Maintenance and repair of bridge bearings

Bridge bearings play a crucial role in the stress, displacement, and rotation of the bridge superstructure, directly impacting the overall stability, durability, and safety of the bridge. This paper analyzes common bridge bearing defects and their causes, and proposes maintenance and repair methods.

Bridge bearings are an essential component of the bridge structure, directly affecting its structural safety and durability. In my country, common bridge bearings include: simple bearings (such as asphalt felt), sliding plate rubber bearings, plate rubber bearings, and pot bearings. The functions of bearings are threefold: first, they act as force transmission devices connecting the superstructure and substructure, reliably transferring various loads from the superstructure to the substructure while ensuring free rotation or displacement of the superstructure at the bearing, allowing the beam to undergo necessary free deformation; second, they accommodate structural expansion and contraction caused by environmental changes such as temperature and humidity; and third, they resist structural translation caused by wind and earthquakes, mitigating the adverse effects of vibration on the structure. Damage to bridge bearings has become a significant threat to bridge safety, making the analysis and maintenance of bridge bearing defects particularly important.

I. Common Bridge Bearing Defects

1.1 Incorrect Bearing Type

Common errors in bearing type include: confusion regarding bearing dimensions, mixing up circular and rectangular bearings, and incorrect placement of rectangular bearings along their length.

1.2 Bearing Misalignment

Bearing misalignment is currently the most prevalent problem in bearing installation. It is divided into longitudinal and lateral misalignment. Severe bearing misalignment will cause uneven stress on the bearing and excessive additional internal forces on the beam, among other defects.

1.3 Bearing Damage and Cracks

Bearing damage and cracking include issues such as cracked or extruded asphalt felt in bridges, and displacement of rubber plates; aging and deterioration of rubber in rubber plate bearings, leading to loss of free expansion and contraction capacity of the beam, directly causing cracking of the concrete at beam ends or pier/abutment caps, resulting in chipped corners and edges, and in severe cases, damage to expansion joints; loosening and misalignment at the fixing points of pot bearings, corrosion of the exposed steel pot, and cracked dust covers.

1.4 Bearing Voiding

Bearing voiding is one of the most common and serious quality problems in bearing installation. Voiding of a single bearing will cause excessive stress on other bearings, affecting their durability. Furthermore, it may cause uneven stress on the superstructure, affecting structural stability and safety.

1.5 Excessive Support Deformation

Support deformation refers to compression deformation and shear deformation. Excessive compression deformation of supports can be divided into two types: localized excessive deformation of the support, i.e., eccentric pressure on the support, which leads to support failure; and excessive overall vertical deformation of the support, which may generate extremely unfavorable additional internal forces on the superstructure such as continuous beams, or displacement exceeding the design control range, leading to structural damage; the greater the shear deformation of the support, the worse its durability, thus reducing the service life of the support.

1.6 Other Defects

(1) Unevenness on the side of the plate rubber bearing; (2) Dirt and aging of PTFE sliding bearings, the separation of the PTFE plate from the rubber layer causes the surface sliding coefficient to not meet the requirements, resulting in excessive shear deformation, which aggravates the damage to the rubber layer of the support, reduces the service life of the support, and also increases the constraint on the beam, which has an adverse effect on the structural stress; (3) Uneven and non-close contact between the pot bearing and the pad; (4) Unevenness, cracking, peeling, and breakage of the bearing pad stone, etc.

II. Common Causes of Bridge Bearing Defects

The main causes of bridge bearing defects are as follows:

(1) The bearing itself is a substandard product with low strength and insufficient bearing capacity;

(2) Design issues: Insufficient understanding of bearing performance; failure to verify the appropriate bearing size in the design, leading to premature failure of the bearing due to insufficient compressive and shear strength; unreasonable selection and arrangement of bridge bearings; insufficient reserved width at the bearing edge; low concrete grade of the bearing pad or insufficient reinforcing steel in the pad; insufficient strength of the bolts and nuts used for fixing; improper adjustment measures when the beam has a large longitudinal slope, resulting in the bearing not fitting tightly to the beam and causing damage due to improper stress;

(3) Construction issues: Insufficient awareness of the importance of bridge bearings; lack of attention to them; failure to prevent bearing detachment during hoisting processes. The technical measures and quality control methods are not strict, the construction process is not strictly controlled, the bearing pad and beam top surface are not level or the top surface of the pad is not horizontal, the bearing is not accurately positioned, the connection between the pad and the bearing is not firm, the anti-corrosion and anti-rust treatment of metal bearings is not of high quality, when the beam has longitudinal and transverse slopes, the bearing upper bearing surface (beam bottom slope adjustment wedge block) is not ideally placed, causing the bearing to have eccentric pressure, initial shear deformation and other phenomena;

(4) In terms of maintenance and repair: the sliding surface and rolling surface are not clean, foreign objects are not cleaned in time, the fixed components are loose and not reinforced in time, the bearing or connection surface is immersed in water and corroded due to defects in the waterproof device, which accelerates the aging process, etc.;

(5) Uneven settlement, tilting and horizontal displacement of the superstructure and vibration displacement of the superstructure directly affect the normal use of bridge bearings;

(6) Overloading of vehicles, the actual load exceeds the design load, causing irreversible deformation and damage to the bearings.

