After reading this article you will learn about:- 1. Meaning of Resilient Mounting 2. Insulation of Structures 3. Rubber 4. Practical Considerations 5. Fail-Safe Devices 6. Leak Paths 7. Resilient Seating’s 8. Design Procedure 9. Significant Parameters.
Contents:
- Meaning of Resilient Mounting
- Insulation of Structures
- Rubber used for Resilient Mounting
- Practical Considerations for Resilient Mounting
- Fail-Safe Devices to Ensure Resilient Mounting
- Leak Paths Affects Resilient Mounting
- Resilient Seating’s
- Design Procedure of Resilient Mounting
- Significant Parameters for Resilient Mounting
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1. Meaning of Resilient Mounting:
The method of resilient mounting of structures had its origins in 1915 in the United States of America and, somewhat later, in Canada and the United Kingdom. In those early years, the method of resilient mounting made use of alternate layers of lead and asbestos fibre; but the result were disappointing due to over-hardness.
Another disadvantage of such mounting is that the lead/asbestos pads cannot isolate frequencies below 50 Hz, while train vibrations in the ground are usually of the order of 15-30 Hz. They can be as low as 5-6 Hz on soft clay. The “train rumble” noise given off by the “drumming” of building walls and floors, however, consist of much higher frequencies, sometimes extending to 600 Hz.
Smaller areas of the floor of a building, individual walls, and small parts of a building (e.g., a plant room) are either isolated with smaller mounts than those needed for the whole building, or by using rubber carpets, small helical steel springs, or glass or slag-wool-fibre mats.
Whatever method is adopted for resilient mounting of a building or a part of it, successful results can only be obtained by an extensive instrumental investigation of site conditions, plus detailed calculations involving vibration and structural engineering. The degree of isolation can now be chosen according to the type and intensity of the noises and vibrations.
The buildings requiring resilient mounting may range from blocks of flats, offices and hotels to hospitals, radio and television stations, laboratories, cinema halls, churches halls, shops, etc. The cost of resilient mounting of a building may vary from less than 1% to about 5% of the entire cost of the building.
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Instead of the purely empirical approach to resilient mounting followed in the early years from 1915 onwards, there is now a highly developed area of engineering dealing in detail with vibration and noise insulation, together with its special structural needs, covering a wide range of the types of buildings and soils.
2. Insulation of Structures:
An early method of insulating structures from ground-borne vibrations was to provide a trench in the path of such vibrations.
This method is still being erroneously attempted; but its usefulness is limited, as the following example shows If the wavelength of the sound waves is, say, 15 m, their velocity 300 m/s and the dominant frequency 20 Hz (typical of vibration waves in clay); and ‘if there is a vertical-sided trench 5 m deep in the path of such waves, there would be a reduction of transmitted vibration of about a third, which is not very much.
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To obtain more reduction by this method needs a very much deeper trench involving all the structural problems of keeping it open by props, and free of debris or flood water (to avoid transmission of the vibration across the gap A deep trench, however, is very costly and mostly impractical.
Many different types of materials have been, and are still being, used to insulate buildings from vibrations. Table 1 gives a list of these materials, together with their characteristics related to insulating buildings and similar structures from vibration and noise.
3. Rubber used for Resilient Mounting:
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Rubber is one of the most important materials used for resilient mounting. Rubber is available in various natural and synthetic types suitable for different conditions. Each of these types can, moreover, be compounded with a wide range of variable hardnesses.
The configurated rubber, the cork-modified rubber and the laminated fabric reinforced rubber can be moulded or cut to almost any size. In the same way, the steel plate reinforced rubber mounts can be made in any of the large variety of moulds of the International Standard Bridge sizes.
The rubber is also the obvious choice for the mounting of complete industrial buildings (and other structures) for isolating the building at the foundation or other levels. In the same way, the isolation of individual parts of a structure (e.g., studios) can also be carried out, using rubber mounts, within an existing structure.
Use of rubber for resilient mounting offers several other advantages. For example, good weathering properties (such as being anti-ozonant) can be compounded into the rubber mix.
Long- term reduction in dynamic and static physical properties of rubber (resulting from vibration fatigue and ageing) can be accounted for, and the allowance for it can be made in the calculations at the stage of design. The long-term creep of rubber is largely known. The same is true for the very slow loss of resilience of the rubber by aging. Both of these factors can, if necessary, be introduced into the design parameters.
The commonly-known deterioration of rubber used in personal clothes and domestic objects cannot occur in rubbers designed for engineering mounts. The reason is that the chemical constituents, mechanical distortions, and the external physical and chemical influences in such rubber are entirely different.
