Note: Descriptions are shown in the official language in which they were submitted.
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BUILDING STRUCTURAL ELEMENT
FIELD OF THE INVENTION
This invention relates to a building structural element and a method
of making a building structural element. More particularly this invention
relates to
a building structural element and method of making such an element which is
used in floor systems of buildings.
The invention may also be used for many other building uses, such
as a roof deck over road tunnels, railway tracks and the like. It can be used
for
large multi-bay floor spaces, particularly with high floor to floor dimension
and/or
long spans in one or both directions and/or high floor loadings such as floor
decks
for retail, recreational or other use. It can also be used for bridge decks
and for
any other uses where long spans andlor high loads have to be carried between
supports.
One particular aspect of the invention relates to a floor beam used in
the construction of floor systems of a building and is preferably made from a
combination of two components, preferably made from steel, and a cementitious
material such as concrete.
BACKGROUND OF THE INVENTION
Beams are generally required for floor systems of buildings that span
in excess of 8 to 10 metres. This is typically the upper range or limit that
floor
slabs can extend without using beams for structural support. Supporting
columns
are usually located 6 to 9 metres apart of which a common spacing is 8.4
metres.
This is a suitable office module that accommodates the widths of a column plus
three spaces that may be used for vehicles located between adjacent columns
and used for any parking levels below the office floors (or retail floors,
institutional
floors or other floors) of the building. Beams are required to span the larger
dimension of a rectangular floor panel between a grid of four columns or
between
a pair of external columns on the outside of the building and the core or
central
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part of a multi storey building. The core is typically used to house lift
wells and
other common rooms accessible by people on each floor. The floor systems
spanning between these beams can and do take on many forms which depends
on local availability, economies and the client's, engineer's or builder's
preference
or a combination of preferences of any one of these three entities.
Floor systems that span about 8 metres in one direction between the
supporting beams can vary from formed concrete slabs on conventional formwork
or large table forms, concrete slabs on metal tray forms that are temporarily
propped on supports, secondary steel beams at close centres (2.1 to 3.0
metres)
supporting a relatively thin slab on an unpropped metal tray forms. Also there
are
other proprietary systems such as various stressed plank systems that may be
able to span a 7 to 8 metre distance unpropped.
For the supporting beams the upper range of suitability for reinforced
concrete shallow band beams is about 10 to 12 metres. For pre-stressed shallow
band beams (usually post-tensioned) the upper range of suitability is
generally 12
to 14 metres. However for spans in excess of 12 to 14 metres, special
attention
and detail is required with common solutions being presented by either a steel
floor beam system or a pre-stressed concrete floor beam system.
In the steel floor beam system there are a series of steel beams
separated at their centres between 2.1 to 3.0 metres spanning the larger
distance
of the rectangular floor plan and are in turn supported by "primary" beams
that
span the shorter distance between the support columns. A particular problem
that
most engineers or designers are faced with in using a floor beam system is to
reduce the amount of deflection of each beam either due to dead loads or live
loads or a combination of both. Unnecessarily large deflections and vibrations
of
the floor systems can affect the amenity of the floor.
With the steel floor beam system used at present it has the
disadvantage that each beam needs substantial connections at its supports. The
beams and the connections usually require an applied protective coating to
give
them resistance to fire. Although such steel floor beams can be made to act
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compositely with the relatively thin floor slab that they support using shear
studs,
there is a small ongoing component of dead load deflection due to creep of the
concrete and shear stud interface. Deflections of the steel beams due to dead
loads mostly occur as soon as the loads are applied and can be allowed for by
pre
cambering the beam. However floor deflections and vibrations due to transient
live loads are still an inherent problem particularly for larger spans, as the
composite steel beam is less stiff than a reinforced concrete or pre-stressed
concrete beam that would be used for the same span.
