1. Fundamental Of Tall Buildings

FUNDAMENTAL OF TALL BUILDINGS Dr. Henry LUK

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Why tall buildings?

What is tall buildings? How to design a tall buildings? Source: en.wikipedia.org

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Ancient Tall Structures

Ancient pyramids of Egypt

El Castillo, Mayan pyramid

• The ancient tall structures, which can be considered as prototypes of

present-day high-rise buildings, were protective or symbolic in nature and were infrequently used as human habitats. • Ancient structures such as the Egyptian pyramids and the Mayan temples primarily served more as monuments than as space enclosures.

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• The Pyramid of Cheops was built by piling huge masonry blocks

one on top of another to a peak of 146.7 m, equivalent to a modern 40-story office building. • Ancient structures were constructed using masonry or timber owing to limitation on available building materials. • Limitations: • The spans that timber and stone could bridge, either as beams, lintels, or

arches, were limited. • Wood was neither strong enough for large structures, nor did it possess fire-resisting characteristics. • Brick and stone masonry, in spite of their excellent strength and fire resistance, suffered from the drawback of weight.

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• Monadnock Building Location: Chicago, USA

Completion: 1893 Number of storeys: 17 Height: 60 m

Status: Completed Materials: Masonry Architect: Holabird & Roche; Burnham & Root

Main Contractor: George A. Fuller Co. o Around 2 m thick load-bearing masonry walls at the ground floor were used. o Low net usable area was achieved owing to the excess dead loads and wide crosssections.

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Tall Buildings Development • Technological developments 1. 2. 3. 4.

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Home Insurance Building 10+2 storey (55 m) Steel frame Chicago, USA 1885 / 1890 First skyscraper Demolished

Empire State Building 102 storey (381 m) Braced steel frame New York 1931 Tallest in the world from 1931 to 1970

Construction materials Vertical transportation system - elevator Construction technique Structural form Computer simulation

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Why Tall Buildings • The growth in modern tall building constructions has been

largely for commercial and residential purposes. • Tall commercial buildings are primarily a response to the demand by business activities to be as close as possible. • They form distinctive landmarks so that they are frequently developed in city centres as prestige symbols for corporate organisation. • The rapid growth of the urban population and the consequent pressure on limited space have considerably influenced city residential development.

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What is Tall Buildings Tall building / High-rise building / Skyscraper

• Height

• Construction technology

• Number of storey • Wind effect

Council on Tall Buildings and Urban Habitat (CTBUH) http://www.ctbuh.org/

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a) Height relative to context A tall building is not just about the height, but about the context in which it exists.

b) Proportion A tall building is not just about height but also about proportion (aspect ratio). c) Tall building technologies If a building contains technologies which may attributed as being a product of “tall” (e.g., specific vertical transport technologies, structural wind bracing, etc.), then this building can be classified as a tall building. A building of perhaps 14 or more stories, or more than 50 metres in height, could perhaps be used as a threshold for considering it’s a tall building. CTBUH, Council on Tall Buildings and Urban Habitat, Illinois Institute of Technology, http://www.ctbuh.org/TallBuildings/HeightStatistics/Criteria/tabid/446/language/en-US/Default.aspx

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CTBUH Tall buildings

Buildings of 14 storeys or 50 metres height

Super-tall buildings

Buildings of 300 metres height

Mega-tall buildings

Building of 600 metres height

Emporis Standards High-rise buildings

Buildings of 12 storeys or 35 metres height

Skyscrapers

Buildings of 100 metres height

Ali and Armstrong (Architecture of Tall Buildings, 1995) The tall building can be described as a multi-storey buildings generally constructed using a structural frame, provided with high-speed elevators, and combining extraordinary height with ordinary room spaces such as could be found in low-building. In aggregate, it is a physical, economic, and technological expression of the city’s power base, representing its private and public investments.

