Piles and pile caps can be defined in RISAFoundation. Singular piles are used when individual pile locations need to be defined to support slab elements. These are best utilized for mat foundations where the mats are continuously supported by piles.
A pile cap is an element consisting of multiple piles, a cap and a pedestal that are defined at a single node in the model. A pile cap is defined at a single point location, typically where a column reaction is located.
Here we will explain the pile and pile cap interface and modeling techniques.
To see individual pile results see the Pile Results topic. To see pile cap results see the Pile Cap Results topic.
Piles can be defined at the time of drawing by the button from within the Assign Support Type at Points dialog to open the Pile Definition Editor. They can also be defined ahead of time from the Pile Definitions spreadsheet, accessible from the Data Entry toolbar or Spreadsheets drop-down list.
This dialog provides a quick input selection screen to use to build Pile Definitions. Any updates made from within this dialog will automatically update the Pile Definitions spreadsheet. Each of the items here are explained below.
This label provides a unique name for the pile definition.
For general piles, two shape options are provided: Round and Square. For other pile shapes you can approximate them to one of these shapes (H-piles would just be added as a square pile with the side dimension equal to either the depth or width). This shape will affect punching shear calculations. A general pile material is consistent with the analysis and pile checks prior to RISAFoundation version 9.0.
The hot rolled steel shape database is provided when a hot rolled material is selected. Although the full HR Steel shape database is provided, steel piles are typically pipe and H-section shapes.
The wood shapes are available when a wood material is selected. The program assumes a constant cross section along the pile's length in the soil.
The concrete shapes are available when a concrete material is selected. The program assumes a constant cross section along the pile's length in the soil.
The pile materials available are hot rolled steel, concrete, and wood. The material properties are defined in the Materials spreadsheet under the respective material tab. The program will use the material properties to calculate the pile tensile and compressive capacities and stiffnesses.
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Selecting a general pile material and shape is consistent with the analysis and pile checks prior to RISAFoundation version 9.0.
The total length of the pile in soil. The pile length is used for HR steel, Concrete, and Wood pile design calculations.
This is depth that the pile is embedded in the pile cap. This affects the "d" for pile cap design and also punching shear calculations.
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For HR Steel, Concrete, and wood piles, the program will calculate these values based on the material properties and display Auto Calc. For general piles, the user will specify the pile allowable capacities. The program will use these capacities to do a design check of your general piles.
For all pile shapes and materials, the user will specify the pile allowable shear capacity. The program will use the user input value of the shear capacity to do a design check.
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For HR Steel, Concrete, and wood piles, the program will calculate the stiffness values based on AE/L and displays Auto Calc. The Auto Calc stiffness entry may be overridden by a user entered value.
For general piles, this entry allows you to define the stiffnesses for your piles. By default the compression entry is set to completely RIGID. The tension entry is set to 0, meaning there is no resistance to tension.
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For all pile shapes and materials, this entry allows you to define the shear stiffness for your piles. By default the shear stiffness entry is set to completely RIGID.
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The concrete reinforcement can also be specified in the Concrete Pile tab of the Pile Definitions spreadsheet. The user can either use the reinforcement specified in the Design Rules spreadsheet or create a custom rebar layout to apply to the concrete pile.
ACI 318-19 provides explicit equations in Table 13.4.2.1 on allowable compressive strength for piles depending on the pile types. When ACI 318-19 code is selected, the pile type can be modified in Concrete Pile tab, currently RISAFoundation supports pile types (a)-(d) in this table.
Pile caps can be defined at the time of drawing by the button from within the Assign Support Type at Points dialog. They can also be defined ahead of time from the Pile Cap Definitions spreadsheet, accessible from the Data Entry toolbar or Spreadsheets drop-down list.
This dialog provides a quick input selection screen to use to build Pile Cap Definitions. Any updates made from within this dialog will automatically update the Pile Cap Definitions spreadsheet. Each of the items here is explained below.
This label provides a unique name for the pile cap definition. When clicking in this column a red arrow will become available. This will open the Pile Cap Definition Editor and allow you to update all spreadsheet entries via a single dialog.
When ACI 318-19 is selected, ACI 318-19 Section 24.4.3 stipulates that the ratio of deformed shrinkage and temperature reinforcement area to gross concrete area shall be greater than or equal to 0.0018.
