The world’s largest outdoor shake table, which measures how structures perform during earthquakes, is undergoing a makeover. With the help of a $16.3 million grant from the National Science Foundation, the University of California, San Diego’s Large High-Performance Outdoor Shake Table, known as LHPOST, will move from a uniaxial system that could test with only one degree of freedom — horizontal movement — to one that can operate along all six degrees of freedom. Facilitating movement along all six degrees of freedom — back and forth, up and down, left to right, yaw, pitch, and roll — will allow the shake table to reproduce earthquakes more accurately.
Enabling the shake table to move in all six degrees of freedom creates a much better simulation of potential earthquake movements. California’s moment magnitude 6.7 Northridge earthquake in 1994, for example, registered instances where bridge columns had punched through their decks. Such instances point to significant vertical earthquake ground motion, a direction of earthquake movement that the shake table was previously unable to test.
When the shake table’s upgrade is completed in October, its name will be changed to LHPOST6, says Joel Conte, Ph.D., P.E., M.ASCE, a professor of structural engineering at UC San Diego and the principal investigator for the operation and maintenance of the shake table.
The existing shake table was in operation for 15 years, until September 2019. Its primary component is a 145-ton honeycomb steel platen, a platform that measures 40 by 25 ft. The shake table has the world’s largest payload capacity at 20 meganewtons, and because it is situated outdoors, it has no height restrictions on the structures it can test.
It will be upgraded with four horizontal actuators that will deliver biaxial horizontal motions as well as the yaw motion capabilities. Each of the existing six vertical actuators with pressure balanced bearings will be equipped with a high-flow servo valve to enable the vertical, pitch, and roll motion capabilities. Four hydraulic power units, combined with a 10,000 gal. accumulator bank, will operate the table.
Hardware and software will be upgraded to a controller with six degrees of freedom capabilities. The height of the shake table’s four safety towers will also be doubled. These safety towers are located outside of the moving platen and prevent any potential test building from full collapse, which would potentially damage the shake table’s platen and hydraulic power building. (For testing purposes, it is enough to have the specimen building reach incipient, but not full, collapse.)
The building specimens or other structural specimens are clamped or post-tensioned to the shake table platen. If the specimen is being tested under a fixed-base condition, the frame or foundation slab of the specimen is post-tensioned to the shake table platen. The team can also test a building specimen that is on a foundation embedded in soil in a large soil box. In this case, the box is attached to the shake table platen.
The upgraded shake table can also be used for studies on soil behavior during an earthquake. To observe this type of behavior, Conte and his team would use a large soil box, partially or fully filled with soil layers.
“The shake table is a very elaborate closed-loop control system that can be programmed to reproduce some of the largest earthquake ground motion ever measured,” Conte says. The upgraded unit will be able to reproduce the near-field ground motion intensity of a moment magnitude 7 to 8 earthquake and beyond, depending on the specific characteristics of the peak acceleration, velocity, displacement, and duration being tested.
More use for the upgraded shake table
The shake table is just one way that professionals test or validate seismic performance through seismic qualification tests. It is true that smaller physical models and computer modeling and simulation can act as stand-ins. But in the industry, it is widely believed that shake tables are best at testing full-size structures or equipment, such as large electrical transformers and air conditioning units, electrical substations, or nuclear casks, in as close to real-world conditions as possible. The results from these tests also help validate computer models.
The upgraded shake table is also expected to generate datasets that will help develop, calibrate, and validate physics-based computer modeling and simulation for civil structures and infrastructure systems.
While the shake table has many uses, there will be instances that its use might not be indicated, Conte acknowledges. A bridge with long spans and multiple piers, for example, will be subjected to different earthquake excitations at the base of its piers. The earthquake ground motion at the base of one pier could be different from that at the base of another pier. “In order to model these well, you really need multiple shake tables,” Conte says. A small-scale model will have to do as a test sample in such cases because multiple and connected shake tables of the size of the LHPOST are not feasible for practical and economical reasons.
Conte expects that the upgraded shake table will have more collaborations with those working within the industry than the previous iteration had. This is because such users often need shake tables with at least horizontal and vertical degrees of freedom, and the previous table could only move horizontally.
The first scheduled test after the upgrade will be a full-scale 10-story building made from cross-laminated timber. The test is expected to provide critical data about performance of taller wood buildings.
The potential of the upgraded shake table, Conte says, has garnered attention from many industry heavyweights and government agencies. “The upgraded shake table will change the nature of seismic testing in the United States and in the world,” Conte predicts.
The operation and maintenance of the UC San Diego shake table is currently funded through a five-year grant from the National Science Foundation's Natural Hazards Engineering Research Infrastructure program.
This article was updated on Feb. 26 to include funding information and correct how fixed-base specimens are attached to the shake table platen.