It's Still Possible to Build a Better Bridge

I feel for civil engineers. Building roads and sewer systems is unflashy work compared to the always-buzzing realms of particles, information, and nanostructures, where every new design or concept is poised to change everything.

Civil engineers toil so often within government bureaucracy, subject to relentless strain of budgets, where better will so often find itself swapped for cheaper. In a way, it's a vestige of "pure" engineering, where building or designing anything is an act of problem solving, rather than paradigm-shifting or headline-grabbing.

Obviously, that's the reduction of a non-engineer, but also one that combs through a lot of press releases and other assorted boasts from people in the broad field of building stuff. It's not often that you read about something like a new bridge design, like the one being presented this week at the Quake 2014 conference by a team based at the National Science Foundation's Network for Earthquake Engineering Simulation.

The design, which can be constructed using just common construction materials, is billed as a minor revolution in structure prefabrication and earthquake-/damage-proofing. In the future it may be possible to relatively quickly swap out an old deteriorating bridge for a new, much-safer replacement, which is indeed paradigm-shifting, particularly if you're one of the many millions of people perched on the US West Coast, just waiting for the big one.

The advance involves what are known as bridge "bents," which is the collection of beams and columns making up a bridge's supportive guts. Constructing these bents involves a tedious and rather old-school process of on-site concrete pouring, a system involving long waits after each beam or column is poured as the concrete sets and gains strength: pour, wait, pour, wait. It's as if every Lego piece had some sort of timer on it; stick a new piece on and the timer starts, leaving you to just sit there and watch for however long: materials- and seismic- enforced slow motion.

Prefabrication is an old idea, and currently has some limited applications in bridge building. One critical limitation involves earthquake safety. "Pre-fabricating means the pieces need to be connected on-site, and therein lies a major difficulty," said John Stanton, the concept's chief architect. "It is hard enough to design connections that can survive earthquake shaking, or to design them so that they can be easily assembled, but to do both at once is a real challenge." And so we pour and wait.

A structure (and its constituent concrete) being rattled by an earthquake or even just day to day loads experiences different forces pulling and twisting the structure's supports: side to side, up and down. It's not enough to just slap some concrete together and hope it holds up, because it won't: concrete is naturally vulnerable to added tension. That's one way that bridges collapse. So, to tension-proof bridge materials, engineers add the additional feature of pre-tension. A bridge column or beam is compressed, smooshed together from top to bottom like a tall, narrow sandwich. This compressive force, normally implemented by casting concrete around tensioned metal cables or bars, acts against the otherwise destructive forces a bridge might incur during its lifetime, whether it's from semi rigs passing above, or seismic shaking below.

"A good analogy is to think of a series of a child's wooden building blocks, each with a hole through it," Stanton said. "Stack them on top of one another, put a rubber band through the central hole, stretch it tight and anchor it at each end. The rubber band keeps the blocks squeezed together. Now stand the assembly of blocks up on its end and you have a pre-tensioned column. If the bottom of the column is attached to a foundation block, you can push the top sideways, as would an earthquake, but the rubber band just snaps the column back upright when you let go."

The need for pretensioning is one of the major barriers in building prefab bridges in earthquake zones. When you move a bunch of preset concrete slabs to a site, they still need to be joined together. Joints, however, add a weakness; the tension gained in the above-described scheme comes from being able to set wet concrete around that rubber band, a method that clearly doesn't extend to jamming preset concrete pieces together.

The NEES team overcame this by devising giant steel caps that shroud and protect the connection points between prefab, pretensioned beams/columns. "Cyclic tests of the critical connections have demonstrated that the system can deform during strong earthquakes and then bounce back to vertical with minimal damage," Stanton said.

Frankly, anything helps, at least in the United States, home to some 5,237 "structurally deficient" bridges on the national highway system alone. Add in all US roads and highways and the figure climbs to 70,000 bridges that have a "deck, superstructure, substructure, or culvert rated in 'poor' condition." Another 80,000 are considered "functionally obsolete," meaning it's not adequete for its current use. Another 18,000 US bridges are "fracture critical"; if any single component were to fail, then the whole bridge would collapse. A civil engineering technology that might help solve this willfully overlooked crisis isn't exactly a way cool new app, but it might keep some people from dying.