Strut and tie method maximum angle of truss 5

[ENGUCR]

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

 

[KootK]

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.

 

[sticksandtriangles]

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.

 

[rapt]

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.

 

[hokie66]

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.

 

[ENGUCR]

@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

 

[hokie66]

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

 

[ENGUCR]

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

 

[hokie66]

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.

 

[KootK]

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.

 

[ENGUCR]

I am referring to the CEB FIP 1990 model code. I have attached the the snap shot of it. Clau 6.8

 

[KootK]

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.

 

[hokie66]

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

 

[dik]

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

 

[rapt]

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.

 

[KootK]

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.

 

[rapt]

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!

 

[ENGUCR]

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

 

[hokie66]

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

 

[ENGUCR]

In terms of what hokie66. Complexity???