I Bucknall, J Lane, I Palmer page 1 DESIGN PERFORMANCE REQUIREMENTS FOR RAILWAY BRIDGES IN BSEN1990:2002ANNEX A2Ian Bucknall, Network Rail, London, UK John Lane, RSSB, London, UK Ian Palmer, Mott MacDonald, Croydon, UK Abstract BS EN 1990 Annex A2 contains the design performance requirements that are applicable to railway bridges. Compliance with the requirements limiting deformations and deck accelerations is intended to ensure the safe operation of the railway and comfort of passengers. While limiting deformations is required for rail way bridge design to BS 5400-2 and associated railway group standards, checking the bridge behaviour for the dynamic effects of railway traffic has historically been limited to applying a dynamic factor Φto the static load obtained from the design load models (RU and SW/0) in BS 5400-2. However, where bridge decks fall outside of the limits assumed in determining Φ, a dynamic analysis is required to ensure that the deformation and vibration limits set in Annex A2 of BS EN 1990 are met. The background to the differences between the design performance requirements adopted for designs to BS 5400-2 and the requirements of BS EN 1990 Annex 2 and BS EN 1991-2, will be examined. The emerging requirements for clarification and amendment of the design performance requirements in Annex A2 of BS E N 1990will also be highlighted. Notation Φ dynamic factor αload classification factor L effective span length δ deformation
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8/14/2019 1 2 3 Design Performance Requirements for Rail Bridges in en 1990 Annex A2
IntroductionRail bridges are designed to ensure the safe passage of the railway traffic. This is achieved
by ensuring that the stiffness of the bridge is sufficient to prevent excessive deformations that
could lead to overstress in the rails, dangerous twisting of the tracks leading to derailment oran excessive dynamic response of the bridge leading to premature failure. The bridge must
also not cause uncomfortable vibrations for passengers. BS EN 1990 Annex A2[1] specifies
the performance criteria for railway bridges to ensure these levels of safety and comfort are
met.
The limits for allowable deformations specified in BS EN 1990 Annex A2, generally align
with the limits specified in UIC 776-3R [2], although the application of the load classification
factor, α, (BS EN 1991-2[3], 6.3.2(3)P), means that the BS EN 1990 Annex A2 railway loads
are typically greater than those defined in UIC 776-3R as amended by GC/RC5510[4] (now
withdrawn).
Dynamic effects on railway bridges were generally included in the quasi-static analysis
required in BS 5400-2[5] and UIC 776-3R. This design approach means that the static
classified vertical load effects are factored to account for the dynamic effects of the loading
from trains, through the application of a dynamic factor Φ. However, this approach is valid
only for bridges that satisfy certain stiffness criteria, and for standard railway traffic
travelling at up to 200km/h. Where bridges or bridge floors are outside of the stiffness
criteria, or line speeds exceed 200 km/h, a bridge specific dynamic analysis is required in
accordance with the UK National Annex to BS EN 1991-2[6], NA.2.50.
The following sections compare the BS 5400-2 requirements and methodologies with those
of BS EN 1990 Annex A2.
Design Performance – Safety: DeformationIn addition to the structural capacity of the bridges, the bridge is designed to ensure that the
track is not overstressed or that trains are subjected to sudden changes in the track profile
through excessive deformation. BS EN 1990 Annex A2 specifies deformation limits to
ensure that bridge performance will not contribute to an unsafe railway. The limits take into
account the mitigating effects of track maintenance that overcome, for example, the effects of
settlement and creep. The performance requirements are described in the following sections
and comparisons are made with the historical requirements.
Track twist limitationsThe twist of the bridge, measured along the centreline of each track, is limited to minimise
the risk of derailment (see Figure 1). The track twist must be checked on the approach to the
bridge, across the bridge and on departure from the bridge, and also to include the effects of
adjacent tracks being loaded, where the bridge supports more than one track.
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(planned for publication Spring2011), states that the requirements of A2.4.4.2.2 (2) to check the maximum twist shall not
apply and that the only total twist shall be considered. This is because the requirements of
BS EN 1990 Annex A2, A2.4.4.2.2 (2) are considered to be conservative as a traffic safety
limit and UK experience has demonstrated that compliance with the total twist limits is
acceptable.
The total twist limits of BS EN 1990 Annex A2, A2.4.4.2.2 (3) are the same as the limits in
UIC 776-3R. For a track gauge of 1,435m and two axles spaced 3m apart, no wheel shall be
more than 7,5mm out of the plane defined by the other three wheels (see Figure 1). This
7,5mm/3m total twist limit is the equivalent of the 0,0025 radians limit defined in UIC 776-
3R.
