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Railway Bridges – Safety in design for train derailments
Conference Paper · October 2011
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Railway Bridges – Safety in design for train
derailments
Frank Rapattoni1, Prof. Raphael Grzebieta2
1
Parsons Brinckerhoff
2
Transport And Road Safety Research, UNSW
Abstract Derailment of rail cars remains one of the most dangerous hazards for
bridges over and under railway lines with the potential of harm to railway users,
road users and people in nearby buildings. Although their occurrence is rare the
consequences are always severe if not catastrophic. The disasters at Granville
(Australia) and Eschede (Germany) are two examples which demonstrate the
possible consequences of such events with high loss of lives, disruption of
services and costly damages. Prescriptive requirements to safeguard against the
impact by de-railed cars are contained in the Australian bridge design standard
AS5100. The intent of the provisions is to avoid catastrophic failures of bridges
due to loading or collision impact by derailed trains or road vehicles and to
provide a “forgiving” environment to minimise the probability of injury to the
occupants of derailed trains. The provisions are considered to be world’s best
practice but implementation is often difficult and inconsistent. Practice varies
considerably among rail authorities around the world. The UIC Code provides
comprehensive recommendations about risk avoidance and control. Assessments
to provide design solutions to limit risk as low as reasonably practicable (ALARP)
can be used to justify a structural arrangement which does not comply with
AS5100. The Australian Transport Council’s (ATC) “Safe System” approach
came into effect as of 20th May 2011 and requires zero harm at all stages in the
life of a structure. This approach complements the current provisions in AS5100.
Design solutions should comply with the intent of the “Safe System” and the
AS5100 provisions and consider the possibility of train derailments with realistic,
if not worst case, accident models to prevent human trauma. Crash simulation
software can assist in identifying the hazards, likely consequences and test
proposed solutions at the early stages of design.
V. Ponnampalam, H. Madrio and E. Ancich
Sustainable Bridges: The Thread of Society
AP-G90/11_034© ABC 2011
81
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Rapattoni, Grzebieta
Introduction
Derailment of rail cars remains one of the most dangerous hazards for bridges
over and under railway lines and for rail passengers. Although their occurrence is
rare the consequences are always severe if not catastrophic. The disasters at
Granville (Australia) (Fig.1) and Eschede (Germany) (Fig. 2) are two examples
which demonstrate the possible consequences of such events with high loss of
lives, disruption of services and costly damages.
This is a follow up paper to [2]and [3] which outlined the provisions in AS5100
[4] in regards to collision loads and design requirements. In brief,
• The intent of AS5100 is to avoid catastrophic failures of bridges due to loading
or impact by derailed trains or road vehicles and provide a “forgiving”
environment by preventing hard impacts to minimise the probability of injury
to the occupants of derailed trains.
• There is a need for better practical design guidance to ensure best practice in
maximising safety. The current limited prescriptive provisions do not provide
adequate information for designers or rail authorities to make important
decisions in regards to acceptable risks.
A “Safe System” approach in design of road infrastructure is now required under
the new Australian Transport Council’s (ATC’s) safety paradigm shift. This
requires zero harm at all stages in the life of a structure. The new provisions set
down by the Australian Transport Council (ATC), consisting of federal, state and
territory ministers, are firmly based on the “Safe System” approach principles and
are framed by the guiding vision that no person should be killed or seriously
injured on Australia's roads [1]. This came into effect as of 20th May 2011 and
requires zero harm at all stages in the life of a structure. This includes any deaths
resulting from an errant vehicle causing a train derailment because of inadequate
roadside barrier provision.
This paper discusses the current provisions in AS5100 and international codes,
past and current practice and future directions in design using the “Safe System”
approach, which complements the intent of AS5100, towards the achievement of
Zero Harm.
The use of risk assessment models and the latest advances in crash simulation
software to assist in identifying the hazards, likely consequences and test proposed
solutions are also discussed.
