Bridge Bearing-Rocker/Roller Bearing

Rocker cum Roller bearing:

This bearing has both Rocker assembly and Roller assembly. Roller assembly gives translation while rocker assembly gives rotation.  These types of bearings are used seldom in new constructions due to some demerits mentioned in the following para. However, they still exist in a large number of old bridges.

Roller  bearing has a very low coefficient of friction (nearly 0.03), thereby impact of structural movement on substructure is largely reduced. A typical sketch of  rocker and roller bearing for open web steel girders of 45 m span having used in the bridges of Indian Railway is shown in Fig-3, below. Such bearing exists in many numbers of old bridges in Indian Railway.

Rocker cum Roller bearing is provided on the free end of the girder. This type of bearing is generally provided on steel bridges. For bridges with larger spans  30m or more, amount of movement needed and the vertical load transmitted through each bearing is too large to select sliding bearing. In such situation  Rocker and Roller bearing is chosen. These bearings have major advantages that it  transmits least horizontal force to the substructure provided it is cleaned, lubricated and maintained periodically. This bearing has large movement capacity up to 100 mm.

Roller/Rocker bearing has demerits of poor seismic response, subjected to corrosion, permitting rotation about transverse axis only and translation in the long direction. This bearing consists of 02 parts. (Fig-3)

    Rocker assembly (i.e. Saddle plate and Saddle block rotates over knuckle) allows rotation of the Girder about transverse axis (Say Y axis).

    Roller assembly (i.e knuckle, knuckle slab, rollers & Expansion base) permits longitudinal movement ( say along X axis) as per requirement of design.

Therefore Rocker cum Roller bearing permits both rotation and translation.

Rocker bearing (without roller assembly): Fig-2 may be seen below. This bearing can be used on the same span on fixed end. Top part of the bearing i.e  saddle plate & saddle block rotates over knuckle of the bottom part thus permitting only rotation about transverse axis (i.e Y-axis). There is no scope of translation at this end.

                  Fig-2

Periodical cleaning & greasing is required on both end at  the interface of Knuckle and saddle block and between rollers and Base plate for proper functioning of the bearing. Therefore Rocker (without Roller assembly) bearing permits rotation only.

Components of roller cum rocker bearing (Fig-3):

a. Roller: The steel of forging quality be used for the components of the bearings shall comply with Class 3, 3A of IS:2004 and  IS: 1875. These have  machined surface to permit smooth rolling action.

b. Link plate: A bar loosely fixed at each end of the roller assembly for  connecting the individual rollers in a nest and to facilitate movement of rollers in unison.

c. Tooth Bars: Tooth bars are provided on the extreme ends of a group of rollers. The tooth bars rest into grooves provided in the Knuckle Slab on top and expansion slab at the bottom with adequate clearance. The purpose of tooth bars is to restrict the  movement of the rollers within the design movement.

d. Rib and Notch: To arrest transverse movement of the girder and consequent relative motion among knuckle slab assembly,  roller and the expansion base,  a notch in the middle of the roller and a matching rib both on the base plate and Knuckle Slab are provided (Fig-3) . The rib thus guides the rollers to roll only in the longitudinal direction and prevents any transverse movement of superstructure,  keeping the girder in its original alignment. However horizontal force  if any in transverse direction is transmitted to the substructure.   ‘Guide strip’ in the sliding bearings is doing the same function.

Connection of Saddle Plate (Fig-3): It is connected to bottom chord of the girder in tight fit joint. Longitudinal force due to friction will transfer through this connection to the bed block. Connection should be designed suitably with rivets or tight fit turned bolts.

Only Roller bearings: Sole Roller bearings  may be used where longitudinal movement is allowed only while transverse movement and rotation are not permitted or negligible. They consist of cylindrical rollers instead of a rocker in between the sole plate and the masonry plate. 
The cylinder made of high-grade steel rolls over the masonry plate to allow movement of the superstructure. Roller bearings can be in form of a single roller or multiple rollers as shown in the Fig-4, a multiple roller bearing. 

Rotation of Roller bearing:  Single roller bearing will permit both translation and rotation but multiple rollers, without combination of Rocker assembly,  permits only translation. Single roller bearings have a very limited vertical load capacity. Multiple roller bearings, on the other hand, may be able to support very large loads.

Demerits of Roller/Rocker Bearing

• Permits Rotation about transverse axis only. Hence can not fulfil the purpose of multi rotational bearing, not suitable for heavily  loaded structure.
• Does not permit translation along transverse axis, leading to some force is transmitted to the substructure to take into account in the design of connection details.
• Maintenance cost is high. These bearings are made of steel only, thereby they are corrosion prone and require periodic cleaning, greasing, oiling which involves  lifting of girder under traffic block. Freezing of bearing is also not uncommon thus affecting the bridge structure with additional restrained force.
• Seismic response is very poor. Basic material did not have dampening properties unlike elastomeric bearing.
• Not compatible with multi-rotational modern bridge bearings.

        References: 

                  1. Indian Railway Bridge Manual

                  2. Master of Science Thesis Stockholm, Sweden 2012
                  3. Bridge Bearing IRICEN, Pune India
                  4. IRC-83(part-III)
                  5. Bridge Bearings,  Indian Association of structural  Engineers. By Alok Bhowmick