However, the applied load for checking the total twist is different. UIC 776-3R, as amended
by GC/RC5510 (now withdrawn), requires the dynamic factor Φ to be applied to the
specified design loading (i.e. vertical railway traffic loads and centrifugal effects) with partial
factors equal to 1.0.
BS EN 1990 Annex A2, A2.4.4.2.2(1)P states that the loads to apply are, the vertical and
centrifugal effects of LM71 (also SW/0 and SW/2 as appropriate) multiplied by the dynamic
factor Φ and the load classification factor α. This results in an applied railway traffic load
higher than that for designs using UIC 776-3R as the UK National Annex to BS EN 1991-2,
NA.2.48 specifies an α value of 1,10 and NR/L2/CIV/020 specifies an α value of 1,21. Thelatter includes a robustness factor (det = 1.1) to satisfy the High Speed Technical
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Specification for Interoperability requirements to maintain safety and reliability over the life
of the structure and shall be applied to all designs for new and, where possible, replacement
bridge designs.
The appropriate value for the dynamic factor Φ has always been subject to interpretation as
bridge decks deform both transversely and longitudinally and a different dynamic factor can be applicable where the stiffness of the transverse and longitudinal elements is significantly
different and for decks with a significant skew. BS EN 1990 Annex A2 does not provide
additional guidance on the appropriate value of Φ to use, but non contradictory
complimentary information is included in NR/L2/CIV/020, allowing a composite dynamic
factor be calculated to represent the variable influence of the longitudinal and transverse
elements.
Vertical deformation limitationsThe vertical deformation limits specified in BS EN 1990:2002 Annex A2, A2.4.4.2.3 for the
maximum total vertical deformation and rotations at the bridge ends ensure acceptable
vertical track radii and robust structures (see Figure 2a). The deck end rotation limits ensurethat additional rail stresses and uplift forces on rail fasteners are minimised, along with
minimising angular discontinuities at switches or rail expansion devices near the bridge.
Figure 2a. BS EN 1990:2002 Annex A2 vertical deformation limits
Figure 2b. UIC776-3R vertical deformation limits
The total vertical deformation limit in BS EN 1990 Annex A2, A2.4.4.2.3(1) is L/600,
measured along any track, where L is the effective span. A comparable safety limit is not
specified in UIC 776-3R: UIC 776-3R, section 4 only states that the total vertical deflection
must not encroach into the required headroom. UIC 776-3R, section 6 recommends that the
rate of change of angle at the simply supported ends of the bridge deck is checked against the
limits in UIC 776-3R, table 1. The change of angle is limited to 0,010 radian where ballasted
track is provided on both sides of a joint at the end of the bridge, and 0,005 radian where
direct fastened track is provided on oneside of the deck end joint and ballasted track provided
on the other side of the joint (see Figure 2b).
BS EN 1990 Annex A2, A2.4.4.2.3 refers to BS EN 1991-2, 6.5.4 for the rotation limitations
at the bridge ends of ballasted track. BS EN 1991-2, 6.5.4.5.5(3)P limits the vertical
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BS EN 1990 Annex A2, A2.4.4.2.4 specifies the maximum change of radius of curvature and
maximum horizontal rotation in Table A2.8. The limits depend on the number of
consecutive decks and the line speed. The limits are the identical to those in UIC 776-3R.
The loads to be applied in accordance with BS EN 1990 Annex A2 are LM71 (SW/0)multiplied by Φ and α, including wind, nosing, centrifugal and thermal effects. Although not
categorically stated in UIC 776-3R, it is implied that the same loads are to be applied as those
in BS EN 1990, Annex A2, with the exception that α is not required to be applied to the
railway traffic loads for checking the UIC 776-3R deformation limits.
Longitudinal deformation limitations
The longitudinal displacement of the end of the upper surface of the deck due to longitudinal
displacement and rotation of the bridge deck end shall be limited to minimise rail stresses and
to minimise disturbance to the track ballast and the adjacent track formation (see Figure 4).
The check of longitudinal deformation is a new concept introduced in BS EN 1990, AnnexA2, A2.4.4.2.5. The requirements for determination of the required combined response of the
structure and track, are defined in BS EN 1991-2, 6.5.4.5.2.