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Tall Buildings in the World • 10 tallest completed buildings in the world (Skyscrapercenter, Jan 2016)

http://skyscrapercenter.com

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Tall Buildings in the World • 10 tallest completed buildings/buildings under construction in the world

(Skyscrapercenter, Jan 2016)

http://skyscrapercenter.com

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Name: Burj Khalifa Location: Dubai, United Arab Emirates Completion: 2010 Number of storeys: 163 Height: 828 m Status: Completed Materials: Steel/Concrete

http://skyscrapercenter.com

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Name: Shanghai Tower Location: China

Completion: 2015 Number of storeys: 128 Height: 632 m

Status: Completed Materials: Composite

http://skyscrapercenter.com

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Name: Makkah Royal Clock Tower Location: Mecca, Saudi Arabia

Completion: 2012 Number of storeys: 120 Height: 601 m

Status: Completed Materials: Steel/Concrete http://skyscrapercenter.com

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One World Trade Center New York, 2014 541.3 m 94 storey

Taipei 101 Taipei, 2004 508 m 101 storey

http://skyscrapercenter.com

International Commerce Centre Hong Kong, 2010 484 m 108 storey

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Height of Buildings • CTBUH recognises to measure tall building height in three

categories: 1. Height to architectural top 2. Highest occupied floor 3. Height to tip

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1. Height to Architectural Top (widely used) • Height is measured from the level of the lowest, significant, open-air,

pedestrian entrance to the architectural top of the building, including spires, but not including antennae, signage, flag poles or other functional-technical equipment.

CTBUH, Council on Tall Buildings and Urban Habitat, Illinois Institute of Technology

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2. Highest Occupied Floor • Height is measured from the level of the lowest, significant, open-air,

pedestrian entrance to the finished floor level of the highest occupied floor within the building.

CTBUH, Council on Tall Buildings and Urban Habitat, Illinois Institute of Technology

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3. Height of tip • Height is measured from the level of the lowest, significant, open-air,

pedestrian entrance to the highest point of the building, irrespective of material or function of the highest element (i.e., including antennae, flagpoles, signage, and other functional-technical equipment).

CTBUH, Council on Tall Buildings and Urban Habitat, Illinois Institute of Technology

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Tall Buildings in Hong Kong

http://skyscrapercenter.com

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• 10 tallest completed buildings in the HK (Skyscrapercenter, Jan 2015)

http://skyscrapercenter.com

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Central Plaza Wan Chai, 1992 373.9 m

Bank of China Tower Central, 1990 367.4 m

International Commerce Centre Kowloon, 2010 484 m

http://skyscrapercenter.com

HSBC Main Building Central, 1985 178.8 m

BEHAVIOUR OF TALL BUILDINGS

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Tall Buildings • A tall building may be defined as one that, because of its

height, is affected by lateral forces due to wind or earthquake actions to an extent that they play an important role in the structural design. • The influence of these actions must therefore be considered from the very beginning of the design process. • High-rise behaviour: G

P

A high-rise building behaves as a vertical cantilever

Subjected to 1. Vertical loading by gravity 2. Transverse loading by wind or earthquake

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• The key idea in conceptualising the structural system for a

narrow tall building is to think it as a beam cantilevering from the earth. • The laterally directed force generated

either due to wind or seismic actions tends both to snap it (shear), and push it over (bending). • Therefore, the building must have a

structural system to resist shear as well as bending.

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• Tall building under lateral loads (UDL) w L/3

At i-th storey 𝑉 = 𝑤𝐿/3

L

𝑀 = 𝑤𝐿2 /18

At the base 𝑉 = 𝑤𝐿 𝑀 = 𝑤𝐿2 /2

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Point load at the top

Point loads at every storey

Triangular loading

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Effects of Gravity Loading Loading transfer: Slab -> Vertical walls and columns -> Foundations

G

G

G

G

G

G

G

3𝐺

=

𝐺𝑖

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Effects of Horizontal Loading • Single storey frame

Shear

Moment

P

Deflection Δ

P

Ph

• Multi-storey frame P

6Δ

P P

3P

6Ph

• For an n-storey building: • Axial load  n • Lateral shear  n • Overturning moment  n2/2 • Lateral drift  n2

Force / Displacement

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Drift Moment

Force

Storey

• This is why the emphasis of tall building analysis and design

should be placed on the structural behaviour of the systems under lateral loading.

