Common Problems of Spherical Bridge Bearings

Foshan West Railway Station, a comprehensive transportation hub, integrates passenger dedicated line(PDL), inter-city railways, subways, buses and other transportation. The station adopts elevated structure on the whole, and the east and west of throat area, station center area utilizes the bridge and the framework structure. There are totally 344 simple beams and 59 continuous beams, covering single-box bridge , double-box and single-room bridge, single-box and double-room bridge and many other simple beam and a variety of common continuous beam and switch continuous beam. 

Due to the complexity of the bridge structure and the special restrictions of the east and west throat areas, all the bridges are cast-in-site by bracket methods. In order to meet the special requirements of the bridge structure in Foshan West Railway Station, all the bridges of this station adopt the spherical bridge bearing of railway bridge which vertical capacity covers 11 levels from 3000-22500kN and designed allowable rotation is 0.02 rad. 

Thanks AOCOMM for providing such high-quality, high cost-effective spherical bridge bearings. And AOCOMM can actually meet our detailed requirements that the spherical bridge can be delivered by multi-batch which is really convenient and arrives timely for our construction process.

2.1.Common problems of bridge bearings

2.1.1.The thickness of grouting layer can not meet the requirements.

Problems Analysis: According to the requirements of the inspection, the allowable error of the elevation on the top surface of the cushion stone is (-10,0) mm. The deviation beyond standard is led by the lack of the supervision. The elevation of pad stone is too low to cause the thickness of grouting layer beyond standard, and too high to cause the insufficiency of the grouting layer. Because the grouting layer is a kind of rigid compressive material, which thickness beyond standard will lead to cracking due to excessive stress, severely exceeding the standard even gore the upper plate of the bearing to death. That thing will limit free movement to the bearing and girder producing harmful additional internal forces. On the contrary, the insufficient thickness of the grouting layer causes the difficult grouting of the bearing, and the cavity is easily formed between the lower plate and the bearing pad stone resulting in uneven bearing force. In severe cases, the bearing or grouting layer may be damaged. 

2.1.2.The grouting layer is not solid.

Problems Analysis: Insufficient thickness of grouting layer leads to the difficult grouting, easily resulting in less dense of the upper plate of bearing. Bearing mortaring is not in strict accordance with the ratio of preparation or mixing is not sufficient, resulting in poor fluidity of grouting which is easily leading to the the cavity at the bottom of the lower plate of bearing. The less dense or cavity of the grouting layer will lack stress on the contact area to the lower plate or will not be uniformly stressed that both conditions will affect the force(as shown in Figure 1). In severe cases, the grouting layer will crush the lower plate and then causing deformation and harmful additional internal forces on the beam and bearing.

Figure 1 The deformation of bearing

Problems Analysis: Site management is insufficient. Before the tension, the coupling bolts is not removed in accordance with the requirements, therefore the bearing would limit the deformation of the beam(including the plane displacement and the rotation) an then affect the stress on the beam and bearing during the process of tension and operation. In the severe conditions, this kind of appearance would even cause the deformation of the bearing components.

2.1.4.Bearing installation is uneven.

Problems analysis: Without leveling the top of the bearing, during the grouting the uneven load of bearings causes irregular surface. The strength of the grouting materials are not in accordance with the requirements, that is to say, the bearing support is removed before the grouting material is unable to withstand the weight of itself. The uneven installation of the bearing will lead to the non-parallel contact surface among the moving surface of the beam, the spherical cap liner of the bearing and the upper plate of bearing, so the moving of beam will produce the additional horizontal force.

2.1.5.Upper plate of bearing tilts.(mainly refers to the secondary tilt after installation)

Problems Analysis: Due to the intensive steels, the concrete pouring on the beam easily leads to the less dense of the concrete. After completing tension of the beam to remove the bracket, the beam gradually transferred the weight to the bearing. As the contact surface of the bearing is less dense, it gives rise to the uneven bearing force which caused the tilt of  bearing upper plate.

Bearing is one of the most important components of the bridge structure. In the cast-in-place construction of the bridge, it is easy to cause various defects of the spherical bearing due to the long construction period and the lack of supervision. Only strengthening the management of the construction process is the fundamental way to avoid the related problems. 

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