Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MULTI-RAIL PARAPET
The present invention relates to a multi-rail parapet of the type used to
prevent
a wayward vehicle falling over an edge such as a precipice or the edge of a
bridge and
to reduce the level of impact severity.
Known parapet designs consist of a series of uprights that support horizontal
rails that extend along the roadway side of the uprights. In order to provide
protection
for different sizes of vehicles, two, three or sometimes more rails are
provided at
different respective heights above the road level. Parapets are required to
meet certain
standards set by authorities. In the European Union, the standards, such as
EN1317,
require crash testing of the parapet. A test vehicle is propelled into a
section of the
parapet and the effects of the impact are recorded on video and data
collection
devices. Certain parameters are specified in the standard, which are measured,
for
example by inspection of the video and test data, to determine whether or not
the
parapet meets the standard, and to define the parapet designation.
In general, the standards require that the penetration of the vehicle beyond
the
original parapet line is limited to a certain distance, but at the same time a
certain
level of safety is provided to the vehicle occupants, which requires that the
impact
should not cause the parapet, or parts of the vehicle chassis to penetrate the
passenger
compartment. These criteria place opposing constraints on the parapet design.
For a
light vehicle such as a small car (standards specify a weight of 900kg), it is
the lowest
rail of the parapet, which is subjected to the greatest impact. This must be
strong and
stiff enough to limit the penetration of the vehicle through the parapet, but
if too rigid,
the impact loading will be transmitted through the vehicle chassis causing a
greater
risk to an occupant.
The furthest lateral displacement of the parapet or test vehicle is used to
classify the parapet design. For example, a W3 classification is given to a
parapet that
has a lateral displacement of no more than lm. However, it is common for a
parapet
to be mounted on a relatively narrow plinth, for example many bridge parapet
plinths
are 450mm wide. This means that an impacting vehicle will penetrate through a
W3
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parapet such that the wheels on the impacting side of the vehicle could pass
beyond
the plinth. When this occurs, the vehicle will start to roll, which in turn
causes much
greater damage to both the vehicle and the parapet structure. At the same
time, the
tests are used to provide occupant safety ratings A and B. Limiting the
penetration of
the vehicle, such that the wheels remain on the plinth would, in general,
require
stiffening the bottom rail such that a previously acceptable, B-rated parapet
could
increase to an unacceptable level for many parapet installations.
It is an aim of the present invention to provide a multi-rail parapet design
that
alleviates the aforementioned problems.
According to the present invention there is provided a vehicle safety parapet
comprising a plurality of rails supported at varying heights on uprights,
wherein a first
of said rails comprises a steel box-section supported at each upright by
bracketing
means configured to collapse when a horizontal impact load is applied to the
rail such
that the first rail is displaced towards the upright, and wherein at least one
other rail
extends from the uprights by a greater distance than the first rail when the
bracketing
means has collapsed.
When a vehicle strikes the parapet the impact taken by the first rail will
initially cause the bracket to collapse towards the upright. Although the
first rail is a
steel box-section, which has a high inherent stiffness, it behaves like a less
stiff rail
because of the collapsing bracket. In other words the collapsing bracket acts
to
cushion the impact. This means that the initial impact is not sufficient for
the first rail
to cause significant damage to the vehicle at a risk to occupant safety.
However, once
the first rail makes contact with the upright, the stiffness of the parapet
structure is
insufficient for the vehicle to penetrate far through the parapet. At this
stage, the first
rail and collapsing bracket have already reduced the initial impact and
momentum of
the vehicle without significant damage to the vehicle. Now the stiff first
rail limits
further movement through the parapet reducing the likelihood of the vehicle's
wheels
passing beyond the edge.
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A further advantage arises because the other rail(s) extend further from the
upright when the bracketing supporting the first rail has collapsed under the
impact of
a vehicle. These other rails will act to restrain any tendency for the vehicle
to roll
when it impacts the first rail This also means that when a larger vehicle
impacts the
parapet another, stiffer rail will absorb more of the impact, placing less
reliance on the
first rail with its collapsible bracket mounting.
In a preferred embodiment, the first rail is a lower rail, the at least one
other
rail being mounted above the first rail.
In a preferred embodiment, the bracket means comprises at least one sheet
metal cleat member having a vertical portion for securing to the upright, a
horizontal
portion for securing to the first rail, whereby the cleat is operable to
collapse by
plastic deformation.
The cleat may further comprise an inter-connecting portion adjoining the
horizontal and vertical portions at bends such that the plastic deformation
occurs at
one or more of the bends. In this arrangement, when mounted to the upright,
the
inter-connecting portion is set at an angle to both the upright and the rail.
The length
and angle of the interconnecting portion determines the spacing of the rail
from the
upright.
Preferably the bracket means comprises a pair of cleats mounted to the upright
one above the other, a lower cleat supporting the first rail from below, and
an upper
cleat supporting the first rail from above., More preferably, both cleats have
the same
dimensions and are mounted in the same orientation. This arrangement provides
the
advantage that both cleats will deform in the same manner under the impact
load
resulting in a predictable mode of collapse of the bracketing means.
