Learning Strut and Tie model 2

[ahmedt123]

Hello All,

I have recently started reading about the uses and benefits of the strut and tie model. I've perused through some of the older forums discussing this topic and have used some of the sources that professionals have cited.
I have started drawing up my own STM for a deep beam problem, (Simply supported beam with an overhang, and point load applied to overhang) and I am unsure about how to tell the effectiveness of an STM model.
I have come up with two different models, and would like to know how to objectively call one better than the other. One method I have used is running a Non linear FEA on the two models and obtaining force-displacement graphs for both, indicating their capacities. I then related the capacities to the amount of steel used to see the efficiency of either design. Are there any other ways to tell the superior nature of one STM over another other than experience.

Cheers

 

[KootK]

Some desirable features in an efficient STM model:

- Struts should not cross other struts.
- Struts and tie location and orientations should roughly reflect elastic stress trajectories to avoid the need for excessive plastic deformation.
- The angles between struts and ties should not be too shallow (see local code requirements).
- Ties should be set out in a rational manner that reflects typical detailing.
- The model should be complex enough to capture the important behavioral features but not much more complicated than that.

I'm sure that we'll end up with a pretty comprehensive list when all is said and done.

Consider posting your two models for review. It could make for some interesting discussion.



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.

 

[KootK]

This is one of the STM papers that got the whole ball rolling. And, in my opinion, it's still one of the best learning resources: Link. It may not answer your specific question but is probably worth your while if you haven't seen it yet.

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.

 

[hokie66]

The strut and tie modelling of concrete is analogous to trusses in other materials. So I try to think of a steel truss. The main difference is that in steel, the most efficient designs have the diagonal web members in tension, while in concrete the diagonals are usually in compression, as we like to place ligatures normal to the member rather than inclined.

 

[ahmedt123]

Thanks for the help,

KootK, here are 2 models I came up with for this simple problem. There are no values added, except for the inclination angle of the strut, which is 50 degrees.
I would just like some qualitative advice on which one is better. I know the one with orthogonal Ties is easier to detail, but what other points are there as to which one is better.
Ties are shown in red, and struts are dashed lines.

PS. I have taken a look at the book, helpful in drawing up the models.

 

[KootK]

I would say that:

1) model #1 has a leg up when it comes to rational reinforcing layout.

2) model #2 has a leg up when it comes to matching elastic stress trajectories.

3) Point #1 weights more than point #2 so I vote for model #1 as the preferred option.

Take my stress trajectory diagram with a grain of salt. I suck at making them.

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.

 

[TehMightyEngineer]

I'll throw in with Kootk.

Model #2 seems to match my experience with how the stresses will naturally want to be distributed. Because model #1 will "force" the stresses to make that hard 90° then your strut/tie loads will be a lot higher in model #1 vs model #2.

However, I agree with KootK that this will be a more practical rebar layout and also better represent the large compressive forces at the middle support. I'd go with model #2.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[JLNJ]

The thing that gave me the most fits in the last one of these I did was massaging the geometry so that I could properly anchor the ties to and through the node.