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Vibrators to check natural frequency

TWO SHAKING machines have been brought in from building research body BRE to test the Millennium Bridge this week. BRE staff and Arup engineers will excite the bridge with the shaking machines to check its natural frequency, damping, and mode shapes, along with static stiffness.

Tests will begin with the BRE's smaller Grandstand vibrator and move on to the larger No 1 vibrator system later in the week. Both machines use contra rotating masses - where mass and rotation velocity determine the excitation force and frequency - which for the larger vibrator can be up to 10kN and 20Hz.

The vibrators will be attached to the deck of the bridge while the three tests are conducted. The Grandstand vibrator will conduct the tests in both the vertical and horizontal axes.

For each test, 2g accelerometers will be used to measure the vibration response of the bridge. The first test will be to run a frequency sweep to identify the bridges' natural resonant frequency. This test can be used to calculate static stiffness and give an initial indication of damping equipment.

For the second test, the bridge is excited to its natural frequency - as identified by the first test. The vibrator is then switched off and the decay of vibrations plotted to give a confirmation of damping.

The third test uses two accelerometers, one fixed near the vibrator, and one roving. The vibrator is set at the natural frequency and the roving accelerometer used to pick up phase differences along the structure to define the mode shape.

The test results will be used to check Arup's computer models.

The BRE has developed these tests over the last 20 years and has used them on several different structures, including the Humber Bridge. Recently, the No 1 vibrator was used on a tall building test at BRE's Cardington facility.

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