Boundary Conditions

Boundary Conditions define how the model is externally constrained. All models must be attached to some external point or points of support. You may define these points of support as completely restrained or as partially restrained with a Spring. You can also define a spring support that has stiffness in only one direction with tension-only or compression-only springs.

For additional advice on this topic, please see the RISA Tips & Tricks webpage at risa.com/post/support. Type in Search keywords: Boundary Conditions.

Creating and Modifying Boundary Conditions

There are a number of ways to create or modify boundary conditions. You may view and edit the data in the Boundary Conditions Spreadsheet, you may click a node to view and edit its properties, or you can use the Boundary Conditions tool to graphically assign or modify a possibly large selection of boundary conditions.

Modify Boundary Conditions for Nodes

The graphical Boundary Conditions tool discussed here lets you specify and modify boundary conditions graphically. To use this, you typically specify the new boundary condition, then click on Click to Apply and click on nodes one by one or press Apply to Selected after having selected the nodes that you want to assign or modify.

You can modify or assign nodes one at a time by selecting the Click to Apply option and then click on the nodes you wish to modify. You may also modify or assign entire selections of nodes by selecting the nodes first and then use the Apply to Selected option.

The parameters shown below are the same as those on the Boundary Conditions Spreadsheet and are described in Boundary Condition Options. Use the ellipsis button to select the boundary condition.

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To Apply Boundary Conditions

If a model view is not already open, click the Open 3D Views icon on the ‘View’ ribbon to open a new view.
Click the Boundary Conditions icon on the ‘Home’ ribbon (as shown in the following image) and select a boundary condition.

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You can apply the boundary condition by choosing Click to Apply or Apply to Selected.

To choose nodes on the fly, choose Click to Apply. Click/Box the nodes with the left mouse button.

To apply the boundary condition to a selection of nodes, choose Apply to Selected.

Note:

To apply more boundaries with different conditions, press the Boundary Conditions button once more.
You may also view and edit boundary conditions by clicking on a node and editing the values in the Properties panel.
You may also specify or edit boundary conditions in the Boundary Conditions Spreadsheet.
You may undo any mistakes by clicking the Undo button.

Generate Soil Springs

A subgrade modulus may be automatically applied to horizontal plates in a model using the Subgrade Springs tool. The tool will generate compression-only springs in the vertical direction at all plate nodes in the selected group of plates. Nodes must be connected to plates which are perpendicular to the vertical direction or the nodes will be ignored. If a boundary code other than a compression-only spring already exists in the vertical direction at that node, the boundary code will NOT be modified.

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RISA-3D calculates the tributary area for each plate node individually and multiplies that area by the subgrade modulus to determine the spring stiffness for the compression-only spring at that node. The boundary codes for all plate nodes affected are automatically modified in the Boundary Conditions Spreadsheet.

Boundary Conditions Spreadsheet

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The Boundary Conditions Spreadsheet records the boundaries for the nodes and may be accessed by selecting Boundary Conditions from the Data Entry drop down menu in the Spreadsheets tab of the Ribbon Toolbar or by selecting Boundary Conditions from the Data Entry Window.

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The Node Label column contains the label of the node that is restrained.

The remaining columns record the boundary conditions that apply to the node. There are six degrees of freedom for each node (3 translation, 3 rotation), so there are six columns for degrees of freedom.

The boundary conditions are entered in these remaining columns by selecting the cell, clicking and choosing from the boundary options. You may also type them in directly.

Advanced Boundary Conditions

The Advanced tab of the boundary conditions spreadsheet contains information on nodal damping.

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Damping for Spring Boundary Conditions

When the user has defined a two-way soil spring to a node and chooses to run a time history analysis using the Direct Integration solution method, then they may manually assign a Damping value to each of these soil springs.

Note:

If there is not a spring boundary condition for the direction specified, then the user entered damping value will be ignored. You can only manipulate supports in this column if they are simple point supports. Any footings, piles or pile caps are manipulated in the other columns.
These damping entries are only used when the direct integration method is selected for the time history solution. These values are not editable unless this method is already selected in the Global Parameters.

Boundary Condition Options

Free nodes have no restraint in any of the degrees of freedom and need not be listed on the Boundary Conditions Spreadsheet. The following are the valid boundary condition options that may be used for the six degrees of freedom.

