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7/31/2019 US 0926A Pot Bearing Aap

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Technical guide

m=~=Use on bridges, viaducts and similar structures


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The Technical Department for Transport, Roads and Bridges Engineering and Road Safety (Service d'tudes techniquesdes routes et autoroutes - Stra) is a technical department within the Ministry of Transport and Infrastructure. Its field ofactivities is the road, the transportation and the engineering structures.

The Stra supports the public ownerThe Stra supplies State agencies and local communities (counties, large cities and urban communities) with informations,methodologies and tools suited to the specificities of the networks in order to:

improve the projects quality; help with the asset management;

define, apply and evaluate the public policies; guarantee the coherence of the road network and state of the art; put forward the public interests, in particular within the framework of European standardization; bring an expertise on complex projects.

The Stra, producer of the state of the artWithin a very large scale, beyond the road and engineering structures, in the field of transport, intermodality, sustainabledevelopment, the Stra:

takes into account the needs of project owners and prime contractors, managers and operators; fosters the exchanges of experience;

evaluates technical progress and the scientific results; develops knowledge and good practices through technical guides, softwares; contributes to the training and information of the technical community.

The Stra, a work in partnership The Stra associates all the players of the French road community to its action: operational services; researchorganizations; Scientific and Technical Network (Rseau Scientifique et Technique de l'Equipement RST), in particularthe Public Works Regional Engineering Offices (Centres d'tudes techniques de l'Equipement CETE), companiesand professional organizations; motorway concessionary operators; other organizations such as French Rail NetworkCompany (Rseau Ferr de France RFF) and French Waterways Network (Voies Navigables de France - VNF);Departments like the department for Ecology and Sustainable Development

The Stra regularly exchanges its experience and projects with its foreign counterparts, through bilateral co-operations,presentations in conferences and congresses, by welcoming delegations, through missions and expertises in othercountries. It takes part in the European standardization commissions and many authorities and international workinggroups. The Stra is an organization for technical approval, as an EOTA member (European Organisation for TechnicalApprovals).


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Technical guide

m=~=Use on bridges, viaducts and similar structures=

This document is the translation of the work

Appareils dappui po t

Utilisation sur les ponts, viaducs et structures similaires

published in november 2007 under the reference 0734.


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2

This guide was produced, under the supervision of the head of

Stras CTOA (Centre des techniques d'ouvrages d'art engineering-structure technical centre), by a working group

comprised of:

Jean-Franois Derais, Stra

Michel Fragnet, Stra

Gilles Lacoste, Stra

Yvon Meuric, Stra

Florence Pero, Stra

Ludovic Picard, DREIF

Yves PIcard, Consultant.

The following made comments and provided advice:

A. Chabert, LCPC

B. Plu, SNCF

Ph. Deniard, SNCFM. Dauvilliers, LROP

J.B. Datry, SETEC

V. Mauvisseau, SETEC

J. Ryckaert, SETEC

J.M. Lacombe, Stra

D. Lefaucheur, Stra.

Drawings prepared by Jean-Pierre Gilcart (Stra).

Photo illustrations: Stra photo library.

This guide cancels and replaces the technical guide

Les appareils dappui pot dlastomre. Utilisation sur les ponts, viaducs et structures similaires

(Elastomeric pot bearings. Use on bridges, viaducts and similar structures),

september 2000 (F0033)


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Pot bearings - Use on bridges, viaducts and similar structures

Contents

Chapter 1 - In troduct ion .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . 41.1 General remarks. Purpose and content of the present guide...................................................................4

1.3 - Application of standard NF EN 1337-5 to the French national context .....................................................5

Chapter 2 - Make-up of a pot bear ing.. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . 7

2.1 General principles .....................................................................................................................................7

2.2 The constituent parts.................................................................................................................................7

2.3 - Advantages and disadvantages of this type of bearing...........................................................................11

2.4 Key geometrical dimensions ...................................................................................................................11

Chapter 3 Remarks on standard NF EN 1337- par ts 2 & 5 Key points concern ing design . 133.1 - Presentation.............................................................................................................................................13

3.2 - Introduction ..............................................................................................................................................13

3.3 Pot bearings............................................................................................................................................13

Chapter 4 Pr incip les govern ing calculat ions for structures compr is ing pot bear ings .. . . . 19

4.1 Regulatory context ..................................................................................................................................19

4.2 Extreme vertical forces............................................................................................................................21

4.3 Longitudinal horizontal forces for sliding pot bearings............................................................................22

4.4 Longitudinal horizontal forces for restraint pot bearings.........................................................................26

4.5 Other recommendations..........................................................................................................................304.6 Examples of calculations ........................................................................................................................33

Chapter 5 Ver i f icat ion of pot bear ings .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . 45

5.1 Definition documents...............................................................................................................................45

Remarks on verification during service ............................................................................................................47

Appendix 1 - Help in draf t ing Par t icu lar Technical Clauses (CCTP) .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 49

A1 - Examples of clauses to be integrated into the Quality of materials chapter..................................... 49

A2 -Examples of clauses to be integrated into the Bearing justification paragraph of chapter II

Site preparation and organisation of the Particular Technical Clauses (CCTP) ......................................50A3 -Examples of clauses to be integrated into the Implementation chapter .................................................50

Appendix 2 - Pot-bear ing character ist ic summary sheets. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . 53

Bib l iography .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . 59

General documents..........................................................................................................................................59

Standards.........................................................................................................................................................59


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`~=N=J=Introduct ion

1.1 General r emarks. Purpose and content o f t he present gui de

Elastomeric pot bearings were first developed in around 1960, based on a system devised, notably, by Andra, Beyer andWintergerst. In France, they were first used in 1967, and received brief coverage in the 1974 edition of BT41, which outlinedtheir key characteristics at that time and sought to define a possible scope for their use.

Since then, applications have been developed without any real technical basements other than the information found inmanufacturers documentations or provided to designers by manufacturers. Competition between manufacturers has led to adanger of exaggeration as regards potential performance.

The T 47-816 standard was drawn up with clarification in mind, and parts 1 (general principles), 2 and 5 of standard NF EN1337 2, which deal specifically with these products, have since been released and are available.

It was felt that, in addition to these normative documents, there was a need for a guide focusing on the use of pot bearings withbridges, and examining interactions between pot bearings at different supports.

The guide published in September 2000 was based on draft European standards, which were difficult to obtain directly fromAFNOR, hence a degree of ambiguity in the document. This ambiguity was compounded by reference to non-finalisedstructural-design documents in addition to the French standards governing the verification of bearing characteristics.

The situation has now been clarified by the publication of standard NF EN 1337 in its entirety (with the exception of part 8 -Guide bearings and restrain bearings) and of the design standards (of the Eurocodes used in the present guide, at any rate).Moreover, the partial publication of standard NF EN 1337 was followed by the withdrawal, on 31 December 2006, of otherFrench standards covering the same topic, after a period of coexistence.

The present guide is intended to be complementary, and to provide explanations concerning current normative texts at the timeof drafting. It sheds light on the texts, notably by providing certain key specifications concerning use with bridges.

The document comprises the following:

A brief description of the product category and related equipment; The key regulations; The design criteria specified in the CENstandards; A calculation methodology, not for the product itself, but for its use in a bridge project, with a concrete example based on an

a real situation;

The NF EN standards make provision for a certification procedure using CE marking, for which the application proceduresare being put in place; in this new context, we will try to provide explanations concerning the choice of products and thepoints to be checked during the on-site acceptance process;

A programme has been developed for the verification of this type of bearing. There is a corresponding presentation.

1.2 Scope of use

Laminated elastomeric bearings or AAEFs (appareils dappui en lastomre frett) and pot bearings or AAPs (appareilsdappui pot) account for more than 90% of the bearings used with bridges in France. While the reasons for choosing one orother can be perfectly clear at the remoter edges of their respective scopes of use, the matter is a little more delicate wherethose scopes meet.

The choice of bearing depends on a wide variety of factors, including vertical loading, maximal rotation, horizontaldisplacement, durability, cost, type of structure, environment and structural arrangements. For this reason, it is not always easyto define the relative scopes of use of various techniques.

1 This publication (Bulletin technique no. 4), which served as a guide to laminated elastomeric bearings , is no longer available.2 See bibliographic references.


