Loads - Joint Load / Displacement

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

Drawing Joint Loads

You can apply joint loads to joints. 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 Joint Load/Displacement above for more information on joint loads.

To Apply Joint Loads, Mass and Enforced Displacements

1. If there is not a model view already open then click    on the RISA Toolbar to open a new view and click    to turn on the Drawing Toolbar if it is not already displayed.
2. Click the Apply Joint Loads    button and define the load.  For help on an item, click    and then click the item.
3. You may apply the load by choosing jointson the fly or apply it to a selection of joints.

To choose joints on the fly choose Apply by Clicking/Boxing Joints and click/box on the joints with the left mouse button.

To apply the load to a selection, choose Apply Entries to All Selected Items.

Note

• To apply more loads with different parameters, press CTRL-D to recall the Joint Loads settings.
• You may also specify or edit joint loads/displacements in the Joint Loads Spreadsheet.
• You may undo any mistakes by clicking the Undo    button.

Joint Load Spreadsheet

The Joint Load Spreadsheet records the loads for the joints and may be accessed by selecting Loads  Joint Loads on the Spreadsheets menu.

When you open this spreadsheet you may view only one basic load case at a time.  Use the drop down list on the toolbar to specify a different load case.  The current load case is also displayed in the title bar at the top of the spreadsheet.

The Joint Label specifies the joint that receives the load or displacement.  The same joint may 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 or Y for the translational directions, or MZ for the rotational direction.

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.

Note

• If you have a “Reaction” or a “Spring” boundary condition for the same degree of freedom that you have an enforced displacement assigned, NO reaction will be calculated.  See Reactions at Joints with Enforced Displacements to learn how work around this limitation.

Joint 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 joint 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 Joint 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 joint mass on the Joint 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.

Joint 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.

Joint 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.