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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