LEARN FLAT RAFT STEP BY STEP
 
INTRO & LIMITATIONS +
EXCERPTS FROM IS : 2950 - PART 1 - 1981

 

Please take Print Out of Every Step, including this page before commencing Learn. Take a Yellow Marker Pen and Mark the Learning Process while Proceeding further. This is Essential for Learning.
The software performs Analysis, Design, Estimation, Costing & AutoCAD Drawing of RCC Rigid Flat Raft Foundation.

The Software basically requires a User to enter Raft data for Joints, Columns, Beams, Slabs & Continuity. The rest of the things are taken care of by the software.

The results are displayed in the form of BM & SF, Raft Hidden Beam & Slab Schedule, Quantities, Cost & Approximate Bar bending Schedule for Beams.

Graphics option are available for display and tabular Format is available for Editing and Deleting Data.

A User should Delete / Edit Input-Data through the various Program Options only. If any editing is done outside the design environment than Data files may become corrupted. All Data should be Strictly "Entered" as explained in following steps.

Extensive Printing options are available under each display. Printing is straight forward with default set of values ( Arial Font, 8 mm Thick, Bold, Portrait ).
Only Beam Schedule will be Printed in Landscape Orientation.

The best way to go about the software is to Mark on the Foundation Plan, Joint, Beam, Column and Slab Numbers. A Joint represents a Column location or an intersection between 2 Raft Beams. The Beams are represented by its location in the form of Right Hand Side (RHS) & Left Hand Side (LHS) Joint numbers. The Slabs are represented by LEFT BOTTOM & RIGHT TOP joint numbers. All Joints will have X & Y Co-Ordinates, Top Left corner is taken as origin (0, 0). Joint / Beam / Column / Slab numbers should start with " 1 " and should not be repeated.

The Program will generate automatic Joint, Beam, Column & Slab Numbers from the information given in Project File. Some of these Numbers / Members may not be required & shall be deleted in a systematic manner as explained in the following chapters. The Final Plan Graphics should look exactly as the Raft Foundation Plan.
Cantilever Raft beams / Slabs cannot be analyzed.
Beams / Columns / Slabs shall be along two mutually perpendicular axis 
(X and Y). Polygonal (Multi-sided) slabs cannot be analyzed.
Analyze the Structure, assuming Raft Top (Column Base) as Hinged 
(Refer Graphics above). Hence there shall be no Moments on Raft Top. 
 
Structure shall be Analyzed in Either direction. Hence User shall enter 3 Column Load Cases, i.e.

● DL + LL

DL + LL + Wind / Seismic Load in Ton (From X-X Direction)

DL + LL + Wind / Seismic Load in Ton (From Y-Y Direction)

End Column Reactions on Raft Beams (DL + LL + Wind or Seismic) are obtained by running 2-D or 3-D Frame Analysis programs separately. 
User may use our 2-D Frame Analysis software for calculation of End Reactions.
  
Export to Excel :

When the " Analysis Result -> Bending Moment & Reaction " option is Run, a Text file is automatically created. This File will open in Any Text Editor. You can also Open this Text File in EXCEL.
Start Excel -> File -> Open -> Delimited ->Next : Delimiters -> Comma ->Next -> Finish.
Now you will notice that Complete Data is displayed in Excel Spread Sheet.
If more than One File is Created, Corresponding to Each of Load Cases, than Open Excel Sheet for Each File (Load Case). In Excel Sheet Editing, Deleting, Sorting, Printing & Merging of Data/Files/Excel Sheets is Extremely Easy. This way any no. of Load Cases can be Manipulated.

Similar Text files are created in " Shear Corrected BM & SF " (Design BM & SF),
" Raft Beam Schedule " and  " Raft Slab Schedule option for Exporting Results to Excel Spread Sheet & its subsequent Manipulation.
 
Intersecting Joints between two Beams (Main & Secondary) is assumed as Hinged. Hence no Moment transfer is envisaged.
Connection between End Column and Beam is considered as Hinged. Hence no Moment transfer is envisaged between Column and Beam. 
After data input, the user has to switch over to graphic option for visual checking of joints / columns / beams / slab nos. When the data is error free the user can  run the Analysis, Design and Quantity options. The various results are also available through display or print options.
Analysis, Design and Quantity options should be run in strict order, else program will give unexpected results.
Program creates automatic Joint numbers as per nos. of Horizontal & Vertical Grids. Here Grids means Beams coming along Column center lines  as well as all  Internal Beams not aligned with columns. A user has to input Information regarding Horizontal & Vertical Grids while creating Project File.
A user can delete the Joints not required by using Joint Option.
Joints will be automatically re-numbered when "UPDATE" button is clicked or at "EXIT".
Remember to Delete / Edit Corresponding Beam / Column / Slab Member, whose Joint has been deleted.
Always delete Beam / Column / Slab member from the "END" to facilitate further Editing. After Deleting press "UPDATE" button for re-numbering of members.
After Deleting corresponding Beam / Column / Slab Member & Updating, edit the required Joint Numbers of affected Beam / Column / Slab Members.
Go through the "READ ME" Button for better understanding of that particular Option.
Raft Beam & Slab Design is as per IS 456 - 2000. Refer IS 2950 : Part 1 - 1981 / 1965 For Rigid Raft Design / Relative Stiffness Factor.
Hidden Beam Depth < 200 mm not permitted.
Beam Width  > 8000 mm not permitted.
Links (Stirrups) < 75 MM for Hidden Beams is not permitted.
Beam Reinforcement > 4 % not permitted.
Age factor is considered as 1.15.
For Durability aspect of Design, refer our " Super Civil CD " software.
All Columns are placed Centrally with respect to Beams in either direction. There is no provision to offset the column in either direction. If the offset is large than user should re-workout the Beam Span. 
Minimum Computer RAM memory of 1 GB is recommended.
Use Laser OR Ink Jet Printer.
References:

1. IS - 2950 - Part 1 - 1981.
2. BS - 8110 - 1997.
3. IS - 456- 2000 / 1978 / 1964.
4. SP 24 - 1983.
5. Advance Reinforced Concrete Design by P. C. Varghese.
6. Reinforced Concrete Designers Handbook by C. E. Reynolds.
  
