Steel Concrete Composite High rise Buildings
The tall buildings of ancient times were built i n those days mainly as a symbol of pri de and architecture of their nation. On the contrary, today's tall structures are mainly built for human habitats, conceived in response to rapid urbanization and alarmin g rate of growth of population. The clustering of migrated people from villages to the metro cities in search of better opportunities has posed a severe problem to the c oncerned planners as to how to accommodate them in the cities due to expansion restrictions and slower pace of development compared to the influx rate. Expansion in vertical direction seems to be a prospective solution to this problem. The issue of integrating i ntegrating architecture with structural design is a complex one. When structure is expressed, it is expected to look elegant, appealing, and above all, structurally efficient and compact. Harmony between structure and arc hitectural form is the key to the success of expression. From a purely structural point of view, an optimally designed building must utilize and exploit its structural materials to the fullest extent possible. Structure and aesthetics are also related through efficiency, lightness, elegance, and the principles of minimizing weight and using the least material possible to control costs, among other factors. INSDAG has carried out design studies on steel intensive tall buildings, which have indicated encouraging cost savings, lesser life- cycle cost and higher life expectancy of buildings with SteelConcrete Composite Option. Various types of structural framing systems along with factors governing design requirements and the factors to be considered for their optimization has been elaborated. In this paper, it has been explicitly discussed how to integrate the architectural requirement with steel intensive design including showing the optimization of steel -concrete composite option. The salient points for design philosophy with steel- concrete composite option including particular involvement of structural components in structural framin g systems along with optimization of structural framing system will provide the basic idea behind the design of tall buildings. Since in India, the concept of tall building is at its infancy, this paper may provide a practical guideline to practicing engineers involved in engineering of tall buildings. Architecture, ancient structures, beams, braced frames, bundled-tube structure, columns, composite, compression, critical elements, ductility, durability, efficiency, elegance, flexibility, framed-tube structures, girder, mega-tube structures, moment resisting frame, optimization, P-delta analysis, RCC structures, shear lag, shear wall, spandrel, stability, steel structures, tall buildings, tall structures, urbanization have also been touched. The human aspiration to build increasingly tall structures are inherent from the ancient ti mes bearing examples of Pyramids of Giza, Egypt; Mayan temples ofTikal, Guate mala; and Kutab Minar of Delhi, India. These ancient structures are primarily solid in nature and were built in those days mainly as a symbol of pride and architec ture of their nation. On the contrary, today's tall structures are mainly built for human habitats, c onceived in response to rapid urbanization and alarming rate of growth of population. Howeve r, the differences in the usage of buildings, has not changed the basic requirements of strength and stability. The recent trend in technology of high-rise structures can be conceived as a progressive reduction in the quantum of structural materials to be used for creating architectural aesthetics as well as spaces within the structural framework. In the recent era, for construction of highrise structures, the extensive use of RCC and Steel as primary material was employed. Advancement in technology for using both the materials are significant, however having better
higher strength to weight ratio and hi gh ductility, structural steel used as a prime material for construction of highrise structures has got a little edge over RCC construction. In their Environmental Resources Guide, the American Institute of Architects (AIA) recommended steel as an environment-friendly material for construction. Steel has the beneficial material properties like durability, flexibility and high strength to weight ratio. Further, Steel is reusable, recyclable and consumes less energy with minimum wastages and therefore more use of steel in structures ensures sustainable development. The property of higher strength to weight ratio will provide more spaces within the structure and lesser occupancy of structural elements, whereas the property of ductility will contribute significantly to take care of forces during earthquake or if the structure is subjected to any other dynamic external actions. The clustering of migrated people from villages to the metro cities in search of better opportunities has posed a severe problem to the c oncerned planners as to how to accommodate them in the cities due to expansion restrictions and slower pace of development compared to the influx rate. Expansion in vertical direction seems to be a prospective solution to this problem. Integrating Architecture with Structural Steel Design The issue of integrating architecture with struc tural design is a complex one. When structure is expressed, it is expected to look elegant, appealing, and above all, structurally efficient. Harmony between structure and architectural form i s' the key to the success of expression. Ideally, the structure should visually clarify and enrich the form. From the structural point of view, the architectural solutions must utilize and exploit the structure to the fullest extent. Since arc hitecture includes a technological component, the buildings should express the structural system as well as its behavior, materials, and construction. Today we are obliged to search for new ways of expressing and understanding the engineering and scientific language of modern architectural design. Structure and aesthetics are also related through efficiency, lightness, elegance, and the principles of minimizing weight and usi ng the least material possible to control costs, among other factors. To a great extent, when the structure is well proportioned, it will look aesthetically appealing. From the structural point of view. Architectural solutions must utilize and exploit the structure to the fullest extent. In this context, steel structures have their own c haracter in expressing the structural system truthfully emerging from the inherent character of the unique material. Tall buildings are the most complex built structures since there are many conflicting requirements and complex building systems to integrate. Today's tall buildings are becoming more and more slender, leading to the possibility of more sway in comparison with earlier highrise buildings. Thus, the impact of wind and seismic forces acting on them becomes an important aspect of the design. Improving the structural systems of tall buildings can control their dynamic response. The skeleton structure used in modern high-rise construction is the result of the rational use of steel as well as concrete. Among its characteristic features are the reduction of load-carrying members to minimize component sizes and c osts and a clear expression of the division between structural and non-structural elements. The skeleton is c omposed of rigidly connected beams and columns. Steel framed construction with steel-concrete composite action with intermittent floor slabs is particularly suitable form for high -rise multi-story buildings. The great strength of modern building materials makes it possible to build taller, to meet to day's market driven demands for efficient use of high-priced urban real estate and maximizing investment returns.
