Introduction
City
centre construction sites are often complicated by having surface rail or
underground railway tunnels close to proposed foundations and piles. New office
complexes are frequently built directly over existing railway stations where
heavy diesel, as well as underground trains pass or terminate. These essential
services generate ground bourne vibration which enters buildings of
conventional construction and can make the life of occupants intolerable.
The solution for new construction may be to design the building with elastomeric anti-vibration bearings incorporated into the foundations. This has been an accepted building practice since 1963 and many buildings have been isolated on elastomeric bearings of various types.
We offer both laminated natural rubber/steel plate bearings and fabric reinforced bearings. The former been designed to BS5400 and the latter to BS6177. These products have been developed and designed with technology and experience gained during 30 years of production.
The bearings are suitable for use in any buildings where there is a requirement for the structure to be isolated from a source of vibration.
Guidance
General guidance relating to the selection and use of elastomeric bearings for isolating buildings from vibration is published in BS6177:1982. It is recommended however, that the CMS Vibration Solutions Technical Department are consulted for a bearing system proposal since applying the wrong bearing design can compound a vibration problem.
Life Expectancy
Elastomeric Bearings are made to a strict specification and quality control, thereby ensuring a maintainence free working life, generally comparable with the design life of the structure.
Installation
Please note that in some cases, side restraint will be required to ensure the stability of the structure. This can usually be provided by steel dowels which should always have a rubber dowel sleeve or some form of side restraint bearings incorporating sliding (PTFE) faces to account for vertical compression of the main bearings.
Fail safe devices are usually fabricated steel boxes or strip designs but concrete upstands can also be used. For in situ concrete structures a filler e.g Clayboard, polystyrene or other suitable filler material can be used to infill between bearings prior to casting. Alternitavely a galvanised steel base plate with starter bars can be used as a permanent shutter on top of the groups of bearings.
Product Types
Type – V
Type
V Bearings are manufactured to BS 6177 from vulcanised natural rubber
conforming to bridge bearing standard BS5400. Non-metallic reinforcement of high
tensile strength synthetic fabric is vulcanised into the bearing in a similar
manner to steel plate reinforcement of conventional bridge bearings. The basic
material is moulded in 25mm thick sheets, then cut into individual bearing
units and bonded together to the required finished thickness. This thickness
depends on the required natural frequency for the project and can vary from
25mm to 150mm in multiples of 25mm.
Bearings are available in any plan size, however, it is more economical to make use of the standard sizes cut from sheets. These range from a minimum of 100 x 100 x 25mm to 600 x 600 x 150mm generally in multiples of 50mm on plan dimensions. BS6177 gives engineers useful information when considering the installation of bearings for anti-vibration purposes. Items such as fail safe devices, wind loading restraint, fire protection, replacement, etc. are also covered.
Bearings are always used in groups rather than one large “area” of bearing material. This ensures the optimum performance is gained together with other advantages such as ease of installation. For example a pile cap with a vertical load of 8000kN would require 16 bearings of 250mm square plan dimensions. The pile head may therefore have to be flared to give sufficient area for bearings, fail safe devices and dowel bars for side restraint.
The basic material is manufactured in 25mm thick sheets and bonded together to give the required natural frequency at a design vertical load. From tests, the damping ratio has been measured as 0.08 and the loss factor at 0.056 for our bearings.
The recommended temperature operating range is -30ºc to +70ºc.
Type – G Bearings
Type G Bearings
are designed and manufactured to BS5400 and comprise natural rubber vulcanised with
internal steel plates in individual moulded units similar in construction to
normal bridge bearings. As with the V-Type, the dimensions of the bearing will
vary depending on the load capacity required and the natural frequency.
Bearings are available in a number of plan sizes which can have a variation of rubber thickness and metal plates, hence load capacity can vary for the same plan size. It is recommended that CMS Vibration Solutions Technical Department are contacted for any bearing proposal. For example, a range of bearings may be as follows;
- 229 x 152 x 24mm typical load 150kN
- 300 x 240 x 24mm typical load 450kN
- 455 x 240 x 24mm typical load 750kN
As with V type bearings are always used in groups.
The bearings are manufactured in single moulded units and the dimensions are calculated to provide the required natural frequency at design vertical load.
From tests the damping ratio has been measured as 0.028 and the loss factor at 0.056 for type G bearings.
The recommended operating temperature range is –30ºc to +70ºc.
Product Information
