Steel column connection eccentricity moments

Overview

Nominal eccentricity moments that arise from beam end reactions are considered in Tekla Structural Designer's steel column design.

These moments do not come directly from the global analysis but instead are calculated at the ‘load analysis’ post-processing stage as follows:

At each level the eccentricity of each connection is taken as half the depth of the supporting column, plus an additional user defined offset from the column face.
At each level the pinned beam end reactions connecting to the column at each face are determined.
If braces also connect to the same face, the force in the brace will also be taken into consideration if the "Include force in eccentricity moment" brace release property is checked for the appropriate end of the brace.
Taking the beam end reactions (and brace forces if included) on opposite faces multiplied by their connection eccentricities, resultant eccentricity moments are determined.
These moments are then distributed above and below the level based on the column stiffnesses.

The eccentricity moments resulting from live loads can be patterned if required to account for the likelihood that the load is not present on all spans simultaneously.

Note: Eccentricity moments are typically assumed not to be transferred beyond the level at which they are applied.

Design

Note: Patterned eccentricity moments are only considered in the design for the AISC or Eurocode regional codes. If working to other regional codes, while the patterned eccentricity moments are calculated, only the fully loaded pattern (P0) is used in the design.

In general, eccentricity moments are only added to the 'real' moments at the ends of each stack and are only added if they make the design worse.

If you have elected to pattern live eccentricity moments these are considered in conjunction with the eccentricity moments from other types of load, and with the 'real' moments.

As the eccentricity moments are considered localised to each floor the full axial force from other floors is maintained. The axial force at the level under consideration will be slightly reduced with patterning enabled as the live floor loading will not be present on all sides simultaneously.
Since it is not known whether a reduced axial force with more eccentricity moment is a worse case than full axial and a lower (or even zero, in the balanced case) eccentricity moment, the design loops through all patterns in order to consider each eventuality.
The patterned eccentricity moments are considered in all design checks apart from 'Shear', (which is unaffected).
To keep the design details to a manageable level, results for every pattern are not listed; the pattern which produces the governing design forces is listed in the check combination and location tree and details heading, (as shown below).

Define connection eccentricity values

The eccentricities at each level are defined in the column properties and a different eccentricity can be applied to each face.

As long as the option to apply eccentricity at a face is selected, the total eccentricity at that face is taken as half the dimension of the supporting column, plus the additional eccentricity from the face as specified in the Properties window shown above.

If you clear the option to apply eccentricity at a face the end reaction on that face is applied axially.

Note: Face A can be identified graphically, from which the other faces follow, see: Steel member orientation

Pattern of eccentricity moments for live loadcases

To account for the likelihood that live load is not present on all spans simultaneously, eccentricity moments resulting from live loadcases can be patterned if required.

When eccentricity moment patterning is enabled you must then indicate which of the live cases are to be patterned, (you may for example decide not to pattern storage loads.)

For those live cases with patterning enabled, five patterns are considered. These are:

Pattern 0 is for the full live load at all positions i.e. no patterning - this gives the maximum axial force in any one stack with (usually) lower eccentricity moment.

Patterns 1 to 4 are ‘true’ patterns switching live load ‘on’ and ‘off at each pair of positions around the column in order to generate the maximum live eccentricity moments about the major and minor axes of the column.

Note: The same pattern is applied at the top and bottom of the stack, so for example it is not possible to have P1 at the top and P4 at the bottom.

Patterning can be switched on for specific live loadcases in a two-step process as follows:

From the Home ribbon:
1. Click Model Settings > Loading > General
2. Select Use patterning of eccentricity moments for steel columns
3. Click OK
From the Loadcases page of the Loading dialog:
1. Select a live loadcase that you want to be patterned
2. Select Pattern Eccentricity Moments for Steel Columns
3. When patterning has been selected for each of the required loadcases, click OK

Review connection eccentricity moments

Because eccentricity moments do not come directly from the global analysis they cannot be displayed graphically in a Results View, they can only be displayed on a column by column basis by opening a Load Analysis View.

With a Load Analysis View open and the required loadcase or combination selected in the Loading list, you then select the Major, or Minor direction in the Loading Analysis ribbon.

The 'real' moments are displayed as a shaded diagram using solid lines, the eccentricity moments as an unshaded using dashed lines:

The red marker line can be set to a specified distance in the Properties window to allow the real and ecc. moment values above and below the line to be displayed.

Displaying patterned eccentricity moments

When you select a patterned live loadcase a Show ecc. moment pattern box will become available in the Properties window.

After selecting Show ecc. moment pattern you can then click Ecc. moment pattern in order to select the pattern to display from the droplist.

Distribution of eccentricity moments for 'cantilever' stacks

Provided the tip of an upper column stack is restrained, then irrespective of whether it is marked as a cantilever or not, eccentricity moments are shared between it and the lower stack. The moments are distributed in proportion to the upper/lower stack stiffnesses, but ignore the effect on stiffness of the cantilever end marking (simply distributed based on EI/L).

If the tip of an upper stack is unrestrained all of the eccentricity moment goes into the lower stack.

To illustrate, consider the following frame which is loaded at first floor level:

Because the resulting beam end reactions are at an eccentricity e to the column center lines, an eccentricity moment is generated. As stated above, how this then gets distributed between the stacks will depend on the restraint conditions.

Initially the tips of the upper stacks are unrestrained and for the purpose of this exercise one has been marked as a cantilever while the other has not.

By displaying Load Analysis Views for each of the columns, it can be seen that all of the eccentricity moment goes into the lower stacks.

However, if a beam is introduced between the column tips to restrain them as shown below, the distribution of the original eccentricity moments at the first floor level would change.

Because the tips are now restrained, the eccentricity moments are distributed above and below the first floor level in proportion to the upper/lower stack stiffnesses, (irrespective of the cantilever marking).