Railway Bridges – Safety in design for train derailments
83
AS 5100 – 2004 Provisions
Bridges over railways
For bridges over railway lines, AS5100.1 provisions are as follows:
11.3.1 General
For collision from railway traffic, to minimize the likelihood of damage and
collapse of bridges and other structures over or adjacent to railway tracks as a
result of collision from railway vehicles, the following shall apply:
(a)
Unless approved otherwise by the relevant authority, bridges over
railways shall have a clear span between abutments.
(b)
Where the relevant authority agrees that the requirement of Item (a) is
not achievable, supports adjacent to railway tracks may be permitted subject to
the following conditions being met, and in the following order of preference:
(i) Alternative load paths are available through the structure to ensure that the
superstructure does not collapse in the event of removal of the supporting piers or
columns as a result of collision. In this case, the supporting piers shall not be of
heavy construction. They may have independent deflection walls. However, they
shall not have integral deflection walls.
(ii) The support piers are of heavy construction designed to resist the loads
specified in AS 5100.2. Such supports shall be protected from head-on collision by
deflection walls or the like. Any supports that cannot be protected from head-on
collision shall be designed to be removable in accordance with Item (b)(i).
AS 5100.2 Clause 10.4.3 specifies design collision loads of
a) 3000 kN parallel to rails and
b) 1500 kN normal to rails
It is important to note that the loads specified in this Clause allow for
“….moderate derailments and minor collisions but not major derailments of a
300LA train… This may also represent a self-propelled passenger train derailing
at moderate speed” (Ref. Clause C.10.4.3). In effect these loads represent “…
only a glancing blow, not a head-on collision” (Ref. Clause C11.3.2).
A head-on collision of a train at operational speed with a pier will produce much
higher loads than specified by AS5100 and the hard impact would impact severely
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on train occupants due to the high sudden deceleration. The Eschede accident
(Fig.2) shows the possible consequences of this type of collision.
The AS5100 provisions are considered to be world’s best practice to ensure
maximum safety but implementation is often difficult and inconsistent. Strict
compliance has proved impractical in some situations [3], especially when
attempting to modify existing bridges to comply with the provisions.
Underbridges
AS 5100.2 Clause 10.5.1 specifies that “Railway bridges designed to carry 300LA
loads shall be designed for two separate train derailment load cases as set out in
Clauses 10.5.2 and 10.5.3”.
For underbridges the specified train loads will be applied at the most critical
location, at the edge of the bridge deck. It is noted that there are no provisions
intended to retain a de-railed train on the bridge. This is left to the discretion of
the rail authority. Provision of “guard rails” (double rails) is usually specified
by the rail authorities [5] although there is a growing trend towards adopting more
robust containment systems [2].
International Codes
The UIC Code [6] sets out recommendations for managing risk from derailed
trains near structures for different train types and speed. It discusses preventative
and protective measures and recommends suitable measures to reduce as far as is
reasonably practicable the effects of an accidental impact from a derailed vehicle
against the supports of structures located above tracks and supports carrying
superstructures of different classes:
• Class A – superstructures supporting elevated structures that are permanently
occupied such as offices, lodgings or offices and
• Class B - superstructures not supporting elevated structures such as bridges
The UIC Code differentiates between new and existing structures in view of the
difficulty and cost in relocating the supports to improve safety for the latter. The
recommendation in this case is to consider and compare the risk of impacting the
Railway Bridges – Safety in design for train derailments
85
supports with the level of other risks to the safety of train operations to assess
priorities for ameliorative works.
The UIC Code recommendations provide comprehensive design guidelines to
minimise risks however it does not require prevention of a head-on collision with
heavy piers as prescribed by AS5100. In regards to deflecting devices it states
that guide walls are suitable only if they can absorb high horizontal loads and have
a high degree of ductility. Energy absorbing devices are not considered practical
due to the great deformation distance needed to guarantee sufficient energy
absorption for a typical train travelling at the operational speed.
