December_1970 — Page 37

vulcanised bearing

ings are an alternative and convenient method of providing a high degree of isolation at low frequencies.

Isolation of buildings

Existing buildings. The isolation of a complete existing building is a diffi cult matter and in many cases the work required would be elaborate and costly. The isolation of limited areas, however, has been successfully achiev- ed on many occasions, usually by mounting floors or small rooms on rubber carpet mountings or on a rubber or steel spring mountings.

Trenches have been used as a means of preventing external vibra- tions affecting a building and can be effective in the protection of small buildings, for example, seismological observation posts. The method, how- ever, is not usually applicable to a large building, particularly if the ground vibrations are deep-seated rather than close to the surface. Modern theory is that the depth of trench should be about one-third of the wavelength of the vibration. Thus if the velocity of wave propagation in a particular soil were 300 m/sec and the frequency of vibration 20 Hz, the wavelength would be 15 m and the trench would have to be at least 5 m deep; it would also have to be kept open or, possibly, filled with thixo- tropic fluid.

New Buildings. Provision can be made at the design stage for a new building to be specially mounted to protect it from the effects of vibration that cannot otherwise be controlled.

For many years, buildings in the USA have been mounted on sandwich pads of lead and asbestos, inserted under girders and columns. The origi- nal purpose of this treatment was to protect the buildings from the effects of noise from road and rail traffic but it has been claimed that the low-fre- quency vibration problem is also ade- quately dealt with. This view is, how ever, not universally supported.

In London there are now several buildings in which special measures have been taken to combat vibration and low-frequency noise.

In the Barbican Development, the underground railway runs just below the area and the heavy deck of the track itself is supported on rubber units giving a natural frequency of about 6 Hz. The possibility of reson- ance of the prestressed beams of the deck has also been taken into account and the intrinsic damping improved by

Far East BUILDER, December 1970

Fig. 6

the use of a sandwich damping layer of specially selected material.

Exploded view

In the new Marble Arch Odeon Fig. 7 scheme the whole auditorium, weigh- ing about 4,600 tonnes, has been mounted on sandwich pads to protect it from vibration and noise from near- by railway lines.

Assembled view

A block of flats, Albany Court, in London, has been mounted on a low-frequency isolating system to pro- tect it from vibrations from the under- ground railway running directly below the building. A large number of sand- wich pads of rubber and steel were used, the individual loads carried rang- ing from 60 tonnes to over 200 tonnes and the natural frequencies obtained were 7 Hz vertically and 2.5 Hz hori- Fig. 8 zontally. The possibility of wind- excitation of the spring-mounted building was also taken into account. The cost of mounting the building, in- cluding the structural modifications to accommodate the springs, is likely to be more than offset by the amenity value that would otherwise be lost.

The pathological laboratories of St Mary's Hospital Medical School (Fig. 6) are located near to three railway lines, one on the surface and two underground, which interfered serious. ly with work in the old building. Measurements showed that vibrations due to trains were mainly in the range 25-40 Hz. The new building of five storeys was built on a reinforced con- crete platform supported by four columns, each about 1.2 x 1.5 m cap- able of carrying a load of 660 tonnes and resting on a rubber mounting.

The V-shaped bases (Fig. 7) take care of the vertical component of vibra- tion and of the horizontal component at right-angles to the Vs; the horizontal component in the other direction is dealt with by a 50 mm thick rubber

1.5m

Fig. 9 Vibration in the old building.

Fig. 10 Vibration in the new building.

1.2m

pad, clamped to the parallel sides of the column feet (Fig. 8). An airspace of about 50 mm was left between the new building and its neighbours on each side and sealed with rubber strip. The effectiveness of the design is shown by the two record traces (Figs. 9 and 10) of train vibrations measured in the old building and in the electron microscope room of the new.

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