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  #21  
Old Posted May 17, 2009, 1:02 PM
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200 W Lake - Chicago

May 4
Post tension cables being positioned inside of rebar for beams.


Post Tension cable
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  #22  
Old Posted May 17, 2009, 11:39 PM
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Is the green paint marked on some of the strands related to the tensioning (refer to photo previous page)? Yes, probably it is. Each strand is marked prior to the pull in order to be able to record the length of pull made. This serves as a backup to the reading(s) made from the hydraulic jack used to actually pull the strand which could be miscalibrated and otherwise cause a whole lot of damage if a strand gets pulled to far or not enough!!

Speaking of green - you guys sure like that epoxy-coated bar! Other states and provinces have discontinued use as the coating is too delicate and easily damaged. If the knick is not repaired, it could actually accelerate corrosion at that one spot a lead to premature failure of the bar (or so the story goes).
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  #23  
Old Posted Jun 13, 2009, 11:02 PM
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Post tension in the trough that will be a RC beam.




Yes we love the green rebar, there are mountains of salt stored all over the place, these will be dumped liberally over every paved surface as soon as it approaches freezing.
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  #24  
Old Posted Jun 28, 2009, 9:55 PM
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Hi all,

I have worked on several high rise buildings in South Africa where the use of PT cabling is very popular. In fact, having lived in SA for 5 years (I'm Scottish), I have worked on 4 high rise projects, and all have been PT.

There are several advantages over conventional slabs and the financial benefits are substantial. First of all, you require a lot less rebar than a conventional slab with very little top steel needed, and secondly you can release the formwork quickly, usually after 3 days when a compressive strength of 18MPa is reached. Ready mix companies here have all got a PT mix which reaches a high strength earlier and they guarantee 18MPa after 3 days.

I've yet to have a cable snap under normal conditions, although remedial work can be very time consuming and costly if they are accidentally cut. I've had a plumber core drill through one and an electrician cut a band of four with a grinder.

I'm assuming that PT cabling is not very common in the US?
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  #25  
Old Posted Jun 29, 2009, 3:35 AM
Nowhereman1280 Nowhereman1280 is offline
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^^^ No, its fairly common when Reinforced Concrete is used. However, at least in Chicago, its not usually used throughout the entire building, but only in area's with particularly large spans or beams that need extra strength as shown in Harry's above pictures. Its frequently used on buildings that have large cantilevered balconies.
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  #26  
Old Posted Jun 29, 2009, 3:42 AM
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Quote:
Originally Posted by Spizz View Post
Hi all,

I have worked on several high rise buildings in South Africa where the use of PT cabling is very popular. In fact, having lived in SA for 5 years (I'm Scottish), I have worked on 4 high rise projects, and all have been PT.

There are several advantages over conventional slabs and the financial benefits are substantial. First of all, you require a lot less rebar than a conventional slab with very little top steel needed, and secondly you can release the formwork quickly, usually after 3 days when a compressive strength of 18MPa is reached. Ready mix companies here have all got a PT mix which reaches a high strength earlier and they guarantee 18MPa after 3 days.

I've yet to have a cable snap under normal conditions, although remedial work can be very time consuming and costly if they are accidentally cut. I've had a plumber core drill through one and an electrician cut a band of four with a grinder.

I'm assuming that PT cabling is not very common in the US?

When I was with the telco company at NorthWestern (hospital and university in Chicago/Evanston ) the facilities guys almost had a coronary when we were getting ready to core the floor - apparently they were afaid we were going to hit one of these new-fangled post tension things.

Would you have any photos of the actual tensioning ?
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  #27  
Old Posted Jun 29, 2009, 10:28 PM
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I have a lot of pictures at home, but am on contract in Togo, West Africa at the moment so I only have a few that are on my laptop and were taken by cell phone. Not great quality but maybe better than nothing.

You can almost see the bursting steel coils at the live ends on the distribution cables in the first picture to the right hand side. There are some lying about at the foreground of the picture. I didn't notice any in the photographs posted so far, is this not used in USA?





