www.sefindia.org

[Sudhakaran]

Dear sefians,

A tool for analysis and design of raft for irrigation structure attached herewith.

Click on reset(to clear stored values)

Then enter the loads from right edge of raft and distance from right edge of raft to the each load.

Click on analysis and then design and report

It is for uniform width of 1m.



Sudhakaran

[VPandya]

Dear  Er.  Sudhakaranji,
There is a very widely used ACI  Publication on RAFT  ANALYSIS  AND DESIGN.  I use this:

ACI SP 152-1
An Introduction to the
State-of-the-Art Mat/Raft  Foundation
Design and Construction
by E. J. Ulrich, Jr.
Synopsis: Subgrade response is the most important parameter in
analyzing and designing mat foundations. Rational design of a mat
foundation requires the consideration of immediate and long term
subgrade response. "

This publication recommends using PLATE & SHELL ELEMENTS ( PLATE FINITE ELEMENT METHOD) of let us say  STAAD/PRO  for RAFT CONCRETE  and Soil Springs ( Wiinkler Springs) for modeling soil. There are some 3 actual case studies  ( High Rise Buildings) of RAFT  ANALYSIS  & DESGN  supporting  50 stories  Structure.  


Note: Recently I was involved in Checking a  RAFT  ANALYSIS & DESIGN of a Major Structure ( Heavy Loads), and Design Engineers  did RAFT Analysis and Design without using this document ACI SP-152-1, and results were disappointing.  What does it take to use this ACI  SP-152-1 and do it right?  

Copy of Publication Attached.


Vasudeo  Pandya  P.E.

Structural Engineer  

[Sudhakaran]

Dear Vpandya sir,

Thank you for very useful document.



Thanks,

Sudhakaran

[ferozpatthan]

Thank you very much for sharing

[VPandya]

Dear Sefi Engrs,
Recently a Structural Engineer name Mr. Vasantha Kumar contacted by E-Mail  on some query he had on FEM analysis and finding out CORRECT  RAFT THICKNESS from such FEM  analysis. I gave this attached reply.  Please comment on my reply. Comments are more than welcome.  In real world changing large Raft Thickness is always  a project cost item. They do not like it. But IS 456-2000 Section 40.2.3 and 40.2.3.1 Table-20 limit Tcmax  even with Shear Reinf. to 2.0 mpa. I got in one of my FEM analysis  max of SQX and SQY ( Plate Element Shear Stress ) = 2.55 mpa. I recommended CHANGE  THE RAFT THICKNESS from present design thickness of 3.0 m .  At that point they were looking for a GUN TO SHOOT ME.     

Please comment.

-------------------------------------------------------------------------------------------
Dear  Er. Vasantha Kumar ,
Sorry I was busy with  Christmas Holidays and some socialfunctions ( Marriages ) so could not reply to your query early.  ForDirection of Plate  & Shell elements forces in STAAD/PRO  see Section 1.6.1 PLATE  & SHELL ELEMENTS of  STAADTECH. REFERENCE MANUAL ( Available online in Help and type Plate and Shell Elements in searchbox)  .  That tell you what is positive and Negative force in yourout put.  See table 1-3 of same section giving TERMS INCLUDED  IN THESTRESS ELEMENT OUTPUT .  in that table  Shear Stresses SQX , SQY ----  ShearStress  (  Force/length / thickness)  .

So let us say I have done a RAFT FOUNDATION  analysis usingPLATE  &  SHELL ELEMENTS  of STAAD/PRO .   I willdo this to check THICKNESS   of RAFT .
a)  Run Analysis.
b) Go to mode in the  top Manu bar and selectMODE.  Select  Post Process.
c) Select  or High light all plate elements .
d) Go to report , PLATE  STRESSES and select first SQX ( Select from highest to lowest )
e ) This will give you highest  SQX any where in theModel or of all elements . Note it down value of stress  Max  SQX andElement Number.
f)   Now do the same report for Max SQY .  Notedown value of  max SQY and Element number .  

Now CHECK ON  RAFT THICKNESS  ( Assume Raft Thickness is3.0 m )  .
Say output gives you max  of SQX  or SQY is  =  2.52 mpa,  Allowable in IS 456-2000 is  Shear Stress =2.0 mpa .  So 3.0m Raft thickness fails.  In this case,  You need  to increase the RAFT THICKNESS tosay 4.0 m.    So do same check for 4.0m Raft Thickness . Whoknows after this Check you may need even a Thicker Raft, say 5.0 m .  On a projects I worked which had very HEAVY LOADS , The 3.0m Raft Thickness failed by this check.

Regards.

Vasudeo  Pandya  P.E.
Structural Engineer

[es_jayakumar]

Dear Pandya Sir,

"Allowable in IS 456-2000 is  Shear Stress =2.0 mpa".
Here, do you refer to the punching shear and where is this 2.0 MPa from ?

Regards,
E S Jayakumar

[VPandya]

Dear  Er  E S  Jayakumar ,

We had on that project high strength concrete M 40  and above.  So from Table - 20  value for Tcmax ( Maximum Shear Stress in concrete with Shear Reinf. ) is  4.0 mpa.
For Raft ( Solid Slab )  Tc ( Max Allowable Shear Stress in Concrete) is = 4.0/2.0 = 2.0 mpa.  

40.2.3.1For solid slabs, the nominal shear stress shall not exceed half the appropriate values given in  Table 20.

Regards .
Vasudeo  Pandya P.E.
Structural Engineer

[sakumar79]

Dear Er Pandya,
      If shear stress is so high in slab (2 N/sqmm), will it not require provision of shear reinforcement? Is shear reinforcement practical in raft slabs and if so can you share us any sample detailing for the same?

     In my humble opinion, will it not be better to subdivide the slabs in smaller segments by providing more beams so that the raft slab is suitable for detailing without shear reinforcement rather than making the slab thicker?

Yours sincerely
Arunkumar

[es_jayakumar]

Thank you, Pandya sir. Is the raft referred by you slab type or slab & beam type ? I hope slab & beam type will help to reduce the thickness of the raft slab.

Regards,
E S Jayakumar