When other ACI 318 editions are selected, acceptable steel ratios are controlled by ACI 318-14 Section 24.4.3 (ACI 318-11 section 7.12.2.1) which stipulates that the minimum steel ratio for Grade 40 or 50 steel is .0020. For Grade 60 or greater the ratio is 0.0018.
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In addition, the codes limit the maximum steel that can be used. ACI 318-14, for example, limits the minimum strain on the reinforcing steel to 0.004.
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This allows you to define the pile cap material from the Materials spreadsheet.
This allows you to define the design rule from the Design Rules spreadsheet. For pile caps we take information from the General, Footing/Pile Cap, and the Pedestal tabs of the design rules.
This allows you to define a soil overburden on your pile cap. This overburden weight is added to the weight of the slab for pile demand calculations. These pile forces affect flexural and shear design for the cap.
This value defines the center to center spacing of piles. This factor multiplied by the diameter/side dimension of the pile defines this spacing. Typically this factor is used to define capacity reduction factors for closely spaced piles.
This value defines the minimum distance from the centerline of pile to edge of pile cap.
This allows you to choose a pile type for your pile cap from the Pile Definitions spreadsheet.
This selection defines the number and layout of piles. These pile layouts were derived from the CRSI manual on pile caps. Here are the standard layouts from the CRSI manual.
Figure 13-2 from CRSI Design Handbook 2008
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This value defines the total thickness of your pile cap.
This allows you to define a field placement tolerance that is acceptable for your pile caps. This affects two checks in the program: the moment demand calculation and the one way shear capacity calculation.
These factors will reduce the pile capacities from the Pile Definitions spreadsheet. The value in the Pile Definitions spreadsheet will be divided by this value to produce a reduced capacity based on closely spaced pile groups.
The pedestal dimensions are entered in these fields. The x dim and z dim values are the pedestal dimensions parallel to the local x and z axes of the pile cap, respectively. The height is the distance from the upper surface of the footing to the top of the pedestal.
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As discussed previously, the general functionality of the program is that piles would be used in scenarios where a mat slab was drawn and piles are located at specific locations under the slab. Pile caps, on the other hand, would be used at a specific point location to support a reaction coming down.
The Pile Cap feature has some limitations and can not be applied to all situations. If there is a limitation in the pile cap, however, using the singular piles, slabs and pedestals within the program will almost always allow you to overcome them. The piles, slabs and pedestal option allows you to create whatever custom geometries you have.
The pile cap pile layouts are configured based on the CRSI design manual layouts. Thus, these configurations can not be manipulated. However, it is possible to simply create a pile cap for a concentrated force using singular piles, a slab and a pedestal. Simply draw your pile cap as a slab element, add the pedestal using the draw pedestals utility, and locate/draw your piles according to your specific pile configuration. Below is an example where this was done.
Compare this with CRSI layout for 8 piles:
From this example we can also see that locating a pedestal at an eccentric location to the pile group is also possible.
It is also possible to model identical configurations both with the piles, slabs and pedestals and the pile cap. When doing this considering identical loading you may get nearly identical results.
Keep in mind that pile cap entities assume the cap is fully rigid. Slabs supported with piles assume a semi-rigid slab. Thus, the slab entity will place load in the piles based on the rigidity of the cap and this can alter the force distribution into the piles.
Also, the default stiffness for general piles is completely rigid. Thus, you can get very different distributions of force to these two seemingly identical entities. However, if you lower the pile stiffness to a non-completely rigid value then your pile force distribution should be much more similar.
If the slab is relatively thick, such that the slab pile cap behaves in a fairly rigid manner, and the pile stiffnesses are not set to completely RIGID then likely you will get similar pile force distributions between the two pile cap entities.
Many times there are regions within a large mat foundation where large loads are coming down and a pile group is required in this location. The program does not allow you to define a pile cap within a slab, but you can simply draw in individual piles at this close spacing. See the example below, where large column loads are coming down on a mat foundation.
In this scenario the slab is one continuous thickness over the entire mat. If the slab was thickened at the pile groups and around the exterior, we would simply need to draw multiple slab elements adjacent to each other. If two slab elements are adjacent to each other, they will have full fixity to each other. If we thicken the previous model it may look something like this:
Here we can see that this mat slab was drawn in multiple pieces of different thicknesses. The image on the right shows a rendered view with part of the slab cut away to see the differences in thickness.