Expert: Arshad Khan - 4/12/2009

Question
Can we do eccentric  connection between column and beam in etaabs? thanx so much.

Answer
Hi Ravikumar,

The answers to many of the questions you are asking me on ETABS are readily available in the help menu, so please make sue of that. I copy/paste from the help menu below what you are asking about:

Form: Frame End Length Offsets

In ETABS frame section properties are assigned to line objects. However, actual structural members have finite cross-sectional dimensions. When two members such as a beam and column are connected at a point, 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 element. ETABS provides the capability of defining end length offsets along the length of frame elements to account for these finite dimensions of structural elements.

When a line object is used to model a frame section, the line object is assumed to be located at the centroid of the frame section. Thus, when line 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 type line object as follows:

Select the line object to which the end length offset is to be assigned.

Click the Assign menu > Frame/Line > End (Length) Offsets command to open the Frame End Length Offsets form to define end length offsets along the length of frame elements and rigid frame end joint offsets. Be careful that you do not get these two types of offsets confused.

On the Frame End Length Offsets form, select parameters for the following:

Automatic from Connectivity  

If the Automatic from Connectivity check box is checked, 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 elements. It assumes the offset length for all brace-type frame elements to be zero. (You can define your own non-zero offset lengths for brace elements if necessary.) Also, the dimensions of brace elements that frame into the ends of column and beam elements 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

If the Define Lengths check box is checked, enter values for the length of the offset at the start (End I) and end (End J) of the line object.

Rigid zone factor

If the Define Lengths check box is checked and you want to specify a rigid zone for the frame element, enter a value between 0 and 1 in the rigid zone factor edit box. 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 OK button.

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 line object is ignored unless the line object also has a frame section assigned to it.