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  #28  
Old Posted Jun 29, 2009, 10:32 PM
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Sorry Harry, I misread your post, but the same answer applies I guess. Everything is at home in South Africa. I'll be back home end September so will look out some pictures taken during the tensioning process and post here.
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  #29  
Old Posted Jul 3, 2009, 7:04 PM
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Post-tensioned flat slabs are very common here in Texas. Almost all new mid and high-rise residential buildings utilize this structural floor system.
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  #30  
Old Posted Jul 3, 2009, 8:18 PM
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Since the bulk of tall structures here in Tampa have a concrete superstructure, naturally there are lots of post-tensioned slabs being built. I got to see it happen on a project once, as part of my training for my last job... It was pretty cool how it all works. It's also rather amazing how the concrete in the area of the tensioning cables is so much stronger than it would otherwise be.
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  #31  
Old Posted Jul 9, 2009, 12:53 AM
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Dumb question,
would a given floor be tightened all at once, one side and then the other, the middle to the outside, does it even matter?
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  #32  
Old Posted Jul 9, 2009, 1:26 AM
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Quote:
Originally Posted by BlackSash View Post
Dumb question,
would a given floor be tightened all at once, one side and then the other, the middle to the outside, does it even matter?
good question
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  #33  
Old Posted Jul 10, 2009, 2:16 AM
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Quote:
Originally Posted by BlackSash View Post
Dumb question,
would a given floor be tightened all at once, one side and then the other, the middle to the outside, does it even matter?
It does matter a great deal and is one of the major considerations contemplated by the designers. As you note, it can be done from one end or the other, or both. It is not done from the middle - you need to have room at the end at the section to install clamps, jacks, gauges, etc.

There would also be a sequence of tensioning that is followed rigourously - i.e. left-to-right, alternating sides, centre-outward, etc.

The reason for tensioning on alternating ends is that the cable will experience a level of friction (also called a loss) inside the sheath which varies according to distance from the jack, the type of sheath, the "wobble", the number and degree of curves present in the profile, the lubricant used, etc. By alternating the jacking side, the losses are the same at each edge rather than having one side outperforming the other. The alternative is to simultaneously jack both ends, but this may not prove to be economical or practical.
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  #34  
Old Posted Jul 10, 2009, 4:47 PM
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Neat!
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  #35  
Old Posted Jul 13, 2009, 11:16 PM
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Absolutely fascinating thread, particularly to those of us that consider architecture a hobby.

Can someone elaborate on the methods used in presressed concrete?
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  #36  
Old Posted Jul 14, 2009, 12:01 AM
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thank you kelvin- fascinating the way simple things ( "loss" ) can be accommodated in simple ways (alternate tensioning ).
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  #37  
Old Posted Jul 14, 2009, 2:38 AM
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HarryC - yes it is an interesting topic and obviously there are entire books and courses devoted only to this type of construction.

ChicagoChicago - Prestressed does not differ significantly from post-tensioned in concept, just in practice. With prestressed, the cables are stressed first then concrete is placed around the cable(s) rather than casting then stressing. Once the concrete is sufficiently cured, say 1 to 3 days, the cables are cut (the cables will be stressed and anchored to big steel anchor heads at each end of the casting and the cables then pass through the concrete formwork). By cutting each strand, the tension is now applied along the line of action for the strand - in effect putting the concrete into compression along that same line. The cables bond directly to the concrete - they are not encased in a sheath and do not need to be grouted. Prestressed concrete design does not involve losses like friction or wobble; but both prestressed and post-tensioned will have some "long-term" losses such as relaxation, shrinkage, and creep.

Prestressed is generally suited to being done on assembly lines under controlled conditions whereas post-tensioning is perhaps better suited to "field" construction for large flat-work (slabs).

Here is a good view of a "clam shell" style steel form used for prestressed concrete girders. It is essentially two halves that are opened (to allow placement of steel and arranegment of strands) or closed when concrete placement is underway. There is even a walking platform to allow workers to place concrete easily at the top. What you don't see in this picture are the two steel bulkheads that would contain the ends of the pour and through which the steel strands pass through on their way to the anchor blocks.

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  #38  
Old Posted Jul 14, 2009, 6:41 PM
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Kelvin, Thanks for the excellent descriptions.
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  #39  
Old Posted Dec 26, 2021, 12:53 PM
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you may consult this company
https://www.eforce-pt.com/

They are specialized in designing & executing Post Tension slabs.
they are present in Singapore, UAE, KSA & Egypt.
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  #40  
Old Posted Jan 22, 2022, 9:50 AM
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Check this PT Company

You may consult this company
https://www.eforce-pt.com/

they are specialized in designing & Installing Post Tension slabs.
They are present in Singapore, UAE, KSA & Egypt.
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