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LOAD ACTION ON TALL BUILDINGS
• Dead Loads• Live Loads• Wind Loads• Seismic Loading• Construction Loads • Loads Due to Restrained Volume Changes of Materials • Rain, Snow & Ice Loads• Water and Earth Pressure Loads• Impact and Dynamic Loads• Blast Loads• Combination of Loads
a. Dead Load
• Static forces caused by the weight of every element within the structure.
• The forces resulting in dead load consists of the weights of the load bearing elements of the building, floor and ceiling finishes, permanent partition walls, façade cladding, storage tanks, mechanical distribution systems etc.
b. Live Load• ‘Occupancy Loads’ : Loads caused
by the contents of objects within or on a building. Not part of the structure
• Include weights of people, furniture, movable partitions, mechanical equipments (e.g computers, business machines) etc.
• Variable and unpredictable. Change in live loads not only over time but also as a function of location.
c. Wind Loads• Lateral action caused by winds.• Wind velocity in general increases
with height. The taller the building is, the more exposed the building to strong winds.
• Can cause the parts of the external wall or roof to be blown off.
• If the building is slender, it will sway or vibrate in the wind.
• Major problem for the designer of tall buildings.
d. Seismic Loading
• The earth’s crust is not static; its subject to constant motion.
• Seismic motion acts on the building by shaking the foundation back and forth.
• The mass of the building resists this motion, setting up inertia forces throughout the structure
e. Construction Loads
• Loads during construction of a building – example contractors commonly stockpile heavy equipment and materials on a small area of the structure.
• Causes concentrated loads that are much larger than the assumed live loads which the structure was designed.
TALL BUILDING STRUCTURAL SYSTEMS
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- Column and Beam- Capable of resisting axial and rotational beam
- Wall : either solid with peforation or trussed, capable of carrying axial and rotational forces
- Façade envelope or core for example, tying the building together to act as a unit
- Floor : solid or ribbed, supported on floor framing, capable of supporting forces in and perpendicular to the plane
- types of structural systems
• Parallel Bearing Walls• Cores and Façade Bearing Walls• Self Supporting Boxes• Cantilevered Slab• Flat Slab• Interspatial• Suspension• Staggered Truss• Rigid Frame• Rigid Frame and Core• Trussed Frame• Belt-Trussed and Core• Tube in Tube• Bundled Tube
a. Parallel Bearing Walls
• Comprised of plannar vertical elements that are prestressed by their own weight, thus efficiently absorb lateral force action.
• Used mostly for apartment building ahere large free spaces are not needed and mechanical systems do not necessitate core structures.
b. Cores and Facades Bearing Walls
• Planar vertical elements form exterior walls around a core structure.
• This allows for open interior spaces, which depend on the spanning capacities of the floor structure.
• The core houses mechanical and vertical transportation systems and adds to the stiffenes of the building
c. Rigid Frame• Rigid joints are used between an
assemblage of linear elements to form vertical and horizontal planes.
• The vertical planes consists of columns and girders mostly on a rectangular grid
• A similar organizational grid is used is used for the horizontal planes consisting of beams and girders.
• With the integrity of the spatial skeleton depending on the strength and rigidity of the individual columns and beams, story height and column spacing become controlling design considerations
d. Rigid Frame and Core
• As rigid frame but introducing a core structure to increase the lateral resistance of the building as a result of the core and frame interaction.
• The core systems house the mechanical and vertical transportation systems.
e. Self Supporting Boxes
• Boxes are prefabricated three dimensional units that resemble the bearing wall when they are place and joined together.
• The boxes are stacked like bricks in the ’English pattern bond’ resulting in a criss crossed wall beam system.
f. Cantilevered Slab
• Supporting the floor systems from a central core allown for a column-free space with the strength of the slab as the limit of the building size.
• Large quantities of steel are required especially with large slab projections.
• Slab stiffenes can be increased by tacking advantage of prestressing techniques.
g. Flat Slab
• Generally consists of uniformly thick concrete floor slabs supported on columns
• No deep beams allowing for a mimimum story height
h. Interspatial
• Cantilevered story high framed structures are employed on every other floor to create usable space within and above the frame.
• The space within the framed floor is used for fixed operations, and the totally free space above the frame can adapt to any type of activity.
i. Suspension
• Employing hangers instead of columns to carry the floor loads.
