#### MIKE_311

##### Structural

- Feb 15, 2020

- 108

I’m designing a long span pedestrian bridge. It's a multispan steel girder bridge. A computer vibration analysis shows a vertical frequency of 1.9hz which is less than 3hz which is less than the AASHTO allowable in the pedestrian bridge spec. AASHTO allows the frequency below 3hz if the weight of the structure meets the alternate criteria of f>=2.86ln(180/W) or if W>=180e^-0.35f. Does any one know how this works for multispan bridges? Do you just use the largest span length to compute W?

Alternatively, I found the AISC design guide for vibration of steel framed structures, there is a section on pedestrian bridges. If the peak acceleration of a single walker Poe^-0.35 / BW is less than their limit of 5% , using the recommended values for B (0.01), and Po (92lbs), it works out to be the same equation as in AASHTO.

However, AISC has a second formula to check the vibration from running- acceleration < 0.79Qe^-173f / BW, where Q (168lbs). If I assume that W is the span length, the bridge will pass the AASHTO criteria, but fail the one for a runner in the AISC design guide. To pass the runner I need to add a lot of dead load, like 1k/ft more. Which, is just not going to happen.

This is a very large signature structure we are designing, is it adequate to just meet AASHTO? Or should we look at installing some dampers, which could be expensive. I know AASHTO really doesn't go to the level of say the Eurocode for vibration analysis and keeps it simple. Do I need to perform a more refined analysis?

To add, to the assumption of W being the span length, the AISC guide does discuss vibrations of floor system and increasing W 1.5x if the adjacent spans are great that .7x the span under consideration which leads to me to believe that I am correct in using the span length for W but it makes no explicit mention of how to analysis a continuous span bridge. The AISC guide says the effective weight, W, is taken as the total weight of the bridge so its not clear and need to make sure I'm understanding it correctly. In contrast, the AISC guide has an equation of f = 0.18*sqrt(g/delta), if I use the deflection get from the computer analysis in this equation, the frequency is over 3, which all leads me to believe these equations don't work for multispan bridges.

Has anyone had to address this and can offer any insight, any information would be appreciated.

Edited with more info.