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Laminated elastomeric bearings are ideally suited for reactions of up to 12 MN (calculated at ULS). This value corresponds todimensions in plan of some 700 mm x 700 mm. Above 20 MN, it is preferable to use pot bearings so as to keep size withinacceptable limits. Between these two values, laminated elastomeric bearings may still be used, provided that the dimensionsare increased to 900 mm x 900 mm in the case of a large structure or that two smaller bearings are positioned side by side.Bearing bulk issues mean that the latter approach is easy to implement only with box-section or concrete-slab bridges. It isdifficult to use in the case of girder bridges (composite or made of prestressed concrete).

Where there is considerable rotation, however, laminated elastomeric bearings may be suitable, although it is often necessaryto increase the elastomer thickness, which can result in other problems. In such an event, the use of spherical bearings may betechnically appropriate (see NF EN 1337-7). Having said that, the sliding systems on pot bearings provide better performance,and therefore greater durability, as regards horizontal displacement. The displacement parameter will consequently be a majorinfluence on the choice.

In all events, manufacturing constraints (and notably press sizes) mean that French-made laminated elastomeric bearings arecurrently limited in size to some 1,000 mm x 1,000 mm x 300 mm, while certain manufacturers elsewhere can produce itemsas large as 1,200 mm x 1,200 mm x 300 mm).

While laminated elastomeric bearings cost less than pot bearings, it should be borne in mind that the cost of the bearingsrepresents only a small percentage of the total cost of the structure.

In seismic zones, laminated elastomeric bearings are preferable, even where vertical loading is considerable. The absence of afixed point and the flexibility offered by such bearings makes for better overall performance in the event of moderate tremors.Although strong tremors could cause the laminated elastomeric bearings to tear, they are less costly to replace than potbearings.

1.3 - Appl icatio n of s tandard NF EN 1337-5 to t he French nati onalcontext

The EN standards do not define all of the characteristics, but leave it up to individual Member States to detail their applicationon structures at national level. This text is covered by a technical memo (Note dinformation technique) on the nationalapplication of the standard (Note dinformation Technique sur lApplication Nationale de la norme NF EN 1337) published byStra 3, part of the contents of which have been prepared by the T47A standardisation commission. The content of thisdocument is not reproduced here, and the reader is advised to obtain it and read it in addition to the standard.

N.B.: The present guide applies to highway structures. In the event of use under railway bridges, the appropriate entitiesshould be contacted.

3 See bibliographic reference.


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`~=O=J=Make-up of a pot bearing

2.1 General principles

From the outset, pot bearings quickly became popular, and are now the most widely used bearings after laminated elastomericbearings. This is because they can cope with considerable vertical loading while requiring little space, particularly in terms ofthickness. Another advantage is their simple design, which allows production to be rationalised.Pot bearings have a cylindrical elastomeric pad4 confined within a pot, the latter having a piston cover that transfers the loadto the pad. The elastomer is deformed at constant volume only, which means that it can withstand considerable loads as well asthe rotations generated by the structure.

Pot bearings therefore comprise three main components, each of which can incorporate specific equipment. To these threeparts, which allow three degrees of freedom of movement, may be added a fourth designed to provide one or two additionaldegrees of freedom for displacement. Although suited in design terms for pot bearings, such fourth parts can also be used with

other types of bearing.

2.2 The cons ti tuent parts

A pot bearing comprises (figs. 2.1 & 2.2):

2.2.1 Lower part ( th e pot)

The pot, or shell, may be manufactured in a variety of ways, and its capacity and durability can therefore vary (see fig. 5,standard NF EN 1337-5). The best manufacturing process, and practically the only one used in standard situations, involvesmachining a (usually rolled) steel plate of the same thickness as the pot to be produced.

Another approach is to weld a shell, which means that performance will depend on the quality of the welding. The standard(NF EN 1337-5, 6.2.2 d and e) specifies deep welding, but also allows assembly using standard welding. Given that thestandard does not appear to require systematic verification of welding quality, we advise against the use of welded-shellbearings.

Yet another approach involves bolting the shell to the lower plate.

In certain cases, where sliding is liable to occur, pot plates may be attached to the underlying structure with screws or threadedrods. Other techniques, such as welded feet embedded in the concrete, should be avoided. Such an approach would requireconsiderable raising to allow the pot bearing to be removed from the concrete when it is being changed. To enable thisobligatory removal option, it is important to ensure that there is no need for raising in excess of the 10 mm specified instandard NF EN 1337-1 ( 7.6).

The minimal bottom thickness set by standard NF EN 1337-5 ( 6.2.2) is 12 mm. For machined plates, manufacturersgenerally use thicknesses of at least 20 mm to allow for any deformation caused by the liberation of internal stresses present inthe plate prior to machining.

4 For the scope of use defined in 1, the standard specifies a single pad.


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Fig. 2.1: make-up of a pot bearing

N.B.: the section shows a pot bearing without the sliding system.Remark: where applicable, the measurement device is placed under the pad, and the power-supply input is on the right.

For special applications, the pots or covers can be equipped with measurement systems, allowing the load applied to thebearing to be monitored. These are installed in dedicated spaces at the bottoms of the pots, providing ongoing details of thepressure to which the elastomeric pad is subjected.

Support plate

Guidance system

Sliding part, with type of lateralguidance

Slide plane

PTFE sheet

Piston / pot cover

Elastomeric pad

Internal seal

Pot

Lower plate

Measurement system, where applicable


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2.2.2 The elast omeri c pad

The elastomeric pad is made of natural elastomer or of polychloroprene, as specified in standard NF EN 1337-5 ( 5.3). It iseither vulcanised in moulds whose dimensions correspond to those of the finished item, or cut to size (see adjustmentdimensions in NF EN 1337-5, 7.3.2).

The pad is placed in the pot, which has been machined to the degree of roughness set out in standard NF EN 1337-5 ( 7.4).Grease is also added. These conditions ensure that, under the pressures to which it is subjected, the elastomer behavessimilarly to a liquid.

Fig. 2.2: pot bearings with sliding systems

a) pot bearing with non-guided (multidirectional) sliding plate

b) pot bearing with centrally guided (unidirectional) sliding plate

c) pot bearing with laterally guided (unidirectional) sliding plate

2.2.3 - The pist on or cover

This part, which is made of steel, closed the box, maintaining the elastomer within its confines. The shape and dimensions areappropriate for the pot, as defined in standard NF EN 1337-5, article 6.2.3.

A seal is used to avoid extrusion of the elastomer due to play in the contact zone between the piston and the inside walls of theshell. The technology and materials used for this seal vary from one manufacturer to another. The seal which is often madeof brass is generally slotted into the pad after the latter has been positioned in the pot.

The seal plays a key role in ensuring that the pot bearing functions correctly, notably by avoiding elastomer extrusion, which isone of the rare problems arising with this type of bearing.

It is advisable to add markers so that pot rotation can be verified more easily.


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2.2.4 The sl id ing components

These are comprised of a sheet of cellular PTFE5 slotted into the upper part of the piston and greased at the assembly stage( 5.8), on which slides a stainless-steel plate connected to another steel plate higher up. These components are defined instandard NF EN 1337-2.

As with the pot plate, the upper parts can be attached to the portion of the structure that is in contact with the bearing.To monitor displacement and allow the current data to be recorded when structures are inspected, these sliding plates are fittedwith graduated scales. It is essential that the latter be positioned so as to optimise visibility for visitors. It is also very advisableto set all scales throughout a given structure in an identical fashion, so as to facilitate operations.

Displacement monitoring using a graduated scale

To ensure protection of the slide plane as required by standard NF EN 1337-2, 7.3, the use of a wiper seal system isrecommended.

Where displacement is to be kept unidirectional, a guidance system is used. These are generally either:

lateral, in which case the sliding plate is fitted with two side lips that meet the upper edges of the piston, or central, in which case a key bolt jutting out of the top of the piston fits into a groove in the sliding plate.

It is advisable to seal the space between the stainless-steel plate and the part of the unit to which it is attached, although this isnot required by the standard.

2.2.5 Prot ect ion f rom corro sion

Apart from functional steel-steel contact zones and the inside of the pot, all metal parts are protected from corrosion inaccordance with standard NF EN 1337-9. Systems compliant with Booklet 56 of the general technical specifications (CCTG)and based on one of the ACQPA-certified systems may also be added. Where metals of different electrolytic potentials are

used, it is advisable to ensure appropriate insulation (see NF EN 1337-9, art 4.2) in order to avoid galvanic corrosion.

N.B.: while pot bearings are generally installed piston-up, there is no reason why they should not be installed with thepiston at the bottom and the pot above.

5PolyTetraFluoroEthylene ou Tflonou similaire.