In a preferred embodiment, the parapet includes a second rail having a
trapezoidal section with the shorter parallel side facing the roadway. At
least part of
the longer parallel side of the second rail, facing away from the roadway, way
may be
open.
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It is an advantage that the trapezoidal cross-section provides a particularly
strong and stiff rail section for resisting an impact from a vehicle striking
the shorter
parallel side. Such a section can be manufactured by folding the section from
a sheet
of steel in a cold rolling operation. This is significantly less expensive
than a steel
box section, which is formed by hot-rolling. The open-back section means that
it is
not necessary to form an enclosed section, as with a box-section rail. The
open-back
section also allows the rail to be mounted to an upright by means of a simple
clamping plate arrangement.
Preferably the second rail is mounted above the first rail and provides a
second
barrier. It is an advantage that, with a light vehicle, such as a small or
medium sized
car, the second rail will be impacted by an upper surface of the vehicle. The
second
rail helps to absorb more of the impact and further limit the penetration of
the vehicle.
For larger vehicles, the second rail will be impacted before the first rail
and will serve
to absorb a high proportion of the impact.
In an alternative preferred embodiment, the second rail is another rail having
the same, or similar construction and collapsible support bracketing as the
first rail. It
is an advantage that by providing two rails mounted one above the other on
collapsible brackets, the cushioning effect of an impact will occur for both
small and
medium sized vehicles.
The steel box-section of the first and/or the second rail may be a square
section. Alternatively, the steel box section may be rectangular. Preferably,
the
rectangular box section is orientated with the longer side of the rectangle
horizontal.
Preferably, a third rail is provided mounted above the first and second rails.
It
is an advantage that the third rail restrains rolling of small and medium
sized vehicles
impacting the parapet, and also provides a barrier for larger vehicles.
An embodiment of the invention will be described with reference to the
following drawings.
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Figure l is a cross-section through a 3-rail parapet and an upright.
Figure 2 is a detailed cross-section through a first rail of the parapet of
Figure
1.
Figure 3 is a detailed cross-section through a second rail of the parapet of
Figure 1.
Figure 4 is a cross-section through another 3-rail parapet and an upright.
Referring to Figure 1, a parapet 10 consists of a series of uprights 12, one
of
which is shown. The uprights 12 are mounted to a plinth 14 that defines an
edge of a
roadway next to a drop. The edge may be an edge of a bridge or a precipice.
The
plinth 14 is constructed of concrete with a width of 450mm, as commonly used
on
bridges. An anchoring arrangement 16 is used to secure each upright 12 through
a
baseplate 15 to the plinth 14. The uprights 12 support three rails 18, 24, 28,
which run
past the uprights on the roadway side.
A first rail 18 has a square steel box section supported from each upright 12
by
a bracket arrangement having a lower cleat 20 and an upper cleat 22. This will
be
described in more detail below with reference to Figure 2. A second rail 24
has a
trapezoidal cross-section and is mounted to each upright by means of a
clamping plate
26. This will be described in more detail below with reference to Figure 3. A
third
rail 28 has a square steel box section supported from each upright 12 by
another
bracket arrangement having a lower cleat 30 and an upper cleat 32.
Referring to Figure 2, the first rail 18, which forms the lowest of the three
rails, is supported from below by the lower cleat 20, and from above by the
upper -
cleat 22. The upper cleat 22 has the same shape and dimensions as the lower
cleat 20,
and is mounted to the upright 12 in the same orientation as the lower cleat
20. The
lower cleat 20 is formed of a length of steel strip having two bends 21a, 21
b, which
define a vertical portion 20a, a horizontal portion 20b and an inter-
connecting portion
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20c. The vertical portion is secured to the upright 12 by means of a lower
screw
fixing 34. Similarly the vertical portion of the upper cleat 22 is secured to
the upright
12 by means of an upper screw fixing 36. The first rail 18 is sandwiched
between the
horizontal portion 20b of the lower cleat 20 and the equivalent horizontal
portion of
the upper cleat 22. A vertical bolt 38 passes through the first rail 18 and
both cleats
and is secured by a nut 40.
The lower cleat 20 and the upper cleat 22 are fabricated from steel strip
having
a thickness selected such that when a vehicle strikes the first rail 18 and
applies a
certain minimum horizontal impact load, both cleats will collapse due to
plastic
deformation at the bends 21a, 21b. The impact load at which the cleats
collapse is
determined by design of the cleats. Although the first rail is a steel box-
section,
which has a high stiffness, it behaves like a less stiff rail because of the
collapsing
bracket. This means that the initial impact is not sufficient for the first
rail to cause
significant damage to the vehicle at a risk to occupant safety.
The collapsible bracket arrangement of the lower cleat 20 and the upper cleat
22 are provided on the first rail 18 because it is this rail, which will be
struck by the
wheels or lower parts of a smaller vehicle. It is on smaller vehicles where
the
occupants are seated closer to the vehicle chassis, where there is a
greater'risk of
injury to an occupant when the vehicle strikes a low parapet rail. This is
because the
impact of the rail is transmitted directly into the chassis, which may buckle
or break
such that part of the chassis may penetrate into the occupant's compartment.
