Sherif A. Mourad ١

METALLIC BRIDGESSTR403

Sherif A. Mourad

Professor of Steel Structures and Bridges

Faculty of Engineering, Cairo University

Lecture 9 – 23 April 2012

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Lecture 9: BEARINGSTopics• Why are bearings needed?

• Types of Bearings.

• Steel Bearings.

• Design of Steel Bearings.

• Elastomeric Bearing.

• Design of Elastomeric Bearings.

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WHY BEARINGS?

1- Transfer forces from one part of the bridge to another, usually from the superstructure to the substructure.

2- Allow movement (translation along and/or rotation about any set of axes) of one part of the bridge in relation to another.

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RESTRAIN OR NOT?• Movement is mainly from temperature changes.

• Axial and rotational strains arise from applied loads.

• It is not recommended to fully restrain a bridge against temperature movements. Why?

• If we consider a steel plate girder of cross sectional area 500 cm2. If this girder is subjected to a 30oC rise in temperature and is restrained from expanding axially, an axial stress of

E α ∆T = 2100x(1.2x10-5)x30 = 0.756 t/cm2

is induced in the girder. The corresponding restraining force required is 0.756*500 = 378 ton. Neither the girder nor its supporting structure can carry such a force.

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TYPES OF BEARINGS

Bearings may be classified according to their deformation behavior into three basic types:

a) fixed bearings.

b) hinged bearings

c) expansion bearings.

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FIXED BEARINGSA fixed-end bearing completely restrains the member end from translation and rotation. It is capable of supplying a vertical and a horizontal reaction plus a restraining moment. Considering the expense of fixing a heavy steel member at the ends, the use of such a bearing is usually limited to sites having very strong rocky soils. Typical applications of this type of bearings are found at supports of arch bridges and sloping leg frames bridges.

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HINGED BEARINGSA hinged bearing will permit rotation of the member ends, and this is usually accomplished by a pin. Hinges carrying heavy vertical loads are normally providedwith lubrication systems to reduce friction and ensure free rotation without excessive wearing.

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EXPANSION BEARINGSExpansion bearings permit movement as well as rotation of the superstructure. They are usually provided in two forms: the sliding type and the rolling type. Sliding type bearings are used only for short spans and small loads since they cause high friction forces between the sliding plates.Rolling-type bearings achieve their translational movement by using cylindrical rollers.

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DESIGN REQUIREMENTS

1. Movement.

2. Design Life.

3. Durability.

4. Limit states.

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MOVEMENTThe sources of movement include bridge skew and curvature effects, initial comber, misalignment or construction tolerance, settlement of support, thermal effects, construction loads, and traffic loading. Restraints that restrict the translation movement of a structure may be provided as part of or separate from the vertical load bearings. Restraints may be provided by separate dowels, keys, or side restraints on sliding bearings.

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SYMBOLIC REPRESENTATION

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DESIGN LIFE

Bearings should be designed to last as long as the bridge itself. However, with some non-metallic materials in use today, it is difficult to ascertain this requirement. Inadequate maintenance of metallic parts of bearings may reduce their service life. It is thus important to allow for inspection and replacement of bridge bearings, in whole or in part.

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DESIGN LIFEProvisions should be made for installation of jacks necessary for the removal of bearings, insertion of shims, or any other operations requiring lifting the bridge deck from the bearings. Adequate space should be provided around bearings to facilitate inspection and replacement. If there is a possibility of differential settlement, provisions should be made for jacking up the bridge deck and inserting metal shims.

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DURABILITY

Bearings should be detailed without recesses and enclosures that may trap moisture and dirt. The materials used in their manufacture and the method adopted for protection against corrosion should ensure that the bearings function properly throughout their life.

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LIMIT STATESTo meet the serviceability limit state for bearings the design should be such that they do not suffer damage that would affect their proper functioning or incur excessive maintenance during their working life. In the ultimate limit state, the strength and stability of the bearings should be adequate to resist the ultimate design loads and movements of the structure.

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MATERIALS OF BEARINGS

Bearings may also be classified according to the material used in their fabrication into:

a) steel bearings.

b) elastomeric bearings.

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STEEL BEARINGS

Steel bearings may also be classified into:

a) Roller bearing.

b) Rocker bearing.

c) Knuckle pin bearing.

d) Leaf pin bearing.

