Laminated elastomeric bearings
The first devices
At the beginning of the 1950s, drawn and tinned wire meshes of 4 mm diameter began to be placed between 5 mm thick rubber plates. Under the effect of pressure, the threads of the mesh were printed on the rubber. The sets of stacked elements were held together by metal straps that surrounded the edges.[23].
With the LARGO-PILE devices, the mesh was replaced by sheets after having undergone a first treatment, but the first real supports in the form of rubber circles joined by vulcanization appeared in France in 1956[24] and in the United States in 1957.[25].
These were initially plates made up of groups of individual sheets of rubber adhered to two thin sheets of steel. To form a support device that allowed greater deformation by shear and rotation, the individual sheets (steel-rubber-steel) were stacked and glued together alternating with other steel sheets.[9].
There were three types of devices: uncoated, semi-coated and coated.[9] The intermediate plates were made of plain steel or stainless steel. The individual supports were cut from a motherboard of approximately 1 m², and are recognizable in France by the color of the protective paint "Paint (material)"):[26].
• - Red: quality of road bridges.
• - Green: quality of railway bridges.
• - Gray: with stainless steel intermediate plates.
• - Yellow: building quality.
Typology of current devices
The French standard NF EN 1337-3 defines six types of laminated rubber supports:[27].
• - Type A: support with a single covering ring.
• - Type B: support with at least two intermediate plates (up to five) and fully covered.
• - Type C: support with external metal plates.
• - Type D: sliding support with an outer PTFE (polytetrafluoroethylene) sheet.
• - Type E: sliding support that includes a metal plate on the surface in contact with the PTFE sheet.
• - Type F: support without intermediate plates and in band.
• - Type B: Support with at least two intermediate plates and fully covered.
• - Type C: Composite support with external metal plates for anchoring.
• - Type E: Sliding support with a metal plate on the surface in contact with the PTFE sheet.
Constituent parts
A laminated elastomeric bearing is a.
The elastic material used in the composition of the supports can be natural rubber of plant origin, latex, an isoprene polymer, or synthetic, the most used compound being a chloroprene polymer (polychloroprene or CR for "Chloroprene Rubber" in the standard). There are several formulas that, on the market, carry brand names: Neoprene (Du Pont de Nemours), Butachlor (Ugine)[29]… Over time, the brand name neoprene has become its usual name.
They are S 235 steel sheets or a steel with an equivalent minimum elongation at break. Its thickness may not be, in any case, less than 2 mm.[30].
There are several provisions. In France, they consist of a cellular PTFE plate adhered to the upper part of the elastomer of the support, either on the external elastomer coating (type D device according to NF EN 1337-3), or on an external steel plate (type E device according to NF EN 1337-3). A polished stainless steel sheet attached to an S235 steel top plate slides over the PTFE plate.[30].
When it is desired to avoid displacement, anti-slip devices composed of stops or stop blocks are placed, which should not interfere with deformations: compression, shear and rotation. In particular, the stops must come into contact with a plate (or external ring) whose thickness will be at least equal to the height of the stop (Type C supports of the NF EN 1337-3 standard). In no case should the stop be placed on the elastomer sheet.[31].
Device characterization
Shrink-fit elastomeric bearings are characterized by their dimensions (width, length, thickness), the number of (shrinkable) layers, and the allowable load. Laminated elastomer supports that support support reactions of less than 12 MN, calculated at the ultimate limit state (ULS), have plan dimensions of the order of 700 x 700 mm or less.
The work indicators are as follows:
• - Shear modulus G.
• - Stiffness in compression Cc.
• - Resistance to repeated compression efforts.
• - Static rotation capacity.
• - Adhesion to the cut.
• - Physical and mechanical properties of the elastomer.
• - Mechanical properties of the intermediate retaining sheets between layers.
With reference to the NF EN 1337-3 standard, four types of verification must be carried out in the Ultimate Limit States for laminated elastomer supports, whatever their type:
• - The maximum total distortion at any point of the support is limited.
• - The thickness of the straps must be sufficient to resist the traction they suffer.
• - The stability of the support must be ensured against rotation, buckling and sliding.
• - The actions exerted by the support on the rest of the structure must be checked (direct effect of the support on the structure and indirect effect due to deformations of the support).
Shearing of a laminated elastomeric bearing is the deformation of the entire device due to shear stress. It is measured by the value of the tangent, being the deformation angle.
If given as variables:.
you have to:
A rubber support is sized for a maximum value of = 0.7 called deformation capacity and this maximum value corresponds to the extreme relative displacements between the support and the structure. In most cases, the u/T ratio is a good approximation.
In-situ deformation measurements must take into account the temperature of the bridge, because the length of the deck varies with temperature and, in fact, the device deforms as a function of this parameter.
Problematic
Its main problem is the appearance of cracks or “chapping” if the sheets bulge. The possible origin of these cracks is:[32].
• - Excessive compression.
• - More rarely, poor resistance to the effects of ozone, with characteristic cracking at 45°. The depth of the cracks remains limited to between 1 and 2 cm, and the device can be left in place subject to regular monitoring.
• - A bursting of the layers due to excess compression (rare), which gives a straight crack parallel to the characteristic union plane.
The old supports cut from a motherboard or that have an external ring have metal parts that are in contact with the air. Corrosion of the edges of the corrugated plates or the external straps can therefore occur in the long term. From a certain degree of corrosion onwards, these devices must be changed. To prevent corrosion of the edges of the plates, a protective paint was used, the behavior of which against beading and deformation was particularly poor. However, its chipping or cracking is not serious.[33].
This problem should no longer occur with the current fully lined supports. However, there are outer rings that come with lugs (anti-slip devices) that are not coated, and are therefore subject to deterioration due to corrosion.
The sliding of piles of strapping plates occurs mainly in old supports not adhered by vulcanization. Such a disorder requires rapid intervention to avoid a sudden drop in the leveling of the structure.[34].
Beyond a distortion value of 0.7, support performance is considered abnormal. Thus, it should be considered that these devices can accept distortions of up to 1.5 and it is only at this level that their short-term replacement should be considered.[35].
However, it must be checked whether the tangential force generated by this deformation is compatible with the operation of the support (in the case of thin piers, for example).
Beyond 1.5, there is a risk of a so-called S-shaped deformation, following or leakage of the supports.[35].
The causes of abnormal distortion can be:
• - An error in calculation or positioning of a fixed point.
• - An undervaluation of delayed deformations.
• - A movement of the structure.
• - A fit failure during construction.
• - A movement of the support (due to the push of an embankment).
• - Locking a sliding plate.