Thomas Fisher

Fracture Critical

Dean of the College of Design at the University of Minnesota, Nov 05, 09.

Tagged with government, infrastructure, energy, housing, bridges

...A fracture-critical design has four key characteristics. The first is lack of redundancy, which makes a structure susceptible to collapse should any individual component fail. The I-35W’s undersized gusset plates might not have brought down the span if it had had additional members to carry the structural load. At the time of the bridge’s design, such redundancy no doubt seemed expensive and wasteful. But given the extraordinary costs — financial and human — of collapse, the incremental expense of redundancy would have been cost-effective and wise. Engineers understand this and in recent years have increased the redundancy of bridge designs — but the pressure to reduce initial costs continues to threaten the durability of our infrastructure.

Other weaknesses of fracture-critical design are interconnectedness and efficiency. The
I-35W had both. When the gusset plates cracked near the bridge’s southern end, this overstressed other structural members — all interconnected so efficiently that nothing could interrupt serial collapse. The 10th Avenue Bridge adjacent to the I-35W shows the advantage of less interconnection, less efficiency. Completed in 1929, that bridge consists of independent concrete arches separated by concrete pylons that divide the structure into discrete parts. The concrete columns supporting the road deck seem oversized, making the entire ensemble less than efficient, but more than sufficient to compensate for the failure of any one element. [4] Even if several columns or one of the arches failed, the bridge wouldn’t collapse.

The final characteristic of fracture-critical systems is sensitivity to stress. Had inspectors attached strain gauges to the I-35W gusset plates, they would have detected a gradual increase in stress, with a rapid rise in strain, just before the plates fractured and the bridge fell. Sudden, exponential increase in strain prior to failure is a well-known phenomenon, and a fracture-critical design magnifies its effect. What seems a localized, controllable problem can quickly become catastrophic, because of the nature of exponential growth, doubling with each increment of time.

To understand how lack of redundancy, connectedness, efficiency, and exponential stress relate to each other, consider the concept of “panarchy,” explored by ecologists Lance Gunderson and C.S. Holling. [5] Panarchy explains that human and natural systems move in continuous adaptive cycles, and that exponential growth in connectedness and efficiency actually makes systems less and less resilient, inevitably leading to collapse and then return to a state of greater resilience, with fewer connections and less efficiency. The collapse of fracture-critical designs like the I-35W — which we would be wise to see as part of an adaptive cycle — warns us that we need to replace such structures with designs that are less connected, less efficient, more resilient.

But we’ll need to change more than the design of our bridges. It’s clear in retrospect that the fracture-critical structures of the 1950s and ’60s reflected the larger culture — this was when John Kenneth Galbraith famously critiqued the United States as a nation of private affluence and public squalor. In an era when America could have afforded the best infrastructure in the world, we began instead to channel wealth into private hands and to impoverish the public realm. [6] This was also when a deeper, though subtler shift began to be felt in American culture. The United States had emerged from World War II as the dominant global power and, as many commentators have noted, dominance easily led to hubris, to the pride of pax americana in the ’50s and more recently to theories of American exceptionalism. [7] We now know that our wartime enemies as well as allies have proven to be formidable competitors, and that we can no longer take dominance for granted. In this sense the I-35W stood — and fell — not just as a physical bridge across the Mississippi but also as a symbol of postwar overconfidence. Fracture-critical design epitomizes all the postwar systems vulnerable to sudden failure. The bridge’s collapse warns us that future catastrophic events will surely occur. The I-35W Bridge is both metaphor and omen…

Full Story: Fracture Critical
Source: Design Observer, October 26, 09

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