AS3600-2018 Post Tension (Cl 9.5.2.3) 1

[GB1000]

Hey,

The item below refers to a post tension slab design

I'm slightly confused on the wording which AS3600 uses for this clause.... 'if this stress is exceeded, by providing reinforcement or bonded tendons, or both near the tensile face with a centre to centre spacing not exceeding the lesser of 300mm or 2.0Ds'

On a 200thk slab our tendons generally spaced at 1m, You would not have tendons in a slab spaced at 300mm or 2Ds on a slab so why the wording, particular in reference to tendons

We currently have a 200thk slab, and calculations indicate we need an additional 200mm2/m in the slab, which is relatively small. I've heard some consultancy adding 1/N16 at every 1m (which does not comply with above 300mm max spacing). some adding a layer of mesh (SL92) and then others adding no reinforcement by limiting the crack width and table 9.5.2.3.

Seems everyone is doing something different, would welcome thoughts

 

[rapt]

The wording is very clear. If designers are not following it, the code cannot be blamed. Yes, you would not normally have tendons alone at that spacing, but I have seen cases with very heavily loaded slabs where it does happen, and band beams (which are really thickened slabs) would often have this. So the code has allowed tendons alone as a possibility to try to cover all possible situations. Using this as a reason to question the clause is simply nit-picking.

Maximum spacing of all bonded reinforcement near the tension face, bars or tendons combined, is 2D or 300mm.

Bonded steel at greater than 300mm spacing is not considered to provide control to crack width.

The reason the limits were added is simply because tendons at 1-1.5m centres by themselves provide no control over crack width once the concrete has cracked.

It is even more onerous if you want a Strong Degree of crack control in the secondary direction. 1.5D or 200mm. Gone are the days of designers considering that tendons providing .7MPa P/A with tendons at 2.5m centres provide crack control.

 

[GB1000]

Good point Rapt and appreciate the comment.

How would you define 'short term service loads' as this is what the clause specifies? Would you define this as the as the self weight and tendon uplift (i.e. transfer load), or defined as something else?

 

[rapt]

The load case you have defined is Transfer, and yes it needs to be considered also but would normally only control if there is a lot of prestress relative to SW causing cracking on the reverse face.

AS1170.0 defines short term and long term load factors and load combinations. I would suggest that the short term serviceability combination might be the short term service loading! That would include SW, all dead load and short term LL, or permanent LL plus service wind as a minimum. Plus any other combination you might think is possible as the loading code only defines minimum loading considerations.