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2.3 - Advantages and di sadvantages of thi s ty pe of bearing

2.3.1 - Advant ages

These bearings can cope with considerable vertical loading while requiring little space. Common capacities range from5,000 kN to 30,000 kN, but other capacities may be obtained.

Thanks to the hydrostatic pressure developed, loads are spread in an almost-uniform fashion throughout the structure.

The elastic restoring forces generated are much less significant than with other types of bearing.

They offer a satisfactory level of operating safety, and the rare problems6 reported to us were a result of elastomer extrusioncaused by a defective seal or of rotations beyond the range provided for at the design stage. Problems can also arise as a resultof poor installation (poorly secured, incorrectly positioned, etc.) or of sub-optimal performance of the sliding system:displacement of PTFE, corrosion of stainless-steel sliding plate, application of paint to sliding plate during painting of metalframework, etc. Such factors are, unfortunately, not peculiar to this type of bearing.

2.3.2 - Disadvan tages

The main disadvantage is the limited rotation capacity, which is, nevertheless, adequate for most structures.

Their implementation calls for greater precision and rigour than is usually required in bridge construction.

Their manufacture requires considerable industrial resources, including the use of robots for machining of pots and pistons.Manufacturing tolerances are extremely low, and extremely rigorous quality control is required.

These factors account for the high cost of this type of bearing compared with laminated elastomeric bearings, for example.

They constitute an attractive solution in technical and economic terms where displacement and vertical loading exceed certainlevels, provided that appropriate sliding systems are used.

2.4 Key geometr ical d imensions

Certain key dimensions featuring in the technical instructions are shown in fig. 2.3.

Fig. 2.3: key geometrical dimensions

6 For further details, see booklet 13 on bearings. References can be found in the bibliography.


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`~=P=Remarks on standard NF EN 1337- parts 2 & 5Key points concerning design

3.1 - Presentati on

The standards to be considered are NF EN 1337-5 for pot bearings and NF EN 1337-27 for the sliding parts. Use of the presentguide therefore requires simultaneous access to parts 5 and 2 where multidirectional and/or unidirectional pot bearings areconcerned. Technical memo no. 27 is also required.

Many experts, in France and elsewhere, are of the opinion that the content of these texts is contrary to the spirit of the

Construction Products Directive, which calls for performance standards rather than product descriptions. Nonetheless, thedocuments in question provide highly useful information, and experts in various Member States have done much work aimedat harmonising their content in recent years.

From a technical standpoint, we felt that it would be interesting to make some comments here, particularly since themanufacturers have begun launching compliant products.

Contract managers have everything to gain from an in-depth knowledge of these products, which would enable them to usethem correctly within the scopes of use for which they have been designed.

3.2 - Intr oduct ion

The main purpose of the standards is to define and state the design specifications for the product. Many of these documents,therefore, are not of interest to the designer.

This chapter intentionally lists only the most important points, as well as commenting on certain parts of the documents. It alsopoints out a number of technical choices that must be made by the contract manager.

3.3 Pot bearings

(NF EN 1337-5)

3.3.1 Thick ness of the rol led st eel

(NF EN 1337-5: 5.2)The yield strength of rolled steel depends on the thickness of the plates after processing in the rolling mill: standards NF EN10025 (A 35.501) for thickness 30 mm, and NF EN 10113 (A 35.505) 8 for thickness > 30 mm.

When the plates are trimmed, the yield strength may not be the same as with the initial thickness. The Contract Manager istherefore advised to demand that the initial thickness and grade of the steel be specified for the pot-bearing construction plan(this information is shown on the factory control certificates for the plates used).

Manufacturers sometimes design their bearings to minimal thickness specifications, and manufacture the actual items usingthicker steel, depending on availability. In such cases, care should be taken with regards to the compatibility between themaximal thickness of the finished pot bearing and the space allowed between the supports.

7 There is no CE marking on the sliding parts. Only bearings (pot or otherwise) comprising sliding parts have such markings.8 Since the publication of part 5, these references have changed. Standard NF EN 10113 has been replaced by Standard NF EN 10025 of March 2005, whichmakes provision for other steel designations.


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Figure 3.1: types of piston/shell contact

a) flat contact surface

Force concentrated at periphery

b) curved contact surface

R max [ D/2, 100 mm ]

Contact point of width bc) curved point of contact and force transfer

Hertz stress / fy if Hertz stress is maximal as determined by the standard formula


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3.3.2 - Rotat ion value and pot -piston cont act

(NF EN 1337-5: 6.1.2 & 6.2.3)

When determining the restoring torque of the pot bearing, one must distinguish between long-term rotation (duringconstruction, including installation precision, shrinkage, creep, temperature, etc.) and sudden rotation (operating loads, etc.).These values are to be input into the data form provided for in Appendix B of standard NF EN 1337-1.

If these values are not known in detail at the preliminary design stage, it is advisable to overestimate the required pot-bearingdimensions.

The shape of the pistons point of contact on the pot shell depends on the rotation amplitudes and on the intensity of thehorizontal forces. Standard NF EN 1337-5 allows for two types of contact: flat contact surface and curved contact surface(see fig. 3.1).

In the standard, the flat contact surface type is deemed acceptable for a calculated contact height w, where w < 15 mm (seeNF EN 1337-5 6.2.3.1 and 6.2.3.2). To avoid concentration of forces, which can cause the piston to wear a groove in theshell, it is advisable to use this approach only for multidirectional pot bearings.

The curved contact surface type is to be used with a radius R that is compliant with standard NF EN 1337-5 ( 6.2.3.3) suchthat R [D/2, 100 mm]. Since the standard makes no mention of the conditions for the diffusion of Hertz stress in the edge ofthe piston in maximal rotation position, we recommend complying with the diffusion principle shown in fig. 3.1c. The height,w, obtained is generally greater that that recommended in 6.2.4 of the standard.

3.3.3 Play between the pot and the pi ston

(NF EN 1337-5: 7.3.1)

In line with manufacturers practices, the standard states that play between the shell and piston must not exceed 0.8 mm inplan (or 1 mm if a metal seal is used).

To take account of this play, the stress distribution is as explained in 6.2.3 of standard NF EN 1337-5.

This approximation will not always suffice (large horizontal forces, etc.), and it would appear desirable to seek to reduce play

to 0.5 mm, which should be well within reach for the manufacturers. Verification will be performed using the dimensions ofthe piston and shell in the construction drawings.

3.3.4 The thi ckness of t he pot bot tom

(NF EN 1337-5: 6.2.2)

The standard sets the minimal thickness at 12 mm, and the item must be able to bear the forces defined in 6.2.2a of standardNF EN 1337-5.

It should be noted that, to optimise diffusion of stresses in the bosse (see 3.3.5 below), it may be necessary to increase thisminimal thickness.

Reminder: the pot-bottom plate is, of necessity, thicker in pot bearings fitted with vertical-load measurement devices or

similar systems.


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3.3.5 Actual contact sur face on the structu re

(NF EN 1337-5: 6.1.5 and 6.2.6)

The standard has opted for a diffusion angle of 45 to the vertical for the pot bearings metal components. The angle ofdiffusion in the structure will be that defined by the regulations for the material in contact with the pot bearing. A greater angle(not exceeding 60) may be allowed if assembly with the support is justified.

The standard recommends a contact surface as defined in the previous paragraph, but of lesser dimensions. This surface (lesserand greater) derives from the momentum created by the restoring torque of the elastomeric pad and the torque resulting fromthe horizontal forces at the point of contact between the piston and the shell, and/or at the guidance system. When calculatingthis lesser surface, it would appear logical to use the same methods as for the sliding plates (see 4.2.2.1 of the present guide).In the absence of a specific software tool, we recommend the use of the formulae provided in appendix A of standard NF EN1337-2.

It is this lesser surface that is to be taken into account for the application of article 6.7 of Eurocode 2 (NF EN 1992-1-1)concerning the justification of the adjacent concrete. The centre of the bearing is, therefore, not the centre of the pressure takento be distributed. This discrepancy may be significant where pot bearings are restraint, or even where they are unidirectional.

3.3.6 Thickn ess of the elastomer ic pad

(NF EN 1337-5: 6.2.1.2)

The standard defines the minimal thickness of the pad based on the amplitude of the rotations expected in the pot bearing. Foreach pad geometry, there is a restoring torque value (NF EN 1337-5: 6.1.3). The value of this torque is determined on thebasis of tests. These are, however, conducted only on certain types of pot bearing (elastomer diameter of between 500 mm and600 mm: NF EN 1337-5, D2), and it does not seem possible to determine the results for the entire intermediate and otherranges by extrapolating from the results obtained for such a small number of units.

