Roaby, leading the technical revolution of bridge expansion joint worldwide! Its RB unit finger expansion joint with multi-directional displacement capability (RB finger joint) is a breakthrough product which reaches a high level of expansion joint technology, solving the technical difficulties of expansion joint in large-scale and long-span bridges.


III.Advantages of RB Finger Joint

1. Horizontal rotation performance

RB finger joint
RB finger joint is assembled from one meter long modular components at every one meter perpendicular to the traffic direction. When the superstructure undergoes horizontal rotation or twisting under wind or vehicle loads, RB finger joint will distribute the horizontal rotation or twisting angle into transverse units. The horizontal rotation and twisting capacities of RB finger joint can be customized according to the design demands.



Modular joint

Because of the lamella beam in modular joint is spanning along the bridge width, when bridge superstructure has a horizontal rotation and twisting under some severe load cases, it will endure some plastic deformation under fan shape displacement and squeezing conditions. The following picture was taken at Jiangyin Yangtze River Bridge (Jiangsu Province) on May 27, 2005 shows that the girder end maximum displacement is 417mm under the 0.01 radians horizontal girder rotation and the gap was opened up to 470 mm causing traffic closure (test report issued by the Jiangsu Provincial Traffic Science Research Institute and Hong Kong Polytechnic University).




2.Vertical rotation performance

RB finger joint
The response of the RB finger joint to the bridge deck rotation due to the live load on the bridge is one of the best features of this product. The pivot and the hinge bearing allow the attached plate and fingers to rotate and keep the sliding fingers to be in contact with the stainless steel support at the other end. This feature reduces the noise generated by the vehicles running over the joint and eliminates clanking and bumping caused by fingers spring up from their supports. The rotation capacity can be customized according to the design requirements.


Vertical rotation theory


Cantilever finger joint
The cantilevered finger plates may be lifted above the roadway surface when the girder supporting the cantilevered plate undergoes some vertical rotations due to live load. Under this scenario, the projecting finger plates and the anchors may be prone to fatigue damage under impact from the over-rolling traffic load.


Modular joint
The friction resistance between the sliding bearing underneath the lamella beam and supporting girder can increase significantly when the girder ends rotate. This may cause the supporting bearing fall-out and the lamella beam to deform. The vehicular traffic will "jump" over it, and the increased impact load on adjacent lamella beam may lead to more fatigue damage to the joint.




3.Seismic performance (optional per client's request)


RB finger joint

RB finger joint provides a relatively flexible connection between main bridge structure and approaches.


Under medium earthquake, the structural response of the joint is maintained and force transfer function is also preserved.


Under severe earthquake load, the expansion joint will aid the response of the bridge superstructure in the longitudinal direction, as follows:

(1) When the fingers are closed, the joint will work as a restrainer. (2) When the fingers move past the allowable limits, the studs on the fixed plate will shear off, and the rubber block in between the gap will work as a damper and stopper.



RB finger joint

Because the modular joint is embedded deeper between girder ends:
(1) Under medium earthquake, the steel beams squeeze each other, and then the structure responses and force transfer paths are affected;
(2) Under severe earthquake the joint acts as a direct rigid contact with bridge and approach structure and can cause more damage to the girders.


4.Structural performance

RB finger joint

RB finger joint consists of multi-directional displacement hinge, sliding finger plate, fixed finger plate, support bracket, anchorage, and drainage system. The multi-directional displacement hinge is the key component of RB finger joint, with capacity to accommodate the bridge three-dimensional deformation demands.


The fixed finger and the sliding finger plate components are located at the two opposite sides of the expansion gap. As the bridge deck expands and contracts, the fingers of the sliding part slide longitudinally between the fingers of the fixed part. The steel fingers which span the expansion gap act as a simply supported beam. The sliding finger plate assembly is connected with the multi-directional displacement hinge at one side of the joint and individual fingers spanning the gap always remain in permanent contact with the support on the opposing side of the joint provided by a stainless steel sliding surface and a rubber shock absorber. The flexible and shock-absorbing design of the system helps to reduce the effects of vehicular loading impacts and thus improve the performance of the expansion joint.


