Bearing Replacement and Associated Superstructure Assessment (Eurocode)

Introduction

Bridge bearings are important structural components of bridges. The main purpose of bridge bearings is to connect the deck to the substructure. Some of the most important functions of bridge bearings are listed below.

• Help in dissipation and transfer of dynamic forces to the substructure without causing any wear or damage
• Allow translational and rotational movement of the superstructure and avoid the generation of secondary stresses in the structure.
• Bearings play an important role in the adjustment of the dynamic properties of bridges (eg. friction pendulum bearings).
• Bearings also reduce shear demand on the head of the piers or abutments.

Inspection and Maintenance Needs of Bridge Bearings

In most cases, the life span of the bridge bearings is less than that of bridges. There can be inconsistencies during the installation, which can cause the failure of the bearings even before their life span. Failure of the bridge bearings could lead to catastrophic failures of the bridge. Even though the cost of installing the bearings in good conditions may be high, it excessively small compared to the cost of failure of a bridge structure.

Superstructure assessment:

For PSC bridges, BD 21/01 was used earlier for the assessment of the deck during jacking. However, BD 21/01 was superseded by CS 454, which was released in June 2019 and revised in March 2020. CS 454 includes wind, thermal, and HB load models in the new appendix, previously in BD 37. It also has traffic load models for loaded lengths greater than 50 m, previously in BD 50. The basic assessment process for CS 454 is shown below. Assessment of the PSC bridge can be performed in midas Civil asper CS 454. Detailed assessment verification reports can also be generated in midas Civil. Assessment live loads to be considered are ALL Model 1 and ALL Model 2:

ALL Model 1:

• Based on real vehicle with authorized weight
• Single vehicle or convoy of vehicles
• Suitable for all structures
  

ALL Model 2:

• UDL +KEL, based on Type HA from BD 21
• Not suitable for certain situations (cl.5.6)
  • Structures with transversely spanning trough decks
  • Structures with longitudinal members at the centers of 2.5m or less with low transverse distribution
  • Structures with loaded length less than 2m
    
     

Fig 1: Process flow for assessment of bridge as per CS 454

Table 2: Typical bearing schedule

There are a variety of bearings that has different properties and can accommodate a wide spectrum of vertical and horizontal loadings, deformation, and rotations. BSEN 1337- 1 has the most common bearings defined. A part of the table is shown below.

Definition of Bearings:

Depending on the type of bearing used for the bridge, it can be simulated in midas civil using the elastic link or the general link. For example, if an elastomeric bearing is used for a particular bridge, then it can be modeled as an elastic link in the analysis model. The rotational and translational stiffness of the bearing should be used as the stiffness of the elastic link:

Fig 3: Simulation of elastomeric bearing as elastic link in midas civil

Similarly, other types of bearing (pot PTFE, Pin-roller) can also be defined for analysis using elastic/general links.

Considerations for Bearing Schedule:

Design loads:

The design load section establishes a spectrum of loading from minimum to maximum. Here:

• Maximum vertical load is the sum of all vertical loading applied to the support.

• Minimum vertical is normally permanent loading added to negative loading because of uplift.

• Transverse loading- skidding loading (to BD 37/01 or BS EN 1991-2)

• Longitudinal loading- braking, skidding to temperature-induced expansion and contraction (to BD37/01 or BS EN 1991-5)

A detailed analysis should be carried out in midas civil to determine the maximum design load in accordance with the above-mentioned design standards.

Displacements:

1. Displacement of the bearing is calculated from the point of fixity.

• Coefficient of thermal expansion and contraction as per BS 5400-2, cl 5.4.6

Rotations:

• Displacement of the bearing is calculated from the point of fixity.

Maximum bearing dimension:

• Maximum bearing dimensions depends on space available on bearing shelf. Allowance should also be made for any supporting steel plates and minimum spacing from the front or side edge of the bearing shelf.

Conclusion:

Bridge bearings are an important component of bridges. Failure of bearings can lead to catastrophic failure of the structure. Hence proper maintenance and restoration of the bearing need to be done from time to time. For replacement of bearing, assessment of the deck needs to be carried out to find the design forces, deformations, and rotations requirements of the new bearing system. This can be easily carried out in midas civil due to its versatility in the creation of complex models. CS 454, BD 37/01, Eurocodes are readily available in midas civil which makes the process more convenient.

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