Non-structural elements are the elements that contribute loads to the primary structure, but don't contribute to the stiffness.
There are three (3) ways to define non-structural elements in SACS:
- Dummy Structures
- Appurtenance Structures
- Alternate Method
Dummy Structures
A dummy structure is a structure consisting entirely of a smaller self-contained non-structural element that is attached to the main structure, such as a boat landing or an access deck in the wave zone. These elements are used to generate environmental loads, but are not included in the stiffness analysis. The loading incurred by the dummy structure is distributed to the boundary joints, assuming that the dummy structure is a rigid body. All elements attached to a deleted joint are removed from the model along with all joints designated as deleted joints.
Appurtenance Structures
The structure is not connected to the primary structure. They are intended for a longer structure with a series of connections, such as Risers and J-tubes. The joints of the appurtenance structure are indicated along with the member in the primary structure to which the load is to be transferred. The members of the appurtenance structure are removed from the model by specifying the members or groups. The loads on the appurtenance members will be transferred to the end joints of that member. The end joint loads will be transferred to the primary structure as member concentrated loads at the point closest to the end joint. All references to these appurtenances will be deleted from all output reports.
Alternate Method
The structure is modeled as elements, and it's not deleted for loading purposes only. The stiffness group modulus of elasticity or section stiffness properties is reduced to reduce the stiffness contribution to the stiffness matrix. Users need to ensure each degree of freedom still has sufficient stiffness, so there will be no instability or loss of accuracy in the stiffness matrix analysis. We recommend not retaining the degrees of freedom for the reduced stiffness elements for the mode shape extraction because it can lead to some odd modes in the structure due to the very flexible structure elements.
How to define dummy and appurtenance structures in Precede?
DUMMY STRUCTURES
In Precede, go to the Environment tab > Non-Structural Elements > Dummy Structure 
In the Dummy Structure definition,
The Boundary Joints contain a list of joints that connect the dummy structure to the rest of the structure.
The Dummy Joints contains a list of joints that lie solely within the dummy structure.
If you are looking at Datagen, the joints will be listed under KEEP and DELETE lines.
APPURTENANCE STRUCTURES
In Precede, go to the Environment tab > Non-Structural Elements > Appurtenance Structure
In the Appurtenance Structure definition,
For the appurtenance structure definition, we are going to define DELJNT (delete joints), DELMEM (delete members) or DELGRP (delete group).
Under the Joint Tab,
The 'Joint' and 'Member' table contains a list of the joints to be deleted and the corresponding members that will receive the joint loads.
Under the Member tab,
The 'Remove' list contains all members to be removed.
Under the Group tab,
The 'Remove' list contains all member groups to be removed.
In Datagen,
When you define the dummy or the appurtenance structure, it will run through the Seastate, which is the SACS automatic load generation module. Seastate will output the OCI. file that is going to have all the distributed member loads and joint loads that represent the input waves that were defined. SACS will also remove those dummy structures from the model and distribute the loads that were applied to the structure to the primary steel to retain the structure.
A detailed explanation on this can be found in the SACS Seastate manual, section 2.7.
Also, refer to this article to understand how the dummy structure and delete joint options work in SACS.