Loads - Surface Loads

Surface loads are loads that are spread out over the surface of a plate element or wall panel. RISA-3D allows surface loads directed along the global axes, the local axes, or projected in the direction of the global axes. Loads may be input manually or assigned graphically. See Drawing Surface Loads below to learn how to draw surface loads graphically.

Draw Plate Surface Loads

To apply surface loads to plates enter the load direction and magnitude. Make sure that you are careful to enter the correct BLC number that you want the loads assigned to. See Surface Loads above for more information on surface loads.

Apply Plate Surface Loads

To apply Plate Surface Loads:

1. Go to the Home ribbon.

   

   Click on image to enlarge it

2. Click on the Plate Surface icon to bring up the plate surface load information in the ‘Properties Panel’.

3. Define the load in the Plate Surface Properties Panel. Use the dropdown menu to select load Direction, and BLC.

   For help on an item, click the Help icon and then click the item.

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

   • To apply the load to a few plates choose Click to Apply and click on the plates with the left mouse button.

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

     If nothing is selected, Apply to Selected will assume the full model is selected and apply changes to all plates.

Draw Wall Panel Surface Loads

To apply surface loads to wall panels enter the load direction, start location (from bottom), height, and magnitudes (top and bottom). Make sure that you are careful to enter the correct BLC number that you want the loads assigned to. See Surface Loads above for more information on surface loads.

Apply Wall Panel Surface Loads

To apply Wall Panel Surface Loads:

1. Go to the Home ribbon.

   

   Click on image to enlarge it

2. Click on the Wall Surface icon to bring up the wall surface load information in the ‘Properties Panel’.

3. Define the load in the Wall Surface Properties Panel. Use the dropdown menu to select load Direction, BLC, load distribution (uniform or tapered), and load height (entire wall or partial).

   For help on an item, click the Help icon and then click the item.

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

   • To apply the load to a few wall panels choose Click to Apply and click on the wall panels with the left mouse button.

   • To apply the load to a selection, choose Apply to Selected. If nothing is selected, Apply to Selected will assume the full model is selected and apply changes to all wall panels.

   • To apply tapered wall load, choose Tapered from Load Distribution dropdown menu, and specify Top Magnitude and Bottom Magnitude.

   • To apply partial height wall load, choose Partial from Load Height dropdown menu, and specify load Start Location and Height.

Surface Loads Spreadsheet 

The Surface Loads Spreadsheet records the surface loads for the plate/shell elements and wall panels, and may be accessed by selecting select Surface Loads from the Data Entry menu in the Explorer panel. Alternatively, it can be accessed by going to the Spreadsheets Tab Data Entry Surface Loads.

Click on image to enlarge it

 

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 current load case is also displayed in the title bar at the top of the spreadsheet.

Under the Plate tab the first column contains the label of the plate to be loaded. The second column defines the direction of the load. Direction options are discussed in the next section. The third column holds the magnitude of the load.

Under the Wall Panel tab the first column contains the label of the wall panel to be loaded. The second column defines the direction of the load. Direction options are discussed in the next section. The third and fourth columns hold the magnitude of the load. The fifth and sixth columns define the Start Location (bottom) and Height of the load.

For loads that apply to the full height of the wall, leave the start location and height as zero.

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.

For models integrated from RISAFloor, the Diaphragm Tab records the surface loads applied on the semi-rigid diaphragm. These loads are automatically generated during the integration from RISAFloor to RISA-3D. These lines will be grayed out because they cannot be edited from RISA-3D since they are tied to the RISAFloor analysis. If you detach the model from RISAFloor in RISA-3D, these loads will not carry over since semi-rigid diaphragms are a feature only available for models integrated with RISAFloor.

Surface Load Directions

The direction code indicates the direction of application for the surface load.  The following directions are available:

The following diagram illustrates the difference between local (x, y, z) and global (X, Y, Z) direction loads. The local direction loads line up with the element’s local axis directions, so their direction relative to the rest of the model changes if the element orientation changes. Global loads have the same direction regardless of the member’s orientation. Keep in mind that global loads are applied without being modified for projection. For example, a global Y direction load of 1 kip/sq.ft. applied to an element with an area of 10 sq.ft., which is inclined at 45 degrees, generates a total force of 10 kips.

Projected loads, on the other hand, are applied in the global directions, but their actual magnitude is influenced by the element orientation.  The load is applied to the projected area of the element that is perpendicular to the load.

For example, a "V" direction load is a projected load applied in the global Y direction. The actual magnitude of the load is the entered magnitude reduced by the ratio A/Axz.  "A" is the actual area of the element and Axz is the element's projected area on the X-Z plane, which is always less than or equal to the actual area. See the following figure:

If the "Axis of Projection" in the figure is the Y-axis, then the shaded area is the total element area "projected" onto the plane perpendicular to the Y-axis (which happens to be the X-Z plane). The total load generated is equal to the input magnitude applied to the projected area. The generated load is then applied to the whole area, so the generated load magnitude is reduced accordingly.

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

Surface Loads at Openings (Wall Panels)

When a surface load is applied to a wall panel with openings it is converted into equivalent nodal loads around the openings. The example below is a wall panel with an opening, and a surface load applied to the lower portion of the wall.

The program calculates transient nodal loads according to the following approximation procedure:

1. The centroid of the union of the surface load and the opening is determined.

   

2. Using the centroid location, the surface load within the opening region is split into two parts.

   

3. Each of the two parts the surface load are summed up and smeared as point loads along the edge nodes of the opening. The forces and moments are conserved with this approximation method. These generated nodal loads are transient, and will be deleted when the solution result is deleted. These loads are not visible by default, however if you would like to view them you can select Include Transient WP Loads under Model Display Options. Below is an example: