Design Concept of RC Tall Building
Concrete has become universal building material and more appropriate structural forms such as shear wall and tube structures. The high dead load characteristics are no limited on the height of concrete building. Otherwise, the dead load from the concrete tends to be more significant in minimizing the sway deflection and floor vibration as well as instability problem.
Tall building cannot be defined in specific terms related to height or number of floor. Building to be considered as tall when the structural analyses and design are affected by the lateral loads. The lateral loads begin to dominate the structural system and take on increasing importance in the overall building system when the building heights increase.
Vertical load, lateral load effects on building are quite variable and increase rapidly with increase in height. The strength, rigidity and stability were three major factors to consider in design of such structures. Basically there are two ways to satisfy these requirements that may be by increasing the size of the member beyond to achieve the strength requirement or change the form of the structure into something more rigid and stable.
Classification of Tall Building Systems
The primary important role of structural system for tall Building system is to resist lateral load. The hierarchy of system formation could be roughly categorized with respect to relative effectiveness in resisting lateral loads. Anyhow, in 1984 the methodology for cataloging the tall building with respect to their structural system is rigorous to develop. Then, the classification scheme involves four distinct levels of framing-oriented division (Falconer and Beedle, 1984) were adopted. Braced framed and moment resisting frame system, shear wall system, core and outrigger system and tubular system are commonly used for tall building.
Development of Tube in Tube Tall Building
Tube in tube building is combination of shear wall and framed tube with closely spaced column and deep spandrels. This system is general accepted as a very efficient structural system for tall building. The simplicity of tube system was first introduced by the late Dr. Fazlur Khan of the architectural and engineering firm of Skidmore, Owings and Merrill. The tube system only needed to employ the very basic elements structures, namely beams and strategically deploying the locations of column. What more important is that it is not requires new method of analysis.
Tube structure in high-rise structure is effective system because the bending and transverse shears are supported three-dimensionally at the flange and web surface in the structure. The analysis of tube structures has to be based on three-dimensional analysis using finite element.
The introduction of the tube system has brought a revolution in the design of high rise building. The efficiency for lateral strength and stiffness of this system is due to employment of the exterior wall alone as the wind-resisting element to make the entire building act as a hollow tube cantilevering out of the ground. In essence the system strives to create a tube-like form structure through the exterior wall around the building. The service core for purpose house elevators, emergency stairway, electrical and mechanical equipment is usually equipped inside building. Then, the walls of the core were become as additional stiffness to the building acting like a second tube within the outside tube.
The frame consisting of vertical columns was usually arranged on an uncompromising grid pattern in two perpendicular directions around the entire perimeter of the building exterior to form a perimeter tube. Because the entire lateral load is resisted by the perimeter frame, the interior floor plan is kept relatively free of large column to increase the net leasable area for the building. The tube has become the workhorse of the high rise construction system because it minimizes the structural premium for lateral strength and stiffness, simultaneously accommodating recent trends in architectural forms.
Behavior of Tube in Tube Tall Building
The stiffness of a hollow tube system is very much improved by using the core not only for gravity loads but to resist lateral loads. The floor structure ties the exterior and interior tubes together, and they respond as a unit to lateral forces. The reaction of a tube in tube system to wind is similar to that of a frame and shear wall structure. However the framed exterior tube is much stiffer than a rigid frame. The following Figure 2.1 indicates that the exterior tube resists most of the wind in the upper portion of the building, whereas the core carries most of the loads in the lower portion.
Figure 1: ( a ) Deform shape of frame ; ( b ) Deform shape of shear wall; ( c ) Deform shape of composite structure
The wall deflects in a flexural mode with concavity downwind and maximun slope at the top, while the frame deflect in a shear mode with concavity upwind and maximum slope at the base (Smith and coull, 1991). When frame and wall conducted as composite structure, the deflected shape has a flexural profile in the lower part and shear profile in the upper part. The axial forces cause the wall to restrain the frame near the base and the frames to restrain the wall at the top.
The Figure 2 indicates that the typical deflection, moments and shears curve of wall-frame structure. The deflection curve and the wall moment curve indicate the reversal in curvature with a point of inflexion, above which the wall moment is opposite in sense to that in a free cantilever (Figure 2a and 2b). Figure 2c illustrates the shear as approximately uniform over the height of the frame, except near the base where it reduces to a negligible amount. At the top, where the external shear is zero, the frame is subjected to a significant positive shear, which is balanced by an equal negative shear at top of the wall, with a corresponding concentrated interaction force acting between the frame and the wall.
Figure 2: Typical deformation of the tube in tube building. ( a ) Typical deflection diagram of laterally loaded wall-frame structure; ( b ) typical moment diagram for components of wall-frame structure; ( c ) Typical shear diagram for component of wall-frame structure.