4. Practical Considerations for Resilient Mounting:
Many buildings in the UK, USA, Canada and other advanced countries have already been successfully isolated, in spite of the extra complications, difficulties and apparent hazards.
The difference between a non-isolated building and a similar but successfully isolated one is very marked indeed. A word of caution must, however, be given here. Insulating a building by simply putting some rubber under it can lead to a badly damaged structure and even an actual increase in vibration.
Attempts in the past by non-experts have lead to one or more of the following consequences:
(a) Buildings being split in several places from top to bottom;
(b) Serious structural damage during construction;
(c) Springs with gross creep;
(d) Extra fire hazard;
(e) Over-stressed bent columns;
(f) Short circuiting of the springs and cancellation of their effects; and
(g) Wrong vibration-site surveys using wrong kinds of instruments.
On the other hand, the advantages of properly isolating a building by experts are enormous. Specialist experts are available (at least in advanced countries) for this purpose.
Fire protection of rubber in-dangerous places has received a great deal of attention, in recent years, in connection with the isolation of buildings. Manufacturers of isolation materials, moreover, possess now sophisticated means of testing for safety overloads and other design parameters.
Local Authorities in the UK and some other advanced countries now increasingly demand that either it is arranged for the isolation springs to be replaced after, say, a hundred years, or that they are designed to carry the building safely for 200 years. The springs can certainly be designed to do this, although they may not be very effective as a vibration isolator after a period of 200 years.
5. Fail-Safe Devices to Ensure Resilient Mounting:
There are certain requirements to ensure the structural safety and serviceability of buildings on resilient mountings. A structure that is carried on resilient mountings should be designed on fail-safe principles so that in the event of failure, partial failure or deterioration of one or more mountings, the structure should remain adequately supported and retain its safety and general serviceability under the designed load.
Where there is a provision for the replacement of a damaged or defective mounting, temporary conditions may arise during which margins of safety and serviceability less than the normal ones may have to be tolerated. The provision for this situation should be made when the resilient mountings are designed.
The provision for the replacement of mountings is economically desirable under the following circumstances:
(a) When the structure is altered or extended;
(b) When the use of the structure changes, with alteration to the loading; or
(c) When the expected life of the structure is long enough to suggest a good possibility that a mounting will need replacement because of damage or deterioration.
Consideration should also be given, at the design stage, to making the mountings of a building accessible for inspection. This is necessary in order to monitor the condition of mountings with respect to creep or degradation.
Here is an example of fail-safe devices used for resilient mounting of structures. In this case, the mounting consists of rigid bearing members interposed between resilient bearings. The height of the rigid bearings is such that they normally remain clear of the supported structure resting on the resilient bearings. These are designed to carry the same load as that of the associated resilient bearings.
6. Leak Paths Affects Resilient Mounting:
No form of isolation mounting can remain effective if it is “short circuited” by a rigid connection between the resiliently-mounted structure and its surroundings, thus allowing vibrations to “leak” across this path.
Moreover, clearances around a mounting should be such as to permit movement to its loaded profile without restriction. Care must also be taken to ensure that restraint to a mounting is not caused through interference either by structural paths or by the accumulation in surplus grout or other debris in its vicinity.
Service connections (connections for the supply of water, electricity, fresh air, etc.) to a mounted structure should either be sufficiently flexible to act as isolators themselves, or incorporate flexible couplings to give the safe effect.
When it is necessary to seal any small gaps between the mounted structure and its surroundings (e.g., an adjacent un-mounted structure), the sealing must be done with a material of adequate flexibility to accept any probable movement.
Due allowance must also be made in the design for any consequent effects on the efficiency of the mounting. In order to ensure that a mounted structure remains completely isolated from any adjoining structure, sufficient space should be left between them to allow for movements of either of these buildings.
Such movements may occur due to any of the following reasons:
(a) Variations in temperature;
(b) Deflection and sway under load;
(c) Foundation settlement; and
(d) Creep and deflections of mountings.
The last also includes the deflections due to possible deterioration of the mountings. In addition to these movements, the normal building tolerances and clearances should also be taken into account.
7. Resilient Seating’s:
All buildings are subject to some movement; but the extent of this is very difficult or even impossible to predict accurately, mainly because of variation in the constructional material (and its properties) from the original design.
This may result in the development of cracks due to any of the following causes:
(a) Shrinkage;
(b) Differential settlement;
(c) Creep; and
(d) Thermal movements within the structure.
