Swivel joints

These support elements have curved surfaces that allow the upper element to rotate. As the bridge expands, they are tilted to allow movement of the deck in a horizontal direction. They are usually built with steel, and are usually used in highway bridges.[13]​.

Elastomeric supports

Elastomeric supports are the most widespread type today. They are made of elastic compounds and lack mechanisms with moving parts, since the deformation of the rubber itself allows the controlled movement of the bridge. They can be manufactured at low cost and require very little maintenance, unlike other metal devices that have moving parts. It is also common to reinforce them with steel to make them more resistant if necessary.[14]​.

Sliding supports

Sliding supports have a flat surface to allow horizontal movement and a spherical surface to allow rotation, behaving like a spherical ball joint. Although they used to be made of metal, sliding supports now tend to be made from sheets of polytetrafluoroethylene, a material with a low coefficient of friction and great resistance to wear.[13]​.

Ball joints

As the name suggests, these supports are shaped like a spherical cap. They only allow rotation and prevent movement in the horizontal and vertical directions.[13]​.

metal plate appliances

They are the simplest and oldest devices. They are found in older structures with expandable lengths rarely exceeding 20 m and load locations distributed over a fairly large number of points. They consist of a lead or zinc plate for older ones or two plates that slide over each other. In case of rotation by bending, the contact surface is reduced to a line.[15]​.

Rocker devices

This type of support consists of several parts, one of which has a curved, cylindrical or spherical surface, called a rocker, on which another flat or curved element can roll.

There are more modern devices whose friction surfaces are made of PTFE (polytetrafluoroethylene) plates.[18]​.

Roller devices

They ensure both longitudinal displacement and rotation of the transverse axis due to the bending of the board. There are several types of devices: simple roller, simple truncated roller (or boulder), roller with flattened sides (or pendulum), multiple rollers.[16]​.

Cranks

The connecting rods allow double articulation. Therefore, they are generally used in the union of two elements whose movements are independent. You can find them on suspension bridges, cantilevers, etc[19]​...

Due to its design, possible disorders will be sought at the level of the contact surfaces (axles, etc.), ovalization due to wear, abnormal play, blocking due to corrosion, displacement. In case of significant translation, the connecting rods cause unevenness between the elements.[19]​.

Problems with metal supports

The damage that the metal supports may suffer is due to the corrosion of the metal parts or the lamination of the rollers when present. There are three types:[20]​.

This problem can have serious consequences and appropriate protective measures must be taken (installation of safety wedges under the structure, leaving a space between the wedge and the soffit, for example). This inclination value must be measured based on the ambient temperature on the day of measurement and calculated for an extreme position of the bridge (maximum and minimum temperatures).[20]​.

The causes of these tilt failures can have several origins:

The function of the teeth (screwed or welded) is to ensure rolling without slipping. Therefore, the teeth are not fundamentally necessary, but their breakage is indicative of a design or function problem. For fixed supports, these teeth absorb the force and, in this case, their breakage is serious.[20]​.

Maintenance failures can be:[20]​.

Mesnager Joints

The French engineer Augustin Mesnager invented this type of joint in 1907. They consist of steel bars that cross in an X in a narrow section of concrete arranged between the parts to be articulated. The bars are arranged alternately in one direction or another and are concurrent on the same line: the axis of the joint. The resistance comes exclusively from the transverse bars, excluding the concrete core with a narrowed section, which only has a function of protecting the reinforcement.[7]​[21]​.

The operation of the joint leads to cracking of the core and therefore the steels are poorly protected against corrosion. Therefore, there are fewer and fewer Mesnager joints on structures in service.[7]​.

Joints Consider

Invented by the engineer Armand Considère at the beginning of the century, they are temporary joints that consist of a concrete element provisionally reduced to a small core.[7]​.

The presence of reinforcement then made it possible to cover the joint and reconstitute the column section. As a result, they are difficult to identify even if they still exist in certain structures in service, which may explain disagreements between different publications dealing with the subject.[7]​.

Freyssinet gaskets

Freyssinet joints or joints with narrow concrete sections very quickly replaced Considère joints. They equip many structures, and depending on the intensity of the horizontal forces, they have or do not have through steel bars (bolts). The dimensioning has been included in the calculation standards since 1964. Disorders are rarely observed in these joints.[8]​**·[21]​·**[22]​.

Caquot Gaskets

Caquot (or Considère-Caquot) joints were introduced around 1928. These are cylindrical or spherical joints on a plane or cylinder. Known as rolling joints, they serve the role of a fixed support apparatus. If the horizontal forces are important, steel bars are arranged that cross the contact line perpendicularly.

double joints

Caquot or Freyssinet joints are sometimes split to add translational capacity to their normal rotational capacity.[17]​.

