Form: Frame Assignment - End Length Offsets
In ETABS frame section properties are assigned to frame objects. However, actual structural members have finite cross-sectional dimensions. When two members such as a beam and column are connected at a joint, there is some overlap of the cross-sections. In many structures, the dimensions of the members are large, and the length of the overlap can be a significant fraction of the total length of the frame section. ETABS provides the capability of defining end length offsets along the length of frame sections to account for these finite dimensions of structural components.
When a frame object is used to model a frame section, the frame object is assumed to be located at the centroid of the frame section. Thus, when frame objects (frame sections) intersect in a model, it means that the centroids of the associated frame objects intersect. In a real structure, that is not always the case. For example, it is not unusual for one or more floor beams in a building to frame eccentrically into a column. ETABS provides the capability of defining frame joint offsets to account for these eccentric connections.
Assign end length offsets to a frame object as follows:
Select the frame object to which the end length offset is to be assigned.
Click the Assign menu > Frame > End Length Offsets command to open the Frame Assignment - End Length Offsets form to define end length offsets along the length of frame objects and rigid frame end joint offsets. Be careful that you do not get these two types of offsets confused.
On the Frame Assignment - End Length Offsets form, select parameters for the following:
Automatic from Connectivity option. If this option is selected, ETABS will automatically calculate the offset length. ETABS bases the end offset length at the end of the beam on the maximum section dimensions of all columns that connect to that end of the beam. This is the default. ETABS automatically calculates offset lengths for beam and column-type frame sections. It assumes the offset length for all brace-type frame sections to be zero. (You can define your own non-zero offset lengths for brace sections if necessary.) Also, the dimensions of brace sections that frame into the ends of column and beam objects are not considered when calculating the end offset dimension for a column or a beam.
Beams. When ETABS automatically calculates the end offsets along the length of a beam, it bases the end offset length at an end of the beam on the maximum section dimensions of all columns that connect to that end of the beam.
Columns. When ETABS automatically calculates the end offsets along the length of a column, it bases the end offset length at an end of the column on the maximum section dimensions of all beams that connect to that end of the column.
Note the following about ETABS automatically calculated end offsets along the length of frame members:
When more than one beam frames into a column, ETABS bases the end offset in the column on the deeper beam.
End offsets in beams are controlled by the size of the column below. The column above is not considered.
Define Lengths, End I, End J option. If this option is selected, enter values for the length of the offset at the start (End I) and end (End J) of the frame object.
Rigid zone factor option. If this option is selected, the edit box can be used to enter a value between 0 and 1 to specify a rigid zone for the frame section. The rigid-zone factor specifies the fraction of each end offset assumed to be rigid for bending and shear deformations. When a fraction of the end offset is specified rigid, the outside portion of the end offset is assumed rigid, that is, the portion at the end of the frame member. By default ETABS assumes the rigid end factor to be zero; that is, the end offsets are fully flexible and they have the same frame section properties as are assigned to the rest of the member.
The rigid zones of the end offsets never affect axial and torsional deformations. The full element length is always assumed to be flexible for those deformations.
Click the Apply button. Any previously assigned end length offset will be replaced by this assignment.
When the Apply button is used, the Frame Assignment - End Length Offsets form will remain open until it is closed by clicking the Close button. This allows selection of another frame object(s), to which a different assignments can be made.
If only one assignment is being made to only one set of selected objects, the OK button can be used to both apply the assignment and close the form.
Output forces for the end of a frame member are provided at the inside face of the end offset along the length of the member. No output forces are produced within the end offset.
Any end offset assigned to a frame object is ignored unless the frame object also has a frame section assigned to it.
See Also