how to measure vibration of pedestrain bridge

[dgkhan]

Contractor who build the pedestrian bridge, also did the connection design of the bridge. Now main consultants say that the connections are flexible and as per there computer Model Bridge will vibrate. I took few people with me and walked over the Bridge. Do not feel anything. By the way is there any gage to measure vibrations?

 

[kelowna]

You will need an accelerometer (same technology as a seismograph for earthquakes). There are specialized companies that can install and process the information. Where to locate the instrumentation and in which direction to record the vibration (vertical, horizontal transversal or horizontal longitudinal) would come from the specific geometry and concerns.

The footbridge might vibrate under different loading conditions than you and a few other people walking on it. There are also minimum code requirements in terms of dynamic response of the footbridge that a change on the connection might affect.

Didn't the consultant approve the connection design? That is usually the process.

 

[271828]

Yes, this is done all the time. Google "accelerometer" and "vibration analyzer." In summary, one can put an accelerometer on the slab and then walk on it. The analyzer measures the voltage and converts it to acceleration. The results are waveforms (accel vs t) and spectra (accel vs frequency). YOu can look at the results and compare those to tolerance limits such as AISC DG11, 1.5%g for interior footbridge or 5%g for exterior one. Someone who's not a vibe specialist probably can't do this in a reasonable amount of time, though, so you'll have to hire someone if you want it done.

BTW, you won't be able to feel the vibe when you walk. Stand still and have someone else walk. You'll also need to walk at different frequencies. One frequency won't cause much vibe whereas another will.

As for modeling, in most cases, the connections are considered rigid for vibe. There's not enough force to break the friction. This is standard today for FEA of floors, footbridges, etc. Tell the EOR to see the latest British SCI design guide on the subject.

 

[rowingengineer]

vertical or horizontal, agree with "e" in regards to everything else. I attached a document i found good to get an understanding of what is going on.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it