What are Inclined Supports and how do I use them?
Most supports are defined with respect to the global coordinates system (i.e., the global X, Y, and Z axes). Inclined supports, however, allow the user to define a non-global, local axis system for the support if restraint is required in other directions. This is done by specifying a "reference point" in space towards which the local x axis of the support points.
There are three ways to define the reference point:
Option 1 - By explicitly entering the global X, Y, and Z coordinates.
Option 2 - By specifying relative distances x, y and z in the global directions from the node to the reference point.
Option 3 - Specifying an existing node whose coordinates define the reference point.
The ideal method for defining the reference point will differ depending on the particular scenario you are working with (structure type, geometry, etc.) Below three examples have been provided demonstrating each.
Example One
Consider a cylindrical structure such as a tank modeled using plate elements (for which the origin is at the center of the tank, as indicated by the label shown in the screenshot below):
Suppose your goal is to assign supports at the base which are normal to the surface of the tank. The local x axes of these supports, all being perpendicular to the tank itself, by definition must be oriented toward the center of the circle--in other words the origin at (0, 0, 0). Because all the nodes can be assigned a support with the same reference point, it is convenient to use Option 1 and explicitly enter the coordinates (0, 0, 0). This can be done under the Inclined tab in the Create Support dialog box by selecting the "Ref" radio button as shown below:
Once the reference point has been defined, the support type, directions, and spring constants (if any) can be specified as one would typically do for any other non-inclined support. Finally, it can be assigned to all the nodes at the base of the tank:
The command syntax in the input command file appears as follows:
SUPPORTS1 TO 36 INC REF 0 0 0 FIXED BUT FY FZ MX MY MZ KFX 5.5
Example Two
The above example utilized the coordinate method for defining the reference point because all the nodes shared the same one. Consider a scenario in which nodes must be assigned reference points that do not share the same coordinates, such as the wall shown below.
As you can see from the origin label, the wall is not aligned with any global plane (it is at a 45-degree angle relative to the X-Y and Z-Y planes). Unlike the cylindrical tank, if you want to assign supports at the base nodes normal to the surface, the same reference point coordinates cannot be used for each. This is where Option 2 comes in handy, however, because the same relative distances can be assigned to each support. For example, the x, y, and z relative distances (1, 0, -1) will work for each joint at the base of the wall, and can be entered by selecting the "Coordinate" radio button as shown below:
Once the support is defined, it can be assigned to all the nodes at the base of the wall. The command syntax in the input command file appears as follows:
SUPPORTS1 2 5 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 -42 INC 1 0 -1 FIXED BUT FY FZ MX MY MZ KFX 5.5
Example Three -
Lastly, let us consider the example shown below consisting of a simple frame structure for which the members are not aligned neatly with any global axis:
Suppose your goal is to assign a support to node # 6 oriented with its local x axis aligned with the member defined between joints 3 and 6. There is no quick and easy way to calculate the coordinates of a reference point or relative distances to a reference point, however no such calculations are necessary because node # 3 already defines a valid reference point. Option three can therefore be used to define the inclined support, specified by selecting the "RefJt" radio button as shown below:
SUPPORTS6 INC REFJT 3 FIXED BUT MX MY MZ KFX 30000
For more information on inclined supports please refer to section TR.27.2 in the STAAD.pro Help documentation, as well as Application Example # 19.