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CASE
S T U IE S
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can encompass shear walls, columns and beams
3. Types
of
Tubular Structures
attempting to make them act as one unit. The main
future of a tube is closely spaced exterior columns
connected by deep spa ndrels that form a spatial skeleton
and are ad vantageou s for resisting lateral loads in a three
dimensional structural space. Window openings usually
cover about 50 of exterior wall surface. Larger
openings such as retail store and garage entries are
accommodated by large transfer girders. The tubular
concept is both structurally and architecturally
applicable to concrete buildings as is evident from
DeWitt-Chestnut Apartment Building in Chicago
completed in 1965, the first
known
building engineered
as a tube by Khan [ 3, 4, 7, 8 ,9 , 101.
The adoption of the framed tube in steel required an
examination of fabrication methods. While concrete is
field-molded, steel needs to be welded, which is not cost
effective in the field. The development of a framed tube
module involving one column and half spandrel beams,
which can be field-bolted, made possible the app lication
of the framed tube principle to steel. The tree units are
shop-fabricated in jigs where welding can be d one under
controlled conditions. The erection of this unit is both
highly efficient and faster than normal construction. The
60
State Street Building of Skidmore, Owings and
Merill, designed in 1977, in Boston is one of the many
existing examples of framed tube construction in steel
Figure 1) [ I l , 121. With this concept, one can examine
the freedom for forming exterior surfaces. The shape
was derived from consideration of massing with respect
to neighboring tall buildings and visual impact. The
building was conceived as a concrete frame tube, but
was later changed to steel, thus attesting to the
interchangeability of m aterials in this concept.
The shape of the tubes may be designed in a number of
ways depending on the layout of the building. Several
types may be distinguished from the point of view of
structural design and the layout of walls. They will be
lined up acco rding to their effectiveness and the implied
suitability for large o r sm all heights or slenderness ratios
as follows.
3.1
Framed tube system
The framed tube system consists of closely spaced
exterior columns and spandrel beams, which are rigidly
connected together [2, 4, 131. The monolithic nature of
reinforced concrete is ideally suited for such a system.
Depen ding on the height and d imensions of the building,
exterior column spaces sh ould be on the order of 1.5 to
4.5 m
on
center maximum. Spandrel beam depths for
office or residential buildings are typically
600
to 1200
mm. The resulting system approximates a tube
cantilevered from the ground with openings punched
through the tube walls. The closely spaced columns and
deep spandrels have a secondary benefit related to the
exterior cladding for reinforced concrete constructions.
Exterior columns eliminate the need for intermediate
vertical mullion elements of the curtain walls, partially
or totally. An early exam ple of the framed tube system is
the DeWitt Chestnut Apartments in Chicago, as
mentioned e arlier.
The tube is suitable for both steel and reinforced
concrete construction and has been used for buildings
ranging from 40 to 100 stories [14]. The highly
repetitive pattern of the frames lends itself to
prefabrication in steel, and to the use of rapidly movable
gang forms in conc rete, which allows rapid construction.
The framed tube in structural steel requires welding of
the beam-column joint to develop rigidity and
continuity. The formation of fabricated tree elements,
where all welding is performed in the shop
in
a
horizontal position, has made the steel frame tube
system more practical and efficient Figure 2) [2].
Figure 1.
6
State Street Building, Boston.
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363
framed tube at the perimeter Figure 7a-b). The sizes of
the columns are 600 x 600 mm on the lower towers and
600 x 900 m on the taller tower, and spaced 3.5 m on
the centers. The spandrel beams of the higher tower is
designed as flat beams with a height of 350
mm,
whereas the sizes of the spandrel beams of the lower
towers are 600 x 750 mm. The tow ers also contain inner
cores, which consists of shear w alls. The w idth
of
these
shear walls in the 3 6-story tower is 400mm , whereas the
width of the shear walls in the 52-story tower is 600
mm
[9, 10, 15,
16,
171.
Figure
2
Typical tree erection unit..
The closely spacing of columns throughout the
height of a framed tube is usually unacceptable at the
entrance levels for architectural reasons. Therefore a
limited number of column s can be transferred w ith little,
if any, structural premium because the vierendeel action
of the fagade frame is generally sufficient to transfer the
load. However, if the transfer is too severe requiring
removal of a large number of columns, a 1- or 2-story
deep transfer girder or truss may be necessary.
Temporary shoring is required to support the dead and
construction loads until a sufficient number of
vierendeel frames are constructed, or in concrete
construction, until the girder has achieved the design
strength Figure 3)
[
13.
