Meaning
The geometric path of post-tensioning steel cables that are not grouted into the surrounding duct determines the structural response of a prestressed concrete member. The unbonded tendon profile defines the vertical and horizontal layout of these cables, which determines the load-balancing forces exerted on the concrete. This geometry is chosen to counteract the bending moments caused by dead and live loads.
Flexural Behavior
Continuous members require variable eccentricity to match the changing moment diagrams under load. The unbonded tendon profile is draped to sit near the bottom of the slab in span regions and near the top over columns. This draping generates upward forces that reduce both deflections and bending stresses.
Stress Distribution
Tendon force remains nearly uniform along the entire length of the cable due to the absence of grout. Friction during stressing still occurs, meaning the unbonded tendon profile must be designed to minimize sharp curves that cause force losses. Engineers must calculate the curvature and wobble coefficients to ensure that the required prestressing force is delivered to the critical sections.
When the profile is too complex, friction losses prevent the cable from reaching its design tension. This can lead to under-prestressed areas that are vulnerable to early cracking.
Installation Layout
Securing the tendon ducting to support chairs at precise heights ensures correct placement during concrete pouring. Deviations in the unbonded tendon profile from the design coordinates can significantly alter the load-carrying capacity of the slab. Frequent inspections during construction verify that the cables are locked in place before concrete placement begins.