The American Railways Engineering Maintenance-of-way Association
(AREMA) [9][provides detailed recommendations on design of “crash walls” for
piers but has no explicit provision prohibiting head-on impact of a derailed train
with piers.
Past and Current Australian Practice
Bridges over railways
The preferred bridge type is a single clear span over railways as this prevents any
possibility of a collision. However, this is not always possible due to the
increased span length and/or cost of alternative bridge types which may be
required. Long spans may not be practical at some locations as girders may be
too deep and a through-truss, through-girder or cable stayed bridge may prove too
costly or unsuitable for the location.
The currently most common bridge type specified is multi-span with simply
supported precast beams on heavy reinforced concrete piers designed for the
above collision loads. This choice is made mainly due to the lower cost of this
system, using economical prestressed concrete (PSC) beams. For this type of
construction, AS5100.1 Clause 11.3.1 (b) (ii) requires the use of deflection walls
to avoid the possibility of head-on impact with the heavy pier. If this is not
possible, AS5100 requires that a pier-redundant bridge with frangible piers be
adopted to prevent a hard impact with the train (Cl. 11.3.1 b (i)).
In order to justify the use of a bridge with heavy piers not complying with Clause
11.3.1 (b) (ii), a risk analysis is required to demonstrate that the risk to life from
an impact by a derailed train is “as low as reasonably practicable” (ALARP) and
can be accepted. The test for ALARP is often very difficult as tradeoffs are
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Rapattoni, Grzebieta
usually required between safety and cost and involves subjective judgments about
what is a “socially acceptable risk” and what cost is justifiable to reduce the risk.
It is considered that risk analysis should be used as a last resort when it is not
possible to contain risks in a design within a reasonable budget as this approach
requires numerous assumptions in regards to
• Probability of train derailments over the design life of the structure
• Consequences due to derailment – likely fatalities and property damage
• Subjective judgments of what is an acceptable risk
Whilst designers are responsible to design to the required safety standard, rail and
road authorities are responsible for making the decision about acceptable risk
using guidelines which reflect the community’s acceptance or aversion to risk. A
typical matrix delineating boundaries of acceptable risk is shown below.
Acceptable Risk for Bridges over Railways
1.E-01
INTOLERABLE
1.E-03
1.E-04
TOLERABLE IF ALARP
year (F)
Frequency of N or more fatalities per
1.E-02
1.E-05
1.E-06
NEGLIGIBLE
ALARP - As Low As Reasonably
1.E-07
N umber of fatalities (N )
(S
Source: Main Roads WA)
An alternative strategy proposed when it is not possible to contain risks in a design
within a reasonable budget is to use the “Safe System” approach [1], using a
holistic Crashworthy System and a Vision Zero philosophy ([10],[11],[12]) which
now governs road design and indeed present day Occupational Health and Safety.
That is, to reduce the kinetic energy of the system such that incidents can be
contained to a tolerable, survivable level. Hence, for example, the speed of the
train along the line should be reduced to a speed at which, if a derailment does
occur, the impacted pier or support could withstand the impact load and cause no
injury to people. This speed limit could be maintained until such time that the
structural system can be designed to tolerate a higher speed derailment incident.
Railway Bridges – Safety in design for train derailments
87
Pier-redundant bridges
The authors are not aware of any bridges in Australia over railways designed in
accordance with Cl. 11.3.1(b)(i). The reluctance to use this bridge system is
mainly due to the preference of using robust piers which can withstand a moderate
or glancing impact from a derailed train without failure as well as unfamiliarity
with this type of bridge and the higher construction cost of the superstructure.
A pier-redundant bridge requires a continuous superstructure, typically using
composite steel girders, which can be designed to span the resulting extended span
after a pier collapse, without a total collapse. The bridge would be designed for
all dead loads and limited live loads as deemed appropriate (20% minimum in
AS5100). The overhead design clearance should be increased to allow for the
resulting high deflections to avoid impact by the train with the superstructure.