• The cables carry the gravity loads to trusses cantilevering from a central core.
j. Staggered Truss
• Story-high trusses are arranged so that each building floor rests alternatively on the top chord of one truss and the bottom of the next.
• Besides carrying the vertical loads, this truss arrangement minimizes wind bracing requirements by transferring wind loads to the base through web members and floor slab.
k. Trussed Frame• Combining a rigid (or
hinged) frame with vertical shear trusses provides an increase in strength and stiffenes of the structure.
• The design of the structure may be based on using the frame for the resistance of gravity loads and the vertical truss for wind loads similar to the riogid frame and core case.
l. Belt Trussed Frame and Core
• Belt trusses tie the façade columns to the core, thus eliminating the individual action of frame and core.
• The bracing is called cap trussing when it is on the top of the building and belt trussing when around lower sections.
m. Tube in Tube• The exterior columns and
beams are spaced so closely that the façade has the appearance of a wall with perforated window openings.
• The entire building acts as a hollow tube cantilevering out of the ground.
• The interior core (tube) increases the stiffenes of the building by sharing the loads with the facade tube.
n. Bundled Tube
• An assemblage of individual tubes resulting in a multiple-cell tube.
• The increase in stiffnes is apparent and allows for the greates height and the most floor area.
STRUCTURAL SYSTEMS FOR TALL BUILDINGS OF DIFFERENT HEIGHTS
Efficiency of structural systems of tall buildings
Building Cases Year Stories Slender kN/m2 Structural
Empire State Building, New York 1931 102 9.3 2.02 Braced rigid frame
John Hancock Centre, Chicago 1968 100 7.9 1.42 Trussed tube
World Trade Centre, New York 1972 110 6.9 1.77 Frame tube
Sears Tower, Chicago 1974 109 6.4 1.58 Bundled tube
Chase Manhattan, New York 1963 60 7.3 2.64 Braced rigid frame
US Steel Building, Pittsburgh 1971 64 6.3 1.44 Shearwalls+outrigger+belt trusses
IDS Centre, Minneapolis 1971 57 6.1 0.86 Shearwalls+outrigger+belt trusses
Boston Co. Building, Boston 1970 41 4.1 1.01 K-braced tube
Alcoa Building, San Francisco 1969 26 4.0 1.24 Latticed tube
U.S Steel TowerJohn Hancock Centre
Empire State BuildingAlcoa Building, San
FranciscoBoston Co. Building, Boston
VERTICAL LOADING SYSTEMS OF TALL BUILDINGS
• The main function of the vertical loading systems is to transfer the dead and live loads of the superstructure to the substructure.
• Systems of transferring the loads:- Structural Wall System- Skeleton frame System- Suspension System- Composite Wall Frame System- Cantilevered Floor System- Transfers System
Structural Wall System
• Loads are transmitted to the ground via floor and wall (designed as load bearing wall).
• Masonry and brick load bearing were common during the late 19th and late 20th century.
• Now load bearing walls are made from reinforced concrete ;high performance concrete (HPC) .
• Usually of precast concrete panels systems and cast in situ concrete buildings using ‘tunnel forms’.
• Usually residential type because the internal wall layout do not need to be changeable such as in office building.
Skeleton Frame System
• Loads are transferred to the beam and column grid to the ground.
• Using RC or Steel frame.• Faster to erect especially when
structural steel is used
Suspension System
The floors of the building are suspended over a long span.
Ability to provide a column free floor.
Three types:i. Hanger systemii. Bridge Systemiii. Catenary System
i. Hanger System
• Loads are transmitted upwards through vertical tensile members to outrigger arms.
• The loads are then transferred from the outriggers to one or more piers that transmit the loads to the ground.
• The tensile members can be hangers or cables and the pier tower are either monolithic reinforced concrete load bearing walls or steel framed tower.
• e.g Sabah Foundation Building ; Hong Kong & Shanghai Bank Building, Hongkong.
Central Pier Tower
Curtain Wall Facade
Hanger Elements
Tension Ring
Sabah Foundation Building Structural System
Hongkong & Shanghai Bank Building, Hong Kong
ii. Bridge System
• The floor slabs are suspended between two or more towers or mega columns.
• No intermediate columns used to support the floor slabs.
• e.g ; Tabung Haji Building, Petronas Twin Towers, OCBC Singapore.
