A floating or hanger column is a vertical member that is supported by a transfer beam and doesn’t transfer its load directly to the foundation. This type of construction is widely used in multi-story commercial and residential buildings because it allows for the column layout on individual floors to be modified where larger, open spaces are required by the building’s program. Although these types of members are not automatically supported, they can be easily created in RISAFloor using one of two methods.
Method 1: You can model them as Lateral columns in RISAFloor with gravity columns below. RISAFloor requires that all beams are supported below so the gravity columns act as a dummy support member.
You can then solve the model in RISAFloor and the loading will be incorrect (because of the dummy gravity columns) but once to export to RISA-3D only the lateral members will be exported. As a result, you will have the correct geometry. Once in RISA-3D, simply re-solve the gravity load combinations to finalize the hanger design.
Method 2: This method will give accurate floor mass values in RISAFloor. Model in a dummy floor 6 inches below the floor that is supported by the hangers. This floor would contain all of your dummy non-hanger columns that trick RISAFloor into thinking that the true hanging columns are supported. The dummy columns can stop at the dummy floor level rather than spanning all the way down to the base.
Then, solve your model and then take the reactions from the dummy columns and re-apply them as point loads to the true hung floor. Solve again and the design for the hung columns will be correct. Using this method, you can ignore the dummy column reactions after the first solve.