Engineering News Spring 2016
Going to Bat for Bats
In the comic book world, Batman is a fictional superhero who roams the streets after dark to protect the citizenry. But the Caped Crusader's accomplishments pale in comparison to the real-world ecological superhero that courses through the sky each night, battling bugs, promoting pollination, and fertilizing fields and forests—the humble bat. Bats munching on mosquitoes can help us enjoy the outdoors, but their value extends far beyond the palliative. The world's only flying mammals are vital to our food supply, our economy, and our planet's natural ecosystems.
So when Cornerstone Structural Engineering Group, headed by President Todd Goolkasian, SE, B.S. '85 (civil engineering) was contracted by California's Tulare County to design a state-of-the art bridge replacement project, the company went to bat for the furry flyers with a design that not only included bat habitats, it also won the firm a prestigious National Engineering Excellence Award from the American Council of Engineering Companies.
Leading to the redesign were concerns over the stability of the original 1948 structure spanning the Kings River on Avenue 416 in the City of Dinuba. Goolkasian explained that in the 1940s and '50s "bridge expansion joints were typically designed into the structure to allow the bridge to shrink and expand under ordinary temperature changes. As it turned out these expansion joints provided just the right gap for bats to roost in comfortably. Now, with our more stringent seismic design codes, fewer expansion joints are used as they make the bridge more vulnerable to collapse during an earthquake. So today's structural engineers design bridges to be continuous over much longer span lengths, as we did for this 740-foot-long bridge. But eliminating these joints created the unintended effect of 'evicting' their long-term tenants!"
To obtain the necessary regulatory agency permits for the project, Goolkasian's team had to fulfill the California Department of Fish and Game's requirement to provide a biological habitat that is at least as good as what previously existed. The firm could have installed "bat houses" around the bridge—small wooden boxes on poles—but instead they decided to think outside the box and sought the advice of noted Chiroptera expert Greg Tatarian.
"Working with Mr. Tatarian," Goolkasian said, "Cornerstone developed a design that included the types of nooks and crannies studies had shown to be favored by bats for their day and night roosting—the types of crevices that the vintage concrete girder bridge with its multiple expansion joints had naturally provided. Since the smooth underside of the cast-in-place, post-tensioned, concrete box girder replacement bridge did not lend itself well to bat habitat, we incorporated several new elements into the underside of the bridge: concrete boxes were installed between columns for day roost, and spaces behind precast 'winged' bridge elements and other depressions were integrated to serve as night roost habitat."
Preserving bat habitat wasn't simply a design requirement to be checked off; its successful implementation is critical for California's Central Valley farmers. Tulare County leads the nation in agricultural production—$8.1 billion in 2014—and nearly half those earnings are generated from fruit and nut commodities. Bats annually save the farmers in this region millions of dollars by consuming prodigious amounts of harmful insects, protecting the crops and thereby reducing the amount of pesticides entering the ecosystem. Protecting the bat habitat is crucial to the region's wellbeing. Luckily, our bug-loving friends have found cozy digs in their new home.
In Tatarian's first post-construction monitoring survey, he noted that the design and implementation "has been successful, even in the first bat reproductive season since completion of the bridge." Population of four species of bats has increased from 1,200 to more than 4,000—and a new species, E. fuscus (big brown bat), has now taken up residence. Cliff swallows are also finding safe haven in the bridge's integrated crevices.
"While the need to provide bat habitat on bridge replacement projects was not a new requirement," said Goolkasian, "the 'old-school' solution would have been to provide a cheap, stand-alone environment—bat boxes. Knowing that my client (the County of Tulare) wanted a solution that would not require any maintenance or future expense caused us to rethink how we might incorporate the bat habitat more permanently into the replacement bridge design. In the end, we were able to come up with a much more robust concrete solution that fulfilled our ethical responsibility to ensure that the replacement bat habitat would be functional for the entire design lifetime of the replacement bridge."
Seeing the thousands of bats tucked comfortably into the bridge’s carefully crafted spaces, and judging by the company's national recognition for the project, it would appear they are batting a thousand.