varies linearly along the length of the segment.Nonprismatic frame sections may be defined with the properties varying along the element length. In addition, the element length may be divided into any number of segments of unequal lengths. Most common situations can be modeled using one to five segments. The variation of the bending stiffnesses may be linear, parabolic, or cubic over each segment of length. The axial, shear, torsional, mass, and weight properties all vary linearly over each segment. Section properties may change discontinuously from one segment to the next. Click here to review figures and example input for nonprismatic frame sections.
TIP: When a single nonprismatic frame section property definition is applied over the length of several frame objects, the Assign > Frame > Nonprismatic Property Assignment Parameters command can be used to specify the total clear length of the nonprismatic section and the relative distance along the nonprismatic section to the start of the clear length of the frame object.
If needed, see the Frame Section Property Data Form topic for help related to the Property Name edit box, the Display Color and Change button, the Notes Modify/Show Notes button, the Section Shape drop-down list and the Sketch. As the name suggests, when the Show Current Segment Only check box is checked, the section selected in the spreadsheet area at the bottom of the form is displayed in the Sketch.
Show drop-down list. Use this drop-down list to toggle the Nonprismatic Section Segments display area between Plan and Elevation.
Show Aligned at This Cardinal Point drop-down list. Select a cardinal point along which to align the display of the section segments.
Start Section and End Section drop-down lists.
Select the name of a previously defined/default prismatic section from the Start Section drop-down list. This defines the end closest to joint i.
Select the name of a previously defined/default prismatic section from the End Section drop-down list. This defines the end closest to joint j.
The starting and ending sections may use the same named section if the properties are constant over the length of the segment. The material would normally be the same for both the starting and ending sections and only the geometric properties would differ, but this is not required.
Length Type drop-down list. Select Proportional or Absolute from the Length drop-down list.
Length edit box. When a nonprismatic frame section is assigned to an element, the actual lengths of each segment for that element are determined as follows:
The clear length of the element, Lc, is first calculated as the total length minus the end offsets: Lc = L - (ioff + joff). In that equation, L is the full length of the frame element and ioff and joff are the lengths of the end offsets along the length of the frame element at the i and j ends of the element, respectively.
If the sum of the absolute lengths of the segments exceeds the clear length, the absolute lengths are scaled down proportionately so that the sum equals the clear length. Otherwise the absolute lengths are used as specified.
The remaining length (the clear length minus the sum of the absolute lengths) is divided among the segments having variable lengths in the same proportion as the specified lengths. For example, for two segments with variable lengths, vl, specified as vl = 1 and vl =2, one third of the remaining length would go to the first segment, and two thirds to the second segment.
El22 and EI33 Variations drop-down list. Nonprismatic column/beam/brace section properties are interpolated along the length of each segment from the values at the two ends. Select linear, parabolic, or cubic from the EI22 and EI33 Variations drop-down lists to specify the variation of bending stiffnesses for the section.
ETABS calculates the linear, parabolic or cubic variation for EI33 as follows:
Linear: The value EI33 varies linearly along the length of the segment.
Parabolic: The value varies linearly along the length of the segment.
Cubic: The value 
varies linearly along the length of the segment.
This usually corresponds to a linear variation in one of the section dimensions. For example a linear variation in the width of a rectangular shape yields a linear variation for EI33. A linear variation in the depth of a rectangular shape yields a cubic variation for EI33. Finally, a linear variation in the depth of an I-shape yields a parabolic variation for EI33.
The interpolation of the bending stiffness in the 1-2 plane, EI22, is defined in the same manner to that for the 1-3 plane.
The remaining stiffness properties, other than EI33 and EI22 are always assumed to vary linearly between the ends of each segment. Similarly the mass and weight densities are always assumed to vary linearly between the ends of each segment.
If a shear area is zero at either end, it is taken to be zero along the full segment, thus eliminating all shear deformation in the corresponding bending plane for that segment.
Copy Current Row and Paste Append button. Place the cursor in a row and click this button to copy the row and add it to the spreadsheet area.
Effect on End Offsets Along the Length of Frame Elements
Frame section properties vary only along the clear length of the element. Section properties within the longitudinal end offset at the i-end of the element are constant using the starting section of the first segment. Section properties within the end offset at the j-end of the element are constant using the ending section of the last segment. Note that if a longitudinal end offset rigidity factor is specified, the specified part of the end offset is rigid and the rest has the section property described herein.
Access the Frame Section Property Data for Nonprismatic sections as follows:
Click the Define menu > Section Properties > Frame Sections command to display the Frame Properties form. Then do one of the following:
Click the Add New Property button to display the Frame Property Shape Type form ; click the Nonprismatic button/icon and click the OK button (or double click the Nonprismatic button/icon).
Highlight an existing Nonprismatic section in the large display area on the left-hand side of the form and click the Add Copy of Property or Modify/Show Property button.