Loads - Nodal Load / Displacement

You can specify nodal loads, and enforced nodal displacements and nodal mass in any of the global degrees of freedom. Loads and displacements can be applied in any non-global direction by defining components of the load in the global directions. This can be accomplished graphically or in the spreadsheets. See Drawing Nodal Loads below to learn how to apply nodal loads/displacements/masses graphically.

Draw Nodal Loads

You can apply nodal loads to nodes. You must enter the load direction, magnitude and type. Make sure that you are careful to enter the correct BLC number that you want the loads assigned to. See Nodal Load/Displacement above for more information on nodal loads.

Apply Nodal Loads, Mass and Enforced Displacements

To apply nodal loads, mass and enforced displacements:

1. Go to the Home ribbon.

   

   Click on image to enlarge it

2. Click on the Nodal icon to display the nodal load information in the ‘Properties Panel’.

   

3. Define the Nodal Load information:

   1. Click the L, D, M arrow and choose the load type from the menu.

   2. Click the Direction arrow and choose the direction.

   3. Click the BLC arrow and choose the Basic Load Case.

   4. Type in a Magnitude, (k, k-ft), if applicable.

   5. Click the Inactive arrow and choose whether the Nodal Load should be ‘Active’ or ‘Inactive’.

      For help on an item, click the Help icon in the upper right corner of the program (as shown in the following image).

      

4. You can apply the load by choosing nodes on the fly or apply it to a selection of nodes.

   • To choose nodes on the fly, click the Click to Apply button, and click on or box the nodes with the left mouse button.

   • To apply the load to a selection, click the Apply to Selected button.

   • If no nodes are selected, Apply to Selected will assume the full model is selected and apply changes to all nodes. If any node is selected, Apply to Selected will only apply to the selected nodes.

Nodal Load Spreadsheet

The Nodal Load spreadsheet records the loads for the nodes, It can be accessed by choosing Nodal Loads from the ‘Data Entry’ section of the ‘Explorer’ panel.

When you open this spreadsheet You can view only one basic load case at a time. Use the drop down list on the toolbar to specify a different load case.

The Node Label specifies the node that receives the load or displacement. The same node can be listed any number of times.

The next column indicates the value is a load or an enforced displacement. Enter "L" if it’s a load, "D" if it's a displacement and “M” if it is a mass.

The direction code indicates in which of the global directions the value is applied. Valid entries are X,Y or Z for the translational directions, or MX, MY orMZ for the rotational directions.

The Magnitude column holds the value of the load, displacement or mass. The appropriate units for the magnitudes are displayed at the top of the column. Which units apply depends upon whether the value is a load, displacement or mass, and whether the direction is translational or rotational.

The last column allows you to set the specified load as an "Active" load that will be considered for the analysis, or an "Inactive" load that will not be included in the analysis.

Nodal Mass

For more sophisticated dynamics modeling, you can enter your mass directly as a mass rather than have the program convert it from a load. Using nodal masses offers several advantages such as being able to define directional mass and also the ability to specify mass moment of inertia’s to account for rotational inertial effects.

The units used for Nodal Mass are derived from the current Force and Length units as specified on the Units settings. For example, if the current force units are Kips and the current length units are Feet, you will need to specify your mass as kips / g and mass moments of inertia as kip-ft2 / g where g is the acceleration of gravity given in those units (feet per seconds squared).

When specifying a nodal mass on the Nodal Loads spreadsheet, enter an “M” for the load type. The directions are defined relative to the global axes. Enter translational mass using the global X, Y, or Z codes and mass moments of inertia by specifying the global MX, MY or MZ.

Nodal masses only allow dynamic response in the direction that they’ve been applied. This can be a very effective way to prevent local modes. A good example is a floor diaphragm modeled with plate/shell finite elements. If the mass is only specified for the two lateral directions, you will prevent any unwanted vertical modes. Care must be taken in limiting dynamic response using directional mass for complicated structures. A structure that has “coupled modes” will not give the “real” dynamic response when mass is only specified in one or two directions. A coupled mode is a mode that has mass participate in two or three directions at one time.

Nodal masses also allow you to account for rotational inertia effects by specifying a mass moment of inertia. These are particularly important when you’re using a rigid diaphragm and you’ve also lumped all your mass at one point (typically the center of mass). The rotational inertia effects contribute to the torsion on the diaphragm and should not be neglected. The following table shows some typical diaphragm shapes and the formulas to calculate their mass moment of inertias. Note that you can use the axis transformation equation to calculate the mass moment of inertia for diaphragms that are combinations of these basic shapes. For very irregular diaphragms, a more general equation is given based on the in-plane moment of inertia and the area of the diaphragm.

Mass Moment of Inertia About an Axis Through the Center of Mass

In the table below C.M. is the center of mass point. M is the total Mass of the area (typically including self weight, dead load, and a percentage of the live load) and is assumed to be uniformly distributed throughout. Ixx is the moment of inertia about the X-X axis. Izz is the moment of inertia about the Z-Z axis. A is the area. MMIo is the mass moment of inertia about some other point.