Rigid diaphragms in RISA-3D distribute lateral loads across a floor without requiring the engineer to apply loads individually at each lateral member. Lateral loads can be generated automatically using RISA’s built-in load generator or applied manually when the engineer requires greater control over the load location, load category, or loading condition.
When the built-in load generator is used, RISA-3D calculates the total lateral load, wind or seismic, and applies it as a point load. The program also creates additional nodes representing the structure’s center of gravity and offset locations relative to that center of gravity. These nodes allow the program to apply the primary lateral load as well as accidental torsion load cases associated with the specified eccentricities.
This example demonstrates how to manually create comparable loading conditions to the built-in load generator for a rigid diaphragm in RISA-3D.
This workflow applies specifically to models that use rigid diaphragms. It is not intended as general guidance for selecting a diaphragm type or for manually applying wind and seismic loads in all RISA-3D modeling situations. Engineers should first confirm that a rigid diaphragm is appropriate for the structure and analysis assumptions.
Before manually applying lateral loads, clear any generated lateral loads and nodes from the model. This prevents duplicate loading conditions.
To clear the generated loading, open the Advanced tab, select the applicable lateral loading type, set the Building Code option to None, and then select Generate. RISA-3D will remove the generated point loads and associated nodes from the model.
Tip: RISA-3D will not remove generated nodes that are still referenced by other modeling elements, such as manually applied loads, boundary conditions, or members. Before clearing the load generators, verify that these nodes are not being used elsewhere in the model.
Determine the intended load application location and add a node at that point. Because the lateral load is being applied manually, the engineer must select a location that is appropriate for the structure’s geometry and intended load path.
To apply the load at the structure’s center of gravity similar to the generated loading workflow, first determine the center of gravity location. Engineers can calculate the center of gravity outside of the program or use built in tools to help determine the location.
The rigid diaphragm provides in-plane stiffness to the added node. For a typical diaphragm in the XZ plane, it restrains the node for the X, Z, and MY degrees of freedom. External boundary conditions should not be added in these directions as that can unintentionally restrain the entire diaphragm.
For additional background on how rigid diaphragms interact with model nodes, see the related section of the Help File.
The rigid diaphragm does not restrain the added node for the vertical or out-of-plane degrees of freedom. To prevent having an unsupported node and potential model instability, add external boundary conditions at the load application node for the Y, MX, and MZ degrees of freedom.
These boundary conditions can be defined as either reactions or springs. They are intended only to stabilize the node, and should not participate in the load path or receive load.
If the added node is not stabilized in the Y, MX, and MZ directions, those degrees of freedom may appear as LOCKED in the Node Reactions Spreadsheet after the model is run. Locked reactions produce instability warnings and do not report reaction magnitudes, so they should be resolved with the appropriate boundary conditions.
After solving the model, it is best practice to review the reactions to ensure that no degrees of freedom remain locked and that added restraints have zero reaction. Zero reaction at the added restraints confirms that they are providing stability to the model while not affecting the intended load path.
After the node and boundary conditions have been defined, apply the lateral load directly at the designated node.
When assigning the load, verify that the selected load category matches the intended loading direction and load combination reference. For example, use ELX or ELZ rather than the general EL category when applying seismic load separately in the global X or Z direction. These separate designations allow engineers to reference this loading independently in the load combinations.
For more information on basic load cases, load categories, and load combinations, see the article, Difference between Basic Load Cases, Load Categories, and Load Combinations, linked below.
Add additional lateral loading conditions as required for the project. This may include accidental torsion cases or other eccentric loading conditions.
Additional cases can be created by repeating the process above with alternate load application locations. Depending on the intended loading condition, accidental torsion may also be represented by applying an MY moment at the existing load application node.
For each additional load case, again verify that the selected load category is appropriate. For example, use the applicable preset eccentric load categories, such as ELX+Z, ELX-Z, ELZ+X, and ELZ-X, when defining eccentric lateral load cases in the model.