Wood-Armer & Twisting Moments

[Trenno]

Just getting myself up to speed with everything discussed and presented in thread744-341206

I understand twisting moments can be significant and therefore considered carefully in design.

However reading my RC textbook... Link

"In general, for reinforced concrete slabs, it is required to design for Mx and My only. Special reinforcement detailing is sometimes necessary to cover twisting moments at the exterior slab corners."

Setting up a test file in RAM Concept and quick hand calculations shows that using Wood-Armer can significantly change the design actions present.

So why would this textbook say that? When is it critical to consider the twisting moments? Always? Or only at the slab corners?

My gut says that the Wood-Armer method is much more comprehensive and correctly generates design actions for reo in the orthogonal directions.

 

[rowingengineer]

Depend on the context if you use the traditional method of analysis resultants, than inflection reo would be required. Maybe that is what your text is referring, or maybe it is wrong.

If you use a method of analysis that produces Mxy moment as apart of equilibrium, than you must include it in your design.

Wood-armer isn't the only solution for converting Mxy to Mx and My resultants. It can be conservative in situations and consideration to its use should be given.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[asixth]

I reckon the textbook you are using wouldn't have be written with FE analysis using plate elements in mind. Mxy definitely can't be ignored. If using plate elements where the results are given as Mx, My and Mxy, Mxy must be distributed to Mx and My when designing your strips.

When using Ram I keep the wood-armer on.

I also reduce the torsional stiffness to 10%. It gives a more realistic deflection plot.

 

[Trenno]

Since digging a little deeper on these forums I know have a solid understanding of the topic. Thanks RE.

Asixth - am I correct in saying that the default option of "none in regards to torsion design in ram is making the program ignore Mxy? Here's hoping just allows for the extra twisting moments in the Mx and My moments.

 

[handex]

If you look at the differential equations that govern plates, the load is distributed into 3 components, Mx, My and Mxy. Provided the slab has enough ductility the applied load can be resisted by a combination of these components. In the traditional strip based methods, all of the load is proportioned to be carried by the Mx and My components, and although the slab would exhibit Mxy moments in reality they are not part of the load path used for design. This satisfies equilibrium and the design works. However most modern FEM programs use plate or shell elements that include Mxy stiffness, hence it is part of the load path and must be designed for, either by wood armer or any of the other conversion methods. I am pretty sure in ram concept you can model the slab as "no torsion", which essentially assignes zero stiffness to the Mxy stiffness of the plate elements and forces the load to be distributed in Mx and My. In this case wood armer would not be required because Mxy isn't part of the load path and would be 0. Once again, Mxy moments will actually exist in the slab but won't require any additional reo for strength, but possibly require some in corners etc to minimise cracking.

 

[rapt]

Handex,

The Mx and My moments in strip methods are not the same as Mx and My in FEM.

FEM produces node moments. To convert them to useable orthogonal design moments for an element, Mx and My cannot fully describe the situation for an element unless the moments at the corners are basically symmetrical. So for most elements a twisting moment Mxy is needed to define the twist in the element.

You will find in a regular structure that the FEM Mx and My, after being adjusted by a contribution from Mxy will be basically the same as the strip method Mx and My.