Example problem 2_modelling of combined footing (part 1)

 

Minimum needed version to run this example:

STAAD Foundation Advanced 2024 (24.00.00.248)

 

Unit System Used: kip-in or kip-ft (Imperial)

 

Problem statement:

In the attached STAAD.pro file there are four steel columns (M1, M2, M3 & M4) modelled of sections W16x77, each with a height of 10 feet. The c/c distance between columns M1, M2 & M3, M4 are 20 ft. Each of the column members carry dead and live loads. Model the combined footing of size 376.5 in x 127 in x 21 in, under columns M1, M2 & M3, M4 along with the material and soil data using STAAD Foundation Advanced. Import the STAAD.Pro file into the application.

 

Considerations:

The steel column is supported on the top of the combined footing through pedestals and the combined footing is founded at 4.75 feet below the top of the soil level.

 

Material Data:

Characteristic compressive strength of concrete: 4000 psi

Strength of reinforcing steel: 60 psi

 

Soil Data:

Below mentioned soil data to be used,

Dry density of soil = 0.12 kip/ft³

Wet density of soil = 0.13 kip/ft³

Friction Coefficient = 3

Subgrade modulus = 120 kip/ ft²/ft

Gross SBC = 5 kip/ft²

Allowable settlement = 0.5 in

Soil level is at 3 ft above the combined footing top.

 

Water Table Data:

Water level is at 1.5 ft above the combined footing top.

 

Solution: Step by step process

Step 1: Make sure that the STAAD.Pro file is not open. SFA will not be able to import a STAAD.Pro file which is already open.

Step 2: Open the STAAD Foundation Advanced program and click on “Import STAAD.Pro Model” option. Please refer to Figure 1 below.

Figure 1

Step 3: In the form that appears, please browse to the STAAD.Pro file in your computer by clicking the “Browse” button. As defined in the problem statement, the soil level is defined as -2 ft. We consider the datum at the same level as the top of the pedestal and hence, we specify it as 0 ft. The water level to be specify as -3.5 ft. Now, click on the “Create” button.

The following figure (Figure 2) shows the sequence of the steps to be followed.

Figure 2

Step 4: If the STAAD.Pro file is unanalysed, SFA will not be able to find the reaction information which will be created as loads on the foundation. Hence, in this case the application asks the user if he would like to run the STAAD.Pro analysis. Say “Yes”. Please refer to Figure 3 below.

Figure 3

If the STAAD.Pro file is already analyzed, then program will not ask this question.

Step 5: Now the “Model Import” box appears. Press, the “Import” button, as shown in Figure 4 below.

Figure 4

Step 6: You will now see that the physical forms of the columns that you have defined in the STAAD.Pro files now appears on the SFA viewing area. Please refer to Figure 5.

Figure 5

Step 7: Now, create the first combined footing under the columns M1 and M2. Click on Geometry > Combined > Object and then select M1 and M2 on UI. This is a way of creating the combined footing using the Object method. Please refer to Figure 6 below.

Figure 6

Step 8: The “Create Rectangular Combined Footing” box appears. Please specify the modelling parameters as shown in Figure 7 and press “Create” button.

Figure 7

Figure 8

Note for the user: “Column Alignment” is set as “Top”, as it is assumed that the column transfers its load on the top of pedestal. We said “Yes” for pedestals below steel columns, which means that if there is a gap between the bottom of the column and the top of the foundation slab, that would be filled by the pedestals. In version 2024, the user does not have a way to model the pedestals.

Step 9: Now, our first combined footing is under the columns M1 and M2 has been modelled. You will see that a representative base model has been created under the columns and.

Figure 9

You will also notice a bluish representation on the top of the foundation slab. This is representative of the water present in the soil up to the level of water specified, which is 1.5 ft. above the footing top.

Step 10: Now, we want to create the second combined foundation under the columns M3 and M4. However, we will use a different method in creating this foundation. So, clicks on Geometry > Combined > Point. This is a way of creating the combined footing using the point method. Please refer to Figure 10 below.

Figure 10

Step 11: The “Create Rectangular Combined Footing” box appears. Please specify the modelling parameters as shown in Figure 11 and press “Create” button.

Figure 11

Note for the user: “Column Alignment” is set as “Top”, as it is assumed that the column transfers its load on the top of pedestal. We said “Yes” for pedestals below steel columns, which means that if there is a gap between the bottom of the column and the top of the foundation slab, that would be filled by the pedestals. The plan coordinates of the center of the foundation are X=10 ft and Z = 0 ft. Since, the foundation is top fixed the Y is -5 ft, which the top level of the foundation the slab

Step 12: Now, we will go for material creation, as is specified in the problem statement. Go to Specification > Concrete > Select, as shown in Figure 12.

Figure 12

Step 13: The following box pops up on the screen. Select the check box besides 4000 psi to select the concrete strength. Now, click on “Add to Model”.

Figure 13

Step 14: Now, select reinforcing steel grade by going to Specification > Rebar Steel > Select.

Figure 14

Step 15: In the box that pops up, select appropriate steel grade and select “Add to Model”.

Figure 15

Step 16: Now, let us specify the soil properties as has been instructed in the problem statement. Let us click on Specification > Soil > Create.

Figure 16

Step 17: In the “Create Soil” box, specify the following parameters. Once done, please press the “Create” button.

Figure 17

Step 18: Click on Support > Assign (Soil). Refer to Figure 18.

Figure 18

After clicking on “Assign” button “Assign Soil Support’ property page will get displayed, in this make sure that “Use Same For Overburden” option should be selected.

Figure 19

Step 20: Select both the combined footings and click on “Assign” button.

Figure 20

Step 21: Once, you have clicked on the “Assign” button, two brownish layers appear beneath bottom and above the foundations to show that the soil support and soil overburden have been assigned to both the footings. Please refer to Figure 21.

Figure 21

Concluding remarks: Now, this foundation has been modelled along with its associated material properties, soil support and overburden soil pressure. Now, this model is ready to be set for analysis and post-analysis checks.