Knights of Columbus Building
Corner core RC tower ; contains either stairs or risers and toilets
Floor slab structural girder and beams
Central core RC tower; contains lifts shafts
Tabung Haji Building OCBC Singapore
iii. Cantenary System
• Example: Federal Reserve bank of Minneapolis
• Consisted of a pair of catenary members that span between two towers.
• Both catenary members lie on the long facades of the building
d. Composite Wall-Frame System
• The skeleton frame and structural load bearing walls are used together.
• Several arrangements:i. - the wall are arranged to form a core.
- the frame surrounds the core walls. - tubes configuration.ii.- the walls are located at opposite ends
of square shaped plan; generally C- shaped.
- the frame is located between the end walls.
iii.- the walls are located at the corners of square shaped or rectangular shaped plan.
- the corner walls are L-shaped. - the frame is located within the plan
and four corner walls.
e. Cantilevered Floor System
• Floor slabs rest on beams cantilevering from a central tower.
• The loads of the building is transferred to the foundation through the central tower.
• E.g: Nagakin Capsule Tower, Marina Building, Miami & Turnig Torso Malmo
f. Transfer System
• For building where lower floors have lesser columns than the rest of the building.
• The transfer is in the form of a horizontal elements.
• Consists of mega column and mega beams at lower floor
• Skeleton frame is positioned above the transfer mega structures.
• The loads are transferred from the transfer mega beams to the mega columns and then to the foundation.
Minimizing Vertical Loads
• Foundations costs may be lower if the total vertical loads can be reduced• Some of the ways are:
- reducing the floor plan area as the building increases- using lighter materials in the upper floors- using steel instead of R.C for the structural system- reducing the cross-section area of the structural members in the upper floors- placing the heavier M&E plant in an adjunct building e.g using district cooling system.
HORIZONTAL LOADING SYSTEM
• Horizontal (lateral) forces act on the superstructure and substructure of buildings.
• Two types of horizontal forces:i. Wind Forcesii. Earthquake Forces
WIND FORCES
• Wind is variable both in direction and strength.
• Wind exerts loads on the tall structure causing it to oscillate or sway like a pendulum.
• Oscillations must be kept to a minimum:- to ensure occupants’
psychological and physical comfort.
- to prevent deterioration of joints in the
curtain walling and building services.
Earthquakes create lateral forces on a tall building causing it to sway.
Cause the ground to move horizontally and vertically
Earthquake-resistant building has to absorb or counteract the forces.
EARTHQUAKES
Preventing Oscillation of Tall Building
Three main ways:
1. Structural methods by either stiffening or having heavier mass.- Shear Walls- Moment Resistant Frame
Systems : eg tube systems- Bracing- Diagrid systems
2. Counteracting the oscillation by either damping devices or top-to-bottom structural tie members.- Guying methods- Damping Devices : Passive dampers or Active dampers
3. Aerodynamic methods.
a. Structural Methods
i. Shear Walls
• Structural elements to induce stiffenes in the building.
• Monolithic walls of reinforced concrete, brick or masonry can be used to provide stiffenes ; walls with a moment resistant frame.
• Location of the shear walls are:- Central core of building- Ends or corners of building- At certain wall position inside the building
a. Structural Methodsii. Moment Resistant Frame Systems
(also known as Skeleton Frame)• Three dimensional grid of linear
column and beams – connected each other using rigid or semi rigid connections.
• Usually used ‘tubes systems’ – load bearing columns of the exterior perimeter are placed together to form a ‘tube’.- single tube ; tube within tube; bundling of tubes; braced tubes
• e.g : Xerox Building USA, Sears Towers USA
a. Structural Methods
iii. Bracing• Adding braces to the frame.• The bracing can be in different locations in the
structure.• The bracing configurations includes:
- some vertical and/or horizontal bays of the frame are braced.- solid beam bracing- used to brace shear walls together.- vertical truss – consists of mega column, mega beam and mega brace single plane truss arrangement that is located along the height of the moment resistant frame.- a mega space truss that housed floor slabs, minor columns and beams.
• E.g: Bank of China, Hong Kong, John Hancock Building, USA, HongKong Shanghai bank, Hong Kong
a. Structural Methodsiv. Diagrid Systems• Consists of a grid of diagonal
members that cross each other.• The distrubution of the load is
similar to that experienced in a single layer grid dome.
• The diagrid is tied to the core by the floor elements along the height of the building.
