Unbalanced Moments
Gravity loads on columns consist of an axial load and moments about both axes where the column design moments are caused by unbalanced loading. Such moments are created when there is an imbalance in the loads on one side of the column versus the loads on the other side. Each beam induces a moment equal to the reaction times the connection eccentricity. The unbalanced moment is the difference in the reactions on opposite sides of the column multiplied by the eccentricity.
Stub cantilevers also induce moments into the column that are considered in the design. The Live Load component of the cantilever moment is subject to the skip loading performed to calculate the controlling design loads. Furthermore, stub cantilevers framing off of the side of a beam induce torsion into that beam; if that beam is supported by a column, the torsion in the beam is transferred to the column as a moment in the column.
The connection eccentricity is determined based on criteria you specify. The eccentricity is specified as either an absolute dimension or as an increment to be added to one-half of the column dimension in that direction. The latter eccentricity is adjusted internally if the column is sloping.
If you do not want moments considered in the design, an absolute dimension of 0.0 can be specified. This does not eliminate moments due to stub cantilevers.
The default eccentricity assigned to modeled hanging columns in RAM Modeler is 0.0. You may assign a non-zero eccentricity value to hanging columns using the Layout – Column – Assign Eccentricity command.
If the option to skip-load the Live Load around the column is selected in the Criteria – Design Defaults command, the program will automatically skip-load, or pattern, the Live Load around the column to determine the worst case of axial load and unbalanced moment. For example, by placing the Live Load on only two sides of the column, the resulting axial load is reduced, but the unbalanced moment in both axes is increased. This may produce a worse design condition than the fully loaded column with smaller unbalanced moments. As shown in Table 6 below, 34 different cases of skip-loading (pattern loading) of the Live Load from the beams around the four sides of the column, top and bottom, are considered. Table 5 shows the skip-load conditions associated with the load cases from Table 6.
If the option is not selected the program will not skip-load the Live Load around the column. Only Load Conditions 1 and 10 in table 5 will be investigated (the Live Load applied to all four sides of the column), which means that only Load Cases 1 and 18 in Table 6 will be used to determine column design axial loads and moments. In these cases there will only be design moments if the beam reactions on opposite sides of the column are unequal.
See Load Cases for more information on the load cases and conditions considered by the program.
