Strut & Tie Modelling: Nodal Stresses & Mohr's Circle
Strut & Tie Modelling: Nodal Stresses & Mohr's Circle
(OP)
Hi All,
I'm doing a quick strut and tie analysis and ran into an issue with the numbers that I can't seem to get my head around.
See attached rough hand calcs.
I'm working out the stresses at a non-hydrostatic node, in my first trial I have a support width of 282.3 mm. I work out the principal compressive stress in the node to be 27.2 MPa, which exceeds the allowable stress of 21.6 MPa.
I then increase the support width to 500 mm to check to see if this works. I adjust my geometry and again go through working out the principal stress. As you can see, my guess of 500 mm results in the normal stresses on the two faces being almost equal to each other. This results in a Mohr's circle with an origin of 65.7 MPa and a radius of 54.1 MPa. The maximum principal stress is 119.8 MPa and minimum is 11.6 MPa - what's going on here???
Is someone able to explain this anomaly, or more likely, point out the error in my ways?
Thanks in advance.
Robbie
I'm doing a quick strut and tie analysis and ran into an issue with the numbers that I can't seem to get my head around.
See attached rough hand calcs.
I'm working out the stresses at a non-hydrostatic node, in my first trial I have a support width of 282.3 mm. I work out the principal compressive stress in the node to be 27.2 MPa, which exceeds the allowable stress of 21.6 MPa.
I then increase the support width to 500 mm to check to see if this works. I adjust my geometry and again go through working out the principal stress. As you can see, my guess of 500 mm results in the normal stresses on the two faces being almost equal to each other. This results in a Mohr's circle with an origin of 65.7 MPa and a radius of 54.1 MPa. The maximum principal stress is 119.8 MPa and minimum is 11.6 MPa - what's going on here???
Is someone able to explain this anomaly, or more likely, point out the error in my ways?
Thanks in advance.
Robbie






RE: Strut & Tie Modelling: Nodal Stresses & Mohr's Circle
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Strut & Tie Modelling: Nodal Stresses & Mohr's Circle
1) It would be great to see the STM model of the entire structure under consideration for context.
2) It would be great to see the node in the context of it's surrounding struts, ties, and supports for context.
3) I've never seen a node with shear on it.
4) A triangular node generally has compression on all three faces.
5) What country's code is being used? I've not encountered a Mohr's circle approach in the past.
I suspect that rectifying #3 and #4 will go along way to resolving the issues.
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 & Tie Modelling: Nodal Stresses & Mohr's Circle
Thanks for your help, for some reason it took posting this to realise that one of the forces I was using was incorrect
Nevertheless, here are some answers to your questions:
1) See attached screenshot of the strut and tie model - basically a cantilever wall.
2) I've also zoomed in on the node and shown the CORRECT forces - I would be interested in your take of how I treat nodes with more than 3 forces as i'm fairly new to this type of analysis.
3) The shear arises from the fact that the surface of the node is not perpendicular to the strut force (non-hydrostatic). The strut force therefore is broken down into a normal force and a shear force.
4) In this case I have five compression forces entering one node, hence I have a pentagonal node - I break this down into 3 triangles to allow me to work out the principal stresses in the node.
5) I'm based in Australia and therefore am using AS3600-2009. As far as I understand it, when evaluating the capacity of a node, you have to make sure that the principal compressive stress is less than the allowable stress in the concrete. Mohr's circle is used to work out the principal stresses with a non-hydrostatic node.
Just out of interest, I did a quick analysis of my triangle using both the correct and incorrect forces and varied the width of the reaction (see attached graphs). Interestingly the correct forces resulted in a fairly smooth graph, but using the incorrect reaction resulted in some vertical asymptotes - hence why I was getting huge stresses!
Interested in your thoughts regarding the above
Robbie
RE: Strut & Tie Modelling: Nodal Stresses & Mohr's Circle
RE: Strut & Tie Modelling: Nodal Stresses & Mohr's Circle
1) Simplicity. No CCCCC nodes.
2) Rational reinforcement layout. No need for diagonal reinforcing at member one.
Below, I've shown your model and mine for comparison's sake.
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 & Tie Modelling: Nodal Stresses & Mohr's Circle
Robbie.
RE: Strut & Tie Modelling: Nodal Stresses & Mohr's Circle
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 & Tie Modelling: Nodal Stresses & Mohr's Circle
RE: Strut & Tie Modelling: Nodal Stresses & Mohr's Circle
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.