Note:

Models that contain compression-only or tension-only springs must be iterated until the solution converges. Convergence is achieved when no more load reversals are detected in the springs. During the iteration process, each spring is checked, and if any springs are turned off (or back on), the stiffness matrix is rebuilt and model is resolved. This can take quite a bit longer than a regular static solution.
With this iteration procedure it is possible to run into a phenomenon known as "clapping". This occurs when a compression-only spring keeps going between a tension force present (where the spring is removed) to a compression force present (where the spring is re-added). This can be somewhat common in non-linear solutions. If you run into this phenomenon, you may see the program give Error 1162. The fix is to very slightly adjust either the loading or the spring/element stiffness to get the offending spring out of the range where it can bounce back and forth. Another option would be to move the location of the soil spring slightly so it's not right at an inflection boundary.
You can enter the first letter of the option ("R" for Reaction, "S" for Spring, etc.) rather than typing out the entire code. RISA-3D fills in the rest automatically. The exception is the Tether entry, where the full word does have to be entered. After Tether, the primarynode needs to be entered (for example, Tether N2)

Boundary Condition at ALL Nodes

The entry "ALL" may be entered in the Node Label field. The boundary conditions entered on this line will be applied to ALL the nodes not otherwise listed. This is useful if you should want to lock certain directions of movement for all or most of the nodes. For example, if you are solving a 2D frame defined in the XY plane and you're only interested in the planar action, you could enter "ALL" and put an "F" (for Fixed) for Z translation, X Rotation and Y Rotation. See the following figure:

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Note:

If a node is explicitly listed with boundary conditions, those boundary conditions override the "ALL" conditions for all 6 directions. The "ALL" specified boundary codes apply only to those nodes NOT otherwise listed on the Boundary Conditions Spreadsheet.
When using the "ALL" command, boundary conditions are no longer graphically displayed.

Reaction Boundary Condition

The "R" code, for Reaction, specifies full restraint for the indicated direction. No movement will be allowed in the indicated direction for this node. Furthermore, the reaction will be calculated at this node, for this direction.

Fixed Boundary Condition

The "F" code, for Fixed, specifies full restraint for the node in the indicated direction. The difference between "Fixed" and "Reaction" is that for the "Fixed" code, no reaction is calculated. The "Fixed" condition actually removes the degree of freedom from the solution, which is why the reaction value is not available. If you aren't interested in the reaction value, using the "Fixed" code will result in a slightly smaller model and less output.

Spring Boundary Condition

The "Snnn" code, for Spring, models a spring attached to the node in the indicated direction. The "nnn" portion of the code is the numerical magnitude of the springs' stiffness. The units for the spring stiffness depend upon whether the spring is translational or rotational. The appropriate units are shown at the top of the column.

For example, if a spring of stiffness 1000 Kips per Inch were desired in the X direction at a particular node, for that node you would enter 'S1000' for the X direction boundary condition.

Compression-Only Springs

The "CSnnn" code, for Compression-Only Springs, models a one way "compression-only" spring attached to the node in the indicated direction. This spring has stiffness for negative displacements and NO stiffness for positive displacements. The "nnn" portion of the code is the numerical magnitude of the springs' stiffness. The spring stiffness units are the same as those for a normal spring. Compression-only springs are useful as soil springs when analyzing foundations that may have uplift.

For example, if a compression-only (CS) spring with a stiffness of 500k/in were desired in the Y direction at a certain node, you would enter 'CS500' for the Y direction boundary condition.

This means that all displacements at this node in the negative Y direction will be resisted with a stiffness of 500k/in. However, the node is free to move in the positive Y direction.

When a model contains T/C only springs, the program must iterate the solution until it converges. Convergence is achieved when no more load reversals are detected in the T/C only springs. During the iteration process, each T/C only boundary condition is checked. If any springs are turned off (or turned back on), the stiffness matrix is rebuilt and model is resolved. For models with lots of T/C only elements, this can take a bit longer than a regular static solution.

Tension-Only Springs

The "TSnnn" code, for Tension-Only Springs, models a one way "tension-only" spring attached to the node in the indicated direction. This spring has stiffness for positive displacements and NO stiffness for negative displacements. The "nnn" portion of the code is the numerical magnitude of the springs' stiffness. The spring stiffness units are the same as for a normal spring.

For example, if a tension-only (TS) spring with a stiffness of 500k/in. were desired in the Y direction at a certain node, you would enter 'TS500' for the Y direction boundary condition.

This means that all displacements at this node in the positive Y direction will be resisted with a stiffness of 500k/in. However the node is free to move in the negative Y direction.

When a model contains T/C only springs, the program must iterate the solution until it converges. Convergence is achieved when no more load reversals are detected in the T/C only springs. During the iteration process, each T/C only boundary condition is checked. If any springs are turned off (or turned back on), the stiffness matrix is rebuilt and model is resolved. For models with lots of T/C only elements, this can take a bit longer than a regular static solution.

Tethered Nodes

You may tether any or all of the node degrees of freedom to another node. See Tethering Nodes for more information.

Story Drift Nodes

The Boundary spreadsheet is also used to record nodes to be used for story drift calculation. For example, to indicate that a particular node is to represent the fourth story level for X direction drift, you would enter “STORY 4” for the X direction boundary condition for the node. These STORY entries may only be made in the translation degrees of freedom. See Drift for more information.

Boundary Conditions at Wall Panels

If the edge of a wall panel is to be viewed as continuously pinned or fixed, then the boundary condition for that wall must be set in the wall panel editor. Situations can arise where there is a difference between the wall panel edge boundary condition and the boundary condition defined at a node along that edge. In these situations the node boundary condition will always govern for that node. However, the rest of the edge will be based on the wall panel's boundary conditions.