Post-Tensioning in Slabs

Overview

This lecture covers automating post-tensioning in slabs using SAFE software, focusing on balancing 60–80% of self-weight in a residential-style concrete floor slab.

Model Setup and Materials

• The slab is 8 inches thick, with 12-inch square columns and 8-inch walls, all made of 5000 psi concrete.
• Pre-stressing tendons are 0.5-inch diameter strands (0.153 in² area), using 416 grade 270 steel.
• The jacking stress applied is 216 ksi, with total losses estimated at 40.5 ksi.
• The floor plan measures approximately 108 ft by 92 ft, with 8 X-grids and 6 Y-grids spaced 8–28 ft apart.

Tendon Layout and Design

• Banded tendons run in the X direction, one tendon per band; the program reports the needed number of strands per band.
• Distributed tendons in the Y direction are spaced at 38 inches on center.
• The SAFE program auto-generates tendon vertical profiles to optimize self-weight balancing.
• Tendons in X (A layer) use a parabola profile; those in Y (B layer) use a reverse parabola.

Load Patterns and Cases

• Default load patterns: Dead (self-weight multiplier = 1) and Live (no self-weight multiplier).
• Additional patterns: PT final (post-tensioning after losses) and PT transfer (before long-term losses).
• Load cases are defined automatically and all are analyzed as linear static cases.

Support Lines and Design Strips

• Support lines define slab span load paths and are aligned with columns/walls using grid lines.
• In the X direction, support lines use layer A; in the Y direction, layer B.
• Design strips inherit layers from support lines and are auto-widened for analysis.
• Only column strips are used for post-tensioned slabs; middle strips are not added.

Tendon Editing and Review

• Tendons can be viewed and edited via selection; the program displays their vertical profiles and allows adjustment.
• Anchorage and stressing ends are indicated by different icons.
• For banded tendons, the number of required strands is shown; for distributed tendons, the strand count and per-foot force are calculated.
• Tendon stresses and losses can be reviewed: stressing loss is 27 ksi, long-term loss is 13.5 ksi.

Applying Additional Loads and Analysis

• Uniform gravity dead load (35 psf) and live load (50 psf) are applied to the slab.
• Analysis is performed, and results include slab deformations and stresses for Dead, Live, PT final, PT transfer, and hyperstatic cases.
• Design results include reinforcement envelopes, shear, flexural, and stress checks with options for transfer, normal, and long-term conditions.

Key Terms & Definitions

• Post-tensioning — Applying force to pre-stressing tendons after concrete has set to balance part of the slab’s self-weight.
• Tendon — Steel cable or strand used to apply post-tensioning force within concrete.
• Jacking Stress — The initial stress applied to a tendon during post-tensioning.
• Band Tendons — Group of tendons closely spaced, typically in one direction, used to balance loads.
• Distributed Tendons — Tendons spaced evenly across a slab, typically in the perpendicular direction to banded tendons.
• Design Strip — Analytical tool defining slab regions for post-tensioning and reinforcement layout.
• Support Line — Line indicating load path through slab spans, guiding tendon or reinforcement placement.

Action Items / Next Steps

• Review and practice defining slab geometry, material properties, tendons, and load patterns in SAFE.
• Analyze a sample slab for post-tensioning design using the described steps.
• Optional: Explore various stress and reinforcement check outputs in SAFE.