What is the bearing capacity of spherical bearings for railway rail transit bridges
The bearing capacity of spherical bearings for railway rail transit bridges is its core performance indicator, which needs to be strictly matched with complex working conditions such as bridge self weight, train load (including dynamic load), temperature deformation, earthquake action, etc. The bearing capacity ranges from hundreds to thousands of tons, depending on the support type, structural de
The bearing capacity of spherical bearings for railway rail transit bridges is its core performance indicator, which needs to be strictly matched with complex working conditions such as bridge self weight, train load (including dynamic load), temperature deformation, earthquake action, etc. The bearing capacity ranges from hundreds to thousands of tons, depending on the support type, structural design, and application scenario (such as conventional railway, high-speed railway, heavy-duty railway bridges). The design, grading, and guarantee logic of its bearing capacity need to be analyzed in conjunction with the special requirements of railway bridges (safety, stability, durability):
1、 Classification criteria for carrying capacity: divided by railway type and bridge scale
The bearing capacity of railway bridge spherical bearings is not a uniform standard, but is divided into different levels according to industry standards such as "Railway Bridge Spherical Bearings" (TB/T 3320-2018), based on the "design vertical bearing capacity", to adapt to the load requirements of different railway scenarios.
2、 The 'design logic' of carrying capacity: balancing three core loads is required
The bearing capacity design of spherical bearings for railway bridges does not only consider "vertical load-bearing", but also needs to simultaneously resist three types of loads: vertical load, horizontal load, and angular deformation, to ensure that they do not fail under complex working conditions. The specific design focuses are as follows:
1. Vertical bearing: The core is to "bear the self weight and train weight"
Vertical load is the main force on the support, accounting for 70% -80% of the total bearing demand. During design, two key loads need to be covered:
Load: The structural weight of the bridge itself (such as the beam body, bridge deck pavement, railings) is a long-term constant load, accounting for 60% -70% of the vertical load (for example, a 32m high-speed railway box girder with a self weight of about 8000kN requires supports to bear at least 2000kN per support, which are shared by four supports);
Variable load: train load (including static load and dynamic load)+additional load (such as wind and snow load, bridge deck construction load), among which "dynamic load" is the key - when high-speed trains pass through at high speed (350km/h), impact load (dynamic load coefficient 1.3-1.4) will be generated, and the support needs to bear an additional 30% -40% of the dynamic load (for example, a support with a static load of 1000kN needs to be able to withstand a dynamic load of 1300-1400kN).
Standard requirement: The vertical bearing capacity of the support design should be ≥ 1.2 times the sum of "load+variable load", with a safety margin reserved to avoid fatigue damage during long-term use.
2. Horizontal bearing: resist "lateral thrust and seismic force"
Railway bridges may experience horizontal loads due to temperature changes (thermal expansion and contraction of the beam body), centrifugal force of trains (curved bridges), and seismic effects. Spherical bearings need to have sufficient horizontal bearing capacity to prevent bearing sliding or damage:
Temperature and centrifugal force horizontal load: The horizontal load of a conventional railway bridge is about 5% -10% of the vertical load, while the horizontal load of a high-speed railway can reach 10% -15% of the vertical load due to the large centrifugal force of the train (the smaller the curve radius, the greater the centrifugal force) (for example, a high-speed railway bearing with a vertical load of 5000kN needs to withstand a horizontal force of 500-750kN);
Earthquake horizontal load: For bridges located in high earthquake prone areas (such as Southwest and North China), the bearings should be designed with a horizontal shear capacity according to the "earthquake intensity" - in an 8-degree earthquake zone, the horizontal bearing capacity should be ≥ 20% of the vertical load (such as a vertical 5000kN bearing with a horizontal shear capacity ≥ 1000kN) to avoid beam displacement and bearing detachment during earthquakes.
Design guarantee: A "horizontal limit device" (such as PTFE sliding plate+stainless steel plate, controlling horizontal displacement ≤ 50mm) is installed inside the support, and "steel component strength design" (such as Q355 steel for the upper and lower seat plates of the support, with tensile strength ≥ 470MPa) is used to resist horizontal forces.
