Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02493141 2008-08-27
IMPROVEMENT IN LOAD SUPPORTING STRUCTURE
This invention relates to load supporting structures of the type in which
concrete deck panels are supported by laterally spaced beams.
BACKGROUND OF THE INVENTION
The support of decks on laterally spaced beams is of course widely
used primarily in bridge decks but also in many other constructions wherein a
concrete deck is applied over supporting beams such as in a parking lot or
other
building.
Conventionally the deck is reinforced with steel so that the steel
accommodates tensions in the concrete structure. Thus the conventional deck
includes a layer or mat of supporting steel adjacent the bottom of the cast
deck and
a second layer or mat of reinforcing steel adjacent the top surface of the
cast deck.
The steel layers are in tension and are required since, as is well known,
concrete is
very weak under tension. Reinforcing steel has however the significant problem
that
it can corrode in the presence of chlorides thus reducing the life of the
structure and
requiring periodic expensive maintenance to repair or replace due to the
corrosion.
In U.S. Patent 5,339,475 (Jaeger) there is shown and claimed a
technique in which tension members are connected across the beams to prevent
expansion or movement of the beams in the transverse direction and the layer
of
concrete is cast over those tension members. Under load, within the structure
of the
concrete, is therefore formed a compressive arch so that loads from the upper
surface of the concrete deck are transferred downwardly and outwardly to the
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girders and then to the ends of the tension members attached to the beams. The
tension members are attached into the concrete by upstanding studs which are
cast
into the concrete so that the concrete is prevented by the tension members for
moving outwardly at its bottom surface.
The compressive arch thus formed does not require that the deck itself
be in any way arched since the compressive arch is formed in the body of the
deck
by compressing areas of the deck as required to accommodate the loading point
relative to the tension points caused by the transverse tension member.
The compressive arch thus formed within the body of the deck thus
generates areas of micro tension within the structure since some parts are
compressed while other parts are not compressed. It is known therefore that
the
concrete may be reinforced by short length or staple fibers to transfer the
micro
tensions across the concrete and reduce cracking. This technique therefore
obviates the necessity for the reinforcing steel within the structure of the
deck since
the loading within the deck is compressive rather than in tension.
The tensioning members are generally steel straps, which are
generally welded to the girders, but these are located at or immediately
adjacent the
bottom surface of the concrete deck so that they are much further from the
corrosive
chlorides which enter the top surface of the deck. Thus they are much less
susceptible to corrosion. In addition it is in some cases possible to expose
at least a
part of the length of the steel straps underneath the bottom of the concrete
deck,
particularly in the central area between the two support beams.
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This technique of forming a compressive arch using transverse tension
mernbers has been widely disseminated and is becoming accepted with a number
of
installations being tried.
The above technique as disclosed in this patent has the disadvantage
that all the concrete must be cast in place with the disadvantages of forming
and
disassembling shuttering which is highly labour intensive and can be difficult
when
the deck is located in an in-convenient location.
In U.S. Patent 5,850,653 (Mufti) issued December 22, 1998 is
disclosed a modification of the above technique in which the entire deck is
pre-cast.
In this arrangement therefore the tension straps are embedded in the pre-cast
concrete deck panel during the casting process. In order to attach the pre-
cast deck
panel to the beams, the beams have welded thereon a series of shear transfer
studs
which can be welded as individual studs or as an array of studs with the pre-
cast
panel having cooperating recesses in the underside which are filled with grout
to
receive and retain the deck panel.
This arrangement has the disadvantage that it requires on site welding
of the studs and the studs are located in grout rather than in the concrete
itself. This
technique has not achieved significant success.
SUMMARY OF THE INVENTION
It is one object of the present invention, therefore, to provide an
improved load supporting structure, such as a bridge deck, which utilizes the
above
compressive arch technique to avoid the use of corrosive tensioning elements
within
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the concrete and which has advantages of improved installation.
