Drift check

Tekla Structural Designer
2021
Tekla Structural Designer

Drift check

By default, this check is applied to all columns (of all materials) and all walls at every level in the model.

Note: The drift check is only relevant to models that use the ACI/AISC head code.

Results are presented in terms of levels, with detailed results for individual column/wall stacks/panels also being available.

For levels set as floors a Check for drift option is displayed in Level Properties. By unselecting this option, individual levels can be excluded from the check reporting.

In this check the story 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 as follows:

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).

Note: 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)

A workflow for running the check is outlined below:

Run the check

Load combinations are automatically checked on a level by level basis for drift by running a static analysis or design.

Review the check status and details

Once the check has been run, an overall status is reported in the Design branch of the Status Tree, the overall status being the worst status from each of the individual levels.

Right clicking on the status opens a context menu.
From this menu you can display the drift check summary in a Review Data Table.
By unselecting the Summary Only option in the ribbon, the table can also display detailed results on a stack by stack basis if required.
The checks can be investigated in more detail by clicking the Details button at the end of any row.

This displays the stability coefficients in both directions (RatioDir1,RatioDir2). These are calculated as follows:

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).

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)

Review the check graphically

From the right click menu in the Status Tree you are also provided with an option to graphically review the checks in a Show/Alter State View.

The color coded legend makes it easy to see individual failing members. By hovering the cursor over a member, its drift results are displayed in the tooltip.

From the same Show/Alter State view, by changing the Mode in the Properties window, you also have the option to switch off checks for specific stacks if you require.

Switching off inappropriate checks and merging stack lengths

After reviewing you might choose to switch off checks for selected columns and walls, or individual stacks/panels within selected columns and walls: this can be done directly via the Properties window, or graphically in a Show/Alter State view (as described in the previous section).

You might also choose to take advantage of the option to merge short stacks automatically where they are less than a specified limit. Stacks check lengths can also be manually adjusted by merging them together if required.

An option is also provided which allows you to switch off checks for specific levels if you decide they are inappropriate, (for example if the story height is very short). The checks at that level are still performed, but they are excluded from the tabular review data display.

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