Repairs following an extreme incident would be extensive, consisting of
temporary propping, jacking the superstructure to its original level, rebuilding the
piers and repairing the deck but considered acceptable given the rarity of this
event and saved lives.
From a structural point of view, this bridge system is viable, as demonstrated by
two bridges over the Murray River at Berri and Hindmarsh Island [2] and may
prove to be more economical in some instances due to the lower cost of the
substructure which would not be required to be designed to withstand the collision
loads.
Future Directions - Towards a “Safe System”
A “Safe System” approach in design of road infrastructure is now required under
the new Australian Transport Council’s (ATC’s) safety paradigm shift [1]. The
(ATC) “Safe System” approach came into effect as of 20th May 2011 and requires
zero harm at all stages in the life of a structure. If that is not possible for the
funds available then, either the project is abandoned or speeds are reduced to
ensure that incidents can be contained to survivable levels. Prescriptive code
provisions should be considered as minimum requirements and used as guidelines.
Design solutions should comply with the intent of the “Safe System” and the
AS5100 provisions and consider the possibility of train derailments with realistic,
if not worst case, accident models to prevent human trauma. The following is a
discussion of the recommended design approach and considerations towards the
achievement of this objective.
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Rapattoni, Grzebieta
“Safe System” approach
Considering a risk avoidance strategy or “Safe System” approach, the design
should be carried by:
• Considering design options to prevent or limit derailments within the sphere of
influence of possible impact in the vicinity of a structure, such as rail geometry,
clearances, location of switching points and crossings as well as track/train type
if possible.
Design Codes [6] recognise that the probability of derailment is much higher at
curves, crossings and switching points in particular, based on studies of past
crashes. Generally, the bridge design engineer’s scope does not include rail track
design or setting operating train speed. However, this is considered to be
important in minimising risk and should be considered in a holistic approach to
design through close interaction with the rail track designers and operations
managers at the early stages of design. Design standards should highlight this
requirement as design changes at advanced stages of design are often prohibitive.
1. Considering bridging options which avoid potential collisions with a derailed
train or that are less vulnerable to impact (including speed reduction).
The following Table 1 identifies the risk level for typical types of bridges over
railways and possible ameliorative measures:
Table 1 – Type of Bridge over Railways and Risk level
BRIDGE
TYPE
RISK LEVEL
AMELIORATIVE MEASURES
COMMENT
Single Clear
Span
NIL
Not required
Preferred
Multi-span
with simply
supported or
non pierredundant
spans with
frangible piers
VERY HIGH
Essential use of deflection walls
to avoid impact with the
frangible piers
This type
must be
avoided due
to the high
risk of failure
Multi-span
with frangible
pier and PierRedundant
Superstructure
LOW
Impact with
frangible piers will
most likely lead to
bridge collapse and
fatalities (Fig. 1).
Impact with
frangible piers
should not lead to
train passenger
fatalities.
Reduce rail speed to ensure that
a derailed train will not crash
into the pier.
Desirable use of deflection walls
to avoid impact with the
frangible piers and their failure
due to a minor derailment
impact.
Frangible piers with enough
robustness to avoid failure due to
Requires
continuous
superstructur
e designed
for DL and
limited LL
(20%
Railway Bridges – Safety in design for train derailments
Multi-span
with Heavy
Pier
89
Frangible piers may
fail due to collision
loads from a minor
derailment.
minor impact loads
minimum)
for extended
span.
HIGH
Essential use deflection walls or
other impact absorbing system to
avoid head-on impact with the
piers.
Design of
deflection
walls
between
tracks to
redirect
derailed train
is difficult
and costly.
Hard impact with
heavy pier can lead
to passenger
fatalities (Fig. 2)
Desirable use of continuous pierredundant superstructures.
Speed reduction for impact at
tolerable levels.
