Drift

You may calculate and report inter-story drift based on calculated node displacements.  The calculations will be performed in the two horizontal translation directions, but not for the vertical direction.

Drift Definitions Spreadsheet

The Drift Definitions spreadsheet defines where the drift calculations will be performed. If the model contains diaphragms, the nodes connected to that diaphragm are automatically considered in the drift calculations. Drift calculations can be performed at specific elevations (where diaphragms don't exist). However, these elevations must be manually entered in the Drift Definitions spreadsheet. These elevations are defined with respect to the vertical axis of the model.

In addition, you can specify individual nodes to be used in drift calculation. Node definitions are included for a number of reasons. They allow models, defined in older versions of the program (which only used defined "story joints"), to produce identical drift results in the current version. They also allow you to get drift reporting at specific points of interest, even if these points don't line up well from floor to floor or don't behave the same as other nodes at that elevation. See Drift Modeling Tips for more information.

Inactive Diaphragms

Drift is not reported for elevations that have been flagged as "Inactive" on the Diaphragms Spreadsheet. However, the inactive diaphragm is still considered for the drift calculation for the floor above, in the determination of story height.

No Wind / Drift Checkbox

When the No Wind / Drift box is checked on the Diaphragms Spreadsheet, then that diaphragm will be completely ignored for drift calculations. Drift will not be reported at that level. It will also not affect the story height calculation for the floor above or below.

Define Elevation for Drift Calculation

To Define an Elevation for Drift Calculation:

1. Open the Drift Definitions spreadsheet from the ‘Explorer’ panel.

2. Add a new row to the spreadsheet:

   1. Right-click in the spreadsheet to open an options menu.

   2. Click on either Add New Row to End or Insert Row Above Selected, depending on where you want the new row to appear in the spreadsheet.

3. Click the Type column down-arrow in the new row.

   

   Click on image to enlarge it

4. Click on Elevation in the list that appears.

   This indicates the type of drift definition you are creating.

5. Click in the Elevation [ft] column of the new row and type the value for the elevation, as shown in the following image.

   

   Click on image to enlarge it

Define Node for Drift Calculation

To Define a Node for Drift Calculation:

1. Open the Drift Definitions spreadsheet from the ‘Explorer’ panel.

2. Add a new row to the spreadsheet:

   1. Right-click in the spreadsheet to open an options menu.

   2. Click on either Add New Row to End or Insert Row Above Selected, depending on where you want the new row to appear in the spreadsheet.

3. Click the Type column down-arrow in the new row.

   

   Click on image to enlarge it

4. Click on Node in the list that appears.

   This indicates the type of drift definition you are creating.

5. Click in the Node Label column of the new row and type in the name, as shown in the following image.

   

   Click on image to enlarge it

For additional advice on this topic, please see the RISA Tips & Tricks webpage at risa.com/post/support. Type in Search keywords: Story Drift.

Drift Results

Open the Story Drift Spreadsheet

Once the solution is performed you may view the drift results in the ‘Story Drift’ spreadsheet.

To open the Story Drift spreadsheet:

1. Ensure you have “Solved” the model so that the Results section appears in the Explorer panel.

2. Click on Story Drift in the Explorer panel under the Results section.

   This opens the Envelope Story Drift results report/spreadsheet to the first tab.

   

   Click on image to enlarge it

   This report lists the drift for all defined Diaphragms, Elevations and Nodes that exist in the Drift Definitions spreadsheet. The results are reported in the order in which they appear in the Drift Definitions spreadsheet.

3. Click on the X Direction Service tab (if not already open) to view its results.

   Service level and strength level drifts are reported on different tabs of the Envelope Story Drift results spreadsheet. This is because seismic drift checks are usually checked purely for the strength level Load Combinations, while wind drift is usually checked against service level Load Combinations.

How Drift is Calculated

• Inter-Story Drift: To calculate inter-story drift for a particular direction, the displacement at the lower level is subtracted from the current story displacement. 

  For example, to calculate X direction drift for story 2, the X displacement for the node representing story 1 is subtracted from the X displacement for the node representing story 2.

• Diaphragm and Elevation Definitions: For Diaphragm and Elevation definitions, the drift calculations are only done at locations where columns or walls have nodes at the current level and at the level below.

• NodeDefinitions: For NodeDefinitions, the calculations are done for the current defined drift node compared to the nearest defined drift node at a the level below. Node definitions never look at the nodes in a Diaphragm or Elevation definition. These are user-defined drift nodes which act completely independent of the more automated Diaphragm and Elevation drift results and offer flexibility for cases where the Diaphragm or Elevation options don't give the output needed.