According to one aspect of the present invention there is provided a
load supporting structure comprising:
a plurality of laterally spaced apart beams including two which define a
space therebetween;
each beam having a support surface extending along an upper surface
of the beam; and
a concrete deck mounted on and spanning the space between the
beams, the deck comprising:
a pre-cast concrete panel having two opposed outer edge portions
supported on the respective support surfaces;
a plurality of tension members at positions spaced apart longitudinally
of the panel each extending across the concrete panel at or adjacent a lower
surface
thereof;
each tension member having adjacent respective ends thereof at least
one panel anchoring member attached to the tension member and embedded within
the panel at the outer edge portions thereof such the tension members prevent
spreading of the panel so that loads are transferred from an upper surface of
the
panel to the beams by forming a compressive arch within the panel;
and a cast in place concrete deck slab covering the pre-cast panel;
and each tension member having adjacent respective ends thereof at
least one slab anchoring member attached to the tension member and embedded
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within the deck slab at the outer edge portions thereof such the tension
members
prevent spreading of the slab on the panel so that loads are transferred from
an
upper surface of the slab to the beams by forming a compressive arch within
the
slab.
Preferably each of the slab anchoring members and the panel
anchoring members comprises an upstanding shear transfer stud with a bottom
end
fastened to the respective tension member.
Preferably each shear stud has a shaft and an upper head of greater
transverse dimension than the shaft.
Preferably the tension member extends to a position beyond an outer
edge of the panel and wherein the at least one slab anchoring member is
located on
the tension member at the position beyond the outer edge of the panel. This
avoids
on site welding.
Preferably the compressive arch in the panel and in the slab avoids the
requirement for transversely extending tension reinforcement in the panel and
in the
slab. That is the conventional steel reinforcing bar which provides tension
reinforcement can be omitted thus avoiding the corrosion problems associated
with
steel.
Preferably a lower surface of the concrete panel is recessed at a
center area between the outer edge portions such that tension member is
exterior to
the panel at the center area. This reduces the weight of the panel and also
removes
material which is not associated with forming the compressive arch.
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Preferably each of the beams has on its support surface a support pad,
which may be formed of an elastomeric material, extending longitudinally along
the
beam and defining the support surface.
Preferably the tension member is free from fixed connection directly to
the beam. This is no longer necessary since the cast slab may be fastened to
the
beams, or the slab may be continuous across a series of beams such as in the
deck
of a parking lot.
Preferably the beam has slab anchoring members thereon for
connection of the slab to the beam for communication of forces from the
compressive arch in the slab to the beam.
The concrete of the slab and the panel may contain staple fibers for
micro-crack reinforcement due to the tensions in the concrete caused by the
formation of the compressed arch in part of the concrete while other parts
remain
uncompressed. Alternatively the concrete may contain non-corrosive rebar
material.
According to a second aspect of the invention there is provided a
method of forming a load supporting structure comprising:
providing a plurality of laterally spaced apart beams including two
which define a space therebetween;
each beam having a support surface extending along an upper surface
of the beam; and
providing a pre-cast concrete panel having a body of concrete with a
center area and two opposed outer edge portions shaped and arranged such that
a
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transverse width spans the beams and such that a longitudinal length extends
along
the beams;
providing in the pre-cast panel a plurality of tension members at
positions spaced apart longitudinally of the panel each extending across the
concrete panel at or adjacent a lower surface thereof;
each tension member having adjacent respective ends thereof at least
one panel anchoring member attached to the tension member and embedded within
the panel at the outer edge portions thereof;
applying the pre-cast panel so as to span the beams with the edge
portions resting on the support surface of the beam such the tension members
prevent spreading of the panel so that loads are transferred from an upper
surface of
the panel to the beams by forming a compressive arch within the panel;
and each tension member having adjacent respective ends thereof at
least one slab anchoring member attached to the tension member and exposed
from
the panel;
cast in place onto the panel a concrete deck slab so as to cover the
pre-cast panel;
and embedding the slab anchoring members within the deck slab at
outer edge portions thereof such the tension members prevent spreading of the
slab
on the panel so that loads are transferred from an upper surface of the slab
to the
beams by forming a compressive arch within the slab.
According to a third aspect of the invention there is provided a pre-cast
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concrete panel for mounting on laterally spaced apart beams defining a space
therebetween comprising:
a body of concrete with a center area and two opposed outer edge
portions shaped and arranged such that a transverse width spans the beams and
such that a longitudinal length extends along the beams;
a plurality of tension members in the pre-cast panel at positions
spaced apart longitudinally of the panel each extending across the concrete
panel at
or adjacent a lower surface thereof;
each tension member having adjacent respective ends thereof at least
one upstanding shear transfer stud with a bottom end attached to the tension
member and embedded within the panel at the outer edge portions thereof such
that
the tension members prevent spreading of the panel so that loads are
transferred
from an upper surface of the panel to the beams by forming a compressive arch
within the panel;
and each tension member having adjacent respective ends thereof at
least one further upstanding shear transfer stud with a bottom end attached to
the
tension member;
wherein said at least one further upstanding shear transfer stud is
exposed from the panel.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, which illustrate exemplary
embodiments of the present invention:
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Figure 1 is a transverse cross section of a load supporting structure
according to the present invention including a pre-cast concrete panel and a
cast
deck slab.
