Usually the overhead line conductors exert bending and torsioning forces on the poles.
The part of the pole will be stretched while another part will be compressed.
Since the concrete does not withstand stretch efforts the steel reinforcing bar will take this effort .In order to facilitate this the vertical reinforcing bars are pre-tensioned maintaining the concrete compressed. By stretching further the compression force acting on concrete will decrease but remain still compressed. The spun wire–a wounded steel rod pre-tensioned-will reduced the stretch forces in the torsioning phenomenon.
The spun pre-stressed concrete pole may withstand 20-30% more effort.