From a purely structural point of view, an optimally designed building must utilize and exploit its structural materials to the fullest extent possible. For a rec tangular building, for instance, four massive columns at each corner interconnected by gigantic diagonals can be used to create an effective structural system. However, a system with interior columns in the core area interconnected by closely spaced exterior columns is a better, more efficient structural solution. Other structural alternatives include super-frames, telescopic tubes, or a hollow mega-tube structure or bundled tube structures. Whatever t he structural solution is, it must be architecturally functional, efficient, and aesthetically pleasing. Although there are many structural design alternatives for tall buildings, the design philosophy for steel-framed buildings that mainly exhibit strong structural expressions are discussed in this paper. Design of Steel Intensive Tall Buildings i) Design Requirements The task of the structural engineer is to arrive at suitable structural systems, to resist al l types of external actions including gravity load, lateral forces, fire, and blast loadings. The design of components for tall buildings involves the following general design r equirements: - Strength: stresses of different members to be within permissible limits - Serviceability: displacements and deflections of members to be within specified limits - Human Comfort: vibration of the floor and the structural system to be within specified limits. - Stability: sufficient safety against overturning, lateral shift, buckling and PDel ta effects - Ductility: sufficient safety against yielding under extreme events ii) Various Types of Structural Framing Systems
The essential role of the lateral resisting frame sys tems is to carry the wind and earthquake loads, as well as to resist the amplified actions / stress resultants generated by P-Delta effects induced within the structural members of tall building and to transfer the forces efficiently to the foundation system.
Moment resisting frames are column and girder plane frames with fixed or semi-rigid connections. The strength and stiffness are proportional to the story height and column spacing. Various combinations of different materials including composite materials are used for construction of tall buildings such as concrete moment r esisting frames, steel moment resisting frames and steel-concrete composite moment resisting frames. C oncrete encased steel columns may also be used. Steel beams encased in concrete and steel beams connected to slabs by shear connector are also used. Moment resisting frames could also be built with columns connected to flat plates, in concrete. Slab and walls could also be designed as moment resisting frames. Steel moment resistant frames could be fabricated using 3 story panels of beam-column subassemblies. Usually these are kept to bend panels so that, points of inflection are generated at midpoints of columns where girders and joints are field bolted. When the width is restricted within 4m, transportation of structural segments are controllable.
The types of braced frames may be single diagonal, cross braces, k-braces and eccentric or knee braces. Lattice types of bracings are also used. Concrete braced frames are often not used, since shear walls are superior for construction and lateral resistance. Steel braced frames are used in interior cores, so that connections could be easily made with wall panels. Composite braced frames may have steel bracings in concrete frames or concrete bracings in steel frames. Concrete encasement of columns and composite floor beams are also used in
practice.
Shear walls are plane elements made up of reinforced concrete thin walls having length and thickness providing lateral stiffness. The shear and overall flexural deformations are design constraints, along with the stress levels, axial and bending. Concrete shear walls may be cast in place or pre-cast. Pre-cast panel walls are also used within a concrete or steel frame to provide lateral resistance. The ductile shear walls used in earth quake resistant design have to be detailed carefully. Coupling beams should have diagonal reinforcement to develop shear resistance. Steel shear walls are also used sometimes, by connecting them to framework by welding or high strength bolts.
