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COMMON TYPES OF STRUCTURAL SYSTEMS IN TALL BUILDINGS Moment resisting frame system

Braced frame system Shear wall system Advanced structural forms- tubular systems 2.1 RIGID

FRAMES It is a system that utilizes the moment resisting connection between the columns and beams

throughout its perimeter to resist lateral loads applied. It may be used to provide lateral load resistance

for low rise buildings. Generally its less stiff than other systems. This is also known as sway frames or

moment frames. 2.2 BRACED FRAME To resist lateral deflections, the simplest method from theoretical

standpoint is intersection of full diagonal bracing or X bracing. It works well for 20 to 60 storey height but

does not give room for opening such as door and windows. To provide more flexibility for placing of doors

and windows, K bracing system is preferred. If we need to provide larger openings, we can use full storey

knee bracing system. It is found to be efficient in energy dissipation during earthquake loads by forming

plastic hinges in beam at the point of intersection of bracings with the beams. 2.3 SHEAR WALLS The

lateral loads are assumed to be concentrated at floor levels. The rigid floors spread these forces to the

columns or walls in the building. Specially designed reinforced concrete walls parallel to directions of load

are used to resist a large part of the lateral loads caused by winds or earthquakes by acting as deep

cantilever beams fixed at foundation. These elements are called shear walls. They extend over the full

length of the building. Frequently, buildings have interior concrete core walls around the elevator, stair

and service wells. Such walls may be considered as shear walls. The above systems discussed are not

efficient for building greater than 60 storeys 2.4 ADVANCED STRUCTURAL FORMS They are: Framed

tube structure (a) Braced tube structure (b) Tube in tube structure (c) Bundled tube structures (d) (d)

3. TUBE Tube is a system where in order to resist lateral loads (wind, seismic etc.) a building is designed

to act like a hollow cylinder cantilevered perpendicular to ground. This system was first introduced by

Fazlur Rahman Khan. The first example of a tubes use is 43-storey khan designed Dewitt-chestnut

apartment building in Chicago. Fazlur Khan defined a framed tube structure as a 3 dimensional space

structure composed of 3,4 or possibly more frames, braced frames or shear walls joined at or near their

edges to form a vertical tube like structural system capable of resisting lateral forces in any direction by

cantilevering from the foundation This laid the foundation for tube structural design of many later

skyscrapers including John Hancock Centre, Wilis tower, W.T.C, Petronas tower 3.1 CONCEPT OF

TUBULAR SYSTEM The main idea of tubular system is to arrange the structural elements so that the

system can resist the loads imposed on the structure efficiently particularly the horizontal loads. In this

arrangement several elements contribute to the system i.e. slabs, beams, girders, columns. Unlike mostoften, the walls and cores are used to resist the horizontal loads, in tubular system the horizontal loads are

resisted by column and spandrel beams at the perimeter of the tubes. Many tall buildings have adopted

this system and the very first building designed using tubular concept was sears tower. The exterior

framing is designed sufficiently strong to resist all lateral loads on the building, thereby allowing the

interior of the building to be simply framed for gravity loads. Interior columns are comparatively few and

located at the core. The distance between the interior and the exterior is spanned with beams or trusses

and intentionally left column free. This maximizes the effectiveness of the perimeter tube by transferring

some of the gravity loads within the structure to it and increases its ability to resist overturning due to

lateral loads. Tubular structure is a structure with closed column space between two to four metres and

joined by deep spandrel beam at the floor level Group of columns perpendicular to the direction of

horizontal load is called flanged frame and group of columns parallel to the direction of horizontal load is

called web frames. Since the columns are close to each other and the spandrel beams are deep, thestructure can be considered as perforated tube and behaves as cantilevered tube. The flanged frame

columns will resist the axial forces (tension and compression) and web will resist the shear forces. Taking

a pure rectangular tube, as shown in figure 1. The thickness of the wall is t, the length of tube is b and

width is d. The contribution of flanged frame and web to resist the horizontal load for pure rectangular

tube can be obtained by calculating exact moment of inertia as When MI of flanges about their own axis is

neglected, the MI of tube becomes For square tube, b=d, then the section modulus becomes Stress at

extreme fiber where M is the overturning moment at the floor level and d is the between the two extreme

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fibers Portion of overturning moment carried by flange Portion of overturning moment carried by web

Therefore, it is obvious that the largest portion of overturning moment is carried by flanges i.e. 75% of M

and the remaining 25% by webs. This is due to the fact that Z of the square tube is a function of the square

of the distance between the extreme fiber and width of high rise buildings is usually large. Hence, tubular

system is an efficient system to resist horizontal loads. 4. FRAMED TUBE STRUCTURES Frames consist

of closely spaced columns, 2 to 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 structureis provided by stiff moment resisting frames that form a tube around the perimeter of the building. The

gravity loading is shared between tube and the interior columns. This structural form offers an efficient,

easily constructed structure appropriate for buildings having 40 to 100 storeys. W.T.C When lateral loads

act, the perimeter frames aligned in the direction of loads acts as the webs of 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 faade as small windowed and is repetitious and

hence use of fabrication in steel makes construction faster. E.g.: Aon Centre and W.T.C towers 5. BRACED