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ROLLER BEARINGSConsist of one or more steel cylinders between parallel upper and lower plates. Gearing or some other form of guidance should be provided to ensure that the axis of the roller is maintained in the desired orientation during the life of the bearing. Roller bearings with a single cylinder can permit translation parallel to the longitudinal bridge axis and rotation about a horizontal axis in the transversal direction. Multiple cylinders on the other hand require another element such as a rocker or a knuckle bearing to permit rotation.

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ROLLER BEARINGS

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ROCKER BEARINGSConsist primarily of a curved surface in contact with a flat or curved surface and constrained to prevent relative horizontal movement. The curved surface may be cylindrical or spherical to permit rotation about one or more axes. Rocker bearings on their own do not permit translation and are usually used at the fixed end of a bridge to complement roller bearings. They can also permit rotation by the rolling of one part over another.

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ROCKER BEARINGS

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KNUCKLE PIN BEARINGSConsist of a steel pin housed between an upper and a lower support each having a curved surface which mates with the pin. Transversal lateral loads may be transmitted by flanges on the ends of the pin. This type of bearing permits rotation by the sliding of one part on the other.

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LEAF PIN BEARINGSConsist of a pin passing through a number of interleaved plates fixed alternatively to the upper and lower bearing plates. They permit only rotational movements, but can be used in conjunction with roller bearings to provide rotation and translation.

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PHOTO

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DESIGN OF STEEL BEARINGSThe maximum contact stress (f in t/cm2) between a roller and a flat surface is given by the following Hertz formula:

V = reaction in tons r = radius of roller, cm E = Young’s modulus of steel

(t/cm2)υ = Poisson’s ratio of steel L = roller length, cm

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DESIGN OF STEEL ROLLERSince the contact stress is confined and limited to a small area, it is permissible to use a high allowable stress, even exceeding the ultimate tensile strength of the material. For fixed, sliding and movable bearings with one or more rollers, the allowable contact stresses shall be as given below when the surface of contact between the different parts of a steel bearing is a line:

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DESIGN OF STEEL ROLLER

Thus for bearing rollers made from structural steel St 44, resting on flat plates, using f = 6.5 t/cm2, E = 2100 t/cm2, and υ = 0.30 the above equation gives:

V = 0.0550 (d)x(L)Note: for bearings employing more than two rollers, the maximum permitted design loads given above for single rollers should be reduced by 20 % to allow for uneven loading of the rollers caused by dimensional differences.

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MULTIPLE ROLLERSWith two or more rollers, an independent pin must be provided to allow end rotation of the bridge due to bending deflection.To save space between rollers, they can be flat sided. Such rollers should be symmetrical about the vertical plane passing through the centre and the width should not be less than one-third of the diameter or such that the bearing contact doesn’t move outside the middle third of the rolling surfaces when the roller is at the extreme of its movement.

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MULTIPLE ROLLERS

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BASE PLATEThe rollers are seated on a base plate which distributes the vertical load to the concrete abutment or pier. The area of this plate is computed from the allowable bearing stress on the concrete which is 70 kg/cm2 for concrete C250 and 110 kg/cm2 for C350. Anchor bolts connecting the base plate to concrete are designed to transmit transversal or longitudinal frictional forces resulting from movements. If the bearing is subjected to negative reactions, they are designed to carry tension.

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HINGED BEARINGSThe design of hinged bearings is similar to that for a roller bearing except that the contact stress used for the pin design is computed from Hertz formula for the case of bearing between two cylinders. If the pin is made of cast steel, the diameter is given by:

d = 1.334 V/Lwhere d = diameter of pin in cm, V = vertical load in ton, L = length of pin in cm.

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ELASTOMERIC BEARINGThe main component of elastomeric bearings is a rubber pad that distributes the loads from the superstructure to the substructure and uses its material flexibility to accommodate the rotation and longitudinal movement of the superstructure. Translational movement is accommodated by shear in the elastomer, one surface of which moves relative to the other. Rotational movement is accommodated by the variation in compressive strain across the elastomer.

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ELASTOMERIC BEARING

Bridge bearings may be divided into four basic categories;1. Elastomeric pads.2. Pot bearings.3. Sliding surfaces.4. Curved sliding surfaces.

