In the heart of the worst U.S. earthquake zones, an alarming number of older, low-rise concrete buildings have not been retrofitted for earthquake safety. These two-story to five-story structures may meet the building-code standards of their day, but that day is long past. Today's building codes reflect later earthquake engineering research and incorporate structural elements that allow concrete buildings to bend and stretch a bit during an earthquake. Older designs lack those details.
"There are hundreds of thousands of buildings that have not been retrofitted that ... are very dangerous," said structural engineer Reginald DesRoches, chair and professor in the School of Civil and Environmental Engineering at Georgia Tech.
Technically, these older, brittle buildings are called "non-ductile concrete" or "non-ductile reinforced concrete" buildings. ("Ductile" means flexible, while "reinforced concrete" refers to concrete embedded with material such as steel mesh and rebar.) During a 7.8-magnitude quake, such as the recent one in Nepal, these buildings may not hold up well — or at all.
"We know from past earthquakes, non-ductile reinforced concrete buildings don't perform well in earthquakes, all around the world," said DesRoches, whose research is supported by the U.S. National Science Foundation. "They collapse."
A concrete failure
Non-ductile concrete construction was prevalent in most of the United States until 1980; it ended as states and localities began enforcing new, improved ductility requirements, such as more closely spaced steel reinforcements. An exception was the earthquake-prone West Coast, which began phasing out non-ductile concrete construction in the 1950s.
Although many brittle buildings have been retrofitted over the past several decades, there are still buildings that are hazards. In Los Angeles, there are as many as 50 older concrete buildings likely to collapse in a major earthquake, according to the "most conservative" estimate of reporters Rong-Gong Lin II, Rosanna Xia and Doug Smith in a piece in the Los Angeles Times. Depending on the time of day a temblor hit, the casualty list from those 50 collapses could be in the thousands, they said in the October 2013 report.
Even in earthquake-prone areas, information about which buildings seem to be most at risk can be hard to get. In L.A., for example, the majority of older concrete buildings had not been inspected for seismic safety before the Times report came out. Since then the city has begun inventorying older concrete buildings and also has been discussing how to implement a retrofit program. In 2014, University of California researchers gave city officials the addresses of all non-ductile concrete buildings they had identified in the city, and the resulting list is online. The researchers also noted that they "did not determine whether any specific building represents a greater collapse hazard than other buildings."
This inventory of pre-1976 concrete buildings in Los Angeles was part of a broader "NEES Grand Challenge" project funded by the NSF Network for Earthquake Engineering Simulation (NEES) program. The project brings together researchers, educators, engineers, and public policy experts to develop strategies for identifying hazardous older concrete buildings and promote mitigation strategies. (NEES is a network of 15 large-scale, experimental sites that feature such advanced tools as shake tables, centrifuges that simulate earthquake effects, unique laboratories, a tsunami wave basin and field-testing equipment.)
Making bendy buildings
A saying in the world of seismic engineers is that "earthquakes don't kill people, buildings do." This is no doubt why the U.S. Federal Emergency Management Agency (FEMA) calls non-retrofitted buildings "the single biggest contributor" to earthquake risk in the United States.
In Nepal, which is recovering from the devastating April 25 earthquake and aftershocks, some buildings are made of unreinforced concrete, but most structures there — especially in rural places — are constructed out of unreinforced masonry (URM), said DesRoches. URMs, which are made of brick, adobe or other masonry materials, are extremely vulnerable in earthquakes. So much so, building codes in the United States prohibit new construction of this kind in areas of high seismic activity. (If you're concerned about a URM building, FEMA has a risk reduction article available.) [Smart Materials Improve Earthquake-Resistant Bridge Design]
A main problem with older concrete buildings during earthquakes is the failure of load-bearing columns. In newer buildings, columns have more and better-placed "ductile details," such as closely spaced steel ties or spiral reinforcement. To make older buildings more ductile, retrofitting is needed.
The most common retrofits are concrete shear walls and bracing systems such as diagonal steel reinforcements, said DesRoches. But these can be expensive, and require the use of heavy machinery as well as the temporary evacuation of buildings.