Advantages of Tube in Tube Tall Building
The tube concept has numerous significant advantages over other framing systems, not only for reasons of economy and efficiency, but also for structural reasons, such as:
- The wind- resisting system being located on the perimeter of the building meant that maximum advantage is taken of the total width of the building to resist overturning moments.
- Since the wind-resisting system is concentrated on the perimeter, it is generally possible to design the interior framing for gravity loads only. As a result, there is a greater freedom in locating columns and beams within the core, and their size is considerably less. Consequently, the core framing may be arranged to best suit the many non-structural requirements within the core, which in turn leads to a significant gain in rentable space.
- The tube system leads to an identical framing for all floors, because the floor members are not subjected to the varying internal forces due to lateral loads.
- In a framed tube with close column spacing and deep spandrels, the tube has an enormous load distribution capacity, which leads to a nearly uniform column loading, permitting many columns in each tube wall to be identical.
- From a practical point of view, the final analysis and design of the tube can proceed unaffected by the lengthy process of resolving detail layout and service requirements in the core area.
Some Important Factors Inherent in Tube Concepts Building
The tube concept in itself does not guarantee stiffness in the frame adequate to satisfy deflection and vibration limitations. Fortunately, in most instances there is sufficient space available along the perimeter to use deep girders and wide, closely spaced columns.
Unless the columns are inside the building facade, serious problems may arise with respect to the effects of temperature difference in the columns of the building.
The floor diaphragm becomes a vital element, and is required not only to distribute wind forces to the side walls of the tube, but also to provide lateral support to all columns.
Reaction of Vertical and Horizontal Component due to Wind Load
One of the major distinguishing characteristic of a tall building is the need to resist large lateral forces due to wind or earthquake. The wind load resisting system must do this, and at the same time must prevent excessive deflections or accelerations and must help to provide stability. A lateral system is generally considered to be efficient if the provision of the lateral load resistance does not increase floor and column sizes beyond those required for gravity loads.
The lateral loads on this building arise mainly from wind pressure effects and their magnitudes increase with the height of the building. The lateral load resisting systems should not only have adequate strength and stiffness against lateral loads, but should be able to resist tendencies to become unstable due to toppling, sliding and uplift.
The core structure is one of the main lateral load resisting systems for the tube in tube tall building. It is located centrally to the building plan and services run throughout the whole building in the shafts. The slab, that supports by the core should own sufficient lateral dimension to prevent overturning of core due to lateral loads.
The selection of large columns with deep spandrel beams is clearly provides a remarkably stiff outer tube which, interacting with the core, results a much smaller deflection under the effect of the wind load. Then, the very tall building is great need for stiffness to act against the lateral wind loads. The overall strategy for transferring lateral loads is to collect all the lateral loads acting on the facade and transfer them horizontally along planes through floor levels to the main lateral load resisting element at the centre of the building. The load is transferred in the vertical direction along the shortest path before transferred to the foundation. The strategy for overcoming the toppling effect is to use the dead load of the core to counteract the lateral load effects.
The lateral load transferred to the floor planes by the secondary system is first picked up by the primary column and spandrel beams on the facade. The loads are then transferred horizontally by the floor system acting as a diaphragm to the central core. The wind load is first picked up by the glazing on the facade and the spandrel beams which span between the primary columns. Some of the load is transferred to the primary columns which take the load directly down, while the rest is passed to the floor slabs and then to the core where it will be taken down to the foundations. The floor structure acts as a horizontal diaphragm and through horizontal compression it transfers the load to the core. So, the floors of this tall building must have sufficient rigidity and strength to act as a diaphragm to transfer and distribute the horizontal forces to another vertical element in tall buildings.
Shear Lag Phenomenon Caused by Lateral Load of tube in Tube Tall Building
A true cantilever of the tube for a tall building is significant to resist the all lateral forces in the exterior walls. To easily illustrate the behavior of the frame tube when subjected to lateral loads (Figure 3a), if the tube loaded on side AB, then the whole frames (facades) AB and CD are called flange frames and the frames AD and BC are called web frame. When a framed tube is under lateral load, it can be found that the force in the corner column is much larger than the force in the center column of the flange frame especially at the ground level or first floor. On the other hand, the forces in 3b. This phenomenon is called shear lag. This phenomenon causing the structural system behaves differently than would be predicted by the engineering bending theory. This is due to the shear flexibility of the flange frames. Thus, the ratio of the stress at the center column to the stress at the corner column is defined as shear-lag factor.