The double Caquot joint allows movements of several centimeters, while the double Freyssinet joint allows only small movements. Every repair operation must take care to preserve the stability of the work.[17]​.

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]​.

Typology of current devices

The French standard NF EN 1337-3 defines six types of laminated rubber supports:[27]​.

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:

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:

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]​.

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:

A pot type support consists of a metal base in the shape of a low cylindrical pot (meaning of the word "pot" in French), on which a rubber cushion with a peripheral gasket is placed to guarantee its tightness.[12]​.

Metal bases are generally machined from a very thick plate, which forms a monolithic assembly.[36]​ The piston (or cap) provided with a very small movement clearance in the base rests on the cushion to transmit the loads with the possibility of rotation. The cushion, trapped between the piston and the base, theoretically behaves like a fluid.[12]​.

Most of these pads are made of natural rubber, although some older ones are made of neoprene. Peripheral seals are usually made of brass or stainless steel.[12]​.

With this configuration, the function of the device is limited to behaving as a fixed support. To allow the displacement function, a PTFE (polytetrafluoroethylene) disc is fixed to the upper part of the piston, on which a steel plate will slide, whose surface in contact with the PTFE, honeycomb or not and greased, is generally made of stainless steel. This steel plate is fixed to its support by gluing, screwing or welding.[12]​.

The devices can be equipped with a guide element that allows sliding in all directions (multidirectional device) or in only one direction (unidirectional device).

They can be anchored either by simple friction with the structure or by means of bolts and sealing lugs. These anchors must be arranged in such a way that they allow in principle the disassembly of the device.

Typology of pot-type devices

The devices are essentially differentiated by their type of guidance, which can be central or lateral.

The central guide normally consists of a central key fixed to the piston by tightening screws or by welding, or by machining the sliding plate and engraving on the piston.

The side guide can be bolted, welded or machined from solid side plates.

Maintenance

The degradation of the anti-corrosion paint on the metal is the main disorder found. It is measured using a device specially designed for this purpose. Depending on the state of the protection, it is possible to plan a long-term repair of the protective paint.[37]​.

It is very rare to find cracked metal parts.

The in-service rotation of the support apparatus must be measured regularly, since in the event of a malfunction, the rubber cushion may come off the base and cause the board to tilt.[37]​

The possible causes are:

The metal plate and PTFE plate can wear out, which could limit or even prevent any sliding of the structure. It can also be observed that the stainless steel or PTFE piece loses its laminar shape.[38]​.

The laminated elastomeric support is suitable for reactions limited to 12 MN, calculated at the ultimate limit state (ULS). This value corresponds to plan dimensions of the order of 700 x 700 mm. Beyond 20 MN, pot type supports are preferable because they limit the size of the device. Between these two values ​​it is possible to maintain the elastomer supports, either increasing the dimensions to 900 x 900 mm for large structures, or combining two smaller supports.[39]​.

This last solution is only easy to use on box section bridges and concrete slab bridges for reasons of load-bearing size. It is difficult to foresee for "beam" bridges (mixed or prestressed concrete).[39]​.

On the other hand, in the case of significant rotations on the support, the elastomer device may be adequate, but it is often necessary to excessively increase the thickness of the elastomer, which poses other problems. Regarding horizontal displacements, the sliding systems of pot type devices offer better quality and, therefore, greater durability: therefore, it is the displacement criterion that will influence the choice of the type of support.[40]​.

Manufacturing restrictions (mainly press dimensions) mean that the largest dimensions of elastomer devices are currently limited to approximately 1000 x 1000 x 300 mm for French manufacturing and 1200 x 1200 x 300 mm in exceptional cases.[40]​.

In seismic zones, even for lowering heavy loads, it is preferable to use laminated devices. Indeed, in the absence of a fixed point, and given the flexibility they provide, the overall behavior of a structure subjected to a moderate amplitude earthquake is better. Under strong earthquakes, they will tear and will be less expensive to replace than pot type devices.[40]​.

In Europe

The standardization of supports began at the European level following the "Directive (European Union law)") on Construction Products dated December 21, 1988,[41]​ which was part of the so-called new approach Directives.[42]​.

The objective of these directives was to eliminate technical barriers in the field of construction. In this context, the works must comply with the six essential requirements (Essential Requirements or ER):[43]​.

European rules are adopted by a weighted majority, that is, at least 71% of the votes cast (excluding abstentions) are in favor of the text. They must be included in the national collection of member countries, which must eliminate all contradictory provisions from their standards.

The scope of the NF EN 1337 “Structural supports” standard is very general and concerns both buildings and all civil engineering structures, including bridges and viaducts:[43]​.

They are one of the most important components of bridges.

