By Roger K. Lewis
Saturday, October 15, 2005
With storms and earthquakes taking lives and destroying structures along the U.S. Gulf Coast, in Central America and in South Asia, it's worth pondering the "what" and "why" of our behavior in creating structures exposed to nature's periodically awesome forces.
We know how to design structures to withstand Category 5 hurricanes and 8.0-magnitude earthquakes. Yet we rarely design and build to cope with such extreme conditions.
Hurricane-force winds can deform, violently shake and ultimately tear apart anything -- trees, overhead utilities, bridges, buildings -- with parts or connections that are too weak to resist wind-induced stresses. We know how to calculate those stresses.
Hurricane winds propel rain into buildings through joints or seams that are not well sealed, and through windows, doors or roofs torn open by the storm. We know how to keep roofs and walls intact. We also know how to securely tie down objects that gusts could otherwise toss around.
Storm surges -- a combination of above-normal water levels, powerful currents and churning waves -- produce flooding while exerting destructive pressure on inundated structures. If the pressure is great enough, it can cause buildings to collapse or levees to fail. We know how to avoid exposing structures to flooding and storm surge pressures.
Earthquakes shake structures from side to side, as do hurricanes, and sometimes up and down as the earth moves. When the magnitude, frequency and duration of the shaking are great enough, abnormally high oscillations and stresses occur. A structure will collapse if stresses in critical structural components -- bearing walls, columns, girders, beams, connectors -- become excessive, causing those components to deform or come apart.
We know how to design and construct buildings strong enough, with the right mix of stiffness and flexibility, to absorb and dissipate such stresses, no matter how violent the earthquake or hurricane.
So if we know how to build structures that can't be demolished by nature, why don't we do so routinely?
Civil and structural engineering philosophy, as well as public policy, is based on statistics and economics. Historical records allow scientists to estimate the probability of a natural disaster occurring. Engineers then design structures to stand up to forces unleashed by the most probable events, not all possible events. Building structures to remain whole and safe, no matter how extreme the conditions, would be prohibitively expensive.
Thus we design and build in anticipation of 50-year or 100-year floods instead of 500-year floods; Category 3 hurricanes instead of Category 4 and 5 hurricanes; 125 mph winds instead of 175 mph winds; and minimal earthquakes or, sometimes, no earthquakes at all. No building in the Washington region has been designed to withstand a tornado, yet tornadoes have touched down here.
Even oil rigs scattered across the Gulf of Mexico, which look so structurally robust, are not engineered to weather the most powerful storm.
There are millions of structures throughout the world that can collapse when subjected to natural forces substantially less than the most extreme imaginable, as last weekend's earthquake in Pakistan painfully demonstrated. Yet retrofitting and reinforcing all those structures clearly is impractical and economically unfeasible.
Humans are compulsive gamblers when it comes to dealing with nature. How else can you explain why people inhabit flood plains; low-lying, hurricane-plagued coastlines; or known earthquake zones? Why would someone choose to build on a steep, unstable hillside or live near an active volcano?
People explain their behavior by citing what are for them compelling reasons to live where they live: It's been the family home for generations, it's familiar, it's the view, it's fertile or it's affordable.
We willingly take risks, betting that the cataclysmic event will never happen. And usually we are unwilling or unable to pay for reducing the risks to near-zero, either by building for the worst-case scenario or moving to higher ground. Instead, some of us do the next best thing: We buy insurance.
Regrettably, much of the world's population and property is uninsured.
Will the natural disasters of recent weeks affect how people and nations behave? I predict that fundamentally little will change. We will still design structures based on probabilities. People will continue living with nature's risks. And at some point, we will again lose the bet and pay the price. We just don't know when it will happen and what it will cost.
Roger K. Lewis is a practicing architect and a professor of architecture at the University of Maryland.