Why do wires from pylons sag instead of being tight?

Question

i really need to know within the next hour for a project. Please help.

Answer 1

The more tension that is put on the guy wires attached to a tower or pylon, the higher the compressive buckling force is on the tower. Since the guy wires are only used to reduce the bending moment and compressive buckling stress in the tower or pylon due to wind and seismic loads, adding an additional large compressive load in the structure due to the guy wire tension would defeat the purpose of the guy wires.
So, guy wires are tensioned as little as possible, so that they will provide a degree of stability, and yet not overload the structure. When used to stabilize a tower, this usually results in a lot of sag in some cases.
Guy wire system designs are tricky.

Answer 2

Gravity.

The wire has its own weight to support, and this pulls it down. The very middle of the wire has the most weight to support and it sags the farthest. Pulling the wire horizontally exerts the least amount of upward force because the relationship depends on the cosine of the angle of the force applied. In this case it is 90 degrees and the cosine is zero at this point. If a wagon is pulled up a hill, the rope attached to the wagon does not sag because the hill is at an angle and so is the force. Gravity always points straight down, so the angle is less than 90 degrees here. If gravity is negated however, a very small force will eliminate all sag, even if the line is strung horizontally. If an underwater cable is lighter than water, it will sag upwards. Finally, the downward force creates an arch, because this is the most efficient weight bearing structure. Inverted arches have been used in archetecture for thousands of years.

Answer 3

The tension required to make them taut would be quite high to be able to take that the pylons and cables would have to be much heavier. It may be impossible due to law of diminishing returns. In addition taut wires may not behave well in windy conditions.

Answer 4

since metals expand when heated & contract when cooled, sagging gives them room to contract in cold weather.If they were to be taut they would snap when cooled i.e in cold weather.