Are there any pitfalls in the selection of plate bearings, bowl bearings, and ball bearings? How to clarify the load limit and applicable scenario boundaries?

In the field of bridge engineering, the selection of bridge bearings is crucial, as it is like the "joints" of a bridge, directly affecting the stability, durability, and safety of the bridge structure. However, in practical engineering, there are many misconceptions about the selection of plate, bowl, and ball bearings, especially in the judgment of load limits and applicable scenario boundaries. A slight mistake may pose a safety hazard to the bridge. Next, we will delve into the misconceptions that these three types of supports are prone to in these areas.

Plate type support: the "exclusive" for small spans and light loads, but where is the boundary?

Misunderstanding of Load Limit Values

Plate bearings are usually perceived as suitable for small load scenarios. Taking the common GYZ type circular plate rubber bearing as an example, a standard specification (such as a diameter of 300mm and a thickness of 54mm) bearing can achieve a vertical compressive bearing capacity of over 1000kN (about 100 tons). By adjusting the thickness of the rubber layer and the number of steel plate layers, it can adapt to the load requirements of different bridges within a certain range. However, in practical selection, some people often simply believe that as long as it is a small span bridge, a random selection of slab bearings is sufficient, while ignoring the accurate calculation of actual loads. For example, in a small highway renovation project, a 20m span simply supported beam bridge was designed with plate bearings that only considered the self weight of the upper structure of the bridge, without sufficient allowance for vehicle loads (highway - Class I/II). After opening to traffic, the supports experienced excessive deformation and even local cracking due to frequent heavy truck traffic. According to the "Plate Rubber Bearings for Highway Bridges" (JT/T 4-2019), the vertical bearing capacity of the bearings should be accurately calculated based on the self weight of the upper structure of the bridge and the vehicle load during design, and a safety reserve of 10% -20% should be reserved. This means that in practical engineering, for small span bridges with the possibility of heavy traffic, even if slab bearings are used, their bearing capacity needs to be rigorously calculated, and cannot be roughly estimated based solely on experience.

Misunderstanding of applicable scene boundaries

It is generally believed that slab bearings are suitable for small span bridges (≤ 30m), such as pedestrian overpasses in cities and simply supported beam bridges with smaller spans. But in continuous beam bridges, their applicable boundaries are often misunderstood. When the span of a continuous beam bridge is ≤ 40m, slab supports can be preferred, but not all continuous beam bridges within this span are suitable. For example, in a continuous beam bridge located in a high-intensity earthquake zone with complex geological conditions, even if the span is within 40m, if slab bearings are used, their seismic performance is relatively limited, making it difficult to meet the displacement and energy dissipation requirements of the beam body during earthquakes. In this case, more suitable types such as seismic resistant basin type bearings should be considered in conjunction with seismic design requirements. In addition, in areas with large temperature differences, if using plate bearings, natural rubber materials should be selected to adapt to environments above -25 ℃. If the local climate conditions are not taken into account when selecting, rubber materials that are not suitable for low-temperature environments are selected, and the bearings are prone to losing their normal deformation capacity due to low-temperature hardening, leading to problems such as beam cracking.

Basin type support: the "darling" of large-span heavy loads, but not omnipotent

Misunderstanding of Load Limit Values

Basin type bearings have high bearing capacity, with a vertical bearing capacity of over 50MN, and can withstand large loads generated by large-span bridges and heavy vehicles. Its vertical bearing capacity is divided into 31 levels, ranging from 0.8MN to 60MN. However, in practical applications, some people mistakenly believe that as long as it is a large-span bridge, simply selecting a high bearing capacity bowl shaped support can meet the requirements. For example, in the side span design of a large cable-stayed bridge, bowl shaped supports with high bearing capacity but mismatched displacement were selected. Although the support can withstand vertical loads, during the operation of the bridge, factors such as temperature changes and vehicle loads can cause displacement of the beam, exceeding the displacement design value of the support, resulting in severe wear of the PTFE sliding plate of the support, affecting the normal sliding function of the support, and thus having adverse effects on the stress of the bridge structure. According to the "Pot Bearings for Highway Bridges" (JT/T 391-2019), in addition to meeting the vertical bearing capacity requirements, pot bearings also need to be accurately selected based on the expected displacement of the bridge (such as different specifications such as ± 50mm in the longitudinal direction and ± 50mm in the transverse direction at 1000-3000kN), to ensure that they can meet the horizontal displacement requirements of the beam while meeting the vertical load.

