Raft Foundation and Piled Raft Foundation Analysis & Design Example
Raft Foundation Example
This example is based on the "Quick Start Guide Concrete Complete" model which is automatically installed in the default ProtaDataXXXX folder.
In this example we will be inserting a raft foundation and then piled raft foundation underneath the shear wall lift core.
The raft foundation is created as slabs and then analyzed using Finite Element Foundation Analysis.
Foundation Design Settings & Raft Modelling
>> Ensure that Building Analysis is successfully completed.
>> Go to Foundation Design Settings > General tab . These settings generally affects the FE Raft Foundation analysis & Design.
>> Change Allowable Stress of Soil to 150 kN/m2 > Click OK to save and exit.
>> Go to Storey : 0 (Plan) view - as all foundation elements can only be modelled in Storey 0 (foundation level).
>> Create a 400mm deep slab cover inside the lift core (Type 1, Cover = 100mm & Dead/Imposed Load = 0)
Soil Subgrade Coefficient (soil stiffness) is automatically fetched from Foundation Design Settings. User can check " User Defined" & manually input a different coefficient.
>> Create 500 mm wide cantilever slabs around the edge of the core (change to Type 12 & click along the edge of the corewall)
FE Raft Foundation Analysis
The raft can now be analyzed.
>> From the Analysis tab > click FE Raft Foundation Analysis
Use the Subgrade Reaction Values Assigned to Slabs for Soil Stress Checks : If checked, user-defined subgrade coefficient in Slab Properties will be used.
The mesh options are similar to FE Floor Analysis options.
>> Click on Floor Mesh and Analysis
The FE analytical model with resultant slab mesh and the applied superstructure loads will be shown.
The core wall forces from the superstructure are automatically evaluated and transfer as nodal axial and moments forces shown as red arrows.
>> Click OK to accept the analysis and close the mesh window.
>> Ensure the Allowable Stress of Soil to 150 kN/m2.
>> Click Display Analytical Model
FE Analytical Model to show results
The FE Floor Analysis dialog will close & the FE Foundation Model View will be created.
>> Click on the FE Foundation Model View & the associated Analytical Model ribbons will appear.
Alternatively, the FE Foundation Model view can be created by going to Analysis tab > Analytical Model dropdown > Select "FE Foundation Model"
>> To have a less cluttered view with nodal loads hidden, Go to Member tab > Deactivate nodal Loads 
>> Go to Contours tab > Select Threshold Contours
>> On the Effects list on the right of the screen, scroll down to locate & select Soil Pressure (as shown above)
>> On the Load Combinations list on the right most of screen, select load combination G+Q (as shown above)
The threshold contours shows areas where resultant soil pressure is pass in Green & fail / overstressed in Red.
Since factored load combination are plotted, the allowable soil pressure will multiplied by ultimate factor of 1.25 default in Foundation Design Settings, i.e. 150 (allowable stress) x 1.25 = 187.5 kN/m2. The actual soil pressure will be compared with this factored allowable soil pressure in the Threshold contour.
We check soil pressure first to ensure that soil is not overstressed before we even look at the steel requirements.
Place the mouse cursor on any mesh & the soil pressure will be shown at each mesh end.
It is necessary to make the raft larger to decrease the resultant soil pressure to pass as shown in below steps :
>> Go to ST 0 plan view by clicking on Storey: 0 (Plan) view tab
>> Increase the raft size by increasing the cantilever length to 1100 mm (select each individual slab > bring up the Properties > Cantilever tab > change L-Cant > click Update > Close)
>> Then re-run the FE Raft Foundation Analysis. Similarly go the FE Foundation Model > Contours tab > check the Soil Pressure Threshold. The soil pressure should now pass (all green)
>> The raft can now be designed in the same way as Finite Element meshed slabs by viewing the As (d) contours.
Example, click Contours & select As(d)1-bot (as shown below)
The design of raft foundation is similar to a flat slab. The design procedure can be summarized as follows :
- Display & examine the various steel area required contours & specifying user-defined and / or threshold steel area contours.
- Create FE slab strips (fixed band strip) to automate the selection of reinforcements, if preferable.
- Column punching shear check should be performed for all columns & discrete walls (not merged core walls). Go Design tab > Column Punching Check > Refer to Punching Shear Check for Raft Foundation
Refer to this article and the attached flat slab manual for further details : Working with Flat Slabs
The wireframe of members (e.g., column) is based on the centroid of the section
Raft Foundation Reports
The relevant reports can be obtained by going to Drawings & Reports ribbon > Reports dropdown menu :
- Geotechnical Report Pre-design Summary : contains the foundation data such as soil bearing capacity, coefficient of subgrade reactions, total loadings, average & maximum soil pressures.
- FE Foundation Report : contains FE Analysis Results, FE nodes & shells data, Slab Strip Reinforcement design calculation, various contours diagrams & punching shear checks.
- Base Reactions Report : prints out the foundation analysis forces of columns and walls transferred to the raft foundations. These forces are obtained from the base reactions of the Building Analysis analytical model.
Piled Raft Foundation Example
We will change the raft foundation to a piled raft foundation.
For piled raft foundation, a single slab must be modelled, i.e. you cannot break the slab into different pieces.
Creating a pile raft slab using Rectangle Polyline
>> Go to Storey : 0 (Plan) view - as all foundation elements can only be modelled in Storey 0 (foundation level).
