Bridges should be designed so that a total movement due to temperature change of 11⁄4 in per 100 ft can take place. Also, provisions should be made for changes in length of span resulting from live-load stresses. In spans over 300 ft long, allowance should be made for expansion and contraction in the floor system.
Expansion bearings may be needed to permit such movements. (See also Art. 11.26.) In addition, to control of the movements, at least one fixed bearing is required in each simple or continuous span. A fixed bearing should be firmly anchored against horizontal and vertical movement, but it may permit the end of the member supported to rotate in a vertical plane. An expansion bearing should permit only end rotation and movement parallel to the longitudinal axis of the supported member, unless provisions for transverse expansion are necessary Allowable bearing on granite is 800 psi and on sandstone or limestone, 400 psi, when the masonry projects 3 in or more beyond the edge of the bearing plate. For smaller projections, only 75% of these stresses is allowed. For reinforced concrete, the basic allowable stress ƒc is 30% of the 28-day compressive strength. When the supporting surface is wider on all sides than the loaded area A1, the allowable stress may be multiplied by A /A 2 1 2, where A2 is the area of the supporting surface.
Bearings for spans of 50 ft or more should be designed to permit end rotation. For the purpose, curved bearing plates, elastomeric pads, or pin arrangements may be used. Elastomeric bearings are generally preferred. At expansion bearings, such spans may be provided with rollers, rockers, or sliding plates. Shorter spans may slide on metal plates with smooth surfaces.
In all cases, design of supports should ensure against accumulation of dirt, which could obstruct free movement of the span, and against trapping of water, which could accelerate corrosion. Beams, girders, or trusses should be supported so that bottom chords or flanges are above the bridge seat.
Self-lubricating bronze or copper-alloy sliding plates, with a coefficient of friction of 0.10 or less, may be used in expansion bearings instead of elastomeric pads, rollers, or rockers. These plates should be at least 1⁄2 in thick and chamfered at the ends.
Rockers generally are preferred to rollers because of the smaller probability of becoming frozen by dirt or corrosion. The upper surface of a rocker should have a pin or cylindrical bearing. The lower surface should be cylindrical with center of rotation at the center of rotation of the upper bearing surface. At the nominal centerline of bearing, the lower portion should be at least 11⁄2 in thick. The effective length of rocker for computing line bearing stress should not exceed the length of the upper bearing surface plus the distance from lower to upper bearing surface. Adequate web material should be provided and arranged to ensure uniform load distribution over the effective length. The rocker should be doweled to the base plate.
Rollers are the alternative when the pressure on a rocker would require it to have too large a radius to keep bearing stress within the allowable. Rollers may be cylindrical or segmental. They should be at least 6 in in diameter. They should be connected by substantial side bars and guided by gearing or other means to prevent lateral movement, skewing, and creeping. The roller nest should be designed so that the parts may be easily cleaned.
Effective bearing area for rockers and rollers equals effective length times effective width. Effective length of bearing area may be taken equal to effective length of rocker, or to roller length plus twice the thickness of the base plate. Effective width of bearing area may be taken as 4 times the base-plate thickness for rockers, or the distance between end rollers plus 4 times the baseplate thickness for rollers. The vertical load may be assumed uniformly distributed over the effective bearing area, except for eccentricity from rocker travel.
Sole plates and masonry plates should be at least 3⁄4 in thick. For bearings with sliding plates but without hinges, the distance from centerline of bearing to edge of masonry plate, measured parallel to the longitudinal axis of the supported member, should not exceed 4 in plus twice the plate thickness. For spans on inclines exceeding 1% without hinged bearings, the bottom of the sole plate should be radially curved or beveled to be level.
Elastomeric pads are bearings made partly or completely of elastomer. They are used to transmit loads from a structural member to a support while allowing movements between the bridge and the support. Pads that are not all elastomer (reinforced pads) generally consist of alternate layers of steel or fabric reinforcement bonded to the elastomer. In addition to the reinforcement, the bearings may have external steel plates bonded to the elastomeric bearings. AASHTO prohibits tapered elastomeric layers in reinforced bearings.
The AASHTO ‘‘Standard Specifications for Highway Bridges’’ contain specifications for the materials, fabrication, and installation of the bearings. The specifications also present two methods for their design, both based on service loads without impact and the shear modulus at 73F. The grade of elastomer permitted depends on the temperature zone in which the bridge is located. The specifications also require that either (1) a positive slip apparatus be installed and bridge components be able to withstand forces arising from a bearing force equal to twice the design shear force or (2) bridge components be able to sustain the forces arising from a bearing force equal to four times the design shear force. If the shear force exceeds one-fifth the dead-load compressive force, the bearing should be fixed against horizontal movement.