Misunderstanding of applicable scene boundaries

Basin bearings are commonly used in medium span bridges (30-100m), such as continuous beam bridges on highways, urban overpasses, etc. They are also suitable for the side spans or auxiliary piers of large-span bridges (>100m). But in some special scenarios, its applicability is excessively expanded. For example, in some wide bridge designs, if ordinary bowl shaped supports are simply used, problems may arise. Due to the relatively limited rotational performance of bowl shaped supports in the transverse direction of the bridge, for wide bridges, the deformation and torsion of the beam in the transverse direction under load are relatively complex. Ordinary bowl shaped supports may not be able to adapt well to this multi-directional deformation demand, resulting in excessive local stress on the supports and phenomena such as detachment and bias. At this point, it is necessary to comprehensively consider factors such as the width, curvature, and slope of the bridge, and choose more suitable spherical bearings or make special design improvements to bowl bearings. In addition, in some space limited scenarios where there are strict restrictions on the height of supports, pot type supports may not be suitable due to their relatively large height dimensions. If forcibly selected, it will bring difficulties to the design and construction of the lower structure of the bridge, and even affect existing facilities in the surrounding area.

Spherical bearings: suitable for complex working conditions, where are the boundaries?

Misunderstanding of Load Limit Values

The vertical reaction force that spherical bearings can withstand ranges from 1000kN to 20000kN, and can adapt to large load requirements. However, in actual selection, some designers have insufficient understanding of its horizontal load bearing capacity. Although spherical bearings have good rotational performance in all directions and can adapt to curved bridges, sloping bridges, inclined bridges, wide bridges, and large-span bridges, in the design of bridges in areas with strong winds or frequent earthquakes, if only vertical loads are considered and horizontal loads are ignored (such as the horizontal load that the bearing can withstand being 10% of the vertical reaction force that the bearing can withstand), it may lead to displacement exceeding the limit or even damage of the bearing under horizontal force. For example, a large-span bridge in a coastal area did not fully consider the horizontal force generated by wind loads when designing spherical bearings due to strong sea winds. After operating for a period of time, the bearings showed significant horizontal displacement, which seriously threatened the safety of the bridge. Therefore, in the design of bridges in such areas, it is necessary to strictly follow relevant specifications, accurately calculate horizontal forces including wind loads, seismic effects, etc., and ensure that the horizontal bearing capacity of spherical bearings meets the requirements.

Misunderstanding of applicable scene boundaries

Ball bearings are particularly suitable for low-temperature areas due to the absence of rubber low-temperature brittleness and other influences. However, its applicability needs to be carefully considered in environments with high humidity and corrosive media. There are many steel components in spherical bearings, which are prone to corrosion in such harsh environments. If anti-corrosion measures are not in place, it will reduce the bearing capacity and rotational performance of the bearings. For example, in some chemical industrial parks, there are acidic gases and corrosive liquids in the surrounding environment. If ordinary spherical bearings are used without special anti-corrosion treatment or corrosion-resistant materials, the bearings may experience severe corrosion in a short period of time, affecting the service life of the bridge. In addition, in some small bridge renovation projects that have strict limitations on the installation space of bearings and require extremely high installation accuracy, spherical bearings may not be suitable due to their relatively complex structure and large installation space requirements. If forcibly adopted, it will not only be difficult to install, but also difficult to ensure installation accuracy, and cannot fully utilize the performance advantages of spherical bearings.

In the process of selecting bridge bearings, a thorough, detailed, and comprehensive analysis must be conducted on the load limits and applicable scenario boundaries of slab, bowl, and ball bearings. Designers should strictly follow relevant specifications, comprehensively consider various factors such as bridge span, load type, environmental conditions, seismic requirements, etc., avoid falling into selection errors, ensure the selection of the most suitable bearings for each bridge, and lay a solid foundation for the safe and stable operation of the bridge.