TALL STRUCTURES

INTRODUCTION

Tall Structures has been fascination mankind since early ages.

Human beings urge to stand tall which has been expressed through

construction of tall monuments, high places of worship in the beginning.

Further the invention of elevators, air-conditioning systems etc., have

made the living in tall buildings comfortable.. Emergence of new efficient

structural systems, high strength materials, construction technology has

made the dream of scaling the sky realistic for nearly 0.7 kms at present.

Scarcity of living space has prompted engineers to conceptualize vertical

cities in this century.

DEFINITION:

The tallness of the building is relative and cannot be defined is

absolute terms either in relation to height or the number of stories. But

from a structural engineers point of view the tall building can be defined

as one that, by virtue of its height, is affected by lateral forces due to wind

or earthquake or both to an extent that they play an important role in the

structural design.

‘A building whose height creates different conditions in the

design, construction and use than those that exists in common buildings

of a certain region and period’ by the council of Tall Buildings and habitat.

WHY DO WE NEED A TALL BUILDING?

Tall structures have fascinated mankind since

They stand as a mark of prestige and civilization.

They serve as landmark in the global picture.

The increasing demand for shelter is met without encroaching

on the agricultural land.

FACTORS AFFECTING GROWTH, HEIGHT AND

STRUCTURED FORM OF A TALL BUILDING:

State of Art of Service Systems.

Availability of Material.

Construction Technology.

Building Name Location Year Height in

metres Material Use

Pyramids Egypt 2500 B.C. 147 Stone Tomb

Singer USA 1907 187 Steel Office

Metropolitan Tower USA 1909 206 Steel Office

Woolworth USA 1913 242 Steel Office

Chrysler USA 1929 319 Steel Office

Empire State Building USA 1931 381 Steel Office

World trade Centre USA 1972 412 Steel Office

Sears Towers USA 1973 442 Steel Office

Petronas Towers Malaysia 1996 452 Mixed Multipurpose

Tapei Towers Taiwan 2005 515 Mixed Multipurpose

CHRONOLOGY OF INCREASE IN HEIGHT OF TALL BUILDINGS:

Building Name Location Year Height in metres Material

Tapei Towers Taiwan 2005 515 Mixed

Petronas Towers 1 Malaysia 1996 452 Mixed

Petronas Towers 2 Malaysia 1996 452 Mixed

Sears Towers USA 1973 442 Steel

World trade Centre 1 USA 1972 417 Steel

World trade Centre 2 USA 1972 415 Steel

Empire State Building USA 1931 381 Steel

Central Plaza Hong kong 1992 374 Concrete

Bank of China Tower Hong Kong 1989 369 Mixed

Amoco Building USA 1973 346 Steel

TEN TALL BUILDINGS IN THE WORLD

IMPACT OF TALL BUILDINGS ON MANKIND:

Living on horizontal cities has a different effect on the people

when compared to their life in tall buildings. Many are the benefits that

can be listed out as follows

Brightness is ensured because of the height.

Fresh air is available.

Dust free clean environment is possible.

Privacy is ensured.

Noise pollution is made less.

Land is used in a better way.

A landmark is created.

Stands as a mark of respect (Prestige and Progress of the country)

Generates interest in tourism to the place.

It has also been reported that there are many practical and

psychological problems in living of high rise structures. They are

mentioned below:

Space and operation of lifts cost problems.

Safety of children is less.

Does not provide a garden for residence of top floors.

Reduces family interaction.

The vandalism and crime rate increases.

Fear of failure of the structure is implicit.

Elderly people find it inconvenient to live.

Increases traffic problem in the nearby area.

Increase in land value.

Children feel isolated and their study performance is found to be

affected.

Due to restricted social space, melancholy is created.

COMPONENTS OF A TALL STRUCTURE:

Structural systems

Mechanical systems

Electrical systems

Partition walls and claddings

Foundation

STRUCTURAL SYSTEMS

Systems for resistingVertical Loads Horizontal Loads

Systems for resisting

VERTICAL FRAMING SYSTEMS:

These systems function primarily to carry vertical loads. In short

they can be called as vertical load transfer systems which may be either

columns or bearing walls or hangers or suspended systems.

LATERAL LOAD RESISTING SYSTEMS:

Rigid Frames

Braced Frames

Shear Walls

Wall frame Structure

Tubular Structure

Tube in Tube buildings

Outrigger – Braced Structure

RIGID FRAME STRUCTURES:

Rigid frame structures consist of columns and girders

joined by moment resisting connections. The lateral stiffness of

a rigid frame depends on the bending stiffness of the columns,

girders and connections in the plane of the bent. The rigid

frames principal advantage is its open rectangular

arrangement which allows freedom of planning and easy fitting

of doors and windows.

RIGID FRAME STRUCTURES Contd.,

If used as the only source of lateral resistance in a building in

its only typical 6m x 9m bay size, rigid framing is economic only for

buildings up to 25 stories. Above 25 stories the relatively high lateral

flexibility of the frame cells are uneconomically large members in order

to control the drift.

