UCSD vibration table test could change building codes for earthquake protection

An 18-foot steel pole vibrates in a vise grip on a vibrating table at the University of California, San Diego.

The column stands perpendicular to the building, and the vibration table stimulates the pressure generated by the weight of the building and the movement of the strong earthquake.

Eventually, the shaft flexes and bends, which can cause the structure to bend or even collapse.

Building codes will likely change soon, after trying the vibrating table, because the shaft is used on many California buildings.

The experiment is detailed in an article published in the Journal of Structural Engineering.

Engineer Jay Harris of the University of California, San Francisco is a co-author of the article, and said the problem is in the spine attached to the shaft, called the “web,” which has been shown to be very thin.

“So it gets very thin — the thinness is getting taller. The shaft itself is at risk of bending too much and losing some of its load bearing capacity,” Harris said.

Courtesy of the National Institute of Standards and Technology

In this undated illustration, the wide steel column (cross-section shown on the left) was not able to withstand the pressure of the building’s weight, along with the strong earthquake, due to the thin contact network.

“If you lose the ability to withstand these gravitational forces, you could be at risk of some kind of collapse. Whether it’s a partial or total collapse, it will be specific to the building and the skeletal structure. And where this shaft is located… what it holds.”

One consequence of shaft bending is what is called “story drift”. It occurs when the upper floors of a multi-storey building sway in an earthquake, causing the building to bend.

Ironically, the use of poles, which failed the vibration table test, became very popular after the Northridge earthquake. The engineers believed that the larger beam width would prevent movement of the side buildings in the event of an earthquake.

“The Northridge earthquake was a huge shock to the engineering community, when it came to earthquake designs. What we were using was fragile,” said Chia Ming-yong, an engineering professor at UC San Diego, another co-author of an article in the journal about vibration table testing.

Now that the wide pillars have proven their own problems, Harris said these pillars will be phased out with new building codes.

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