AMSYSCO Post Tension Blog » elongation

Missed Stressing Operation

nkhosa — Sun, 18 Dec 2011 17:00:51 +0000

Q: What happens if you forgot to stress an unbonded post-tensioning tendon at the construction joint and have already poured the subsequent pour?

A: The most preferred option is to stress the tendon in the subsequent pour. However, the following items should be factored in:

a) The 7% Elongation Range must be revised.

Since you have a longer pull, the elongation calculation needs to be revised also. This should be noted in the elongation records from the PTI-Certified Inspector. A thumb-rule is to just add Min1+Min2 and Max1+Max2 together. For example,

First pull of 50′ would be Min1 = 3.72″ and Max1 = 4.28″
Second pull of 100′ would be Min1 = 7.44″ and Max1 = 8.56″
New elongation range for 150′ pull would be Min1 = 3.72″ + 7.44″ = 11.16″ and Max1 = 12.84″.
The new range is an close approximation. A more accurate range can be calculated with PT software. This is especially important for very long pulls.

b) The force in the tendon will likely be less.

Since the stressed tendon length is now longer, your angular and wobble friction have likely increased somewhat due to the increased number of spans. In other words, the final average force will drop in both pours for that particular tendon.

Previously, the 50′ pull may provide 27.5 kips of force in Pour #1.
Previously, the 100′ pull may provide 27.3 kips of force in Pour #2.
Now, the 150′ pull may provide only 26.9 kips of force in Pours #1 and #2.

c) The intermediate anchor has been abandoned.

The force within the 150′ tendon will be transmitted uniformly through the construction joint. Conversely, the 50′ tendon transmits force only within Pour #1 and the 100′ tendon transmits force only within Pour #2 (i.e. the construction joint “locks-off” the force within each pour). This is important should be there be any repair/renovation conducted in the future.

- Neel Khosa, Vice President, AMSYSCO

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Video : Applying paint mark for Tendon Elongation

admin — Thu, 16 Jun 2011 23:17:34 +0000

This short clip shows how to apply paint-marks prior to stressing of post-tension tendons. It’s recommended to use a straight-edge, metal to help with elongation measurements. Refer to the Post-Tensioning Institute’s “Field Procedures Manual for Unbonded Single-Strand Tendons” Chapter 7 for additional information.

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How do I know if my Post-Tensioning Tendons have been stressed?

nkhosa — Thu, 16 Jun 2011 22:32:20 +0000

As a post-tensioning inspector or installer, have you ever been in a situation whether you are unsure whether some PT tendons have been stressed? By not having a PT tendon stressed, this could have detrimental effects on the structure.

Here are three pointers that may help you out:

1. PAINT-MARKS : Compare the location of the paint-marks to other tendons of the same length. If you see a major difference in one or more paint-marks, then the flag should go up. If the location of the paint marks are similar, then either ALL of tendon have been stressed or NONE of them have been stressed. If you have a deviation, then you may have an unstressed tendons or an elongation issue. (Related Video : Applying paint mark for post-tensioning tendon).

2. BITE-MARKS : Check whether there are “bite-marks” on the tendons. During the stressing operation, the hydraulic jack “grabs” the stressing tail of the PT strand and and creates a series of small bite-marks. The picture below shows two tendons that have been stressed (click picture to enlarge). This visual inspection helps if your paint-marks have been washed away.

Bitemarks into Post-Tensioning Strand

3. ELONGATION RECORDING : AMSYSCO highly recommends that the inspector (or testing agency) record elongation during the stressing operations…rather than several hours later. This step increases the quality control of the project and eliminates a potential unstressed tendon.

- Neel Khosa, Vice President, AMSYSCO

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Post Tension Friction-Loss Calculations

nkhosa — Thu, 10 Mar 2011 22:30:06 +0000

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

Video on Field Friction Test

Material Properties of Post-Tension Strand

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Material Properties of Post-Tension Strands

rkhosa — Sat, 30 Jan 2010 04:05:16 +0000

The following is a list of basic formulas for 270 ksi, 7-wire Prestressing steel strand (per ASTM-A416) used in Post-Tensioned concrete.

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Assume 0.5″ diameter strand has cross-sectional area of 0.153 sq.in. and weight of 0.525 lbs/ft.

