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4.2.1 Specifications

Currently RMS uses the following quality assurance specifications for design, manufacture, supply, and installation of bearings (Table 4.1):

Table 4.1: Specifications for bearings in RMS

Code Name Date of issue

B280 Unreinforced elastomeric bearing pads and strips 2006

B281 Laminated elastomeric bearings 2006

B282 Pot bearings – structural steel 2007

B283 Pot bearings – stainless steel 2005

B284 Installation of bridge bearings 2008

B280 and B281 set out the requirements for the supply of unreinforced elastomeric bearing pads and strips, and laminated elastomeric bearings, respectively. Generally they conform to AS 5100.4, except specifying some requirements in more detail. For elastomeric pads and strips, the main differences between B280 and AS 5100.4 are outlined in Table 4.2.

Table 4.2: Differences between RMS specification for elastomeric pads and strips and AS 5100.4

Item AS 5100.4 B280

Tolerances in plan dimension +4/-2 mm ±2 mm

Tolerances: thickness ±0.2 mm + 10% the design thickness ±1 mm

Parallelism of top and bottom surfaces 1.5 mm or 0.3% diameter 1 (0.5) mm for pad (strip) Number of representatives Determined by quality control and previous

test data 2 for each 25 pads, 6 per lot of strips

Tolerances on compressive stiffness Not specified ±20% the mean value of test samples Tolerances on shear stiffness ±20% the design value ±20% the values given in table B280.2 for 2

pad sizes and 4 strip sizes (Pads 350x250x25, 450x250x25; Strips: 400x65x20, 400x125x25, 400x150x25)

Acceptance criteria for pads and strips Not mentioned clearly Rejected if splitting, permanent deformation, significantly irregular or unsymmetrical surface bulging

For laminated elastomeric bearings, RMS specification B281 requires that all bearings shall be tested in compression and shear, and one per 20 identical bearings shall be tested in rotation. In addition, the tolerance in compressive stiffness is required to be 15% of the mean value of all bearings in a batch.

B282 specifies the requirements for the design, fabrication, testing, protective treatment, supply and installation of structural steel pot bearings and associated attachment plates. The main differences compared to AS 5100.4 are summarised below:

 Materials used for sliding surfaces and guide bars shall conform to ASTM 240M while they shall conform to AS 1449 type 316 2B in AS 5100.4.

 The gap between piston and cylinder shall be no less than 0.5 mm while in AS 5100.4, it requires this gap to be no greater than 1.0 mm.

 A provision for lubricant used in a pot bearing was added in which B282 specified that a silicone compound conforms with the test method ASTM D217 or ASTM D972.

 For an elastomeric disc, the maximum gap between pot and disc in the unloaded condition shall be 0.2% of the diameter of the disc or 0.5 mm while it is not specified in AS 5100.4.

 The number of representatives for testing is 1, 2 or 3 for a number of identical bearings of less than or equal to 10, less than or equal to 25, or greater than or equal to 50, respectively.

AS 5100.4 does not specify this provision clearly.

 A coefficient of friction test was added which depends on the bearing pressure. If the bearing pressure is 5, 15, 20, and greater than 30 MPa, the respective coefficient of friction shall be no greater than 0.04, 0.025, 0.02, and 0.015.

 Protective treatment methods and procedures to avoid corrosion of steel parts were added.

B283 is similar to B282 except it is used for stainless steel and the provisions relating to steel parts and protective treatment are modified.

B284 sets out the requirements for the installation of bridge bearings. It addresses provisions for installation materials such as grout and mortar, the installation tolerances and the installation procedure.

In addition to the above mentioned specifications, RMS often uses parts of European Standard EN 1337 for assessment and design of pot or spherical type bearings (email from Mohamed Anzar on 1 April 2010).

4.2.2 Design and Construction

There are a number of recommendations made available through recent publications by RMS to improve the performance of bridge bearings. The recommendations were made based on various modes of failures observed in NSW.

RMS internal instruction CBE 97/5 (RTA 1997a) requires the consideration of using more durable materials such as pot type bearings made of stainless steels, where possible, within 1 km of the coast or under an equivalent environment (manufacture cost will be 4-5 times higher than normal steels). This arose from the fact that one of the most common failure modes in mechanic bearings in NSW is corrosion of metal parts. Furthermore, in a recent internal report on the performance assessment of bridge components (Austroads 2001), it was recommended that the manufacture of metal bearings could be improved by increasing the resistance to corrosion by using cast iron or stainless steel and improving the paint protection system.

