By Luke Miner, PE
There is great interest today in bike, pedestrian, and multi-use trail projects, especially in urban areas. California is currently making a big push to support projects that increase the use of active modes of transportation through the Active Transportation Program (ATP). Senate Bill 1, signed by the Governor of California in April, directs $100 million annually from the Road Maintenance and Rehabilitation Account to the ATP beginning in the 2017-18 fiscal year.
Meanwhile, programs like “Rails to Trails” are taking advantage of unused railway corridors by converting them into bike and pedestrian trails. Historically, these rail lines followed natural amenities like creeks and rivers, making them good candidates for multi-use trails. However, where these railroad tracks crossed a river, they left an old bridge designed in a different era that must be retrofitted for this new function.
The railroads typically constructed large, massive piers to support very heavy trains. Piers built before about 1940 may even be plain concrete or cut stone, with no steel reinforcing.
The good news is that trains are much heavier than people so old railroad bridges, which were designed to resist forces from heavy winds and braking, are designed the same today. The bad news is that these old bridges were also designed to be rigid and unyielding, whereas today we design them for flexibility and ductility during earthquakes. And while the old piers were designed for heavy train loads that outweigh the proposed pedestrian loading, they were not designed for lateral seismic loads.
Modifying for Pedestrian Use
Two issues could sink your plan to modify a railroad bridge for pedestrian use.
One issue is scour. Often the pier depths are not suitable for the scour predicted during flooding because they were either not constructed very deep below the riverbed or they are supported on weak piles (possibly timber piling).
If scour can be managed, the next concern, especially in California, is seismic loading. Sometimes the foundations are actually wide enough that they remain stable during seismic events. Psomas is applying a seismic design technique called “rocking” to re-use the piers of an old railroad bridge in Los Angeles. The idea is that even if an earthquake causes the piers to rock in place, the pier will not collapse because its center of gravity remains inside the footprint of the footing. The reason these kinds of piers survive seismic events is that they are so wide at the base that they won’t fall over, even though they may tip dramatically.
The behavior of wide-based piers described above can be greatly enhanced by rebuilding the pier to make it lighter and reusing its foundation or its footprint. This technique maintains the historic and aesthetic value and ensures that the piers do not collapse under seismic loads.
Do you have a trails project over old railroad piers? Give Psomas a call to evaluate if re-use is the right option.