Figure 3: Shear lag effects in tube structures. ( a ) Cantilever tube subjected to lateral loads; ( b ) Shear stress distribution;
The stress distribution of the flange and web column which on opposite sides of the neutral axis are subjected to tensile and compressive forces when under the action of lateral load, as indicated in Figure 3b. From the mention figure, the frame parallel to the direction of the lateral load AD and BC were subjected to the usual in plane bending and shearing or racking action. The prime action is the flexibility of the spandrel beams that produces a shear lag that will increases the stresses in the corner column and reduces those in the inner columns of both the flange panels AB and DC and the web panels AD and BC (shown by the solid line).
The primary resistance comes from the side web panels, so that the column A and B are in tension and C and D are in compression. The interaction between the web and flange of frames occurs through the vertical displacement of the corner columns. Thus, the displacement corresponds to vertical shear in the column of the flange frames. When the column C experiences a compression deformation, it tend to compress the adjacent column C1, as shown in Figure 4 since the two column are connected by the spandrel beam. However, the compressive deformation will not be identical since the flexible connecting spandrel beam will bend and the axial deformation of the adjacent column will be less. The deformation of column C1 will in turn induce compression deformation of the next inner column C2, otherwise less deformation will occurs. Thus, each successive interior column will suffer a smaller deformation and lower stress than the outer ones.
Figure 4 Side view of axial deformation of flange of frame’s causing shear lag effect
The shear lag effect produces bending of the floor slabs and causing the plane cross section could no be longer remains in plane. Consequently the deformation of interior partitions and secondary structural components were occurred which increase cumulatively throughout the height of the building. Therefore, several structural formulations are in vogue which strives to achieve tubular action with a minimum of shear lag effect and yet accommodate the window penetrations in the exterior tube walls.
Framed Tube Behavior
The primary lateral load resistance of the frame tube is provided by the overall bending of the tube by introducing the tensile and compression forces in the tube windward and leeward faces. The perimeter column can be considered as continuous wall elements were look like square hollow. Under lateral load, the perimeter columns were predominant the bending mode of the building. From the stress distribution diagram as shown in Figure 5, it is easy to visualize the behavior of the tube building having plan forms other than square.
The efficiency of the tube system is directly related to the geometry of the shape building, such as the overall depth-to-width ratio and the height-to-width ratio. The behavior of the tube can be compared to hollow cantilever, the overall action of the cantilever bending due to lateral load because of shortening on leeward column and elongation on windward columns plus a shear deformation brought about by the local bending of column and spandrel.
The principle behind an efficient of frame tube system is designed the bracing system were minimizes the shear type of deformation and make the wholes building bending essentially as a cantilever. Actually, the in plane of exterior wall is significant provides the efficient system for carrying the lateral loads. It is because the system essentially eliminates the shear type of deformation. Anyhow, it is needed to introduce the shear-resisting element between the windward and leeward columns to make the overall column work as integral parts of tube. This requirement can be achieved by provided a system of closely spaced column and deep spandrels along the building periphery.
Figure 5: Axial stress distributions in square hollow tube
Vortex shedding phenomenon
The effect lateral load due to wind on the building are increase over the height. The horizontal swings may be not dangerous but may causing motion sickness to the occupants. Tall buildings may not only be subjected to wind excitation in a direction parallel to the wind but also in a direction perpendicular to it. The major criterion for design of building is the crosswind response and dynamic forces arising from the wind loads. The structure has a significant dynamic response to wind because of the effect buffeting. However, when subjected to bending under this loading, the system of the building may conventionally acts as a vertical cantilevered tube resisting the lateral force.
This phenomenon of alternate shedding of vortex is shown in Figure 6. The building under the wind pressure will bend slightly and moving at its top. It first moves in the direction of wind, says with a magnitude of 0.61m, and then starts oscillating back and forth. The top goes through its neutral position, then moves approximately 0.61m in the opposite direction, and continues oscillating back and forth until it eventually stops by the damping that inherent inside the structure.
Figure 6: Vortex shedding phenomenon
Behaviour of Interacting Vertical and Horizontal Component of Buildings
Column and Spandrel Beam
The action of column and spandrel beam may happened under the action of combined vertical and lateral loads. The beam and column stiffness may be increased dramatically by reducing the clear span and increasing the depth as recommended in Council on Tall Building and Urban Habitat. It recommend that the column spacing should be on the order of 1.5-4.5m center to center, maximum and a beam depth typically is the range of 600-1200mm.
The columns and spandrel beam rigidly connected together as an entire assemblage, continuously around the building and may efficiently distributing the column stress. The amounts of transfer deformation from one column to next column are depending on the stiffness of the connecting beam (Alex Coull, et. al, 1991). Therefore, the column and connection perform the important interacting role between frame tube and core. The large displacement value can be avoided by taking rigidity connection of column and beam (Anderson, 1991) in the analysis.
