CATENERY

THE PRINCIPLE OF CATENERY AND APPLICATIONS

A catenery is the theoretical shape of a suspended or hanging chain or cable will assume when acted on only by its own weight. Such a member experiences only tension forces and is very competent for spanning a space. The opposite would be the classical arch, a design feature that ideally only experiences compression. Both structural concepts were widely implemented until the advent of steel beam.

The use of suspension cables owed is origins towards the suspension bridges which has win through in rudimentary forms since the most ancient beginnings of civilisation. The modern suspension bridges had its origin with the discovery of steel in the 1880’s, and since that time has experienced continuous development where in spans reached 1260metres.

The engineering desirability of such a structure is derived from the fact that the most economical means known for spanning large distances is the steel cable. This is owed to the physical fact that steel wire or cable in tension is four to six times stronger than steel in any other form. It is this fact which makes achievable theoretical bridges spans far in excess of those  presently constructed, and create a large amount of the appeal of cables supported roof structures, the principal difficulties in suspension structures includes vibration, flutter and uplift due to wind or unequal live loading. Though these problems exist in suspension bridges, they are more critical even when suspension principles are applied in building structures. This occurs because the extra necessities of breadth and weather tight enclosed space require that a greater extent of firmness be maintained. This coupled with the fact that the large tensile stresses developed in the cables must ultimately be resisted by some large edge beams, added roof weight, and secondary tie members, all of which can compromise structural clarity and economy. In effect, the method of delivering the cable stresses to the ground, and the means by which the problems of flutter, vibration and uplift are controlled become primary consideration in the design of cable structures.

The three methods use to reduce the tensile stresses within cable structures and also which help them to maintain their shape includes:

A doubly curved saddle made up of two networks of which is resisted by edge beams or arches supported on columns.

A circular form with cables suspended radially from a central tension ring in midair to an exterior compression ring supported by wall or columns.

A singly curved network of cables spanning along one direction and slung between edges beams at each end of the cables.

THEORY OF SUSPENSION CABLES

The general form of equilibrium of suspension cables includes the following:

Catenery of equilibrium, this describe a cable loaded uniformly along its length that it will take the form of a catenery .this also applies also to the case of a cable supporting its own weight only.

Parabola of equilibrium, this describes a cable loaded uniformly along its chord length that it will take the form of a parabola.

Funicular polygon of equilibrium, this describe a system of concentrated loads or point loads that  will take the form of a funicular polygon.

The Structural systems of cable networks.

The cable networks consist of two families of cables perpendicular to each other and are at opposite curvature to one another. The geometric breakup produces a doubly curved surface whose parts are sufficiently uniform to permit a repetition of similar building components and a resulting ease of fabrication and erection.

There are cable structures of two types, these include; Cable tension structure and cable stayed structure.

Cable stayed structure, include an intermittent trusses supported by strands or rods that have greater stiffness than wire ropes and hence reduce deflection. It support structures pulling from above, and also pushes from above using compression struts. They do not form part of the structure they only suspends structures. Cable-stayed bridges carry the perpendicular main-span loads through nearly straight diagonal cables in tension. The towers convey the cable forces in the direction of the foundations through vertical compression. The tensile forces in the cables also put the deck into horizontal compression

The cable tension structure: they form part of the structure, using transverse cables and longitudinal cable system such that they are at perpendicular and at opposite curvature in other to relieve the excess tensile stresses the longitudinal cables that act as anti sag cables or support to maintain the catenery shape and to transmit the tensile stresses on the suspension cable to the vertical stress cables or column around.

CABLES AND MATERIALS

Cable net structures offer the possibilities of covering unsupported spans with considerable ease.

As a regulation the constructional element are steel pylons, steel cables networks or wooden grids, as well as roof covering of acrylic glass or translucent, plastic-reinforced sheeting.

FIXING OF CABLES AND MATERIALS

Cables are fastened into the boundaries of the steel network, the eaves, or side of slab to be propped, and are laid over pin-jointed and generally diagonally placed steel supports, and then anchored.

Aerial supports; cable supporting element are necessitated which are stayed from underneath, divided up the load of the main prop up cables to diminish the cable cross-sections.

The transfer of load of the stress cables are usually done by casting component connected to cables through fixing bolts, the cable fixing is a said to be secured by self-locking nuts or by making use of pressure clamps.

The advantages of cable structures used in suspending and tensioning load bearing structure

Suspended structures encompass the rationale of reducing the span of supporting beams or eliminating cantilevered structures. Tensioned structures similarly, diminish the span of beams and, hence also the section modulus which has to be well thought-out in determining their cross-section, they also need a mask or pylon to supports them, in trussed structures, require an area support.

The suspension or support of load bearing structures make available a means of dropping the cross sections, of the structural members, thereby make possible delicate and network designs to be developed. As a regulation, this is only achievable in steel and timber frame structures. The cables prop up tensile forces only.

In engineering cables are, either an assemblage of three or more ropes twisted together for extra strength, the oldest of engineering structure, suspension bridges were constructed by ancient peoples by means of vines for cables as well as mounting the roadway in a straight line on the cables. A a great deal of  stronger type was pioneered in India about the 4th century AD that make use of cables of plaited bamboo and afterwards of iron chain, by the roadway suspended. The technique of cable spinning for suspension bridges was invented by the French engineer Louis Vicat, Vicat's technique engaged a travelling wheel en route for carrying the uninterrupted cable strand commencing from the anchorage on one side up above the tower, down on a fixed sag (catenery) to the midpoint of the bridge, up and over the tower on the farther side to the farther anchorage, where a team received the wheel, anchored the strand, and returned the wheel, laying a fresh strand. From these successive parallel strands a cable was built up. A cable-braced bridge was build up by German engineers at Cologne, Düsseldorf, and somewhere else in the 1950s and '60s; in this variety a lone tower at the midpoint chains the roadway by way of a number of cables. an additional development of the 1960s, aimed at reducing time of construction, was cable made-up in the shop.

Disadvantages of cable structures

The members requires some degree of freedom, which denote that it requires independent restraint necessary to maintain it geometric stability, this freedom tends to make the structure to experience sway when there is an external force acting on it

The cable structures system requires anticlastic cable system to maintain it geometry of catenery in other to remain in equilibrium, in that state which it will sufficiently transfer the tensile stresses along cables to the columns or compression reducing members

The cable net structure can only be formed from steel and wood frame network

It has to allow compression along it curvature, which in long run, it sags the cable, the use of anticlastic cables those not prevent sags over a long period of time.