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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 in compression
� Tent:
– a cable structure consisting of a waterproofing membrane
supported 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 roof must be maintained in tension -easily achieved if the roof is saddle-shaped
� Example: hyperbolic paraboloid, with curvatures in opposite senses in directions at right angles
– cables hung in direction BD
– a second set of cables placed over
them, parallel to direction AC and put in
to tension
– cables from the second set press down
on those from the first one, putting them
into tension as well ⇒⇒⇒⇒ fully-tensioned network
Cables: roof structures
� One of the first doubly curved saddle-shaped cable supported roof was the Dorton Arena in Raleigh, North Carolina, built in 1952
� The building has dimensions of 92 m x 97 m
� The roof is suspended between two parabolic arches in reinforced concrete intercrossing each other, and supported by columns
� The cable network consists of 47 prestressed cables with diameter varying from 19 mm to 33 mm
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Suspension bridges
� Suspension bridges: the earliest method of crossing large gaps
� Early bridges realised from a walkway suspended from hanging ropes of vines
� To walk a lighter bridge of this type at a reasonable pace requires a particular gliding step, as the more normal walking step will induce travelling waves that can cause the traveller to pitch (uncomfortably) up and down or side-to-side.
Suspension bridges
� Suspension bridge realised following the simple design of 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 the cable 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 distributes concentrated loads over greater lengths
� Alternatively: restrain vertical movement of the catenaries by inclined cables attached to the top of the towers 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 for traffic for a few months only before collapsing.
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Cable-stayed bridges
� A cable-stayed bridge consists of one or more piers, with cables supporting the bridge deck
� Basic idea: reduce the span of the beam (deck) several times compared to the clear span between the piers
� Steel cable-stayed bridges are regarded as the most economical bridge design for spans ranging between 200 and 400 m
� Shorter spans: truss or box girder bridges
� Larger spans: suspension bridges
Cable-stayed bridges
� Reducing the span of a beam greatly improves the maximum stress 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 the building 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 of Herculaneum
Multi-storey buildings: beginnings
� Beginning of the 19th century - forefront of the industrial revolution in England:
– demand for large factory buildings of several storeys and large
clear 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 taller office 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 in Chicago
� Built between 1889 and 1891
� 16 storeys, 60 m high
� Tallest masonry building until 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 exterior walls for each square meter of floor
Multi-storey buildings: skeleton frames
� Home Insurance Building
� Built in 1884 and demolished 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 make use of steel skeleton frame instead of masonry walls
⇒⇒⇒⇒ significant reduction of dead weight (1/3 of that of a masonry building)
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Multi-storey buildings: skeleton frames
� Steel skeleton frames
– loads carried by a steel frame composed of columns and beams
rigidly connected between them
– large clear spaces
Traditional load-bearing wall construction
� Outside load-bearing wall support:
– dead weight of the walls
and 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 resultant force must lie in the middle third of the
thickness of the wall ⇒⇒⇒⇒very thick walls in the lower storeys
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Load-bearing wall construction
� In modern load-bearing wall construction, lateral forces due to wind are resisted by walls aligned in the direction of the wind
� Such walls are much more effective, because they have a much larger moment resistance
� Transverse walls acts as vertical cantilevers against lateral forces
� In modern construction,load-bearing walls are from reinforcedconcrete
Multi-storey buildings: gravity and lateral loads
� The load-bearing walls must be in the same position in plan to act as a vertical cantilever
� In order to provide clear floor spaces, doors, corridors, lift wells and staircases
� Most buildings realised as a combination 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 resisting
vertical loads only
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Multi-storey buildings: gravity and lateral loads
� Lateral forces on external cladding are transmitted to the bearing walls
– directly, through external cladding
– indirectly, via floors
� Floors must be stiff and strong in their plane in order to allow lateral forces acting on gravity frames to be transmitted to load-bearing walls
� Usually floors are realised from cast in place reinforced concrete to give a monolithic slab over full plan of the building
F F
stiff floor flexible floor
Multi-storey buildings: types of structures
� As the height of the building increases, the more important are wind and earthquake loads in comparison with gravity loading
– In a multi-storey building, acting as a vertical cantilever, bending
stresses 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 lateral loads through flexural strength of members
– clear spaces, but
– large deformations of the structure
– large stresses due to bending
� Braced frames resist lateral loads through direct (axial) stresses in the triangulated system
– obstruction of clear spaces, but
– small deformations (rigid structure)
– smaller stresses due to more efficient
structural behaviour
Multi-storey buildings: braced steel frames
� Concentrically braced frames with diagonal bracing
� Concentrically V-braced frames
� Eccentrically braced frames
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Multi-storey buildings: steel structural systems
Multi-storey buildings: steel structural systems
� Braced frame efficient in reducing lateral deformations at the lower storeys, but becomes inefficient at upper storeys 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 between the 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 columns are "involved" into the truss-like action (axial stresses) through the outrigger truss
Multi-storey buildings: steel structural systems
� Exterior framed tube: closely spaced columns at the exterior of the building, rigidly connected to deep beams
� Acting like a giant rectangular steel hollow section
� Shear-lag effect - non-uniform stresses on web 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 tubes to reduce the shear lag effect
� Sears Tower, Chicago
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Multi-storey buildings: steel structural systems
� Exterior diagonal tube: giant truss-like behaviour
Multi-storey buildings: steel structural systems
� Exterior diagonal tube: John Hancock Center, Chicago