Brian Brenner, P.E., F.ASCE, is a professor of the practice at Tufts University and a principal engineer with Tighe & Bond in Westwood, Massachusetts. His collections of essays, Don’t Throw This Away!, Bridginess, and Too Much Information, were published by ASCE Press and are available in the ASCE Library.
In his Civil Engineering Source series More Water Under the Bridge, Brenner shares some thoughts each month about life as a civil engineer, considering bridge engineering from a unique, often comical point of view.
The city of Longview, Washington, has constructed a series of pedestrian bridges for squirrels. The bridges span from treetop to treetop across busy city streets. The design intent is to provide safe passage for the squirrels.
The first crossing, the “Nutty Narrows Bridge,” was built by a contractor in 1963. It is about 60 feet long and now spans over Olympia Way near the city center. The structure, at least the main span, looks like a suspension bridge. But upon closer observation, the main span does not serve any structural function. It seems that it is there to provide the appearance of a suspension bridge, but not the actual structural performance of a suspension bridge.
This may be of visual interest to motorists on Olympia Way below. Or perhaps this configuration provides comfort for crossing squirrels. The bridge may seem to be more solid than it actually is with this provided suspension bridge affectation. Although it is conjecture, squirrels may be aware of a previous bridge folly crossing the Tacoma Narrows not too far away. In that case, they would appreciate crossing a bridge that looks more stable even if it isn’t.
The full structure actually is a suspension bridge even with its faux main span. The squirrel crossing deck is supported by a single cable draped between trees.
There is not a lot of material available in the literature that focusses on squirrel bridge design. To address this gap, let’s assume that the original designers adapted parts of the bridge design code to make it suitable for squirrels.
Designing for dead load weights would be about the same, but live loads clearly would differ greatly. In the U.S., most highway bridges are designed in accordance with the American Association of State Highway and Transportation Officials LRFD Bridge Design Specifications. For vehicular loading, this code specifies a Design Truck, which is to be used for the live load analysis of a bridge. The Design Truck has three concentrated loads of 8,000 pounds, 32,000 pounds, and 32,000 pounds, as shown in the figure below.
If an average squirrel weighs about 2 pounds, then only one axle of an AASHTO LRFD Design Truck would account for approximately 16,000 squirrels. A full Design Truck would be the equivalent of about 82,000 squirrels. That is a lot of squirrels. Therefore, it would be over-conservative to use AASHTO vehicular loading. Furthermore, we can assume that squirrel bridges in Longview Washington, while being in the “pedestrian” design category, do not need to account for emergency vehicles like ambulances or firetrucks. While many pedestrian bridges are designed for emergency vehicular use, the vehicles are not operated in treetops.
Although an average squirrel weighs two pounds, what about the nuts? During certain times of the year, a squirrel will be busy transporting large quantities of acorns to and from nut storage facilities. A squirrel may carry several times his or her weight in nuts. So to be safe, perhaps a separate nut loading factor of three should be applied to the squirrel weight to account for the nuts.
Impact loading is another important design consideration. “Impact” accounts for the dynamic effects of vehicles moving across bridges. This movement imparts loads in addition to the weight of the vehicles. It is possible to perform a complex dynamic loading analysis to evaluate the additional loads due to impact, but it is impractical. Such analysis depends on many factors, such as vehicle speed, configuration, damping, bridge stiffness, span lengths, and other factors. When complete, the analysis is still probably imprecise because it is based on a somewhat closed set of assumptions, and the actual conditions can vary significantly from what is assumed.
Therefore, bridge design codes tend to simplify the calculation by treating impact load as a fraction of the vehicular weight. It is not necessary to perform a complex analysis, at least for standard bridges. In the case of AASHTO, the impact is generally taken as 33% of the vehicular live load. For our assumed squirrel bridge design code (Squirrel AASHTO, or “SqAASHTO”)(see note 1 below), possibly 33% is too low a percentage, because squirrels are observed to jump up and down a lot more than trucks, and they are fidgety. Therefore, let us consider an impact loading factor of 100%. Remember that the weight of the squirrels to be used should include additional load in the fall as they transport nuts to their nut storage facilities. When designing squirrel bridges, it is always important to account for the nuts.
Other bridge loading conditions would probably be similar to the vehicular code, with some modifications suitable for squirrels. For example, the case of Wind on Live Load in AASHTO is based on wind applied to trucks. But this effect would be expected to be less because squirrels are smaller than trucks. Although, they may be proportionately bigger when they are carrying nuts.
Based on the popularity of the Nutty Narrows Bridge, Longview has constructed several additional squirrel crossings. A map showing the location of all the bridges is here.
The John R. Dick Squirrel Bridge has a design similar to the Nutty Narrows Bridge (see note 2 below). It is also a rope suspension bridge that provides a faux main span. In this case, the main span is patterned after a cable-stayed bridge.
Similarly, the S & R Squirrel Bridge has a main span that looks like the Fremont Bridge in Portland, Oregon, one of the world’s longest tied arch spans. In the squirrel bridge version, the supporting piers extend below the deck, impacting the illusion that the structure does anything structurally at all. Bridges usually do not have piers hanging in midair.
Also downtown is the Safety Awareness Squirrel Bridge. This structure provides a sort of approximation of a truss with safety postings on the sides, for the humans below and not the squirrels.
Other squirrel bridges have spans that look like continuous truss bridges and covered bridges. A drive around downtown Longview provides viewers with a nice assortment of bridge structural types hanging between the trees.
When designing a squirrel bridge, there is not a lot of information available to help cost the project. The Longview squirrel bridges appear to employ economical designs, with simple cables and some decorative but inexpensive central spans. Other jurisdictions have used more elaborate design approaches. A squirrel bridge crossing a motorway in the Netherlands was detailed as an actual cable-stayed bridge at a cost of about $200,000 (US).
Unfortunately, this bridge was not connected to treetops like the ones in Longview. The squirrels needed to climb up onto the deck to cross. For the first two years, wildlife cameras indicated that a total of five squirrels crossed the bridge. This is about $40,000 per squirrel. In time, more squirrels learned to use the bridge, and fewer were flattened on the motorway.
Longview is justifiably proud of its squirrel infrastructure, and the many bridges have helped the small city to become a Northwest tourist attraction. The municipality offers visitors an annual festival, Squirrel Fest, held in August. The festival provides “a wide variety of nutty activities,” and it is a showcase for presentation and discussion of the city’s squirrel bridges. One can imagine that prominent squirrel bridge engineers from around the world meet each year at the festival to discuss the latest developments in squirrel bridge design and construction.
It is possible that with increasing concerns about sustainability and a resilient infrastructure, other communities will seek to better protect their flora and fauna, including squirrels. They can look to Longview, a community that has taken the lead in effective squirrel bridge design.
Future designers should remember when designing a squirrel bridge, always account for the nuts.
(1) If it is preferable to compare to railroad design with locomotives instead of highway vehicles, the U.S. AREMA design code would apply. Then we can identify a squirrel version of this approach as “SqAREMA.”
(2) I was hoping this article would have at least two footnotes.