Determination of pre-tensioning forces due to wedge removal pre-transfer

[tkd77]

Hi All,
A potentially unique problem (in my experience). Am working for a large precast company who regularly manufacture pre-stressed flooring products, typically by a long line method with mass tensioning / detensioning. We have recently purchased some self-stressing moulds for a series of unique floor units. Whilst I am happy with our methods of calculating the required extensions, and stress checks in general, we have a situation that I have not come across before (and may be worrying about unnecessarily). See below for summary of points:
1) One of the self-stressing moulds is very short - circa 10m for a 9.5m unit
2) The design for this unit requires a high level of initial prestress (210kN for a 15.7mm superstrand with UTS = 1860N/mm2)
3) Whilst this checks out for both the limits on initial prestress and prestress at transfer, I'm wondering whether we should be concerned about additional forces caused by the additional tension required to remove the strand wedges (post-curing, but prior to transfer). Initial stressing and transfer is by mass tensioning / detensioning.
4) My challenge is that if one works out the additional force in the strand needed to release the wedges over the length of strand outside of the unit (assuming that the strand bonded within the unit does not elongate), we are essentially calculating the force to extend around 500mm of strand by around 5mm....so the resulting force are very large. So large in fact, that I question whether this is a 'real check'...as it blows all the other checks out the water.

So, query is..should we be doing this, and if so, are we doing it right. Any assistance gratefully appreciated. Not involved in the original mould procurement or permanent works design...just tasked with sorting this out......(help!)

Thanks in advance!

 

[rapt]

Yes, you have a problem. The unit itself will elongate slightly due to the increased strand force, but probably not enough to matter.

Sounds like the oxy is needed to relieve the stress and then cut the strands! Not sure if this is an accepted solution these days.

 

[tkd77]

Hi Rapt,
Thanks for that. You've confirmed the conclusion Iarrived at earlier today. Am going to go for cutting the strand whilst adding in some additional wedges to minimise the risk of it shooting through when cutting. Scary thing is that this only came to light because I heard the PM diwcussing mould procurement by chance... Thanks again, would be good to seek your advice on some other questions (but I'll deal with the current crisis first!). Are you US or UK based?

 

[Ingenuity]

Are you US or UK based?

You have insulted him twice...he is from a previous convict colony...AU

tdk77: If your forms are TRUE self-stressing forms, cannot you simply retract the hydraulic form ram and collar such that the stressed strand multi-use wedges will release, and hence transfer of prestress to the section, then abrasive or oxy/acetylene cut the strand tails to required length.

 

[rapt]

If Ingenuitys' solution is not possible, If you heat the strand over the free length rather than just cutting at a point to start with, you will get the benefit of a gradual release, and not have to worry so much about strands shooting through, though you should have protection just in case the oxy operator is incompetent, or slips!

I thought we were still regarded as a convict colony. When did the previous come in?

 

[Ingenuity]

If the self-stressing forms do NOT have a hydraulic form ram for release, then with only a 500 mm length of stressed (and exposed) strand between the stressing buttress and the precast product, there is not a lot of elastic energy over such a length when cut instantaneously, even with an abrasive grinder. I have de-tensioned 12+ strand bundled unbonded tendons in a single cut with an exposed length of several feet - BUT the unraveling of the strand wires is the real hazard - so you I usually provide wood boards sandwiched over the strand/s and secured

However, shock loading 'release' of the strand to the product will be dynamic and result in more strand slip. If you can heat up the strand for gradual release, as rapt states, that would be preferred, albeit slow.

 

[tkd77]

Ha..apologies!
If it's any consolation I'm based in the north of the UK, and secretly harbour dreams of moving down under.

The moulds don't allow detensioning with the rams without first releasing the wedges (as I understand it, but will be checking!). The oxy makes sense, but suspect our H&S people might keel over... our foreman has already suggested it, and has probably already started getting the kit ready