SUSPENDED ROOF

STRUCTURES

Submitted by-

Chetan VasaniAshish VarghesePravin FatingUmesh TipleGaurav Meshram

CONTENTS Introduction Classification Flexible membrane system Cable network structure Types of cable network

roofing system Cable supported roof Cable suspended system Shapes of cable suspended system Single and double layer system Cable net mesh patterns Synclastic surfaces Anticlastic surfaces Cable cum air supported system

Advantages Disadvantages Reduction in flutter Reduction in flutter Water-tightness Structural strand and rope Cable fittings Zinc-poured sockets Swaged socket fittings Swaged socket fittings

INTRODUCTION

HISTORY: Steel-rope cables have been used for

suspension bridges for more than 100 years.

The animal skin, and its tension membrane action, and the spider’s web have encouraged man to develop tension system for his use.

The principle of using tensioned cables in the construction of roof was first used in A.D. 70, in Roman Colosseum.

After that, it was used in much refined manner Raleigh arena in 1953.

GENERAL: It is a structure whose principal

supporting elements are tension members, draped (hung) between anchorages.

Main supports are ropes or steel cables.STRUCTURAL CONSIDERATION: Requires more planning and calculations

of stresses while designing. Cables made out of high tensile steel

wires are uniformly stressed. Special equipments are required to

construct. Wind forces act greatly on structure.

This can be easily overcame by some structural arrangements.

CLASSIFICATION

SUSPENDED ROOF STRUCTURES CAN BE DIVIDED INTO TWO TYPES:

1. FLEXIBLE MEMBRANE SYSTEM.2. CABLE NETWORK SYSTEM.

SPACE STRUCTURES

SKELETON(BRACED)FRAMEWORKS

STRESSED SKINSYSTEMS

SUSPENDED(CABLE OR

MEMBRANE)STRUCTURES

FLEXIBLE MEMBRANE SYSTEM This type of system is suitable for

temporary structures and has been used in many exhibition buildings.

CABLE NETWORK SYSTEM In this systems vertical supports,

anchorages and stabilizers are needed to generate the roof surface.

Vertical supports are needed to provide the required vertical clearances within the structure, because cables sag below their supports.

The cables are supported on posts, or towers or on walls.

Anchorages are required to resist the tension in the cables.

For attachment to the anchorages, each cable is usually equipped with end fittings, often threaded to permit a jack to grip and tension the cable and allow the use of a nut for holding the tensioned cable in place.

In addition, bearing plates are also needed for distributing the cable reaction.

TYPES OF CABLE NETWORK ROOF SYSTEMS

The cable network systems are usuallyclassified as:1. Cable supported system2. Cable suspended system3. Cable-cum-air-supported system

CABLE SUPPORTED ROOF In this system, the deck is covered by

girders or trusses, which, in turn, are supported at one or more points by cables.

This type of construction is advantageous where long span cantilevers are needed.

The steel cables are effective members of the roof structure itself, and not just conveyors of forces from the structure to the anchorages.

In this type of construction (tension structures), the cables themselves resist the various external loads.

Ex. For hangers, Tilburg Railway Station

CABLE SUSPENDED SYSTEM In cable suspended roofs, the system of

cables carries the roof load directly and as such has a primary structural function.

In this case, the main roof structure, instead of being supported, is actually suspended from steel cables above the roof, which transmit the tensile forces to appropriate anchorages.

Ex. Raleigh arena.

SHAPES OF CABLE SUSPENDED SYSTEM

THE CABLE SUSPENDED SYSTEMS CAN BE FURTHER CLASSIFIED AS:

1. ACCORDING TO THE NO. OF CABLES IN THE SYSTEM-(SINGLE OR DOUBLE LAYER)

2. ACCORDING TO THE CURVATURE OF THE IMAGINARY MEMBRANES STRETCHED OVER THE CABLES-(SYNCLASTIC OR ANTICLASTIC)

SINGLE OR DOUBLE LAYER SYSTEM

Suspension systems consisting a single layer of cables are referred to as single layer systems.

When viewed in plan, the cables in a layer may be arranged in parallel, radial or mesh pattern(i.e. cable nets).

Double layer cable systems having cables of one layer directly below corresponding layers of the other layer can be utilized for forming anticlastic or synclastic surfaces.

CABLE NET MESH PATTERNS

SYNCLASTIC SURFACES A surface formed by a real or imaginary

membrane stretched over the cable layer may be synclastic or anticlastic.

A synclastic surface has the same kind of curvature in all the directions; that is, concave or convex in in any direction.

ANTICLASTIC SURFACES

An anticlastic surface has opposite curvature in the principal orthogonal directions; that is, concave in one direction and convex in the other.

CABLE-CUM-AIR-SUPPORTED SYSTEM These roofs are mostly tent or balloon-

type structures which are supported by a combination of cables and are air inflation.

ADVANTAGES They are economical for long span structures. It is lightest roof structure as compared to

others. These structures provide good acoustics and

ventilation. There is higher factor of safety against fire

than conventional roof trusses which fails at certain temperature.

The variety of roof forms and building shapes are possible with this system.

Small amount of scaffolding is required. In double layer system, the space between

upper and lower sets of cables can be used for various pipes, conduits, ducts and air-conditioning equipment.

DISADVANTAGES This requires cable stretching machine. Specialized anchor systems are

required. During storm it vibrate and structure

gets disturbed. So it needs prevention from flutter.

During high temperature cables expand which requires re-adjustment.

Special contractors are required.

REDUCTION IN FLUTTER One of the way to reduce flutter is to

specify a heavy rigid roof deck, such as precast-concrete slabs(which are post tensioned after erection), firmly secured to the cables.

A second way is to use diagonal stays. A much stiffer structure can be obtained by using a double layer construction. With diagonal struts between the primary and secondary cables, truss action can be developed.

WATER-TIGHTNESS It is desirable to place cables either

completely below or completely above the roof surface. If cables must penetrate roof, the joints should be caulked and sealed with a metal protected rubber.

The cable roofs can also be clad with light metal sheets such as corrugated al., tongued and grooved timber boards or lightweight slabs or panels covered with waterproof membrane.

Gunite concrete may be sprayed on metal roofing to provide water-tightness.

STRUCTURAL STRAND AND ROPE

Structural strands and ropes are most commonly utilized as cables.

STRAND: It is an assembly of steel wires wrapped helically around a center wire in one or more symmetrical layers and has a cross section.

For better protection against corrosion, the strands are locked.

ROPE: It is composed of a plurality of strands wrapped helically around a core.

CABLE FITTINGS Many kinds of terminal attachments are

available to facilitate the connection of cables to the structure and to anchorages.

There are two types of terminal fittings; viz.,

1. Swagged sockets.2. Zinc-poured sockets. Other types of fittings used with cables

consist of turnbuckles with both eye-fittings and jaw-end fittings, wire rope clips, wire rope thimbles and anchor shackles.

ZINC-POURED SOCKETS

These are most widely used type of end- terminal.

In this type of fitting, the wires at theend of the cables are broomed out, cleaned and usually immersed in a flux solution.

The wires are then placed in the socket basket and pure molten zinc is poured in the basket surrounding the individual wires.

SWAGED SOCKET FITTINGS

Swagged fittings are often used on the smaller ropes and strands.

In this type, the cleanly cut cable end is inserted into a close-tolerance hole in the end of a steel fitting which is then placed in a die block of a hydraulic press.

The fitting is of a softer steel than the cable wires at that when the die block is under controlled pressure, the softer steel of the fitting flows plastically around the wires.

After an initial squeezing, the fitting may be squeezed several times with the fitting rotated in the die between squeezings.