CABLE STRUCTURES. SUBMITTED TO: AR.KARAMJIT S. CABLE SYSTEMS. MAJOR SYSTEM FORM ACTIVE STRUCTURE SYSTEMS.
MAJOR SYSTEM FORM ACTIVE STRUCTURE SYSTEMS.
Non rigid, flexible matter shaped in a certain way and secured by fixed ends, an support itself & span space. The transmit loads only through simple normal stresses; either tension or through compression.
Two cables with different points of suspension tied together form a suspension system. A cable subject to external loads will deform in a way depending upon the magnitude and location of the external forces. The form acquired by the cable is called the FUNICULAR SHAPE of the cable.
# Form Active Structure Systems redirect external forces by simple normal stresses : the arch by compression, the suspension cable by tension. The bearing mechanism of form active systems vests essentially on the material form.
# The natural stress line of the form active tension system in the funicular tension line.
# Any change of loading or support conditions changes the form of the funicular curve.
Form active systems because of their dependence on loading conditions are strictly governed by the natural ‘flow of forces’ and hence cannot become subject to arbitrary free form design.
# The high tensile strength of steel, combined with the efficiency of simple tension, makes a steel cable the ideal structural element to span large distances.
# Cables are flexible because o their large shall lateral dimensions in relation to their lengths. As uneven stresses true to bending are prevented by flexibility the tensile load is evenly divided among the cable strands.
In order to understand the mechanism by means of which a cable supports vertical loads, one may first consider a cable suspended between two fixed points, located at the same level and carrying a single load at mid span. Under the action of the load the cable assumes a symmetrical triangular shape and half the load is carried to each support by simple tension along he two halves of the cable.
A large sag increases the cable length, but reduces the tensile force & allows a reduction of cross-section. A similar sag requires a larger cross-section. Hence the total volume of cable (product of cross-section & length), must be minimum for some optimal value of sag
Optimal sag equal half the span for a given horizontal distance & corresponds to a symmetrical 45o – triangle cable configuration with thrust = p/2.
If the load is shifted from midspan position, the cable changes shape.
# If two equal loads are set on the cable in symmetrical positions the cable adapts itself by acquiring a new configuration with three straight video.
# As the number of loads increases, the funicular polygon approaches a geometrical curve – the PARABOLA large number of loads are evenly spaced horizontally.
If the equal loads are distributed evenly along the length of the cable, rather than horizontally, the funicular curve differs from a parabola, through it has the same general configuration. It is a catenary.
A cable carrying its own weight ad a loads evenly distributed horizontally, acquires a shape that is intermediate between a parabola & catenary. This is the shape of cables in the central span of suspension bridges.
The principal methods of providing stability are the following:
(i) Additional permanent load supported on, or suspended from, the roof, sufficient to neutralize the effects of asymmetrical variable actions or uplift Figure 14a).
This arrangement has the drawback that it eliminates the lightweight nature of the structure, adding significant cost to the entire structure.
(ii) Rigid members acting as beams, where permanent load may not be adequate to counteract uplift forces completely, but where there is sufficient flexural rigidity to deal with the net uplift forces, whilst availing of cables to help resist effects of gravity loading (Figure 14b).
A cable truss system has a triangulated structural form which increases stiffness, particularly under non-symmetric loading.
Today the longest suspension bridge has a span of 1410 m. (4226 ft.); the longest suspension roof; the Burgo Paper Mill in Mantcia has a span of 163 m. (535 ft.). The roof was designed like a suspension bridge. The cable flexibility is not wholly advantageous as in bridge. Excessive vibrations can not be tolerated in a building. Water proofing of the roof is difficult. Most suspension roofs are therefore prestressed to reduce their flexibility & some also have concrete roofs.
The first modern roof was an Arena. Load bearing cables are suspended from two intersecting arches, anchored against one another. At night angles to the load bearing are secondary cables prestressed to ensure tautness even on a hot day. Corrugated sheets supported on the cable network.
Yale University-skating rink
Structures using suspended cables have a functional advantage for arenas, because the shape is better suited to an array of banked seats than that of a dome. A suspension roof requires a smaller volume of air than a dome. This can produce imp. economics in air-conditioning & heating.
Roof over sports arena, Munich by Fvei Offo. Approximate span of the structure is 130 m. (430 ft.). The tentlike simplicity of this prestressed cable structure is deceptive. The roof-over the entire sports arena cost about $48 million. The design required a great deal of theory as well as model analysis.
Memorial Auditorium in Litica, New York. Span – 73 m (240 ft.). Two sets of cables, are separated by struts that cause them to act in conjunction. The amount of prestress for upper and lower cables varied. Vibrations in one set of cables are different or out of phase with the other and the opposing forces damp the vibration of the structure.
A double layer of cables covered with pre-cast concrete slabs. These were loaded temporarily with a large weight of building. Materials to prestress the cables, and the joints between concerete slabs were then filled with cement mortar to auction the prestress. Rainwater was pumped off the roof.
Cable-stiffened cantilever roof. The structure is several 100% stronger than the cantilever on its own. The cable provides the tensile component of the resistant moment, so that the cantilever becomes the compression member, and the distance between the cantilever & cable of the support provides the lever arm of the resistance moment.
Because of their identity with the natural flow of forces, the form active structure system is a suitable mechanism for achieving long spans and forming large spaces.