This effect (i.e., the development of cracks due to the causes mentioned above) can be countered by the use of resilient separating members interspersed between structural members where the above conditions are likely to occur. Such materials are known as resilient seating’s.
Resilient seating’s allow for extra movement due to the causes mentioned earlier. At the same time, such materials also ensure a small but uniform transfer of stresses from sub-structures to all beams, etc. Resilient seating’s can be designed for use on all types of structures, and employed between any combination of concrete, steel, brick and timber to eliminate the high stress at support edges.
Sliding resilient seating’s (known also as “sliding bearings”) can be employed where large structural movements are naturally present. Basically, these are bearing pads combined with a low- friction sliding surface. Perhaps the best-known application of sliding resilient seating’s is for the expansion parts of bridges. Other applications of these include floating roofs, beam.
8. Design Procedure of Resilient Mounting:
The following steps give a general idea of the design procedure for a resiliency mounted structure:
(a) Site vibration survey:
This includes the survey for all types of trains (passenger, express, local, goods, and heavy- freight trains), and several examples of each. The recordings must be vibrograms of velocity or displacement. Moreover, the natural frequency of the pick-up must be lower than the lowest frequency of the ground waves.
It is important to remember in this connection that meter readings from the “high-frequency” accelerometers do not provide information about the low-frequency waves, nor about the member of cycles contained by them. They do not even provide adequate information about pak transients. It is needless to add that all of these information’s are required for the design of insulation.
(b) The next step in the design is the calculation of the amount of vibration reduction needed. For living units, vibration must preferably be reduced to about the “boundary of perception”. The natural frequency of the building springs (based on the dynamic- not static-elasticity of the springs) is also determined during the implementation of this step. The wind say calculations of the building are based on the static stiffness.
(c) The next step is the determination of the dead weight of the building and its centre of mass. If the latter is eccentric for a rigid building, the springs must be modified to keep the structure dead upright, and to prevent its cracking.
(d) Since the springs are quite resilient compared with the much more rigid ordinary foundations, the structure must often be made stiffer to accept wind loads without undue distortion. If adequate consideration is not given to this during the design stage, the distortions due to winds may seriously damage lightweight partitions, external cladding, or windows at a later stage.
(e) In vulnerable places, all rubber springs must be protected against fire, and the design should take into account this requirement.
(f) A structure that is carried on resilient bearings should be designed on fail-safe principles. This is necessary in order to ensure that in the event of a failure, partial failure or damage to the springs, the building remains adequately supported and retains its safety and general serviceability under the design load. In addition, the much softer low-frequency bearings must be provided with individual side stability.
(g) Long-term side stability of the building mounted on springs is maintained by thin side-acting rubber pads, which also take over the function of wind braces.
(h) Before the springs are delivered to the intended site, they must be given overload tests to ensure safety. Moreover, load/deflection graphs of the springs should also be recorded. The dynamic behaviour of the springs should be calculated from these graphs, and checked.
(i) The last step in the design of resiliently mounted structures consists of comparing all variations of stiffness in the springs (shown by the tests mentioned above with the designed stiffness of the building members, and adjusting any excess variations.
9. Significant Parameters for Resilient Mounting:
The characteristics of bearings for resilient mounting can be tailored over a very wide range. To meet the particular requirements of the static deflection, shear deflection and stress, the following can be adjusted.
(a) Compounding the rubber mix;
(b) Its fabrication;
(c) Moulding pressure;
(d) Vulcanization; and
(e) Bearing sizes.
Where large column loads/areas of bearings are concerned, it is advisable to employ a number of smaller bearing units in module form to ensure equal dynamic and static bearing properties throughout the structure.
The significant parameters of the dynamic behaviour of resilient bearings are as follows:
(a) Dynamic stiffness:
This is a function of the dynamic modulus of elasticity, pad area and its thickness. It is very different from the static stiffness.)
(b) Shape factor:
(Laminated construction makes the pad far less susceptible to excess internal stresses, long-term fatigue and loss of shape compared with the homogeneous rubber pads of the same overall dimensions.)
(c) Loss factor:
(This is a measure of the damping ratio of the pad).
(d) Model factor:
It has been found by laboratory tests that the test-model springs are substantially less stiff dynamically than the dimensionally larger, but similarly proportioned, springs used in civil engineering. Some recent examples of well-isolated buildings in the United Kingdom, Australia and Belgium are given in Table 2. The isolation scheme of the building concerned and the type of bearings used are also mentioned in this Table.
