cracked torsional slab stiffness 1

[handex]

Hey guys,

Just looking for a reference on how cracking effects a slabs torsional stiffness?

For designing slabs we currently (and conservatively) set the Mxy stiffness to zero, so that it doesnt pick up any twisting moments.

Does anyone know if any of the current FEM programs reduce torsional stiffness once it has cracked?

 

[BAretired]

That's what computer programs are for. You can model a good approximation of how things really behave, rather than a gross approximation to how they behave to make the arithmatic easy.

I have no doubt that computer programs model a good approximation of how things would behave if everything was truly elastic.

Two way slabs do not behave elastically because they crack. And when they crack, moments re-distribute to suit the reinforcement arrangement chosen by the designer. The important thing to remember is that reinforcement must be placed such that overall statics will be satisfied.

Satisfying an exact elastic distribution of moments achieves no useful purpose and is therefor unnecessary. Much better is to place the reinforcement in a way which makes life easier for the contractor and still achieves total safety for the occupants of the building.

It has nothing to do with making the arithmetic easy. It has more to do with common sense.

BA

 

[rapt]

BAretired,

What you are saying is correct for ultimate strength/failure conditions. Internal redistribution will redistribute actions to the provided reinforcement pattern as long as a load path to the supports is supplied and sufficent reinforcement is supplied for this load path.

However, for crack control, the cracking will occur based on the elastic moment pattern. If the slab is reinforced to a different pattern, cracks at locations where insufficient reinforcement is supplied compared to the elastic moment will be wider than expected unless more reinforcement is supplied. If the cracks are allowed to be wider, deflections will be worse than expected. So the elastic moment pattern is important for serviceability calculations! It is not just done for calculation accuracy!

 

[asixth]

good article and thanks to those members of this forum who contributed to the latest concrete in australia magazine.