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Strut and tie method maximum angle of truss

Strut and tie method maximum angle of truss

(OP)
Generally the truss angle of strut and tie system is restricted to a maximum limit about 60 deg. What happen if someone goes beyond this upper bound limit. What is the concept behind max limit? I am struggling with a deep pile capping beam where the strut and tie system angle is almost about 75 degress
Thanks

RE: Strut and tie method maximum angle of truss

I believe that the maximum angle is intended to result in struts oriented roughly in line with anticipated diagonal tension shear cracks. I don't feel that the limit applies to cases like yours where the strut angle is predefined by geometry to run from the load above to the pile below. Applying the limit in that scenario would be akin to prohibiting direct bearing which would be nonsensical.

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.

RE: Strut and tie method maximum angle of truss

If this was my case, this would tell me I needed to add more tie locations to break down this angle. Convert your strut and tie model to have more nodes. This often necessitates a vertical tie, which is often not done in pile cap designs.

RE: Strut and tie method maximum angle of truss

Are you talking about shear Truss analogy limits or Strut Tie limits.

In Shear Truss analogy, the upper limit is often 60 degrees, but that is for flexural members.

For Strut Tie, the strut can technically be vertical! The maximum angle of the strut from the vertical is normally about 60 degrees. Any shallower than this and it basically becomes flexural.

RE: Strut and tie method maximum angle of truss

Some clarification is needed here. In strut and tie analysis, the applicable angle is always between the strut and the horizontal...unless the load orientation is different from vertical.

So the important limitation on angle is a minimum limit, not maximum.

RE: Strut and tie method maximum angle of truss

(OP)
@KOOTK & Hokie66
I am afraid that whether the ties at bottom of the cap will be formed. Doubt that when the strut angle is becoming more and more, the ties will become mobilized.

Other than that my another point is that even any space truss with much steeper angle, is unstable and we don't practice

@rapt
I am referring to a strut and tie system

@stickandtriangles
Thanks but your system with vertical tie is not practical in pile caps. Identifying of vertical ties in a big chunky rc volume is not practical

RE: Strut and tie method maximum angle of truss

anushka,
I don't believe you understand truss behaviour. The steeper the angle of a compression member, the smaller the horizontal force.

RE: Strut and tie method maximum angle of truss

(OP)
Sorry i have used wrong word. It's ineffective. As the tie takes less tie force compared with the strut force. So isn't it ineffective as the rf carry less force and imaginary strut carry comparatively larger compression forces. Would this would be the reason behind this upper bound limit

RE: Strut and tie method maximum angle of truss

I don't know where your upper limit comes from. There is a minimum angle between strut and tie, usually specified as 25 degrees, as per ACI. There is no logic to a maximum angle, as KootK pointed out above.

RE: Strut and tie method maximum angle of truss

Not asking much of the tie is a good thing. Taken to the extreme, the strut would be vertical and you'd need no tie at all. I'm afraid that I don't understand your stability concern.

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.

RE: Strut and tie method maximum angle of truss

The code clause actually reads as being non-mandatory with regard to maximum strut angle. And, as I mentioned above, I don't think that the recommendation rightfully applies to this situation anyhow.

I'd be curious to know more about the method of model selection based on energy principles mentioned in the clauses that you posted. Sounds like some sweet, sweet intellectual Halloween candy to me...

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.

RE: Strut and tie method maximum angle of truss

That is quite an old document, and was in the early days of formulation of strut and tie principles. Why not use later recommendations?

RE: Strut and tie method maximum angle of truss

I think the use of energy modelling would apply to strut&tie models where the model had continuity and may be indeterminate, like with truss systems. Don't know, I've never encountered one.

Dik

RE: Strut and tie method maximum angle of truss

Anuska,

What you have printed is commentary, not code limits. The text on the right is the Code and you have not printed that. Greater than 60 is not a problem, up to 90 degrees which is a straight through compression without tie required. Less than 45 degrees is a problem as it is then too low an angle and the tie forces and strut compression stress become unmanageable as it approaches a flexural design.

RE: Strut and tie method maximum angle of truss

My understanding is that the shallow angle limits have much to to with strain compatibility. Or strain incompatibility as the case may be. Once the strut and tie are darn near parallel, it becomes tough to argue that one is in compression and the other tension.

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.

