JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
CIVIL ENGINEERING
COMPARISION OF VARIOUS STRUCTURAL SYSTEMS FOR TALL STEEL BUILDING IN INDIAN SCENARIO
1NILESH M. GAUTAMI, 2PROF. SUMANT B. PATEL, 3J. P. LAKHLANI
1M.E. (Structure Engineering) Student/ Dept. of Structure engineering/ Birla vishvakarma mahavidyalaya/ Gujarat technical university/ VallabhVidyaNagar City/ VallabhVidyaNagar / Gujarat/ India.
2Associate Professor Dept. of Structure engineering/ Birla vishvakarma mahavidyalaya/ Gujarat technical university/ VallabhVidyaNagar City/ VallabhVidyaNagar / Gujarat/ India.
3Director of Lakhlani associates / Rajkot/ Gujarat/ India.
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ABSTRACT : Tall building developments have been rapidly increasing worldwide and also in India. Steel has a more advantages material in world today it gives innovative framing systems, easy to assembling, high strength to weight ratio availability of various strength and wider selection of sections and it is a environment friendly so steel is being used in worldwide tall building. In past designers are only considering a gravity loads for the design of building. But now improvisation in seismic and wind study, lateral forces are added in design of building.
Objective to fined the economical structural systems for tall buildings in Indian scenario. In this paper reviews the various steel structural systems – moment resisting frame systems, Braced frames systems, Outrigger belt truss systems, Shear wall frame systems, Framed tube systems, Truss tube systems, The bundled tube systems for tall buildings.
Keywords: Moment resisting Frame, Cross-Braced Frame, Outrigger belt truss system, Shear wall frame, Framed tube, Truss tube, The bundled tube.
ISSN: 0975 –6744| NOV 11 TO OCT 12 | Volume 2, Issue 1 Page 78
JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
CIVIL ENGINEERING
ISSN: 0975 –6744| NOV 11 TO OCT 12 | Volume 2, Issue 1 Page 78
JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN
CIVIL ENGINEERING
1. INTRODUCTION :
According to last population survey 1.21 billion peoples are counted with a increase growth of 1.344%. If Indian population growing this speed many problems are obtain in which one of the problem is to provide shelter facility to all. In metro cities land prizes are very high so we are forced to constructing our city vertically. In India we are starting a multi-story or High-rise building construction just last 20 years. In India generally we are using a concrete as a construction material.
Steel is a common building material used throughout the construction industry. It primary purpose is to form a skeleton for the building or structure essentially the part of the structure that holds everything up and together. Steel has many advantages when compared to other structural materials such as concrete, timber, plastics and newer composite materials. Steel is one of the friendliest environmental building material-steel is 100% recyclable [1]. In India we are using steel in industrial building. Using steel in Tall building construction is now in starting stage.
Development in the structural systems of tall buildings has been a continuously evolving process since the growth in tall buildings began in1880s. From structure engineer’s point of view tall buildings may be defined as one that, because of its height, it is affected by the lateral forces due to wind or earthquake to an extent that they play an important role in the structural design. The factors are responsible for the high-rise building development are availability of urban land, advances in construction technology and high strength materials, efficient structural systems and computational techniques etc.[2].
In case of tall structure, stiffness is more important than its strength. It is necessary to ensure that the top storey displacement, lateral drift and human comfort level in tall structure are within the permissible limit. To satisfy these design requirements different structural systems are developed for high-rise building as shown in Fig 1 It is also important to ensure that in the selected system structural members can be utilized up to the full design capacity with design requirements [2].
Fig.1: Classification of Tall building by Fazlur khan
2. BEHAVIOUR OF DIFFERENT STRUCTURAL SYSTEMS.
Behavior of different structural systems is unique and each system is having different load transferred mechanism. Generally in case of high-rise structures, stiffness requirement in terms of inter storey drift and top storey displacements are important criteria to control. Therefore selected structural system should be such that the design requirement should satisfy along with the full utilization of the structural elements.
Moment resisting frame
Moment Resisting Frame is an assembly of the columns and girders, connected by the moment resistant connection. The main advantage of this system is that the internal planning is not obstructed because of its rectangular arrangement. Lateral stiffness of the system depends on the bending stiffness of the girders, columns and connections [2]. Moment resistant connection is costly in steel building compare to concrete building. This system can be economically used up to 25 stories. As the number of storey increases drift of the structure become difficult and costly to control. Lateral loads are resisted by the rigid frame action that is by the development of the shear forces and bending moments in the frame members and joints. Storey shear is generally resisted by the columns by the bending in double curvature with points of contra flexure at approximately mid storey height levels. The moments acting on a joint are resisted by the bending of the attached girders in double curvature with points of contra flexure at approximately mid span. This type of deformation generally has shear configuration as shown in Fig 2.
The overall moment due to lateral load is resisted by the couple generated by the axial tensile and compressive forces in the columns, this type of deformation has flexural configuration as shown in Fig 3. Generally deflected shape of a high-rise moment resisting frame has a shear configuration as shown in Fig 4.
Fig. 2: Shear deformation of structure
Fig. 3: Bending deformation of structure
Fig. 4: Deformation of rigid frame under lateral loading
Moment resisting frame can be solved by two cycle moment distribution, portal or cantilever method to get approximate member forces caused by horizontal loading. Storey drift and total drift is the sum of three components i.e. storey drift due to girder rotation, storey drift due to column rotation and storey drift due to overall bending. The girder rotation and column rotation are a part of shear deflection. Drift component due to shear deflection is generally more compared to overall bending component. The contribution due overall bending deformation is approximately 15 to 20%, due to girder rotation is 50 to 60% and due to column rotation is 15 to 20 % of the total drift. Additionally there is a forth component which also contributes to the drift is a deformation of joint. In moment resisting frame the sizes of joints are small compared to length of column and beam therefore drift component due to joint rotation is generally neglected. But is case of very tall structure consisting of closely spaced columns and deep spandrels, the effect of joint deformation is taken into account. This effect is called panel zone deformation.