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ELASTOMERIC BEARINGThe rubber used is either natural rubber or synthetic rubber

(neoprene). Because of their relative simplicity and minimal fabrication effort, elastomeric bearings are now widely used in new bridges. Elastomeric bearings are available in two basic forms;

1- Plain elastomeric pads which are single unreinforced pads of elastomer of relatively thin section;

2- Reinforced elastomeric pads comprising one or more layers of elastomer bonded to reinforcing plates in sandwich form. Two main types of reinforcements:

i) Steel.ii) Polytetraflouroethylene (PTFE) also known as Teflon.

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ELASTOMERIC BEARINGA steel reinforced elastomeric bearing consists of discrete steel thin plates strongly bonded between adjacent layers of elastomer. The design of this type of bearings consists of finding the plan dimensions, number of elastomeric layers and their corresponding thicknesses, and steel plate thicknesses. Because these calculations depend largely of the properties of the rubber used, the design of these bearing types is usually taken from their manufacturer's certified design tables.

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ELASTOMERIC BEARING

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ELASTOMERIC BEARING

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PHOTO

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Elastomeric PadElastomers are used in both elastomeric bearing pads and steel-reinforced elastomeric bearings. The behavior of both pads and bearings is influenced by the shape factor, S, defined as:S=A/Pwhere • A is the plan area• P is the area of the perimeter free to

bulge.

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Elastomeric PadElastomeric bearing pads and steel reinforced elastomeric bearings have several advantages. They have a low cost and require minimal maintenance. Further, the components can sustain higher values than the design loads, which is useful in case of extreme events that have a low probability of occurrence (earthquakes, for example). Natural rubber or neoprene may be used in the bearings.

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Elastomeric PadElastomers are visco-elastic nonlinear materials and thus their properties vary with strain level, rate of loading and temperature. Elastomers are flexible under shear and uniaxial deformation, but are very stiff against volume changes. This feature allows for the design of a bearing that is stiff in compression but flexible in shear. The shear stiffness of the bearing is the most important property, since it affects the forces transmitted between the superstucture and substructure.

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Elastomeric PadThere are two types of elastomeric pads:• Unreinforced; can accommodate small to

moderate compressive loads with limited or no rotation and translation, so they are best suited for short bridges (less than 40 m).

• Reinforced; Steel reinforced elastomeric bearings have uniformly spaced layers of steel and elastomer.

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Reinforced Elastomeric Pad

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Reinforced Elastomeric PadThe steel reinforcement within elastomeric pads makes their behavior quite different from plain elastomeric pads. The bearing accommodates translation and rotation by deformation of the elastomer. Under uniaxialcompression, the flexible elastomer would shorten significantly and sustain large increases in its plan dimension, but the stiff steel layers restrain this lateral expansion. This restraint induces a bulging pattern and provides a large increase in stiffness under compressive loads.

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Reinforced Elastomeric PadA steel reinforced elastomeric bearing may support relatively high compressive loads while accommodating large translations and rotations. The stress in the steel plates and the strain in the elastomer are controlled by the elastomer thickness and the shape factor of the bearing. Large rotations and translations require taller bearings. Translations and rotations may occur about either horizontal axes, thus these bearings are suitable for bridges where the direction of movement is not precisely defined.

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Pot BearingThe basic components of a pot bearing are a shallow cylinder, a pot, an elastomeric pad, a set of sealing rings and a piston. Pot bearings are fixed against all translation unless they are used with a PTFE sliding surface. The pot and piston are made from structural carbon steel, whereas the sealing ring is usually made of a single circular brass ring or a set of two or three flat brass rings. The brass rings are placed in a recess on the top of the elastomeric pad.

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Pot BearingVertical load is carried through the piston of the bearing and is resisted by compressive stress in the elastomeric pad. The pad is deformable but almost incompressible and is often idealized as behaving hydrostatically, however, in practice; the elastomer has some shear stiffness. Deformation of the pot wall is a concern, since this deformation changes the clearance between the pot and the piston and may lead to binding of the bearing or to elastomer leakage.

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Pot Bearing

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Pot BearingRotation about any axis is accommodated by deformation of the elastomeric pad. Pot bearings are usually designed for a maximum compressive strain of 15% in the elastomer due to rotation. To achieve 0.02 radians, the ratio D/t must not exceed 15. Increasing the pad thickness accommodates larger rotations but increases the required depth, and thus the cost of the pot. During rotation, the elastomeric pad compresses on one side and expands on the other, so the elastomer is in contact with the pot wall and slips against it.

