1、Section 14 BEARINGS 14.1 SCOPE This section contains requirements for the design and selection of structural bearings. The selection and layout of the bearings shall be con- sistent with the proper functioning of the bridge, and shall allow for deformations due to temperature and other time dependen
2、t causes. The loads induced in the bearings and structural mem- bers depend on the stiffnesses of the individual elements and the tolerances achieved during fabrication and erec- tion. These influences shall be taken into account when calculating design loads for the elements. Units used in this sec
3、tion shall be taken as KIP, IN, RAD, OF and Shore Hardness, unless noted. 14.2 DEFINITIONS Bearing-A structural device that transmits loads while facilitating translation and/or rotation. Bronze Bearing-A bearing in which displacements or ro- tations take place by the slip of a bronze surface agains
4、t a mating surface. Cotton Duck Reinforced Pad (CDP)-A pad made from closely spaced layers of elastomer and cotton duck, bonded together during vulcanization. Disc Bearing-A bearing which accommodates rotation by deformation of a single elastomeric disc, molded from a urethane compound. It may conta
5、in a device for partially confining the disc against lateral expansion. Double Cylindrical Bearing-A bearing made from two cylindrical bearings placed on top of each other with their axes at right angles to each other, in order to pro- vide rotation about any horizontal axis. Fiberglass Reinforced P
6、ad (FGP)-A pad made from dis- crete layers of elastomer and woven fiberglass, bonded together during vulcanization. Fixed Bearing-A bearing which prevents differential longitudinal translation of abutting structure elements. It may or may not provide for differential lateral trans- lation or rotatio
7、n. Knuckle Bearing-A bearing in which a concave metal surface rocks on a convex metal surface to provide ro- tation capability about any horizontal axis. Longitudinal-The direction associated with the axis of the main structural trusses or girders in the bridge. Metal Rocker or Roller Bearing-A bear
8、ing which carries vertical load by direct contact between two metal sur- faces and which accommodates movement by rolling of one surface with respect to the other. Movable Bearing-A bearing that facilitates differential horizontal translation of abutting structural elements in a longitudinal and/or
9、lateral direction. It may or may not provide for rotation. Plain Elastomeric Pad (PEP)-A pad made exclusively of elastomer. Pot Bearing-A bearing which carries vertical load by compression on an elastomeric disc confined in a steel cylinder and which accommodates rotations by defor- mations of the d
10、isc. PTFE Sliding Bearing-A bearing which carries vertical load by contact stresses between a PTFE sheet or woven fabric and its mating surface, and which permits move- ments by sliding of the PTFE over the mating surface. Rotation about the Longitudinal Axis-Rotation about an axis parallel to the l
11、ongitudinal axis of the bridge. Rotation about the Transverse Axis-Rotation about an axis parallel to the transverse axis of the bridge. RMS-Root mean square. Sliding Bearing-A bearing which accommodates move- ment by slip of one surface over another. Steel Reinforced Elastomeric Bearing-A bearing m
12、ade from alternate laminates of steel and elastomer, bonded together during vulcanization. Translation-Horizontal movement of the bridge in the longitudinal or transverse direction. Transverse-The horizontal direction normal to the lon- gitudinal axis of the bridge. 14.3 NOTATIONS A B = Plan area of
13、 elastomeric bearing (in2) = length of pad if rotation is about its transverse axis, or width of pad if rotation is about its longi- tudinal axis (in) 385 386 HIGHWAY BRIDGES 14.3 c = Design clearance between piston and pot wall (in) D = Diameter of the projection of the loaded surface of the bearin
14、g in the horizontal plane (in) Dd = Diameter of disc element (in) D, = Internal pot diameter in pot bearing (in) D1 = Diameter of curved surface of rocker or roller unit D2 =Diameter of curved surface of mating unit dj = Diameter of the j* hole in an elastomeric bearing E = Youngs modulus (ksi) E, =
15、 Effective modulus in compression of elastomeric E, = Youngs modulus for steel (ksi) e = Eccentricity of loading on a bearing (in) F, = Allowable fatigue stress range for over 2,000,000 F, = Yield strength of the least strong steel at the con- G = Shear modulus of the elastomer (ksi) H, = Maximum ho
16、rizontal load on the bearing or re- straint considering all appropriate load combina- tions (kip) hn = Thickness of i* elastomeric layer in elastomeric bearing (in) h, = Thickness of thickest elastomeric layer in elas- tomeric bearing (in) h, = Total elastomer thickness in an elastomeric bear- ing (
17、in) h, = Thickness of steel laminate in steel-laminated elastomeric bearing (in) I = Moment of inertia (in4) L = Length of a rectangular elastomeric bearing (par- allel to longitudinal bridge axis) (in) M, = Maximum bending moment (K-in) n = Number of interior layers of elastomer PD = Compressive lo
18、ad due to dead load (kip) PTL = Compressive load due to live plus dead load (kip) PL = Compressive load due to live load (kip) Pm = Maximum compressive load considering all ap- R = Radius of a curved sliding surface (in) R, = Radial distance from center of bearing to object, such as an anchor bolt,