Floor system is assumed to act as rigid diaphragms for analytical purpose. The cross sectional shape is maintained at each level and rigid body movement in plan is happening. All horizontal displacement may be expressed in terms of two orthogonal translation and rotation (Alex Coull, et. al, 1991). The primary action of the floor system is to transmit the horizontal forces between the frame tube and core system. The consideration of the interacting floor slab is accounted for all the possible displacement using three dimensional analyses.
Frame Action of Column and Slab System
Frame action of tall building may developed due to a portion of the slab as a shallow beam continuous with the columns. Typically, concrete floor in tall building often consist of a two-way floor system. The floor slab distributes the lateral load to the various resisting elements through the forces in its own plane. Seldom, the force distribution of slab effected by the in-plane deformation. The assumption of the floor slabs as a fully rigid connection is used almost all of tall building structural analyses.
It is difficult to specify the slenderness ratio of the building with concern to the slab in the finite-element analysis due it is involvement of many factors that effect its behaviour. The stress concentration at the connecting joint between the columns and slabs is one of the problems in tall building analyses with neglect to lateral load. The non-linear behaviour of the structure is initiated through the concrete cracking and steel yielding. While, the shear reinforcement at the column slab joint is used to improve the joint behaviour and to avoid early stiffness deterioration under lateral cyclic loading.
Slab, Shear Wall and Column
The applicable height range of slab and shear wall system can be increased by the inclusion of the frame action between column and slabs. The walls can either in planar, open sections, or closed section around elevator and stair cores. The frame action of column and slabs is taken into account in the lateral load analysis because this action is significantly related to the structure’s element stiffness. Typically, the resistance to overturning of the frame is offered by the shear wall is in a range 10 – 20 percent. But, many engineers still ignore the frame action while designing the building and assuming that the wall will carry the whole lateral loads. In keeping with the current trend of all structural action, it is advisable to include the frame action in the analysis.
Coupled Shear Wall
When two or more shear wall is interconnected by system of beams or slabs, the total stiffness of the system were exceeds the summation of the individual wall stiffness alone. This may due to the connecting slab or beam restraining the individual cantilever action of each wall by forcing the system to work as composite section. Where shear wall are compatible with other functional requirement, such walls can economically resist the lateral forces. However, the planar shear wall is only efficient if its in the plane of lateral load.
Wall around elevators, stairs and utility shafts offer an excellent resistance to lateral and gravity loads. Closed or partially closed section of shear walls are also efficient in resisting the torsion, bending moments and shear forces in all directions of the building.
To achieve the full-strength capacity of the wall, the coupling beam is needed to possess a high degree of rotational capacity. The shear or diagonal splitting is a common mode of failure in RC beams with relatively low span-depth ration and moderate reinforcement (Subedi et al, 1986). There are three basic modes of failure that can be identified in coupled shear wall structures, depending on the degrees of interaction and the behavior of the coupling beams. The failure modes of the coupled shear wall are:
- Flexural failure of coupling beams
- Shear or diagonal-splitting failure of coupling beams
- Rigid action of coupling beams
Flexural Failure of Coupling Beam
This mode of failure occurs in walls with relatively shallow coupling beams reinforcement with small amount of main bars. The wall will deform with the formation of flexural cracks in the tension wall as shown in Figure 7a. The coupling beams near the highly stressed levels will develop flexural cracks at the junctions with the walls. As the load is increased, the flexural cracks will progress deeper into the wall. Some cracks may also develop along the height of the wall and spread into more coupling beams. As the load is increased, the failure of the wall will occur by the crushing of the compression wall at the most highly stressed corner (Figure 7a).
Shear or Diagonal-Splitting Failure of Coupling Beams
This mode of failure occurs in walls with relatively deep and moderately reinforced coupling beams. The process of failure starts with the formation of flexural cracks in the tension wall. The coupling beams at the highly stressed level might show some minor flexural cracks at the junction with the wall.
As the lateral load increased, the main characteristic of failure is the formation of diagonal splitting cracks in the coupling beam around the highly stressed levels. These inclined cracks start near the centre of the coupling beam and spread across the compression diagonals (Figure 7b).
Further increment of load will show some progress in the already-formed flexural cracks in the wall and some new flexural cracks along the height. The spread of diagonal splitting into other coupling beams will follow as load continues to increase. The failure of the wall will occur with the crushing of the compression wall at the most highly stressed corner.
Rigid Action of Coupling Beams
The failure of coupled shear wall is characterized by the crushing of the highly stressed compression corner with only partial damage or node at all the coupling beam. This is because of the composite action of the walls brought by the stiffer connection by the coupling beams. The tension wall will develop a large number of cracks along the height of the structure. The failure of the wall will resemble a simple cantilever beam under the action of the lateral load (Figure 2.7c).