• At the top of the building the diagrid terminates to either a ring beam or the core itself.
• Hearst Tower ; Swiss Re Building, London; Hubbell Lighting Headquarters - Greenville, S.C
b. Counteracting The Oscillation
i. Guying Method• Top-to-bottom structural tie
members are installed to the main vertical structure to prevent swaying of the structure.
• The structural tie members are either steel cables that are stretched or monolithic R.C Fins that are extended between the ground and the top of the tall buildings.
• Usually used for towers.
b. Counteracting The Oscillation
ii. Damping Services• The devices are used in the structure of lighter tall buildings (normally of
steel frame construction) and tall ‘pencil’ thin towers and spires of super tall buildings.
• Several types of dampers”* Passive Dampers : is tuned to react to the movement of the building
- viscoelastic dampers- passive tuned mass dampers- pendulum tuned mass dampers
- liquid tuned mass dampers- viscous liquid dampers
* Active Dampers : require sensors to detect the movement and initiate the mechanical hydraulic piston actuators that push against the damper mass and structure.
Passive Dampers
i. Viscoelastic Dampers• Viscoelastic material is
placed at various points in the structure ; often a rubber or neoprene pad sandwiched between the faces of two steel members.
• The pad provide shear resistance to the oscillations forces.
• Eg. Former World Trade Centre New York.
ii. Passive Tuned Mass Dampers• These are sliding or horizontal moving mass
of steel or concrete tuned to move in reaction to the horizontal movement of the building.
• The slab lies on a bed of oil and held in position by heavy springs (or hydraulic pistons) attached to the structural frame of the building.
• The movement of the building causes the mass to compress some spring and extend the others.
• The extended springs pulls on the building frame while the compressed springs pushes on the time.
• This counteracts the movement of the building
• Usually located at top of building where the swing of the oscillation is most.
Passive Dampers
iii.Pendulum Tuned Mass Dampers• A suspended mass acting as pendulum is used.• The pendulum mass is held by pistons.• Act similar to the passive tuned mass damper.• Need high head room• E.g : Taipei 101 building.
Passive Dampers
iv.Liquid Tuned Mass Dampers• Consists of two large tanks or
more whose water contents flow from tank to tank in response to lateral forces that sway the building.
• The sloshing forces of the water on the sides of the tanks as it moves from one tank to another counteract the swaying forces.
• Water tank for firefighting or air conditioning system of the building can be used for this purpose.
Passive Dampers
v. Viscous Liquid Dampers.• Similar to the use of
hydraulic pistons in cars to absorb vibrations from the road.
• Special hydraulic pistons contains viscous liquid are placed at suitable locations throughout the buildings.
• E.g: Torre Mayor Building, Mexico City
Passive Dampers
Active Dampers• Require sensors to detect the movement and initiate the mechanical
hydraulic piston actuators that push against the damper mass and structure.
• Require external mechanisms and electricity to move them in response to the horizontal movement of the building.
• Computers are used to fine tune the responses to the swaying of the building.
Hybrid Damper• Passive active damper that have both the passive damper and active damper• The passive damper is used to for the initial movement up to a dynamic movement point
where it turns off and the active damper activates to respond to the movement.• Now used as an earthquake measure rather than resist wind induced oscillation.• E.g: Fukuoka Building, Japan.
c. Aerodynamics Methods• The building cross sectional plan is designed
to have minimum air turbulence that could cause oscillation.
• The reduction or air turbulence can be obtained by:i. have circular plan rather than rectangular or square plan of the building.ii cutting or rounding off the corners of the building.iii. Providing for perforation at the corners of the building or in the building.iv. Having channels in the buildings silhouette that allow the wind to be channeled away from the
face of the building• e.g: Shanghai World Financial Centre
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NEXT WEEK TASK
Total Design Approach***not to be assess
DESIGN
TOTAL DESIGN APPROACH
requires a team approach between
the various disciplinesof design, material
fabricationand building construction
building must cope with vertical forces of gravity and horizontal forces of wind above grund and the seismic forces below ground
building envelope has to accommodate the differences in temperature, air pressure and humidity between exterior and interior environments
the structural elements of the building must responds to all this forces where members must be arranged and connected to one another in such manner as to absorb the forces and guide them safely with a minimum effort to the ground.
Design Parameters
Design Process and Tools
Design Elements of Tall Building Form