19、for which clearance must be provided (in) = Shape factor of one layer of an elastomeric bearing - Plan Area Area of Perimeter Free to Bulge (in) (D2 = for a flat plate) (in) bearing (ksi) cycles (ksi) tact surface (ksi) propriate load combinations (kip) S - LW - for rectangular bearings without 2h,
20、(L + w, holes D 4hmax - - for circular bearings without holes t, = Pot wall thickness (in) W = Width of the bearing in the transverse direction w = Height of piston rim in pot bearing (in) = Effective angle of friction angle in PTFE bear- Ao = Maximum service horizontal displacement of the A, = Maxi
21、mum shear deformation of the elastomer 8 = Instantaneous compressive deflection of bearing , = Maximum compressive deflection of bearing (in) e = Instantaneous compressive strain of a plain elas- tomeric pad ei = Instantaneous compressive strain in i* elastomer layer of a laminated elastomeric beari
22、ng 8 = Component of maximum service rotation in di- rection of interest on an elastomeric bearing under load for Article 14.6.5.3 (in) ings = tan- (HJPD) bridge deck (in) (in) (in) D L = Maximum rotation due to dead load (rad) = Maximum rotation due to live load (rad) = Maximum rotation considering
23、all appropriate load and deformation combinations about trans- verse axis (rad) = Maximum rotation considering all appropriate load and deformation combinations about longi- tudinal axis (rad) = Maximum design rotation considering all appro- priate load and deformation combinations includ- ing live
24、and dead load, bridge movements, and construction tolerances (rad) 8, p, = Coefficient of friction uD =Average compressive stress due to dead load uL = Average compressive stress due to live load (ksi) un = Average compressive stress due to total dead plus u, = Maximum average compressive stress (ks
25、i) (ksi) live load (ksi) 14.4 MOVEMENTS AND LOADS Bearings shall be designed to resist loads and accom- modate movements. No damage due to joint or bearing movement shall be permitted under any appropriate load and movement combination. 14.4 DIVISION I-DESIGN 387 Translational and rotational movemen
26、ts of the bridge shall be considered in the design of bearings. The se- quence of construction shall be considered and all critical combinations of load and movement shall be considered in the design. Rotations about two horizontal axes and the vertical axis shall be considered. The movements shall
27、in- clude those caused by the loads, deformations and dis- placements caused by creep, shrinkage and thermal ef- fects, and inaccuracies in installation. In all cases, both instantaneous and long-term effects shall be considered, but the influence of impact need not be included. The most adverse com
28、bination of movements shall be used for de- sign. All design requirements shall be tabulated in a ratio- nal form such as shown in Figure 14.4. ings shall have lateral strength adequate to resist all ap- plied loads and restrain unwanted translation. Combinations of different types of fixed or movea
29、ble bearings should not be used at the same expansion joint, bent or pier unless the effects of differing deflection and rotational characteristics on the bearings and structure are accounted for in the design. 14.5.1 Load and Movement Capabilities The movements and loads to be used in the design of
30、 the bearing shall be clearly defined on the contract drawings. 14.5.2 Characteristics 14.4.1 Design Requirements The minimum thermal movements shall be computed from the extreme temperature defined in Article 3.16 of Division I and the estimated setting temperature. Design loads shall be based on t
31、he load combinations and load factors specified in Section 3 of Division I. The design rotation, Om, for bearings such as elasto- meric pads or steel reinforced elastomeric bearings which do not achieve hard contact between metal components shall be taken as the sum of -the dead and live load rotati
32、ons. -an allowance for uncertainties, which is normally taken as less than 0.005 rad. The design rotation, O, for bearings such as pot bearings, disc bearings and curved sliding surfaces which may de- velop hard contact between metal components shall be taken as the sum of -the greater of either the
33、 rotations due to all applicable factored loads or the rotation at the service limit state. -the maximum rotation caused by fabrication and in- stallation tolerances, which shall be taken as 0.01 rad unless an approved quality control plan justifies a smaller value. -an allowance for uncertainties,
34、which shall be taken as 0.01 rad unless an approved quality control plan jus- tifies a smaller value. 14.5 GENERAL REQUIREMENTS FOR BEARINGS Bearings may be fixed or movable as required for the bridge design. Movable bearings may include guides to control the direction of translation. Fixed and guid
35、ed bear- The bearing chosen for a particular application must have appropriate load and movement capabilities. Those listed in Table 14.5.2-1 may be used as a guide. Figure 14.5.2-1 may be used as a guide in defining the different bearing systems. The following terminology shall apply to Table 14.5.
