CABLE STRUCTURES. SUBMITTED TO: AR.KARAMJIT S. CABLE SYSTEMS. MAJOR SYSTEM FORM ACTIVE STRUCTURE SYSTEMS.
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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.
OPTIMAL SAG :
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.
GEOMETRIC FUNICULAR FORMS :
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.
FUNICULAR POLYGONS :
#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.
SPECIAL DESIGN CONSIDERATIONS:(And Corrective Measures)
DYNAMIC EFFECTS OF WIND ON TYPICAL FLEXIBLE ROOF STRUCTURE :
PREVENTIVE MEASURES :
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).
LIMITATIONS DUE TO VIBRATIONS & CHANGING LOADS :
STIFFENING TRUSSES :
A cable truss system has a triangulated structural form which increases stiffness, particularly under non-symmetric loading.
Double-layer prestressed cable-truss system
DESIGN OF SUPPORTING ELEMENTS :
APPLICATIONS OF CABLE SYSTEMS :
Yale University-skating rink