>> Create a rectangle polyline (as shown above). This will be the outline of the piled raft slab.
You can also use the Polyline function to make irregular shape of piled raft slab. Refer : Working with Slab/Column Edge >> Create a 400mm deep slab cover inside the lift core (Type 1, Cover = 100mm & Dead/Imposed Load = 0)
Creating Piles
>> Delete any existing strip footing and pad footing (as we will be ignoring the soil bearing capacity of the soil)
>> Select the raft slab then right-click, select Insert Pile
>> You would be asked to pick the pile Reference Point. Left-click on the lower-left corner at intersection of GL B/2.
After selecting the reference point, the Pile Properties is dialog is used to position the piles. A preview of the layout is displayed to assist in this process.
>> Enter Eccentricities, Ex & Ey = 0 mm. This means that the first pile would be exactly at the position of the ref point picked.
>> Enter Pile Spacings, Sx & Sy = 1250 mm. This would result in a regular pile layout with spacing of 1250 mm in the horizontal and vertical plan view.
>> Enter Compression Safe Working Load = 300 kN
If you want to specify a different pile location, you will have to edit the pile position (Ex, Ey) after creating it by choosing each pile in turn > Right-Click > Properties.
Pile safe working load for vertical combinations will be factored as stated at the bottom of the dialog box.
FE Piled Raft Analysis & Pile Report
>> Run FE Raft Foundation Analysis
>> Check ‘Ignore the Bearing Capacity of Soil’ (as shown above)
This means that the raft is supported completely by the piles with no contribution from the soil. It will then function like a pile cap.If this option is unchecked, then raft will also sit on the soil. We can then expect the resultant piles forces to be decreased as load is shared with the “non-suspended” raft.
Alternatively, you can enter a very small number for Soil Subgrade Coefficient, i.e soil stiffness (example 0.01)
>> Click on Floor Mesh and Analysis > OK to close meshing dialog after successful mesh
>> Click Pile Results Report in Post-Analysis Processes and Report tab
>> Choose the default options in Pile Report Options > OK > Report will be generated (example as below)
The pile capacity is Safe Working Compression Load multiplied by the Ultimate Strength Factor in Foundation Settings, i.e. 300 kN x 1.25 = 375 kN
If there are failed piles, the solution maybe to either increase the pile compression capacity or increase the number of piles (or both)
>> Close the Report Preview
>> Click Display Analytical Model in the Post-Analysis Processors and Report tab > a new FE Foundation Model view will be created
>> Click on the FE Foundation Model view & the associated Analytical Model ribbons will appear.
>> Got to Contours tab > Deactivate all Contours to get a clearer view (as shown below)
>> Go to Members tab > Deactivate "Loads" > This will turn off all the superstructure nodal loads for better view (as shown below)
>> Activate " Pile Forces" > This will turn on the display of Pile Forces
>> Select 1st load combination G+Q, so the pile forces will now be shown for this selected load combination
>> Select Envelop (min) and Envelop (max) - which will show the minimum and maximum values of all load combinations (as shown below)
Pile negative value means compression and positive value is tension. These values will be checked with respective compression or tension limit.
The piles will colored coded to reflect Pass in Green or Fail in Red. These are same values and limits shown in the Pile Results Report.
>> The raft (now totally suspended) can now be designed in the same way as Finite Element meshed slabs by viewing the As (d) contours in the Contours tab.
The design of piled raft foundation slabs is similar to a flat slab. Refer to this article and the attached flat slab manual for further details : Working with Flat Slabs
Punching shear checks cannot be performed for piled raft foundation, as shear perimeter is complex & influenced by irregular piles location.
>> Click on Storey : 0 (Plan) view
>> Go to Review Tab > Click Visual Interrogation
>> Select Pile Capacity Status for Criteria for Coloring > OK
Pile capacity status will be shown as above, i.e. Green = PASS, Amber = > 90% Capacity and Red = Fail
Changing Pile Properties
The properties and location of the piles can be changed :
>> Ensure to go to Storey : 0 (Plan) view
>> Select a single pile or Hold down CTRL key to multiple select piles
Alternatively, you can go to Select Filter 
options in Modelling tab, and turn on only Piles, after which it's easy to click and drag a window to select piles only.
>> Right-click > Choose Properties
This will bring out the Pile Properties dialog again.
You can now change the properties of the piles such as size, location (Eccentricities) & Safe working load.
After changing the pile properties, ensure re-perform FE Foundation Analysis. It is not necessary to re-perform Building Analysis if there is no changes in superstructure (e.g. members sizes or loads)
Release Column Top Node in X and Y direction for FE foundation Model
Option is checked :
- Top Nodes of the columns are free to move & rotate. The effect of moment from the column stump is considered & the raft foundation will rotate.
- Results are closer to a completely rigid pile cap assumption.
- Due to effect of moments, the pile axial forces will considerably different from one another.
- Generally, this may more conservative approach to get maximum pile forces, as moments will cause pile(s) further away from the column stump to have higher axial loads.
Option is unchecked :
- Top nodes of columns are restrained to X and Y translations.
- Minimum raft or pilecap rotation is obtained as the moment from the column stump essentially ignored.
- Pile axial forces are closer to one another, i.e. more evenly distributed.
Raft Foundation Webinar Recording
The following webinar covers the basic procedures in modelling, analysis & design of a sample raft / mat foundation using ProtaStructure :
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