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Multihazard Design: How to Prepare for Dual Disasters

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While one natural disaster does not predict another and a double disaster is unlikely to repeat itself, Hurricane Irene and the Virginia earthquake, much like the Japanese earthquake and tsunami in March, are chilling reminders that past experience is seldom an infallible guide to the worst-case scenarios that

inform structural design decisions. Engineers would do well to ponder the difference that a slightly stronger earthquake could have made on preparations for and damage from the storm that followed.

Taking natural disasters one at a time, 20th-century style, should no longer be an option. Structures are best designed by considering the effects of all potential disasters in parallel—but not simultaneously—a process called multihazard design. If we want new structures to stand for the next 50 to 100 years and age with grace, then we must manage structure and infrastructure health comprehensively, making as-sessments throughout the life cycle.

Hurricanes are known quantities on the East Coast: New York City's buildings should perform well if exposed to the design-level winds envisioned by the International Building Code (IBC) currently in effect. They remain at risk, however, from the potential of wind-borne debris generated by nearby, older buildings. Seismic design of East Coast buildings is a different story. Significant earthquakes have occurred so infrequently that no meaningful seismic design codes were in effect in New York City before 1995. The mayor's seismic code committee, which was convened in the early 1990s to redress this oversight, had reasonable cause, based on expert opinion, to draft codes approximating those of the West Coast.

Of course, equating West and East coast risk was counterintuitive and a political impossibility.

Even if codes were universally applied or more conservative, they would need to be updated to reflect new data and improved knowledge and technologies.

Surprisingly, during the 1994 Northridge earthquake, code-compliant, steel moment-frame connections failed, not only because the earthquake was novel in terms of demand but because many aspects of this connection type—such as the three-dimensionality of the stresses on these details—were not well understood at the time.

Flooding is an increasing threat, too. After Katrina, storm-surge estimates associated with the various Saffir-Simpson Hurricane Scale (one to five) categories were revised upward. In New York City, the East River has risen eight inches since 1973 and is expected to continue at this rate. Despite FEMA guidelines to the contrary, however, we continue to build in floodplains and hope for the best. After Irene, engineers should focus on the risk of ensuing flooding and its potential effect on structural systems, especially foundations, as they continue to design for structural capacity to resist ground shaking and winds.

What engineers know for certain is that both earthquake ground motion and hurricane flooding compromise soils and foundations. Most lethal of all, the “scour effect,” or the invisible erosion of foundation soil, is a progressive collapse mechanism that escalates as many small, untreated episodes accumulate. If an earthquake and hurricane coincide, all these effects are amplified—not to mention water penetration from a compromised building envelope, which can also cause major damage.

Signs of Damage

Experienced inspectors know where to look for signs of invisible damage and how to confirm their hypotheses with non-destructive testing. In addition to hiring inspectors to check for damage and assess future risk, building owners should look at their foundations, learning from bridge engineers how to mitigate potential water damage and scour by improving soil, deepening piles, building stronger foundations and erecting contingent structures, such as sheet piling and retaining walls, to control water velocity. At the same time, all of us can help to educate the general public about the vulnerability of the built environment, because technical expertise can never remedy the threats a culture refuses to recognize.

Mohammed Ettouney is a principal at the structural engineering firm Weidlinger Associates Inc., New York City, and co-author of the two-volume book, “Infrastructure Health in Civil Engineering” (CRC Press). Anurag Jain is a principal at Weidlinger Associates in California.

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