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6.5.8 Bearing Pads

Bearing pads are used to distribute concentrated loads and reactions over the bearing areaand to allow limited horizontal and rotational movements to provide stress relief. Their use hasproven bene cial and often may be necessary for satisfactory performance of precastconcrete structures.

Several materials are commonly used for bearing pads:

1. AAS T!"grade chloroprene pads are made with 1##$ chloroprene%neoprene& as the only elastomer and conform to the re'uirements of theAAS T! Standard Speci cations for ighway Bridges %1(()&* Sect. 1+. ,nertllers are used with the chloroprene and the resulting pad is blac- in color andof a smooth uniform te ture. /hile allowable compressive stresses aresomewhat lower than other pad types* these pads allow the greatest freedom inmovement at the bearing. 0ote: chloroprene pads* which do not meet theAAS T! Speci cations* are not recommended for use in precast concretestructures.

. 2ads reinforced with randomly oriented bers have been used successfully formany years. These pads are usually blac-* and the short reinforcing bers areclearly visible. 3ertical load capacity is increased by the reinforcement* butcapability of rotations and horizontal movement is somewhat less thanchloroprene pads. Some random oriented ber pads possess di4erent propertiesin di4erent directions in the plane of the pad. Therefore* unless proper planningand care is used in their installation* it may be prudent to specify those padsthat have been tested to e hibit similar properties in di4erent directions. Thereare no national standard speci cations for this material. 5anufacturers havedeveloped appropriate design and performance documentation.

6. 7otton duc- fabric reinforced pads are generally used where a higher

compressive strength is desired. These pads are often yellow"orange in colorand are reinforced with closely spaced* horizontal layers of fabric* bonded in theelastomer. The horizontal reinforcement layers are easily observed at the edgeof the pad. Sect. 1+.1#. of the AAS T! Standard Speci cations for Bridges and5ilitary Speci cation 5,8"7"++ 9 discuss this material.

. 7hloroprene pads laminated with alternate layers of bonded steel orberglass are often used in bridges* but seldom in building construction. Theabove mentioned AAS T! Speci cations cover these pads.

;. A multimonomer plastic bearing strip is manufactured e pressly for bearingpurposes. ,t is a commonly used material for the bearing support of hollow"coreslabs* and is highly suitable for this application. ,t is also often used for bearing

of architectural precast concrete cladding panels.

). Tempered hardboard strips are also used with hollow"core slabs. These padsshould be used with caution under moist conditions. ,n addition to progressivedeterioration of the pad* staining of the precast concrete unit may occur.

%trade name Te?on& coated materials are often used in bearing areaswhen large horizontal movements are anticipated* for e ample at @slip@ oints ore pansion oints. The T=> is normally reinforced by bonding to an appropriate

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6.5.8.1 Design eco!!endations

Bearing pads provide stress relief due to a combination of slippage and pad deformation. ,nelastomeric bearing pads %1 through above&* research C16D has shown that slippage is themore signi cant factor. This research has also shown that the ratio of shear to compressivestress on the pad reduces signi cantly under slow cyclic movements* such as those producedby temperature variations. =ollowing are recommendations which* along with =igures ).;.16and ).;.1 * can be used to select bearing pads.

1. Ese unfactored service loads for design.

. At the suggested ma imum uniform compressive stress* instantaneousvertical strains of 1# to #$ can be e pected. This number may double if thebearing surfaces are not parallel. ,n addition* the time"dependent creep willtypically increase the instantaneous strains by ; to 1##$* depending on themagnitude of sustained dead load.

6. =or stability of the pad* the length and width of unreinforced pads should beat least ve times the thic-ness.

. A minimum thic-ness of >FG in. is recommended for all precast memberse cept slabs.

;. =igure ).;.1 may be used to estimate shear resistance of chloroprene*random ber reinforced and cotton duc- pads.

). The portion of pad outside of the covered bearing surface as well as theportion which is not under load because of rotation of member should beignored in calculating shape factors* pad stresses* stability and movements.

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Figure 6.5.1% #ingle layer bearing pads free to slip

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