In certain special cases, caution should be exercised as regards the restoring torque values provided by manufacturers.

Although this is not mentioned in the standard, it would appear important where certain types of piston joint are concerned tomaintain the entire contact surface between the pad and piston at all times, since partial loss of contact can have ramificationsfor the performance of the piston joint. This verification is not necessary in the more common situations, where contact occurs

at average pressures greater than or equal to 10 MPa for unidirectional and restraint units (which are subjected to stronghorizontal forces) and at around 5 MPa for multidirectional units. Care should, however, be taken with the verification ofabutment bearings subjected to hyperstatic reactions, with account being take of the uncertainty surrounding the value of therotation restoring torque.

3.3.7 - Sett lement

(NF EN 1337-5: Appendix B)

It should be remembered that the differential settlement between pot bearings can typically exceed 1 mm, and that this shouldbe taken into account during structural calculations.


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Pot bearings - Use on bridges, viaducts and similar structures Technical guide

3.4 Sli ding element

(NF EN 1337-2)

3.4.1 Choosing t he guidance-system posi t ion: central or l ateral

There is no single compelling reason to choose one system over the other. It is, however, highly inadvisable to have horizontal

plates of PTFE in contact with the key bolt.

3.4.2 Dimensio ns of s l i ding plates

One should not hesitate to oversize the sliding-plate lengths. This allows, on the one hand, for factory presetting, and, on the

other hand, for inaccuracies resulting from the calculation, the actual installation and the installation temperature. As specified

in technical memo (Note dinformation technique) no. 279 on national application of standard NF EN 1337 published by Stra,

the correct interpretation of 5.4 b of standard NF EN 1337-1 being as follows: Displacement must be increased by 20 mm

in both directions. Moreover, the minimal displacement to be taken into account is 50 mm in the structures main displacement

direction. These values do not apply if the bearing is blocked mechanically.

It is also advisable to allow additional length of 10 cm on either side (see 4.5.4 of the present guide).

3.4.3 Fr ict i on coeff ic i ent

(NF EN 1337-2: 6.7 & table 11)

The values provided are a function ofp. For a given vertical load, the friction coefficient is calculated on the basis of the

stress at ULS. This coefficient is to be recalculated at SLS to determine support dimensions.

Attention is drawn to the considerable variation of the friction coefficient as a function of the compression stress on the PTFE

for bearings subjected to great load variations. With a minimal load at ULS or SLS, therefore, friction is liable to be greatly

increased.

For simplicity, account will not be taken of the correction factor of 2/3, except where there is special reason to do so, and for

applications in French overseas dpartements and territories (DOM-TOM), where actual bearing temperatures never fall

below -5C (see technical memo no. 27 on structures, published by Stra).

For guides, the friction coefficient is independent of the contact pressure, and attention is drawn to the fact that values can vary

greatly depending on the materials used.

3.4.4 - Checking of t he s l idi ng plates and pisto n for defor mation

(NF EN 1337-2: 6.9.2)

Checking should be systematic.

N.B.: The incidence of this calculation is of particular importance where, for example, the diameter of the PTFE is less than

that of the pad (L < D).

3.4.5 - Instal lat i on

(NF EN 1337-2: 9 and NF EN 1337-11)

It is advisable to refer to the standard T 47.816-3 for pot bearing installation.

Nevertheless, the plate horizontality tolerances and guidance-system alignment tolerances must be taken into account where

guided pot bearings are concerned. The guidance tolerance defined in 9 of standard NF EN 1337-2 is 0.3% (see technical

memo no. 27).

9 See bibliographic reference.


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Pot bearings - Use on bridges, viaducts and similar structures Technical guide

3.5 Slidi ng and safety at the bearing-st ruc ture interface

Standard NF EN 1337-1 ( 5.2) specifies the friction-coefficient values to be taken into account in the more common

situations. These values are modified in line with technical memo no. 27 (and with the conditions in the note), as follows:

k/= 0.6/1.8 = 0.33 for a steel-concrete interface,

k/

= 0.4/3 = 0.13 for a steel-steel interface (prepared surfaces).

These values do not apply to railway structures or to structures located in seismic zones, for which there are special

recommendations.


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`~=Q==Princi ples governing calculat ions for st ructu rescomprisi ng pot bearings

4.1 Regulatory cont ext

4.1.1 - General

This chapter looks at calculations relating to elastomeric pot bearings, their justification and their environment (influence ofpot bearings on support calculations, etc.).

The main innovations of standard NF EN 1337-5 are as follows:

the forces applied to bearings are calculated at ULS;

the differences between how flexible and rigid supports are taken into account are more pronounced; the friction coefficients for the bearings are deducted from ULS internal forces end moments; for the spread of horizontal forces with favourable and unfavourable forces, horizontal precision is not taken into account

when friction coefficients are being calculated;

the rule concerning resistance to a horizontal force equal to 5% of the maximal vertical force is discarded;

friction forces on guidance systems must be taken into account for calculations.

Over and above questions concerning the numerical values (maximal stresses, maximal rotations, friction coefficients, etc.),two key points appear to be particularly problematic for designers:

which vertical loads should be taken into account when calculating the maximal horizontal force for a sliding pot bearing? How should horizontal forces be calculated for pot bearings on restraint supports?

In standards NF EN 1337-2 and 1337-5, bearing calculations are done at ULS only. Basic combinations are therefore used,taking account of permanent actions and of actions due to road loading, temperature (uniform and gradient) and wind.

In addition to these verifications, additional elements are also required:

accidental combinations where piers are liable to be hit by boats or goods vehicles, and seismic combinations whereapplicable;

in certain special cases, for example where a beam rests on permanent bearings during construction.

For the calculations below, the combinations provided by the following texts have been used:

Appendix A of standard NF EN 1993-2: this appendix provides the calculation rules for the bearings on steel structures. Itcan, nevertheless, be applied to all types of bridge, since it is to be transferred to standard NF EN 1990. This appendixspecifies, notably, how to take account of uncertainty regarding bearing installation temperatures and how to integrate it intothe calculation temperature variation T

d:

NF EN 1991-1-5 and its national annex: this standard specifies the values to be used for uniform temperature actions TNand temperature gradient TM. It also explains how these two actions should be combined so as to take account of theirsimultaneous nature and obtain the characteristic overall effect Tk;

Appendix A2 of standard NF EN 1990 defines the combinations to be used in calculations concerning supports andbearings in particular.

To begin with, basic combinations are provided table 4.1:


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No.

+ 1,35 {UDLk +TSk + q fk,comb} + 1,5 min{FW* ; 0,6 FWk} 1

+ 1,35 {UDLk + TSk + q fk,comb } + 1,5 {0, 6 Tk} 2

+ 1,35 gr1b 3

+ 1,35 gr2 4(1)

+ 1,35 {gr3 or gr4} + 1,5 {0,6 Tk} 5

+ 1,35 gr5 6

+ 1,5 FWk 7

+ 1,5 Tk + 1,35 { 0,4 UDLk + 0,75 TSk + 0,4 q fk,comb} 8

(1) Including braking

Table 4.1: list of main basic combinations

The horizontal forces in the previous combinations are to be calculated as follows:

- braking:

Standard NF EN 1991-2 defines the braking force to be applied to the deck as a fraction of the maximal load that can be placedon the most heavily loaded lane in load model 1 (NF EN 1991-2 4.4.1). These fractions correspond, respectively, to 10% forUDL and 60% for TS.

For a class 2 structure with a main lane 3 m wide, the total braking force as a characteristic value for a deck of length L is:

HK = 324 + 1.89 x L where L is expressed in metres and HK in kN

The braking force varies between 340 kN and 400 kN, approximately, for small structures measuring 10 m to 50 m in length,and can reach a maximal value of 900 kN for structures 305 m in length between expansion joints. This value is considerablygreater than that generally used in older regulations (300 kN for braking by truck Bc, for example). Where structures areequipped with laminated elastomeric bearings, braking forces are spread across all of the decks bearings, which should notpose any problems in respect of pier reinforcement. On the other hand, for major structures with restraint supports subjected tothe near-totality of the horizontal forces, pier design can be difficult with such high braking values. Where piers are high andflexible, it is advisable to use several restraint supports. In other cases, the restraint support should be located on a short pier oron an abutment, which can give rise to difficulties as regards the design of the expansion joint (and sliding plates) on theabutment at the other extremity of the structure.