STRUCTURAL FORM AND FLOOR SYSTEMS

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Common Structural Forms • Frame structures • Rigid frames • Braced frames • Infilled frames • Shear wall structures • Linked shear walls • Coupled shear walls

• Core wall structures • Tubular structures • Framed-tube structures • Tube-in-tube structures • Exterior diagonal tube • Bundled tube • Mega-braced framed systems

• Wall-frame structures

• Transfer structures

• Outrigger-braced systems

• Hybrid systems

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Rigid Frames • A rigid frame structure consist of columns and girders joined by

moment-resisting connections. • The lateral stiffness is governed mainly by the bending stiffness of columns, girders and connections in the plane of the bent. • Rigid framing is generally economic Beam/girder for buildings of up to about 25 storeys. Column

Beam-column joints

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Braced Frames • Braced frames may be considered as vertical trusses resisting

lateral loads primarily through the axial stiffness of columns and braces. Chord members Web members • The columns act as the chords in resisting the Single diagonal bracing overturning moment. Double diagonal • The diagonals work as web members resisting the horizontal shear in Chevron axial compression or tension. Storey-height knee

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Shear Walls • Concrete

continuous vertical walls may serve both architecturally as partitions and structurally to carry gravity and lateral loads. Shear walls • Their very high in-plane stiffness and strength make them ideally suited for tall building structures. • A shear wall structure may be economically up to about 35 storeys.

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Wall-frame Structures • A wall-frame structure consists of shear wall structure and rigid

frame structures. • The walls and the frames are

constrained to adopted a common deflected shape by the horizontal rigidity of the girders and slabs. • It Is appropriate for building in the 40 to 60-storey range.

Shear walls Rigid frames

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Outrigger-braced Structures • An outrigger structure consists of a central core (braced frames

or shear walls), with horizontal cantilever “outrigger” trusses or girders connecting the core and the outer columns. • The outriggers are made

one or often two stories deep. • It have been used for buildings from 40 to 70 storeys height.

Outrigger trusses

Braced core

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Framed-tube Structures • The lateral resistance is provided by very stiff moment-resisting

frames that form a “tube” around the perimeter of building. • It has been used for buildings ranging from 40 to 100-storeys. Core (inner tube) Columns to carry gravity loads

Hull (outer tube)

Framed-tube to carry gravity and lateral loading

Framed-tube

Tube-in-tube

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Mega Frame/Trussed Systems • Mega frame/trussed systems consist of RC or composite

columns, braces, and/or shear walls with much larger crosssections than normal, running continuously throughout the height of the building.

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Common Floor Systems • Reinforced concrete floor systems: • One-way slabs • Two-way slabs • Flat slabs • Waffle flat slabs • Steel framing floor system • One-way beam system • Two-way beam system • Three-way beam system • Concrete-steel composite floor systems

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Reinforced Concrete Floor Systems

One-way slab

Two-way slab

Flat slab

Waffle flat slab

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Steel Floor Systems

One-way beam system Two-way beam system

Three-way beam system

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Concrete-Steel Composite Floor Systems

Steel decking composite slab Composite frame system

Composite frame and steel decking

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Typical Structural Form

Plan of office block

Residential block

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Foundations • Shallow foundations • Pad footing • Strip footing • Raft footing • Deep foundations/Pile foundations • Steel H-piles/Steel tubular piles • Socketed steel H-piles • Precast prestressed spun concrete piles • Driven cast-in-place concrete piles • Bored piles • Mini-piles

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Weight of Materials in Tall Building The materials weight (and thus cost) increases non-linearly with increasing building height due to the influence of lateral loads. Appropriate structural form should be selected to reduce the cost.

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Construction Materials • Common construction materials • Concrete • Steel • Composite • Timber • Masonry

http://en.wikipedia.org

DESIGN CONSIDERATIONS

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Aims of Structural Design • Safety and Serviceability • Not only must a structure safely support the loads to which it is subjected, but is must support them in such a manner that serviceability issues are not so great as to frighten the occupants or cause structural damages. • Cost • The designer must always bear in mind to lower cost without sacrifice of strength. Savings can be achieved by minimising material weight, construction time, maintenance cost and maximising structural performance. • The structural cost typically accounts for 20%–30% of the overall building cost.

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• Practicality • The designed structure must be fabricated and erected without great problems arising both in construction and in future maintenance. The engineer should understand fully the method of construction and the availability of manpower and construction facilities. • Probability • Uncertainties in loading conditions, material properties and structural behaviour do exist in constructed facilities. Whilst it is certainly the desire of the engineer to provide a safe and serviceable structure, there is always a risk element in the design decision making process that does not guarantee 100% safety resulting in risk free structures.

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Design for structural systems Difficult task which requires creativity, originality and experience of the engineer

Member level design Design for structural members Routine and time consuming task which often an iterative process.

Overall Design process

System level design

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Structural System Design • A structural system is an assemblage of structural members. • These members are interconnected to each other to transfer

forces from top to the foundation.

