Tubular Systems for Tall Office Buildings With Special Cases From Turkey

8/18/2019 Tubular Systems for Tall Office Buildings With Special Cases From Turkey

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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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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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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 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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Tubular Systems for Tall Office Buildings With Special Cases From Turkey