This maximal braking force will most likely be reduced in the national annex, since standard NF EN 1991-2 allows this. Thelevel could well be reduced to 500 kN, except where the structure is to bear military loads in accordance with the STANAGstandardisation agreement (Char Mc 120).

- thermal effects:Te, min Te, max

Deck material Concrete Composite Steel Concrete Composite Steel

Brittany Provence Cte dAzur -10 C -10 C -20 C

Centre North South-East -15 C -15 C -25 C

East - Alps -20 C -20 C -30 C

40 C 45 C 55 C

Table 4.2

The effects of temperature are defined in section 4 of the EN 1991-1-5. The temperature differences Te, max and Te, min incharacteristic values are to be calculated on the basis of the deck material and of the region where the structure is to be located.These temperatures are to be determined using the maps provided in the national annex to NF EN 1991-1-5. Until thesebecome available, the values in table 4.2 may be used.

Temperature variations resulting from these maximal and minimal temperatures are to be calculated on the basis of atemperature T0 equal to 10C in the absence of dedicated project specifications.

1,35 Gk,sup + Gk,inf + P + S + C


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For calculations concerning the securing of bearings or their sliding plates during installation, standard NF EN 1991-1-5specifies an additional value to be added to the temperature variations. This value equals 20C, or 10C if the installationtemperature is specified. These values may be modified by the national annex.

The expansion coefficients provided in the Eurocode are 1 x 10-5/C for concrete decks, and 1.2 x 10-5/C for steel decks (NFEN 1991-1-5 Appendix C). For decks on composite structures, paragraph 5.4.2.5 (3) of standard NF EN 1994-2 specifiesthat this coefficient must be taken as 1.2 x 10-5/C for expansion calculations, and as 1 x 10-5/C for temperature-gradientcalculations.

It should also be noted that, even where the Eurocodes do not specify this explicitly, the spread of forces in the supports andtherefore in the piers must be calculated using the instantaneous concrete modulus.

4.1.2 Regulat ory con si derati ons and Parti cul ar Technic al Clauses (CCTP)

Standard NF EN 1337-1 requires contractors to supply a schedule of the forces applied to bearings (Table B1 or B2), inaddition to the calculation sheets for the equipment concerned.

The calculation sheets must include the verifications required by article 6 of standards NF EN 1337-5 and 1337-2, includingthe verification in paragraph 6.9.2, which is often forgotten.

It is advisable to add the following text to the bearing justification paragraph in chapter II Site preparation andorganisation of the Particular Technical Clauses (CCTP).

The justifications of elastomeric pot bearings, supports and foundations will be conducted in accordance with the rules inchapter IV Principles governing calculations for structures comprising pot bearings of the Stra document Elastomeric potbearings - Use on bridges, viaducts and similar structures (seebibliography).

Moreover, to facilitate verification of the contractors calculation sheets, it is highly advisable to require that summary sheetsproviding the bearing characteristics be supplied with the Particular Technical Clauses (CCTP) (see appendix 2 of the presentguide).

4.2 Extr eme verti cal f orces

4.2.1 General

Designers attention is drawn to the fact that the maximal reaction for a bearing cannot generally be obtained simply bydividing the total maximal reaction for a given pier or abutment by the number of bearings. It is necessary to take account ofthe transversal rigidity of the structure and of the eccentricity of the loads in relation to the bearings.

Justification of the elastomer in the pot bearings and the sliding systems is to be performed at ULS (see NF EN 1337-5, 6.1except verification as specified in 6.1.2.3).

Justification of the pier (or abutment) and of the foundations under the pot bearing is to be conducted at SLS and ULS.

The above conditions relate to the elastomer of all pot bearings and to the PTFE of sliding pot bearings. It is also essential toconduct verifications to ensure that the sliding plates of unidirectional and multidirectional pot bearings do not lift.

4.2.2 Pressur e in th e PTFE

(concerns sliding pot bearings only).

4.2.2.1 Maximal pressure in the PTFE

Standard NF EN 1337-2 ( 6.6 and 6.8.3) limits the pressure on the PTFE to MPaf

m

k 3,6440,1

90==

at ULS. The stress on the

PTFE is to be calculated for a limited surface Ar to take account of the eccentricity of the load. Appendix A of standard NF EN1337-2 provides details of how to calculate Ar.

On the other hand, the value of the limited pressure must be reduced by 2% for each degree over 30C, if the latter temperatureis liable to be exceeded near the bearing.


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For simplicity, at altitudes of less than 1,000 m, the pressure fPTFE

will be limited to MPaf

m

k 5540,1

9085,085,0 =

for

concrete or composite structures and to MPaf

m

k 5140,1

9080,080,0 =

for steel structures (this distinction is made

because all materials do not have the same degree of sensitivity to temperature changes).

4.2.2.2 Average pressure in the PTFE

If vertical load amplitudes allow, it is preferable to have average pressure at ULS of around 41 MPa 42 MPa resulting fromthe combination of maximal permanent loads so as to limit the effects of friction (see 4.3.2 below).

This value can serve as a guideline at the preliminary design stage.

4.2.3 Average pressure in the elastomer

Standard NF EN 1337-5, 6.2.2.1, limits the average pressure on the elastomer to MPaf

fM

ude 4630,1

60, =

at ULS.

M

ude

ff

, at ULS

4.2.4 - Incidence on th e struct ure

The stresses under pot bearings are generally considerable. Section 3.3.5 of the present guide covers justification of theconcrete under the pot bearing.

4.3 Longitudinal hori zontal for ces for sl i ding pot bearings

4.3.1 - General

Sliding pot bearings can function in a variety of ways:

With a flexible support, where the deck moves as a result of a variation in its length, a horizontal force builds up gradually inthe pot bearing until such time as a given value H, known as the sliding threshold, is attained. When this value is reached,sliding occurs, thereby releasing part of the force that had built up. A new state of equilibrium is brought about, with ahorizontal force H < H;

With an infinitely rigid support such as an abutment, any movement of the deck caused by a variation in its length resultsin the horizontal force H (sliding threshold) being reached immediately in the pot bearing;

With a very flexible support such as a high pier, (see 4.6.2), the sliding threshold may well never be reached. The maximalhorizontal force H1 that can be attained is < H (sliding threshold).


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The horizontal force H that can be reached by a sliding pot bearing just before sliding occurs is obtained from the associatedvertical force V:

H = (max + PP + PL) x V

where:

max is the maximal friction coefficient of the pot bearing for the vertical load V (paragraph 4.3.3 defines the vertical load Vrequired for the maximal value of H to be obtained);

PP is the standardised pot-bearing installation precision, which corresponds to a possible deviation from the horizontal of0.003 rd (positive PP in the formula above);

Where applicable, PL includes the slope designed into the slide plane (e.g. where there is a sloping abutment and thedifference in level between the expansionjoint extremities is limited) and that resulting from the load under consideration(value generally negligible except where structure is very flexible or where construction kinematics is complex where thetransfer to permanent bearings is concerned), as well as installation errors greater than 0.003 rd (prefabricated structures,poor securing, etc.).

4.3.2 Numerical values f or calcu lat ion

4.3.2.1 Friction coefficients for pot bearings

The friction coefficient of a sliding pot bearings sliding system depends on a number of parameters:

Contact pressure (and therefore the associated vertical force);

Nature of the materials used in the slide planes; Wear of the sliding pot bearing; Temperature; Degree of aggressiveness of the environment.

Standard NF EN 1337-2 ( 6.7) specifies the friction coefficients to be used for calculations relating to lubricated plates ofcellular PTFE at usual temperatures. These coefficients are nominal calculation values to be used for justifications at ULS (forbearings, supports and foundations) and at SLS (for supports and foundations only).

The maximal value of the friction coefficient is provided by the formula: max =1 2

10

,

+

k

p

where:

k = 1 for stainless steel;

k = 1.5 for aluminium;

p: contact pressure on the PTFE.

Table 4.3 below may also be used:

Contact pressure p (MPa) 5 10 20 30

Cellular PTFE / austenitic steel or layer of hard chromium 0,08 0,06 0,04 0,03

Table 4.3:frictioncoefficients for calculations.

In accordance with paragraph 6.7 of standard NF EN 1337-2, the above friction coefficients may be multiplied by 2/3 in zoneswhere the actual minimal temperature is never lower than -5C. This would not appear to apply to Metropolitan France (see 3.4.3).

Where the environment is aggressive, it would appear wise to increase the friction coefficients used for calculation, as well astaking physical precautions designed to protect pot bearings. However, the extent of this increase has yet to be decided upon.

