What is the external load acting on a pre-stressed beam

In the elastic range, a variation in the external load acting on a pre-stressed beam results in a change in the location of the pressure line. If the load is increased, the shift in the location of the pressure line continues at a relatively uniform rate upto the point where cracks develop in the tension fiber. The rate of movement in the pressure line decreases (even though the external load is increased) when the cracking load is exceeded. This change in the action of the internal moment continues until the movement of the pressure line ceases.