Fig. 1. Collapse of Bold St Bridge at Granville
(NSW, Australia), 1977, after demolition of
pier by a derailed train
Fig. 2. Collapse of Bridge at Eschede
(Germany), 1998, after initial impact with
pier by a derailed fast train
The following Table 2 identifies the risk level for typical under-bridge types and
possible ameliorative measures:
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Rapattoni, Grzebieta
Table 2 – Type of Underbridge and Risk Level
BRIDGE
TYPE
RISK LEVEL
AMELIORATIVE
MEASURES
COMMENT
Beam & slab
or box girder
bridges
VERY LOW
Impact with
supporting beams not
possible
Nil. Very low risk of bridge
collapse.
“Guard-rails” to contain train
on bridge after minor, lowspeed derailment
Will not retain train on
the bridge in a fast
derailment unless ballast
walls high enough and
designed for collision
loads.
Throughtruss bridge
HIGH
Impact with truss
either within bridge
or approaches can
cause bridge failure
(Fig. 3)
Use deflection walls within
the bridge to avoid impact
with the supporting truss and
deflection walls on
approaches to avoid head-on
impact with end of truss.
Reduce speed to minimise
risk of derailment
Most vulnerable unless
ameliorative measures
implemented.
Safe bridge system with
ameliorative measures
and/or speed reduction.
MEDIUM
Design for AS5100 collision
loads within the bridge and
use of approach deflection
walls to avoid head-on
impact with the girder’s end.
Reduce speed to reduce
derailment risk and kinetic
energy.
Vulnerable unless
ameliorative measures
implemented
Safe bridge system with
ameliorative measures
as it will contain a derailed train on the
structure
Concrete
Throughgirder bridge
Impact with girder
upstand on approach
constitutes a hard
impact with possible
bridge collapse.
Impact within bridge
can dislodge bridge
off bearings
Fig. 3. Derailed train causing collapse of truss bridge
1. Designing for collision loads by considering realistic, possible derailment
scenarios, not just applying prescribed design nominal loads to parts of the
structure, as prescribed by AS5100.
Railway Bridges – Safety in design for train derailments
91
This may be done by:
• Considering a derailed, out of control train hurtling towards the bridge and
predicting the likely impact scenario.
• Reference to technical papers, available literature and data dealing with past
incidents to understand crash dynamics.
• Collision simulation with the latest software to assist in identifying the hazards,
likely consequences and test proposed solutions. This would offer a very
desirable alternative if models could be calibrated to reflect real collisions.
This is discussed below.
• Designing to ensure that the intent of the “Safe System” approach and AS5100
provisions are implemented if a collision is possible. The prime prerequisites
are avoidance of bridge collapse and avoidance of hard impact by a derailed
train by providing a “forgiving” environment to minimise the probability of
fatalities either within the train or within the sphere of influence of a
derailment.
Perhaps the most challenging task in designing new bridges over railways is the
prevention of head-on collision with a heavy pier. It is considered that research to
develop an appropriate impact absorption system for a given design train and
impact speed, should be undertaken by rail authorities.
Mathematical collision simulation studies of derailments [7] done before the
advent of fast computers attempted to simulate train derailment scenarios and
provided the basis for the development of train containment barrier designs in
Texas, USA. Collision simulation with the latest software can assist in identifying
the hazards, likely consequences and test proposed solutions.
For example, a
recent investigation of collisions between trams and cars [8], [10], [11] using
MADYMO and LSDYNA software shows the power of such programs in
analysing collisions and consequences in terms of impact on humans and property
damage.
Conclusions
Derailment of rail cars remains one of the most dangerous hazards for bridges
over and under railway lines with the potential of harm to railway users, road
users and occupiers of nearby buildings. Although their occurrence is rare the
consequences are always severe if not catastrophic. The intent of the prescriptive
requirements provisions in AS5100 is to avoid catastrophic failures of bridges due
to loading or collision impact by derailed trains or road vehicles and to provide a
“forgiving” environment to minimise the probability of injury to the occupants of
derailed trains. The provisions are considered to be world’s best practice but
implementation is often difficult and inconsistent.
