AS3600 Shrinkage reo in addition to flexural reo 1

[geopat69]

Hi.
Just wanted to get some views on a basic suspended reinforced concrete deck that has a high degree of restraint.

I have designed the flexure reo to As3600 for service stresses. Then I checked checked this same amount of reo assuming my structure has a high degree of restraint (ie using 0.6% reo for high degree of restraint).

In summary the quantity of reo I have for flexure by itself is more than enough to handle the high degree of restraint for when shrinkage occurs.

The question is, should shrinkage reo be additive to the flexure reo? I was always led to believe that if you satisfy shrinkage with your flexure reo..."you're fine"... But I have my doubts.

Surely the mere action of shrinkage and the stiff columns means that tension builds up in the slab (ie so now we have the original flexure moment now with tension included). Does this exaserbate service cracking?

 

[hokie66]

They are not additive, but shrinkage reinforcement Has to be continuous to resist direct tension, while flexural reinforcement might not. That is, where flexural reinforcement is minimal or not lapped, you would need to add bars.

 

[geopat69]

Hi hokie and thank you.

Why do you think the tensile bar stresses from shrinkage cracks need not be additive.

I mean.. If a sufficient amount of crack control reo is provuded and let's say many cracks much less than 0.3mm develop... There still is tension in the bar (as it crosses the crack and trying to hold the crack from opening up).

I can't follow the logic.

 

[rapt]

geopat69,

They are not additive.

But the areas required for restrained shrinkage are minimum quantities and they are total in the cross-section, not the amount in each face if it is in the secondary direction in a one way slab. You need to do section analysis allowing for both flexure and shrinkage to assess the ability of that reinforcement to provide sufficient crack control.

 

[geopat69]

Thanks Rapt.

I'm using FEA with plates to simulate the deck. I have also modelled my supports that create the horizontal restraints.

I got the flexural service momemnts in the deck(easy bit) . And I have used negative thermal loads to simulate shrinkage (based on full slab thickness). But the tension axial loads in the concrete are huge!

Are you suggesting cracked section analysis incorporating these huge tension loads (with service moments). If so.. The stress in the steel will have truly exceeded my 240Mpa limit.

 

[geopat69]

Just to clarify the above comment of mine and maybe I'm wrong with saying this, but if you allow say 150MPa in the steel due to shrinkage, I'd presume there's not much leftover for the service moment. Recalling that max stress is about 240MPa (depending on bar and spacing)

 

[rapt]

Is your model a true representation of what is happening?

Are you modelling a single level in a multi level project. If yes, then the restraints are not real except at the bottom and top levels.

Are your restraining members cracking? If yes, then your restraint model is wrong.

Your floors are cracking, so you element model being elastic uncracked is wrong.

And you cannot do the section analysis for tension and bending separately. You need to model combine bending and tension in your model.