Jacking Stress in Two way residential slabs 2

[d2783]

Hi everyone,

I am involved in the design of a building which has two PT concrete floors; 1st floor parking slab and second floor podium slab which supports the four storey wood structure.
The podium slab is 12.5 in thick and it supports dead load of 200 psf in excess of self weight and live load of 80 psf. The typical span i have is 30 ft. I am using ADAPT-PT for the design (EFM). I did not do the initial stress analysis while using ADAPT 7.2. But in new version of ADAPT PT 8 I was able to separate the self weight and super imposed dead load. I found that my initial stresses are very high for all the PT runs. When I say high means in the range of 6 squrt f`c, while allowable is 3 sqrt f`c. Precompression I have is in the range of 170 to 200 PSI. I would really appreciate if anybody can suggest me how critical these stresses are. Will concrete crack if I ignore these stresses.
And I can not do stage prestressing in this project.
Did anybody face the problem of cracks during jacking?
Should I ingore this? because my service stresses are fine.

Thank you,

 

[civilperson]

No, place reinforcement in top, (tension), locations for resisting cracking or change the location of strand to middle or higher at ends.

 

[rapt]

d2783

Very modern of Adapt to allow this after 23 years to try to catch up with their opposition who have been allowing it since 1986. Too bad about all the old designs without this capability.

When you say 6 squrt f`c do you mean the full concrete strength of the transfer concrete strength. If it is the full strength then the stress is very high. On top of this remember that it is an average stress, not a real life stress. In any case, for this transfer stress level I would have reinforcement at the transfer tension face/s as civilperson suggested.

In other words, no you cannot ignore it. Yes I have seen several cases of severe cracking at transfer.

 

[d2783]

Thank you civilperson, for the reply,

So, should I put the bottom steel at support, because it gives me high tension at bottom of the support.
And in reality there will be shoring when they stress the tendons. So if I ignore the self weight in my analysis that would give me even worst results.

Thank you rapt for the reply.
yea finally adapt included that option in the analysis.
yes the initial stresses are in the range of 6 sqrt f`c. while the ACI permits only 3 squrt f`c.

Oh so I can not ignore this stresses.
but should i provide the steel at the bottom of support?

Thank you very much for the replies, civilperson & rapt

 

[rapt]

d2783

The limit in the code is not for fc, it is for the concrete strength at the time of stressing, fci'.

And, yes you do need to provide this reinforcement.

 

[civilperson]

The pre-stress jacking will lift the slab off the shoring so the self weight will effect the stress in the concrete. The negative moment over the interior support does need tension steel in the top. Tension caused by prestressing at the top face of the exterior supports, (transfer locations), also needs reinforcement if stresses exceed the limit.

 

[rapt]

CivilPerson,

I think you are missing the point. He has checked transfer stresses including the SW and the stresses are too high. To limit crack widths, reinforcemernt at the bottom at the supports will be needed.

If, as is usually the case in USA and with Adapt PT, the stresses at the supports have been checked on the basis of averaging over the full panel width, then the real stresses will be significantly higher then calculated and crack control reinforcement must definitely be supplied at the tension face.

 

[shin25]

The bonded crack control reinforcing needs to carry the entire tension stress in the section at .6fy. Sometimes this translates to a good amount of reinforcing. Also, at the very end of the member, it becomes difficult to develop the rebars within a very short distance.

It is therefore, should be a good idea to reduce the tension stress at release. You may try to increase the release strength of the concrete and at the same time modify the p/t layout a little bit to reduce the stress.

 

[shin25]

Also, it feels like the thickness of your slab for the loading and span length that you have mentioned is little too thin. Have you checked the ultimate moment capacity of the slab vs. required moment capacity?