Figure 4. Definition of longitudinal deformations
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BS EN 1991-2, 6.5.4.5.2(1)P requires that the deformation, at the bridge end between two
adjacent decks, or between the deck and the substructure, under traction and braking, is
checked. The loads to be applied when checking deformations are the larger of the
longitudinal traction and braking loads derived in accordance with the UK National Annex to
BS EN 1991-2, NA.2.45.2 or BS EN 1991-2, 6.5.3. The longitudinal deformation limits
depend on the track details and are typically 5 mm for continuous welded rails without railexpansion devices or with a rail expansion device at one end of the deck, or 30 mm for rail
expansion devices at both ends of the deck.
In addition to checking the longitudinal deformation under longitudinal loads, the
longitudinal deformation of the bridge ends under the vertical loads also needs to be
considered. BS EN 1991-2, 6.5.4.5.2(2)P specifies that up to two tracks may be loaded with
classified vertical traffic loads (i.e. LM71 (SW/0) multiplied by α) with any associated
dynamic effects neglected. The longitudinal deformation limits depend on whether the
combined response of the track and structure has been considered, being 8 mm when the
combined behaviour of the structure and the track is taken into account, or 10 mm when the
combined behaviour of the structure and the track is neglected.
Design Performance – Safety: Vibration
The design performance of railway bridges has historically been assured through the
application of a dynamic factor Φ to the static load obtained from the design load models
(RU and SW/0) in BS 5400-2. For the traffic that generally uses mainline railway bridges in
the UK, a mix of passenger trains with a maximum speed of 200 km/h, and freight trains with
a maximum speed of 120 km/h, has been assumed to comply with the BS 5400-2 load
models. The application of a dynamic factor Φ to the loads calculated in accordance with the
requirements of BS 5400-2, has in practice been deemed to provide an adequate level of
safety for the dynamic effects in most cases. The dynamic performance of a railway bridge ischecked indirectly by complying with, the live load deflection limits, and limits for the first
natural frequency of bending (n0), set out in UIC 776-3R. These limits are the same as those
included in the UK National Annex to BS EN 1991-2, Figure NA.12.
However, where train speeds exceed the maximum values stated above, or where resonance
can occur, the dynamic factor calculated from BS 5400-2 may not be adequate and there
could be a risk of overloading a bridge or of premature fatigue failure. To address this
situation, the UK National Annex to BS EN 1991-2 contains a procedure to establish whether
a bridge-specific dynamic analysis is necessary.
Where a dynamic analysis is required, the deformation and vibration limits set out in BS EN1990 Annex A2, are intended to ensure the safe performance of railway bridges subject to
passenger trains travelling at speeds greater than 200 km/h. The High Speed Load Model
(HSLM) was introduced to replicate the dynamic characteristics of real high speed trains for
this purpose. The design requirements within BS EN 1990 Annex A2 were developed for
typical Continental European slab-type railway bridge decks. However, there are railway
bridge types that are prevalent in the UK (half-through types for example), which would not
comply with the BS EN 1990 Annex A2 limits at speeds lower than 200 km/h and the UK
National Annex to BS EN 1991-2, NA.2.50, requires consideration of these railway bridge
types and for speeds below 200 km/h also.
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The need for dynamic analysisThe UK National Annex to BS EN 1991-2, NA.2.50 provides the flow charts to determine
whether a dynamic analysis is required. In addition to the quasi-static analysis, the design
may be checked to ensure resonance does not occur (as this could lead to unacceptable
deformations and a reduced fatigue life), and that the deck response (acceleration) is within
the limits set out in BS EN 1990 Annex A2. The UK National Annex to BS EN 1991-2,Figure NA.12 is applicable for simple structures (i.e. those structures that exhibit only
longitudinal line beam behaviour) and the UK National Annex to BS EN 1991-2, Figure
NA.13 is applicable for both simple and complex structures (i.e. those structures that require
deck/floor elements to distribute axle/wheel loads to primary longitudinal elements).
The UK National Annex to BS EN 1991-2, NA.2.50 allows all choices and options relating to
dynamic analysis to be specified for the individual project. Consequently, where a dynamic
analysis is necessary, reference to NR/L2/CIV/020 is required. Much of the information
contained in NR/L2/CIV/020 was established following research undertaken for the West
Coast Mainline Upgrade project. Details of the background research supporting the
requirements and limits stated in NR/L2/CIV/020, for railway bridge types common on theUK railway network, are described in the paper Permissible Deck Accelerations for Rail
Bridge Dynamic Assessments, Norris. P et al.[8].
