Pile caps are rigid plate structures that are used to transfer superstructure load from columns to a group of piles. They are usually subjected to bending and shear forces, and shear considerations usually govern the thickness design of pile caps. The three main approaches that are used in the analysis of pile caps are;
- Truss Analogy
- Bending analogy, and
- Finite element analysis
While truss analogy and bending theory can be easily carried using quick manual calculations, finite element analysis usually require the use of computer models. In this article, we are going to explore the potentials of Staad Foundation Advanced Software in the analysis and design of pile caps.
A quick design of pile caps can be done on Staad Foundation Advanced using the Foundation Toolkit option. This approach does not require importing models and can be used for quick stand-alone design when the column load and geotechnical parameters of the soil are available. To use this option, launch the ‘Staad Foundation Advanced‘, click on ‘New Project‘, and select ‘Foundation Toolkit‘ labelled as shown below.
Step 1: Launch the foundation toolkit
Step 2: Create Pile Cap Job
When the Foundation Toolkit opens, go to ‘Main Navigator‘, and from ‘Project Info‘ drawdown list, select ‘Create Pile Cap Job‘ as shown below.
Step 3: Select design code, units, and pile layout
When the ‘Pile Cap Job’ is launched, select the desired code of practice, unit, and click ‘Next’. The pile layout can be left as predefined.
Step 4: Define the load
On clicking ‘Next’, the dialog box for load comes up. Make sure that the unit is consistent as desired, and for this exercise, I am applying a factored column load of 3500 kN. If there are other forces such as moment and shear coming from the column, you can define them also.
Step 5: Define Load Combination
Since we are dealing with an already factored load, select ‘User Defined‘ from the drawdown list of load combination. If you have defined dead load, live load, wind load etc in Step 4, you can select the desired code of practice for the combination of the loads. Since I defined my factored load as dead load, I assigned a factor of 1.0 to dead load at ULS and SLS (actually I am not interested in SLS in this design). Then click ‘Next‘
Step 6: Define the design parameters
In this case, the column dimensions are taken as 450 mm x 450 mm, and the thickness of the pile cap was taken as 1300 mm. Other design parameters specified are as shown below.
Step 7: Select pile arrangement
The diameter of the pile was selected as 750 mm, with a spacing of 2250 mm. The safe working load of the pile was taken as 900 kN. You can also input the uplift and lateral load capacity of the pile. The edge distance is taken as (diameter of pile/2 + 150 mm) – where 150 mm is the overhang from the edge of the pile to the edge of the pile cap. Then click on ‘Calculate‘.
This brings the possible pile arrangements based on the safe working load and the superstructure load. For this tutorial, the arrangement below was adopted. The simple idea behind it is simply (Column load/pile safe working load). Note that for practical purposes, serviceability limit state load should be used when selecting the number of piles. Then click ‘Ok‘ and ‘Next‘.
Step 8: Finish the model
Step 9: Carry out the Design
Clicking ‘Finish‘ returns you to the ‘Main Navigator‘ page, where you can click on ‘Design‘ to carry out the design of the pile cap.
Step 10: View the output
The output page is where you can view the geometry drawing, details and schedule drawing, calculation sheet, and graphs.
The design approved the 1300 mm thick pile cap provided, and provided Y16@100 mm c/c reinforcement. You can go ahead and print the calculation sheet which you can download below.
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