Workflow for AISC Design Guide 11 Methodology for Vibration in Areas with Sensitive Equipment
Designing steel framed floors for vibration sensitive equipment can be a challenge if you have little experience with the various tolerance limits provided by equipment manufacturers. Have you ever wondered what to make of the various peak acceleration limits or generic velocity limits listed for a microscope or MRI machine? Is the design approach the same for a surgical suite as it is for a laboratory? And how does the presence of human activity affect your design approach?
In the June 2020 SE University session, Brad Davis presented Steel Framed Floor Design for Vibration Sensitive Equipment. Brad described the various response metrics used to characterize floor vibrations for sensitive equipment applications. He also reviewed the various forms of tolerance limits for sensitive equipment and showed how to apply the vibration evaluation methodology in Chapter 6 of AISC’s Design Guide 11: Vibrations of Steel-Framed Structural Systems Due to Human Activity (2nd edition) which he co-authored.
Brad helped delineate a workflow for evaluating the floor framing to compare the tolerance limits to those of the predicted response. The first step is determining the tolerance limit of the equipment and matching form of the response. This could include a peak acceleration specific limit which would be used to predict a waveform peak acceleration, or it could be a generic velocity limit which would lead to predict a one third octave spectral velocity. This information can sometimes be found in the equipment specifications, but communication with the supplier may need to be established to clarify specs which can often be ambiguous. Once the tolerance limits are established, you would next define the analysis cases which apply to the scenario. You may have a bay that supports both a laboratory and an adjacent corridor which will dictate the type of activity level considered in the analysis. The typical brisk walking pattern in the corridor will be different than the walking speed in the lab. This will lead to the next step which is selecting the walking speed category for each analysis case. In the previous example, a fast walking speed would be selected for the corridor, while the walking speed in the laboratory would most likely be very slow if only short pathways exist, or maybe slow walking if the room is more open.
Next, characterize the structure by determining its natural frequency, fn, the effective weight, W, and damping ratio, β. Many of these parameters are found in Chapters 3 and 4 of the design guide. Using these parameters, determine the response assuming the equipment and walker at midbay. Then, scale the response based on the actual location of the equipment and walker location. The response may need to be scaled twice if both the walker and the equipment are located away from midbay. Chapter 6 includes the equations needed to calculate the response and scale it, as needed.
Finally, you can compare the given tolerance limit to the predicted response to determine if the framing can successfully support the vibration sensitive equipment. Brad noted that design aids are included in the design guide and can be used to simplify this procedure. For more information and examples problems for steel framed floors supporting vibration sensitive equipment, refer to AISC’s Design Guide 11: Vibrations of Steel-Framed Structural Systems Due to Human Activity (2nd edition).