Do STM struts need to be symmetric about strut axis???

[KootK]

When you're looking at a strut in a strut & tie model, does the strut need to be geometrically symmetric about the axis of the strut? Here's how I've come to wonder about this:

1) To solve a 2-D strut and tie problem, I developed a suitable truss model. The nodes in this truss model obviously define the axes of my struts & ties.

2) Tie layouts and bearing plates etc. define the geometries of my nodes.

3) When I connect all the dots from one node to the next, I end up with tapered struts with cross sections NOT quite symmetric about the axes that I defined in step #1.

Is this okay? In virtually all of the examples that I've reviewed, the struts end up looking pretty close to symmetrical -- at least graphically. That being said, it seems as though it would be a pretty big fluke for them to truly be symmetrical about the truss model axes.

I worry that, if the centroids of my struts are not coincident with the member axes of my truss model, there will be moments in the struts that haven't been accounted for.

Anybody know the answer to this? So far, I can't find anything that says a strut HAS to be concentric about it's longitudinal axes...

 

[Tomfh]

Any chance of posting a sketch?

 

[OCI]

That's an interesting question.

 

[ishvaaag]

Obviously if you are reducing a 3D problem, and lack of simmetry in the struts cause this to be the case, you won't be capturing the effects out of plane. Any action outwards the plane needs be modeled part of the strut and tie model, be it through complementary struts, stiffness or bending action as you say etc.

In floors within the arrangements of codes this shouldn't be a problem in general, this meaning a more accurate analysis be showing the thing needs scarce changes or not any, this mainly out of the general engagement of all the structural model in a widespread structure, but was the strut and tie something isolated as a fork-like support etc and the forces high it could more likely lead to problems, or at least a deficient design.

 

[PST09]

Thinking about the problem I would tend to agree with your hypothesis that there may be additional moments that you have not modelled.

Would having a slightly unsymmetrical strut not mean that the beam properties (I, R, Z etc) would be slightly different for unsymmetrical struts? Your 2d model accounts for symmetrical struts hence beam properties would be standard.

Have a look at this is might shed some light on your problem

http://courses.washington.edu/me354a/Unsym.pdf

 

[KootK]

Okay, here's a sketch of the situation at hand. That being said, my question is really more about the need for S&T struts to be symmetric about their longitudinal axes in general. When I've reviewed S&T problems in papers and journal articles, they don't seem to be symmetric either. In fact, for all but the simplest models, I think that one would have a difficult time forcing all of the struts to be symmetrical.

Also: I'm considering a 2-D situation where the structure is symmetrical out of the page. The moments that I'm concerned about would be in-plane moments. I realize that, practically, the confinement around the struts would restrain these moments. Still though, it seems to me that, if you're gonna model it as a truss, you need to either stay true to the concentrically loaded members / joints or you need to explicitly account for this some other way.

Thanks

 

[hokie66]

I wouldn't worry too much about symmetry of the compression strut in a mass concrete element like a pile cap. It becomes more important in corbels.

I would concentrate on the anchorage of your tension ties at the node.

 

[dik]

No need to be symmetrical unless very extreme and near edges of the concrete mass.

Dik

 

[AHaddad1]

you can easily eleminate your worries by analyzing at a diagonal (i.e. 45deg) 2D section. The horizontal length of the strut would be increased by a factor of 1.414 where the vertical stays the same.

After calculating the tension tie force at a diagonal in plan you can project that force on both axis to see how much top reinforcement you need in both directions. If the horizontal force at a 45deg angle is F, then the projected force in each direction would be F/sqrt(2) or F/1.414

The development length of the tension tie bars in a pile cap is what you need to worry about the most in making sure there is enough horizontal length for the bar past the center of the pile before the 90 degree bar hook, the latter would determine the final size of the pile cap + 3 inches for pile installation tolerance, I like to add 4" instead of 3"

GLTU

 

[hokie66]

In KootenayKid's case, with uplift on the piles, he not only needs to develop the horizontal (top) bars, but the pile bars need to be anchored at or above the node.

 

[AHaddad1]

Definitely hokie66, The pile vertical dowels should be hooked above the top horizontal reinforcement.

 

[KootK]

Yeah, we might drop the top steel down a bit just to ensure that we can get the requisite development on the pile rebar.

We do stress checks on the struts assuming a uniform stress distribution. Surely, if it's okay to have an eccentric strut and therefore a non-uniform distribution, this should be spelled out explicitly somewhere in the code??

 

[AHaddad1]

The Strut and Tie method is clearly explained under Appendix A of the ACI code, I like to call it the tied arch method.

As long as your span to depth ratio is less than 4, the deep beam analysis aka strut and tie method as defined in Appendix A of the ACI 318 applies (I do not have the book with me here, I am going by memory) I believe assuming an average stress on the compression strut is acceptable (its been a while)

 

[hokie66]

KK,

Another truss model for your application would be to use tension diagonals rather than compression. From the pile tops up to the centre, then back down to the opposing diagonal pile.

 

[KootK]

Interesting suggestion hokie. That means diagonal steel though, right?

 

[hokie66]

Yes, but it could all be bent in the shop in roughly triangular shapes, made so one fits through the other. That way the only compression struts you have are the vertical one in the middle and horizontal ones across the bottom.

 

[dik]

FYI

 

[dik]

ditto...

 

[KootK]

Thanks so much for the papers dik -- that's a great score. They're even Canadian too!

 

[dik]

The methodology works for most codes... just a matter of modifying it.