STRUT-AND-TIE MODELING - Suggestions for Self-Teaching

[Illbay]

I am interested in learning strut-and-tie modeling (STM) for the analysis and design of reinforced concrete.

STM has been a "thing" since at least the late 1990s, and has actually been incorporated formally into building codes via ACI 318 and other national codes for nearly 15 years. But as practicing structural engineers outside academia are notoriously resistant to novel approaches to analysis and design - I actually had a near-retirement-age colleague a decade ago who still used "bond force" for determining development of steel reinforcing - you're pretty much on your own as far as learning and attempting to implement such methods in the design office. And I don't do enough heavy-duty RC design these days to have many such opportunities.

But lately I've done quite a bit of RC design and in one application I think STM would have been quite helpful. But I simply could not get a working model. I attempted to use >>Fachwerk<<, a nifty Java applet written by a German engineering professor, but it is not the kind of thing you'd want to use to LEARN the concept; you pretty much have to know what you're doing already. My attempt was a miserable failure.

The key seems to be having a full understanding of the expected performance of the real-world structure or element prior to beginning to work out your model. In fact, that to me is the value of STM, that it helps you gain insights into how reinforced concrete structures work. But there are some advanced concepts as well that need to be understood in order to apply STM, especially in the realm of "nodes" and especially "nodal zones."

And it's not like with stiffness-based structural analysis methods that are well-represented in the software literature, where you can depend on the software "smarts" to do the work for you as long as you specify everything correctly. Even FEM won't lead you astray so long as you get your loads, geometry and material properties correct. But STM isn't like that. You literally must know exactly what you're doing going in. The danger isn't so much in putting together a model that yields bogus results, it seems, as getting a model to WORK at all!

Obviously there are lots of examples in the literature, but going through what I've downloaded - and it's a lot! - is heavy slogging. Again, the academic-types approach the topic as if everyone understands certain fundamental principles, without bothering to explain them. In that respect it is very much like trying to read through a treatise on FEM. But at least with FEM you can rely on well-written software to do the heavy lifting regarding concepts whose nuances might elude your mental grasp.

Seems with STM, if you don't know what you're doing, you cannot rely on software to help you. It's as much an art as a science. Therein lies the beauty of it, of course, and the reason I'd like to make some headway in understanding and applying it.

My query is addressed particularly those who are doing actual applied design rather than academic research. How did you come to learn STM? What resources would you recommend? Do you have a suggested approach? What concepts should you learn early-on that you can apply in design, and what if anything can you leave off till a little later to master for doing more complex design work? Should you try to do this on your own with spreadsheets or Mathcad or Matlab or the like, or is there existing software, either open-source or proprietary, that you would recommend?

Any comments or suggestions are welcome, including those allowing as how I am F.O.S.

Thanks.

 

[Akeee]

Hello,
You can use FEM to help you draw the compression and tension struts/diagonals too, using principle stress directions that will show you the maximum compression and tension stress lines. In STM there are a lot of ways to build that truss, but you need to know which is more realist in your example, so there is an experience factor yes, but i dont know why you say that you cant make at least one to work. Detail the problem more please, with some pictures or something.

 

[KootK]

I'm perpetually on the lookout for good STM design guidance. I agree that, despite being incredibly simple in concept, STM can be surprisingly difficult in application. I recommend this paper as a starting point: Link. It's actually one of the first treatises on STM design. In my opinion, it's still the best when it comes to explaining the fundamentals of the method. I suspect that is because the authors were trying to sell a novel approach that had not yet gained widespread acceptance.

As for tools, I use a hybrid MathCAD & handcalc approach. I use MathCAD to determine allowable node and strut stresses etc so as to cut down on the tedium. At the same time, unless you're running the very same model over and over again in practice, there seems to be little profit in trying to code general purpose STM tools for start to finish design.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SlideRuleEra]

Eight years ago the Portland Cement Association had a continuing education paper, "Strut-and-Tie Model for Structural Concrete Design". Too late to get "credit" for it, but may be of interest for your purpose:

https://www.structurepoint.org/publication/pdf/pdh-Strut-And-Tie-Design-Method.pdf

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Akeee]

Hello,
It appears to be a very good book, thank you KootK!

 

[mike20793]

Great paper KootK. Thanks for sharing.

 

[Guest262653]

I'd suggest working your way through the numerous examples that are published for the main STM models. The following books/reports were helpful for me:
- ACI SP208 - Examples for the design of structural concrete with Strut and Tie Models;
- ACI SP273 - Further Examples for the Design of Structural Concrete with Strut-and-Tie Models;
- FIB Technical Report 61 - Design examples for strut-and-tie models;
- Dimensionnement des constructions selon L'Eurocode 2 à l'aide des modèles bielles et tirants - Principles et applications;
- FIB Model Code for Concrete Structures 2010.

I'm not very confident in developing a new model from scratch as the serviceability requirements are not dealt with explicitly using STM. I'd rather stick to a proven model, backed up with experimental evidence, rather than trust a linear elastic FEM model. But then again, I have never needed one...
By the way, I did use Fachwerk 3D once for a large pile cap and it worked really well to get a force evaluation on the struts and ties of the model(no nodal checks, unfortunately).

I developed a few spreadsheets (in Portuguese, sorry...) for the design of 2 and 4 pile caps and short corbels to the European Standards, whose links I enclose. Probably the extent of my ignorance will be in its full splendour for the whole world to see, but here they go anyway:
-

https://www.dropbox.com/s/e8l9pim9jxzvpnb/Maciço 2 estacas_EC2_v03.xlsx?dl=0

-

https://www.dropbox.com/s/2t5cch0qmzkkp0y/Maciço 4 estacas_EC2_v03.xlsx?dl=0

-

https://www.dropbox.com/s/n0i8g7xt562fod7/Consolas curtas_v01.xlsx?dl=0

Hope it helps.

 

[Akeee]

muito obrigado !

 

[Guest262653]

De nada! (You're welcome).

 

[NNAA]

olá avscorreia,
só agora dei conta deste forum e gostaria de lhe pedir, se possível, se me podia facultar as normas que menciona?
- ACI SP208 - Examples for the design of structural concrete with Strut and Tie Models;
- ACI SP273 - Further Examples for the Design of Structural Concrete with Strut-and-Tie Models;
- FIB Technical Report 61 - Design examples for strut-and-tie models;
- Dimensionnement des constructions selon L'Eurocode 2 à l'aide des modèles bielles et tirants - Principles et applications;

agradecido

 

[Guest262653]

These documents and standards are all available from their respective editors.

 

[NNAA]

Thank you (Obrigado)