Strut and tie model

[Aldar]

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.

 

[dik]

I would try to change the shape, for a start... The large horizontal load should not have such a nicely constructed 'wedge' without any vertical loading... 'bad' geometry!

Your problem is not so much an S&T problem, but a compressive one.

Dik

 

[Aldar]

the geometry is fixed. i can't change it. i didn't get why it's a bad geometry, could you explain better? vertical loading is the weight of concrete.

 

[BAretired]

What are the units...inches, Imperial tons?

BA

 

[csd72]

Aldar,

The concrete is shaped like a sled in the direction you are pushing!

These things are never fixed, expept when they are college assignments!

 

[ishvaaag]

You can't solve this properly. You have an unbalanced counterclockwise moment around the reaction hinge at left that you can't balance without significant weight of the footing downwards, much than the area normally would provide if say 1 m thick. Faltering a proper scheme of equilibrium, no struct and tie scheme can be meaningful.

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.

 

[ishvaaag]

In short, I undertook the challenge without much want of using my wit, but my means. A thorough inspection of the problem data should have discovered that without any resource to computing power.

 

[ishvaaag]

In short, I used the lazy method of using my means instead of using my wit. A thorough inspection of the problem should have shown that without any resource to computing power.

 

[ishvaaag]

Hmm I didn't see the other post in place, hence the repeat. Sorry.

 

[BAretired]

If this is a homework assignment, then forget it...we don't do homework problems on this site.

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

 

[Aldar]

ok, all the units are cm. the width of the foundation is 4m. The forces are in short tons. the force on the left is the stabilizing force( it will be a T beam). Which i think will depend on how much pressure the soil can take. it's not a homework. Thank you for your help!

 

[BAretired]

the force on the left is the stabilizing force (it will be a T beam)

What will be a T Beam...the force? I don't understand what you mean.

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

 

[hokie66]

I think the T-beam is just what resists the horizontal reaction at the left end (not passive soil pressure). So the 1080 tons, which I will choose to convert to about 9800 kN, is just a compressive force in the top of the slab. If you resist this by a column 350 deep (I don't think engineers should use centimetres) x 4000 wide, the stress is about 7 MPa, so manageable.

So it seems to me that the block on the right is where you would employ a strut and tie model. Is that correct?

 

[BAretired]

Where the @!%** is the T Beam? Would somebody please clarify? I don't see a T Beam.

BA

 

[hokie66]

I don't know, but I think it is part of a structure external to the block we are seeing, and provides the reaction across the 4 metre width of the 350 deep section. Must be a big T-beam, but he didn't ask for advice about that.

 

[BAretired]

Aldar,

What exactly is your problem? Explain it properly if you expect any kind of useful input.

BA

 

[Aldar]

Ok, first of all. calm down =)
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_?

 

[ishvaaag]

No soil friction. Turns around support at left. It is not stable.

 

[hokie66]

But ishvaaag, the resisting moment of the concrete mass about the left support is greater than the overturning moment due to the 774 ton load, so why do you say it is unstable?

 

[Aldar]

i don't understand why you think it's unstable! it's a real life example. i just need to understand how the applied force is distributed inside the block, taking into account soil and left beam support.