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Gerardo
Student
Laredo, Texas
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Under what circumstances should a civil engineer use bridge bearings? Why are they so indispensable?
This might be one of the most intriguing questions that I have ever had about my career. Every day I pass by enormous bridges and I can't think but visualize how bridges can withstand great amounts of forces. Essentially, a bridge bearing is a component of a bridge that provides a resting surface between the bridge deck and the bridge piers. I would like to know a bit more as to why they are critical in complex bridge. #college #engineering #engineer #civil-engineering #bridge
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Brett LeGrow
Project Engineer/Office Manager at Huitt-Zollars, Inc.
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Brett’s Answer
To put it simply, it allows for controlled movement. This reduces the stresses between the bridge and it's substructures. Think of 2 pieces of concrete grinding against each other. Eventually, they wear out at the point of contact. With bearings, this doesn't happen.
Spruce Cox
Retired Aerospace Engineer and Rocket Scientist
76
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Spruce’s Answer
A bridge structure is always under varying load conditions caused by one or more of the following: construction loads (usually from one-time events during structure assembly), dead loads (from supporting the bridge itself), live loads (from weight and motion of traffic on the bridge), and environmental loads (from wind, rain, sun, lightning, etc). All loads are integrated and the worst case static and dynamic load conditions are analyzed against the planned bridge structure, plus factors of safety. After evaluation, a final bridge structural design is defined.
Bearings are used at any joint where the worst case load conditions cause stresses that exceed structural capabilities. These locations are identified during the design process above, and design trade-off analyses help determine the most practical solution between strengthening the structure or incorporating bearings or other load alleviation solutions. Design decisions are made for each such location and the solutions are incorporated into the bridge structure prior to final design.
Bearings do not directly alleviate loads. Locations where loads exceed limitations must be redesigned and allow specific movements of the structure to remove a loading condition from structure. Bearings are selected based on the structural element involved and the direction and magnitude of the movements.
Common bearings used in bridges today are described here, along with the type of movements for which they are designed: (1) pin bearings allow rotation of a structure around the axis of the pin, (2) rocker bearings allow small movements of a structure along the axis of the rocking motion, (3) roller bearings allow large movements of a structure along the axis of the rolling surface, (4) one type of laminated bearing has multiple sheets of stainless steel – maybe one foot square – with same size sheets of a particular hard, smooth plastic that allow small movements of a structure along an axis parallel to the bearing surface but do not allow downward movement perpendicular to the bearing surface, (5) spherical bearings allow small movements along or around any axis tangential to the surface of the sphere.
Bearings are used at any joint where the worst case load conditions cause stresses that exceed structural capabilities. These locations are identified during the design process above, and design trade-off analyses help determine the most practical solution between strengthening the structure or incorporating bearings or other load alleviation solutions. Design decisions are made for each such location and the solutions are incorporated into the bridge structure prior to final design.
Bearings do not directly alleviate loads. Locations where loads exceed limitations must be redesigned and allow specific movements of the structure to remove a loading condition from structure. Bearings are selected based on the structural element involved and the direction and magnitude of the movements.
Common bearings used in bridges today are described here, along with the type of movements for which they are designed: (1) pin bearings allow rotation of a structure around the axis of the pin, (2) rocker bearings allow small movements of a structure along the axis of the rocking motion, (3) roller bearings allow large movements of a structure along the axis of the rolling surface, (4) one type of laminated bearing has multiple sheets of stainless steel – maybe one foot square – with same size sheets of a particular hard, smooth plastic that allow small movements of a structure along an axis parallel to the bearing surface but do not allow downward movement perpendicular to the bearing surface, (5) spherical bearings allow small movements along or around any axis tangential to the surface of the sphere.