Final answer:
In pre-tensioned concrete members, the steel cables are stretched to 70% to 80% of their ultimate strength to ensure structural integrity while leaving room for material expansion and contraction, reducing thermal stress.
Step-by-step explanation:
When forming a pre-tensioned concrete member, steel cables are stretched to between 70% to 80% of their ultimate strength and secured between end anchorage points through a casting bed. This pre-tensioning process introduces compressive stress into the concrete, which counteracts tensile forces that the member will encounter in service. The coefficient of linear expansion for steel is almost identical to that of concrete, allowing them to expand and contract at similar rates under temperature changes, thereby reducing thermal stress.
The ultimate strength or breaking stress of a material is the value of stress at which it will fracture. For steel, the ultimate stress can be as high as 20.0 × 108 Pa. Pre-stressing the steel to 70% to 80% of this value ensures that the steel cables provide enough compressive force without reaching the point of failure.
One must consider the thermal stress in engineering designs where no gaps for expansion can be left. In cases where materials must endure both compression and tensile stress, understanding the materials' breaking stress and properties like elongation under load (AL) is critical for ensuring structural integrity and safety.