Vibration Monitoring of Large Structures

CLlVE WILLIAMS, Plymouth Polytechnic, Department of Civil Engineering, Devon. United Kingdom

Structures today are more flexible than traditional buildings due mainly to a better understanding of struc- tural behavior, computer-analysis techniques and modern lightweight construction materials. Thus, they are more susceptible to dynamic disturbing forces.

As design and construction techniques change, proto- type structures must be observed in order to evaluate structural performance and check validity of design data. It is necessary to provide the designer with feedback on the actual performance of the structure and how it reacts to applied loading. Dynamic testing can provide an overall 'picture' of the structure as well as the dynamic charac- teristics and susceptibility to dynamic loading.

Structure and Response The interaction of loading, structure and response can

be represented as shown in the diagram below:

Force, Stiffness, Deflections, Accelerations,

Damping Stress,

With information gained from the study of two of the above examples, it is possible to determine details of the third Thus by inputing known loads, at known frequencies. and monitoring the response of the structural system, details can be found of the properties of the system However, there is more work carried out on theoretical analysis and model work than on full-scale testing. this IS

because of the physical problems involved and the expense of testing

Experimental Techniques Experimental techniques fall into two categories, the

first using ambient vibrations and the second using forced- vibration methods. Both techniques are able to provide data on performance in slightly different forms. Modern instrumentation available to the engineer has made the results from his task more precise with new techniques providing more information on the structure tested.

Ambient vibration. Observations have been made in quantitative terms since the 1930's but have only really become significant since the 1960's when the instru- mentation and techniques improved.

The method of using ambient vibrations for structures can be conveniently considered with respect to multi- story structures Wind loading provides an adequate form of excitation as it can be assumed that the wind spectrum is constant over the frequency range of interest From the tests, estimates of natural frequency, mode shapes and damping can be found Monitoring a multistory building undergoing high wind loads would provide the performance data required to complete the picture of response Earth- quakes, wave pressures on sea walls, wave and current loading on offshore structures all give ambient-vibration loadings

Fig. 1 -The Residentfa/ Nautical Coliege. f / p O f m

Experimental Techniques 29

0

REF = 0.02 V2/HZ

- 10

-20

db

- 30 Fig. 2-Power spectrum from Residential Nautical College data east-west direction

- 4 0

-50

1 2 3 4 5 6 7 8 9 10

FREQUENCY (Hz)

f = 1.35 HZ f = 1.95 Hz f = 5.23 Hz f = 6.49 HZ DIRECTION E-W D I R E C T I O N N-5 DIRECTION E-W D I R E C T I O N N-S

Fig. 3-MOde shapes of Residential Nautical College

Forced vibration. Application of a known force at a known frequency to excite a structure is a simple but very effective concept. The most common forms of 'exciters' used are rotating-mass vibrators or hydraulic actuators, both of which can produce uniaxial sinusoidal forces. When such devices are rigidly attached to structures such as buildings, bridges, platforms or dams, they can be used to excite the important natural frequencies. Large vibrators of this construction have provided sufficient force to produce major structural damage to structures.

A number of other methods for imparting a known amount of energy to a structural system have been em- ployed but they do not have both the frequency and amplitude control of those mentioned above. Methods using cables with winches, explosives and rockets have been used to excite structures. Some structures can be excited by manual means, such as a person using his inertia at the right frequency to provide the energy input.

Dynamic Characteristics of a Multistory Building

As part of a full-scale testing program of structures, an 11-story building was tested under ambient and forced vibration The structure tested was the Residential Nautical College building in Plymouth, Devon, England, (Fig l ) , a reinforced concrete-frame building 33-m high It has a small eccentric lift core with end-shear walls and precast concrete cladding panels

Data were first obtained on the h 'ding during ambient- vibration tests in a light winc i r w l $17 c i ; :he natural

30 December 1984

frequencies and associated mode shapes were obtained. Figure 2 shows a power-spectrum plot made with data from a transducer placed in the weak axis (east-west) direction, on the top floor, towards the edge of the building. The plot contains information on the translational fre- quencies in that direction and also the torsional frequencies. From analysis of the data recorded at each floor level in turn, the mode shapes were determined (Fig. 3). An estimate of damping in a structure can be obtained for the separate frequencies using autocorrelation analysis.

Care should be taken during analysis of the data when frequencies are close together. The analysis assumes a wide-band white noise input to the system; the wind can be assumed to provide this over the small frequency range considered for autocorrelation function calculations.

Tests on the building in a high wind yielded information on the level of accelerations produced. This data were recorded over a four-hour period when the maximum 30-s gust was recorded at 27 rnls, with an average speed of 20 mls over the four-hour period. The anemometer was placed on top of the building, 6 m above the roof. Ampli- tude-probability density functions were obtained from the acceleration data recorded. They revealed that the acceleration had a probability of 0.04 lying outside the range of & 12 mmls. This is equivalent to a deflection of kO.2 mm.