Framed tubes are 3-dimensional space frameworks made by c onnecting intersecting plane frames at the corners by stiff corner columns. Framed tubes behave like giant flange frames and perpendicular web frames carrying axial loads and shear. The flange frames are normal to wind, while web frames are parallel to the wind. The axial forces in the columns in the flange frames are obtained by beam theory. However, due to flexibility of spandrel girders, and columns, there is a shear lag effect, in the box beam cantilever, with a hyperbolic type stress distribution in the framing system acting as a web. In the framing system, act ing as flanges, the axial stresses in columns are magnified also in a parabolic type stress distribution. Thus, it has been observed that in general, the corner c olumns may be subjected to almost four times the axial stress when compared to an ideal cantilever tube. Framed tubes have columns fairly closely spaced with variations ranging from 1m to 3m.
The main indicator in arriving at the cost of a structural system is its unit weight usually expressed in kg/ sq.m. In other words, the weight is directly associated with the overall efficiency of the system in carrying gravity / imposed vertical as well as lateral loads. The stiffness of the system is not only associated with moment of inertia but also with the weight. An ideal structural system could be the one in which the steel required to carry the gravity / imposed vertical loads alone, are also capable to carrying the wind loads. Optimization could be made so that the wind could be carried by keeping stresses within the difference between allowable stresses for gravity plus wind and stresses due to gravity alone, usually an increase of one third of the strength is permitted. However, this is not always possible, as height to width ratios, may not allow this design condition to be achieved. Some premium for wind is often required. For buildings within about 13 to 14 stories tall (usually 50 m in height), this is often achievable. The one third increase allowed in the allowable stresses may be just sufficient to carry the lateral load due to wind. For buildings in the range of20 to 50 storey height (usually 70 m and above), this is not always possible. The structural engineer is required to use innovative schemes with additional reinforcement of structural components like shear wallframe, shear truss-frame and framed tubes and outrigger braced systems. This premium for wind is often optimized and controlled by an optimum design of beams and columns and floor systems to match the allowable stress limits and drift or horizontal sway. The factors, which are affecting optimum analysis and design of tall buildings seriously, are indicated in the following section:
The efficiency of the structural system is often determined by its height to width ratio. The larger width for any given height us ually means larger stiffness. This implies, larger the bay width, larger will be the lever arm for flange frames in framed tube type of tall building. Usually, the optimum height to width ratio is within the range between 5 and 7. For shear truss-framed
buildings, usually the width of the truss should be less than about twelve times relative to its height. So, the optimum and stable design of tall buildings will change significantly depending upon the available height to width ratio.
The dimension of girder span often determines the steel quantity for the floor framing. Smaller spans for exterior framing system including various types of framed tube systems, will generally form more efficient framing system. Spans in the range of 1 m to 3 m are often used for optimum analysis and design. This will form the well-knit skeleton of exterior framing system to act as a flange or a web of a gigantic cantilever tube. However, for interior parts, the spans of the girders may be more depending upon the arrangement of column systems.
The proportions of members of the frame play a leading role in efficiency, with deeper members being more effective in resisting drift. On the contrary, deeper members (predominantly in case of beams) may affect architectural cost as well as cost due to increased floor heights and the corresponding increase in lateral loads. During optimization of design, these should be included while calculating total cost of tall buildings. Larger column spacings and deeper spandrel may generate more efficient framed tube system. The orientation of the wider columns sh ould be along the plane of the frame to extract optimum use of section taking the advantage of maximum available stiffness. Column spacing could be arranged in suc h a way, that all gravity steel can effectively carry wind, with very little increase in weight for girders. Floor framing should be so arranged that most of the beams should frame directly into colu mns with a little or no eccentricity. Thus, gravity loads could be directly carried avoiding provision of extra girders.