Swivel joints

These support elements have curved surfaces that allow the upper element to rotate. As the bridge expands, they are tilted to allow movement of the deck in a horizontal direction. They are usually built with steel, and are usually used in highway bridges.[13]​.

Elastomeric supports

Elastomeric supports are the most widespread type today. They are made of elastic compounds and lack mechanisms with moving parts, since the deformation of the rubber itself allows the controlled movement of the bridge. They can be manufactured at low cost and require very little maintenance, unlike other metal devices that have moving parts. It is also common to reinforce them with steel to make them more resistant if necessary.[14]​.

Sliding supports

Sliding supports have a flat surface to allow horizontal movement and a spherical surface to allow rotation, behaving like a spherical ball joint. Although they used to be made of metal, sliding supports now tend to be made from sheets of polytetrafluoroethylene, a material with a low coefficient of friction and great resistance to wear.[13]​.

Ball joints

As the name suggests, these supports are shaped like a spherical cap. They only allow rotation and prevent movement in the horizontal and vertical directions.[13]​.

metal plate appliances

They are the simplest and oldest devices. They are found in older structures with expandable lengths rarely exceeding 20 m and load locations distributed over a fairly large number of points. They consist of a lead or zinc plate for older ones or two plates that slide over each other. In case of rotation by bending, the contact surface is reduced to a line.[15]​.

Rocker devices

This type of support consists of several parts, one of which has a curved, cylindrical or spherical surface, called a rocker, on which another flat or curved element can roll.

There are more modern devices whose friction surfaces are made of PTFE (polytetrafluoroethylene) plates.[18]​.

Roller devices

They ensure both longitudinal displacement and rotation of the transverse axis due to the bending of the board. There are several types of devices: simple roller, simple truncated roller (or boulder), roller with flattened sides (or pendulum), multiple rollers.[16]​.

Cranks

The connecting rods allow double articulation. Therefore, they are generally used in the union of two elements whose movements are independent. You can find them on suspension bridges, cantilevers, etc[19]​...

Due to its design, possible disorders will be sought at the level of the contact surfaces (axles, etc.), ovalization due to wear, abnormal play, blocking due to corrosion, displacement. In case of significant translation, the connecting rods cause unevenness between the elements.[19]​.

Problems with metal supports

The damage that the metal supports may suffer is due to the corrosion of the metal parts or the lamination of the rollers when present. There are three types:[20]​.

This problem can have serious consequences and appropriate protective measures must be taken (installation of safety wedges under the structure, leaving a space between the wedge and the soffit, for example). This inclination value must be measured based on the ambient temperature on the day of measurement and calculated for an extreme position of the bridge (maximum and minimum temperatures).[20]​.

The causes of these tilt failures can have several origins:

The function of the teeth (screwed or welded) is to ensure rolling without slipping. Therefore, the teeth are not fundamentally necessary, but their breakage is indicative of a design or function problem. For fixed supports, these teeth absorb the force and, in this case, their breakage is serious.[20]​.

Maintenance failures can be:[20]​.

Mesnager Joints

The French engineer Augustin Mesnager invented this type of joint in 1907. They consist of steel bars that cross in an X in a narrow section of concrete arranged between the parts to be articulated. The bars are arranged alternately in one direction or another and are concurrent on the same line: the axis of the joint. The resistance comes exclusively from the transverse bars, excluding the concrete core with a narrowed section, which only has a function of protecting the reinforcement.[7]​[21]​.

The operation of the joint leads to cracking of the core and therefore the steels are poorly protected against corrosion. Therefore, there are fewer and fewer Mesnager joints on structures in service.[7]​.

Joints Consider

Invented by the engineer Armand Considère at the beginning of the century, they are temporary joints that consist of a concrete element provisionally reduced to a small core.[7]​.

The presence of reinforcement then made it possible to cover the joint and reconstitute the column section. As a result, they are difficult to identify even if they still exist in certain structures in service, which may explain disagreements between different publications dealing with the subject.[7]​.

Freyssinet gaskets

Freyssinet joints or joints with narrow concrete sections very quickly replaced Considère joints. They equip many structures, and depending on the intensity of the horizontal forces, they have or do not have through steel bars (bolts). The dimensioning has been included in the calculation standards since 1964. Disorders are rarely observed in these joints.[8]​**·[21]​·**[22]​.

Caquot Gaskets

Caquot (or Considère-Caquot) joints were introduced around 1928. These are cylindrical or spherical joints on a plane or cylinder. Known as rolling joints, they serve the role of a fixed support apparatus. If the horizontal forces are important, steel bars are arranged that cross the contact line perpendicularly.

double joints

Caquot or Freyssinet joints are sometimes split to add translational capacity to their normal rotational capacity.[17]​.