When the cleats 20, 22 collapse far enough for the first rail 18 to come into
contact with the upright 12, a much stiffer barrier is presented to the
vehicle. The first
rail and collapsing bracket have already reduced the initial impact and
momentum of
the vehicle without significant damage to the vehicle. At this point the
parapet is
designed to absorb more of the impact such that the upright 12 begins to yield
and
transfer load via the baseplate 15 to the anchorage 16. The stiffer barrier
will result in
less deflection of the parapet structure for a given impact, such that the
wheels of a
vehicle impacting the parapet side-on will not pass beyond the plinth 14. If
the
wheels were to pass beyond the plinth, the vehicle would tend to roll and when
this
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occurs there is a much greater risk of damage to the vehicle as well as to the
parapet
structure. If an impacting vehicle deflects a parapet structure beyond the
edge of a
bridge, and the structure itself is damaged, there is a much greater risk of
materials
falling off the bridge and causing a hazard to whatever may be below. With the
present invention the stiffer parapet structure reduces the possibility of
this hazard,
while retaining a degree of occupant safety in the impacting vehicle.
Referring to Figure 3, the second rai124 has a trapezoidal cross-section with
a
shorter parallel side 24a facing the roadway and secured to the uprights at
positions on
the opposite, longer parallel side. The trapezoidal section of the second rail
24 is not
a closed section, but has an open back between short flange portions 24b, 24c.
To
secure the second rail 24 to an upright 12, the clamping plate 26 overlaps the
flange
potions 24b, 24c so as to trap them between the clamping plate 26 and the
upright 12.
The clamping plate is secured to the upright by means of screw fasteners 42.
The trapezoidal cross-section of the second rail 24 is somewhat larger than
the
cross section of the square box-section of the first rail 18. It has been
found that this
cross-section provides a strength and stiffness of rail that is comparable to
that of a
square box-section, but can be fabricated from sheet steel using a cold-
rolling process,
making a much less expensive, but equally effective rail. However, unlike the
hot-
rolled square section rails (such as the first rai124 and the third rail 28),
the
trapezoidal second rail will absorb more of the impact of a vehicle by
deformation of
the rail cross-seetion. A vehicle impacting on the parapet 10 of Figure 1
will, in
general, strike the second rail 24 first, and this rail will start to
collapse, absorbing
some of the impact load and "softening" the impact. The trapezoidal shape of
the
cross-section ensures that the cross-section does not collapse completely
under the
impact, a greater portion of the impacting load being transmitted to the
upright 12
than would be the case if, for example, and equivalent size of square or
rectangular
section was used.
Referring again to Figure 1, the third rail 28 is mounted to the upright 12 by
means of a lower cleat 30, of the same type as used on the first rail 18, and
a top cleat
32. The top cleat 32 has a single right-angle bend. This cleat is stronger
than the
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lower cleat 30 and will not collapse in the same way. The third rail 28 is
much higher
above the roadway than the first and second rails, and its main function is to
act as a
barrier for larger vehicles. In this case it is appropriate to use a stiffer
rail
construction than for the first and second rails, the principal objective
being to limit
the penetration of the vehicle past the original line of the parapet. However,
it is
convenient to use the same cleats wherever possible, which is why the lower
cleat 30
is of the same type as used for mounting the first rail 18.
Referring to Figure 4, in an alternative arrangement a parapet 100 consists of
uprights 112 mounted through a baseplate 115 to a plinth 114 by anchoring
means
116, in the same manner as described above for the parapet 10 of Figure 1. The
uprights 112 support three rails 118, 124, 128 on the roadway side. First and
second
rails 118, 124 are of substantially the same rectangular steel box section
supported
from each upright 112 by collapsible bracket arrangements having,
respectively, a
lower cleat 120, 125 and an upper cleat 122, 126. As in the parapet 10 of
Figure 1,
the third rail 128 has a square steel box section.
The parapet 100 is designed to provide a cushioned impact to both small and
medium sized vehicles by way of the first and second rails 118, 124. It has
been
found that, by providing two lower rails supported by collapsible brackets,
the parapet
can more effectively withstand the impacts of a wider range of vehicle sizes.
Thus
most of the impact of a small vehicle will be imparted through the first,
lower rail
118, in the same manner as described above for the parapet 10 of figure 1.
However,
for a medium sized vehicle the second rail 124 will absorb more of the impact.
The
third rail 128 helps to restrain rolling of impacting vehicles and acts as a
barrier to
absorb an impact from a larger vehicle. .
The first and second rails 118, 124 are of rectangular cross-section, mounted
with the longer side of the rectangle horizontal. This provides a stiffer rail
than the
square rail 18 of the parapet 10 of figure 1. Although the first and second
rails 118,
124 are intended to cushion the initial impact of a vehicle, the collapsible
brackets
provide the cushioning effect. A stiffer rail ensures that the rail itself
does not bend
under the initial impact, but transmits the impact to the collapsible
brackets. Once the
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brackets have fully collapsed, it is desirable for the rails to be relatively
stiff to
prevent the parapet from being deflected too far and to keep the vehicle on
the bridge.
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