Help. Tekla Structural Designer Ground Stiffness 1

[ArranB]

HI All,

First time posting here.

I've got an issue with Tekla Structural Designer that i was hoping somebody had experience with. I'm designing a ring beam foundation using the mat feature and one of the properties is 'ground stiffness'. Now I've inputted a spring linear stiffness of 12000kN/m2/m under the advice of the Geotech working on the project. They stated that the higher the value for the spring linear stiffness the better the foundation will perform, however i have found the complete opposite. The higher the number the worse the ground is and the ring beam foundation fails more.

I'm a bit stuck with this and its critical to the foundation design working. I've played about with the numbers ans the spring linear stiffness needs to be about 1000kN/m2/m to work. This goes against what I've been told with regards to high values performing better.

https://files.engineering.com/getfi...0-9be1-56582e235f11&file=Ground_Stiffness.JPG

(hopefully the link works to show screen shot)

Can anybody shine any light on this? Any help will be greatly Appreciated.

Thanks,
Arran

 

[Greenalleycat]

I have no experience with Tekla specifically but do have experience with soil springs in my preferred software

Define 'foundation design working' - we cannot really help unless you give more info as to what is 'not working'
There are multiple parameters at play

Generally, a higher soil stiffness will give you lower foundation demands in exchange for a higher soil bearing pressure
So this is more likely to make your foundation beam 'work' at the cost of your soil 'not working'

A lower stiffness requires the foundation beam to span between soil reactions that are further apart
Hence, foundation demands are higher so less likely to 'work' but with a greater chance of limiting soil bearing pressures to within acceptable bounds
A softer soil (lower spring stiffness) will also cause greater rotations within your superstructure so drift limits could become critical
This could also cause strength issues in your foundation (and superstructure) if you have a building that has significant P-Delta effects from the additional drift

 

[ArranB]

Thanks for the speedy response and help.

The higher ground stiffness figure causes the foundations to fail in bearing.

So for example at 12000kN/m2/m the foundations fail in bearing by a ratio of about 1.5. At 1000kN/m2/m they pass in the bearing with a ratio of 0.9. GBP is 80kN/m2.

It confused me as the Geotech stated that a higher value would help, but it didn't.

Once again thanks for the help.

 

[Greenalleycat]

No worries. I think my response already addressed that so hopefully you're sorted now
This shows the importance of not assuming (from the geotech's side) and seeking clarity (from your side) when making claims about...anything really
Sounds like you'll need to make your foundation wider or look at alternative foundation options (piles say)
Alternatively, you could try be more precise in your model

I don't know if Tekla allows it, but SpaceGass (what I use) has a variety of spring options, including the ability to limit maximum forces on springs
In reality, when you overload soil it's not like it disappears
It just deflects a bit more but can't really go anywhere most of the time as you're probably failing in the middle of the foundation
If there are confining soils etc then you will end up using the foundation stiffness to dissipate load across a larger area of soil
You may find that this is sufficient for your foundation to work when considering real-world performance rather than just the model prediction which shows a hot spot in a single worst-case area

 

[TLHS]

Ask them what the bearing criteria is based on. A lot of time it's deflection and settlement based. If that's the case but your analysis is showing small deflections then you have a basis mismatch between the bearing capacity input and stiffness input. You should talk to the geotech about what you're seeing in your analysis and what it means.

If there's a true bearing capacity where there's a physical bearing failure you should definitely consider that, but make sure that's what the number you've been given represents.