Long Span Pedestrian Bridge Vibration
Long Span Pedestrian Bridge Vibration
(OP)
I am working on the design of a long span (approx. 200') pedestrian bridge consisting of a steel box truss and concrete deck. The natural frequency of the bridge in the vertical mode is approximately 1.5 Hz, so it does not meet the AASHTO ped. bridge spec minimum requirement of 3 Hz - and there is really no way to get the bridge to 3 Hz with the constraints I have been given. The bridge does, however, meet the alternate mass criteria of f>=2.86ln(180/W), since the bridge is quite massive.
My question is, how comfortable are you all with relying on alternate frequency criteria for justification of the bridge frequencies? Do we need to consider expensive dampers? I have seen bridges of this span with similar depth, width, and member sizes in service without dampers, and I'm wondering how it is justified.
My question is, how comfortable are you all with relying on alternate frequency criteria for justification of the bridge frequencies? Do we need to consider expensive dampers? I have seen bridges of this span with similar depth, width, and member sizes in service without dampers, and I'm wondering how it is justified.





RE: Long Span Pedestrian Bridge Vibration
RE: Long Span Pedestrian Bridge Vibration
RE: Long Span Pedestrian Bridge Vibration
AISC Degin guide 11 Floor vibration due to human activity.
SSEC Practice to prevent floor vibration by Farzad Naeim, this one explain a little about vibration impact on humans.
Tips for desiner of office buildings by Thomas Murray.
RE: Long Span Pedestrian Bridge Vibration
If you do end up installing them perhaps you can temporarily disconnect them and get people to jump around on the bridge.
RE: Long Span Pedestrian Bridge Vibration
RE: Long Span Pedestrian Bridge Vibration
RE: Long Span Pedestrian Bridge Vibration
RE: Long Span Pedestrian Bridge Vibration
Since you know the frequency of the structure, a quick thought would be to assign the bridge 2% damping, apply the live load as a periodic function with the same frequency as the bridge and see what sort of excitation of deflections you get due to the resonance. Obviously conservative since, outside of a platoon marching in step across your bridge, the foot fall pattern will be distributed across a range of frequencies. But that would at least give you a ball park estimate of the worst case scenario.
RE: Long Span Pedestrian Bridge Vibration
RE: Long Span Pedestrian Bridge Vibration
The Blanco et al. paper was an interesting read, as were the parts of "Vibrations in structures: induced by man and machines" by Hugo Bachmann and Walter Ammann that address the same issue. I haven't found a way to get a hold of the Bachman paper cited in the Blanco et al work as giving the deflection and acceleration limits without buying the entire conference proceedings, though the deflection criteria are mentioned in the book above. When I have more time next weekend, I think looking into whether there are circumstances differentiating when each limit is appropriate is worthwhile. Using the same references, it also seems 2% damping would be on the low end.
Characterising the vibration of the bridge while including everyone at the same foot fall frequency would still be a reasonable starting point in evaluating whether there is a real need for dampers in my opinion.
RE: Long Span Pedestrian Bridge Vibration
Of course if the fundamental frequency is close to step frequency it will not take a platoon. I've stood near one end of a local pedestrian bridge and nearly became nauseated due to one person walking from the other end. The amount of movement did not seem to be any different when larger groups crossed together. I did some quick calcs and verified the fundamental frequency nearly matched that of pedestrians.
RE: Long Span Pedestrian Bridge Vibration
He said he became "nauseated", but that is not the worst impact on humans, the impact could be more severe. It is not casual that the codes state limits number of hours operating some equipment.
So the calculations and the decision have to be carefully evaluated.
To show some effects of vibration i attached two tables taken from Design manual soil mechanics, foundations earth structures - NAVFACENGCOM
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RE: Long Span Pedestrian Bridge Vibration