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Concrete Article May 19, 2011
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Last year, Applied Ecological Services Inc., Michael Van Valkenburgh Associates and HNTB Corp. submitted an entry to the ARC International Wildlife Crossing Infrastructure Design Competition. The competition brief asked for collaboration among ecologists, landscape architects and structural engineers to develop innovative concepts for safe, efficient, cost-effective and ecologically responsive highway crossings for wildlife. One juror stated that the entry, “is not only eminently possible, it has the capacity to transform what we think of as possible.”

The confluence of wildlife and vehicles produces a scenario that is far too often deadly for wildlife and in some cases for humans. In the last 15 years, there has been a 50% increase in vehicle collisions with wildlife; this has lead to higher levels of personal injury, property damage and rising insurance premiums. Highways can serve as lethal barriers that fragment habitat and reduce wildlife viability. A Federal Highway Administration study reports that there are an estimated 1 million to 2 million collisions between motor vehicles and large mammals every year in the U.S. Such collisions result in 200 fatalities annually. The Western Transportation Institute at Montana State University said these kinds of accidents generate $8 billion a year in medical costs and vehicle repair bills.

For the competition, a wildlife crossing over I-70 at West Vail Pass, roughly 90 miles west of Denver, was chosen among 25 other sites in 16 states and Canadian provinces. Site selection considered the ecological importance of the affected habitat, the number, frequency and severity of animal-vehicle collisions, traffic volume, site visibility and the priority of the site to the local department of transportation.

The need to adapt

While the competition brief was anchored by the West Vail Pass site, our goal was to develop a concept that was adaptable to any location, as there is not a state or province in North America that does not have a serious problem with vehicle-animal interactions. Our sensibility was that cost was the primary barrier to widespread implementation; although we set out to develop a design that was scalable and adaptable to diverse sites, our design focus had pragmatism and austerity at its core.

The formulation of our design was rooted in the collaborative interaction between ecologists, landscape architects and structural engineers. What became clear in our earliest design meetings was the need for modularity: the ability of our design concept to adapt to unforeseen changes in conditions. With so few examples of wildlife crossings in North America, there is no definitive knowledge base to assist in selecting the optimum location, physical dimensions and habitats necessary for a successful crossing. We set out to design a modular system that could be adjusted over time, a design that could be widened, reconfigured, split into multiple crossings or even moved to a more favorable location.

Animal specific

From an ecological standpoint, the crossing must be designed for use by the range of wildlife specific to the site. The target species for the crossing at West Vail Pass included elk, mule deer, bighorn sheep, mountain goat, moose, black bear, lynx, coyote, American marten, marmot and the snowshoe hare. At our site visit for the crossing near Vail, we had no difficulty confirming that elk, mule deer and black bear regularly crossed I-70 in the general location of the bridge anticipated by the design competition. Given this mix of target species, it was clear that the crossing must support a variety of habitats. For example, lynx are most likely to cross through forested areas, deer and elk to cross at the edges of meadows, while coyotes and bears will cross through open meadows. This need for multiple wildlife entry points and terrain preferences prompted us to contemplate a much larger, wider crossing.

We brought meadow ecology across the bridge as a primary track and created forested islands and forested thoroughfares next to the meadow area. In this way, we were able to combine meadow, forest and the intersection between the two in a single crossing.

Our design also contemplated the large-scale impacts associated with the overall site, both from the perspective of steep terrain and the potential for landslides and avalanches, together with enhanced potential for fires given lodgepole pine forest mortality and a mountain pine beetle epidemic. We formulated projections of how the landscape might look in the future and used those projections in our sighting, sizing and orientation of the crossing. Since the trees most likely to survive will cluster along the drainage ways and the meadow, we aligned the wildlife crossing with what we believe will be the remaining forested corridors.

From a landscape-architecture perspective, our design deviates a bit from what we would ordinarily propose. Landscape architects typically seek to balance cut and fill on a site. But to protect what currently exists and minimize disruption at the site, we chose to rely on fill material imported to that location.

Hypar hype

A robust structural system was necessary to support an array of habitats. We determined the structural system must span 165 ft and support a minimum of 3 ft of soil—which is over five times heavier than vehicular loads. We needed a structural system that was highly cost-effective, modular and could be easily constructed with limited equipment in difficult terrain. Given the remoteness of many potential wildlife-crossing sites, we strove to limit on-site construction activities as much as possible.

A typical bridge comprises four distinct elements; abutment, pier, beam and deck. We felt that the only strategy that would be sufficiently cost-effective was to synthesize all of these functions into a single, prefabricated, over-the-road transportable element.

Borrowing from the work of Robert Maillart and Felix Candela, we imagined that a new precast concrete form in the shape of a hypar (hyperbolic parabaloid) vault configured in a manner to achieve arch behavior would be ideal. The variable depth and double curvature of the hypar gives the system tremendous strength with minimal weight. Our proposed vault form is arch-like in concept but configured in such a way as to minimize depth at the crown and springing and maximum depth at the quarter span. The resulting system is a robust three-hinged arch form capable of supporting large asymmetric loads while tolerant of differential settlements or ground movements.

The near vertical leg of the hypar vault serves as the abutment, the near horizontal leg as the beam and slab working together. With arching behavior, piers become unnecessary. A temporary midspan support allows for construction of the bridge in halves, minimizing impacts to vehicular traffic during construction. By maximizing the depth of the hypar at the joint between vertical and horizontal leg, an ideal structural system, reminiscent of Maillart’s Salginatobel bridge, is achieved. All hypar vaults are to be standardized at 8 ft wide to allow for over-the-road trucking without special permits.

Thin-shell concrete structures, pioneered in the 1940s and 1950s, are rare given the high labor costs associated with complex one-off forming and cast-in-place construction. The use of a single precast concrete form for the hypar vault overcomes this deficiency. Our structural system is made up of this one thin-shell structural element, fabricated, transported and erected over and over again, reducing construction costs and schedule.

For widespread implementation, we also wanted a structural system that could utilize existing U.S. fabrication capability. Nearly half of all new bridges in the U.S. are constructed of precast concrete and there is a Precast/Prestressed Concrete Institute (PCI)-certified bridge precast factory within a four- or five-hour drive of any location in the U.S. By adapting the technology that is already used in precast concrete for bridges, we hope to create a new set of forms and a new design that will be amenable to mass fabrication in pre-existing precast concrete factories throughout the country. What is required for implementation is a new set of forms and a modified strategy to place reinforcement. Our goal is to develop a new type of AASHTO girder with standardized modular forms as a function of span length.

On stand-by

As yet, there is no funding to construct this wildlife crossing at West Vail Pass, as the competition was for a hypothetical crossing. Given the scale of the problem and proven effectiveness at reducing collisions, we are certain that there will be many opportunities for this and other innovative wildlife crossings in the future. Our design concept also may have applicability in more conventional applications for vehicular bridges and low-cost shelters in remote regions.

We have much to learn in order to optimize our design and identify corridors where wildlife crossings will be most beneficial. It will be critical to assess the effectiveness of these early projects, both as structural systems and as functioning wildlife crossings, and adapt accordingly.

About the author: 
Apfelbaum is chairman and senior ecologist with Applied Ecological Services Inc. Rock is a senior associate with Michael Van Valkenburgh Associates. Zoli is technical director of bridges for HNTB Corp. and a recent MacArthur fellow.
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