Vibrations for Elevated Running Track in Gym

[SteveGregory]

I have not used the AISC Design Guide on Floor Vibrations and I am in the process of getting familiar with them.

I have a pre-engineered metal building being used for a gymnasium and the building vendor will be designing a running track that wiil be suspended from the metal bldg framing. At the outer wall, the track framing will tie into the frame columns on the sides and the wind columns on the ends. The open side of the track will be hung from the frame members and the roof framing.

I need some help providing the vibration design criteria for the engineers at the metal building company. This part of the design was given to them.

 

[dgkhan]

I assume you need to provide the natural frequency of vibration of running track to PEMB designers. Model the framing of running track and it's load in STAAD or SAP. They wil give raliegh frequency of the track system. alternatively you can assume it as a pedestrrain bridge and there are some formulas to calculate vibrations and frequency in probably AASHTO related publications.

 

[SteveGregory]

I will provide the PEMB designers with a minimum natural frequency and a maximum peak acceleration as %g. This will be the required design criteria. They will be performing all of the analysis and design.

I just need some experienced guidance developing the criteria. I have the AISC DG 11.

 

[271828]

Make the peak acceleration limit equal to that for an indoor footbridge and see if the resulting structure is reasonable. You can probably relax a bit above this limit. Look in DG11 to see the limit. I think it's 1.5%g, but mine's at the office.

The natural frequency needs to be high enough so that the step frequency can't match it. The first harmonic of hte walking or running force is VERY large, so you need to ensure that it can't cause resonance. See if the DG provides a step frequency range for running. If it doesn't say anything, I think I'd use 5 Hz.

 

[SteveGregory]

Should I require the engineers to design the track for both walking and rhythmic excitation? There is a floor at track level with a number of rooms at one end of the track.

In chapter 4 (walking), the peak acceleration is 1.5%g for interior footbridges and the stiffness criteria is 5.7 k/in.

In chapter 5 (rhythmic), the peak acceleration has a range of 4-7%g (use 5%g?) for rhythmic activity only and for aerobics only (light floor), there is a min. natural frequency of 9.2 Hz. Does this make sense?

 

[271828]

I'd design the track for someone running unless you think they might have aerobics classes or something similar on there. Design the floor at the track level for rhythmic.

Re-read Ch. 5. There's a way to have fn<9.2 Hz.

BTW, if you don't have it, you DEFINITELY need to buy Dr. Tom Murray's FloorVibe to help with floor vibe analysis in general. It's cheap and is extremely helpful.

In your particular case, it would have to be used with caution and I would actually make a FE model if I were doing it--I've done this *exact* thing before and the FE modeling approach worked out well. Build a model of the entire bldg with the track hanging from the RFs.

 

[SteveGregory]

Running does not appear as an option in chapter 5. I picked aerobics as an activity that I thought was similar to running. Both have a similar level of impact.

The frequency 9.2 Hz came from a table in chapter 5. It is based on typical aerobics values being used in equation 5.1. Actual values may reduce this value.

I won't be doing any FE modeling or any design work at all. I am only setting up the design criteria for the PEMB engineers who will design the track.

 

[271828]

Steve, forces due to aerobics aren't really similar to forces due to walking and running. I think you'll never get the track to work if you design it for Ch. 5 criteria. I'd definitely specify the track and exercise room areas separately.

Track, using the footbridge and 5 Hz (unless you can find something else) criterion.

Aerobics room using Ch. 5 criteria, although I don't think you want to say 9 Hz min natural frequency. There's a way in there for calculating the peak acceleration. I'd use that instead. Dig in there and see where their fnmin comes from and I think you'll see why you don't want to use it. Might have to go back to Ch. 2 or 3 (forgot which) to see the background. The only fn limit I'd place on those areas would be one to make sure their step frequency can't match the natural frequency. 5 Hz sounds good to me here also.

 

[DaveAtkins]

I worked on a project almost EXACTLY like the one you are describing a couple of years ago. I designed the roof members supporting the inside edge of the track, not the PEMB supplier. They ended up quite heavy, as I recall, something like W 18 X 71, spanning 25' or so. I used the criteria for a footbridge, I think. I was surprised to find that the criteria for walking on a footbridge was more stringent than for aerobics.

Again, I designed the roof members, and let the PEMB supplier design the frames. I gave the PEMB supplier the reactions at the frames.

DaveAtkins

 

[271828]

Dave, how was support flexibility accounted for in the natural frequency calculations?

 

[DaveAtkins]

I assumed rigid supports for my beam designs. The track was only about six or eight feet wide, and the PEMB frames spanned a very large distance, across an Olympic size ice rink with bleachers (I can't remember the span). Anyway, I reasoned the beam ends were being supported very close to the column portion of the PEMB frame, and would not move vertically.

DaveAtkins

 

[271828]

Dave, sound like it might've been reasonable in your case, but I wouldn't typically do this. On the project I worked on, I did this as a preliminary step on the way to modeling the entire structure. Adding in the MB RF flexibility dropped the natural frequency quite a bit, although I don't remember how much. I don't remember the distance out from the column, but I think it was a bit farther out than that--10-12' perhaps.

 

[SteveGregory]

Dave,
If I understand your first post, you checked the design for both walking (footbridge) and rhythmic (aerobics) and found that the walking controlled. Our track is 11'-8" wide and the span is about 120'.

Do these specifications make sense?

Rhythmic: The Minimum Required Fundamental Natural Frequency shall be 9.2 Hz. Peak Acceleration from Rhythmic Excitation shall be less than 5%g with a participant load of 4.2 psf.

Walking: Peak Acceleration from Walking Excitation shall be less than 1.5%g with a 92 pound force. The track floor system shall have a minimum floor stiffness of 5.7 kip/inch.

 

[271828]

Not really, but it seems like you didn't like my first answers anyway, LOL!