Yield Line Vs, Strip method for RC slab analysis 1

[mar2805]

Gretings!
Im interested if someone could give me some insight on these two methods and how they compare to each other.
Im aware of FEM power when analyzing slab structures but when a calculation by hand is required wich one of these methods would you use and why? Wich are pros and cons, and also how they compare to elastic methods like FEM?
Some book recomendations, articles...
Many thanx!

 

[haynewp]

You can ignore torsion for the slab design itself through using strip or yield line method, but I am not sure that it is always safe to ignore the transfer of torsion from a slab system to the supporting vertical elements.

 

[mar2805]

"Mx=q*Lx^2/16
My=q*Ly^2/16
If the slab is simply supported on 4 sides and Lx=Ly."

Is valid if the slab is not restarined against uplifting e.g. if there are no beams under the slab edges!

If there are beams under the slab edges, beams will have its torsional stifness wich will couse negative bending moments in the corners of the slab.
Uplifting is prvented, torsion is present in the slab, bending moments reduce in both direction, but negative momets appear at the corners where the torsion is at max.

 

[asixth]

mar2805,

I agree. The beams will go into torsion causing negative bending moment in the slab at the supports.

What are the dimensions of the edge beams and how have you calcuated the torsional stiffness.

 

[mar2805]

Thats a good question and more importantly how to calculate value of negative bending moments (due the torsion) that appear at the corners?

...I think yield line method has offered an solution to negative moments at the corners (negative yield line), but dont know if elastic methods have somekind of formula for this...also dont know if strip method can calculate these moments due torsion.

 

[rapt]

Hokie66,Thanks for sorting out the NS4U's problem. I only just returned from a training trip plus holidays and have not been checking Engtips. You are definitely correct with flat slabs as distinct from 2way slabs (British definition Flat slab -slab system supported on columns 2way slab - slab system with continuous supports in 2 directions.).

RE Mxy moments, they cannot be ignored in design. They are not Torsion as is defined as compatibility torsion, they, combined with the Mxx and Myy moments on an element, are required to define moment condition on an element when using Orthoginal moments. If you design on principal moments, Mxy is zero but you cannot reinforce based on principal moments as their angle is different for every element. For design for flat slabs, Mxy must be combined with Mxx and Myy in the 2 design directions using Wood Ahmer or similar to get proper design moments. FEM design programs that ignore Mxy in design are definitely wrong and significantly unconservative.
I do not think that setting torsional constant to 0 or a low value is correct either. if you are going to do an FEM analysis you should do it, not fudge it into something meaningless, but then you need to design for all of the effects from it.
When Mxy moments are included, the design moments from FEM are the same as from strip analysis.

Elastic Analysis:
Strip methods should be used logically to model as close as possible to the actual elastic action of the slab, as should Yield Line if you want reasonable performance for crack control and deflections. Otherwise as stated by someone above, you will get large cracks and larger than expected deflections.

The same is true for FEM analysis and design. One problem with FEM is that some people are basivally doing a Yield Line reinforcing layout based on an FEM analysis (eg PT flat slabs based on banded/distributed average designs). This is not logical. You cannot base a Yield Line solution on an FEM analysis.

RE Deflections:
You can get very good estimates of long term deflections from strip analysis/design if you reinforce to the elastic moment pattern for the slab and allow for the transverse distributions of moments. Much better than most FEM programs give if you use RAPT to do it, but you have to understand what you are doing.