Neel Khosa and Rattan Khosa of AMSYSCO, Inc. recently contributed to an article about the use of Post-Tensioning in the Minnesota Twins Target Field. Joe Ales, Jr. from Walter P. Moore and Troy Blizzard from Mortenson Construction also contributed quotations about the design and construction of Target Field.
The abstract was published in the April 2011 issue of ‘Structural Engineering & Design’ magazine and the full article was able as a web-exclusive on GoStructural’s website.
PC Strand is a high-strength steel cable used in Post-Tensioned concrete. This video shows the PC Strand prior to the plastic extrusion process (unbonded PT).
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.
Our blog will announce AMSYSCO news and provide commentary about the Post-Tensioned Concrete and Commercial Construction industries. Our audience includes Architects, Structural Engineers, General Contractors, Concrete Contractors and Ironworkers. Please visit our main website for our Post-Tensioning reinforcement products and services.
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