Drift

You may calculate and report inter-story drift based on calculated displacements in the X-direction.

Drift Definitions Spreadsheet

The Drift Definitions spreadsheet defines where the drift calculations will be performed. Drift calculations may be performed at specific elevations . However, these elevations must be manually entered by the user in the Drift Definitions spreadsheet. These elevations are defined with respect to the vertical axis of the model.

In addition, the user may specify individual to be used in drift calculation. 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 users 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 at that elevation. See Drift Modeling Tips for more information.

Note

Elevations are treated interchangeably for drift calculations. must be in exactly the same location on the upper and lower floors for drift to be calculated.
When Elevations are entered manually, there MUST be defined at that elevation.
When definitions are used, the drift calculation for the is based only on the defined . Therefore, you must also create a definition for the story below (unless it is at zero ft) in order to get correct behavior.
The Drift Definitions spreadsheet is currently applicable to rigid diaphragms only.

To Define an Elevation for Drift Calculation

On the Drift Definitions specify Elevation as the Type of drift definition you are creating.
Enter the value for the elevation.

Note

The program will also assume a default base elevation of 0. This will NOT be done if there are any drift definitions defined for elevations below 0.

To Define a for Drift Calculation

On the Drift Definitions specify as the Type of drift definition you are creating.
Enter the label.

Note

This will be included in the drift calculations for both horizontal deflections.

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

Once the solution is performed you may view the drift results in the Story Drift spreadsheet. Access the Story Drift spreadsheet by selecting it from the Results menu. This report lists the drift for all defined Diaphragms, Elevations and that exist in the Drift Definitions spreadsheet. The results are reported in the order in which they appear in the Drift Definitions spreadsheet.

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 representing story 1 is subtracted from the X displacement for the representing story 2.

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

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

For story height, the vertical axis is used to determine the distance. For example, the story Y coordinate values are used to calculate heights for X direction drift.

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 the user 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 can be ignored by the user.

Service level and strength level drifts are reported on different tabs of 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.

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 will report 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 will account for the inelastic deflection (i.e. the Cd factor) by amplifying the 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.

Note

These seismic checks are only performed for strength level load combinations.
Stiffness reduction (per AISC Direct Analysis Method) does NOT change the drift code checks in RISA even though it may cause significant increase the reported drift.
Load Combinations with Overstrength (Omega) seismic loading are not included in the drift reporting at all.
To negate the effect of the redundancy factor (rho), the deflection results are divided by rho before drift calculations are perform. This only occurs, of course, for load combinations where rho was used to amplify the seismic loading. This is done because the redundancy factor is not intended to affect story drift calculations per section 12.3.4.1 item 2 of ASCE-7 2016.

2nd Order / 1st Order Deflection Ratio

The program will report the ratio of 2nd order deflection to 1st order deflection for the 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 will color 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 will not be 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 elevations, the drift is reported for aligned connected by columns or walls. This doesn't help for inclined columns because those will very specifically NOT align from level to level. If drift reported is required for these , then they should be defined using the option instead. When defined this way, the drift from level to level will be reported based on the nearest at the level below.

Multi-Story Columns

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

Troubleshooting NC results

There are times when drift results will be 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.
When there are no aligned drift between that 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.