The friction-loss calculations are important submittals for post-tensioning tendons. The calculations determine what the tendon’s final force is after short-term and long-term friction losses.
The key input parameters for “Long-Term Losses” are as follows:
- Post-Tensioning System (Unbonded or Bonded)
- Type of Strand (usually Low-Lax)
- Ultimate Strength of Strand (usually 270 ksi)
- Modulus of Elasticity of Strand (usually 28,500 ksi)
- Estimate of initial average compression (depends on concrete member)
- Concrete Strength at 28 days (varies, but typically 5000 psi)
- Average Weight of Concrete (Normal or Light-weight)
- Estimate Age of Concrete at Stressing (usually 3 days)
- Modulus of Elasticity of Concrete at Stressing (ex. 57 x sqrt of specified 3-day compressive strength of concrete of 3000 psi = 3122 ksi)
- Modulus of Elasticity of Concrete at 28 days (ex. 57 x sqrt of specified 28-day compressive strength of concrete of 5000 psi = 4030 ksi)
- Estimate of Average Relative Humidity (varies by geographical region)
- Volume to Surface Ratio of member (depends on concrete member)
The key input parameters for “Friction & Elongation” are as follows:
- Coefficient of angular friction (varies by PT supplier)
- Coefficient of wobble friction (varies by PT supplier)
- Ultimate Strength of Strand (usually 270 ksi)
- Ratio of Jacking Stress to Strand’s Ultimate Strength (0.80 per ACI-318)
- Anchor Set (usually 0.25?)
- Cross-Sectional Area of Strand (varies, but usually 0.153 sq.in. for 0.5? diameter strand)
- Total Number of Strand per Tendon (1 for unbonded, varies for bonded)
- Stressing Configuration (Single-End or Double-End)
- Length and Drape of Tendon per Span (depends on design of concrete member)
The output will include the following:
- Long-Term Losses
- Elastic Shortening
- Shrinkage
- Creep
- Relaxation
- Final Average Force in Tendon
- Total Elongation
- Critical Stress Ratios
ACI-318 and PTI Manuals go into further detail about analyzing the results of the friction-loss calculations.
– Neel Khosa, Vice President, AMSYSCO
Related Articles:
Material Properties of Post-Tension Strand
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