Bridge inspections have come under scrutiny after the collapse of the I-35W bridge over Mississippi, with Minnesotans wondering how a bridge that passed an inspection in May could collapse less than three months later. The technology involved in bridge inspections may surprise some, as they still rely on hammers, steel chains, and eyeballs for most of the analysis. Also, the design itself may have been the major contributing factor, according to a former New York City bridge safety engineer:
As canoeists paddled below, state bridge inspector Eric Evens stood in a cherry picker next to the two-lane steel bridge over the St. Croix River near Scandia on Friday, eyeballing rust, cracks, bolts and rivets.
Computer-aided design and other innovations have changed the way structures are built, but bridge inspections haven't changed much over the years. In this high-tech era, the trained eye and the rap of a hammer to listen for the sound of bad metal are still considered among the best ways to examine bridges.
Many inspectors insist that the great majority of bridges are safe and that their work is thorough. But others say the failure to predict last week's catastrophe in Minneapolis points to flaws in the system and the need for better technology to detect problems.
It turns out that bridge inspections are more art than science. The inspectors listen to the sounds that bridges return from hammer taps and chains dragged across girders more than using high-tech measurements of density and corrosion. Most of the normal routines rely on visual inspections -- eyeballing for corrosion, cracks, and fatigue.
How effective is that? In 2001, the Federal Highway Administration decided to test inspectors on a subtle but significant flaw in a bridge. Only 4% found it.
Technology may be a limited solution, however. The Admiral Emeritus spent years in the space program in quality engineering. He held the highest level of expertise in non-destructive testing, which was used on the Shuttle program to test support structures for engines and the like. That kind of testing was difficult enough in a hangar designed for access to the spacecraft's interior, but would be much more difficult to apply to bridge inspections in the field, especially when inspecting a bridge like the St. Anthony Bridge that collapsed last week.
The Star Trribune points out another issue that led to the collapse. The bridge's design and construction in the 1960s is a potentially critical factor, not for its age but for the philosophy of design at the time. Samuel Schwartz, who closed his share of bridges in New York City, explains that bridges built before and after that period have redundancies designed into them. Not so with the post-war period going into the 1960s, however, when American builders and government officials felt that bridges had been overdesigned and excesively costly. After a number of collapses in the 1970s, the designs went back to redundancies -- but apparently no one thought to add to the existing bridges, even those high-traffic structures like the I-35W bridge.
Another change from the time of the design also may have been a factor. As late as the 1960s, rail traffic handled much of the transportation of goods to market, and trucks were used for mostly local distribution. That has changed in the 40 years since, and heavy vehicles make up a larger percentage of highway traffic. The assumptions for use made in the 1960s stopped baing valid two decades or more ago. The heavier traffic going across the bridge would accelerate fatigue and bring failure even closer.
With all of that, though, the question still remains why the inspections did not catch whatever caused the failure. That question cannot be answered until we know what specifically did cause the collapse, and that may take months to learn. Until then, we have to consider what to do with bridges of similar design and age, and figure out how to inspect them properly.