RMS internal instruction CBE 98/8 (RTA 1997b) enforces the use of replaceable bearings in RMS funded bridges except where unreinforced elastomeric strip bearings are used in conjunction with prestressed concrete plank bridges.

Regarding possible construction faults that may cause bearings not to function properly, RMS bridge policy circulars BCP 2005/03 (RTA 2005b) and BCP 2005/04 (RTA 2005c) require the use of attachment plates in pot type bearings and the epoxy mortar pad in elastomeric bearings for pre-tensioned concrete girders, respectively, in order to adjust for the lack of parallelism between the contact interfaces due to longitudinal grade, cross-fall and hog or camber of the superstructure.

In terms of quality RMS also sets out a series of hold points and witness points for quality control.

4.2.3 Testing

Table 4.3 shows the failure criteria used by RMS when testing the new bearings for quality control purposes. The predominant failure modes that have been observed in service are also used as the criteria for acceptance or rejection of the bearings.

Table 4.3: Failure criteria for different bearing types Number Failure criteria during laboratory testing in accordance with RMS

specification Spherical Pot B282, B283 Elastomer

B280, B281

1 Splitting or permanent deformation of elastomer Yes Yes Yes

2 Tearing, cracking or permanent deformation of the PTFE sliding

surface Yes Yes Not applicable

3 Cracking or permanent deformation of any metal component of bearing Yes Yes Not applicable 4 Abrasive marks indicating abnormal contact between any metal

surfaces Yes Yes Not applicable

5 Cracks or permanent deformation of the sealing ring Not applicable Yes Not applicable

6 Significantly irregular or unsymmetrical surface bulging Not applicable Not applicable Yes Source: Austroads (2001).

For pot bearings, the tests required for each type of bearing are shown in Table 4.4.

Table 4.4: Tests required for different pot bearings Bearing type Vertical Lateral Friction Rotation

Fixed Yes Yes No Yes

Free sliding Yes No Yes Yes

Guided sliding Yes Yes Yes Yes

By comparison, AS 5100.4 does not require friction tests.

4.2.4 Failures

According to Austroads (2001), the failure modes outlined in Table 4.5 were observed during inspections in the period from 1995/96 to 1998.

Table 4.5: Common faults in different types of bearings in NSW

Bearing type Common faults

Confined elastomeric (pot type) Leakage of elastomer (less than 5% inspected bearings). Breakdown of protective system.

Bearing type Common faults Unconfined elastomeric,

laminated Cracks in elastomer. Excessive bulging or splitting under vertical loads. Slippage and irregular contact of bearing surfaces. Changes in rubber properties with aging. Ozone cracking.

Fabricated steel including roller

and rocker Main problem has been corrosion of the steel, particularly at the sliding interfaces leading to a frozen bearing (approximately 50% of the total number of bearing inspected). Roller or rocker bearings fabricated with stainless steel components including bolts are generally free of corrosion.

Low friction PTFE to stainless

steel Warping of stainless steel sheet due to rusting of the metal backing plate. Wear of PTFE layer.

In addition, the modes of failure of common types of bearing in NSW listed in Table 4.6 were extracted from RMS’s Bridge Inspection Procedure Manual (RTA 2007b).

Table 4.6: Common faults of bearings observed in recent inspections in NSW

Bearing type Common faults

Elastomeric bearings Cracking, splitting or bulging of elastomer; misalignment or lateral movement, excessive shear deformation or loss of bearing function; missing, distorted or corroded dowels;

deterioration of materials (Figure 4.14 and Figure 4.15).

Metal bearings Corrosion with pitting; debris and rust build-up stops bearing movements; cracking or spalling in the supporting concrete and loss of support under the bearing; misalignment or excessive movement; failed shear keys (Figure 4.16 and Figure 4.17).

Enclosed/concealed bearings Leakage of rubber; broken seals; excessive vertical and/or horizontal movements;

corrosion of ferrous members; cracking or spalling of supporting members (Figure 4.18 and Figure 4.19).

Figure 4.14: Cracking and deterioration of elastomer

Figure 4.15: Splitting and bulging of bearing pads

Figure 4.16: Corrosion of a rocker bearing

Figure 4.17: Corrosion and loss of support

Figure 4.18: Corrosion and leakage of rubber

Figure 4.19: Broken seal in pot bearing

4.2.5 Repair and Maintenance

As noted in Section 4.2.4, RMS has a Bridge Inspection Procedure Manual (RTA 2007b), part of which is for bearing inspection. For each type of bearing, four condition states are given with clear descriptions and illustrative photos. The manual is a very useful resource for condition

management of bridge bearings.