RB finger joint is assembled from unit segment perpendicular to the traffic direction. A typical unit segment is one meter long. The short length of the unit segment offers the advantage of quick replacement of sections in individual lines if required (possible in a single night shift) without closure of other lanes of traffic. The maintenance and life cycle costs are significantly reduced when compared to the type of modular joint.



Modular joint

The lamella beams of modular joint are installed transversely along the whole bridge width. The gap between lamella beams are fitted with an elastomeric extrusion which does not carry the vehicular traffic. A mis-allignment of the lamella beams can cause severe impact from vehicular traffic. The type of joint is more prone to fatigue damage. The supporting beams under lamella beams are arranged along the bridge in about 1.5 meter interval. If the lamella beam is broken, a large opening in the bridge deck can be created and may cause serious traffic accident.



5.Performance of non-slip surface, drainage and damage control

RB finger joint
RB finger joint has excellent anti-skid performance. The fingers are closely spaced at 5-9 cm, which gives the joint surface good gripping characteristics. The top surface can be machined into specific patterns with different groove depths to improve the traction and drainage condition and to reduce traffic noise. The finger spacing or finger sizes can be reduced or a cover plate is provided per customers' needs if there are motorcycle or bicycle lanes on the bridge.

The water-tight drainage channel of RB finger joint is located in the gap between the two sides below the finger plates and covers the full length of the joint. The drainage channel is connected to the bridge surface drainage system at either or both end as governed by the geometry of the joint. The water-tight elastomeric seal is non-reinforced extruded neoprene compound with suitable high strength technical characteristics and good aging resistance.

In addition to the flexible and shock-absorbing design of the system mentioned above, two other design features also help to control traffic noise and to reduce the fatigue damage from vibration under vehicular load impacts. These include two rubber bearing plates are adopted under the base plate of multi-directional displacement hinge and one canvas rubber pad is also installed under the finger plate.




Modular joint
The rubber seal of modular expansion joint is directly exposed to the weather and vehicular traffic and is more vulnerable to the environmental attacks. The debris is easily accumulated in the gap and the movement is blocked. In addition, large area of water accumulation and icing are other common problems, which may cause serious accidents in rainy and low temperature mountain areas.





6.Longer life cycle design concept

RB finger joint
RB finger joint has longer life cycle design concept. The joint is assembled from short one meter long unit component across the bridge deck. Each unit is relatively independent. There are not special requirements for temperature during installation. The installation gap between finger plates can be adjusted according to installation temperature on site.

The advantage of this feature is to allow easy and quick installation or replacement of sections as required (possible in a single night shift) without closure of other lanes of traffic. The maintenance and life cycle costs are significantly reduced compared to the traditional type of modular joint. (about 1/5 of the traditional modular joint).



Modular joint
The maintenance cost for modular joint is much higher. The edge beams and lamella beams of modular joint span across the whole bridge width. Once there is a partial damage, the repair involves much larger area of the joint. The traffic management is also adversely affected; often the whole bridge has to be closed for repair, which has huge negative impact on the public.



7. Smaller girder ends block-out

Smaller girder ends block-out 

RB finger joint

(1) Small girder ends gap.
The girder end gap is significantly reduced. Generally speaking, the girder end gap could be the movement capacity.
(2) Low installation depth
The cut-in depth is usually about 20cm. For 1000 mm movement demand, the installation depth is no more than 35 cm (ultra-thin design is possible per request for special structure). The smaller block out eases construction difficulty and enhances bridge safety.


Modular joint

(1) The girder end gap is normally 2 times of the movement demand.
(2) Deep installation depth requirement.
The installation depth for modular joint is usually over 35 cm. For joint with 1000 mm movement capacity, installation depth is about 60cm. It creates difficulties for both the design and construction, hence increase in the costs.