92
Rapattoni, Grzebieta
A “Safe System” approach in the design of road infrastructure is now required
under the new Australian Transport Council’s (ATC’s) safety paradigm shift. This
requires zero harm at all stages in the life of a structure and complements the
provisions in AS5100. Zero Harm can be achieved by:
• Considering design options to avoid or limit derailments. A risk avoidance and
“Safe System” design approach is recommended at the early stages of rail track
design with close interaction between rail track designers, operations managers
and bridge designers to achieve maximum safety. This may include speed
reductions if insufficient funds are available;
• Considering bridging options which avoid potential collisions. The selection of
an appropriate bridge type and speed reduction can often avoid most risks;
• Designing for collision loads by considering realistic, possible derailment
scenarios, not just applying design loads to parts of the structure, as prescribed
by AS5100;
• Designing to ensure that the intent of the “Safe System” approach and AS5100
provisions are implemented if a collision is possible. The prime prerequisites
are avoidance of bridge collapse and avoidance of hard impact by a derailed
train by providing a “forgiving” environment to minimise the probability of
fatalities either within the train or within the sphere of influence of a
derailment.
• Collision simulation with the latest software to assist in identifying the hazards,
likely consequences and test proposed solutions at the early stages of design;
• Undertaking research to develop appropriate deflection walls or high energy
absorbing systems to avoid head-on impacts with heavy piers;
• Using risk analysis as a last resort after all possible engineering solutions have
been exhausted. The UIC Code provides comprehensive guidelines and
recommendations to minimise risk.
Railway Bridges – Safety in design for train derailments
93
References
[1] ATC, National Road Safety Strategy 2011-2020,
http://www.atcouncil.gov.au/documents/atcnrss.aspx
[2] Rapattoni, F, (2009) “AS 5100 Bridge Design Standard – Focus on Safety for Railway
Bridges” Proceedings of AUSTROADS Sixth Conference, Auckland, May 2009.
[3] Rapattoni, F, (2004) "Safety first for Bridges – by design", Proceedings of AUSTROADS
Fifth Conference, Hobart, 2004.
[4] AS 5100 – 2004 Bridge Design Standard. Standards Australia
[5] Engineering Standard Structures ASC 310 V2.2 (2010) Rail Corporation
[6] UIC Code (2002) “Structures built over railway lines – Construction requirements in the
track zone
[7] Hirsh, T,J, Harris,W,J, James, R,W, Lamkin, J, Heping Zang (1989) “Analysis and design
of Metrorail-Railroad barrier system” RR 3780-2 on RP TTI 3780, Texan Transport
Institute , Texas A&M University.
[8] Grzebieta, R, H, Rechnitzer, G, (2000) “Tram Interface crashworthiness” Proceedings of
International Crashworthiness Conference 2000, London (UK) Sept 2000
[9] American Railways Engineering Maintenance-of-way Association (AREMA)
[10] Rechnitzer, G, and Grzebieta, R,H, (1999) “Crashworthy Systems – a paradigm shift in
road safety design”, Transport Engineering in Australia, IEAust, Vol.5, No.2, Dec. 1999,
(also presented and in proceedings of “Aus Top Tec” Topical Technical Symposia, Society
of Automotive Engineers Australia, Melbourne, Aug 1999).
[11] Grzebieta, R,H, and Rechnitzer, G, (2001) “Crashworthy Systems – a paradigm shift in
road safety design (part II)”, Transport Engineering in Australia, IEAust, Vol. 7, Nos. 1&2,
Dec 2001.
[12] Grzebieta, R,H, and Rechnitzer, G, (2001) “Vision Zero - The need for crashworthy
systems”, Proc. 24th Australasian Transport Research Forum, Zero road toll - A Dream or a
Realistic Vision?, April, Hobart 2001,
http://www.patrec.org/web_docs/atrf/papers/2001/1419_Grzebieta%20&%20Rechnitzer%2
0(2001).pdf
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