RE: Strut and tie method maximum angle of truss

When they are parallel the compression strut force and the tie force are infinite! So physically impossible

Same as a tied compression arch. There is a limit beyond which thou shall not go as the arch can easily snap through and it becomes a tension membrane structure!

RE: Strut and tie method maximum angle of truss

(OP)
Thanks all. It's a good conversation. In addition mean while I found a reason behind this upper limit from a literature article that it's used to distinguish a poor and a good representative model. So it's not mandatory. To share it with you I have attched it. Thanks again

RE: Strut and tie method maximum angle of truss

I can't say that I find the "good" model any better than the "bad" model.

RE: Strut and tie method maximum angle of truss

(OP)
In terms of what hokie66. Complexity???

RE: Strut and tie method maximum angle of truss

I think that the "good" model represents the stress distribution in the member in reality more accurately.
It anushka's example, the D regions are at the point load at the top of the wall and the 2 supports. The compression stresses spread out from the point load down through the wall, then in the middle of the wall the compressive stresses will be fairly uniform, before the stresses concentrate to the supports.
I quick FE model would show this behaviour.

RE: Strut and tie method maximum angle of truss

For my own fun I did a quick FEM. As others expected the good model seems to capture the behavior of linear elastic analysis pretty well.

Vertical stress picture (notice struts form in a way consistent with "good model"


Horizontal stress picture (notice tie where expected in "good model"

RE: Strut and tie method maximum angle of truss

I think that the fundamental difference here is that the good/bad stuff is a load spread problem where, in all likelihood, the pile cap situation is not (a sketch would help to confirm). With the pile cap situation, the load has no real cause to change direction significantly. You do get some spreading and re-concentrating of the load which is related. In some codes, that is covered by bursting stress provisions in bottle shaped struts. People complain about it all the time here as it's a detailing pain. If I recall correctly, I think that the codes of both the UK and AU have such provisions. This in no way limits your strut angle, however, because you could have a bottle shaped strut running off in any old direction.

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.

RE: Strut and tie method maximum angle of truss

From what I see of the first FEM picture, the "good model" lines bear no relationship to the stress contours. Someone drawing those lines on it is dreaming!

Based on the "good model" you need horizontal tension reinforcement at about 20% of the depth from the top, at the kink from slope to vertical. Personally I think it is a waste of good reinforcement to put tension tie reinforcement there!

Yes, there may be bursting effects near the top where the compression stresses spread out. Yes some side face reinforcement over the full depth of the section if a beam. But tension tie reinforcement at 1/4 depth!!

RE: Strut and tie method maximum angle of truss

Agree, rapt. I see no merit in the "good model". As KootK said, the load has no reason to change direction, so its most likely direction is straight between points of loading and support. Sometimes, I think these new-fangled finite element models just cause confusion in interpreting how a structure actually works.

RE: Strut and tie method maximum angle of truss

Hokie,

Agreed, stresses do not change angle suddenly without a good reason, and I cannot see one there! Maybe a gentle curve in the compression strut, but sudden angle changes. Someone is smoking something that is not good for their logic thought processes!

RE: Strut and tie method maximum angle of truss

I think that the "good" model and the FEM results both have merit. Below, I've tinkered with proportions and orientations to create examples where I feel that the "good" model aligns well with intuition (mine at least) and what is often found in the literature. None of this changes my opinion of the original pile cap issue however.


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.

RE: Strut and tie method maximum angle of truss

I think the difference is that bursting at a post-tensioned anchor is a much more concentrated force in a relatively thin element than bearing of a column on a big footing/pile cap. Confinement is the issue.

RE: Strut and tie method maximum angle of truss

Quote (rapt)

Someone drawing those lines on it is dreaming!

Haha thanks rapt. I agree the vertical lines are questionable between the "good" and "bad" model.

The main thing that made me like the relationship between the "good" model and linear elastic FEM was the presence of horizontal tension in the FEM model (horizontal stress picture). This is not captured in the "bad" model.

RE: Strut and tie method maximum angle of truss

HGokie,

Agree the PT busting effect is very different. It is the dispersion of a very concentrated compression at the anchorage over the full width of the section causing transverse bursting stresses over a length of the member. It is very different to a pile cap situation.

RE: Strut and tie method maximum angle of truss

It's different to the pile cap situation. It's not that different too the good/bad model situation. That was my intended point.

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.

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