Strip Footing

ProtaStructure performs the strip footing calculations for all load combinations and draws moment and shear diagrams as a combination envelope. Besides these, calculations are also performed based on coefficient of subgrade reaction interval.

Before inserting a footing:

1. The building model must be completed and analysed, so that the column and wall axial loads and moments are determined.
   
2. The foundation storey 0 (ST: 0) must be set as the current storey.
   

Strip Footing Definition

To define a new Strip Footing:

1. Insert the foundation beam(s) that will be used in the strip footing by going to top "Modelling" tab > Beam > Foundation Beam

     The beams must be defined such that vertical members (i.e. columns and walls) are covered in the strip footing (at their bottom endpoints).

2. Multiple select all the beams that will be included in the strip footing. (Note that, beams can be selected in any order)

3. Right-click to display the shortcut menu and select "Insert Strip Footing" option. The “Strip Footing” dialog will be displayed. 

     Alternatively, click on "Insert Strip Footing" icon at the top ribbon. 
4.  Press the "Design" button to design the footing. Refer to the Strip Footing Design section below for more information on this topic.

5.   After completing the design of the strip footing, press the "OK" button to complete the insertion of the footing. 

Editing an Existing Strip Footing

In order to edit an existing strip footing:

1. Select an existing strip footing,
   
2. Right-click to load the "Properties" form and modify the design of the strip footing,
   
3. Press the "OK" button in the "Properties" form to update the footing.
   

You can repeat this process to as many members as you wish. One member at a time can be edited by this method.

Beams of the Strip Footing

Beams that will be included in the strip footing must be defined prior to insertion of the strip footing. These beams must be defined based on the following rules and constraints:

Beam Dimensions and Eccentricities

The dimensions of the beams included in a strip footing must be identical. Beams with different section dimensions and beam on different axis cannot be used in a single strip footing.

Strip Footing Design

After selecting the footing beams, the "Strip Footing" dialog will be loaded when "Insert Strip Footing" option is picked in the shortcut menu displayed by right-clicking.

The "Strip Footing" dialog can also be displayed when the "Properties" option is picked in the shortcut menu displayed by right-clicking after selecting an existing strip footing.

Strip footing calculations are performed based on beam on elastic foundation method (“Winkler Method”). Calculations can be done either for all load combinations or for an envelope of all load combinations. Also, envelope calculation can be done for an interval of coefficient of subgrade reaction.

In order to determine concrete grades, steel grades and rebar size that will be used in Strip Footing Design, first pick “Materials” option in “Building setout” tab. Then, press “Foundation Floor” tab in “Material” dialogue.

1. Loading and geometry information of all related columns, walls and beams will be loaded into “Strip Footing” dialog. Only the information in white field can be modified.
   
2. Insert the label in "Strip Footing Label".
   
3. Select the combination using the pulldown menu in “Loading” section for which the calculations are to be performed.  If you want to obtain a design envelope for all combinations then check “Design Envelope” checkbox.
   
4. Enter the value of “Subgrade Coefficient of Reaction” into the “Subgrade Reaction Coefficient” field.
   
5. If you want to perform the calculations for an interval of subgrade reaction values, check “Step” checkbox. Then enter the second subgrade coefficient value into the second field enabled. By using the spin buttons, indicate the number of steps for interval calculation.
   
6. For the determination footing width, select the appropriate method of “Selection of Soil Stress for Calculating the Footing Width”.
   
7. Click on “Design” button.
1. Note that, “Design Envelope” and “Step” boxes must be checked together, if you want to do the calculations both for subgrade reaction and combination envelope.
   
8. Results of the calculation will be loaded into “Strip Footing Design Results” form.
1. Diagram of Soil pressure, shear and moment diagrams are shown. 
2. Click "Design Report" to review the analysis and design
3. Click "OK" to complete the design & close the dialog. 
9. The main Strip Footing dialog will now reflect the design :  
1. Footing Width will be auto-designed to least value to pass the maximum allowable soil stress. 
   
2. Review the designed width carefully and over-write or round up this width if required. Do not reduce the width over-wise it will be insufficient. 
   
3. If the footing width is modified, click "Design" to recheck the design; followed by re-generating the Design report.  
10. You can perform the design of the foundation beams by selecting “Foundation Beams” in “Design” tab.
    

Selection of Soil Stress for Calculating the Footing Width

Prior to commencing the design of the strip footing, it’s necessary to determine the method of which soil pressure under the footing will be calculated.  For this purpose, select the appropriate method among the methods given in the “Soil Pressure Selection Criteria for Calculating the Footing Width” field. 

Default method is “Maximum Soil Stress” that is used normally for soft soil. Especially for firm soil, other methods, namely, “Average in L/4 Column Region”, “Average in L/2 Column Region” can also be used. Nevertheless, these two methods must be used with care as calculated soil pressure can decrease without any control resulting in an unsafe design.

Left and Right Extensions

There are two fields reserved for the extensions at the beginning and at the end of the strip footing.

Enter the cantilever (extension) lengths “Right Cantilever” and “Left Cantilever” fields, if you want to include such extensions in the design.

Footing Width

If you do not provide any values in this field before calculations, required footing width will automatically be calculated. You can ideally enter width values other than the calculated one in this field. But in this case you must click on “Design” button again. If the required width is greater than the value you entered, then calculated value will be used. If the entered value is greater than required width, then entered value will be used.

Footing Depth

Footing Depth must be specified before design. This value can be changed if necessary, but design must be repeated in this case.

Footing Bar Size

Enter the steel diameter to be used in footing design. If bar spacing is calculated too dense, specify a larger diameter.

Column List

Information about a column residing on a strip footing alignment can be adjusted using the column list table. Do the necessary modification by directly clicking in the table cells. Data fields in the table are explained below.

Column Name

Name of the column defined in Graphic Editor is written here.

Column Section (b-column/e-column)

Length (b-column) of the columns along the strip footing direction, while length (e-column) is perpendicular to the strip footing direction. Support moments and shear forces calculated in the beams are reduced by increasing values of this parameter.

Axial Loads And Moments

Orthogonal Span Width

If the strip footings are inserted as a 2D grid, then column axial loads must be shared between two footings in two directions. For this purpose, enter the distance of the column in orthogonal direction into the “Other Dir. Len.” field. This will reduced axial loads can be seen on the “Strip Footing Results” form. 

Definition of Orthogonal Span Width.

In the example sketch above: “d1” is the orthogonal span width for column S1 when the strip footing is calculated along axis A.  Similarly, orthogonal span width for column S2 along axis B is “d1 + d2”. The value for column S3 along axis C is “d2 + 2*d3”, unlike others.