Draw the shape of the Pressure line.

Solution

The locations of resultant force at sections A-A, B-B  and C-C have already been determined. Joining these locations, we can draw pressure line curves for the case (a) and (b), as shown in Figure 5.9. In Figure the distances of pressure line ordinates have been shown from tendon line whereas in Example the locations of  resultant force at various sections in both cases were calculated from the top fiber.

We have seen in the previous example that the pressure line drifted towards the higher side of  the beam at a sectioc where the bending moment due to external loads was greater in magnitude. In a similar manner, it can be shown that a larger resultant load would be represented if  the pressure line moved up even higher. Keeping the same reasoning, for a uniform load applied upward rather than downward, the result would be a downward movement of the pressure line. Therefore, a change in  the external moments  in the elastic range of a pre-stressed beam results in a movement of the pressure line in the beam.