Since the friction coefficient decreases as contact pressure increases, it is important to avoid excessively large sliding systemson sliding pot bearings.


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4.3.2.2 Pot-bearing precision

Standard T 47-816-3 defines the tolerances for deviation from the horizontal at the time of installation of between0.2% and 0.3% depending on the type of structure and the installation method. For the purpose of maintaining homogeneitywith standard NF EN 1337-5, preference should be given to the value of 0.3 %.

4.3.2.3 Friction coefficients for the guides

Standard NF EN 1337-2 ( 6.7) specifies the friction coefficients to be used in calculations concerning the guides ofunidirectional sliding pot bearings:

Non-cellular PTFE: max = 0.08

Composite materials: max = 0.20

(The latter value is to be used in the absence of additional tests taking account of aging).

For transversal forces, see 4.5.3 of the present guide.

4.3.2.4 Simplified coefficients

For the preliminary design of bearings subjected to longitudinal loads in the more common situations and notably on

rectilinear structures less than 600 m in length one can, for simplicity, to use a single friction coefficient covering friction onthe sliding surface and on the guiding system:

max = 3.5% of the maximal vertical load on the support, and

max = 4% of the permanent vertical load (or 4.5% - 5% for light decks offering considerable wind resistance e.g. where noise screens have been installed).

The Particular Technical Clauses (CCTP) can provide other numerical values depending on specific parameters (e.g.aggressive environment or low temperatures).

4.3.3 Loads to b e taken int o account when calcu lat ing h or izont al forces at ULS

Vertical operating loads to be taken into account when calculating the maximal horizontal force that can be reached by asliding pot bearing depend strictly on the supposed operation of the pot bearing in its context (see 4.3.1) and therefore on thestructures studied. In the more usual cases, the sliding thresholds can generally be reached, and the permanent loads representa very large percentage of the vertical loads.

Calculating the maximal horizontal forces on the basis of extreme vertical loads enhances security, but generally leads only toa slight increase in actions.

For simplicity, one can therefore determine the extreme horizontal forces on the basis of the maximal vertical loads,thereby also enhancing security.

Depending on the specific characteristics of the structures under consideration (e.g. flexible piers), the Particular TechnicalClauses (CCTP) can require other vertical loads to be taken into account when the maximal horizontal forces are beingdetermined.


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Remarks:

1 For a very rigid support such as an abutment or very rigid pier, where any deck displacement (due to by a variationin deck length) can cause the pot bearing to slide, the vertical load to be taken into account is the maximal load.

2 For a moderately-rigid support, sliding occurs only after a certain amount of variation in deck length has occurred.Given:

The time needed for the temperature to change sufficiently so that the requisite variation in deck length can occur;

The presumably very short time during which operating loads (and their characteristic maximal values) areapplied;

the calculation of the horizontal forces on the basis of maximal vertical loads would not appear desirable.

Let us consider a support that is on the point of sliding freely (sliding threshold, H). If this support is also subjectedto operating loads with their characteristic maximal values, the new horizontal force H1 required before sliding canoccur (proportional to the vertical load) will be greater than H. The time during which operating loads (and theircharacteristic values) are applied is generally too short for sufficient variation in deck temperature to occur such that

this new force H1 can come about. Sliding will therefore be more likely to occur when operating loads are reduced.

Moreover, to take account only of the permanent loads would be too favourable (imagine the structure beingsubjected to a traffic jam on a sunny afternoon).

In this case, the vertical loads to be taken into account are therefore between the permanent and maximal loads, andthe calculation can be performed using combination no. 8 of table 4.1 in the present guide.

3 For a very flexible support for which the sliding threshold would never be reached, the theoretical maximalhorizontal forces do not depend solely on the vertical loads, and calculations must take account of support rigidity(see 4.6.2). In such cases, sliding pot bearings can be replaced by restraint pot bearings.

4.3.4 - Incidence on th e struct ure

4.3.4.1 Serviceability Limit State

Justification of compound bending of supports (piers, abutments and foundations) on which sliding pot bearings have beeninstalled can be conducted on the basis of the following associated forces:

Extreme vertical forces (maximal and minimal) corresponding to rare combinations of SLS for the sliding pot bearing underconsideration;

Horizontal forces calculated as specified previously (see 4.3.1, 4.3.2 & 4.3.3). These forces result from variations in decklength, with horizontal and vertical forces calculated for associated loading. The Particular Technical Clauses (CCTP) canrequire that other vertical loads be taken into account when the extreme horizontal forces are being determined;

Forces resulting from friction on the guidance system, where applicable.

A spatial study of the structure may be necessary to enable the intensity of the guidance forces be determined. If such is thecase, the forces are calculated with alternating lateral play of 2 mm in the guidance system from one support to another andsimulation of deck movement in extreme-temperature position, prior to which alternate deviation in plan of0.003 rd has beenimposed in line with the theoretical orientation of the guidance systems (the effect of these two inaccuracies being combined inthe most unfavourable manner).

N.B.: for simple, straight structures, one can simply take these forces for a support to be 100 KN or 1% of the maximalreaction at ULS on the entire support, whichever is the greater.

It is also essential to evaluate the horizontal forces caused by the wind. While these will not necessarily determine the designof the support itself, they need to be known so that the bearing guides can be verified.


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4.3.4.2 Ultimate Limit State

Basic combinat ions

The internal forces and moments to be taken into account for calculations are provided in table 4.1 of the present guide. Itshould be noted that combination 7 of table 4.1 (wind being the main source of forces) may determine design and is essentialwhen the dimensions of pot shells and of guidance systems for sliding bearings are being defined.

The principles governing the calculation of horizontal forces at ULS are as described in the previous paragraph.

Concerning structures with high, slender piers, the attention of designers is drawn to the fact that a second-order calculation isoften required. The displacement of the top of the pier is no longer negligible, and the eccentric nature of the vertical reactioncreates an additional bending moment in the pier shaft. With this type of pier, rotations caused by displacements are notalways negligible, in which case they should be added to the deck rotations. Given the considerations raised in remark ofparagraph 4.3.3, it can be taken that the sliding threshold corresponding to the near-permanent loads has not bee exceeded. Theforces to be taken into account at the top of the pier are therefore as follows:

Horizontal force: calculated on the basis only of near-permanent deck loads and, where applicable, of friction on theguidance systems;

Vertical force: calculated on the basis of the most unfavourable effect, taking account of the operating loads or otherwise.

Accidental combinat ions

The accidental combinations to be taken into account are defined in paragraph 6.4.3.3 of standard NF EN 1990.

The accidental action FA under consideration may be caused by a significant increase in the friction coefficient (loss of PTFE,clogging or painting of the stainless steel plate, etc.). In this case, the friction-coefficient numerical values defined in 4.3.2should be replaced by that corresponding to metal-on-metal partial friction, which can easily reach a global value of 10% oreven 15% .

The Particular Technical Clauses (CCTP) can provide other values depending on the specific characteristics of the structure.

The increase in the friction coefficient concerns only one pot bearing at a time.

The other accidental action are not looked at here, and will be defined in specific documents.

4.3.4.3 Immediate and time dependent forces

It should be remembered that the Eurocodes state that the effects of temperature should be calculated using the instantaneousconcrete modulus.

For pot bearings, horizontal forces, even those due to permanent loads, are not constant. They are reduced, or even cancelledout, by variations in deck length. When performing calculations concerning the foundations of these piers and abutments, onecan therefore take all horizontal forces to be instant, thereby also enhancing security.

In specific cases e.g. where the structure has a number of restraint supports a distinction can be made between immediateand time dependent forces when calculations relating to these supports are being performed.

4.4 Longitudinal hor izontal forces for restraint pot bearingsBelow, the term restraint pot bearing will refer to:

Either a pot bearing with horizontal displacement blocked in both directions; Or a unidirectional sliding pot bearing with displacement blocked in the direction under consideration.

4.4.1 - Hor izontal for ce due to a var iat ion in d eck length

4.4.1.1 - General

In the case of a structure supported both by restraint pot bearings and sliding pot bearings, the horizontal force taken up by therestraint pot bearings is obtained by taking account of the longitudinal and transversal equilibrium of the structure.

Distinct friction coefficients are allocated to the various sliding pot bearings on the basis of whether they act favourably orunfavourably on the overall equilibrium of the structure as regards the effect being considered.

These other calculation coefficients relate to the following phenomena:


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The friction coefficients of the materials vary from one pot bearing to another; The horizontal forces do not necessarily come completely into play for all of the pot bearings; The pot-bearing installation precision (horizontal precision), which can have a favourable or unfavourable effect.

