Pedestrian bridge dynamic behaviour

[SayGoodDay]

Hi

Can anyone teach me how to calculate the natural frequency and resonant frequency for the pedestrian bridge. Any example.

Thank you.

SayGoodDay

 

[bridgebuster]

Get the AASHTO GUide Specifications for Pedestrian Bridges, they have an example.

 

[SayGoodDay]

Sorry, forgot to say, I am from Australia, and I use the AS5100, it don't have the example. Any paper in the web with examples? Thanks.

 

[bridgebuster]

Unfortunately, I won't be back in the office for at least a week - that's where my AASHTO is; I don't any example at home

 

[SayGoodDay]

When you have time. Thx.

 

[Dinosaur]

Is your span a simple span? You may approximate the behavior to get a ball park number by finding a global moment of inertia and then look up frequencies based on your support conditions. You will also need a mass per unit length. If your bridge has varying moment of inertia (and mass), which is very likely, the precomputed values in books won't be much use because they almost always assume uniform area and moment of inertia. Good Luck.

 

[bridgebuster]

Here's an example based on AASHTO

 

[graybeach]

Bridgebuster,
In your example, could you say that the fundamental frequency is greater than 3 Hz, so it is OK?

Also, I have heard some engineers opine that the alternative formula, f?2.86 ln (180/W), should not be used to justify a fundamental frequency less than 3 Hz. What do you think?

 

[bridgebuster]

graybeach,

That example is straight from AASHTO. I designed three ped bridges in recent years, including one that is cable-stayed,but to tell you the truth, I've never really thought about the frequencies.

I don't recall what the frequencies were but they met the code and there haven't been any complaints of excessive vibration. All three are heavily travelled.

 

[unclesyd]

Here is reference to an article by Peter Curran that discusses the design of the Millenium Bridge.

http://en.structurae.de/refs/items/index.cfm?ID=r0013926

Enter Millenium Bridge in search.

http://en.structurae.de/index.cfm

 

[jheidt2543]

Some pedestrian bridges in our area are also used during the winter months as snowmobile bridges. What effect does the type of traffic have on bridge vibrations; foot, bike, snowmobile, trail grooming equipment etc...? We are using only the bridge dead load to find the natural frequency, but the applied live load in each case is decidedly different.

 

[gwynn]

Walking vibration is an issue due to the periodic nature of the load. I would picture a snowmobile or bike as a steady load moving across the bridge. Since this doesn't have the same sort of periodic motion, I do not see that there would be significant dynamic excitation.