RE: Ram Concept Vibration Analysis

There is a 12.1 ft slab cantilever on the south side of the floor and the large RMS Velocity values are concentrated at the end of the cantilever. The values in the other bays are much lower. Higher vibration response should be expected in the cantilever area.

Here are some other things that you may want to review or consider:

I recommend reviewing the fundamental frequencies tabulated in Layers – Vibration - Vibration Analysis – Tables – Vibration Frequencies and ensure that enough modes are included in the analysis. In general, the number of modes should be entered to include all modes up to about 15 Hz or, in the case of stiffer structures, all modes that are less than 2x the fundamental frequency.

Also, review the “Excitation Response Factor Threshold.” The procedure below is what we have recommended in the past to reduce analysis time for a modal analysis:

1. Calculate the limiting response factor at a performance level that is deemed unacceptable (this can be done using acceleration or velocity as the performance criteria):

1. For RMS acceleration, a response factor of 1 is equal to 0.005 m/s^
2. For RMS velocity, a response factor of 1 is equal to 0.0001 m/s (in this example, if they desired to limit the maximum RMS velocity to 0.0004 m/s, the limiting response factor would be 4).

• Select “critical nodes” as the excitation nodes option.
• Set “Excitation Response Factor Threshold” to approximately half the limiting response factor value calculated in step 1.
• Select “Vertical DOF at All Nodes” as the response node calculation.
• On the Excitation Areas Plan draw Excitation Areas to define the areas prone to footfall, which normally is not the entire floor. For example, use an Excitation Area in the MRI room and adjacent corridors and common areas.

This procedure will speed up the analysis dramatically, but will only consider a small subset of the nodes as excitation nodes. The idea is that the most probable critical nodes are excited, and nodes that are less likely to be critical will not be part of the analysis.

I see that you have selected to use the stiffness matrix from the final load history step in the Load History Analysis. You may have seen the note of caution below in the manual:

“Since the load history calculations use stiffness adjustments to account for long-term effects, it is not recommended to use a load history step that has any prior load history steps with any significant duration as this can underestimate the short term stiffness.”

Note that the Sustained Service load history step applies 0.5 of the live load over a duration of 5000 days. You may also want to review the value used for the shrinkage restraint %. Some notes on the load history parameters can be found here:

http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/6191

Please post back if you have any other questions on the vibration results.