• Story Height: For story height, the vertical axis is used to determine the distance. 

  For example when the Y-axis is specified as the vertical axis, the story node Y coordinate values are used to calculate heights for X and Z direction drift.  If Y direction drift is being calculated, the Z coordinate values for the story nodes are used to calculate heights.

• Structure Base Elevation: The base elevation of the structure is assumed to be zero. If the 0 ft elevation should NOT be used as the base of the structure for drift calculations, then you should enter in an Elevation entry in the Drift Definitions spreadsheet to define the elevation of the base of the structure. This applies whether the base is a positive or negative value. If the value is positive, then there will be a reported drift (probably zero) at the base elevation that you can ignore.

Story Drift and Drift Ratio %

The Drift at a given level is equal to the deflection at that level minus the deflection at the level below. For the image below: 

1st Level Drift = X₁

2nd Level Drift = X₂ - X₁

The Drift Ratio (%) is equal to the drift at that level divided by the height from that level to the level below. For the image below:

1st Level drift ratio % = (X₁ / H₁ ) *100%

2nd Level drift ratio % =((X₂ - X₁) / H₂ ) *100%

Seismic Drift Checks

Seismic drift are reported for all codes for load combinations containing earthquake loads, but the program reports failures in red text for the ASCE 2005, 2010, and 2016 codes, based on the following table of allowable seismic drift versus Risk Category / Occupancy Category.

Drift Category	I or II	III	IV
High Drift Design	2.5%	2%	1.5%
Masonry Cantilever	1%	1%	1%
Other Masonry	0.7%	0.7&	0.7%
Other	2%	1.5%	1%

When the ASCE and IBC codes are used, the Story Drift for the strength level combinations accounts for the inelastic deflection (i.e. the Cd factor) by amplifying the node deflections at each level by C_d/ I per Section 12.8.6 of ASCE-7. Where C_d is the Deflection Amplification factor intended to convert the elastic deflection levels given in the analysis to the inelastic levels used for seismic drift checks. Similarly, I is the importance factor (based on Risk / Occupancy Category) which was presumably used to amplify the seismic forces applied to the structure.

For codes other than the US codes, the drift results are not modified for C_d, I or rho, and are never reported in red text to indicate a failure.

2nd Order / 1st Order Deflection Ratio

The program reports the ratio of 2nd order deflection to 1st order deflection for the nodes used in the drift calculation. This is useful when determining whether it is required by the AISC code to use the Direct Analysis Method. In general, if the ratio is greater than 1.5 (or 1.7 if stiffness adjustment has been turned on) then you MUST use the Direct Analysis Method on your structure.

To facilitate this check, RISA colors the check in red whenever the ratio exceeds the 1.5 limit (or 1.7 when stiffness adjustment is used). This is done regardless of what has been chosen for the HR steel code. While the 1.5 or 1.7 may not be a code trigger in other HR steel codes, it remains because it can be an indicator of when 2nd order effects become troublesome.

There is an internal tolerance of 0.0005 inches, below which the deflection is not reported in drift results. This is because while the drift is essentially zero, testing showed cases where minor increases in these small values could result in high 2nd order / 1st order deflection ratios. Which were falsely indicating 2nd order effects as reaching high or troublesome levels.

Drift Modeling Tips

Inclined Columns

When using diaphragm or elevations, the drift is reported for aligned nodes connected by columns or walls. This doesn't help for inclined columns because those nodes will very specifically NOT align from level to level. If drift reported is required for these nodes, then they should be defined using the Node option instead. When defined this way, the drift from level to level will be reported based on the nearest node at the level below.

Multi-Story Columns

In the image below, node N2_3 should have its drift calculated based on a story height that is twice what the other nodes at that level would be based on. In that case, the user should use Elevation or Diaphragm definitions for the majority of the nodes, but use Node definitions for N2_3.

Troubleshooting NC results

There are times when drift results are reported as "NC" (which stands for Not Calculate). This can occur for a number of reasons:

• The Story Drift for that floor is below the 0.005" minimum tolerance for reporting.
• If a diaphragm is flagged as inactive then all results for that diaphragm will report NC.
• When there are no aligned drift nodes between that diaphragm / elevation and the one below.
• The 2nd / 1st Ratio will report NC for any load combinations where a P-Delta analysis was not included in the solution.