Vibrations

So I played around with the model, making adjustments and seeing how that effected the natural frequency of the structure. The goal is for a structure with a natural frequency greater than 3 Hertz (cycles per second). (or ~1.3 Hz according to Arup, see pps below)

Case 1) Box girder 3.3 meters wide by 0.5 deep with 15 mm thick top and bottom flanges and 10 mm thick webs. No pins in the deck. The columns are hollow tubes of 300 mm diameter with 10mm thick walls. Result was a natural frequency of 0.45 Hz.

Case 2) Same as case 1, but with 3 pins (elastomeric or spherical bearings) in the deck (The pins were added to allow for thermal expansion/contraction). Result was a natural frequency of 0.39 Hz.

Case 3) Same as case 2, but with columns that are solid rectangles 150 mm x 500 mm. The columns are oriented at alternating 90 degree angles. Result was a natural frequency of 0.45 Hz.

Case 4) Same as case 3, but with a solid steel deck 3.3 meters wide by 0.5 meters deep. It is obviously ridiculous to use anything like this much steel, but it is an extreme case to see if adding steel to the deck would help. Result was a natural frequency of 0.15 Hz.

Case 5) Same as case 3, but with beefier underwater piles. The hollow tube piles were 1 meter in diameter with 50mm thick walls. Result was a natural frequency of 0.89 Hz.

Conclusions:
- Increasing the plate thickness of the box girder does not help and actually hurts. Presumably the extra weight overrides any benefit provided by the added stiffness.

- The main way to increase natural frequency of the structure is to add larger (and/or presumably more) columns.

- In order to prevent uncomfortable oscillation, we need to increase the natural frequency of the structure from where it is right now.

PS - Here is the link for the main company that makes hydraulic dampers called Taylors Devices: http://www.taylordevices.com/dampers-seismic-protection.html The site includes many nice photos of sexy ladies standing next to the products.

PPS - Taylor Devices buffers were installed on the Millenium Bridge in London after it had vibration problems. Arup, the engineering firm for this bridge, has a nice webpage on the bridge, vibration issues, and solutions. Click on "challenge" at the following link: http://info.arup.com/millenniumbridge// Note Arup says "The same excessive sway movement could occur on other bridges, future or existing, with a lateral frequency under ~1.3 Hz and with a sufficient number of pedestrians."