STRUCTURAL ENGINEERING FORUM OF INDIA [SEFI]

prof.arc · ... Joined: 26 Jan 2003 Posts: 703

If one does not model the foundation & soil but considers only the
superstructure over the foundation, then forces on foundation & soil,
could be worked out based on static check.
The external forces applied on the superstructure are ONLY vertical -
distributed over space for dead and live loads. This will result in
vertical reactive loads on the foundation & soil. The CG of applied
vertical loads will coincide with the CG of reactive loads.
A similar argument holds good for horizontal loads. The horizontal
loads applied at a height will result in horizontal reaction and as
upward & downward reactive forces due to moment caused by the lever
arm of horizontal force.

ARC

On 1/5/11, truenotesrakesh <forum@sefindia.org> wrote:

rg.gupta · ... Joined: 26 Jan 2003 Posts: 94

DEAR ARC
YOU HAVE RIGHTLY SAID BUT IT IS VALID ONLY IF THE DESIGN IS DONE AS
RIGID FOUNDATION(CONVENTIONAL FOUNDATION) NOT AS FLEXIBLE FOUNDATION
DONE BY COMPUTER USING SUBGRADE REACTION OF SOIL.
CAN ANY BODY DESIGN THE FOUNDATION INCLUDING ISOLATED ,STRIP & RAFT AS RIGID BY THE USE OF COMPUTER ?
REGARDS
R.G.GUPPTA

hemal · ... Joined: 01 Apr 2008 Posts: 129

Dear R.G.Gupta Sir,

Most of the structure design software treats ISOLATED FOOTING as rigid (uses SBC for design) and RAFT FOUNDATION as FLEXIBLE FOUNDATION (uses SOIL SUBGRADE REACTION for design).

Long ago, STAAD was having both (RIGID & FLEXIBLE i.e. SBC or Ks) options for RAFT FOUNDATIONS, but present versions only have FLEXIBLE option. Actually, as per my knowledge, design of RAFT foundation using RIGID method will be uneconomical (we can tell it conservative).

Regards
Hemal Mistry

--- On Thu, 6/1/11, rg_gupta <forum@sefindia.org> wrote:

VPandya · General Sponsor Joined: 09 Nov 2009 Posts: 842

Dear  SEFIN'S,
May be I do not understand the question. Using modern day SOFTWARE'S  like  STAAD/PRO , RISA-3D  and  SAP-2000 there is no problem of  doing AALYSIS and DESIGN of :
a) Flexible Raft Foundations.
b) Rigid  Raft Foundation.
c) Applying Super-Structure Moments due to Lateral Load on  Super Structure.

Recently I did 100ftX45ft X 3 ft  thick  Raft Foundation supporting 4 Liquid(Oil)  and
Water containing Tanks.   I use  PLATE AND SHELL ELEMENT  OF  RISA-3D (Same in STAAD/PRO) to Model Concrete, Spring Element to Model Soil Springs (Winkler Springs). Applied Overturning Moments as Upward and Downward Forces at the nodes  of junction between Tanks and Raft.  You can nicely apply Loads in a Circular fashion for Circular Tanks.
Using real thickness of Concrete raft with PLATE AND SHELL ELEMENTS with Soil Spring Elements , Computer takes care if  RAFT is FLEXIBLE  or  RIGID.
In these modern day software's like RISA-3D or  STAAD/PRO Soil-Springs in Tension
due to  Super Structure Overturning Moments  are eliminated and loads are transfered to Compression-Springs only. There by giving correct   SOIL-BEARING PRESSURE
due to  Vertical Loads (DL + LL) and  Overturning Moments (M).

This particular Raft I analysed and Design is already  Built and Supporting those 4 tanks. This project is in HIGH SEISMIC ZONE.  So there are large  Overturning Moments.  Remember in USA we have to submiit our  Design and Drawings to Permitting Authority to get permit.  They check our design. They acccepted  my Foundation Design.
These Modern Day software's  like RISA-3D and STAAD/PRO are varified by a large
users group and if properly used eliminate need for  making lots of
old fashion assumptions  like flexibility or Rigidty Raft, Soil Springs in tension.

ONE CHECK WE HAVE TO MAKE BY LONG HAND IS  RESISTANCE OF  LATERAL LOADS
AT FOUNDATION:  This load (Lateral Shear) from super structure is resisted by
value of  PASSIVE PRESSURE (Given by Soil Consultant) on THICKNESS OF  RAFT
and  FRICTION  value between RAFT and, Soil also given by Soil Consultant.
Some times these two resistancences are not  enough for high Lateral Shears. In that case provide  DEEP  GRADE BEAMS attached at the bottom of RAFT  to develope
more Passive Resistance.


Regards.

Vasuddeo Pandya  P.E. ; S.E. ; MASCE
Structural Engineer

ibarua · General Sponsor Joined: 26 Jan 2003 Posts: 1039

9th Jan 2011

Treating a raft foundation as rigid is not only wrong but is also dangerous.

The correct way to design rafts is to treat them as beams on an elastic foundation, by means of the following methods:

(i) Miklos Hetyeni's closed solution
(ii) Finite grid method (J.E. Bowles)
(iii) Finite element method.

Indrajit Barua.

On Fri, 07 Jan 2011 14:26:02 +0530 "hemal"  wrote
[quote]     Dear R.G.Gupta Sir,

Most of the structure design software treats ISOLATED FOOTING as rigid (uses SBC for design) and RAFT FOUNDATION as FLEXIBLE FOUNDATION (uses SOIL SUBGRADE REACTION for design).