Braced frame
When the number of storey is more than 25 to 30 then it is uneconomical to provide the moment resisting frame because of the large member sizes required to control the drift. In such case the simplest and economical way to increase the structure’s lateral stiffness is by introducing bracings in the building. Historically, bracings have been used in the most of the world’s tallest building as a lateral load resisting system. Different types of bracings can be used in the structures i.e. single diagonal, double diagonal, V, inverted V, K bracing, eccentric bracing, buckling-restrained bracing etc as shown in Fig 5.
In this structural system diagonals and girders act as a web and columns act as a chord of the vertical truss. Lateral loading on a building is reversible therefore bracing will be subjected to both tension and compression, but they are usually designed for the compression and checked for the tension. Horizontal shear is resisted by the axial forces in the diagonals and girders, and external moment is resisted by the axial tensile and compressive actions in the columns. Due to the axial deformations in the columns under lateral loading frame will deflect in flexural configuration and due to the axial deformations in the girders and diagonals frame will deflect in shear configuration. The resulting deflected shape is a combination of both flexure and shear deformation. Major disadvantage of this system is the obstruction of the bracings in the internal planning and the location of windows and doors. Generally bracings are provided internally along with walls around the elevator, stair, and service shafts. Braced bent can be approximately solved by using method of sections or by method of joints. The total drift is the sum of three components i.e. drifts due to the column axial deformation, drift due to diagonal deformation and drift due to girder deformation. The diagonal and girder deformation are part of shear deformation. The shear deformation is more compare to column axial deformation (flexural deformation).
(a) Concentric
(b) Eccentric
Fig. 5: Types of Bracings
In this structural system diagonals and girders act as a web and columns act as a chords of the vertical truss. Lateral loading on a building is reversible therefore bracing will be subjected to both tension and compression, but they are usually designed for the compression and checked for the tension. Horizontal shear is resisted by the axial forces in the diagonals and girders, and external moment is resisted by the axial tensile and compressive actions in the columns. Due to the axial deformations in the columns under lateral loading frame will deflect in flexural configuration and due to the axial deformations in the girders and diagonals frame will deflect in shear configuration. The resulting deflected shape is a combination of both flexure and shear deformation. Major disadvantage of this system is the obstruction of the bracings in the internal planning and the location of windows and doors. Generally bracings are provided internally along with walls around the elevator, stair, and service shafts. Braced bent can be approximately solved by using method of sections or by method of joints.
The total drift is the sum of three components i.e. drifts due to the column axial deformation, drift due to diagonal deformation and drift due to girder deformation. The diagonal and girder deformation are part of shear deformation. The shear deformation is more comparing to column axial deformation (flexural deformation).
Outrigger belt truss
Outriggers have been historically used in the sailing ships to resist the wind loading and the same concept has been used in the high-rise structure as a lateral load resisting system [3]. Outrigger structural system consists of a central core either of braced frames or shear walls and horizontal cantilever outrigger trusses or deep girders connecting the core to the periphery columns. In case of lateral loading, the rotation of the central core will be restrained by the deep outrigger by producing tension in the windward columns and compression in the leeward columns as shown in Fig 6.
The restraining action of columns will produce the restoring moment in the structures. This restoring moment reduce the bending moment in core and lateral displacement of the structure. If this outrigger truss is assumed to be infinitely rigid then axial deformation of the columns is equal to rotation of the core multiplied by their distances from the center of the core.
Fig. 6: Outrigger-braced structure
Diagrid
Diagonals (bracings) are very effective in resisting the lateral forces in high-rise structures. In diagrid system the diagonal’s mesh is located along the entire exterior perimeter surfaces of the building that forms an exterior tube which maximize the lever arm to resist overturning moment. Diagrid structures are more effective to reduce the shear deformation because diagonals in this system carry shear by axial force action as shown in Fig 7.
Diagrid system has higher tensional rigidity because exterior diagonal mesh acts as a single unit. In this system all the conventional vertical columns can be eliminated because the diagonal members can carry gravity loads as well as lateral loads due to their triangulated configuration in uniform manner. In diagrid structure central core is not always required because diagrid provide both bending and shear rigidity to the building. The diagrid segments can be prefabricated and properly planned to minimize onsite connection difficulties. Diagrid structure does not obstruct the internal planning of the building and it is most suitable for the twisted external facades. Many tall buildings have used diagrid system on the external facade.
Fig. 7: Load path (a) under gravity loading (b) under lateral loading
Framed -tube
The framed tube is one of the most significant modern developments in high-rise structural form. The frames consist of closely spaced columns, 2 - 4 m between centers, joined by deep girders. The idea is to create a tube that will act like a continuous perforated chimney or stack. The lateral resistance of framed tube structures is provided by very stiff moment resisting frames that form a tube around the perimeter of the building. The gravity loading is shared between the tube and interior columns. This structural form offers an efficient, easily constructed structure appropriate for buildings having 40 to100 storey. When lateral loads act, the perimeter frames aligned in the direction of loads act as the webs of the massive tube cantilever and those normal to the direction of the loading act as the flanges. Even though framed tube is a structurally efficient form, flange frames tend to suffer from shear lag. This results in the mid face flange columns being less stressed than the corner columns and therefore not contributing to their full potential lateral strength. Aesthetically, the tube looks like the grid-like façade as small windowed and is repetitious and hence use of prefabrication in steel makes the construction faster.