Still shakin’ it
Mobile phones and laptops that are old news within a year, cars with plastic fenders — it feels like the pace of designed obsolescence is quickening. So it is truly impressive when something built decades ago continues to be adaptable to modern needs.
Such is the case with the Pacific Earthquake Engineering Research Center (PEER) shake table, located at the Richmond Field Station. The world’s first modern shake table, the 20’x20’ reinforced concrete table was unveiled in 1972, soon after the 1971 Sylmar earthquake.
Initially, the table shook vertically and horizontally; upgrades now enable it to move in six degrees of freedom: three translational degrees (vertical, lateral and longitudinal) and three rotational degrees (pitch, roll and yaw) — making it the largest six-degrees-of-freedom table in the country. Over the years, specimens weighing up to 150,000 pounds have been hoisted by a 10-ton bridge crane to be tested on the table.
The table was pivotal in proof-of-concept tests for energy dissipation devices such as rubber-bearing isolators and friction dampers; the testing was essential in demonstrating those devices could be beneficial to the seismic performance of a structure. Subsequent tests of building systems and anchoring and bracing systems have led to the adoption of new industrial standards and technology.
Computational features controlling the shake table continue to be enhanced. PEER researchers use open-source modeling and simulation software to analyze how distinct parts, or subassemblies, of a structure perform during seismic events. An emerging method, called hybrid testing, allows researchers to model how, for example, the top of a skyscraper will behave in an earthquake, then dial in the shake table to replicate that behavior, and then test specimens that would be located at the top of the skyscraper — essentially performing accurate piecemeal testing.
And now, with a new 19’x27’ rolling skylight, the sky is truly the limit for future tests.