Long ago, STAAD was having both (RIGID & FLEXIBLE i.e. SBC or Ks) options for RAFT FOUNDATIONS, but present versions only have FLEXIBLE option. Actually, as per my knowledge, design of RAFT foundation using RIGID method will be uneconomical (we can tell it conservative).

Regards
Hemal Mistry

--- On Thu, 6/1/11, rg_gupta <forum@sefindia.org> wrote:
      --auto removed--

Posted via Email

lele_raj · ... Joined: 26 Jan 2003 Posts: 145

I partially with Indrajit. Raft footing on soft, low BC soil is normally designed as beams on elastic foundation. But for hard, high BC soil, soil deformations are normally minimal and do not impact on the behavior of raft footing under load. In such a case, raft footing can be considered as "rigid". I've heard, but neither practised, nor read in the literature, of a raft footing on a hard rock strata being treated as an elastic plate.

Best regards,

Rajendra (Raj) Lele

From: ibarua <forum@sefindia.org>
To: general@sefindia.org
Sent: Sun, 9 January, 2011 4:30:05 PM
Subject: [SEFI] Re: RAFT Foundation

9th Jan 2011

Treating a raft foundation as rigid is not only wrong but is also dangerous.

The correct way to design rafts is to treat them as beams on an elastic foundation, by means of the following methods:

(i) Miklos Hetyeni's closed solution
(ii) Finite grid method (J.E. Bowles)
(iii) Finite element method.

Indrajit Barua.

On Fri, 07 Jan 2011 14:26:02 +0530 "hemal" wrote

sarfaraj.husain · ... Joined: 26 Jan 2003 Posts: 90

i agree with Mr. Lele.....his explanation with technical term "do not impact on the behavior of raft footing" is almost perfect.....

regards
sarfraj

[SEFI] Re: RAFT Foundation
lele_raj to: general  01/09/11 04:59 PM

Please respond to general

I partially with Indrajit. Raft footing on soft, low BC soil is normally designed as beams on elastic foundation. But for hard, high BC soil, soil deformations are normally minimal and do not impact on the behavior of raft footing under load. In such a case, raft footing can be considered as "rigid". I've heard, but neither practised, nor read in the literature, of a raft footing on a hard rock strata being treated as an elastic plate.

Best regards,

Rajendra (Raj) Lele

From: ibarua
To: general@sefindia.org
Sent: Sun, 9 January, 2011 4:30:05 PM
Subject: [SEFI] Re: RAFT Foundation

9th Jan 2011

Treating a raft foundation as rigid is not only wrong but is also dangerous.

The correct way to design rafts is to treat them as beams on an elastic foundation, by means of the following methods:

(i) Miklos Hetyeni's closed solution
(ii) Finite grid method (J.E. Bowles)
(iii) Finite element method.

Indrajit Barua.

On Fri, 07 Jan 2011 14:26:02 +0530 "hemal" wrote   :  Dear R.G.Gupta Sir,

Most of the structure design software treats ISOLATED FOOTING as rigid (uses SBC for design) and RAFT FOUNDATION as FLEXIBLE FOUNDATION (uses SOIL SUBGRADE REACTION for design).

Long ago, STAAD was having both (RIGID & FLEXIBLE i.e. SBC or Ks) options for RAFT FOUNDATIONS, but present versions only have FLEXIBLE option. Actually, as per my knowledge, design of RAFT foundation using RIGID method will be uneconomical (we can tell it conservative).

Regards
Hemal Mistry

--- On Thu, 6/1/11, rg_gupta wrote:
--auto removed--

--

Posted via Email

ibarua · General Sponsor Joined: 26 Jan 2003 Posts: 1039

10th Jan 2011

I've so far never come across the need for a raft foundation on hard rock.

Indrajit Barua.

On Sun, 09 Jan 2011 16:58:41 +0530 "lele_raj" wrote

prof.arc · ... Joined: 26 Jan 2003 Posts: 703

A food for thought !!

Is a raft necessary on Hard Soil ?
I thought Raft / Piles are used when soil is iffy to support footings of columns
in that case, raft may have to be designed as a plate resting on flexible soil
ARC

On Sun, Jan 9, 2011 at 5:00 PM, lele_raj <forum@sefindia.org (forum@sefindia.org)> wrote:

VPandya · General Sponsor Joined: 09 Nov 2009 Posts: 842

RIGIDITY  OF  RAFT  MEANS  RIGIDTY  OF  CONCRETE  ELEMENT  and not the Hardness
or Softness of supporting Soil.
For same Vertical Loads (Dl + LL ) and OVERTURNING MOMENT (M) a  Very Thick
Concrete (say 11 Ft.) will have no flexure stresses (Bending Moment) in Concrete
and  a nominal thickness ( 3 ft.) willl bent  like a beam bending and will have flexure
stresses in concrete.
Thick Concrete ( 11 ft.) will rotate like a Rigid Plate on Soil Springs with no flexure
Stresses. For Vertical Loads only Shear Stresses in Concrete no Flexure Stresses.
It is the Rigidity or Thickness of Concrete Element and not the  stiffness of  the Soil,
Soft Soil or Hard Soil is the issue in such analysis.   If you have a Concrete footing
sitting on Rock Surface you will have only  Direct Bearing Stresses and no flexure stresses.

Vasudeo Pandya
Structural Engineer.