Deformations of a moment resisting frame under lateral

load

The point of contra flexure is normally located near the midheight of the

columns and midspan of the beams.

The connections in steel moment resisting frames are important design

elements. Joint rotation can account for a significant portion of the lateral

sway. The strength and ductility of the connections are also important

considerations especially for frames designed to resist seismic loads.

APPLICATIONS:

Moment resisting frames are normally efficient for buildings up to 30

storeys in height. The lack of efficiency for taller buildings is due to the moment

resistance derived primarily through flexure of its members.

EXAMPLE: World trade centre, Osaka, Japan 252 m high, 55 storeys.

BRACED FRAME:

Rigid frames are not efficient for buildings taller than 30 storeys

because the shear racking component of deflection due to the bending of

columns and girders causes the drift to be too large. The braced frame

attempts to improve upon the efficiency of a rigid frame by virtually eliminating

the bending of columns and girders. This is achieved by adding the web

members such as diagonals or braces. The horizontal shear is now primarily

absorbed by the web and not by the columns. The webs carry the lateral shear

predominantly by the horizontal component of axial action allowing for nearly a

pure cantilever behavior.

Behavior:

In simple terms, braced frames may be considered as cantilevered

vertical trusses resisting lateral loads primarily through the axial stiffness of the

columns and braces. The columns act as the chords in resisting the overturn

moment, with tension in the windward column and compression in the leeward

column. The diagonals and girders work as the web members in resisting the

horizontal shear with diagonals in axial compression or tension depending upon

their direction of inclination. They undergo bending when the braces are

eccentrically connected to them because the lateral loads on the building is

reversible, braces are subjected to turn, to both compression and tension.

BEHAVIOR OF BRACED FRAMES

TYPES OF BRACES

ECCENTRIC BRACESCONCENTRIC BRACES

The braced frames can be grouped under above two categories

depending upon their ductility characteristics. In concentric braces, the axes of

all members, that is, columns, beams and braces intersect at a common point

such that the member forces are axial. The eccentric braces utilize offsets to

deliberately introduce flexure and shear into framing beams to increase ductility

CONCENTRIC BRACES

ECCENTRIC BRACES

SHEAR WALL STRUCTURE:

Concrete and masonry continuous vertical walls may serve both

architecturally as partitions and structurally to carry gravity and lateral loading.

This very high plane stiffness and strength makes them ideally suited for bracing

tall buildings. In a shear wall structure, such walls are entirely responsible for the

lateral load resistance of the building. They act as vertical cantilevers in the form

of separate planar walls and as nonplanar assemblies of connected walls around

elevators, stairs and service shafts. They are much stiffer horizontally than rigid

frames, shear wall structures can be economical up to about 35 stories.

SHEAR WALL STRUCTURE

WALL FRAME STRUCTURES:

When shear walls are combined with rigid frames the walls, which tend

to deflect in a flexural configuration, and the frames, which tend to deflect in a

shear mode, are constrained to adopt a common deflected shape by the horizontal

rigidity of the girders and slabs. As a consequence the walls and frames interact

horizontally, especially at the top, to produce a stiffer and stronger structure. The

interacting wall – frame combination is appropriate for buildings in the 40 – 60 storey

range, well beyond that of rigid frames of shear walls alone.

TUBULAR STRUCTURE:

The lateral resistance of framed tube structures is provided by very

stiff moment resisting frames that form a ‘tube’ around the perimeter of the

building. The frames consist of closely spaced columns, 6 – 12 ft ( 2 – 4 m)

between centers, joined by deep spandrel girders. Although the tube carries all

the lateral loading the gravity loading is shared between the tube and interior

columns or walls. When lateral load acts, the perimeter frames aligned in the

direction of loading act as the webs of the massive tube cantilever and those

normal to the direction of the loading act as the flanges. The tube is suitable

for both steel and reinforced concrete construction and has been used for

buildings ranging from 40 – 100 storeys.

TUBULAR STRUCTURE

TUBE IN TUBE STRUCTURE:

This variation of the framed tube consists of an outer framed

tube the hull together with an internal elevator and service core. The hull

and core act jointly in resisting both gravity and lateral loading. In a steel

structure the core may consist of braced frames, whereas in a concrete

structure it would consist of an assembly of shear walls. To some extent

the outer framed tube and the inner core interact horizontally as the

shear and flexural components of a wall frame structure with the benefit

of increased lateral stiffness.

TUBE IN TUBE STRUCTURE

OUTRIGGER BRACED STRUCTURE:

The efficient structural form consists of a central core, comprising

either braced frames or shear walls, with horizontal cantilever “ outrigger

trusses “ or girders connecting the core to the outer columns. When the

structure is loaded horizontally, vertical plane rotations of the core are

restrained by the outriggers through tension in the windward columns and

compression in the leeward columns. Outrigger-braced structures have been

used for buildings from 40 – 70 storeys high, but the system should be

effective and efficient for much greater depths.

OUTRIGGER BRACED STRUCTURE