36、2-1: S = Suitable U = Unsuitable L R = Suitable for limited applications = May be suitable but requires special considera- tions or additional elements such as sliders or guideways. Long. = Longitudinal axis Trans. = Transverse axis Vert. = Vertical axis 14.5.3 Forces in the Structure Caused by Rest
37、raint of Movement at the Bearing Horizontal forces and moments induced in the bridge by restraint of movement at the bearing shall be taken into account in the design of the bridge and the bear- ings. They shall be determined using the calculated movements and the bearing characteristics given in Ar
38、ticle 14.6. 14.5.3.1 Horizontal Force Horizontal forces may be induced by sliding friction, rolling friction or deformation of a flexible element in the bearing. The force used for design shall be the largest one applicable. Sliding friction force shall be computed as Hln = ln (14.5.3.1-1) 14.5.3.1
39、388 HIGHWAY BRIDGES Irreversible Re versi ble Irreversible Reversible Upper surface Lower surface Vertical max. perm. min. Bridge Name or Kef. Beariog Identification Mark Number of bearings required Seating Material Upper Surface Lower Surface Allowable contact pressure Average (KSI) Mge Load Design
40、 load effects (KIP) under transicor loads (N) I Longitudinal I Overall height Longitudinal I Trans vers e 1 Longitudinal Transverse Loogi tudi na1 Transverse Longitud na1 Tran svcrse Trans ver se I Vertical I Tran s v ers e I Longitudinal I FIGURE 14.4 14.5.3.1 DIVISION I-DESIGN 389 Table 14.5.2-1 B
41、earing Suitability Type of Bearing Rotation about bridge Movement axis indicated Resistance to Loads Long Trans Trans Long Vert Vert Long Trans Plain Elastomeric Pad Fiberglass Reinforced Pad Cotton Duck Reinforced Pad Steel-reinforced Elastomeric Bearing Plane Sliding Bearing Curved Sliding Spheric
42、al Bearing Curved Sliding Cylindrical Bearing Disc Bearing Double Cylindrical Bearing Pot Bearing Rocker Bearing Knuckle Bearing Single Roller Bearing Multiple Roller Bearing S S U S S R R R R R S U S S S S U S S R R R R R U U U U S S U S U S S S S S S S S U S S U S U S U S S S U U U U L L U L S S U
43、 L U L U U U U L L S S S S S S S S S S S S L L L L R R R S R S R S U U L L L L R R R R R S R R R U FA 91 ldlng Sur+aoi Cyllndrloal baring Sphor loa1 bar Ing Rocker Be or i ng Pot Ba ar i ne Elastomeric Bearing FIGURE 14.5.2-1 Typical Bearing Components 390 HIGHWAY BRIDGES 14.5.3.1 where: H, = maximu
44、m horizontal load (kip) p. = coefficient of friction P, = maximum compressive load (kip) The force required to deform an elastomeric element shall be computed as: H, = GAA,/h, ( 14.5.3.1 -2) where: G = shear modulus of the elastomer (ksi) A = plan area of elastomeric element or bearing (in2) As = ma
45、ximum shear deformation of the elastomer (in) h, = total elastomer thickness (in) Rolling forces shall be determined by test. 14.5.3.2 Bending Moment The bridge substructure and superstructure shall be de- signed for the largest moment, M, which can be trans- ferred by the bearing. For curved slidin
46、g bearings without a companion flat sliding surface, M, shall be estimated by: M, = pP,R (14.5.3.2-1A) and for curved sliding bearings with a companion flat sliding surface, M, shall be estimated by: M, = 2p.PmR (14.5.3.2-1B) where: M, = maximum bending moment (K-in) R = radius of curved sliding sur
47、face (in) For unconfined elastomeric bearings and pads, M, shall be estimated by: M, = (0.5 E,I)e,/h, (14.5.3.2-2) where: I = moment of inertia of plan shape of bearing (in4) 8, = maximum design rotation (rad) E, = effective modulus of elastomeric bearing in com- pression (ksi) The load deflection c
48、urve of an elastomeric bearing is nonlinear, so E, is load-dependent. However, an accept- able constant approximation is: E, = 6GS2 (14.5.3.2-3) where: 14.6 SPECIAL DESIGN PROVISIONS FOR BEARINGS The stress increases permitted for certain load combi- nations by Table 3.22.1A of this specification sh
49、all not apply in the design of bearings. 14.6.1 Metal Rocker and Roller Bearings 14.6.1.1 General Design Considerations The rotation axis of the bearing shall be aligned with the axis about which the largest rotations of the supported member occur. Provision shall be made to ensure that the bearing alignment does not change during the life of the bridge. Multiple roller bearings shall be connected by gearing to ensure that individual rollers remain parallel to each other and at their original spacing. Metal rocker and roller bearings shall be detailed so that they can