The floor framing system usually constitutes about 20% of the total weight of the structure. So, it is a necessity to optimize this subsystem prior to commencement of actual analysis and design of the entire building. Span to d epth ratios, spacing of beams, thickness of slab, composite action, and openings for mechanical ductwork, should be carefully considered in the design of floor system to achieve maximum efficiency. Span to depth ratios for floor framing are usually workable within the range of 20 to 24. T his is the minimum depth for consideration of strength and stiffness. Open web trusses / c astellated beams / beams with openings could be used for beams with long spans. C omposite action between trusses / beams and slabs should be developed by providing shear connectors. Two way grid systems are usually avoided, since cost of fabrication will be sign ificantly high. Wherever feasible, widest possible spacing of beams and largest possible spans for slab should be used. Profiled deck with or without embossment can be used to introduce c omposite slab. Due to integral action with steel members, the composite floor systems with profiled deck will have larger stiffness and can contribute as a structural element or diaphragm with significant enhancement of the stiffness for the floors. This contributes to overall stability of tall buildings in resisting lateral loads like wind, earthquake etc. in addition to blast or impact loads. Solid slabs are better than slabs with cellular openings. In case of solid slab, the diaphragm stiffness action is better.
This is an important consideration for framed tube system for tall buildings having significant height. This effect can be minimized by using deep spandrels and wide columns and s maller spacing between columns so that distribution of lateral load is better with minimized effect of shear lag. Transfer beams are used at lower levels to carry less number of openings. The stiffness between column and girder should be balanced. Sometimes, deeper built up I shaped beams are used to increase stiffness or alternately the concept of huge diagonal / cro ss bracing can be introduced connecting several storey and closely spaced vertical columns at
certain levels. This will reduce the shear lag effect substantially. Field welding should preferably be avoided, by using 3 storey sub-assemblies of column-girder components which are field bolted at points of inflection. This will reduce t he transportation and erection costs also. High strength steel may not be beneficial in all cases, since the cost of fabrication may be hi gh. Reduction in total number of pieces to be assembled will result in cost savings.
This paper elaborates the concept of architecture in tall buildings with steel-concrete composite option and design philosophy for steel intensive tall buildings. It has been elaborated that how to address the architectural solutions which must utilize and exploit the structure to the fullest extent to make the structure effective and e fficient. It has also been explained how to compromise between slenderness and effective design of the structure. The rational use of steel and concrete in high-rise structures for achieving good forms of architecture were also covered. To attain best architecture in steel intensive construction, the blend between optimum number of columns with their geometric arrangement and the type of structural frames were also covered. Various types of structural framing systems for tall buildings have been described along with limitations and advantages of using each system. The factors influencing optimization of structural systems for tall buildings were also discussed elaborately including their practical application at construction site. Since the cost of a structural system becomes the governing factor for selection of an e ffective and efficient system for a tall building with a better value of Life Cycle Cost (LC C), the choice of selection of structural system has also been addressed. With the rapid increase in use of steel in different sectors particularly in commercial as well as residential sectors, the enhancement in knowledge of tall building is an absolute necessity. Since there is a constant upsurge and growing demand for construction of high-rise buildings in India, this paper will help and provide general guidelines to practicing engineers in India in selection of aesthetically pleasing, effective and efficient structural systems for steel intensive tall buildings. References - Design Study Report of G+40 Storeyed Residential Building at Mumbai with RCC & SteelConcrete Composite options & Assessment of their Cost Effectiveness, October 2004 Institute for Steel Development & Growth - B+G+20 Storied Residential Building with SteelConcrete Composite Option, May 2003 Institute for Steel Development & Growth - G+3 & G+6 Storied Residential Buildings with Steel-Concrete Composite Option, September 2003 - Institute for Steel Development & Growth - BS 5950 - 2000 (Part 1) : Code of Practice for Design - Rolled & Welded sections - BS 5950 - 2000 (Part 3) : Code of Practice for Design of Simple and Continuous Composite Beams - Euro Code 3 (Part 1.1): Design of Steel Structures - Euro Code 4 (Part 1.1): Design of Composite Steel & Concrete Structures - Bandyopadhyay T K, "Basic Concepts in Composite Structures': Refresher Course on Composite Construction using Structural Steel, 17- 21 Jan 2000, organized by Institute for Steel Development & Growth (INSDAG) and Jadavpur University, Kolkata - R Narayanan, "Composite Steel Structures", Advances, Design and Construction, Elsevier, Applied Science, UK, 1987 - Steel Framed Multistoried Buildings: The Economics of Construction in the UK, CONSTRADO Study Report - 1985, 2nd Edition, p 3 - Teaching Resource for "Structural Steel Design" Volume 1, 11& III: INSDAG Publication prepared by IIT Madras, Anna University and Structural Engineering Research Centre (SERC), Chennai under Dr. R Narayanan's leadership - IS 875 -1987 (Part I to 11
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