Decision of making a structural system depends on

1. understanding of the system level behaviours; 2. limitations of all possible alternatives; and 3. design requirements.

http://en.wikipedia.org/wiki/File:Skyscraper_structure.png

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Factors of Consideration • Function of the building • Number of storey / Height of building • The spans involved • Special consideration is necessary if there is a requirement for long spans or large, clear floor areas. • The vertical loading • The presence of heavy point loads on floors or the need to accommodate cranes. • The horizontal loading • Attention must be given to the way in which horizontal loading is to be resisted. This aspect of design is of particular importance for very tall building.

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• The service required • These include water, electricity and gas and often nowadays significant computing facilities, and are usually accommodated under the floors. • In situations where large volume of services are needed (e.g. hospitals), special forms of flooring permitting easy incorporation of the necessary pipework and ducting may be necessary.

• The ground condition • Clearly the type of ground on which the building is to be erected will dictate the form of foundation that must be used and this in turn must be taken into consideration when selecting the super-structure. • The structural performance, practicality and cost.

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Structural Member Design • Once a structural system is defined, the detailed design is then

performed on the member level. • Given the geometric layout of a structural framework, a structural analysis is then carried out to obtained its structural responses. • Depending on the internal force action on each individual member, a specific size of each member is then estimated and designed in according with a design standard.

Beam member

Beam-column member

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Structural Analysis • Internal forces (axial, shear, moment, torsion) in each structural

members can be obtained via structural analysis. 1. 2. 3.

Classical analytical approach Approximate approach Computer simulation (Finite element method / FEM)

Braced frame structure

Rigid frame structure

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Design Standards in HK

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Structural Elements Reinforced Concrete Design

Steel Design

• Beams

• Tension members

• Slabs

• Compression members

• Short columns

• Beams

• Walls

• Beam-columns

• Footings

• Steel connections

• Pilecaps

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Limit State Design • A structural engineer has to design structures that are both

safe and economic. • It is difficult to assess at the design stage how safe and economic a proposed design will actually be in practice since there are too many uncertainties. • Uncertainties fall roughly into groups: • Loading; • Material strength; and • Structural behaviour.

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• Limit state design is a modern approach for structural design

based on the concept of probability. • It aims to ensure an acceptable probability that a structure will perform satisfactorily during its design life. • Two main limit states Ultimate limit state (ULS) Ultimate limit states concern the safety of the whole or part of the structure at ultimate loading conditions.

Serviceability limit state (SLS) Serviceability limit states correspond to limits beyond which the whole or part of the structure becomes unserviceable under working loads.

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• It requires that a member be designed such that

(  Load)  Capacity/     f

Design Load

m

Design Capacity

where γf and γm reflect the degree of uncertainties in the various loads and the resistance. • The above approach is for ULS checking. On the other hands,

SLS checking in principle uses mean values instead of characteristic values and almost always does not apply partial factor of safety.

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Design Load

• Characteristic load = magnitude of load that is sufficiently larger than the average load so that only a very low probability it will be exceeded during the design life. • Design load = Characteristic Load x γf

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Design Capacity •

Characteristic strength = value of the strength of the material that is sufficiently lower than the mean value so that only a small portion of the materials in the structure is expected to fall below it. • Design strength = Characteristic Strength / γm

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References • Bryan Stafford Smith, Alex Coull (1991). Tall Building Structures: Analysis and Design. John • • • •

• • •

Wiley & Soons, Inc. Bungale S. Taranath (2004). Wind and Earthquake Resistant Buildings: Structural Analysis and Design. CRC Press, Taylor & Francis Group. Bungale S. Taranath (2010). Reinforced Concrete Design of Tall Buildings. CRC Press, Taylor & Francis Group. Bungale S. Taranath (2012). Structural Analysis and Design of Tall Buildings, Steel and Composite Construction. CRC Press, Taylor & Francis Group. Lin, T.Y. and Stotesbury Sidney D. (1981). Structural Concepts and Systems for Architects and Engineering, 2nd ed. Van Nostrand Reinhold. Mark Sarkisian (2012). Designing Tall Buildings, Structure as Architecture. Routledge, Taylor & Francis Group. Mehmet Halis Günel and Hüseyin Emre Ilgin (2014). Tall Buildings Structural Systems and Aerodynamic Form. Routledge, Taylor & Francis Group. Dave Parker and Antony Wood (2013). The Tall Buildings Reference Book. Routledge, Taylor & Francis Group.