Local and global models

Separate from a global model, it may be necessary to create a more detailed local model to more effectively evaluate localized behavior for the design of certain components, connections, or systems. Using this approach, sensitive response measures (internal forces, stresses, strains, displacements, etc.) at critical structural locations may be more accurately captured and then averaged for design purposes. Data and design specifications from a local model may then supplement a global model, which should sufficiently capture global behavior through coarse discretization.

Local model

An example of when global response measures may be applied to a detailed local model for design purposes is when global forces are resolved for a steel frame structure, then applied to local systems for connection detailing.

Of critical concern when creating a detailed local model is the application of boundary conditions. When global-model displacements are applied to the local model, local-model boundary joints should be supported by restraints or stiff springs. Displacement may then be applied according to the appropriate load pattern through Assign > Joint Loads > Ground Displacements.

Local-modeling tips and procedures are as follows:

1. Create joints within the global model which will coincide with the joints and boundary conditions of the local model.
2. Analyze the global model to obtain displacements and rotations at these boundary joints.
3. Develop the local model using shell objects. For each global-model joint which will coordinate with a local-model boundary, create multiple joints (L1, L2, ..., Ln) in the local model.
4. For each group of joints at a local-model boundary, create a dummy joint (G) in the same location, then assign a body constraint to joint G and L1 through Ln.
5. Assign restraints (U1, U2, U3, R1, R2, and R3) to joint G, then assign displacements obtained from the global model to joint G through Assign > Joint Loads > Displacements.
6. Analyze the local model for the applied displacement loads. As an alternative, forces may be applied to joint G locations, though supports must be modeled.

Boundary joints should be located far enough from the region for which accurate results are to be obtained as to not influence results.