Earthquake in Chile gives engineers some pointers

When structural engineer Anuj Bansal designs systems for hospitals, university libraries and apartment complexes, seismic safety is a key consideration. But the best way to find out what it takes for a building to withstand a massive earthquake is to analyze the aftermath of an actual event.

That’s why Bansal, who heads the Los Angeles office of Degenkolb Engineers, traveled to Chile with three colleagues to survey the damage resulting from last month’s 8.8 quake that killed 497 people, caused $30 billion in damage and shifted the city of Concepcion 10 feet to the west.

The engineers blogged during their 10-day trip at www.degenkolb.com/blog.

Bansal returned to Los Angeles on Wednesday and shared his observations.

Why did you want to see the damage firsthand?

The Chile earthquake was one of the largest earthquakes in the last 100 years and provided a unique opportunity to enhance our understanding of the seismic behavior of buildings and structures. It’s the difference between reading a report and seeing it -- there is no comparison.

Do you learn the most from buildings that failed?

Collapsed buildings are less valuable because the evidence of how they started to unzip is hidden beneath many layers of debris. Buildings that are partially damaged are extremely valuable from a structural engineering perspective. Closer observation of failed elements reveals the mode of failure.

What about buildings that are relatively intact?

They tell us what worked and what did not.

For example, we saw two identical buildings on opposite sides of the street in Valparaiso. Each building had a tall, skinny tower and then a larger main body. One building had significant damage at the interface, while the other didn’t. They had added a seismic joint between the two elements in one building but not the other. It was pretty clear what worked.

When you examine a building, where do you start?

We walk around the building and note the differences in the structure on all sides. We also look at the ground to see any signs of seismic settlement or liquefaction. We note details such as rebar size and spacing, tie bar size and spacing, shear wall length, and quality of concrete.

Each area of damage explains the type of failure. For example, if we observe large diagonal cracks in a shear wall, which is designed to handle building loads, we know that is a shear failure. By the time we walk out of the building we usually have a pretty good hunch about what happened.

Some of the most heavily damaged buildings were only a few years old. Why?

We were perplexed by this. Newer buildings generally mean compliance with newer codes, which have generally more stringent requirements for seismic design.

Downtown Concepcion had gone through a significant construction boom in the last few years. This area is within about one mile from the edge of the Biobio River. There is a possibility that the soils contributed to the amplification of the ground motions, although we did not see any signs of this ourselves.

Another new yet heavily damaged structure was the Alto Rio apartment building in Concepcion, which broke in half. What did you think when you saw it?

The initial reaction was just awe for an earthquake that could do this level of damage.

We met a grateful father who showed us a picture of his daughter and granddaughter who were trapped in the building for 14 hours. They were rescued and were thankfully doing well. It gave us a greater appreciation of the responsibility we carry when we design buildings in our daily professional lives.

Are the lessons from Chile relevant to Southern California?

Some of our buildings are over 100 years old, use un-reinforced masonry, brittle concrete and have other undesirable characteristics. We have made a lot of progress in retrofitting buildings here when mandated by our cities and jurisdictions.

However, many of our cities are still struggling with mandating retrofits because of political and economic considerations. After seeing the damage to similar buildings in Chile, we need to find the will to deal with these problems.

karen.kaplan@latimes.com