BRIDGE ELASTOMERIC BEARINGS
Elastomeric bearings, primarily made from rubber compounds reinforced with steel plates, were introduced in the mid-20th century as a solution to manage movement and load distribution in bridges. These bearings allow for controlled movement and rotation, forces generated by thermal expansion, traffic loads, and structural deflections. Initially, natural rubber was widely used, but over time, synthetic alternatives like neoprene became preferred due to their improved durability and resistance to environmental factors like ozone degradation.
The use of elastomeric bearings revolutionized bridge bearing design by offering a cost-effective and low-maintenance alternative to earlier systems, such as roller or sliding bearings. Despite these advancements, several issues related to material degradation, installation tolerances, and performance under varying loads and temperatures have surfaced over time.
HOW TO USE ELASTOMERIC BEARINGS IN BRIDGES
Elastomeric bearings are typically placed between the superstructure such as a girder or deck, and the substructure like piers or abutments of the bridge. Their role is to accommodate:
When properly designed and installed, these bearings provide a long service life with minimal maintenance. They are typically designed to handle specific shear deformations and compressive forces, depending on the span and anticipated movement of the bridge.
REASONS FOR CONSTRUCTION ISSUES
Several factors have contributed to the construction issues observed in elastomeric bearings:
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Material quality and wax migration: Historically, the use of natural rubber bearings caused issues with wax migration. This wax, designed to prevent ozone degradation, would sometimes coat the bearing surface, significantly reducing the friction and causing the bearings to slide. This issue has been addressed in modern bearings by switching to neoprene or using different anti-ozonants.
Contact surface deviations: Irregularities in the concrete pedestals or the girder surfaces where the bearing is placed can cause uneven pressure distribution. This often leads to a situation where the bearing cannot maintain proper contact with both surfaces, increasing the likelihood of slippage or reduced performance.
Inclined bearing placement: When the bearing is placed at an incline, due to poorly aligned girders or piers or inclination exceed allowable values, additional shear forces act on the bearing. These forces can deform the elastomeric layers and steel plates inside the bearing, leading to excessive movement or slippage, which compromises the structure's stability.
"WALK-OUT" OF BEARINGS
The "walk-out" phenomenon occurs when the elastomeric bearing shifts from its intended position due to forces exceeding its frictional capacity. This is often caused by construction inaccuracies like uneven or inclined surfaces, and excessive horizontal forces beyond what the bearing was designed to accommodate. “Walk out” may happen even without material issues such as wax content or poor rubber quality.
In cases where the difference between the contacting surfaces exceeds the allowable contraction or deformation of the bearing, the bearing may lose contact with surface, resulting in partial or complete displacement. This phenomenon is more pronounced in cases when the bridge undergoes large thermal expansions or contractions. In such cases, the bearing "walks out," leading to severe structural issues if not addressed promptly.
PREFACE FOR CASE STUDIES
Ongoing challenges with installation precision, and structural behavior under construction methods and thermal loads highlight the need for strict quality control during installation. Previous material issues like wax migration in natural rubber and surface deviations are being mitigated with improved materials and design standards. By ensuring that the bearing is correctly aligned and fully contacting both surfaces, the risk of "walk out" can be minimized, ensuring the long-term functionality of the structure.
Civil Engineer - Caucho Industrial Verdú
5 个月Thank you for sharing this detailed and informative post on elastomeric bearings. It provides an excellent overview of the evolution of these crucial components in bridge design. Your insights into the importance of proper installation practices will be invaluable to those of us working in this field. It’s also quite common to see in some structures how the loads have ejected the bearings. At Caucho Industrial Verdú, in some cases, we opt for reinforced bearings, with checkered plates. Since the exact load requirements cannot always be precisely defined, these bearings are best suited for structures with minor loads.
Very interesting article. These issues have to be taken seriously before a catastrophe occurs. It is extremely important that the bearings are handled appropriately.
Construction Supervisor
6 个月2024 SEP 22