, Taiwan 1997 85 348 Prof. Dr. Zahid A. Siddiqi, UET, Lahore. 14. Aon Centre, Chicago 1973 80 346 15. The Center, Hong Kong 1998 73 346 16. John Hancock Center, Chicago 1969 100 344 17. Shimao International Plaza, Shanghai 2005 60 333 18. Minsheng Bank Building, Wuhan, China UC06 68 331 19. Ryugyong Hotel, Pyongyang, N. Korea 1995 105 330 20. Q1, Gold Coast, Australia 2005 78 323 21. Burj al Arab Hotel, Dubai 1999 60 321 22. Nina Tower I, Hong Kong UC06 80 319 23. Chrysler Building, New York 1930 77 319 24. Bank of America Plaza, Atlanta 1993 55 317 25. U.S. Bank Tower, Los Angeles 1990 73 310 Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Chicago Tribunal Tower (1922) Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Chrysler building Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Tall buildings have a unique appeal, even a mystery associated with their design. Developments in the last five to seven decades have produced many slender high-rise buildings, demanding that particular attention to be paid to their complex behavior under lateral loads. High-rise architecture is continuously changing, and prismatic shapes that were once very popular have given rise to terraced, set-back and splayed elevations. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Computers have given the structural engineer of today the tools to respond to this changing architecture with daring structural solutions. No longer does the structural engineer require that the building be regular in plan and the interior and exterior columns line up with each other. Ego and competition still play a part in determining the height of a building, but various other social and economic factors, such as increase in land values in urban areas and higher density of population, have led to a great increase in the number of tall buildings all over the world. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. 380m high Empire State Building (1930) Prof. Dr. Zahid A. Siddiqi, UET, Lahore. 70 storey RGA Radio Tower (1939) Prof. Dr. Zahid A. Siddiqi, UET, Lahore. In masonry structures, the percentage of area occupied by the vertical structural elements, i.e., columns, walls and braces, was inordinately large compared to the gross floor area. The area occupied by the walls of the 17-story building in Chicago is 15 percent of the gross are at ground floor with wall thicknesses of 2.1 m. Two technological developments, the elevator and model metal frame construction, removed the prevailing limitations on the height of the buildings, and the race for tallness was on. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Today, with the use of computers, buildings are planned and designed which have little or no historic precedent. New structural systems are conceived and applied to extremely tall buildings in a practical demonstration of the engineer’s confidence in the predictive ability of the analysis, the methods used, and the reliability of computer solutions. The development of metal trusses made it possible to roof column-free interior spaces easily and economically. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Lever Building, NewYork (1953) Prof. Dr. Zahid A. Siddiqi, UET, Lahore. Seagram Building (1957) Prof. Dr. Zahid A. Siddiqi, UET, Lahore. The configuration tries to simultaneously satisfy (1) the requirements of site, (2) the requirements of the building program, and (3) the requirements of appearance. For a building to be successful, it should do the following: 1. Create a friendly and inviting image that has positive values to building owners, users, and observers. 2. Fit the site, providing proper approaches to the plaza with a layout congenial for people to live, work and play. Prof. Dr. Zahid A. Siddiqi, UET, Lahore. 3. Be energy efficient, providing space with controllable climate for its users. 4. For office buildings, allow flexibilities in office layout with easily divisible spaces. 5. Most spaces oriented to provide best views. 6. Most of all, the building must make economic sense, without which none of the modern high-rise development would be a reality.