Ground Liquefaction Takes Toll; Heritage Buildings Suffer Most
David T. Biggs, P.E., S.E., Dist.M.ASCE, HTMS, a consultant and former principal of Ryan-Biggs Associates, a structural engineering firm in Troy, N.Y., continues his diary of experiences surrounding the 6.3 magnitude earthquake that struck Christchurch, New Zealand. Biggs happened to be delivering a seminar lecture on structural engineering and seismic forensics when the earthquake hit on Tuesday, Feb. 22, and he immediately went to work assisting New Zealand engineers in the assessment of buildings for safety.
Read Day One and Two of Biggs' diary if you have not already.
The day started with a 7 a.m. check-in. About 200 engineers have shown up from around New Zealand and descended upon the Command Center set up at a modern building, the Art Gallery. The concrete-framed building did extremely well in the earthquake despite having an approximately 50-foot tall glass wall. The primary problems with the command center were falling ceiling tiles and light fixtures. That’s actually a very common occurrence for many buildings, and has been a source of numerous minor injuries.
Prior to check-in, the two New Zealand TV stations and Sky, the international news station, plus one New Zealand radio station asked for interviews from this visitor from the United States. (Click to watch video of one such interview.) Some wanted comparisons to the World Trade Center disaster based upon my experience as part of the FEMA-ASCE Building Performance Assessment Team. Other questions included concern over the ability to build buildings safely in seismic regions and what should be done to rebuild the damaged buildings from this quake. My opinion was that modern design codes do well for new buildings. However, Christchurch has a particular problem with ground liquefaction that needs to be addressed for retrofit and new construction. Regarding rebuilding, the older New Zealand heritage building stock constructed from 1885 to about 1935 was significantly damaged by this quake. Loss of those buildings would make the city very different is they were not rebuilt in an architectural style that reflects the charming character of Christchurch.
After the media attention, it was back to work. The day was spent surveying damage in the heavily damaged Central Business District. Our team included Dr. Jason Ingham, my host from the University of Auckland, and Lisa Moon, a Ph.D. student from the University of Adelaide, Australia. Lisa is working jointly with Auckland U. Of the approximately 200 buildings we surveyed using a Level 1 exterior inspection, our estimate is that about 40 percent of the buildings are likely candidates for demolition. Most of these are unreinforced buildings of two stories.
We saw more of the spots the world is seeing on the news. One was the cathedral, an historic stone building; one was the Canterbury TV office, a multi-story concrete frame structure; and the third was the PPG (Pine Gold Guinness) building, a five-story concrete shear wall building. The cathedral is devastating to the city because it was iconic. Unfortunately, the deaths there were probably people taking a tourist visit into the spire or having lunch out on the steps. The TV building pancaked, trapping many. Japanese rescue teams were the first to fly in to assist in the search and rescue operation. It’s reported 20 Japanese students may be in the rubble. The PPG building overturned and collapsed. The word among the teams is that liquefaction may have been a major contributor to that collapse. Some good news has come from that site as four or five survivors have been found after two days. Unfortunately, our travels took us past the two buses that were crushed by falling buildings.
The liquefaction issue keeps coming up at various sites. We observed a seven-story frame building that appears to have undergone full building rotation. Survey measurements have not been taken but the tilt meter app on my iPhone indicated all the columns with a 1- to 2-degree lean. I estimated one end of the building settled approximately 6 inches and the other lifted about 9 inches. It will be interesting to learn what the surveyors come back with in terms of accurate readings.
Speaking of readings, the 20-story Grand Chancellor Hotel we surveyed the first day has been monitored for 24 hours now. It has a significant lean to the southeast. No perceptible changes were detected. Many predicted it was collapsing in creep mode. I predicted they were wrong and that the structure would lock up. Got this one right but the next large aftershock will change that.
Today’s aftershocks were less severe. Some are still startling, while other times we often asked each other if it was actually a shock. Usually, the aftershocks are most noticeable if you are stationary or in an elevated structure. Seems like the most noticeable ones occur overnight.
A few overall observations, some drawn from other inspectors:
1. The parking structures did quite well, the precast and the cast in place. Most had moment frames in one direction and concrete shear walls in the other. The steel-framed structures and timber-framed residences did well also.
2. Some tall buildings had problems with emergency lighting in stairwells. (Tall here is generally 10-20 stories). These hindered evacuation. I was reminded of the 1994 World Trade Center bombing and similar problems where those issues were subsequently dealt with, and the 9/11 tower evacuations were safer and faster and probably saved lives just by providing better lighting and bright paint in the stairwells.
3. The inspection process is difficult to organize with many volunteers. Whomever is chosen to be the leader must possess skills of technical knowledge, firm command, and a vision. We’ve met several New Zealand people that are superb.
4. The people on the ground in Christchurch are very collegial and professional, like the New York City engineering volunteers following 9/11. It speaks highly of our profession wherever we are in the world.
5. There have been no significant reports of looting or crime. In general, New Zealanders are very nice, very kind people and thankful for assistance. Even the politicians battling for government control in this fall’s elections have been working together to help their country.
6. We found a steel-arch vehicular bridge that buckled out of plane due to liquefaction pressure on the back walls; other inspectors reported buckling of girder bridges from the same effects.
Our long day ended at about 8:30 p.m. Tomorrow we are anticipating more inspections and another special assignment.
Read on for Day Four of David Biggs' diary.