In addition to the member design provisions of each design code, building structures are also required to meet certain drift limitations. The Drift Control Module provides you with a means to see how each of the lateral members contributes to the resistance of that drift. For the Tutorial, drift at the Roof Level in the X and Y directions is the primary concern. In this section you will define a virtual load case in both directions, pair those load cases with the governing seismic load cases and review the results in order to determine which members provide the greatest resistance to that drift. You will also determine how to improve the overall structural performance.
This is an optional module and is not included with the RAM Frame basic module. In order to perform this section of the Tutorial you must have the RAM Frame – Drift control Module installed and the hardware lock programmed for that module. You can skip this section otherwise.
Defining Virtual Load Cases
To define the virtual load cases for analyzing roof drift:
- Select Mode – Analysis – Load Cases.
- Select Loads – Load Cases. The Load Cases dialog box will open.
- Type VX in the Label edit box.
- Click the Virtual Work option button.
- Click [Add] and the Virtual Load Case Story Forces dialog box will open.
- For the Roof Level Type 100 in the Force column (100 can also be used for SI models).
- Leave the Dir. Angle set to 0.
- Leave the X and Y coordinates at the default. This represents the calculated center of mass for the respective level.
- Leave the forces for the other floors set to 0.
- Click [OK].
This returns you to the Load Cases dialog box.
- Type VY in the Label edit box.
- Click the Virtual Work option button.
- Click [Add].
- For the Roof Level Type 100 (100) in the Force column.
- Type 90 for Dir. Angle.
- Click [OK].
- Click [OK] to dismiss the Load Cases dialog box.
Next, you need to analyze the new virtual load cases:
- Select Process – Analyze.
- When the Analyze dialog box opens, make sure that all of load cases are selected by clicking [Select All].
- Click [OK].
Defining Load Pairs
In Order to pair the virtual loads with real load cases and perform the Drift Control Analysis, you must now enter the Drift Control Mode of RAM Frame:
- Select Mode – Drift Control.
- Select Loads – Load Pairs. The Load Pairs dialog box should appear:
In the center portion of the box under Define Pairs:
- Type X Pair for Label.
- Type 1.0 for Factor.
- Type or select W1 for Real column.
- Type or select V1 for the Virtual column.
- Click the Blue Down Arrow to establish the pair.
- Type Y Pair for Label.
- Type 1.0 for Factor.
- Type W2 for Real.
- Type V2 for Virtual.
- Click the Blue Down Arrow to establish the pair.
- Click [OK].
This takes you back to the graphics screen. To analyze the load pairs:
- Select Process – Analyze.
The screen should now display a color coded image of the structure. Members shown in warm colors (i.e. red, orange, yellow…) are participating more in the resistance of the roof drift based on the current load pair (X Pair) and the current evaluation method (Total Displacement). In this case, you can see the beams of the north moment frame do the most work, indicated by their color.
To review the results for the other load pair:
- Click the center drop-down list in the toolbar that indicates the current load pair. Select Y Pair from this list. The screen should be updated to reflect the results of Y direction loads.
Another way of reporting participation is to divide a member’s participation by its own volume. In this way you can see where increasing a member provides the most benefit.
To review the results as Total Displacement/Volume:
- Select Process – Results – Total Displacement/Volume or use the third drop-down list.
A dramatic change should occur in the screen output. The walls that were red or yellow are now blue. This happens because while that member does a lot to resist drift in the Y direction (even at the roof), it is also has a much larger volume than the steel members. Now the lower level braces the frames should be the highest participating members by volume.
There are several other features in the Drift Control Module as described in the Drift Control portion of the RAM Frame manual. Examine some of those options now.
To review an individual member to evaluate its participation:
- Select Process – View/Update.
- With the cursor select a brace in the lowest level of the EBF.
The Brace View/Update dialog box should appear. In the center of the box you can see the participation of that beam due to the current load pair.
As in the other modes, you can investigate other member sizes by selecting them from the list and clicking analyze.
- Select another brace size from the list.
- Click [Analyze].
- The participation values will change slightly.
- Click [Update Database] if you want to make the change official.
- Click [Close] to dismiss the View/Update dialog box.
The fact that one or more members are drawn red is only an indication that those members are working the hardest on a relative scale. It does not mean that they are failing in any way. An optimized structure in terms of drift is one where the majority of the members are all performing equally. If a model has a few red members and the rest are blue, that is an indication that the red members are overworked while the rest of your framing isn’t helping that much with respect to drift control.
As with any Update Database command, the analysis results considered when recalculating the participation factors are from the previous analysis run, and those results are invalidated by any modification to the stiffness matrix. Another analysis run should be performed before the Drift Control results are accepted.
To rerun the analysis and review the new sizes:
- Select Mode – Analysis – Load Cases.
- Select Process – Analyze.
- When the Analyze dialog box opens, make sure that all of load cases are selected and Click [OK].
- Select Mode – Drift Control.
- Select Process – Analyze.
The participation factors should be shown on screen. The colors may be a little different than they appeared before.
Reports
Reports can be generated from the Drift Control Mode just like the other modes. The printed output is generated using the Reports menu. To print the Displacement Participation / Volume Summary report:
- Select Reports – Displacement/Volume Summary.
The Displacement Participation / Volume Summary report should appear:
- Hit the Page Down Key to see the rest of the report.
- Click
![]()
to exit the report.
to exit the report.Take time now to review the various reports available in the Reports menu.
- Select File – Exit to exit RAM Frame and return to RAM Manager.
This completes the RAM Frame portion of the tutorial. Proceed to the next section to perform the concrete member design. If you do not own that module you can skip ahead to the RAM Foundation section of the Tutorial.
- Continue to RAM Concrete Analysis Tutorial
- Go back to RAM Frame Steel - Seismic Provisions Tutorial
- Up to the RAM Structural System Tutorial