Sliding bearings can function in a variety of ways:

With a very rigid support such as an abutment, deck displacement can cause horizontal force to reach the threshold value, H,

instantly; With a flexible support, a variation in length causes the horizontal force to increase gradually until it reaches the sliding

threshold. Once this value, H, has been reached, sliding occurs. A new state of equilibrium is brought about, with horizontalforce once again lower than H;

Lastly, with a very flexible support, it is possible that the sliding threshold will not be reached, in which case the bearingwould function like a restraint bearing.

4.4.1.2 Friction coefficients for an isolated sliding bearing

It should be remembered that, for verification of an isolated bearing, the relation between the attainable force, H, and theassociated vertical force is as follows:

H = (max + PP + PL) V where: V is the vertical load applied to the bearing;

max is the maximal friction coefficient for the bearing, which depends on the vertical load, V (see paragraph 4.3.2.1); PP is the bearing installation precision, with a possible deviation in terms of horizontal precision (PP is positive in the above

formula);

PL is the slope designed into the slide plane, where applicable.

The installation precision for the bearing is 0.3% for concrete cast in-situ decks. For prefabricated concrete decks, the valuespecified in article 9 of standard EN 1337-5 is replaced by the more realistic 1%.

4.4.1.3 - Friction coefficients for a set of sliding pot bearings

For the calculation of the spread of horizontal forces in sliding bearings, the friction coefficients to be used are as follows (NFEN 1337-1, 6.2):

a= 0.5 max (1 + )

r= 0.5 max (1 - )

where:

max maximal friction coefficient for a sliding bearing taken as isolated (seeprevious paragraph);a friction coefficient where the friction is unfavourable vis--vis the effect being considered;

r friction coefficient where the friction is favourable vis--vis the effect being considered; rate of decrease depending on the number, n, of sliding bearings contributing to the longitudinal stability of the

structure, in accordance with the table below:

n

4 1

4 < n < 10 (16-n)/12

10 0,5

Table 4.4

Here, the installation precision, PP, is ignored, since it has already been taken into account in the weighting of the frictioncoefficients a and r. In accordance with the previous paragraph, therefore:

H = ( + PL) V where = a or r

Example: a four-span bridge with two restraint bearings on the central pier and sliding bearings on the other supports:n = 8 hence = 2/3a = 0.5 max 5/3 = 5/6 max


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r= 0.5 max 1/3 = 1/6 max

4.4.2 - Hor izont al for ce due to b raking

According to paragraph 6.7 of standard NF EN 1337-2, sliding bearings must not take up any of the horizontal forces due tobraking. Theoretically, therefore, these forces are taken up fully by the restraint pot bearings. This is a pessimistic hypothesis,

since the sliding bearings actually act in taking up these forces. However, the proportion concerned is not quantifiable.

4.4.3 Maximal calculated ho r izont al for ce

4.4.3.1 Loads to be used when calculating horizontal forces due to variation in decklength

The forces applied to restraint supports depend on the forces taken up by the other deck bearings, most of which are slidingbearings. It is therefore necessary to evaluate all of these forces before the force at the restraint support can be determined.

In the case of sliding bearings, the horizontal forces result from friction on the slide plane, which depends on the verticalreaction. For simplicity, the maximal permanent vertical loads (1.35 Gmax) can be used for all of the sliding bearings when theforce applied to the restraint support is being calculated.

For unidirectional sliding bearings, friction on the guides will be evaluated as specified in 4.3.4.1. The friction coefficientsmax for these guides will be weighted as are those corresponding to the vertical loads (see 4.4.1.3), where n is the number ofunidirectional sliding bearings.

4.4.3.2 Calculating the sums of horizontal forces

Although they are not always associated, the sums of the forces resulting from the following actions may be calculated:

Variation in deck length (including friction forces on the guides); forces resulting from braking or from the wind.

The main combinations to be formed are combinations 4, 5 and 7 in table 4.1. For curved bridges, centrifugal force must alsobe taken into account for the combinations.

4.4.4 Incidence on t he struct ure

Two calculations SLS and ULS are required for justification of piers and abutments, which is not the case for the bearingsthemselves. It is therefore essential that the horizontal forces be determined at these two limit states, with different frictioncoefficients where applicable.

4.4.4.1 Serviceability Limit State

Justification of compound bending of supports (piers, abutments and foundations) on which restraint pot bearings have beeninstalled can be conducted in a simplified fashion on the basis of the following non-associated forces:

Extreme vertical forces (maximal and minimal) corresponding to rare combinations of SLS for the restraint pot bearingunder consideration;

Horizontal forces calculated as specified previously (see 4.4.3.2), but at SLS and therefore without weighting of 1.35.

4.4.4.2 - Ultimate Limit State

Basic combinat ions

The internal forces and moments to be taken into account for calculations are provided in table 4.1 of the present guide.

The numerical values of the friction coefficients to be taken into account are defined in paragraphs 4.4.1.2 and 4.4.1.3.

As with SLS calculations, justification of compound bending of supports (piers, abutments and foundations) on which restraintpot bearings have been installed can be conducted in a simplified fashion on the basis of the following non-associated forces:

Extreme vertical forces (maximal and minimal) corresponding to the basic combinations of ULS for the restraint pot bearing

under consideration; Horizontal forces calculated as specified previously. Once again, these forces result from:


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variations in deck length (see 4.4.1), taking account of the maximal permanent vertical loads (1.35 G max) on the slidingpot bearings;

braking (see 4.4.2) or wind.

The Particular Technical Clauses (CCTP) can require non-simplified calculation, with the horizontal forces calculated forother loads, associated or otherwise.

Accidental combinat ions

The internal forces and moments are provided in article 6.4.3.3 of standard NF EN 1990.

Pot-bearing guidance and attachment systems are not usually designed to resist accidental actions. Supports must, however, beequipped with independent stops that prevent excessive movement of the deck relative to the pier or abutment. These stops aremandatory in seismic zones, and strongly recommended where there is a risk of strong impact involving boats. Moreover, it isalso often possible to check whether the bearing would survive being hit by a vehicle and whether the deck would remainstable if part of the support were to be destroyed.

In addition to any accidental combinations, it is also possible to propose accidental-combination verification with simulationof a bearing failure (bearings sensitive to the effects of horizontal forces), centred on an anomaly relating to the value of thefriction coefficient (e.g. max = 10% or 15% for one, and only one, of the bearings).

The failure could, for example, occur as a result of premature wear of the PTFE, clogging or painting of the sliding parts.

4.4.5 Ver i f icat io n of t he maximal hor i zontal forc e taken up by a restraint pot bear ing

For restraint pot bearings (or unidirectional pot bearings blocked in the direction of displacement), the allowable horizontal forceguaranteed by the supplier must be at least equal to the horizontal internal forces used in calculations. In this calculation, thehorizontal force for the bearing will be increased to take account of the non-uniform spread between supports on a given line.

As specified in paragraph 4.4.3.1 of the guide, it is necessary, in complex cases, to perform a spatial calculation taking accountof the various types of play and rigidity so that the forces on each pot bearing blocked in a given direction can be evaluated.

For restraint bearings, in addition to the horizontal forces listed, it is also necessary to add the transversal forces resulting fromthe effect of temperature on the other bearings of the restraint support.

Important:

If a pier serving as an restraint support is equipped with an restraint bearing and with one or more unidirectionalbearings blocking longitudinal movements, the longitudinal horizontal force from the deck will be concentrated almostexclusively on one of the bearings.

This is because the play (between the pot and piston and/or between the sliding components) resulting from manufacturingtolerances does not allow simultaneous contact on all of the bearings of the support concerned.

In ideal circumstances, a single restraint bearing would therefore suffice. However, two restraint bearings can be used incertain cases (e.g. flexible pier subjected to torque) and, more generally, to optimise the distribution of horizontal forceson the restraint support.

Moreover, where there are two restraint bearings on the same pier, the spread of forces between them should be consideredunequal, even if the pier is very flexible.


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4.5 Other recommendations

4.5.1 - Just i f icat io n of the metal components of th e pot bear ing

Justification of pot bearings metal components must be performed in accordance with standards NF EN 1337-2 and 1337-5.In the absence of specifications concerning certain points, the relevant parts of Eurocode 3 (NF EN 1993) are to be applied.

