Concrete Design Twisting Moments (Wood - Armer)

[Gumpmaster]

Just a quick survey:

1) Do you, in your typical concrete design, consider twisting moments?

2) Have you ever considered twisting moments?

3) If you do consider twisting moments, what method do you use for design (Wood-Armer?)?

4) What region are you in (USA, NZ, Europe...)?

I think that the inclusion of twisting moments in design is not universal, and varies greatly by region, but it would be interesting to see everyone's experience with this.

 

[KootK]

1) Yes.
2) Yes.
3) Wood-Armer
4) Canada

It sort of depends on what you mean by "consider". In an elastic, two way FEM analysis, twisting moments will almost always be generated. To disregard those moments altogether is to produce a flawed design, without question. It doesn't make sense to reinforce for the twisting moment explicitly however. You tend to wind up with light amounts of reinforcing in places where reinforcing looks odd. Dealing with the twisting by converting it to conventional flexure somehow (Wood-Armer) is better.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[asixth]

thread744-372518

This question gets asked alot. Mxy in FEM analysis needs to be accounted for in design.

Nobody should be using Mxx and Myy as there orthogonal strip design actions and ignoring Mxy.

 

[jrbaus]

Refer to Page 7 of the recent publication below. It has a good discussion on this exact issue and states that ignoring the moment generally (but not not always) results in <10% increase in moment.

http://www.structural-safety.org/media/364894/cross-newsletter-no-36-1-.pdf

Regards Jake

 

[TehMightyEngineer]

1) Typical? No FEA? Yes.
2) Yes.
3) Wood-Armer
4) USA

Maine EIT, Civil/Structural.

 

[Gumpmaster]

1) No
2) Yes
3) Wood-Armer
4) USA

I agree that the twisting moments are an integral part of achieving equilibrium and that they should be included in design. It's not the standard practice at my current company though. Designs are primarily for crack control with ACI 350, and that adds quite a bit of conservatism. The majority of engineers in my office have actually never heard of Wood-Armer.

 

[Gumpmaster]

Asixth, I think this is common practice outside the USA, but not necessarily common practice in the US, apart from maybe automated design of PT slabs. Not that that's right, but I think that's probably how it is. I don't think most ACI 318 based textbooks even mention twisting moments or Wood-Armer. I'm hoping to maybe get a better feel for if that's true or not.

 

[TehMightyEngineer]

I think most people just hand-calculate the design of complex concrete using the strip-method thus avoiding FEA, Mxy, and so on.

Maine EIT, Civil/Structural.

 

[AlanLord]

Yes,

And Advance Design America software does it for you

 

[PicoStruc]

Have same answer than KootK

But I add that A23.3-2004 Concrete code REQUIRED to consider twisting moment using Wood-Amer Method for elastic plate method (FEM). See Clause 13.6.4

 

[rapt]

There is a difference between analysing for Mxy and allowing for it in design. ALL FEM software will generate Mxy moments. It is the design routines that need to be programmed to allow for it in design.
US Design software tends to ignore it. European always allows for it. e.g. RAM Concept ignores it by default but they added an option for the user to include it in design a few years ago. Sofistik, Oasys, Cedrus and Orion from Europe all allow for it automatically.

Some design codes specifically say to include it (Canadian, NZ), while other does not see the need to define it specifically and instead insist that the design satisfy equilibrium. To satisfy equilibrium, you must include all actions from the analysis and Mxy must therefore be included in design.

I did an interesting comparison with a square grid once between RAPT (2D frame) and Orion (FEM) and the total panel moments were exactly the same as long as Orion included the effects of the Mxy moments in the design moments (which it does automatically).

The actual effect will vary depending on the coarseness of the mesh and the symmetry of the structure and the location in the structure.

 

[Ingenuity]

Some historical perspective...not related to Mxy, but interesting none the less...ACI-318 up until 1971 (I think) permitted the user to design for 85% of statics for RC flat plates! Yep, Nichols' classic 'total static moment' of wL²/8 was not accepted by some in the profession (Turner, Eddy etc) in 1915 era, and it took more than 50 years for the "less than equilibrium" provision of ACI-318 to be removed.

 

[slickdeals]

1. Yes
2. Yes
3. Wood-Armer (turned on in Ram Concept Strip design preferences)
4. U.S.A/India