Pedetrian bridge vibration

[broekie]

I am working on designing a pedestrian bridge based on the new AASHTO LRFD standards. According to section 3.6.1.6, the fundamental frquency of my bridge has to be higher than 3 Hz. I am having a hard time making this work and it's going to cause me to have a really deep section, almost to the point of seeming somewhat ridiculous.

For the fundamental frequency of the bridge, I used the formaula that the frequency is:

2/pi/L^2 * sqrt (E*I*g/w).

My bridge right now has a frequency of 1.7 Hz. In order to increase that I have to increase I (bigger section), but that also increases w, the weight.

I turned to the AASHTO Guide Specifications for Pedestrian bridges. That tells me in Section 1.3.2 that if I don't meet the 3 Hz requirement, that the bridge should be proportioned such that the fundamental frequency should be greater than:

f > 2.86 * ln (180/W),

where W is the weight of the supported structure in kips. My structural weight is 330 kips. If I plug that into this forumla, I get that my frequency has to be greater than -1.7 Hz. I guess I meet that, but it doesn't make sense to me. This methodology has been adopted by the Florida DOT.

Can anyone shed any light on this subject for me or have you run into anything similar? Thanks.

 

[plasgears]

The fundamental formula for natural frequency is:

f = sqrt[k/M]

K is the spring constant, and M is the mass. You can either stiffen the structure or reduce the weight to increase freq. If you design toward max stress instead of max deflection, then you may have a way out. However, code may have the last word on this.

 

[FeX32]

You don't necessarily need a bigger section or more weight to increase the nat. freq.
Try to think about increasing the product E*I. 'I' can be increased without increasing the weight.
This would increase the stiffness resulting in a higher nat. freq.


Fe

 

[graybeach]

broekie,
Be careful with this. I have heard that the f > 2.86 * ln (180/W) formula was not meant to justify natural frequencies of less that 3 hz. The formula is based on the work of Professor Murray of Virginia Tech. Maybe you could contact him about it. You might need to add tuned mass dampers or use a truss for your bridge.