Here’s the most direct, in fact the only, way to create guaranteed disaster-proof structures:
Step 1. Determine the purpose(s) the structure will serve.
Step 2. Identify the hazards that place it at risk.
Step 3. Don’t build it.
Short of that, this year’s spate of earthquakes provides a compelling reminder (aside from the fact that Mother Nature is still in charge): There’s no such thing as a structure without vulnerabilities, which makes terms like “earthquake-proof” misleading.
That doesn’t mean we throw our hands up and resign ourselves to a world of pancake collapses. It does mean understanding just what degree of structural resilience we can create, what it costs and what other ways there are to mitigate against whatever hazards concern us.
Just as “bullet-proof” vests are really PPE designed to provide protection from a specific level of ballistic threat, structures can be designed and built to a range of hazard-resistant specifications. Whether individual PPE or structures that could house hundreds or thousands of people, enhanced protection generally means not only increased cost, but possibly operational constraints for the end-user.
Several hazards can degrade structural integrity, but wind, earthquakes and blasts are the most significant in terms of catastrophic potential, not just in terms of property damage, but also for the potential of turning the structure itself into a lethal hazard.
Wind resistance is by far the most common protective feature in building codes, and blast resistance may be more appropriate for certain buildings in certain areas, but we’ll concentrate on seismic mitigation here.
Life safety vs. damage control
Most building codes and for that matter, most safety standards in general, address life safety: sufficient structural integrity to allow occupants to get out alive.
Current seismic codes are no exception. They’re designed to keep the structure from killing its occupants, either by direct collapse or by eliminating escape routes (or in the case of a bridge, to not catastrophically drop its load into whatever it’s spanning). That means a focus on structural components, which is appropriate for life-safety considerations.
Unfortunately, many in the public, including most building users, have unrealistic expectations. They see seismic reinforcement as equivalent to “earthquake-proof” and expect such structures to be functional after a major earthquake.
In fact, for seismic reinforcement beyond life safety (damage control), as much attention needs to be focused on non-structural components. In “earthquake country,” emergency managers commonly preach the virtues of non-structural seismic mitigation to the public, typically emphasizing building contents (such as fastening tall or heavy contents to the frame, installing positive closure devices on cabinets or securing gas-powered water-heaters). Most of this, however, still addresses life-safety issues, that is, keeping building’s contents from killing the building’s occupants.
Non-structural considerations to provide post-quake operational status must revolve around building systems: electrical, mechanical, plumbing, the systems that allow productive use of the space. Aside from the life-safety aspects of securing heavy system components, much of this is not part of seismic building codes, even for critical facilities.
A recent example
All of this was amply demonstrated in the M8.8 subduction-zone earthquake that struck just off the Chilean coast in February 2010. Chile has seen two giant earthquakes in 50 years and three in less than 100 years. It has more seismologists per capita than any other country and has a number of structures built to modern seismic codes.
A survey team from Degenkolb, an engineering firm specializing in performance-based design for government and institutional end-users, assessed both building and code performance.
As discussed in considerable detail on their blog, Degenkolb engineers indicate that most buildings performed well relative to code, that is, there were few catastrophic failures. They also described substantial non-structural damage in a range of buildings that either hampered or prevented operational use.
As Degenkolb principal Anuj Bansal described, consider having to evacuate hospital patients several floors by hand and flashlight in Curicó, or look up at the voids, in hospitals and other structures in several cities, created when the appropriately named drop ceilings lost their entire contents: light fixtures, conduit, sprinkler piping and more.
Bansal added that we in the United States typically put more stuff up there (such as network cables and other comms stuff) than the Chileans do.
Even with structures reasonably intact, it was probably the pre-dawn timing of the quake that kept the casualty count relatively low. The scenes of ceiling contents piled on the floor also reinforced the “Drop, Cover, Hold On” steps of self-protection when the earth starts shaking.
What this means for other industrialized seismic areas
Like Chile, the U.S. Pacific Northwest sits onshore of an active subduction zone with the potential to produce a giant quake. Scientists estimate probabilities from 15% to 33% of an M9 Cascadia Subduction Zone quake in the next 50 years.
Unlike Chile, Washington and particularly Oregon do not have many structures built to current seismic codes. Perhaps of equal concern, are current codes realistic for such earthquakes?
Most seismic codes in place in the United States are based on the overwhelmingly prevalent seismicity in California, which is very different from that seen in subduction-zone quakes.
In such events, shaking can last for several minutes, and most structural and non-structural seismic reinforcement just isn’t designed to withstand that much intense shaking. That was demonstrated in Chile as well.
So what can we do?
Sustainable development incorporates resilience, with the San Francisco Planning and Urban Research Association (SPUR) providing a promising example. As part of a community plan for response and recovery, identifying which structures need to be operational immediately and which can follow (and how much later they can follow) is a great idea that’s lacking in most cities.
When building or renovating critical facilities for seismic resilience, consider non-structural components. FEMA has a series of guidelines for installing seismic restraints for mechanical (FEMA Publication 412) and electrical (FEMA 413) equipment, and ducts and pipes (FEMA 414).
And while all this is in progress, consider another lesson from Chile: Incorporate some education for users and the general public about realistic expectations.