TUBE STRUCTURES Further improvement of tubular structure can be made by cross bracings the frame

with X-bracings over many storeys. As the diagonals of the braced tube are connected to the column at

each intersection, they virtually eliminate the effects of shear lag in both flange and web frames. As a

result, the structure behaves under lateral loads more like a braced frame reducing bending in the

members of the frame. JOHN HANCOCK BUILDING Hence spacings of columns can be increased and

the depth of girders will be less, thereby allowing large size windows than in conventional framed tube

structures. In braced tube structures, the braces transfer axial load from more highly stressed columns to

less highly stressed columns and eliminates difference between load stresses in columns. E.g.: Chicagos

John Hancock building, The Citigroup Center, Bank of China Tower 6. TUBE IN TUBE STRUCTURES

This is a type of framed tube consisting of an outer-framed tube together with an internal elevator and

service core. The inner tube may consist of braced frames. The outer and the inner tubes act jointly in

resisting both gravity and lateral loading in steel framed buildings. However, outer tube usually plays a

dominant role because of its much greater structural depth. This type of structures is also as hull and core

structures. 7. BUNDLED TUBE The bundled tube system can be visualized as an assemblage of individualtubes resulting in multiple cell tube. System allows for greatest height and most floor area. E.g.: Sears

Tower In this system, introduction of internal webs greatly reduces the shear lag in the flanges. Hence

their columns are more evenly stressed than in the single tube structures and their contribution to the

lateral stiffness is greater. SEARS TOWER 8. ADVANTAGES OF TUBULAR SYSTEMS IN TALL

BUILDINGS Offers some clear advantage from materials standpoint. Designed well, tubular forms have

been known to utilize the same amount of material as would have been employed for a structure that is

half as large or framed conventionally. Allows greater flexibility in planning of interior space since all the

columns and lateral system is concentrated on the perimeter of structure. This allows a column free space

in the interior Regularity in the column schedule allows off-site fabrication and welding where speed can

be achieved while still confronting to quality Wind resisting system since located on the perimeterof the

building meant that maximum advantage is taken of the total width of the building to resist overturning

moment Identical framing for all floors because floor members are not subjected to varying internalforces due to lateral loads 9. CRITICISM TO TUBULAR STRUCTURAL FORMS While it allows high rise

construction with greater efficiency, it significantly reduces the size of the opening in the building. So

while occupants are working or living at greater heights, their view of world outside is rather obstructed.

The finite rigidity of the girders and the connections also lead to a deviation from the standard linear

distribution of axial forces across the flange and web columns assumed in beam theory. This non linearity

in axial force distribution is known as shear lag and is an effect that has to be accounted for in the design

of tubular structures since it has an effect on overall lateral stiffness of the structure. 10. SHEAR LAG

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EFFECT In actual tubular structure, the distribution of axial forces along the flanged frame columns at

one floor is not uniform and the distribution of shear forces along the web is not linear. This is mainly due

to the flexibility of the tubular structures and is called shear lag effect Along the flanges, this non linearity

can result in the corner or exterior columns experiencing greater stress than the center or interior

columns. This is known as positive shear lag. However negative shear lag has been discovered to exist and

this is opposite of positive shear lag and the corners are less stressed than the center columns. This

deviation from traditional beam bending behaviour naturally has implications on bending stiffness of thebuilt up structure. The presence of shear lag thus prevents the full potential in terms of rigidity of the

structure to be adopted. In order to qualify the magnitude and presence of shear lag, two measures have

generally been used Involves the ratio of slopes of the stress distributions with and without shear lag for

web panels Ratio of stressbetween the corner and the center column for flange panels. In either case, as

the ratio gets to unity, shear lag ceases to exist. 11. STRUCTURAL ANALYSIS Strength of tubular form

arises from the utilization of the entire building perimeter to resist lateral loads. The goal is for the entire

perimeter to function compositely and behave as a large cantilever beam anchored rigidly into the ground.

In order to achieve this composite behavior, columns are closely spaced with deep spandrel beams

connecting them. However due to the thinness of the tube wall and finite rigidity of spandrel beams, the

assumption of classical beam bending are violated and structure cannot be accurately analyzed as pure

cantilever beam bending beam. Two assumptions in traditional bending theory which are violated by the

structural behavior of tubular structures are 1) Plane section remaining plane 2) Linear stress distribution

in web Under lateral loading, plane sections do not remain plane due to differential elongation of columns

along the flange which result from local deformation of connecting spandrels. This leads to non-linear

axial stress distribution in both flange and web panel which is called shear lag. Side view of axial

deformation of flange of frames causing shear lag effect Another reason why tubular structure cannot be

readily analyzed as a cantilever beam is the occurrence of shear deformation. The finite rigidity of

columns and spandrel beams give the structure a finite shear rigidity that allows shear deformation to

occur. This shear mode of deformation is ignored in classical bending theory. Hence analysis of tube

structures is based on 3 dimensional analysis using finite element. 12. CONCLUSION 1. The tubular

systems are one of the most effective way of construction of high rise buildings 2. They are extensively

used in the construction nowadays 3. The Burj Khalifa, which is the tallest building uses the bundled tube

design concept