Sag and Tension Calculations For Conductor/Earth Wire For River Crossing

Sag and tension calculations for conductor earth wire are done for the river crossing by following steps :

  1. Determination of Equivalent Span :
    Based on anchor spans L1 & L3 and crossing span L2, the equivalent span for river crossing portion is determined by the following formula :
    Eq. Span =
    L13 + L23 + L33 / L1   + L2   + L3 (Refer Figure 10-II)

  2. Sag and tension calculation for conductor/earth wire for above equivalent span is done from the following formula :
    T2 (T - K + aEa (q2-q1)) = W2 L2 aE / 24 x q2
    T = Tension at temperature q2 (kg)
    K= Constant
    a = Area of conductor/Earth wire (mm2)
    E = Modulus of Elasticity kg/mm2
    a = Linear Coeft. of expansion (per degree celcius)
    W = weight of conductor (kg/m)
    L = Equivalent span (m)
    q = Wind load factor = P2 + W2 / W2 = 1 (At no wind condition)
    q1 = Initial condition temperature

  3. Calculation for conductor

  1. Initial condition for conductor is taken as 32C and No wind and T0 tension under these conditions is taken as 22% of ultimate tensile strength of conductor.

  2. From the above value of T0, we calculate the constant 'K which is fixed for all further sag-tension calculations.

  3. Considering the calculated value of K, the tension at 0C under No-wind & full wind and 32C full wind and 75C No-wind is determined from the above formula by hit & trial method.

  4. Sag at various tension :
    = WLA2 / 8T  where LA is actual span

  1. Calculation for Earth wire

  1. The earth wire sag at 0C and no-wind should be 90% of the Conductor sag at 0C and No-wind. Value of tension at 0 and no wind is determined by the following formula
    T = {WL2 / 0.9 x Sag of Conductor at 00N/W}
    for equivalent span
    where L = Equivalent span in meters.

  2. As in the case of conductor, the tensions at 0C (No-wind & full wind Condition), 32"C full wind conditions and 75C no-wind condition are determined.

  3. Sag = [WLA2 / 8TA] where LA = Actual span.

Fig. 10-II