Prof Dr-Ing Markus Hecht, Rail Vehicles Section, Berlin Technical University
THIS SUMMER will see the completion of the three-year Safetram project to improve the safety of light rail passengers and crew in the event of a collision. Funded by the European Union as part of the 5th Framework Research Programme, Safetram began on July 1 2001.
Thanks in part to the Kyoto protocol and related efforts to reduce CO2 emissions and energy consumption, tramways and light rail lines are experiencing a renaissance around the world. Factors driving the growing number of tram and tram-train projects include higher capacity and lower energy consumption than bus, together with lower costs and better accessibility than a classic heavy metro.
Light rail's energy-saving advantages include lightweight body structures and regenerative braking, but ironically the same lightweight structures can cause problems in terms of crashworthiness. On unsegregated routes, trams must share road space with other users, and are often driven on sight with no fixed signalling. Both of these factors increase the risk of a collision.
Safetram was formed specifically to develop cost-effective improvements in passive safety - that is by ensuring a collision-resistant survival space within the vehicles, lowering impact forces and lengthening the impact duration to avoid jerks. The main objective was to prove the feasibility of managing collision energy and acceleration for typical trams and tram-train vehicles (which we called periurban) within a range of technological constraints and at an acceptable cost.
The project has been able to build on heavy rail crashworthiness programmes carried out by railway operators, the Brite/Euram Safetrain Project, experience from the automotive industry, and the European Crossrail project. Work has been shared by 13 partners representing research institutes, manufacturers, and operators (Table I), under the leadership and co-ordination of Bombardier Transportation Portugal.
Safetram began with a statistical and risk analysis of accidents, and a review of the design and manufacture of crashworthy front ends. For accidents with road vehicles, we also undertook a study of automotive legislation on structural and interior passive safety and a review of road vehicle legislation, which will influence the behaviour of trams in collisions between the two vehicle types, and highlighted the applicability of common safety concepts.
To capture the collision behaviour and optimised crush characteristics for different collision scenarios, the overall dynamics problems of city trams and periurban vehicles were simulated using two-dimensional multi-body dynamic models.
This modelling process, followed by dynamic testing of components and complete shells and validation of the results will lead to the development of standards which can be incorporated into future vehicle designs. To this end, the project is being co-ordinated with the CEN TC256 Working Group 2 on European standards for crashworthiness structural requirements.
The results from Safetram will form the basis of CEN European Standard 12663 Part II for tramcar categories IV and V. This will help greatly towards harmonising tram operators' requirements for passive safety, and we believe it will also contribute decisively to eliminating obstacles in the functioning of the single market for rail-based mass transit vehicles.
Risk analysis
At present, much of the safety of tram operations relies on dynamic factors, such as high-performance braking. To achieve the required lightweight vehicles, the bodyshells of city trams are generally expected to withstand buffing loads of 200 to 400 kN without structural deformation, or 600 kN for periurban cars, compared to at least 1500 kN for main line railway vehicles.
Nevertheless, tramway accidents continue to happen frequently, resulting in injuries to staff and passengers, pedestrians, car drivers and other road users. Over the past decade, six European operators alone reported 19000 accidents resulting in 3050 casualties.
Two different sources were used to assess risk, which we defined as the product of the frequency of a given type of accident and the severity of its consequences. For city trams, we used a database of accidents on existing networks. As there is no comparable database for tram-train projects, we used DB and SNCF statistics for accidents to regional train services.
An assessment of tramway accidents in the countries represented by the project partners, plus Belgium, quickly revealed that the greatest risk to passengers and staff came from the strong emergency braking rather than an actual collision with another vehicle. Our analyses determined the risk of injury to a vehicle occupant per passenger-km as 10-9 for a collision between a low-floor tram and a lorry, and 10-7 for emergency braking.
In order to reduce the severity of injuries sustained by the tram occupants, the partners believe that it is important to improve the passive safety of the vehicle. Crashworthy vehicles are designed to crush in a controlled and progressive way, ensuring minimal loss of survival space for both the driver and the passenger area. By managing the collision energy and redesigning the tram interior, it is possible to improve the safety of the occupants. Deceleration rates are controlled and limited to an acceptable level by ensuring progressive crush deformation.
For the next stage of the programme, four collision scenarios were agreed for each of the two main vehicle types:
C1: City tram emergency braking (2·73m/s2);
C2: Collision between two identical city trams at 20 km/h;
C3: City tram collision at 25 km/h with a light van standing at a 45í angle to the track;
C4: City tram collision with a periurban vehicle at 10 km/h;
P1: Periurban vehicle collision at 25 km/h with an 80 tonne rail vehicle fitted with side buffers;
P2: Periurban vehicle collision at 22 km/h with a crashworthy train weighing 129 tonnes;
P3: Collision between two identical periurban vehicles at 36 km/h;
P4: Periurban vehicle collision at 40 km/h with a rigid 16·5 tonne lorry on a level crossing.
For P2, a 'crashworthy' train was defined as one having modern force-displacement characteristics in its leading end.
In order to capture the collision behaviour and purpose-optimised crush characteristics for the different scenarios, the overall dynamics of the vehicles were simulated using two-dimensional multi-body dynamic models.
Design and testing
Based on the initial research, the Safetrain project team created two new design concepts for city and periurban tram vehicles. In order to test different materials, it was decided to build the city tram bodyshell from aluminium (Fig 1) and the periburban vehicle from steel (Fig 2).
To simplify the application of crashworthiness principles, considerable emphasis was placed on modularity. For example, the cabs were designed as complete modules with clear mechanical interfaces, and the deformation elements were designed as separate modules. The use of replaceable components to absorb energy in a collision will improve reliability and reduce the repair and life-cycle costs of trams and LRVs.