Design should allow for misalignment of girders because of fabrication or erection tolerances, camber, or other sources. It should also provide for subsequent replacement of bearings, when necessary. Also, it should ensure that bearings are not subjected to uplift when in service.
A beam or girder flange seated on an elastomeric bearing should be stiff enough to avoid damaging it. Stiffening may be achieved with a sole plate or bearing stiffeners. I beams and girders symmetrically placed on a bearing do not require such stiffening if the widththickness ratio bƒ / tƒ of the bottom flange does not exceed
PTFE pads are bearings with sliding surfaces made of polytetrafluoroethylene (PTFE), which may consist of filled or unfilled sheet, fabric with PTFE fibers, interlocked bronze and filled PTFE structures, PTFE-perforated metal composites and adhesives, or stainlesssteel mating surfaces. The AASHTO standard specifications contain specifications for the materials, fabrication, and installation of the bearings.
The sliding surfaces of the pads permit translation or rotation by sliding of the PTFE surfaces over a smooth, hard mating surface. This should preferably be made of stainless steel or other corrosion-resistant material. To prevent local stresses on the sliding surface, an expansion bearing should permit rotation of at least 1 due to live load, changes in camber during construction, and misalignment of the bearing. This may be achieved with such devices as hinges, curved sliding surfaces, elastomeric pads, or preformed fabric pads.
PTFE sliding surfaces should be factory-bonded or mechanically fastened to a rigid backup material capable of resisting bending stresses to which the surfaces may be subjected.
The surface mating to the PTFE should be an accurate mate, flat, cylindrical, or spherical, as required, and should cover the PTFE completely in all operating positions of the bearing.
Preferably, the mating surface should be oriented so that sliding will cause dirt and dust to fall off it.
Pot bearings are used mainly for long-span bridges. They are available as fixed, guided expansion, and nonguided expansion bearings, designed to provide for thermal expansion and contraction, rotation, camber changes, and creep and shrinkage of structural members.
They consist of an elastomeric rotational element, confined and sealed by a steel piston and steel base pot. In effect, a structure supported on a pot bearing floats on a low-profile hydraulic cylinder, or pot, in which the liquid medium is an elastomer.
To facilitate rotation of the elastomeric rotational element, either PTFE sheets are attached to the top and bottom of the elastomeric disk or the element is lubricated with a material compatible with the elastomer. To permit longitudinal or transverse movements, the upper surface of the steel piston is faced with a PTFE sheet and supports a steel sliding-top bearing plate. The mating surface of that plate is faced with polished stainless steel.
Pot bearings have low resistance to bending in their plane. Consequently, a sole plate, beveled if necessary, should be provided on top of the bearing and a masonry plate should be installed on the bottom. A member should not be supported on both a pot bearing and a bearing with different properties.
To ensure contact between the piston and the elastomer, minimum load should be at least 20% of the design vertical load capacity.
Pedestals and shoes, if required, usually are made of cast steel or structural steel. Design should be based on the assumption that the vertical load is uniformly distributed over the entire bearing surface. The difference in width or length between top and bottom bearing surfaces should not exceed twice the vertical distance between them. For hinged bearings, this distance should be measured from the center of the pin.
AASHTO recommends that the web plates and angles connecting built-up pedestals and shoes to the base plate should be at least 5⁄8 in thick.
If pedestal size permits, webs should be rigidly connected transversely to ensure stability of the components. Webs and pinholes in them should be arranged to keep eccentricity to a minimum. The net section through a pinhole should provide at least 140% of the net area required for the stress transmitted through the pedestal or shoe. All parts of pedestals and shoes should be prevented from lateral movement on the pins.
Nuts with washers should be used to hold pins in place. Length of pins should be adequate for full bearing.
Anchor bolts subject to tension should be designed to engage a mass of masonry that will provide resistance to uplift equal to 150% of the calculated uplift due to service loads or 100% of loading combinations for which live load plus impact is increased 100%, whichever is larger. The bolts, however, may be designed for 150% of the basic allowable stress.
Resistance to pullout of anchor bolts may be obtained by use of swage bolts or by placing on each embedded end of a bolt a nut and washer or plate. Minimum requirements for number of bolts for each bearing, diameter, and embedment are given in Table 11.26 for ASD and LRFD. The LRFD specifications does not set minimums.