Bridge Vibration: Can a bridge bounce?

Delaware River Bridge at Lackawaxen PA- one of the first suspension bridges built by John Roebling and Russell Lord. In use since 1849 and listed on the National Register of Historic Places. Photos courtesy of HABS HAER

To start off this terminology page under the word Bridge Vibration, there is a story that is connected with this bridge. The Lackawaxen Suspension Bridge was built in 1849 by Russell Lord and John Roebling and spans the Delaware River bordering Pennsylvania and New Jersey. Together with the suspension bridge in Nuremberg (Germany), they are the only bridges of its kind in the world where the roadway is supported by odd-numbered towers. This in addition to the fact that they were both built before 1870. But the story goes where teenagers in the 1920s and 30s would try and race their cars across the bridge, then stop abruptly in the middle, creating a wave of vibration that would shake the wooden planking of the 300 foot structure! A dangerous stunt for it could lead to the bridge to collapse, but in those days, a carefree attitude towards bridges did not include liability issues like we have nowadays.

And this is where we look at the topic of Bridge Vibration. Can a bridge vibrate and if so, to what extent can it vibrate in order to make it safe?  This is in connection with some discussion about bridge vibration and how unsafe it is. Three bridges- two in Battleboro, New Hampshire and one bridge at Sylvan Island in the Quad Cities were the primary focus of this issue. City officials in the Moline (IL) portion of the Quad Cities feared that bridge vibration would mean that the potential for bridge collapse was there and subsequentially closed the bridge to all traffic this past May. The 1911 Pratt through truss structure, once serving as the lone key access to Sylvan Island, is now scheduled to be replaced this fall with a concrete structure that is not meant to bounce. The Charles Dana and Anna Hunt Marsh Bridges in Battleboro were on the hot seat lately due to suspected negligence on the part of the New Hampshire DOT and vandalism by someone who justified his actions on bridgehunter.com and claimed that a 250 foot bridge does not bounce. Both bridges are scheduled for replacement in 5-10 years time.  This leads to the question of whether a bridge can or has to bounce. After inquiring about this with some bridge engineers who have worked with this topic, the answer to that question is yes, but with certain restrictions. Says Todd Vierendeel, who works for an engineering firm in South Dakota:  “All bridges will experience some amount of deflection under load. The repeated loading and un-loading of spans due to transient loads (truck and pedestrian) can generate the sensation of vibration, or “bouncing” as has been described here. Excessive deflection and/or vibration can cause structural issues, but it’s actually not desirable primarily from a user comfort perspective.”  Billy Wulff, a bridge engineer from Quickborn, Germany, compares this sort of vibration to a plank sitting on top of a box whose expansion and contraction is restricted in contrast to the plank.  Therefore, “…engineers build in tolerances (which they calculate) of movement.” He also added that engineers have tried to construct lighter bridges, using the same materials for both the structure and the flooring, in order for it to not move.

The danger to such vibration is that too much of it, combined with the usage of light-weight materials for bridge construction will lead to structural failure. Many bridges have collapsed because of what Wulff calls misunderstanding of science combined with unsuitable materials. The most classical example was the collapse of the Tacoma Narrows Bridge in Washington in 1940. There, too much light materials for the bridge deck on “Galloping Gertie” resulted in excessive bouncing in 40 mph wind, resulting in the bridge’s collapse.  Video of the collapse can be found here.  To regulate the vibration on all bridges, transportation agencies have bridge design codes to ensure that there is a certain tolerance to the bridge vibration; this applies to agencies responsible for highways and railroads, but as Vierendeel states, the guidelines are more stringent for pedestrian bridges as they can notice the vibrations more than automobile drivers or train conductors.

But for older bridges, like the aforementioned bridges, he adds that more care is needed to ensure that the vibrations do not cause any discomfort among drivers and pedestrians, namely because of the increase in loads going across the bridge.  Therefore, some adjustments, like additional beams, weight restrictions, and extensive maintenance are needed to prolong the bridge’s life. As for bridges that are closed to vehicular traffic but open to pedestrians, such bouncing is considered normal for as they were used to vehicles crossing it, its tolerance may have increased over the years, making the bouncing sensation more pronounced. Yet, as many experts have mentioned, it does not mean that the bridge is unsafe and sometimes, additional support and retrofitting the bridge deck to reduce the vibrations is what is needed to prevent any discomfort of bridge users. This happened with the Millenium Bridge in London, where fluid and mass dampers were retrofitted to reduce the vibration frequency caused by many people crossing the bridge.

So to answer the questions, yes it is possible for bridges to vibrate when crossing, but only within tolerances that are imposed in bridge designs approved by transportation agencies. Should bridges witness any excessive vibrations, it is possible to fix the problem by adding support to the decking and retrofitting bridge parts to ensure that the vibrations are at a minimum. There is no such thing as a bridge not vibrating because of factors involving temperature differences combined with the volume of traffic crossing the bridge.  This leads to the question of the necessity to tear down bridges that vibrate when it is all part of the way the structure functions. Sometimes some minor repairs to bridges like the ones mentioned combined with continuous maintenance is all that is needed to ensure that the bridge lasts longer and the tax payers do not have to suffer as a result of tearing down a bridge for something whose quality cannot match that of the one that is destined for the scrap heap.  While it may be too late to save the Sylvan Island Bridge, it is something officials in New Hampshire and other states should consider before deciding on replacement over restoration.

The author would like to thank Billy Wulff and Todd Vierendeel (the names and occupation were changed to protect their identity) for their help in clarifying this topic.

 

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