Nominal eccentricity moments arise in precast columns due to the reactions from pin ended beams being applied at eccentricities to the column centerline. The eccentricity moments are calculated for all regional codes, and can be interrogated in the Load Analysis View.

If working to Eurocodes, these eccentricity moments are also taken into account when precast columns are transferred to Tedds for design.

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

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.

Design to Eurocodes

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.

There are some limitations in the calculation of precast 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,

And is applicable to both steel and precast column 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.

Example - point load applied at the top of column stack

As the same approach is used in both steel and precast columns, a steel column with a point load applied is used in this example.

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.

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

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

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

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

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.