III. Maintenance and Repair of Bridge Bearings

3.1 Inspection of Bearings To ensure the safe, effective, and normal use of bridge bearings, it is essential to strengthen the inspection of bridge bearings. The inspection mainly includes the following: Contents:

(1) Check if the support components are intact and clean, and whether there are any breaks, misalignments, or gaps;

(2) Check if the movable support is flexible, whether the actual displacement is normal, and whether the anchor pins of the fixed support are intact;

(3) Check if the support pad has cracks;

(4) Check if the asphalt felt of the simple support is aged, cracked, or ineffective;

(5) Check if the shear displacement of the support is too large;

(6) Check if the support has excessive compression deformation;

(7) Check if the rubber protective layer of the support shows signs of aging such as cracking or hardening, and record the location, width, and length of the cracks;

(8) Check if the rubber plate protrusion between the stiffening steel plates of each layer of the support is uniform and normal;

(9) For PTFE sliding plate rubber supports, check if the support is dirty or aged, whether the upper PTFE sliding plate is intact, and whether the support is... The stainless steel plate on the top surface of the support has slid out;

(10) Check whether the fixing bolts of the pot bearing are cut off, whether the nuts are loose, whether the exposed part of the steel pot is corroded, and whether the dust cover is intact.

3.2 Daily Maintenance of Bridge Bearings

The daily maintenance of bridge bearings mainly includes:

(1) All parts of the bearing should be kept intact and clean. They should be cleaned at least once every six months to remove oil stains and garbage around the bearing, prevent water and snow accumulation, and ensure the bearing functions normally.

(2) The rolling surfaces of rolling bearings should be lubricated regularly (generally once a year). Before applying oil, the rolling surfaces should be wiped clean.

(3) Steel bearings should be derusted and protected against corrosion. Except for the hinge shaft and rolling surfaces, all other parts of the bearing should be painted for protection.

(4) For fixed bearings, the anchor bolts should be checked for firmness. The support pads should be flat and tight, and all connecting bolts should be tightened promptly.

(5) Various rubber bearings should be frequently cleaned of wastewater, and water accumulated on piers and abutments should be removed. Rubber bearings should be prevented from contacting grease. Residual machine oil on the beam bottom and pier/abutment caps should be cleaned to prevent the rubber from aging and deteriorating, thus losing its function. (6) Pot bearings should be cleaned regularly and dust covers should be installed to prevent dust from falling in or rain and snow from seeping into the bearings. The exposed parts of the bearings should be coated with red lead anti-rust paint regularly for protection.

3.3 Maintenance of Bearings

If a bearing exhibits the following defects or malfunctions and cannot function properly, it should be repaired or replaced promptly:

(1) If the bearing plate is warped, deformed, or broken, it should be replaced; if the weld is cracked, it should be repaired.

(2) If a plate rubber bearing experiences voiding or uneven compression deformation, it should be adjusted.

(3) If a plate rubber bearing experiences excessive shear deformation, exposed intermediate steel plate, rubber cracking, or aging, it should be replaced promptly.

(4) If an asphalt felt pad bearing loses its function, it should be replaced promptly.

(5) Damaged bearing pad stones should be repaired promptly, ensuring their strength meets design requirements.

When adjusting or replacing plate rubber bearings, steel plate bearings, or asphalt felt pad bearings, the following method can be used: Install jacks at the bottom or end of the beam next to the bearing to appropriately lift the beam (plate), allowing the bearing to be free from stress, and then adjust or replace it. After adjustment or placement of the new bearing, lower the beam (plate) to its operating position. When it is necessary to raise the bearing, the following methods can be selected according to the amount of raising:

(1) Insert steel plates (within 50mm) or cast steel plates (50-100mm);

(2) Cast reinforced concrete bearing pads in situ. The height of the pads should be set as needed, generally greater than 100mm.

The main function of bridge bearings is to transfer the dead load and live load reaction forces on the bridge span structure to the bridge piers. At the same time, they ensure the required displacement and rotation of the bridge span structure so that the actual stress condition of the structure matches the calculation diagram used in the design. Therefore, ensuring the normal use and integrity of the bearings is very important. It is essential to strengthen the maintenance and repair of bridge bearings to avoid premature damage, ensure bridge safety, and extend the service life of the bridge.