The wind drift check

Tekla Structural Designer
2020
Tekla Structural Designer

The wind drift check

The Wind Drift check is performed during the structure static design and also when any 3D analysis is run in isolation. If a sub-set of combinations are considered for analysis then only those combinations are considered in the drift checks, (allowing engineers working on larger structures to investigate and optimize the lateral load resisting systems more rapidly).

The check is performed for (wind) combinations using the combination SLS (Service Level) factors - (which can be < 1.0).

By default the check is applied to all columns (of all materials) and all walls.

Wind drift check design options

In Design Settings > Sway & Drift Checks, there is a 'Check wind cases only' option (default On) that applies to the check as follows:

  • With the setting On, the Wind Drift check only considers the effects of the wind load case(s) in wind combinations.
  • With the setting Off, the check considers the effects of all load cases in wind combinations (which would include drift induced by gravity loads).

On the same page of the Design Options there is also a Merge short stacks option (default Off) that applies to the check as follows:

  • With the setting Off, the check considers all stacks of all columns/walls apart from those that have been manually excluded.
  • With the setting On, any stacks that are less than the limit specified are merged with the stack below, and the check is performed on the merged stack length. The check is not performed on single stack columns/walls that are less than this limit

Manually exclude an entire column or wall or an individual column stack or wall panel from the check

An entire column/wall can be excluded from the check by unselecting Wind Drift check located under All stacks/panels>Sway and Drift Checks in the column/wall properties window.

Similarly an individual stack/panel can be excluded from the check by unselecting Wind Drift check located under Sway and Drift Checks for the specific stack/panel in the column/wall properties window.

Manually adjust the automatically determined stack lengths

Columns are divided into stacks and walls are divided into panels at floor levels where members or slabs connect to the column/wall. For the purposes of Sway/Drift, Wind Drift and Seismic Drift checks only you can override the default stack/panel divisions by merging a stack/panel with a lower stack/panel. The length of the combined stack/panel is then used in the checks.

In order to do this you would have to select Merge with stack below located under Sway and Drift Checks for the specific stack/panel in the column/wall properties window.

Wind drift calculations

For those stacks to which the check has been applied, the lateral drift in each direction (i.e. the difference between top and bottom deflection of the stack) is determined for each wind load case and wind service combination. This drift is then compared against a user-defined limit (the default is 1/300 of the story height, in line with Eurocode 3 recommendations, but you are free to specify a limit of your choice). Different limits can be applied to different stacks if required.

The checks are performed using results from a 1st order linear analysis (with no Reduced stiffness factor), which are generated by running any of the Design (Gravity), or Design (Static) commands from the Design ribbon.

Note:

For non-linear models with Tension Only bracing it is essential that the 'X Brace' pattern is used to input the braces as brace pairs rather than creating them individually. For linear analysis one of the braces in a brace pair is automatically inactivated, ensuring that the model's lateral stiffness, and hence lateral drift, is reasonably correct. If braces are input individually this will not be the case.

Review wind drift checks

Any stack failures are flagged in the Design - Wind Drift branch of the Status Tree in the Project Workspace.

  • Double clicking on a failing column in this list causes it to be highlighted in red in the current view.
  • Double clicking on the Wind Drift heading at the top of the list causes all failing columns to be highlighted in red
Note:

Wind drift failures remain highlighted in the current view until you press Esc to clear the highlight.

In addition to viewing the summary in the Project Workspace, the wind drift checks can be interrogated in a variety of ways:

Report wind drift checks

If required, the wind drift results table can be included in printed output by adding the Analysis> Wind Drift chapter to your model report.

Note:

The Wind Drift chapter has a Settings option that can be used to dramatically reduce the number of report pages. By default all columns and walls are output, but you can specify instead the most critical 'XX' columns and walls, (where 'XX' is a number of your choice).

The same Settings option also allows you to choose between reporting only the critcal stack/all stacks, and reporting only the critical combination/all combinations.

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