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Pot BearingLateral load is transferred from the piston to the pot by contact between the rim of the piston and the wall of the pot. The contact stress may be high because the piston rim may be relatively thin to avoid binding when the piston rotates and the rim slides against the pot. The pot wall must transfer the load down into the base plate (combined shear and bending). The load is then transferred to the substructure through friction under the base of the bearing and shear in the anchor bolts.

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Sliding SurfacesLubricated bronze and polytetrafluorethylene(PTFE) are commonly used as components of bridge bearings. Sliding surfaces develop a frictional force that acts on the superstructure, substructure, and bearing. The frictional force, F, can be computed as F= µNwhere µ is the coefficient of friction and N is the normal force on the sliding surface.

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Sliding SurfacesLubricated bronze sliding surfaces are used to accommodate very large translation, and the load capacity is also big as it is only limited by the surface area. The coefficient of friction is typically 0.07 under initial lubricated conditions. However, it increases to 0.1 as the surface dissipates with time and movement. Coefficient of friction in the order of 0.4 may be expected after the lubrication has completely dissipated.

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Sliding SurfacesRecommended design coefficients of friction for bearings with stainless steel sliding on pure PTFE continuously lubricated are given in the Table below. For design purposes, the coefficient of friction for pure unlubricated PTFE on stainless steel should be taken as twice the values given in the Table.

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Sliding Surfaces

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Sliding SurfacesPTFE sliding surfaces are used to accommodate large translations, and, when combined with spherical or cylindrical bearings, large rotations. They develop substantially smaller friction forces than lubricated bronze bearings. However, they require greater care in design and greater quality control in construction and installation. PTFE is used with mating surfaces made of very smooth stainless steel (for all flat surfaces and many curved surfaces) or anodized aluminum (for some spherical or cylindrical surfaces).

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Sliding SurfacesThe stainless steel is larger than the PTFE surface to achieve full movement without exposing the PTFE. The steel plate is typically place on top of the PTFE to prevent contamination with dust or dirt. PTFE sliding surfaces are often used in combination with a wide range of other bearing systems. PTFE wears under service conditions and may require replacement after a period of time. Low temperatures, fast sliding speeds, rough mating surface, lack of lubrication and contamination of the sliding interface increase the wear rate.

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Curved Sliding SurfacesBearings with curved sliding surfaces include spherical and cylindrical bearings. They are a special case of lubricated bronze or PTFE sliding surfaces. They are used primarily to sustain large rotations about one or more axes, and are fixed against translation. The rotation occurs about the center of radius of the curved surface, and the maximum rotation is limited by the geometry and clearances of the bearing.

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Curved Sliding SurfacesThese bearings may develop horizontal resistance by virtue of the geometry. This lateral load capacity is limited and large lateral loads require an external resisting system. The center of rotation of the bearing and the neutral axis of the beam seldom coincide, and this eccentricity introduces additional translation and girder end moment that must be considered in the design. An additional flat sliding surface must be added if the bearing is to accommodate displacements or to reduce the girder end moment.

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Curved Sliding SurfacesThe moment, M, may be estimated as:M = µ N dwhere d is the distance between the center of radius of the bearing and the center of rotation of the girder. This moment must be considered in the design of the bearings, superstructure and substructure. The inside and outside radii of spherical and cylindrical bearings must be accurately controlled and machined to assure good performance.

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Curved Sliding Surfaces

When using PTFE, a small tolerance between the two radii and a smooth surface finish is required to prevent wear, creep, or cold flow damage due to nonuniform contact and to ensure a low coefficient of friction.

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Selection of Bearing Type1- Define the design requirements (forces,

translation, and rotation limits).2- Identify the bearing types that satisfy the

design requirements.3- Identify the initial and maintenance cost of the

bearings.4- Choose the bearing type that meets the design

requirement at lowest overall cost.5- Consider ease of access for inspection,

maintenance and possible replacement.6- Note that the limits provided are practical to

determine the most economical bearing.

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PRACTICAL EXAMPLES

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PRACTICAL EXAMPLES

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PRACTICAL EXAMPLES

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PRACTICAL EXAMPLES

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CONCLUSION

• Why are bearings needed?• Types of Bearings.• Steel Bearings.• Design of Steel Bearings.• Elastomeric Bearing.• Design of Elastomeric Bearings.