Tension Cable Roof

7/27/2019 Tension Cable Roof

1/19

1

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


2/19

2

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


3/19

3

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


4/19

4

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


5/19

5

Suspension bridges

The Akashi-Kaikyo Bridge, Japan: 1991 m span

Suspension bridges

Golden Gate Bridge, California, USA: 1280 m span


6/19

6

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.


7/19

7

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


8/19

8

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.


9/19

9

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


10/19

10

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)


11/19

11

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


12/19

12

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


13/19

13

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


14/19

14

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


15/19

15

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


16/19

16


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


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


17/19

17


Exterior framed tube:World Trade Center,New-York


Exterior framed tube: World Trade Center, New-York


18/19

18


Exterior framed tube: World Trade Center, New-York


Bundled framed tube:combination of multiple tubesto reduce the shear lag effect

SearsTower,Chicago


19/19


Exterior diagonal tube: gianttruss-like behaviour


Exteriordiagonaltube: JohnHancockCenter,Chicago

Documents

Tension Cable Roof