Strand7 modelling 1

[divyeshrohit]

Hello friends,
I am using Strand7 for the first time, I need to model a 2 storey 2D structure with isolated footings, can anybody help me in modelling isolated footing.

 

[IDS]

Can you give us a bit more detail.

Are you wanting to model the soil or just the structure?
Either way, which particular aspects are you wanting help with?

At the moment I'm not sure what problems are presented by the footings, different from the rest of the structure.



Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[divyeshrohit]

thank you for your reply, I am trying to model a 2D structure for example 2 storey building with isolated footings along with soil, and I want to change the soil properties around the footings and see the change in response spectrum.

 

[glass99]

I assume you are modeling both soil and structure in the one model with solid elements for the soil and beam elements for the structure. What kind of isolation do you have? You can model link elements in strand with friction properties. In fact, link elements have many various properties you can ascribe. But fancy link elements will not do you any good if you are running a linear analysis like a response spectrum. Non-linear behavior has to be approximated with linear spring stiffnesses in RSA and other similar analysis.

 

[IDS]

First have a look at:
ST7-1.10.20.13 Response Spectrum and Power Spectral Density
(You will need to be a current supported user to download)

Also have a browse of the other web-notes at the Strand7 site, there may well be something on modelling base isolation systems.

For a 2D model I would suggest plane strain plate elements for the soil and footings and beam elements for the structure. I do a lot of soil-structure interaction work, and usually use 8-noded plate elements. As noted by glass99, if you are doing a response-spectrum analysis you don't need to worry about non-linear material properties, but the stiffness does need to be appropriate to the strains you are getting.

I don't know whether or not a response spectrum analysis is appropriate for a base isolation system; a post in the seismology engineering section might be best on that question.

Any Strand7 specific questions I'll be glad to help.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[glass99]

IDS - the big problem I always ran into when I did seismic analysis a lot was lack of a proper time history ground motion which is essential for NL analysis. You can pull response spectra from building codes more or less, but time histories required a bigger effort to obtain. This lack of data prevents you from properly designing base isolation or damping systems, so you have to do it the old fashioned way and design massive foundations.

 

[IDS]

It's quite some time since I did any serious seismic analysis work, and that was response spectrum based, but aren't the time histories of many large earthquakes freely available?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[glass99]

IDS: if you want Northridge time history its freely available, but you are supposed to modify the time history for your soil conditions and being somewhere other than LA. I am used to needing a specialist seismic geotech engineer to provide the basic input data. At least this was what we had to do when we did bridge seismic analysis in New York. The footings and substructure was always dominated by seismic, and you could save a bunch of construction by doing a time history analysis, but to do so we had to hire this guy to tell us the ground movements. That was like 10 years ago now, so maybe things have changed.

 

[divyeshrohit]

IDS: actually here i want to see the change in site response that is amplitude v/s frequency and acceleration v/s time by changing the soil properties. stiffness, modulus etc. for a simple structure with isolated footings.

 

[divyeshrohit]

I have given below the image of a structure with pile foundations similarly i want to use isolated footings instead of pile foundations. please help me to design it in Strand 7

 

[glass99]

What is an "isolated footing"? Is it a regular footing with an isolation bearing, single pile with no pile cap, or a pad footing?

 

[IDS]

What is an "isolated footing"? Is it a regular footing with an isolation bearing, single pile with no pile cap, or a pad footing?

My guess is a pad footing, but whichever it is, the answer to the question of how to model it in Strand7 is the same way as any other finite element package:

!) Create the mesh
2) Define boundary and global freedom conditions
3) Define element properties
4) Apply loads
5) Run analysis
6) Extract results
7) Review to see if they make sense.

To give any more detailed advice, without writing a book, we need to know exactly what problems have been encountered in following that process.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[divyeshrohit]

i have added the link for isolated footing, these types of footing are most commonly used india.

 

[yakpol]

Typically footings are modeled with the 6 springs boundary conditions. Most widely used is Method 1 from FEMA 356. See attached.

Regards,

http://files.engineering.com/getfil...a0aa57e&file=FEMA_356_Foundation_modeling.pdf

 

[IDS]

Typically footings are modeled with the 6 springs boundary conditions. Most widely used is Method 1 from FEMA 356. See attached.

I'm sure that's how many people still do it, but I really don't see the point of using springs (with the added complication of how to determine the spring stiffness), rather than just modelling the soil with plane strain plate elements.

divyeshrohit - if you tell us specifically what problems you are having we can probably help.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[yakpol]

Doug, including soil elements makes model non-linear making linear RS analysis impossible. The springs are very simple to calculate using a spreadsheet, all you need is a shear modulus, poisson ratio and footing dimensions.

 

[glass99]

As a non-geotechnical engineer, I am always nervous about first principles modeling of soil because I know that soil is not a linear elastic material, so basic properties are hard to come by. I feel like there is a lot of engineering judgement to turn "sandy clay" into an Young's modulus. A FEMA formula relieves some of that burden.

 

[IDS]

yakpol - soil elements are only non-linear if you make them non-linear. Linear elastic plate elements are a better model of the actual structure than linear elastic springs, are just as easy to apply, and it is easier to determine the appropriate properties.

glass99 - I don't follow your point. Using springs also requires engineering judgement to determine the properties of the soils at any given location, with the added level of engineering judgement required for allowing for scale effects etc. If you are happy to use a simplified rule of thumb to give you a spring stiffness, why are you not happy with the same approach to give you soil properties?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[glass99]

IDS: Even if a geotech gave be linear soil material values, I would still be nervous about a first principles model. Soil simply isn't linear like steel, and the linearity has limits which have to be respected. I have no idea what those limits are. For example, the lateral stiffness of a pile group is primarily a function of the passive pressure which builds up against the pilecap and piles. There is a wedge of soil being pushed up a hill, then active pressures coming back the other way. A linear plate model does not capture this behavior at all. Non-linear modeling to capture this mechanism is complicated. There may be some back figured "equivalent" young's modulus that kind of gets you to the right answer, but its heavy on fudge factors.

 

[IDS]

All this is true. How is it improved with another layer of approximation by replacing the plate elements with springs?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/