tension cable roof
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Basis of Structural Design
Course 5
Structural action:- Cable structures
- Multi-storey structures
Course notes are available for download athttp://www.ct.upt.ro/users/AurelStratan/
Cable structures
Cables - good resistance in tension, but no strength incompression
Tent:
a cable structure consisting of a waterproofing membranesupported by ropes or cables and posts
cables must be maintained in tension by prestressing in order to
avoid large vibrations under wind forces and avoid collapse
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Cables: roof structures
Cables in a cable-supported roofmust be maintained in tension -easily achieved if the roof is saddle-shaped
Example: hyperbolic paraboloid,with curvatures in opposite sensesin directions at right angles
cables hung in direction BD
a second set of cables placed overthem, parallel to direction AC and put into tension
cables from the second set press downon those from the first one, putting them
into tension as well fully-tensionednetwork
Cables: roof structures
One of the first doubly curvedsaddle-shaped cable supportedroof was the Dorton Arena inRaleigh, North Carolina, built in1952
The building has dimensions of
92 m x 97 m
The roof is suspended betweentwo parabolic arches inreinforced concreteintercrossing each other, andsupported by columns
The cable network consists of47 prestressed cables withdiameter varying from 19 mm to
33 mm
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Suspension bridges
Suspension bridges: the earliest method of crossinglarge gaps
Early bridges realised from a walkway suspended fromhanging ropes of vines
To walk a lighter bridge of this type at a reasonable pacerequires a particular gliding step, as the more normalwalking step will induce travelling waves that can causethe traveller to pitch (uncomfortably) up and down or
side-to-side.
Suspension bridges
Suspension bridge realised following the simple designof early bridges:
cables (catenaries)
light deck
hangers suspending the deck on catenaries
Lack of stability in high winds
Very flexible under concentrated loads, as the form of thecable will adapt to loading form
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Suspension bridges
Capilano Suspension Bridge, Canada
Suspension bridges
Improved behaviour under traffic and wind loads:stiffening trusses at the level of the deck, that distributesconcentrated loads over greater lengths
Alternatively: restrain vertical movement of thecatenaries by inclined cables attached to the top of thetowers or inclined struts below the deck
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Suspension bridges
The Akashi-Kaikyo Bridge, Japan: 1991 m span
Suspension bridges
Golden Gate Bridge, California, USA: 1280 m span
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Suspension bridges
Brooklyn Bridge, USA (the largest from 1883 until 1903):486 m span
Suspension bridges: famous collapse
Tacoma Narrows Bridge, USA, collapsed on November 7,1940 due to wind-induced vibrations. It had been open fortraffic for a few months only before collapsing.
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Cable-stayed bridges
A cable-stayed bridge consists of one or more piers, withcables supporting the bridge deck
Basic idea: reduce the span of the beam (deck) severaltimes compared to the clear span between the piers
Steel cable-stayed bridges are regarded as the mosteconomical bridge design for spans ranging between 200and 400 m
Shorter spans: truss or box girder bridges
Larger spans: suspension bridges
Cable-stayed bridges
Reducing thespan of abeam greatlyimproves themaximumstress and
deflection
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Cable-stayed bridges: examples
Rio-Antirio bridge in Greece. Longest span: 560 m.Total length: 2,880 m.
Cable-stayed bridges: examples
The Millau Viaduct, France. Longest span: 342 m.Total length: 2,460 m.