Maya A kar Business Center in Istanbul, Turkey is a
typical example of framed tube systems. The Business
Center consists of two towers , one of which is 19- and
the other 34 stories Figure 4). The plan shape of the 19-
story
A
Block is a rectangle, whereas the plan shape of
the 34-story B Block is a square Figure
5 .
The exterior
columns are spaced at 3.5 m centers and the spandrels
connecting the columns are 4 00
mm
deep. The towers
also have inner cores, designed to resist the gravity
loads only. The w idth of load-bearing walls are 350 mm
and 600 mm respectively in the A and B Blocks [ lo ].
Another recent examp le of the framed tube systems
is the Is Bank General Headquarters, which was
completed in late
2000
Figure 6). The com plex consists
of three towers, two of which are 36-story and the third
is 52 story. Designed by Dogan Tekeli and Sami Sisa
Architect, the tow ers are the tallest buildings of Turkey.
The structural engineer, Irfan Balioglu, designed the
towers to resist an earthquake magnitude of 9.0 on the
Richter Scale. The towers resist the lateral loads by a
Transfer
girder
Dmgonal brace
\Sand
box
Figure 3 Shoring system for a tube structure.
Figure
4
Maya
Akar
Business Center.
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64
Figure
5 .
Maya
ar
Business Center, typical floor plans
3.2
Tube in tube system
This variation of the framed tube consists of an outer-
framed tube, the “hull” tog ether with an internal elevator
and service core. The hull and the core act jointly in
resisting both gravity and lateral loading. In a steel
structure the core may consists of braced frames,
whereas in a concrete structure it would consist of an
assembly of shear wa lls.
Figure 6. s Bank General Headquarters, Istanbul.
To some extent, the outer frame tube and the inner
core interact horizontally as the shear and flexural
comp onents of a w all-frame structure, with the b enefit of
increased lateral stiffness. However, the structural tube
usually adopts a highly dominant role due to its much
greater structural depth [4, 10, 141.
45 2
17 65
‘
99
L
17 65
1
1
Figure 7
s
Bank General Headquarters, structural framing:
a) The 52-story tower.
32 60
5 30 22 00 . 30
0 65
0 6 ~ 5 ~
L0 60 0 60
I
~~ ~
Figure 7 . s Bank General Headquarters, structural framing:
b)
The 36 story tower.
3.3 Trussed tube system
A trussed tube system represents a classic solution for
improving the efficiency of fram ed tube by increasing its
potential for use to even greater heights as well as
allowing greater spacing between the columns. This is
achieved by adding diagonal bracing to the face of the
tube to virtually eliminate the shear lag effects in both
flange and web frames [ l , 141 This arrangement was
first used in a steel structure in 1969, in Chicago’s John
Hancock Center.
Although the trussed tube was initially developed
for structural steel construction, Fazlur Khan applied
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365
similar principles to reinforced concrete construction.
He visualized a concrete version of the diagonal trussed
tube consisting of exterior columns spaced at about 3.04
m centers with block out windows at each floor to create
a diagonal pattern on the faGade. The diagonals could
then be designed to carry the shear forces, thus
eliminating bending in the tube columns and girders.
Currently there exist two high-rises, which are
constructed using this approach. The first is a 50-story
office structure located on Third Avenue in N.Y., and
the second is a mixed-use building located on Michigan
Avenue in Chicago. The first example is a combination
of a framed and trussed tube interacting with a system of
interior core walls Figure 8). All the three subsystems,
consisting of a framed tube, trussed tube, and shear
walls,
are
designed to carry both lateral as the 780 Third
Avenue Building and vertical loads. The building is
173.73 m high with an unusually high height-to-width
ratio of 8:l. The diagonals created by filling in the
windows serve a dual function. First, they increase the
efficiency of the tube by diminishing the shear lag, and
second they reduce the differential shortening of the
exterior columns by redistributing the gravity loads. A
stiffer and much mor e efficient structure is realized with
the addition of diagonals. The idea of diagonally bracing
this structure was suggested by Fazlur Khan to the firm
of Robert Rossenwasser Associates, who executed the
structural design for the building [l, 101. The Chicago
version of the system is a 60-storey multi-use project,
named as Onterie Center Figure 9). Th e building rises
in two tubular segments above a flared base. According
to the designer, diagonal bracing was used primarily to
allow maximum flexibility in the interior layout
needed for mixed use. In contrast to the building in New
York, which is clad with polished granite, Onterie
Center has an exposed concrete framing and bracing
[2,
10, 181. Citicorp Center is a remarkable example of
trussed tube system, which is constructed in steel. The
60-story office building has a 47.8 m s quar e plan, with
all of its com ers jutting out 23 m unsupported from only
four exterior columns, one centered on each side. The
central core also suppo rts the tower. The most direct and
economical way to achieve the 23-m comer cantilevers
on each face of the tower was to provide a steel-framed
braced tube with a systemof columns and diagonals in
compression, channeling the buildings gravity loads into
a 1.5-m wide mast columns in the center of each face
Figure
IO).