Figure 2 is a plan view of one embodiment of the pre-cast concrete
panei of Figure 1.
Figure 3 is a transverse cross section of one end of the pre-cast
concrete panel of Figure 1 when laid on the beams.
Figure 4 is a top plan view of the structure of Figure 1.
Figure 5 is a transverse cross section of a second embodiment of the
structure.
DETAILED DESCRIPTION
In Figure 1 is shown a load supporting structure according to the
present invention which includes a first beam 10, a second beam 11 and a deck
12.
The beams 10 and 11 are shown as steel beams having a horizontal
upper support surface 13 but the beam is maybe formed of other materials
including
concrete. The beams are generally of a conventional nature.
The deck 12 is formed of a pre-cast panel 14 and a cast deck 15 whidh
is cast over the pre-cast panel 14 in situ. The pre-cast panel is shown in
Figures 2
and 3 and comprises a rectangular panel having ends 14A and 14B defining a
length
extending along the beams over a predetermined distance. The panels in general
are arranged end to end along the beams so as to complete a span along the
length
of the beams. The panels may have end components and structures, commonly
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used in such pre-cast members as is well known to one skilled in the art. The
panel
has side edges 14C and 14D defining a width of the panel sufficient to span
the
space between the beams. The width of the panel is less than the spacing
between
the center lines of the beams so that the edge 14D just sits on one edge 13A
of the
support surface 13 as best shown in Figure 3.
At the bottom of the cast panel 14 is cast in place a transverse tension
member 16 in the form of a steel strap extending across the full width of the
panel
with end portions 16A and 16B projecting outwardly beyond the side edges 14D
and
14C. As shown in Figure 3 the end portion 16A extend outwardly to an end edge
16C which is located on the surface 13.
The strap 16 carries one or more steel transfer studs 17 at a position
on the strap which is within the pre-cast panel 14. The studs 17 are of a
conventional nature and include a shaft 17A and a head 17B with a bottom end
17C
of the shaft welded to the strap. Thus the stud projects vertically upwardly
from the
horizontal strap with the head located within the body of the panel and the
stud
having a sufficient length such that it provides an effective anchor within
the panel to
transfer loading from the panel to the strap. In the arrangement shown in
Figures 2
and 3, there are two such studs and in the arrangement shown in Figure 1 there
are
three such studs and it will be appreciated that the number will be at least 1
and can
be significantly great in number. However the number is selected so as to
provide
them effective anchoring action so as to prevent the panel from spreading
outwardly
under loading to the top surface due to the tension within the strap.
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Such studs are well known and the head at the top of the stud which is
of greater transverse dimension than the shaft prevents any possibility of the
panel
lifting away from the strap under loading.
The first of the studs 17 closest the edge 14C or 14D is located above
the edge of the surface 13 and just spaced from the edge over the top of the
surface
13. The remaining stud or studs are located between the span of the beam so as
to
be located over the space between the beams.
The portions 16A of the tension strap carries two further shear transfer
studs 18 which are located beyond the edges 14C and 14D respectively. These
studs are of the same construction as the studs 17 but have a height greater
than
the studs 17 so they project above the top surface 14F of the panel 14.
In the arrangement shown in Figures 2 and 3 there is one such stud 18
and in the arrangement shown in Figure 1 there are two such studs 18 arranged
at
spaced positions along the length of the portion 16A of the strap 16. Again
the
number of studs will vary depending upon the loadings involved as will be well
known to one skilled in the art.
In the finished construction of the deck, the cast in place concrete deck
slab 15 is cast over the panel 14 using the panel 14 as a support for the
casting
without the necessity for additional shuttering. Thus the deck slab 15 is
formed from
cast concrete poured onto the panel 14 and poured onto the top surface 13 of
the
beams. The cast concrete deck 15 has a top surface 15A and has a surface 15B
on
top of the surface 14F of the panel. The cast concrete of the deck or slab 15
enters
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the area beyond the ends 14C and 14D of the panel onto the top surface 13 of
the
beams so as to encounter and engage the studs 18 on the straps 16. Thus the
slab
15 is itself anchored to the tension members or straps 16.
