post tensioning calculations 1

[boffintech]

Considering post-tensioning: specs call for tendons to be stressed within 7% of calculated elongation.

A calculated elongation of 2 3/4” results in a possible 7% variance of 0.1925”.

The rub: The Field Procedures Manual for Unbonded Single Strand Tendons requires that tendon elongations be measured to the nearest 1/8”. If during stressing a tendon is slightly more than 3/16” (0.1875) over the 2 3/4” it is recorded as 3”.

Since tendon measurement is rounded off to the nearest 1/8” a calculated elongation of 2 3/4” has a variance of 1/4” and a possible low of 2 1/2” and high of 3” results.

But is it really a 1/4”? If the actual elongation is measured at 3” this is 9.1% over the calculated elongation. ((2 3/4 - 3)/2 3/4) x 100 = 9.1%

The question: The calculated elongation is 2 3/4”. 7% of 2 3/4” is 0.1925. Does the 0.1925 round off to 1/8” or 1/4” to determine the low/high variance? The 7% variance of 0.1925 is closer to 1/4” than 1/8” but if 1/4” is used it technically results in a 9.1% tolerance.

Any ideas?

 

[henri2]

The tolerance will be plus or minus 1/4 inch if PTI provisions are followed.

As you have determined, 0.1925 in eigths of an inch = [0.1925 inch] x [1]=[0.1925 inch] x [8/8] = [1.54/8] inch. The numerator 1.54 is rounded up to 2 so we have [2/8] inch or 1/4 inch. Even though the upper limit is 9.1% above required elongation this is still consistent with PTI provisions which is based on rounding to nearest 1/8th of an inch.

If you have any questions regarding this matter you should clear it up with the design engineer. I am pretty sure the engineer will concur with the plus or minus 1/4 inch tolerance. A spreadsheet showing min/max acceptable elongations (based on nearest 1/8 inch) will provide same result. Most labs provide their inspectors with this sheet and or a construction master type V calculator.

This reminds me of reporting slump test measuremnts. Let's say the specs call for a slump of 4 +/- 1 inch which means the minimum acceptable slump is 3 inches and the maximum acceptable slump is 5 inches. If an actual slump measurement is 5-1/16 inch will this be rejected? No it will not because you have to report slump measurements to the nearest 1/4 inch. In this example, the slump measuement is recorded as 5 inches and is in compliance.

 

[boffintech]

So a tendon with a calculated elongation of 3 3/8" the correct range of 7% tolerance would be 3 1/8" to 3 5/8" even though technically both the low and high measurements would give you a mathematical 7.4% tolerance?

 

[henri2]

That's correct.

Consider purchasing a construction master V calculator. It can compute the tolerances and max/min acceptable elongation range in 1/8ths of an inch.

 

[rapt]

You have to be sensible gentlemen and use our engineering brains. You do not round the number to the near3est 1/4" if the total is in the order of 2 - 3" and the allowed inaccuracy is 7% which is less than the rounding amount, or even 1/8" in this case.

Or convert to metric and round to the nearest mm, not to the neqarest 6mm.

 

[henri2]

Boffintech, for sample computations on permissible elongation (with +/- 5% tolerance applied) check out Appendix H, of the PCI MNL 116--Manual for QC for Plants and Production of STRUCTURAL PRECAST CONCRETE PRODUCTS. This is one of the references you used for the ICC prestressed concrete exam. It seems that for example where elongation values are not too high, they use an accuracy of 1/16 in, while for higher values they use an accuracy of 1/8 in. Review elongation computation summaries on pages H.3, H.5, H.8, H.11, H.14, and H.16.

rant, in the example we were looking at, rounding to nearest 1/8 in (as opposed to nearest 1/4 in) for calculated elongations of unbonded single strand post-tensioned tendons.

The 1/4 in value was the tolerance for 2-3/4 in elongation when 7% tolerance was applied and reported to nearest 1/8 in.

The 1/8 in tolerance comes from Section 6.3.18 of The PTI Field Procedures Manual for Unbonded Single Strand Tendons which states " the measured elongation tolerance should be within +/- 7% (or as specified) of the values shown on the approved installation drawings. Elongation measurements should be to an accuracy of 1/8 in (3 mm)." So for the example Boffintech used, the structural engineer can specify a tighter accuracy like say 1/16 in, otherwise the PT installer and inspector will most likely go with the PTI provision.

 

[rapt]

That is what I was suggesting. For such a short tendon, 1/16 should be used.


And you do not round the variance as well as the measured value.

 

[henri2]

rapt, do you think it prudent for the structural engineer to have a table in typical details that shows acceptable elongation ranges (min to max acceptable elongations base on 7% tolerance)? This table can benefit the special inspector and PT installer when evaluating elongations shown on the post-tensioned concrete fabricator's installation drawings. With such a table, there will be no argument whether 3/16 in or 1/4 in is the applicable tolerance for a calculated elongation of 2-3/4 in.

In concluding, you write "And you do not round the variance as well as the measured value." While this will be unacceptable for meaningful scientific research and construction entailing strict adherance to tolerances, I do feel it is done a lot in the real world of post-tensioned concrete construction of building structures...and is something the design engineer should be aware of and consider taking preventive action at a pre-construction meeting.

 

[rapt]

Personally, I do not think the installer/stresser should have any document listing the expected extensions or the range of expected extensions on site. Human nature often leads to the recorded value being within the expected range when, in reality, it is not. They should never be placed on the installation drawings.

The structural engineer can have a table of acceptable range to make his life easier.

The engineer should be making his own calculations as to allowable extensions. In my ideal world he is the only one who would know the expected extensions also and would be given site extensions measured as they read, to the nearest 1/16 at the worst.

Unfortunately, that is not the way the industry has gone, and the standards have been set by the industry, not the designers who should be doing it.