With support from NSF, DesRoches' team at Georgia Tech is studying how non-ductile buildings behave during earthquakes and how to place the right retrofits in the right places to keep structures intact on moving ground.
"We want them to not collapse in a moderate to larger earthquake ... We want them to have very little damage in a small earthquake," DesRoches said. A top priority is developing solutions that are cost-efficient, easy to maintain, and won't require buildings to be vacated during upgrades. "This project is focused on trying to find a way ... that's less disruptive than the current approaches. Meaning, we don't have to perhaps vacate the building for months or years to retrofit it. It doesn't change the look of the building dramatically and it's fairly easy and economical to do."
Testing next-generation retrofits
Last year, Georgia Tech's Structural Engineering and Materials Laboratory had a full-scale non-ductile concrete building constructed to test retrofits. (See accompanying video .) The two-story, 3,000-square foot building is a copy of mid- to late-20th-century construction.
"It's very typical construction in parts of the West Coast prior to 1950 and parts of the East Coast prior to 1980. We detailed it, reinforced it based on exactly what would be done at that time, and what we know exist out there, all over the U.S.," he said.
The team conducted a series of tests using a mobile shaker borrowed from the NSF-funded Network for Earthquake Engineering Simulation. "We placed it on the roof of the structure and we clamped it down. It imposes a load laterally onto your building [and] the building is actually experiencing what would it experience in an earthquake."
The tests simulated shaking equivalent to that of two historic earthquakes — the 1940 El Centro, Calif., earthquake and the 1994 Northridge earthquake.
The team sliced the model into four identical bays, to test three different retrofits as well as no retrofit. The first retrofit was a carbon-fiber jacket designed to wrap around columns and confine them; the second was also a carbon wrap, with grout between the wrap and the column; the third was a novel smart material DesRoches developed — a bendable nickel titanium alloy known as a shape memory alloy. "This material can go [through] a significant amount of displacement and then just springs right back," DesRoches said. [In Structural Design, Less is More ]
The researchers also tested a bay without any retrofitting, to see how it performed. That test was very successful — they had to stop the shaker to avoid collapse. "It really confirmed what we knew," DesRoches said. "These structures can't move much."
By contrast, all three retrofitted bays performed well, with each providing subsequently more protection than the previous one (carbon wrap, carbon wrap with grout, shape memory alloy). Data analysis will tell more, but DesRoches believes, based on what he saw during the shake tests, that the shape memory retrofit will provide the most strength and ductility, though it will also be the costliest retrofit.
DesRoches's students are now analyzing the large amounts of data generated by the model building, which was equipped with thousands of sensors. The goal is to create a choice of retrofits and provide building code bodies such as the International Code Council (ICC) with research-based results they can use to update code. The ICCs
It will take time for the research team to finalize the results and get them to the ICC or other bodies that write and update buildings codes and retrofit guidelines — and, the results will need to be replicated by other researchers.
Building strength
With a choice of retrofits, it will be up to real estate owners to determine what kind of performance they want for their structures, which will likely be a function of the kind of building. A hospital, for example, would have to be fully operational during and after an earthquake, but a parking garage would not.
Cities are starting to address the significant challenge of earthquake-proofing older buildings. San Francisco has mandated that all wood frame buildings of five or more units with "soft" first stories be retrofitted by 2020.In Los Angeles, mayor Eric Garcetti is urging state lawmakers to pass a bill that would give tax credits to property owners who retrofit buildings. The bill is one element of Garcetti's earthquake safety effort, which includes a mandatory retrofitting proposal.
There are as many as 17,000 older concrete buildings in California, including private buildings, schools and government buildings, according to a research group called the Concrete Coalition. In Los Angeles alone, there are more than 1,000, according to the Coalition.
"I think many people think there's nothing you can do about an earthquake other than hope it doesn't happen while you're in the building or while you're living in that area," said DesRoches. "I think our test and other tests have shown that, in fact, there are fairly straightforward ways that you can retrofit a structure to significantly improve this behavior, so that either it doesn't collapse or even is fully operational after an earthquake. We want to save lives, but we also want to keep hospitals, schools and businesses as functional as possible."
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