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.

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

Patterning of eccentricity moments

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.

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.

Design

Note: Patterned eccentricity moments are only considered in the design for the AISC or Eurocode head codes. If working to other head 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).

Patterning of eccentricity moments - limitations

There are some limitations in the calculation of column forces when working with patterning of eccentricity moments as described below:

The underlying method of calculating the column forces including eccentricity moment patterning fundamentally relies on the superposition of loadcase results from beams and braces connecting at the top of column stacks.

When the patterning of eccentricity moments is considered for columns, it is a limitation in the current Tekla Structural Designer version that other localised forces occurring at the top of column stacks, and which result either from directly applied loads or supported members other than the ones mentioned above, are not accounted for at those positions.

This includes cases such as,

Supported trusses,
Buttressing walls (against the steel/precast column),
Point loads applied at the top of column stacks,
Stacked columns (discontinuous),

And is applicable to both steel and precast columns as the same approach is used in both.

This is mitigated by the fact that design uses the maximum axial force in the stack and it is only locally incorrect such that this maximum is nearly always found.

Warning: An exception might be when there is a significant uplift force that puts the column in tension.

Example - point load applied at the top of column stack

A steel column has a point load applied at the top of the top stack as shown below:

When an unpatterned combination is reviewed in a Load Analysis View, the localised forces occurring at both the top and bottom of the upper stack reflect the point load applied at the top of the stack. (The value at the bottom being slightly larger due to the column self weight).

However, if eccentricity moment patterning is switched on for the same combination, the localised forces displayed at the top of the column stack do not take into account the directly applied load.

The design uses the maximum axial force in the stack. As this occurs at the bottom of the stack for this example, the design is still correct.

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 checked, 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 uncheck 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 eccentricity moments for live loadcases

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.