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CHAPTER II
LITERATURE REVIEW
2.1
Design Philosophy of Reinforcement Concrete 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.
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2.2
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.
2.3
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 threedimensional 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
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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.
2.4
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.
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Frame Shear wall
(a)
(b)
Figure 2.1
Composite
structure
(c)
( 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 2.2 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 2.2a and 2.2b). Figure 2.2c
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.
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Height
Height
Wall-frame
Wall alone
Height
Frame
Wall
External
Point of
contraflexure
Wall
Horizontal deflection
Figure 2.2
External
Bending moment
Frame
Shear force
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.
2.5
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.
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
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.
2.5.1

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.
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
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.
2.6
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
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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.
2.7
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 2.3a), 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 the web frame are growing smaller toward the center linearly instead in
Figure 2.3b. This phenomenon is called shear lag. This phenomenon causing the
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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.
D
C
A
B
(a)
Actual axial
stress
distribution
Distribution
without shear
lag
Compression
Web frames
Compression
C
D
Distribution
without shear
lag
A
B Tension
Flange frames
Wind force
(b)
Figure 2.3
Column stresses no shear lag
Column stresses with shear lag
Shear lag effects in tube structures. ( a ) Cantilever tube
subjected to lateral loads; ( b ) Shear stress distribution;
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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 2.3b. 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 2.4 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.
C2 C1 C
Figure 2.4
lag effect

Side view of axial deformation of flange of frame’s causing shear
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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.
2.8
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 2.5, 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
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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.
Wind load
Tension
Compression
Figure 2.5
2.8.1
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
17
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 2.6. 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.
Equivalent load
Due to vortices
Vortice
s
Wind
Building deflection
Equivalent load
due to vortices
Figure 2.6
Vortex shedding phenomenon
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2.9
Behavior of Interacting Vertical and Horizontal Component of Buildings
2.9.1
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.
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2.9.2
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 behavior. 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 nonlinear behavior 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 behavior and to avoid early stiffness deterioration under
lateral cyclic loading.
2.9.3
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
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with the current trend of all structural action, it is advisable to include the frame
action in the analysis.
2.9.4
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:
1. Flexural failure of coupling beams
2. Shear or diagonal-splitting failure of coupling beams
3. Rigid action of coupling beams
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2.9.4.1 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 2.7a. 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 2.7a).
2.9.4.2 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 center of the coupling beam and
spread across the compression diagonals (Figure 2.7b).
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.
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2.9.4.3 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).
(a)
Figure 2.7
(b)
(c)
Mode of failure of coupled shear wall structures. ( a ) Flexural
failure of coupling beams; ( b ) Failure of coupling beams; ( c ) Rigid action of
coupling beams