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.
To define the virtual load cases for analyzing roof drift:
This returns you to the Load Cases dialog box.
Next, you need to analyze the new virtual load cases:
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:
In the center portion of the box under Define Pairs:
This takes you back to the graphics screen. To analyze the load pairs:
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:
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:
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:
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.
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:
The participation factors should be shown on screen. The colors may be a little different than they appeared before.
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:
The Displacement Participation / Volume Summary report should appear:
Take time now to review the various reports available in the Reports menu.
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.