The drift check

Tekla Structural Designer
2020
Tekla Structural Designer

The drift check

Note: The drift check is only relevant to models that use the ACI/AISC head code.
By default the drift check is applied to all columns (of all materials), and all walls.

Drift check design options

In Design Settings > Drift Checks, there is 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 Drift/Seismic drift checks located under All stacks/panels>Drift Checks in the column/wall properties window.

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

Manually adjust 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 Drift Checks for the specific stack/panel in the column/wall properties window.

Perform drift checks

The checks are performed either by running Analyze All (Static) from the Analyze ribbon, or 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.

For those stacks to which the check has been applied, the storey drift in each direction (i.e. the difference between top and bottom deflection of the stack) is calculated, and from this the stability coefficient is determined.

Review drift checks

The building's overall drift check status is then displayed in the Design branch of the Status Tree in the Project Workspace.

  • Any column in this branch with a warning status can be double clicked on in order to highlight it in amber in the current view.
  • Double click on the Drift main heading in order to highlight all columns with warnings in amber
Note:

Drift warnings 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 drift checks can be interrogated in a couple of ways:

In the Review Data table, for those stacks to which the check has been applied, stability coefficients in both directions (RatioDir1,RatioDir2) are calculated as:

Stability coefficient = Δ21

Where

Δ2= second order drift

Δ1= first order drift

A Warning status is displayed against any stack in which the ratio exceeds 1.71 and a Beyond Scope status is applied if it exceeds 2.85 (as this indicates the structure lacks stiffness in whole or in part).

Tip:

If there are a lot of columns in the building - in order to quickly determine the biggest coefficient in each direction, simply click the RatioDir1 header until the columns are arranged in increasing order, then repeat for RatioDir2.

Stability coefficients can only be calculated accurately by performing a second order analysis, however indicative values can still be displayed when only a first order analysis has been performed. These are determined from the column's elastic critical load factor, λ:

21) = 1/(1- 1/λ)

Approximate second order drift:

Δ2= Δ1/(1- 1/λ)

Report drift checks

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

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