The city tram concept envisages two stages of energy absorption. The reversable first stage uses hydraulic buffers to absorb 35kJ. The second irreversible stage comprises a crushable aluminium extrusion which can absorb up to 100kJ. Maximum total displacement is about 500mm.
For the periurban scenario there would be four steps. Again the first stage buffers would account for 35kJ. The other stages are all irreversible. Side buffers absorb 160kJ and a central aluminium honeycomb takes a further 64kJ. These elements are both designed as replaceable modules. The final step provides for up to 600kJ to be absorbed by a crush zone at the front of the vehicle structure. Maximum displacement in this case would be 700mm.
The actual energy absorbed in any given collision would, of course, be dependent upon the individual scenario, because the various energy absorption elements are addressed differently.
In addition to the well-established benefits of shortening the design process for special variants and productivity in manufacture, the modular approach gave important advantages in terms of qualification. We could assess the crash behaviour of modules at the component testing stage.
Practical verification
To validate the multi-body and finite-element calculations, full-scale collision tests were carried out at Poland's Zmigród test centre in November 2003, together with sled tests at the MIRA laboratory in the UK. The aim was to verify our calculations with representative practical tests, for which purpose we selected scenarios C2 and P1. Scenario C2 was simplified by colliding a single vehicle equipped with the test cab end into a rigid wall, at a reduced collision speed of 14 km/h.
The result of the city tram test was identical to the behaviour predicted in the calculation phase. There were some slight differences in the deformation of the periurban vehicle which could be explained by a better understanding of the test conditions.
Interior issues
To improve the survivability for vehicle occupants, it is important to limit the acceleration levels felt by passengers and interior equipment. In such cases the standing passenger is most vulnerable.
Current interior layouts in tramcars show a number of deficiencies in the design of the passenger saloons that could constitute serious safety hazards to their occupants during an accident. The Safetram project reviewed various interior layouts, and will propose in its final report a range of safety improvements that can be assessed through dynamic modelling and sled testing.
Protection against injuries sustained in secondary collisions calls for consideration of interior layouts, and of the human response to impact forces and accelerations. The biomechanics of standing passengers is a very novel and challenging field of research.
For the final stage of the work, Safetram used a Hybrid III 50% dummy in a series of sled tests. These were conducted by SNCF at Inrets in August 2003 and by MIRA in December 2003. The behaviour of seated passengers and the driver were then calculated using dynamic modelling to establish the injuries arising from the defined collision scenarios.
Table I. Safetram project partners
Bombardier Transportation Portugal
AnsaldoBreda Costruzioni Ferroviarie Italy
Bombardier Transportation Germany
Alstom Transport SA France
Alcan Alesa Ltd Switzerland
Berliner Verkehrsbetriebe AG Germany
Centrum Nukowo Technicze Kolejnictwa Poland
Deutsche Bahn AG Germany
Instituto Superior Técnico Portugal
Motor Industry Research Association United Kingdom
Régie Autonome des
Transports Parisiens France
Société Nationale des
Chemins de Fer Français France
Technische Universität Berlin Germany
- Fig 1. The aluminium cab module design for a crashworthy city tram
- Fig 2. A steel cab design was selected for the periurban vehicle Images: Alstom
- CAPTION: The collision test with the prototype periurban cab module was carried out at Zmigród on November 7 2003
- CAPTION: Deformable side and central energy absorption elements are fitted to the periurban vehicle as a second line of defence after the hydraulic buffer
- CAPTION: A full-scale collision test with the city tram cab module was carried out by CNTK at Zmigród on November 5 2003 (left). This proved the functioning of the crushable absorption module (inset) bolted to the front of the cab structure (right)
Safetram s'adresse à la résistance des tramways
Financé par l'Union Européenne, le programme de recherche tri-annuel Safetram qui s'achève en juillet avait pour but d'apporter des améliorations de coûts dans le domaine de la sécurité passive des tramways. Cela comprend la création d'espaces de survie à l'intérieur de caisses plus résistantes et l'abaissement des forces d'impact dans l'éventualité d'une collision. S'étant basée sur les résultats de plusieurs crash-tests, l'équipe de projet a mis au point deux enveloppes de caisse modulaires conçues pour des applications urbaines et interurbaines
Safetram nimmt sich Festigkeit von Strassenbahnen vor
Das von der Europäischen Union ins Leben gerufene dreijährige Safetram- Forschungsprogramm, welches im Juli ausläuft, hatte zum Ziel kostengünstige Verbesserungen bei der passiven Sicherheit von Strassenbahnfahrzeugen zu erarbeiten. Dies umfasst die Schaffung von Überlebensräumen innerhalb von stärkeren Wagenkastenelementen und die Verringerung von Aufprallkräften bei einer Kollision. Basieren auf den Resultaten verschiedener Crashtests entwickelte das Projektteam zwei modulare Wagenkastenkonstruktionen für innerstädtischen und Überland-Einsatz
Safetram investiga la resistencia de los tranvías
El programa de investigación trianual Safetram, financiado por la Unión Europea y que finaliza en julio, tenía como objetivo desarrollar mejoras económicas en la seguridad pasiva de los tranvías. Aquí se incluye la creación de espacios de supervivencia dentro de cajas m? s resistentes y la reducción de las fuerzas de impacto en caso de colisión. Bas? ndose en los resultados y pruebas de golpes, el equipo del proyecto ha desarrollado dos diseños modulares de caja para aplicaciones urbanas e interurbanas