 

[boffintech]

Thanks for the responses thus far. However, a new wrinkle has emerged for the example of 2 3/4" that I used. An engineer in the office told me that for any calculated elongation of 3" or less a 1/4" variance is used. He could not cite a reference for this and I can not find this in any of the PTI books that I have. Any ideas?

I'll look into a CM V. I have the CM IV. But I use a Casio programmable to figure % deviation and whatnot.

 

[henri2]

boffintech, if you already own a CM IV, that will work. You can "fraction set" the CM to 1/8ths, 1/16ths etc. The instructions booklet will explain how this is done.

To "fraction-set" it to 1/8ths of an inch, I believe you press the following keys(3 of them) in sequence: convert...8....= After you have done that, mutiply 2-3/4 inch by 7% and see what answer you get for tolerance.

 

[boffintech]

I tested the CM IV and it works as predicted. Thanks for the help on that. I wish I could find a programmable calculator that has fractions and inches like the CM IV.

 

[boffintech]

Let me see if I can type this so this makes sense. At my previous employer I used method A when field checking elongations. At my new company they report differently.

Past Method at Company A (my preferred method)

Member, Tendon Number, Calculated Elongation, Measured Elongation, Tolerance ± 7%, Low, High
1 Blue White Gold, 1, 6 1/4?, 6 3/4?, .44?, 5 3/4?, 6 3/4?

New Method at Company B (not preferred but required to perform)

Member, Tendon Number, Calculated Elongation, Measured Elongation, % Deviation
1 Blue White Gold, 1, 6 1/4?, 6 3/4?, 8.00

Interestingly enough PTI has sample reports that support Method B (page 54 of PTI Field Procedures Manual). However, method B seems to give erroneous results as shown in the example. A 6 1/4" tendon stressed to 6 3/4" in technically "in" tolerance but when % Deviation is shown as 8% it appears to be "out" of tolerance.

What to do?

 

[boffintech]

That should have read

Past Method at Company A (my preferred method)

Member, Tendon Number, Calculated Elongation, Measured Elongation, Tolerance ± 7%, Low, High
1 Blue White Gold, 1, 6 1/4", 6 3/4", .44", 5 3/4", 6 3/4"

New Method at Company B (not preferred but required to perform)

Member, Tendon Number, Calculated Elongation, Measured Elongation, % Deviation
1 Blue White Gold, 1, 6 1/4", 6 3/4", 8.00%

 

[SAVAGE54]

I did not take the time to read all of responses, but I have work as a PCI auditor and am also a design engineer. The problem you are talking about is typical of elongations that are very small.

IT IS IMPOSSIBLE to except the contractor to hold to such a tolerence. YOU must consider the manner and method the elongations will be take. By a non skilled person, with poor eyesight and you except him or her to read a tape measure to the nearest 1/8". They will fake it.

Also the live end and dead end seating can vary. More then a 1/8" I have seen as much as a 1/4" each end. The values used in the calculations are assumtions not real.

What will happen is the records are forged and the engineer never knows. What we typical did at PCI plants for low elongation values was to use the gauge pressure only and neglect the elongation as long as it was CLOSE. I mean +/- 1/4 to 3/8".

As a designer, you should not typically layout P/T strand were elongations are real small. If it is required to have short pulls, please consider the effect of major out of tolerences. The effects are only changes in deflections and cambers. AGAIN not a problem for short spans. There is VERY VERY little effect on the ulimate moment capacity.

Run the calculations.

 

[rapt]

Savage54,

Prestressing should not be done by a non-skilled person and/or a person with poor eyesight. How can he read the pressure gauge for a start? It is a specialist industry and should be done by trained specialists. Nearly every country in the world uses trained and experienced specialists for prestressing works (except North America).

This is a very bad reason for allowing low quality construction work.

If the stresser cannot achieve the assumptions used in design he should not be doing the stressing. The assumptions used in design have been developed from long experience on site. They are achievable with good work practices.

 

[henri2]

boffintech, given the level of sophistication of the industry and to avoid hair-splitting problems, your current company should go with Company A's method. Sell the idea to them.

 

[boffintech]

henri2,

I'm selling but they aren't buying. I believe that this is due to couple of reasons:

1) The PTI manual has a sample report that uses method B.
2) Method B simply reports numbers and makes no judgement as to whether those numbers are in/out of tolerance. Upon receipt of the method B report the EOR must make in/out calls. This shift is decision making (liability issue?) from us to them is apparently desired.

SAVAGE54 FYI On one of my current jobs I have a number of tendons that have very short elongations: 1 5/8" and 2". Surprisingly they come out dead on.

 

[Ingenuity]

boffintech,

"SAVAGE54 FYI On one of my current jobs I have a number of tendons that have very short elongations: 1 5/8" and 2". Surprisingly they come out dead on."

Ah...the pen is mightier than the ram! If this was a USA PT project where the industry practice is to detail the expected tendon elongations on the shop drawings, (and the PT special inspector was "distracted") this may explain why the results are dead on!

 

[SAVAGE54]

The question, I have is how many of you have inspected a tensioning operation. I am not talking about looking at the records but stood there watched the gauge and measured the elongation. I have watched tensioning operations in 42 states, Canada and Mexico.

Also most of the SKILLED people doing tensioning operation CANNOT speak English.

My firm place a bid to be the oversight company for helping the PTI organization develop standards. So I believe I know exactly what I am talking about. I was one of eight engineers developing standards.

I am just trying to discribe to REAL world not what happens in the office with the computers.

The inspector typical will report the actual results in oder to prevent having to explain the problem to a nonexperience engineer or a boss that does not understand.

You all just need to run the numbers. Do the calcations
See what deference it makes when you are 10% out of tolerence.