UC Berkeley Shaking Table
The PEER Earthquake Shaking Table, dedicated in 1972, was the first of its kind and at 20′ x 20′ is still the largest multidirectional shaking table in the US. The shaking table is configured to produce three translational components of motion; vertical and two horizontal, plus three rotational components; pitch, roll and yaw. These six degrees of freedom can be programmed to reproduce any wave forms within the capacities of force, velocity, displacement, and frequency of the system. The shaking table can subject structures, weighing 100,000 lbs, to horizontal accelerations of 1.5 G. The concrete shaking table is heavily reinforced with both traditional reinforcement and post-tensioning tendons.
Structurally, the table may be considered as a 1′ thick diaphragm, stiffened by heavy central transverse ribs that extend below its bottom surface. The hydraulic actuators that drive the table horizontally are attached to these transverse ribs. The structures to be tested, are attached to the table by post-tensioning rods located in 2″ diameter conduits that pierce the table vertically on a 3′ square grid. The table itself weighs 100,000 lbs. By design, the stiff shaking table with a natural frequency of greater than 20 Hertz behaves essentially as a rigid body in the operating range of 0 - 10 Hertz.
The shaking table is driven horizontally by eight 75,000-lb capacity hydraulic actuators and vertically by four 75,000-lb capacity actuators. All of the hydraulic actuators are located in the chamber beneath the shaking table. The actuators have swivel joints at each end allowing for rotation, which accommodates the components of motion perpendicular to their primary direction of travel. The total length of each horizontal actuator, including swivel joints, is 10′- 6′ and the total length of each vertical actuator is 8′- 8″. The length of the actuators assists in decoupling the horizontal and vertical components of motion; further decoupling is accomplished in the control system.
An MTS model 469 controller commands the shaking table′s movement. The controller provides for closed loop control of motion in translation and rotation about the 3 principal axes. The controller is designed so that each of these 6 degrees of freedom can be programmed individually and run concurrently. Recorded earthquake acceleration records are used in programming the command signal to the shaking table.
In operation, the air pressure in the chamber beneath the shaking table is pressurized so that the total weight of the table and structure being tested is supported by the difference in air pressure between the chamber and ambient air pressure. Two airtight doors seal the chamber and permit access while the air in the pit is pressurized. Supporting the shaking table and test payload mass with air pressure allows the hydraulic system to be more efficient in subjecting the system to dynamic loading. The horizontal gap between the edge of the shaking table and the interior foundation wall is sealed by a wide vinyl strip providing the hermetic seal required to float the table with air pressure.
In 1969, Professor Penzien conceived the design of the world′s first modern shaking table with the assistance of Professor Clough, both UC Berkeley faculty. The shaking table they created is the same one operated by PEER today and is still the largest multidirectional shaking table in the United States and among the most sophisticated in the world. Many of the design features originated in Berkeley, were later adopted for shaking tables in Japan, Mexico, Peru, and Yugoslavia.