The main diagonals repeat in eight-story
modules [2,9, 191.
3 4
Modular bun dled) ube system
The most efficient plan shape for a framed tube is a
square or a circle, whereas a triangular shape has the
least inherent efficiency. The high torsional stiffened
characteristic of the exterior tubular system is
advantageous in structurally unsymmetrical shapes.
However, for buildings with significant vertical offsets,
the discontinuity in the tubular frame introduces some
serious inefficiency. A modular or bundled tube
configured with many cells, on the other hand has the
ability to offer vertical offsets in buildings without loss
in efficiency.
The modular tube system allows for wider spacing
of columns in the tubular walls than would be possible
with only an exterior framed tube. It is this spacing,
which makes it possible to place interior space planning
[11. In principle, any closed-formed shape may be used
to create a modular tube. The ability to modulate the
cells vertically can create a powerful vocabulary for a
variety of dynamic shapes. The bundled tube principle
therefore offers great latitude in the architectural
planning of a very tall building [2]. The most
remarkable example of the modular tube system in steel
is the 110-story Sears Tower in Chicago. With a height
of 443 m, the tower consists of 9 tubes. Each tube is
22.9 m square and make up a typical lower floor for
overall floor dimensions of 68.6 m. This square plan
shape extends to the fiftieth floor, where the first tube
terminations occur. Other terminations occur at floors
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66
66 and 90 Figure 11) . The structure acts as a vertical
cantilever fixed at the base to resist lateral loads. Nine
square tubes of varying heights are bundled together to
create the larger overall tube. Each tube comprises
columns at 4.58-m centers connected by stiff beams.
Two adjacen t tubes share one set of column s and beams.
All coIumn-to-beam connections are fully welded. At
three levels, the tubes incorporate trusses, provided to
make the axial column loads more uniform where tube
dropoffs occur. These trusses occur below floors 66
and
90
and between floors 29 and
31 [
1 , 2 , 4 , 9 , 101.
I
Figure 10.Citicorp Center, elevation
Another example of the bundled tube system is the
63-story Rialto Building in Melbourne, Australia. A
number of structural systems for the Rialto Building
were initially investigated and a reinforced concrete
structural system was finally adopted, with speed of
construction being a prime consideration in the
development of formwork and reinforcement details.
The external frame of columns and beams, while being
designed for the direct and live loads applicable, acts as
an external tube in resisting lateral loads. Although plan
shape is unsymmetrical and the columns are
5
m apart,
analysis of the load transfer around the co mers indicated
reasonable three-dimensional action. The tube effect
also provides for some lateral distribution of load from
the more heavily loaded columns Figure
12
[2 ,20] .
I
Figure 11. Sears Tower.
Figure 12. Rialto Towers, Melbourne, structural framing.
4
Conclusions
In the design of tall buildings, in addition to the
requirements of strength, stiffness and stability the
lateral deflections due to wind or seismic loads should
be controlled to prevent structural and nonstructural
dam age and occup ants’ discomfort. The recent trends in
tall building design include tubular systems, which have
been developed by Fazlur Khan in 1960 and have been
efficiently employed on a number of buildings since
then. The term, in the usual building terminology
suggests a system of closely spaced columns tied
together with relatively deep spandrels. The system is a
fully three dimensional system that utilizes the entire
building perimeter to resist lateral loads. Since the en tire
lateral load is resisted by the perimeter frame, the
interior floor plan is ke pt relatively free of core bracing
and large columns, thus increasing the net leasable area
of
the building. The interior framing can be designed
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only for resistance to gravity loads. As a trade-off, views
from the interior of the building may be hindered by
closely spaced exterior columns. This issue is
considered to be the best advantage of tubular systems
from the architectural point of view.
The tube system can be constructed of reinforced
concrete, structural steel, or a combination of the two
materials, which is named to be composite construction.
Also the type of the tubular system, such as framed,
trussed, modular tube or tube-in-tube mostly depends on
the layout of the building, as well as the height of the
building and the loads effecting on the structure. Not
only the structural engineers, but also the architects, who
are closely related with the design of high-rises, must be
aware of the tubular system, to design contemporary tall
buildings.
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