When forming a pre-tensioned concrete member, steel cables are stretched to what percentage of their ultimate strength?

When forming a pre-tensioned concrete member, steel cables are typically stretched to approximately 70-80% of their ultimate strength. This process is critical in pre-tensioning as it allows the steel tendons to develop compressive forces that counteract tensile stresses in the concrete once it hardens. By stretching the cables to this percentage, it ensures that the member can handle the loads it will experience during its service life and enhances its overall performance.

This range effectively balances the need for the cables to provide adequate tension without risking failure, as going significantly higher may lead to increased brittleness and reduced ductility in the steel. In this context, while other percentage ranges exist, they are either too low to offer sufficient strength or too high, compromising the integrity of the steel. Thus, stretching the cables to 70-80% of their ultimate strength is an industry-standard practice that aligns with best engineering practices to ensure safety and efficacy in concrete design.