4.5.2 - Rot atio ns

Standard NF EN 1337-5 limits rotations to 0.03 rd at ULS. It should be remembered that a thickness of conventionalelastomeric pad equal to 1/15th the diameter of the pot allows absorption of maximal rotations of 0.02 rd, which correspond tothe values generally attained on bridges. Beyond 0.02 rd, pad thickness should be increased in accordance with article 6.2.1.2of standard NF EN 1337-5.

4.5.3 Transversal ho r izont al for ces

Forces are generated on the longitudinal guides: resulting from transversal forces (mainly wind); for curved structures (see 4.5.6); by the play and orientation tolerances of the guidance system; or following an error in the orientation of the slide axis at installation (see NF EN 1337-2, 9).

As a result of friction, these transversal forces generate longitudinal forces, which are added to those specified in paragraphs4.3 and 4.4. The friction-coefficient values to be taken into account for the guides are provided in these paragraphs. Thefriction coefficients may be weighted in line with the method set out in paragraph 4.4.1.3.

4.5.4 - Dimensio ns of the s l i ding plates

The lengths of sliding plates are calculated taking account of: temperature changes; shrinkage (concrete or concrete-and-steel structures);

creep (prestressed concrete structures).

The action of the temperature is defined in section 4 of standard NF EN 1991-1-5 and in the national annex.

A calculation incorporating the various coefficients must be performed so that the position of the fixed point can bedetermined (see the example of calculation in 4.6.2 below).

Additional lengths must be added to the values calculated (see paragraph 3.4.2 of the present guide).

4.5.5 Special stru ctur es

The present recommendations should be adapted in the case of special e.g. wide, curved or slanted structures.

4.5.6 Determini ng th e s l ide axes on cur ved stru ctur es

4.5.6.1 Regarding deck deformations

Reminder: Navier-Bresse general deformation formulae

Given the displacement (rotationr

0 and translationr

0 ) of section 0 of curved abscissa S0 and centre of gravity G0, the

displacement (rotationr

1 and translationr

1) of section 1 of curved abscissa S1and centre of gravity G1 is provided by thefollowing:

where ( , , )r r r

i j k unit vectors of axes xyz linked to a section of curved abscissa uniform expansion due, for example, to temperature changes or to shrinkage

and


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Take a curved structure in the normal planer

k . Say that at section 0 of curved abscissa S0 there is an infinitely rigid

support, restraint in translation ( 00rr

= ) and in rotation around the vertical axis directed byr

k ( 0=k0r

r

).

Let us calculate the displacements passing through a support situated at section 1 of curved abscissa S1.

Effect of temperature changes and shrinkage:The translation of the section 1 equals

The rotation around a vertical axis of the section equals

Effect of prestressing (instantaneous and creep-induced deformations):

Translation

The point 1 is a support, and vertical displacement is therefore blocked. From now on, we will look only at the

components of the displacement of1 in the normal planer

k (plane of the structure). The terms inr

k , inr

i G1

and inr

j G1

will therefore not be considered;

Moreover, the support in 0 is blocked in rotation aroundr

k . Therefore Shearing strain deformations are not considered, hence

The translation of section 1 therefore equals

Rotation of the vertical axis

The rotation of the section 1 equals

Here, we will consider only the rotation of the vertical axisr

k , hence

Simplification hypotheses

In addition, we base our approach on the following simplification hypotheses:

N/ES = constant = and M z = 0 (centred prestressed)

The translation in 1 therefore becomesr r

1 i dSS

0

1

= . . i.e.The vertical-axis rotation in 1 therefore becomes

Conclusion

Assuming that the simplification hypotheses are valid(infinitely rigid restraint support, centred prestressed,with prestressing constant along the central fibre and slightbending), it follows that the slide axes of the unidirectional

sliding pot bearings should fan out from the restraintsupport on the basis of the deck deformations.

P1P2

P3

C4

C0

VUE EN PLAN

Point fixe

Figure 4.14.5.6.2 Regarding equipment

Regarding equipment (expansion joints, railings, noise screens, etc.), it is preferable to orient bearings along the axis of thestructure at the abutment level, an approach that is often in contradiction with the arrangement referred to above.

Figs. 4.2: optimal orientations of the slide axes of the abutment unidirectional pot bearings:

a) regarding deck deformations b) regarding equipment


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4.5.6.3 - Recommendations

Various parameters must therefore be taken into account to determine slide-axis orientation.

Regarding deck deformations, it is preferable to orient the slide axes in relation to the fixed point. Failing this, the expansionof the structure is hampered, and considerable transversal forces can appear in the guidance systems if the supports are rigid.This additional friction must be taken into account when the longitudinal horizontal forces are being determined.

Regarding the equipment expansion systems, on the other hand, it is strongly recommended that the slide axes of the abutment

pot bearings be oriented along the length of the structure.

Other parameters can also influence the choice of slide-axis orientation (transversal temperature gradient depending on theorientation of the structure, differential creep between two boxes linked by the slab, etc.)10.

It is not possible to provide general recommendations for all curved structures, since each structure requires specific analysisencompassing the various parameters mentioned earlier and based on structural calculation taking account of support rigidityand the pot-bearing slide axes.

In the final analysis, the choice of orientation involves a compromise between various often-contradictory considerations.Starting at the general structural calculations, therefore, it is important to ensure that the ramifications for the equipment ofthe slide-axis orientation at the level of the abutments are neither forgotten nor underestimated.

Some general principles to be approached with care:

guidance along axis of structure at the level of the abutments; to avoid hard spots, unidirectional sliding pot bearings and restraint pot bearings should not be installed on very rigid piers

(where possible, use multidirectional sliding pot bearings or laminated elastomeric bearings).

4.5.7 - Pot bear ings and laminated elastomer ic bear ings

When the dimensions of pier tops and abutment crossheads permit, it can be a good idea to combine pot bearings andlaminated elastomeric bearings on different supports.

In such cases, the laminated elastomeric bearings should be positioned at the middle of the structure on all of the supportswhere they can take up deck displacement and undergo deformation. They therefore play two roles:

take-up of horizontal forces;

distribution of those forces between all of the supports equipped with laminated elastomeric bearings.

Sliding pot bearings may be placed on the other supports, since low contact pressures mean that the use of sliding laminatedelastomeric bearings would result in higher friction coefficients (see 4.3.2).

The calculation of the horizontal forces to be taken up by the laminated elastomeric bearings is performed in accordance withthe recommendations in the Stra guide Laminated elastomeric bearings11, taking account of rigidity in foundations andsupports.

10 For railway bridges, the slide axes are obviously oriented in the same direction as the tracks at the level of the abutments.11See bibliographic reference.


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Appui mono directionnel libredans le sens de la flche

Appuimultidirectionnel

Appui en lastomrefrett

C0 P1 P2 C6P4P3 P5

Fig. 4.3: typical bearing arrangement

Fig. 4.3 shows a typical arrangement for a six-span structure. If the forces on the guides of the unidirectional abutmentbearings are too great, these should be replaced by the multidirectional variety, and guidance should be provided externally.

4.6 Examples of calcul ations

4.6.1 Numerical appl i cat ion in a s i mple case12

4.6.1.1 Characteristics of the structure

The structure is a bridge built using the successive cantilever method. It is 260 metres long and 10.8 metres wide, and featurestwo traffic lanes and two footways. The application shown below is merely an example (it is not an existing structure, andserves merely to illustrate the approach).

Elevation Transversal section

12 In the example, we have followed the French practice of using the letters H and V, respectively, to indicate the horizontal and vertical forces. Part 3 of thestandard (some other parts differ) uses F and N, respectively.


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C0 P1 P2 P3 C4

mono

multimultimultimulti

mono monofixe

mono

mono

Support conditions,

seen from above

Fig. 4.4: main characteristics of the structure used in the numerical application in 4.6.1

Table 4.5

a) Vertical deck loads for a pair of pot bearings

Vertical loads (MN)for a pair of pot bearings

C0 P1 P2 P3 C4

Minimal permanent-combination ULS 2,87 14,79 15,17 14,79 2,87

Maximal permanent-combination ULS 3,87 19,96 20,48 19,96 3,87

Accidental-combination ULS (minimal reactions) 2,30 16,90 17,36 16,90 2,30

Accidental-combination ULS (maximal reactions) 3,76 19,75 20,27 19,75 3,76

Basic-combination ULS (minimal reactions) 2,33 14,24 14,75 14,24 2,33

Basic-combination ULS (maximal reactions) 6,11 28,84 29,62 28,84 6,11

b) Vertical deck loads for a pot bearing

Vertical loads (MN)for a pair of pot bearings

C0 P1 P2 P3 C4

M

us 0926a pot bearing aap

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