Flexural crack control by limiting the increment in steel stress

[asixth]

Hi guys

I have a question regarding crack control for flexure in prestressed slabs by limiting the increment in steel stress near the tension face. My code requirements limit this increment in steel stress for both bonded tendons and conventional reinforcement to 200MPa (29ksi).

Is this to limit the flexural cracking in member to a prescribed limit that the code deems acceptable (say 0.2mm) and what would be the ramifications to the slab system if this value is exceeded (say 240MPa).

I do intend of researching into this topic further but I am just a little stretched on time at the moment. I have a post tensioned transfer beam (bonded tendons) and I do not particularly want to increase the amount of conventional reinforcement in the section because it is already quite “heavy” in terms of reinforcement rates. I am thinking about adding another tendon if require.

I believe that ACI318 has similar requirements in section 18.4 (2002 version) but limits the stress increment to 250MPa.

Any insight provided will be greatly appreciated.

Thanks

 

[rapt]

asixth,

If you look at the latest version of your code (assuming you are using AS3600), this has changed completely in the 2009 version of the code!

 

[asixth]

Thanks RAPT

My current design is a 700dx2400w band reinforced with 5/5s tendons and 11N36. My increment in steel stress is 238MPa with an AS3600-2009 limit of 200MPa. It is a carpark where the flexural cracking will be visible but in an otherwise controlled environment.

My options are too either 1) replaced the 11N36 with 2 rows 11N28 (230c/c) or 16N36 (150c/c) or 2) Removed 200kN from the transfer load by a more refined load takedown.

Although it is not stated in AS3600 anywhere, I am guessing that flexural crack width based on this increment of steel stress is 0.3mm based on recommendations given in textbooks and BS codes.

The AS3600-1994 commentary and supporting information on serviceability design written by Gilbert also states that a refined procedure where crack widths are calculated can be used where an acceptable model is used (such as BS/EC).

I am going to proceed with the calculation of crack widths for my own learning more than anything. I might need to swollow some pride an put in some additional reinforcement.

Thanks

 

[rapt]

Asixth,

The other solutions would be smaller bars allowing a higher stress in the bars, or another tendon, reducing the stress but also reducing the reinforcement requirements and probably allowing you to reduce the bar diameter as well, thus possibly allowing a higher stress limit.

Currently you are only supplying about 40-50% (depending on 12.7/15.2mm strand) of your total capacity with tendons, the rest with reinforcement. This is relatively low. Thus you have fairly high service stress and high stresses in the reinforceemnt at service. An extra tendon and less reinforcement would help a lot!

If you attended the CIA seminars presented by Gilbert and others over the last few years, he has presented the crack width calculation method used in AS3600 in those seminars. It will also be in the new Commentary later this year (hopefully). It is based on approximately .3mm (actually .35 I think) crack width. The calculation methodology is similar to the Eurocode methodology (but not the same as it allows for shrinkage effects).