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Multi-storey buildings
Why multi-storey buildings? large urban population
expensive land
Multi-storey buildings make more efficient use of land:higher the building (more storeys) - larger the ratio of thebuilding floor area to the used land area
Technological competition (very high buildings)
Until the end of the 18th century most buildings of several
storeys in the Western world were made of: continuous walls of brick or stone masonry supporting the roof
floors from timber beams
The same structural system used in the Roman city ofHerculaneum
Multi-storey buildings: beginnings
Beginning of the 19th century - forefront of the industrialrevolution in England:
demand for large factory buildings of several storeys and largeclear floor areas
cast iron available in bulk
cast iron columns used instead of bearing walls and cast iron
beams instead of timber floor joists
Elevator invented in USA in 1870, enabling much talleroffice and apartment buildings to be constructed
Most multi-storey buildings in USA were still making use
of masonry walls instead of columns
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Multi-storey buildings: masonry
Monadnock building inChicago
Built between 1889 and 1891
16 storeys, 60 m high
Tallest masonry buildinguntil today
Walls at the ground floor:almost 1.80 m thick,occupying more than one-fifth of the width of the
building Wall thickness: rule of
thumb - 0.3m3 of exteriorwalls for each square meterof floor
Multi-storey buildings: skeleton frames
Home Insurance Building
Built in 1884 anddemolished in 1931
10 storeys, 42 m high
Considered to be the first
skyscraper Exterior masonry walls
Cast-iron columns
Wrought-iron beams
One of the first to makeuse of steel skeleton frameinstead of masonry walls
significant reduction ofdead weight (1/3 of that ofa masonry building)
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Multi-storey buildings: skeleton frames
Steel skeleton frames loads carried by a steel frame composed of columns and beamsrigidly connected between them
large clear spaces
Traditional load-bearing wall construction
Outside load-bearingwall support:
dead weight of the wallsand floors above
live loads on the floors
horizontal forces due to
wind pressure Columns support
gravity loads only
To avoid tension on the
brick walls, the resultantforce must lie in themiddle third of the
thickness of the wallvery thick walls in thelower storeys
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Load-bearing wall construction
In modern load-bearing wall construction, lateral forcesdue to wind are resisted by walls aligned in the directionof the wind
Such walls are much more effective, because they have amuch larger moment resistance
Transverse walls acts as vertical cantilevers againstlateral forces
In modern construction,load-bearing walls
are from reinforced
concrete
Multi-storey buildings: gravity and lateral loads
The load-bearing walls must be in thesame position in plan to act as a verticalcantilever
In order to provide clear floor spaces,doors, corridors, lift wells and staircases
Most buildings realised as acombination of:
load-bearing walls resisting lateral forces
frames resisting gravity loads
load-bearing walls
or braced framesload-bearing walls
or braced frames
frames resisting
vertical loads only
frames resisting
vertical loads only
load-bearing walls
for lateral loads
frames resistingvertical loads only
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Multi-storey buildings: gravity and lateral loads
Lateral forces on external cladding are transmitted to thebearing walls
directly, through external cladding
indirectly, via floors
Floors must be stiff and strong in their plane in order toallow lateral forces acting on gravity frames to betransmitted to load-bearing walls
Usually floors are realised from cast in place reinforced
concrete to give a monolithic slab over full plan of thebuilding
F F
stiff floor flexible floor
Multi-storey buildings: types of structures
As the height of the building increases, the moreimportant are wind and earthquake loads in comparisonwith gravity loading
In a multi-storey building, acting as a vertical cantilever, bendingstresses at the base increase with the square of its height
Wind loading increases with the height
Earthquake loading increases with building weight
Reinforced concrete structures:
reinforced concrete frames
load-bearing walls
Steel structures:
moment-resisting frames
braced frames
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Multi-storey buildings: types of steel structures
Moment-resisting frames resist lateralloads through flexural strength ofmembers
clear spaces, but
large deformations of the structure
large stresses due to bending
Braced frames resist lateral loads through
direct (axial) stresses in the triangulatedsystem
obstruction of clear spaces, but
small deformations (rigid structure)
smaller stresses due to more efficientstructural behaviour
Multi-storey buildings: braced steel frames
Concentrically braced frames with diagonal bracing
ConcentricallyV-braced frames
Eccentricallybraced frames
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Multi-storey buildings: steel structural systems
Multi-storey buildings: steel structural systems
Braced frame efficient in reducing lateral deformations atthe lower storeys, but becomes inefficient at upperstoreys due to overall cantilever-like effect
Moment-resisting frame: uniform "shear-like"deformations
Combined moment-resisting frame and braced frame:more rigid overall behaviour due to interaction betweenthe two systems
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Multi-storey buildings: steel structural systems
Braced frame with central braced span: inner columns: large axial stresses due to truss action
outer columns: small axial stresses
Outrigger truss: outer columnsare "involved" into the truss-likeaction (axial stresses) throughthe outrigger truss
Multi-storey buildings: steel structural systems
Exterior framed tube:closely spaced columnsat the exterior of thebuilding, rigidlyconnected to deepbeams
Acting like a giantrectangular steel hollowsection
Shear-lag effect - non-uniform stresses onweb and flanges:middle sections are not
very stressed
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Multi-storey buildings: steel structural systems
Exterior framed tube:World Trade Center,New-York
Multi-storey buildings: steel structural systems
Exterior framed tube: World Trade Center, New-York
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Multi-storey buildings: steel structural systems
Exterior framed tube: World Trade Center, New-York
Multi-storey buildings: steel structural systems
Bundled framed tube:combination of multiple tubesto reduce the shear lag effect
SearsTower,Chicago
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Multi-storey buildings: steel structural systems
Exterior diagonal tube: gianttruss-like behaviour
Multi-storey buildings: steel structural systems
Exteriordiagonaltube: JohnHancockCenter,Chicago