STRUCTURAL ENGINEERING FORUM OF INDIA [SEFI]

abhio · ... Joined: 08 Mar 2010 Posts: 545

Piled rafts are often used for tall buildings. I would request seniors to clarify some doubts that I have faced when proposing pile groups:

Today every large structure is analysed and designed using stiffness method programs with detailed study by FEA where appropriate. However, only specialised consultants are carrying out the analysis of piles/ pile groups in realistic geotechnical FE models. This leads to the following questions:

1. How is the pile-soil interface modelled in these geotechnical models?
2. Is it possible to use pile stiffness calculated from geotechnical models in analysis of the foundation raft and superstructure?
3. Is it possible to model the interaction between different piles in a pile group and between the pile cap/ soil and piles?

I would be grateful if anyone can provide literature to explain these topics.

padamkr · SEFI Member Joined: 26 Jan 2003 Posts: 4

Dear sefians

Further to the queries raised by Mr.� A S Oundhakar , I would like to ask ,

Is there any difference between analysis of Piled Raft and Pile Assisted Raft ?

Regards

Padam Kumar

ENGINEERING SOLUTIONS - DELHI

On 20 November 2012 11:42, abhio forum@www.buonovino.com)> wrote:

jshukla · Progressive Member Joined: 16 Jun 2008 Posts: 45

I feel happy that questions like these are asked in the forum...

Let me go one by one..

1. How is the pile-soil interface modelled in these geotechnical models?

They are modeled using finite element methods i.e. PLAXIS 2D/3D or similar softwares (there are many to list) or Finite difference methods i.e. FLAC 2D/3D. The interfaces are modeled using special interface elements since the diplacement and strength along these elements are specially handled by interface frictions.

2. Is it possible to use pile stiffness calculated from geotechnical models in analysis of the foundation raft and superstructure?

Yes to some extent. Since the displacement demand from superstructure to pile does not have one to one relation in those software, the precise modeling is in questions. However, I have seen some presentation of GSA-RAFT where the soil profile/pile displacement has direct relation with superstructure analysis and iterative procedure is handled within the software to get the displacement competencies.

3. Is it possible to model the interaction between different piles in a pile group and between the pile cap/ soil and piles?

Yes but only possible in the software that involves geotechnical analysis i.e. PLAXIS, FLAC, CRISP but difficult with structural software. However, it can be handled out side the software domain by some multiplying factors but requires geotechnical consultations.

4. Is there any difference between analysis of Piled Raft and Pile Assisted Raft

Yes. Piles in the later case will largely act as settlement reducing rather than carrying large loads. It will be difficult to explain you the main concept here in this post however you may refer to the the ISSMGE TC 18 report which is already shared to this forum in one of the post.

I hope it helps.

regards,

jay

jshukla · Progressive Member Joined: 16 Jun 2008 Posts: 45

Dear Friends,

Those who are interested in the piled raft analysis may look at following link which is considered to be important ref. Difficult to practice in India but things are on the way and may come to India in near future...!!!

Its thesis which elaborates the procedure for analysis of piled raft with piles of different length.

http://sydney.edu.au/engineering/civil/publications/phd_thesis_chow.pdf

I hope it helps.

regards,

jay

thirumalaichettiar · Silver Sponsor Joined: 26 Jan 2003 Posts: 3552

Dear Er.Jay,

Happy to know that you are actively participating. Thanks since we would like to learn from your earned knowledge on Soil engg.

T.RangaRajan.

B.V.Harsoda · Posted: Mon Nov 26, 2012 9:54 pmPost subject:

Research Papers links:-
Studies of site response and soil-structure interaction effects in a tall building in Mexico City
http://www.iitk.ac.in/nicee/wcee/article/11_569.PDF

SOIL-STRUCTURE INTERACTION EFFECTS IN TWO INSTRUMENTED TALL BUILDINGS
http://www.iitk.ac.in/nicee/wcee/article/13_1911.pdf

Regards,
Er. B. V. Harsoda

Econf_Moderator · E-Conference Moderator Joined: 26 Feb 2012 Posts: 23

Forwarded message....

