Stage Stressing 1

[dcStrucEng]

I am hoping to find some guidance on stage stressing of PT girders. I am designing a girder which supports a column at midspan. The column supports two floors above that.

When is stage stressing necessary? When is it not?

In my office, the latest Post-Tensioning Manual we have is the 4th Edition from 1985. There is only about 3 pages of information out of the whole Manual on stage stressing and no design example. Any information would be appreciated. Thanks.

 

[rapt]

dcStructEng,

Normally,
- if the stresses at transfer are too high with full prestress before the floors above are added - either transfer tension face reinforcement is added to control cracking or stressing is staged.

- if the deflection upwards at transfer is too high, resulting in too high an incremental deflection from the transfer deflected position to the total long term deflected position (all allowing appropriately for cracking and long term effects for the deflection calculations at both load conditions - remember factor methods (eg kcs) for long term deflections do not apply to PT, proper long term calculations must be performed).

In other words, the logic is no different to any other design. Just that your loading on the girder is coming from a couple of floors above, not just the one you are designing.

I would think that for 2 floors above, as long as the girder dimensions have been determined logically and are not too this, stage stressing should not be needed, but your transfer and deflection checks will tell you if it is needed.

 

[amsysco]

Another point to add if you elect to stage-stress:
The CGS of each stage-stressing should match the CGC of your concrete member - to avoid introducing eccentricities. Therefore, the PT Supplier should provide a sequence of which tendons to stress with each stage.

AMSYSCO, Inc. (

http://www.amsyscoinc.com)

 

[msquared48]

When I first looked at the title of this string. I thought it said "Sage Dressing". I was extremely disappointed when reality hit me. I love Sage Dressing...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rapt]

Mike,

Sorry to ruin your culinary musings.

Amsysco,

1 It should be decided by the designer, not the PT supplier.

2 It is normal not to maintain the CGS and CGC equal at stage stressing and that is not a problem as long as it is allowed for in both the flexural/shear design at each stage (so it must be determined by the designer in order to do this) and in the anchorage design.

3 The main thing to remember is to stress and grout complete tendons at each stage. Do not do a series of partial stresses on each tendon for each stage. THis will cause a lot of problems and possibly failures as well as being dangerous.