Strut and tie model
Strut and tie model
(OP)
Hi to all,
could someone help me with the strut and tie model. I need to design a big foundation. I attach the picture of the problem. thank you.
could someone help me with the strut and tie model. I need to design a big foundation. I attach the picture of the problem. thank you.






RE: Strut and tie model
Your problem is not so much an S&T problem, but a compressive one.
Dik
RE: Strut and tie model
RE: Strut and tie model
BA
RE: Strut and tie model
The concrete is shaped like a sled in the direction you are pushing!
These things are never fixed, expept when they are college assignments!
RE: Strut and tie model
This is not magic, but seeing the shape with the loads on compression only spring with the area without weight doesn't converge to a solution. It would if equilibrium was feasible, then through inspection one would see the compressive and tensile struts.
RE: Strut and tie model
RE: Strut and tie model
RE: Strut and tie model
RE: Strut and tie model
If it is not homework, then give us an idea how big the foundation is by telling us what your units are. I'm guessing the dimensions are in inches and the loads are in Imperial tons. Is that correct? How wide is the foundation normal to the page?
Hint: Before worrying about strut and tie forces, you should solve for all external reactions.
BA
RE: Strut and tie model
RE: Strut and tie model
Your net applied force is 1854 - 774 = 1080 tons to the left. That means the horizontal reaction required is 1080 tons to the right. That can be carried by a combination of passive pressure and friction.
If, instead of the sloping bottom, you had a rectangular block 12m x 2.5m x 4m, its volume would be 120 m^3 neglecting the 80x160 trench. At 24 kN/m^3 it would weigh 2880 kN (324 tons). With a coefficient of friction of 0.3 between concrete and soil, you can resist 97 tons by friction.
The end bearing area would be (2.5 - .35)4 = 8.6 m^2. The maximum passive pressure available is about 49 tons. Total horizontal resistance available = 97 + 49 = 146 tons << 1080 tons.
The foundation cannot resist the applied forces, so further calculations are meaningless.
BA
RE: Strut and tie model
So it seems to me that the block on the right is where you would employ a strut and tie model. Is that correct?
RE: Strut and tie model
BA
RE: Strut and tie model
RE: Strut and tie model
What exactly is your problem? Explain it properly if you expect any kind of useful input.
BA
RE: Strut and tie model
on the left of the picture the reaction force R is shown. Which is reaction force coming from a T beam. The T beam is not shown on the picture, but represented as R force. The concept is to decrease some of the 1080 to the left by soil friction and bearing capacity, and the remaining force to apply to the T beam. Was i clear_?
RE: Strut and tie model
RE: Strut and tie model
RE: Strut and tie model
RE: Strut and tie model
RE: Strut and tie model
RE: Strut and tie model
Anyways the stability of the foundation is not a question. The question is the flow of stresses inside.
P.S: The T beam will take only the compression forces.
RE: Strut and tie model
If you place the remaining dimensions, with the thickness etc we can see if stays stable or not.
Respect the strut and tie scheme, once stable, you will be guided to a one close good approximation by looking the principal stresses then developing. You may still then establish some others that are also feasible, but further from the real behaviour.
The effect of friction will need to be mimicked either through forces or the more suitable devices provided by the software for boundary conditions. The produced strut and tie schemes will be strongly dependent on the assumptions for the boundary conditions. Then in order to properly produce the model, we need also the properties of allowable soil strength, and friction at the interface, plus if the loads are assumed factored or not etc.
RE: Strut and tie model
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Strut and tie model
Yes, you were clear in describing your concept, but unless we know how much the T Beam deflects under lateral load, we don't know how much force goes to the soil in friction and passive pressure. If the T Beam is very rigid, the soil forces will be negligible.
BA
RE: Strut and tie model
Since I by whatever the reason have some custom of being a dissonant voice sometimes, I also point to the fact of that strut and tie, useful that it maybe, it is, and may be more, entirely superseded by proper FEM analysis. Any strut and tie model is shown to be only a guess (it maybe a simplifying and interesting one) but of less interest than the other, if only because the former is less in accord with the laws that rule the behaviour of the structural materials than the other. I in particular (with the most simple exceptions as say deep footings, caps or beams) dislike the complications brougth by some practical rules to find the scheme in equilibrium, whereas you can do better the other way.
RE: Strut and tie model
RE: Strut and tie model
Are there any better ideas?
BA
RE: Strut and tie model
RE: Strut and tie model
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On the left of the trench, if you are going to attribute some of the resistance to the soil, perhaps you should add Strut a-c.
BA
RE: Strut and tie model
BAretired: There isn't anything wrong, conceptually, with struts and ties overlapping. It just means that, depending on the load conditions, that area could have a load reversal. When he does his truss analysis for the load cases he needs to check it should clarify that area. It may be that it never reverses and it's always tension or compression and he just needs to check worst case, or it may be that he needs to design it as a strut in one case and a tie in another.