Post tension, When is a beam stiff enough?

[aaronPT]

Hi,

I have been designing post tension slabs for around 5 years, sticking to mainly 2D software in transfer situations and FEA for complex transfer slabs/flat slabs or flat plates. To date with no issues (touch wood!)

When I look at a simple office slab, for example 8m main spans, 6.5m end bays, with a 2400w, 350D post tension beam and 160 internal slabs spanning 8m with 200mm deep end bays,

The results differ when comparing 2D to 3D, and it begs the question, When Is a beam stiff enough to ensure one way action of the slab?
With post tension designs the level of reinforcement in shallow beams is nonexistent, with some minor negative reinforcement over the columns, Therefore if we are considering how the structure behaves if it is lightly reinforced are we
Allowing the concrete to crack in the slab, in line with the columns between beams in the top, and in the bottom mid span(Due to 2 way bending moments). As if this occurs then it will kick into the one way situation?. Which what we are designing for with 2D software?

 

[asixth]

What software are you using for your 3D analysis. For a 160 deep slab and 350 deep bands I think it would acting one-way (slab spanning one way to the band and bands spanning one way to the supports).

If using RAM then change the slab to one-way span and orientate in the direction of the span. Or keep as two-way and change the Kmrs stiffness to 0.3.

 

[aaronPT]

I am using inducta,

Yes it is one-way however with shallow bands and post tension slabs with no Reo for strength, if the beam has a deflection of say 10mm then the slab will have 2way action to some minor extent? Just because we stiffen the model in the FEA program does not mean this happens in real life? Is it ok for us to assume if secondary cracking occurs the design has accounted for a one way strength design, therefore besides minor cracking no failure will occur?

 

[asixth]

If it is not reinforced in the secondary direction, then it will crack, weaken and redistribute back to the bands. I do think that a 350d band is significantly stiffer than a 200thk slab, enough so that it can be considered a one-way spanning design.