Final answer:
The statement regarding the need for concrete pads to resist downward thrust in prestressed concrete pressure pipelines is true. These pads help distribute forces into the surrounding soil, analogous to how an arch must be supported to resist thrust.
Step-by-step explanation:
The statement that downward thrust at vertical to flexions on prestressed concrete pressure pipelines should be resisted by earth bearing by use of a concrete pad is True. Downward thrust is a force that occurs due to the weight of the structure and the pressure of the fluid within the pipeline. To ensure the stability and integrity of the structure, this force must be resisted effectively. A concrete thrust block or pad redistributes the pipeline forces into the surrounding soil. Just as an arch transfers loads into compressive stresses and requires strong support to resist the outward thrust, a pressure pipeline also needs proper support to withstand the downward and outward forces.
Contrasting with the normal reaction, which is a force opposing gravity and acting upwards, the thrust in a pipeline caused by internal pressure acts downward and must be counteracted, demonstrating how different forces interact within structural systems.
A similar concept can be seen when examining the force exerted by a boy pushing a box up a ramp. The force he must apply is not simply determined by the height and length of the ramp but also by other factors such as friction and the angle of the ramp.