Missing Moment of the Wire
A thin wire is placed between two points, represented by points A and B. The wire is light in weight, so the support forces at A and B are small.
A = B = G / 2
 In order for a light wire not to sag, it would have to be tensioned with an infinitely large force relative to its length. Consequently, the tensile strength of the wire would need to be infinite to withstand the force induced by such tension. This situation does not occur in reality. However, an analogous case arises in overhead power lines, where the conductors inevitably sag. Thin wires develop tensile stress but no bending moment. Due to the absence of flexural stiffness, the design of the wire is therefore based on its cross-sectional area.
In a cross-section with low bending resistance, tensile stress essentially develops. A cable or electrical conductor has poor load-bearing capacity, let alone compressive capacity, but possesses high tensile strength. Tension generates tensile stress, which makes the design of the wire straightforward. In members with stiffness, both tensile and compressive stresses arise, being of opposite sign on opposite sides of the cross-section. At the center lies the neutral axis, where the magnitude of stress is zero.
 When a luminaire is suspended eccentrically above a roadway, the loading case illustrated in the figure arises. The design software simplifies the determination of cable forces. Subsequently, the safety against dynamic (pulsating) loads on the cable is assessed.
An outdoor luminaire accumulates snow, and wind induces oscillations of the assembly, among other effects. These factors must be considered particularly at the end connections of the cables to the supporting structures. Furthermore, the number of cable shackles at the attachment points must be accounted for. At least two cable clamps are required, and the influence of corrosion on the cable must also be considered. Even a seemingly simple design involves multiple considerations to ensure that the designer can sleep peacefully at night.
From a computational perspective, human fatigue, the deflection of a steel beam, and many other phenomena are analogous. When a greater load is applied to a beam, its deflection increases. Similarly, when a heavier burden is placed on a person, the time required for performance increases. Time is equivalent to fatigue in this analogy, since fatigue extends the duration. The level of an athlete’s fatigue can, to some extent, be inferred from the time expended. Conversely, if the length of the beam increases while the load remains constant, the deflection also increases. In the human case, when the distance grows, the time required for performance likewise increases.
In time comparisons, the matter is essentially one-dimensional. Interestingly, in road running, time is generated through all dimensions (up–down–sideways–time [dilation]). The motion of a cable is a reciprocating progression, with pulleys transmitting the movement across different dimensions. A steel bar, being rigid and fixed, does not possess the material properties of a wire. The wire itself and its sizing are not essential, but rather the reference to the absence of a torsional moment. An analogous situation arises in running, where the so‑called missing moment does not prevent the involvement of multiple dimensions. This is quantifiable, as both physics and physiology confirm.
9.5.2015*17:40 (1091 - 306)
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