3. Corner bearing: Adapt to "beam deformation and avoid stress concentration"
Under load, bridges will produce small turning angles (such as when the beam is bent, both ends will bend downwards, producing a turning angle of 0.001-0.005rad). Spherical bearings adapt to the turning angle through a "spherical sliding pair" (the spherical surface of the upper bearing plate+the spherical cap pad of the lower bearing plate), avoiding excessive local stress on the bearings:
The core of the corner bearing capacity is the "spherical contact area" - the larger the contact area, the smaller the force per unit area (pressure ≤ 25MPa, to avoid yielding of the spherical steel plate);
Standard requirement: The design turning angle of the support must be ≥ 1.2 times the actual turning angle of the bridge (for example, if the bridge turning angle is 0.003rad, the support must be designed to be 0.0036rad or above), ensuring that the spherical contact is uniform at the turning angle and there is no local damage caused by "point contact".
3、 Guarantee measures for carrying capacity: full process control from materials to processes
The bearing capacity of railway bridge spherical bearings is not a "theoretical value", but an "actual reliable performance" achieved through material selection, structural design, testing and certification. The core guarantee measures include:
1. Material: High strength steel is used to ensure the bearing foundation
The core load-bearing components of the support (upper/lower seat plates, ball crown pads, anchor rods) are all made of high-strength steel to ensure that the mechanical properties meet the standards:
Seat plate and ball crown liner: Made of Q355 or Q460 low-alloy high-strength steel, with yield strength ≥ 355MPa (Q355), 460MPa (Q460), tensile strength ≥ 470MPa (Q355), 550MPa (Q460), capable of withstanding tension, compression, and shear under high loads;
Anchor rod (fixed support and pier): Made of 40Cr alloy structural steel, quenched and tempered (hardness HRC28-32), with a tensile strength of ≥ 980MPa, ensuring a firm connection between the support and pier without being pulled out.
2. Structure: Optimize the design to disperse loads
By designing structural details to avoid load concentration and improve overall load-bearing capacity:
Spherical contact optimization: The ball crown liner adopts "medium carbon steel+surface quenching" (hardness HRC50-55), with a spherical roughness of ≤ 0.8 μ m, reducing sliding friction resistance while ensuring uniform contact area (contact pressure ≤ design value);
Strengthening rib design: Strengthening ribs (thickness ≥ 12mm) are set around the base plate to enhance its bending stiffness and avoid deformation under vertical loads (deformation amount ≤ L/500, where L is the side length of the base plate);
Sealing and anti-corrosion: Rubber sealing rings are installed on the outside of the support, and lubricating grease is filled inside to prevent rainwater and dust from entering the spherical pair, avoiding steel corrosion and reducing bearing capacity (design service life ≥ 50 years).
3. Testing: 100% load test before leaving the factory
According to the TB/T 3320-2018 standard, each ball shaped support must undergo a "bearing capacity test" before leaving the factory, and can only be shipped after passing the test:
Vertical bearing capacity test: Apply 1.5 times the design vertical bearing capacity, hold the load for 1 hour, and the support has no obvious deformation or cracks (deformation amount ≤ 0.1mm);
Horizontal bearing capacity test: Apply 1.2 times the design horizontal bearing capacity, hold the load for 1 hour, horizontal displacement ≤ the design allowable value, no sliding or abnormal noise;
Corner test: Apply 1.5 times the design corner, hold the load for 1 hour, the spherical contact is uniform, and there is no local crushing.
4、 Typical application case: Actual matching of carrying capacity
High speed railway simply supported beam bridge: 32m high-speed railway box girder (with a self weight of about 8000kN), using 4 "QZ-2000 spherical supports" (designed vertical bearing capacity of 2000kN/each), with a total vertical bearing capacity of 8000kN, just matching the self weight of the beam body+train load (about 1000kN), with a reserved safety factor of 1.2 times;
Heavy load railway bridge: Daqin Railway Heavy load bridge (axle load 30t, train static load about 1200kN/line), using "QZ-5000 ball bearings" (vertical bearing 5000kN, horizontal bearing 500kN), suitable for the impact load and horizontal thrust of heavy load trains;
Large span high-speed railway cable-stayed bridge: A 100m span cable-stayed bridge (with a beam weight of approximately 20000kN) is equipped with two "QZ-10000 spherical supports" (vertically bearing 10000kN/each), and has a 0.005rad turning capacity to adapt to the bending deformation and temperature displacement of the beam.
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