cairo university faculty of engineering fourth year ... university faculty of engineering fourth...

Cairo University Faculty of Engineering Fourth Year Structural Second Term 2011 - 2012 Metallic Bridges (1) ST403

Bridge Bearings Lecture Notes Prof. Sherif A. Mourad

Introduction: Bridge bearings are an important element of the bridge and require careful consideration. Up to the middle of the 20th century, bridges relied on steel bearings (rollers, rockers, or sliding bearings) to allow movement. With more advanced designs to make better use of the materials employed and the increased used of curved and skewed bridges, it was necessary to develop the bearings to allow for movement in more than one direction. New types of bearings were developed making use of new materials and improved technology. This brief note covers bearings types other than the steel roller and rocker bearings, which are detailed in the textbook, pages 208 to 211, and 214 to 217. It is more elaborate than the information provided in pages 212 and 213. Figure (1) shows the various types of bridge bearings. Bearings are required to fulfill the following functions:

Transfer forces from one part of the bridge (superstructure) to another (substructure).

Allow movement (translation or rotation) of one part of the bridge with respect to another.

Allow free movement in a certain direction but not in others, in order to constrain the movement of the bridge to specific direction(s).

Design Requirements: Bearings ensure the functionality of a bridge by allowing translation and rotation to occur while supporting vertical loading. The main design requirements are: 1. Movements: Consideration of movement is important for bearing design. The 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. Table (1) defines the symbolic representation of bearing displacement and rotation restraints.

Table (1): Symbolic representation of bearing functions.

ST403 – Steel Bridges (1) 4th Year Structural Sherif A. Mourad

Figure (1): Types of bearings


2. 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. Provisions 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. 3. 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. 4. Limit states: To 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. Types of Bearing: Bridge bearings may be divided into four basic categories;

Elastomeric pads. Pot bearings. Sliding surfaces. Curved sliding surfaces.

1. Elastomeric Pads: Elastomers 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:

P

AS

where A is the plan area and P is the area of the perimeter free to bulge. Elastomeric 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. Elastomers 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.


Elastomeric bearings: Elastomeric bearing pads include plain elastomeric pads (PEP), cotton duck reinforced pads (CDP), and layered fiberglass reinforced bearing pads (FGP). Elastomeric bearings can accommodate small to moderate compressive loads with limited or no rotation and translation, so they are best suited for bridges with small lengths (less than 40 m). CDP may support somewhat larger compressive loads than PEP and FGP. Translations less than 25 mm and rotations of a degree or less may be accommodated with GFP, whereas smaller values are possible for PEP, and no significant movements are practical with CDP. Steel reinforced elastomeric bearings: The steel reinforcement within elastomeric pads makes their behavior quite different from plain elastomeric pads. Steel reinforced elastomeric bearings have uniformly spaced layers of steel and elastomer. The bearing accommodates translation and rotation by deformation of the elastomer. Under uniaxial compression, 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. This permits a steel reinforced elastomeric bearing to 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. 2. Pot bearing: The 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. Vertical 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. Rotation 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. This may cause elastomer abrasion and sometimes contributes to elastomer leakage.


Lateral 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. 3. Sliding surfaces: Lubricated 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 NF

where is the coefficient of friction and N is the normal force on the sliding surface. Lubricated 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. Recommended design coefficients of friction for bearings with stainless steel sliding on pure PTFE continuously lubricated are given in Table (2) 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. Table (2): Coefficient of friction for stainless steel sliding on pure PTFE continuously

lubricated. Bearing stress (N/mm2) 5 10 20 30 and over Coefficient of friction 0.08 0.06 0.04 0.03 PTFE 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). The 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. 4. Curved sliding surfaces: Bearings 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. These 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. The moment, M, may be estimated as:

NdM

where 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. 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. Selection of Bearing Type: There are several approaches to selecting a cost-effective and appropriate bearing system for bridges. An important decision is to define the bearing type suitable for the design requirements. Table 3 provides a guide for choosing a suitable bearing type.

Table (3): Bearing Function.


There are several procedures to follow in the design process. The one outlined here follows the reference “Steel Bridge Bearing Selection and Design Guide”, published by the American Iron and Steel Institute. Table 3 is provided to identify the bearing types that satisfy design requirements. The selection procedure is:

1. 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 appropriate bearing type that meets the design requirement at the

lowest overall cost. 5. Ease of access for inspection, maintenance and possible replacement must also

be considered. 6. Note that the limits provided are not absolute, but are practical limits that

approximate the most economical application of each bearing type. Examples of other procedures is the one in Table 4. Note that there are some differences in the types and limits set in each table, indicating that the procedure is subjective and depends on the designer's experience. For example, if we want to accommodate translation more than 100 mm, and the axial load is 8000 tons, then the available options are Flat PTFE and Multiple Rollers. Note that pot bearings were excluded although they can sustain up to 1000 tons, as they cannot accommodate translation. Flat PTFE would be preferable due to the lower initial and maintenance cost as compared to multiple rollers.

Table (4): Bearing Facilities.


As an example on the use of tables provided by a specific manufacturer, a few pages from a manufacturer's manual are provided in Appendix A. They include tables for use in the design of elastomeric reinforced bearings as well as elastomeric/PTFE sliding and sliding/guided bearings. Bearing Arrangement: A typical arrangement for the bearings in plan is to provide for one fixed bearing and one transversally flexible bearing at the "fixed" support, and one longitudinally flexible and one multi-directionally flexible at the "movable" support. This is used for wide bridges and where a fair degree of lateral movement must be allowed (see Fig. 2a). If the bridge is narrow, two fixed bearings with some play for movement in the transverse direction are provided at the fixed support, and one longitudinally flexible and one multi-directionally flexible at the flexible support (Fig. 2b). For a slab bridge with skewed ends, a lot of bearings that are flexible and tiltable in all directions are used, with lateral restraint provided by bearings on the bridge centerlines (Fig. 2c). Figure 3 describes the placement of elastomeric bearing supports for precast beams, where special details are required to ensure that the vertical stress is uniformly distributed along the bearing cross-section (Fig. 3a & 3b), avoiding stress concentration on a partly-loaded bearing (Fig. 3c).

Figure (2): Bearing arrangement on bridge plan view.

Figure (3): Elastomeric bearing attachment to sloping precast beams.

References: Steel Bridges, Metwally Abu-Hamd, 2007. Steel Bridge Bearing Selection and Design Guide, American Iron and Steel

Institute. BS 5400: Steel, Concrete and composite Bridges – Part 9, Bridge Bearings. Bearings and Expansion Joints – Alga Catalog – Milano, Italy. Lee, D. J., Bridge Bearings and Expansion Joints, E&FN Spon, 2nd Ed., 1994.


Appendix A

Sample Bearings Manufacturer Catalog

cairo university faculty of engineering fourth year ... university faculty of engineering fourth...

Documents