Nelson Bustamante: Circular Prestressing – The Constructor

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Circular prestressing refers to the technique employed in reinforcing circular structures such as pipes and tanks by winding prestressing wires in circular patterns. This method stands in contrast to linear prestressing, which encompasses all other forms of prestressing where cables are straight or curved but not wound in circles around a circular structure. In the majority of prestressed circular structures, both circumferential and longitudinal prestress are applied. Circumferential prestress takes a circular form, while longitudinal prestress is essentially linear.

The purpose of circumferential prestressing is to counteract the hoop tension generated by internal pressure within the structure. This prestressing is achieved by placing wires or tendons spirally or in sectors around the circumference of the member, positioned externally to the concrete core. Consequently, the center of the prestressing steel (CGS) lies outside the core concrete section.

When prestressed members are curved in the direction of prestressing, it is termed circular prestressing. Examples include circumferential prestressing in pipes, tanks, silos, containment structures, domes, and shells. In such structures, longitudinal prestressing, parallel to the axis, may also be incorporated.

The uniformity of hoop compression across the thickness of a thin shell leads to the positioning of the pressure line (or Cline) at the center of the core concrete section (CGC). Internal forces under service conditions are analyzed for a unit-length slice along the longitudinal direction.

Circular prestressing finds particular applicability in liquid-retaining structures such as circular pipes, tanks, and pressure vessels. The circumferential hoop compression induced by prestressing serves to counterbalance the hoop tension arising from internal fluid pressure. In reinforced concrete pressure pipes, a significant amount of reinforcement is typically required to mitigate low-tensile stresses and prevent cracking. However, circular prestressing effectively eliminates cracks and enables a more economical use of materials. Moreover, it serves as a preventative measure against shrinkage cracks in liquid-retaining structures.

To mitigate prestress loss due to friction, prestressing over sectors of the circumference can be implemented. Buttresses are employed for tendon anchorage, strategically positioned along the circumference to enhance structural integrity.

CIRCULAR PRESTRESSING
CIRCULAR PRESTRESSING

Overall, circular prestressing offers a robust solution for reinforcing circular structures, providing enhanced resistance to internal pressure, crack prevention, and efficient material utilization. The technique, with its various applications and advantages, remains a cornerstone in structural engineering for liquid-retaining structures and beyond.

buttress - circular prestressing

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