Dynamic analysis design rulesWhere a dynamic analysis is required, the designer is required to check the effect of real
trains (axle loads and spacings to be specified for the individual project) and, where the route
is part of the high speed Trans European Network (TENs route), the load effects attributable
to the High Speed Load Models (HSLMs). Two models, HSLM A and HSLM B, are defined
in BS EN 1991-2, 6.4.6.1.1(4) and (5) respectively. Each HSLM represents a Universal Train
with variable coach lengths. The pair of HSLMs together represent the dynamic load effects
of articulated, conventional and regular high speed passenger trains, which comply with the
requirements for the European Technical Specification for Interoperability on high speed
routes (HS INS TSI).
BS EN 1991-2, 6.4.6.1.1 (6), Table 6.4, specifies requirements for the application of HSLM
A and HSLM B for analysis, in terms of particular structure types and for particular span
ranges. It is permitted to specify additional requirements for the application of HSLM A and
HSLM B in the National Annex. The UK National Annex to BS EN 1991-2, NA.2.54 allows
the individual project to specify additional requirements for the application of HSLM but
these are to be specified for the individual project.
Bridge deck acceleration limitationsBS EN 1990, Annex A2, A2.4.4.2.1(4), provides limits for bridge deck acceleration
associated with particular loading frequencies. The recommended deck acceleration limits
are:
i. bt = 3,5 m/s2 for ballasted track.
ii. df = 5,0 m/s2 for directly fastened track.
These acceleration limits are associated with loading frequencies up to the greater of:
i. 30 Hz.
ii. 1,5 times the frequency of the fundamental mode of vibration of the member
being considered.iii. The frequency of the third mode of vibration of the member.
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The UK National Annex to BS EN 1990, NA.2.3.11.2, advises that the maximum peak values
of bridge deck acceleration and the associated frequency limits, should be determined for the
individual project and Railway Group Standard, GC/RT5112[9], 3.3.2, states that the
recommended peak values for deck acceleration shall be used. The GC/RT5112
recommendations are valid only for typical Continental European slab-type railway bridgedecks. Shake table testing in Germany, commissioned by Network Rail as part of the West
Coast Route modernisation project, has demonstrated that the BS EN 1990, Annex A2,
A2.4.4.2.1(4) limits are onerous for typical UK railway half through bridge types. Therefore
the requirements in NR/L2/CIV/020 include increasing the limit bt = 5,0 m/s2 for ballasted
track for short half wave lengths (i.e. short enough to not disturb more than two sleepers).
GC/RT5112 will be amended to clarify the limitations of the recommended values in EN
1990, Annex A2, A2.4.4.2.1(4), at the earliest opportunity.
Design Performance – Safety: Other RequirementsIn addition to the performance requirements described in detail in BS EN 1990:2002 Annex
A2 and standards referred to therein, BS EN 1990:2002 Annex A2, A2.4.4.1(2)P specifiesthat unrestrained uplift at bearings shall be avoided to prevent premature bearing failure.
Design Performance – Passenger ComfortPassenger comfort depends on the vertical acceleration experienced inside the coach.
Although subjective, research by the Office for Research and Experiments (ORE) has derived
a number of comfort levels; „acceptable‟, „good‟ and „very good‟, with typical levels of
acceleration associated with each. The bridge response, train suspension response and
condition of the track, all influence the vertical acceleration experienced by the passengers.
A simplified approach with rules based on checking the vertical deflections of a railway
bridge, has been established and although not specifically stated, it is assumed the approach isvalid for railway bridges with a natural frequency within the limits in BS EN 1991-2, Figure
6.10.
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Deformation limitationsBS EN 1990, Annex A2, Figure A2.3, provides charts of the span / deflection limits plotted
against span for three or more simply supported spans and for a „very good‟ comfort level
(see Figure 5). The limits may be factored where appropriate in accordance with A2.4.4.3.2
(4), (5) and (6) for other comfort levels, span numbers and span arrangements. The UK
National Annex to BS EN 1990, Annex A2, NA.2.3.11.10, allows the individual project tospecify the level of comfort required. NR/L2/CIV/020 requires a „good‟ level of comfort for
all bridges except for bridges on a primary route, or where the line speed exceeds 145km/h,
where a „very good‟ level is specified.
Figure 5: Vertical deformation limits for very good passenger comfort and a 3 spansimply supported bridge. (BS EN 1990:2002 Annex A2, Figure A2.3)
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