Malita Anders

July 30, 2001

MCEER at UB

 

Seismic Isolation: Bearing Characteristics

Objective: 

The scope of this study includes two types of bearings, Low Damping Rubber Bearings and High Damping Rubber Bearings. These two bearings are the same with the exception of the type of rubber.  Testing will help us to better understand bearings performance under varying parameters.

 

Test Protocol:

            The experiments are displacement controlled; therefore we will assume that the input and output are reasonably close not to alter our results.  This was confirmed by running a test with out a bearing in the machine, and the seeing that the input time history and the output time history were reasonably close.  See the Table 1 below.

In each test are five strain amplitudes, and each amplitude last three cycles, with a constant normal force.  For test two the rate was increased.  Test three reverses strain amplitude.  Test four and five have decreased normal loads.  Test six, or the recovery test has the same parameters as test one, but is performed after some time period.

 

Test #

Strain %

Strain %

Strain %

Strain %

Strain %

Frequency

Normal force

1

5

25

50

100

200

0.01Hz

28 kips

2

5

25

50

100

200

0.5Hz

28 kips

3

200

100

50

25

5

0.01Hz

28 kips

4

5

25

50

100

200

0.01Hz

14 kips

5

5

25

50

100

200

0.01Hz

7 kips

6

5

25

50

100

200

0.01Hz

28 kips

Table 1

 

 

Bearing Characterization:

            For increasing levels of strain one would expect the first cycle to be stiffer than the two following cycles.  This is typical of an unscragged bearing.  After test one the bearing has been scragged and the stiffness of the first cycle should be relatively close to the stiffness of the other two cycles.  Test six should show unscragged behavior because it is performed at a later time.  The stiffness is calculated by finding the slope of the line that connects the positive direction maximum displacement and force to the negative direction maximum and minimum displacement.

            The experimental length vs. the resultant force is called hysteresis graph.  The area of each loop is proportional to the amount of energy dissipated in each cycle.  The area of the loop increases as the force and displacement increase.  From these loops one can see the peak forces reached.  As mentioned before, this usually happens on the first cycle of the largest strain level, for an unscragged bearing.

 

Importance:

            Engineers design the superstructure and substructure to withstand a certain peak response.  The peak response of a bearing sets the design standard.  If the peak response or the bearing cannot be accurately predicted the design may not achieve desired strength or performance.

 

Testing Problem:

            Complete results cannot be drawn due to problems that occurred during testing. However, the problems will be mitigated as soon as possible and complete results will be posted in the final report.