Real Time Nonlinear Hybrid Simulation of MR Damper
The Problem:
The MR damper has been identified as a potentially ideal semi-active earthquake damage mitigation device for steel frame structures. Realtime hybrid testing is being utilized at the CU NEES Fast Hybrid Testing (FHT) facility in an effort to gain and deepen an understanding of the actual performance of such devices and there controllers. The force output of the MR damper is highly velocity dependant so hybrid tests must be conducted in hard realtime in order to have any practical meaning.
In order to realistically test the performance of MR dampers it is essential to evaluate the interaction of the damper with a structure while both components are pushed into a region of nonlinear behavior. Such behavior is a characteristic of strong earthquake motion and is central to the evaluation of these devices.
Perhaps the most obvious and direct approach to a test of this nature would involve installing and testing the dampers in a representative multi-story structure on a shaking table.
Large shake tables with such capabilities do exist but require an enormous financial investment in equipment, construction personnel, repair, reconstruction, and demolition. Even if we were to use a very large shake table, we will still be violating some similitude laws. Furthermore, by testing the entire frame, we would be inducing cracking/damage in other locations which would taint our understanding of the structure. Henceforth the shake table test, whereas a priori would be perceived as the ideal testing method, in reality is not.
The Solution:
Rendering of FHT Laboratory with Projected Image of Hybrid Structure
An alternative to the shake table approach is FHT and as can be seen in the laboratory rendering shown above a significant economic benefit is achieve by reducing the physical portion of the test to only the MR damper which is fully reusable from one test to the next. The remaining portion of the structure is modeled numerically using OpenSees and a constrained and customized implementation of implicit direct time integration. The resulting hybrid structure may be tested repeatedly under realtime conditions during which there is at no time any distortion of the temporal component of the structures response. The preservation of a one-to-one time scaling during these simulations is absolutely essential owing to the rate sensitivity of the MR damper.
Prior to completing an actual FHT the staff at CU NEES routinely conducts a four-level series of simulations providing valuable model verification and validation data. The level 1 test is a fully numerical simulation and facilitates the evaluation of the critical question; "Is the correct physics being modeled and captured especially within the finite element portion of the hybrid structure?" Additionally, level 1 simulations provide a baseline response for subsequent comparison during levels 2 thru 4. The level 2 simulation involves the hybridization of the full structure. The substructure to be tested physically is defined and distinguished from the remainder of the structural system. A simple numerical model of the physical substructure is created for work and analysis on both level 2 and level 3 simulations. During level 3 simulations the numerical model of the physical substructure is combined with a model of the hydraulic test equipment and run on a separate computer in realtime. Finally the level 4 simulation is the actual hybrid test and the simulated test structure and actuator models are replaced with real thing.
| Level | Experimental | Numerical | Audience |
|---|---|---|---|
| 1 | None | Yes | K-12 |
| 2 | Computer | Undergrad | |
| 3 | Computer 1 | Computer 2 | Graduate Students |
| 4 | Laboratory Tests | Computer(s) | Researchers |
The Result:
The results presented here are preliminary with a single MR damper located at the base level of the 3 story structure. Further hybrid testing will be carried out with multiple dampers positioned at various locations within the structure. Additionally, there will be an investigation of different control algorithms applied to the damper during the seismic simulations. These control algorithms will vary the electrical current which is applied to the MR fluid within the damper (changing the dampers dynamic characteristics) during the seismic simulations.
Initial results indicate MR Dampers can indeed mitigate structural damage during strong earthquake motion. The existing hybrid testing platform will provide an ideal environment for validating and refining damper control alogrithms.
We are currenty exploring the feasibility of integrating our hybrid simulation software with OpenFRESCO.
These tests are the culmination of extensive collaborative work carried out by the CU NEES team motivated by the NSF funded project of Dr. Richard Christenson of the University of Connecticut and through the direct support of the National Science Foundation.