The concrete forming the panel 14 and forming the slab 15 are both
free from corrosive reinforcing tensioning members such as steel. The concrete
may include staple fibers of a length and thickness and material as is well
known to
one skilled in the art sufficient to reinforce the concrete to prevent or
reduce micro-
cracking.
Some additional reinforcement may be provided of a non-corrosive
nature such as a mesh or grid of a plastics material which simply assists in
reducing
cracking without providing any significant tensioning effects.
In the installation of the system, the beams are firstly installed and
located at the required positions. The beams may be interconnected by suitable
structural elements so as to ensure that they are located at the required
spacing and
are prevented from movement side to side beyond predetermined acceptable
amounts.
With the beams in place, the pre-cast panel carrying the concrete
material and the transverse tension straps is carefully placed onto the beams
at the
required position so that the side edges 14C and 14D are properly and
accurately
located just extending onto the surface 13. In this position the pre-cast
panel 14 is
itself sufficiently strong to accommodate loads using the compressive arch
loading
principal so that it can act to support the concrete to be cast in place
including the
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necessary equipment to place that concrete. Thus with the panels in place,
there is
no necessity for any further structure elements or shuttering to receive the
casting of
the concrete. Thus with the panels in place, the structure is closed and can
accommodate loading from the engineers and other operators passing over the
panels.
With the panels in place, the concrete is simply poured to form the slab
15. The slab 15 can be terminated at the outside edges of the beams as shown
in
Figures 4 and 5 to form side edges 15C and 15D of the slab or the slab may be
continuous as shown in Figure 1 and extends beyond the beams onto further
beams
which are themselves spanned by additional panels 14.
In some cases it is desirable to provide additional studs 19 attached to
the top surface of 13 of the beams. These studs are arranged either in
clusters or
in spaced array along the length of the beam and act to anchor the slab 15 to
the
beams. Thus if the anchors 19 are used, there is a connection between the slab
and
the beams. As the slab is connected to the panel by the anchors 18 and 17,
this
locates the whole structure onto the beam.
Whether or not the anchors 19 are used, there is no necessity to attach
the tension strap 16 to the beam so that these are loosely placed on top of
the
surface 13 and there is no welding required for permanent attachment or for
transfer
of the loads. To provide a better support of the panel 14 on the surface 13, a
resilient or elastomeric strip 20 can be provided which is relatively narrow
in
comparison with the surface 13 and is located under the edge of the panel
along the
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length of the beam and along the length of the panel. A suitable material is
neoprene, but other similar materials may be used.
As shown in Figure 1, the side edges 14C and 14D are inclined
downwardly and inwardly relative to the panel so that the width of the panel
at the
bottom surface is less than the width of the panel at the top surface. This
provides a
key between the panel and the slab tending to reduce the possibility of
lifting of the
slab relative to the panel.
As shown in Figure 5, there is provided a modification in which the
bottom surface 14G of the panel 14 is recessed upwardly above the strap 16 so
that
portions of the strap 16 between points 16F and 16G are fully exposed to allow
maintenance to reduce corrosion. In addition the material removed in the
center
location has no structural importance since it is below the compressive arch
which is
formed within the panel 14 and the compressive arch formed in the deck defined
by
the cast slab and the panel.
Typical panels may have a width of 1.5 to 3.0 meters. The thickness of
the panel may be of the order of 75 to 100 mm and the thickness of the
completed
deck including the cast slab and the panel will be of the order of 200 to
300mm. The
weight of the typical panel will such that it is simple to handle and can be
readily
positioned at the required location.
While specific embodiments of the invention have been described in
the foregoing, it is to be understood that these embodiments are only
exemplary.
Other embodiments of the invention are possible and are intended to be
included
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within the invention. Thus, while the exemplary embodiments use steel straps
in the
tension members, the straps may be formed of any suitable tension-sustaining
material. Where reference is made to non-metallic, reinforcing fibres, these
may be
any suitable material, for example aramid, polypropylene or glass. The
invention is
thus to be considered limited solely by the scope of the appended claims.
