Do STM struts need to be symmetric about strut axis???
Do STM struts need to be symmetric about strut axis???
(OP)
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...
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...






RE: Do STM struts need to be symmetric about strut axis???
RE: Do STM struts need to be symmetric about strut axis???
RE: Do STM struts need to be symmetric about strut axis???
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.
RE: Do STM struts need to be symmetric about strut axis???
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
RE: Do STM struts need to be symmetric about strut axis???
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
RE: Do STM struts need to be symmetric about strut axis???
I would concentrate on the anchorage of your tension ties at the node.
RE: Do STM struts need to be symmetric about strut axis???
Dik
RE: Do STM struts need to be symmetric about strut axis???
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
RE: Do STM struts need to be symmetric about strut axis???
RE: Do STM struts need to be symmetric about strut axis???
RE: Do STM struts need to be symmetric about strut axis???
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??
RE: Do STM struts need to be symmetric about strut axis???
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)
RE: Do STM struts need to be symmetric about strut axis???
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.
RE: Do STM struts need to be symmetric about strut axis???
RE: Do STM struts need to be symmetric about strut axis???
RE: Do STM struts need to be symmetric about strut axis???
RE: Do STM struts need to be symmetric about strut axis???
RE: Do STM struts need to be symmetric about strut axis???
RE: Do STM struts need to be symmetric about strut axis???
RE: Do STM struts need to be symmetric about strut axis???