   IMPORTANT FEATURES OF FLAT RAFT
Hidden Beam marking in flat Raft Slab is different than the normal Raft plan GA.

1:  Mark the Hidden Beams (Column Strips) in the same way as you Mark the 
     Normal Beams but remember the following differences.

2: Hidden Beam (Column Strips) depth shall = Flat Raft (Middle Strips) Thickness.

3: All Flat Raft Slab (Middle Strips) thickness shall be uniform.

4: Initial (Trial) Flat Raft Slab Thickness = 
   
Max. Span of Beam or Slab in the Floor in MM 7.

5: Keep Column Size as Large as Possible, Larger the Column size lesser 
    the Flat Raft Slab thickness.

6: Minimum Column size allowed is 300 x 300 MM.

7:  Try to Avoid secondary beams, i.e. hidden beams resting on one another.

8: Concentrated loads Loads are not Permitted.

9: Convert wall line load in to UDL as w (0.6 * slab_span). and avoid beam   
     below wall line load. Refer IS 456-2000, clause 24.3.2.

10: Hence convert all internal partitions in to equivalent UDL & avoid beams.

11: Try to Keep Minimum spacing of beams at 2.0 M C/C.

12: In order to get most optimum design keep the slab and beam spans 
       uniformly within 20 % of each other.

13: Spacing of columns shall be as uniform as possible within say 20 % of each 
       other.

14: Avoid Eccentricity of Walls vis a vis Hidden Beams and Columns.      

15: One Way Shears are calculated at effective depth from Column face and 
       Punching shear at deff 2.

16: BMs are calculated at Column face.  
      
BM & SF are calculated at Beam Center Line for Beam to Beam Joint.

17: Raft Slab depth to be revised, if steel area exceeds 245 cm2.

18: In Case of Un-safe Hidden Beam design Reframe the Plan or Revise Slab  
      Depth or Column Size or Concrete Grade.

19: Flat Raft Slab Design is meant for Vertical Loads only.
 
20: Main Steel is to be placed in Longer Direction & not in Shorter Direction.

21: Convert Non Rectangular Column into Equivalent Square Column
       before Designing.

22: Min Reinforcement % in Either Direction in Hidden beam = 0.20 %.

23: Reinforcement Spacing Shall not Exceed 2 x Depth.

24: Use High Grade of Concrete Preferably M 30 or M 35.

25: At Ends and Corners the Flat Raft Slab shall Extend up to or beyond Outer 
       Edge of Column / Beam Else Flat Raft Slab will Fail in Punching. 
       Refer Details as under. 
       It is
Preferable to extend at least 300 MM beyond Beam / Column face.

26: Large openings cannot be provided in Flat Raft Slab.

27: Small openings say <= 300 MM can be located near the center of Slab.

28: Staircase / Lift Well panel shall be treated as normal slab & not as opening
       Program will calculate the Loads automatically.

29. In case of Unsafe Flat Raft Slab / Hidden beam Design, any one or all of the 
       following Corrective measures shall be adopted.

Reframe General Arrangement (GA) / {Beam layout} Plan.
Revise Hidden Beam / Flat Raft Slab thickness.
Increase Column Size.
Increase Concrete Grade.
  

Export to PDF:

A free PDF Creator Program is available with this Software. Designer can directly export to PDF instead of printing various reports.
    
   EXCERPTS FROM IS : 2950 - PART 1 - 1981
IS : 2950 - Part 1 is CP for Design & Construction of Raft Foundation.
In granular Soil, the SBC of Raft is generally very high, however for rafts placed at considerable depth (Like Basements) the punching mode of failure shall be investigated. The Influence of soil compressibility & related scale effects should be assessed.
For Rafts on cohesive soils, the stability against deep seated failures & effect of long term settlement shall be taken in to consideration.
The Depth of Raft Foundation shall not be Less than 1.0 M. 
Rigidity of Foundation tends to iron out uneven deformations & thereby modifies the contact pressure distribution. High order of rigidity is characterized by large moments & relative small uniform settlements. A rigid Foundation may also generate high secondary stresses in structural members. The effects of Rigidity shall be taken in to account in the analysis.
Rigid foundation design is based on the assumption of linear distribution of contact pressure. The basic assumption of this method are :
(a) The Foundation is rigid relative to the supporting soil & the compressible soil layer is relatively shallow.
(b) The contact pressure is assumed as planner, such that the centroid of the contact pressure coincides with the line of action of the resultant force of all loads acting on the foundation.
 
This method may be used when either of the following conditions is satisfied.
(a) The structures behaves as rigid (due to combine action of the superstructure & the foundation) with a relative stiffness factor K > 0.50. (Refer Appendix C)
(b) The Column spacing is less than 1.75 / . (Refer Appendix C)

The Raft is analyzed as a whole in of the two perpendicular direction & analysis is based on statics.