The double Caquot joint allows movements of several centimeters, while the double Freyssinet joint allows only small movements. Every repair operation must take care to preserve the stability of the work.[17]​.

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]​.

Typology of current devices

The French standard NF EN 1337-3 defines six types of laminated rubber supports:[27]​.

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:

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:

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]​.

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:

A pot type support consists of a metal base in the shape of a low cylindrical pot (meaning of the word "pot" in French), on which a rubber cushion with a peripheral gasket is placed to guarantee its tightness.[12]​.

Metal bases are generally machined from a very thick plate, which forms a monolithic assembly.[36]​ The piston (or cap) provided with a very small movement clearance in the base rests on the cushion to transmit the loads with the possibility of rotation. The cushion, trapped between the piston and the base, theoretically behaves like a fluid.[12]​.

Most of these pads are made of natural rubber, although some older ones are made of neoprene. Peripheral seals are usually made of brass or stainless steel.[12]​.

With this configuration, the function of the device is limited to behaving as a fixed support. To allow the displacement function, a PTFE (polytetrafluoroethylene) disc is fixed to the upper part of the piston, on which a steel plate will slide, whose surface in contact with the PTFE, honeycomb or not and greased, is generally made of stainless steel. This steel plate is fixed to its support by gluing, screwing or welding.[12]​.

The devices can be equipped with a guide element that allows sliding in all directions (multidirectional device) or in only one direction (unidirectional device).

They can be anchored either by simple friction with the structure or by means of bolts and sealing lugs. These anchors must be arranged in such a way that they allow in principle the disassembly of the device.

Typology of pot-type devices

The devices are essentially differentiated by their type of guidance, which can be central or lateral.

The central guide normally consists of a central key fixed to the piston by tightening screws or by welding, or by machining the sliding plate and engraving on the piston.

The side guide can be bolted, welded or machined from solid side plates.

Maintenance

The degradation of the anti-corrosion paint on the metal is the main disorder found. It is measured using a device specially designed for this purpose. Depending on the state of the protection, it is possible to plan a long-term repair of the protective paint.[37]​.

It is very rare to find cracked metal parts.

The in-service rotation of the support apparatus must be measured regularly, since in the event of a malfunction, the rubber cushion may come off the base and cause the board to tilt.[37]​

The possible causes are:

The metal plate and PTFE plate can wear out, which could limit or even prevent any sliding of the structure. It can also be observed that the stainless steel or PTFE piece loses its laminar shape.[38]​.

The laminated elastomeric support is suitable for reactions limited to 12 MN, calculated at the ultimate limit state (ULS). This value corresponds to plan dimensions of the order of 700 x 700 mm. Beyond 20 MN, pot type supports are preferable because they limit the size of the device. Between these two values ​​it is possible to maintain the elastomer supports, either increasing the dimensions to 900 x 900 mm for large structures, or combining two smaller supports.[39]​.

This last solution is only easy to use on box section bridges and concrete slab bridges for reasons of load-bearing size. It is difficult to foresee for "beam" bridges (mixed or prestressed concrete).[39]​.

On the other hand, in the case of significant rotations on the support, the elastomer device may be adequate, but it is often necessary to excessively increase the thickness of the elastomer, which poses other problems. Regarding horizontal displacements, the sliding systems of pot type devices offer better quality and, therefore, greater durability: therefore, it is the displacement criterion that will influence the choice of the type of support.[40]​.

Manufacturing restrictions (mainly press dimensions) mean that the largest dimensions of elastomer devices are currently limited to approximately 1000 x 1000 x 300 mm for French manufacturing and 1200 x 1200 x 300 mm in exceptional cases.[40]​.

In seismic zones, even for lowering heavy loads, it is preferable to use laminated devices. Indeed, in the absence of a fixed point, and given the flexibility they provide, the overall behavior of a structure subjected to a moderate amplitude earthquake is better. Under strong earthquakes, they will tear and will be less expensive to replace than pot type devices.[40]​.

In Europe

The standardization of supports began at the European level following the "Directive (European Union law)") on Construction Products dated December 21, 1988,[41]​ which was part of the so-called new approach Directives.[42]​.

The objective of these directives was to eliminate technical barriers in the field of construction. In this context, the works must comply with the six essential requirements (Essential Requirements or ER):[43]​.

European rules are adopted by a weighted majority, that is, at least 71% of the votes cast (excluding abstentions) are in favor of the text. They must be included in the national collection of member countries, which must eliminate all contradictory provisions from their standards.

The scope of the NF EN 1337 “Structural supports” standard is very general and concerns both buildings and all civil engineering structures, including bridges and viaducts:[43]​.

They are one of the most important components of bridges.