Assume 0.6″ diameter strand has cross-sectional area of 0.217 sq.in. and weight of 0.740 lbs/ft.

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Minimum Ultimate Tensile Strength (MUTS) = (Grade of Steel) x (Cross-Sectional Area)

0.5″ inch diameter = (270 ksi) x (0.153 sq.in.) = 41.3 kips

0.6″ inch diameter = (270 ksi) x (0.217 sq.in.) = 58.6 kips

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Minimum Yield Strength = 90% of MUTS = MUTS x 0.90 (per ASTM-A416)

0.5″ inch diameter = (41.3 kips) x (0.90) = 37.2 kips

0.6″ inch diameter = (58.6 kips) x (0.90) = 52.7 kips

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Jacking Force = 80% of MUTS = MUTS x 0.80 (per ACI Code)

0.5″ inch diameter = (41.3 kips) x (0.80) = 33.0 kips

0.6″ inch diameter = (58.6 kips) x (0.80) = 46.9 kips

“Jacking Force” is the force that tendons are stressed to.

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Allowable Initial Force = (Jacking Force) minus (Short-Term Losses) = 70% of MUTS = MUTS x 0.70 (per ACI-318)

Short-Term Losses include:

Angular Profile of Tendon
Horizontal sweeps in Tendon
Wedge-Seating (typically 0.25 inch)
Wobble due to installation (CLICK HERE to view the video on how to calculate Angular and Wobble Coefficients in unbonded post-tensioning tendons.)

0.5″ inch diameter = (41.3 kips) x (0.70) = 28.9 kips

0.6″ inch diameter = (58.6 kips) x (0.70) = 41.0 kips

“Initial Force” is the force at the anchorage after the wedges are seated and stressing jack is removed. The calculated values above are approximate since the actual short-term losses may differ from the theoretical values.

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Final Force = (Initial Force) minus (Long-Term Losses)

Long-Term Losses include:

Creep of concrete (permanent deflection due application of constant load)
Elastic Shortening of concrete
Relaxation of steel prestressing strand
Shrinkage of concrete during curing

0.5″ inch diameter = approx 26.9 kips

0.6″ inch diameter = approx. 38.1 kips

“Final Force” is the force at the anchorage after the long-term losses are accounted for. The calculated values above are approximate since the actual long-term losses may differ from the theoretical values.

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Average Tendon Elongation (approx.) = (P x L) / (A x E)

P = Prestress jacking force (70% of MUTS)

L = Length of steel (inches)

A = Cross-Sectional Area of steel (sq.in.) on mill certificates

E = Modulus of Elasticity of steel (ksi) on mill certificates

For example, using a 100-foot tendon (L = 100 x 12 inches) with Modulus of Elasticity of 28,500 ksi.

0.5″ inch diameter = (28.9 kips x 1,200 inches) / (0.153 sq.in. x 28,500 ksi) = 7.95 inches

0.6″ inch diameter = (41.0 kips x 1,200 inches) / (0.217 sq.in. x 28,500 ksi) = 7.95 inches

***Notice that the 0.5″ and 0.6″ have the same Avg. Elongation***

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Post-Tensioning Institute recommends an allowable elongation range of plus/minus 7% of the Average Tendon Elongation for unbonded post-tensioning tendons.

Min. Allowable Elongation = 93% of Avg. Elongation = 0.93 x (Avg.El.)

Max. Allowable Elongation = 107% of Avg. Elongation = 1.07 x (Avg.El.)

If we use the same 100-foot tendon with average elongation of 7.95 inches, then Min.El. = 0.93 x 7.95 inches = 7.40 inches and Max.El. = 1.07 x 7.95 inches = 8.51 inches.

- Rattan Khosa, President, AMSYSCO

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Download: Stressing Elongation Form

admin — Wed, 11 Nov 2009 21:01:53 +0000

Please download AMSYSCO’s ‘Master Stressing Record’ (Click here to download). The form is used for recording elongations for Unbonded Post Tensioning.

This form is for Inspectors and Installers certified by the Post Tensioning Institute. Check out our VIDEO for the basics in stressing post tensioning tendons.

*** Updated (10/21/2011) – In response to industry demand, we have updated our elongation report to include. The updates include columns to track whether the tendon tail has been cut properly, the encapsulated anchor has been capped (if applicable) and the stressing pocket has been grouted.