Deflection Walls or Crash walls
Deflection Walls or Crash walls are huge wall shaped reinforced concrete structures provided for the protection of piers and abutments in the vicinity of railway tracks. The purpose of deflection wall is to prevent head-on collision or impact from derailed trains on the primary structural support elements. It is also called a crash wall. A direct collision of a train on a non-frangible(primary) pier may lead to total collapse of bridge. Deflection wall helps to avoid catastrophic failures of bridges due to collision impact by derailed trains and minimise probability of injury to the passengers of derailed trains.
This structural system is mostly found in Australia. Australian bridge design code AS5100 mandates protection of piers near to rail lines. These walls are provided for Rail, Road and Pedestrian bridge piers if distance from face of pier to the centerline of existing or proposed tracks is less than 20m. However, this requirement can be ignored for piers within 10m to 20m range, if a risk assessment is carried out to determine the protection requirements and approved by authorities.
Dimensional Requirements of Deflection Walls
AS5100.2 2017, Clause 15.3.4 sets out dimensional and design requirements of deflection walls. A continuous deflection wall of height 3.6m above rail top shall be provided if the distance of pier face from the center line of existing or proposed tracks is less than 4m. If this distance is more than 4m, the deflection wall must project 2m above the top of rail. Additional dimensional requirements of deflection wall specified in the latest design code are as follows:
- Minimum length of wall is 4m with minimum of 2m longitudinal extension on either sides of pier parallel to track;
- Wall must have minimum thickness of 800mm;
- Wall shall extend 1.2 m minimum below ground level;
- Minimum extension from face of pier shall be 150mm laterally;
- Wall shall have rounded ends, smooth face without snagging points.
- Angle with rail tracks and wall shall not exceed 20deg.
Design requirements of Deflection walls
Deflection walls are designed as continuous reinforced concrete walls. It must be ensured that the collision forces on the deflection walls are not transferred back to the primary structural element. When the pier face is within 10m from the centerline of track, following design loads shall be considered in the design of deflection wall. These loads shall be applied simultaneously at 2m above the rail top in the horizontal direction.
- 4000kN parallel to track;
- 1500kN normal to track.
These loads shall be distributed over a length of 2m x0.5m deep. When piers are located between 10m to 20m from track, the protecting deflection wall shall be designed for a horizontal force of 1500kN applied horizontally 2m above rail in most onerous angle to the wall.