The Tamar Suspension Bridge Continuing the full-scale testing program, a suspension

bridge was investigated. The bridge has a main span of 335 rn with two side spans of 114 m and twin hollow rectangular reinforced-concrete main towers to a height of 80 m. The main suspension cables consist of locked coil wire ropes with vertical suspenders supporting a 4.9-m deep Warren stiffening truss. The deck is made of re- inforced concrete supporting a three-lane carriageway 10- m wide and two 1.8-m footpaths running eastlwest.

A laser-monitoring device on the bridge. A prototype laser-based monitoring device developed by the author was used to monitor low-frequency movements of the bridge The device measured the relative displacements between two points in adlacent two-dimensional planes Transducers, such as accelerometers, were available to

TIME (seconds)

0.

10.

Fig. 4-Deflection at the center of the main span of VERTICAL DEFLECTION 20. the Tamar suspension bridge

30 - 40.

VERTICAL DEFLECTION

( m m ~

measure frequencies down to 0.1 Hz, but equipment was not readily available to measure large movements of structures below this frequency. The prototype equipment measured frequencies from 0 to 1 Hz (depending on the amplitude).

The equipment consisted of a laser which was used to produce a spot of light at a given distance. The laser acting as a stationary pointer was directed at the moving structure, where a detector monitored the relative move- ment between the laser position and the part of the structure under observation. An analogy can be drawn to a pencil with its point touching a screen or paper; as the screen moves the pencil traces the deflection in two dimensions on the screen. Replacing the pencil with a laser beam on the screen by an X-Y plotterlphotocell device which can 'lock on' to the laser beam, a deflection- measuring system is created which can monitor low- frequency movements up to 300 mm. By monitoring the voltage signals developed by the X-Y plotter the deflection in the X and Y directions can be recorded in analog form for later analysis in the laboratory.

The laser-based system underwent a comprehensive laboratory and analytical program before it was used on the structure in order to determine its dynamic limitations and accuracy. Tests were also carried out on the laser to assess its directional stability and temperature Sensitivity.

Fig. 5- Wind-induced oscil/affons deflected at center of main span of the Tamar suspension bridge

Quasistatic bridge movements. The central deflection Of the main-span truss of the bridge was monitored with the laser-based device. Here the laser was located at a relatively stationary position on a deep cross beam at the tower location. The laser was placed on the north side of the bridge to minimize the effects of heating from the sun. The position was also monitored for rotations with an accelerometer, but rotations were relatively small and could be allowed for in deflection results.

A typical graph obtained for the passage of lorries over the bridge is shown in Fig. 4. Superimposed on the 'influence-line' diagram for the deflection is a higher frequency signal due to the bridge's natural frequencies and possible atmospheric effects on the beam. The atmospheric effects of temperature gradients were checked with an array of thermistors placed on the line of the laser beam. Over a short period of time, say 30 seconds, no significant effects were observed on the pointing stability of the laser beam.

The bridge movements during a light wind provided the data shown in Fig. 5, where the amplitude of 2 3 mm was observed at a frequency of 0.33 Hz. The frequency response of the system starts to affect the results at this level of operation but from laboratory calibration tests can be allowed for. The true deflection was approximately k 2.8 mm.

Experimental Techniques 31

TEMPEMTURE A I R TEMPERRTURE - hFATHER: HOT WITH P O

CLDUD THEN CLOUDY

NOVEMENT +j

lmm)

IEASTWARD) EAST-WEST MOVEMENT

TIME OF DAY IHOURS)

14.00 16.00 1 8 . 0 0 20 .00 2 2 . 0 0 24 .00 02.00 WEATHER : SUNNY AFTERNWN

AND CLEAR U P T I L A I R ”) or 9 . 0 0 HOURS

TEMPERATURE

EASTHARD) +,

Fig. 6-Temperature and deflections monitored at the top of the lowers of the Tamar suspension bridge

2.59

- 10

db -

- 20

- 3 0

- 40 i. o 2 :o 3.0

FREQUENCY i H z )

Fig. 7-Spectrum analysis from data at position ( I ) for the Tamar suspension bridge

Further tests were carried out investigating the tower deflections. An ‘auto-plumb’ laser on trial from the National Physical Laboratory was used inside the base of the hollow tower with the monitoring device suspended upside- down at the top of the tower. The closed towers provided near perfect operating conditions for the laser system. A number of different loadings were observed on the bridge; the deflections of the tower. due to temperature changes, gave the results presented in Fig. 6. The deflections are in the order of 1 mm per degree Celsius; for temperature rises, the tower splayed outwards at the toD.

Dynamic bridge movements. A quick dynamic testing

32 Decemher lC!84

program was carried out on the Tamar Bridge similar to that used for the Residential Nautical College building described above During the testing sequence the bridge was excited by a northwest wind of between 7 and 12 mls and a fairly continuous flow of traffic Seismometers were used for the investigation, as they could be oriented both vertically and horizontally and used up to 1 km from the recording station Data were recorded on magnetic tape, later analysis from power spectrum plots yielded mode shapes and frequencies Figure 7 shows the data from a location on the bridge which was the control position The first vertical mode was found to occur at 0 32 Hz with the first torsional at 0 83 Hz