***************

My replies provided between ** � ** � below. �

Thanks. �
Jaydeep Wagh

Geotech Consultant

Piled rafts are often used for tall buildings. I would request seniors to clarify some doubts that I have faced when proposing pile groups:

Today every large structure is analysed and designed using stiffness method programs with detailed study by FEA where appropriate. However, only specialised consultants are carrying out the analysis of piles/ pile groups in realistic geotechnical FE models. This leads to the following questions:

1. How is the pile-soil interface modelled in these geotechnical models? �
* *届e pile soil interface strength can be reduced in FEM software Plaxis by specifying the interface strength reduction factor. **

2. Is it possible to use pile stiffness calculated from geotechnical models in analysis of the foundation raft and superstructure? �
* *从岩土桩刚度计算models or analysis is used for further structural analysis of foundation raft by structural engineers FEM software (eg. Safe).**

3. Is it possible to model the interaction between different piles in a pile group and between the pile cap/ soil and piles?�
* *�桩之间的相互作用be manually calculated by charts available for pile group action in most geotechnical books. �These charts are typically provided for uniformly or evenly spaced piles and uniform loading on piles. �The interaction between piles and pile cap can be analyzed manually by methods provided in "Pile Design" by H. Poulos. �Manual calculations may become highly tedious to use for multi layered soil profile and/or for complex loading conditions or for non-uniform pile layouts. �As mentioned in "Pile Design" also, finite element softwares provide a more accurate tool for modeling the interaction between different piles in a pile group and between piles/pile caps. � �**

Posted via Email

knsheth123 · Silver Sponsor Joined: 26 Jan 2003 Posts: 123

Dear Er. Jaydeep Wagh,

Continuing the discussion on Pile Soil Interaction� initiated by� Er. A S Oundhakar:

1. How is the pile-soil interface modelled in these geotechnical models? �

* *届e pile soil interface strength can be reduced in FEM software Plaxis by specifying the interface strength reduction factor. **
The most important is how to derive interface Strength/Stiffness model. What are the guidelines available in the Literature ? Coventionly a Friction Element is used at the Interface.
Please advise for guide lines/literature,� if any to adopt appropriate strength reduction factor for various soil type.�

3. Is it possible to model the interaction between different piles in a pile group and between the pile cap/ soil and piles?�

* *�桩之间的相互作用be manually calculated by charts available for pile group action in most geotechnical books.(Please mention references for charts on Pile - Soil - Pile Interaction)
�� These charts are typically provided for uniformly or evenly spaced piles and uniform loading on piles. �The interaction between piles and pile cap can be analyzed manually by methods provided in "Pile Design" by H. Poulos. �Manual calculations may become highly tedious to use for multi layered soil profile and/or for complex loading conditions or for non-uniform pile layouts. �As mentioned in "Pile Design" also, finite element softwares provide a more accurate tool for modeling the interaction between different piles in a pile group and between piles/pile caps. � �**

K. N. Sheth
Dharmsinh Desai University,
Nadiad

Posted via Email

picajol · ... Joined: 29 Mar 2012 Posts: 65

I a presentation on youtube for GSA-RAFT program analysis wich has somekond of interative process when analysing soil structure interaction.
How is this different from soil structure analysis from programs like STAAD and SAFE?

jshukla · Progressive Member Joined: 16 Jun 2008 Posts: 45

The software developer was having two different modules, one for structural analysis and second one is for settlement estimation. Now it is combined in GSA-RAFT in which pile and soil settlement will be calculated in the geotechnical/ settlement analysis module and then the displacements will be transferred to structural module and so on. The two module will have iterative procedure which will end on the compatible displacement agreement based on structural loading and soil/pile response (here it differs from conventional structural software) . However, the settlement estimation is elastic and mostly based on p-y approach (for pile capacity and soil layering) which are considered to be class 2 predictions. It gives reasonable preliminary estimation but if you want settlement prediction generally for tall building no one relies upon single software. Engineers carry analysis on more than one software and then comes engineering judgement.

I hope it helps.

jay