Pile Spring – MIDAS Support

Intro

One method for modeling pile foundations is to consider the pile foundation stiffness using a 6x6 General Spring.

Support is provided for calculating pile foundation stiffness using the displacement method from the Korean Highway Bridge Design Code 2010.

After entering the specifications of the pile foundation (pile type, upper and lower pile, reinforcement), the piles are placed, and the ground information is entered.

You can check the characteristic values and axial spring constant (Kv), axial perpendicular stiffness (K1 ~ K4) of each pile, and confirm the directional (Global X, Y) coupled spring matrix values in an Excel calculation sheet.

Finally, the 6x6 General Spring, combining the directional matrices, is calculated and imported as a General Spring in midas Civil.

Calculation of the 6x6 stiffness matrix using the displacement method based on pile specifications and ground information input.
Support for matrices under normal conditions, during earthquakes, and during the calculation of the natural period.
Support for types: “Cast-in-place Piles,” “PHC Piles,” “SC Piles,” “Steel Pipe Piles,” and “Soil Cement Piles.”
Support for composite piles (upper and lower piles), pile groups, and single piles.

Version

v1.0.0 : Release

Applicable Standards

Korean Highway Bridge Design Code,2010

Benefits of this plugin

There are several methods for modeling pile foundations, including the fixed point model, virtual fixed point (beta) model, p-y curve model, and 6x6 general spring model.

This plugin calculates the stiffness matrix for modeling using the 6x6 general spring method and automatically generates the general spring in Midas Civil.

The plugin automates the process of inputting pile characteristic values, axial and lateral spring stiffness, and combining the final matrix, which was traditionally done using an Excel spreadsheet.

The calculated values can be verified in an Excel calculation sheet.

How to use this plugin?

To calculate the matrix, enter the pile specifications in the "Pile Information" section and input the soil constants in the "Ground Information" section.

1. Pile Information Input

[Pile Information Input tab]

In the Pile Information tab, input the specifications and arrange the placement of the piles.

It supports single piles, pile groups, reinforced sections, and composite piles (upper and lower), with the following types supported:

by Pile Types: Cast-in-place Piles, PHC Piles, SC Piles, Steel Pipe Piles, Soil Cement Piles
by Construction Methods: Driven Piles (Driving Method), Driven Piles (Vibro Hammer Method), Cast-in-place Piles, Deep Drilled Piles, Pre-bored Piles, Steel Pipe Soil Cement Piles, Rotary Piles

For pile placement, the coordinate system from the 'Korean Highway Bridge Design Code 2010' is followed, and when importing to Midas Civil, it is converted to the Civil coordinate system.

To calculate the matrix, enter the pile specifications in the "Pile Information" section and input the soil constants in the "Ground Information" section.

2. Ground Information Input

[Ground Information Input tab]

In the Ground Information tab, select the soil property constants and the reduction factors for the horizontal subgrade reaction coefficient.

It supports layer selection (clay, sandy soil, gravel) and automatic calculation of shear wave velocity according to the Road Design Manual.

Additionally, options are provided to consider the reduction of the horizontal subgrade reaction coefficient (KH) due to liquefaction layers, slope effects, and pile group effects.

3. Import Data

[Import Data tab]

After verifying the stiffness matrix results, input the 6x6 general spring into Midas Civil.

Type1 and Type2 are settings to align the loading coordinate system of the plugin with the Midas Civil coordinate system.

Note

Flowchart for Calculating Pile Characteristic Values

[Flowchart for Calculating Pile Characteristic Values]

Using this plugin, you can calculate the characteristic values of piles. If not modeling with the 6x6 General Spring and applying a virtual fixed point model instead, the pile characteristic values (beta) outputted in the Excel calculation sheet can be used to determine the virtual fixed point position.

When calculating the characteristic values of composite piles using an Excel calculation sheet, the characteristic values are generally determined using the section properties (EI) of the upper pile and the single ground reaction coefficient (KH).

In the case of this plugin, it considers not only the specifications of the upper and lower piles but also the reinforced sections (covering and filling sections), and calculates the characteristic values of piles by taking into account the ground reaction coefficients of multilayered soil

Calculation Method for Axial Spring Constant (Kv)

[Calculation Method for Axial Spring Constant (Kv)]

The axial spring constant is calculated using the formulas presented in the Korean Highway Bridge Design Code 2010.

For exposed piles, no correction factor is applied. Generally, for composite piles (SC + PHC) used in practice, the calculation considers only the sectional properties of the PHC pile.

For mixed piles, the axial spring constant is calculated using the series spring stiffness formula.

Calculation Method for Pile Axial Perpendicular Spring Constants (K1~K4)

The axial perpendicular spring constants for piles in the Korean Highway Bridge Design Code are calculated as follows:

[Calculation Method for Pile Axial Perpendicular Spring Constants(Korean Highway Bridge Design Code)]

The above formula is the general formula for cases where the single ground (constant horizontal subgrade reaction coefficient) and the pile's sectional properties (EI) are constant.

In this plugin, for multilayered soil and composite piles, the values are calculated using the frame analysis method as follows:

[Calculation Method for Axial Perpendicular Spring Constants (Frame Analysis Method)]

For single-layer soil and single piles, the same results as the general formula in the Korean Highway Bridge Design Code can be obtained.

Conclusion

When conducting modeling that considers pile foundations, this plugin allows you to easily generate general springs.

Additionally, when applying a virtual fixed point model, you can determine the fixed point position using the pile characteristic values from the calculation sheet.