The other type of floor beam system that is generally used today is a
post-tensioned, pre-stressed concrete floor beam system. They include concrete
beams that have a deep aspect ratio, in other words the dimensions of the
concrete beam are such that it is deeper than its width and these concrete
beams
are generally pre-stressed for spans in excess of 10 metres. The concrete
beams
are poured in situ with the slab that they support. Then pre-stressing is
applied by
post-tensioned tendons that are stressed when the concrete has attained
sufficient
strength usually within 3 to 6 days of pouring. The concrete beam and the
adjacent slab panels are usually formed on large table forms that are crane
lifted
from floor to floor. Usually two sets of tables are used to maintain a
preferred floor
cycle of about one week with half the floor area poured at a time to provide
continuity of work for the various trades. Very little prefabrication is
possible other
than the reinforcing cage for the beam that may or may not include the pre-
stressing tendons. Any prefabricated cage needs to be well braced and cradled
to
be able to be crane lifted into the beam formwork. Generally no difficult
connections exist at the supports of the beam as concrete stitches the beam to
the
supports. Where a poured concrete beam is supported at a concrete core that
has been "jump formed" ahead of the main floors, the connection can be a
simple
rebate in the face of the wall of the core and reinforcing bars at the top and
bottom
of the beam are screwed into ferrules anchored into the core wall.
Alternatively
the beam can be seated into pockets left in the core wall.
The pre-stressed concrete floor system is stiffer than the steel floor
beam system required to span the same distance and is thus less susceptible to
floor vibrations and deflections due to transient loads. However as concrete
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4
creeps under sustained load, the incremental deflection of the floor system
that
occurs after the floor is occupied is not only that due to live load and light
weight
partitions, but also a significant proportion of deflection from the dead load
due to
the creep component. Pre-stressing may balance out most of the deflection due
solely to the dead load. However, as the axial pre-stressing imparts a
permanent
axial force to the beam, there are losses of the prestress force from the
resulting
time-dependent shortening that will lead to further incremental deflection of
the
beam.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a building structural
element that substantially overcomes one or more of the above disadvantages.
More particularly the present invention provides a building structural element
having minimal deflections and substantially maintaining pre-stressed force
axially
therewithin and reduces the loss of such prestress force due to axial creep
that
shortens beams as in prior art systems.
According to a first aspect of the invention, there is provided a
building structural element comprising:
a pair of beams, each beam in said pair having a first flange portion,
a second flange portion and a web portion extending between said first flange
portion and said second flange portion;
a plate member adapted to engage one of said first flange portion or
said second flange portion of each beam such that an interior space is defined
by
each beam in said pair of beams and said plate member;
wherein cementitious material occupies a substantial volume of said
interior space to form a non-unitary building structural element and said
building
structural element has residual or no deflection under dead load after
application
of a post-tensioned pre-stressing force.
The building structural element may further comprise one or more
tendons extending along a length of the interior space defined between each
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Received 13 May 2003
., , beam and said plate member which may be a metal tray form soffit. Each
one or
more tendons may be pre-stressed to provide an upwardly directed force to
counteract a portion of the dead load. The first flange portion of each beam
may
support part of a floor span after the element is secured at each end.
Typically,
5 the element may extend between a column and a core of a building. The
element
may end a short distance from the core and a short distance from the column or
alternatively a short distance from a column at each end and is temporarily
supported on false work at its end and possibly also at mid span.
Each beam is preferably constructed of a metal, such as steel, and
the cementitious material is preferably concrete.
According to a second aspect of the invention, there is provided a
building structural element comprising:
a pair of beams, each beam in said pair of beams having a top flange
portion, a bottom flange portion and a web portion extending between said top
flange portion and said bottom flange portion;
a plate member adapted to engage respective bottom flange portions
of each beam in said pair of beams such that an interior space is defined by
each
beam in said pair of beams and said plate member;
wherein cementitious material occupies a substantial volume of said
interior space to form a non-unitary building structural element and said
building
structural element has a residual or no deflection under dead load after
application
of a post-tensioned pre-stressing force.
The plate member may be a metal tray form soffit or other suitable
horizontal soffit surface.
According to a third aspect of the invention, there is provided a
building structural element comprising:
a generally U-shaped channel means having a pair of opposed side
walls and a further portion joining each side wall;
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Received 13 May 2003
6
.., , wherein said pair of opposed side walls and said further portion
define an interior space; and
wherein cementitious material occupies a substantial volume of said
interior space and said building structural element has residual or no
deflection
under dead load after application of a post-tensioned pre-stressing force.
According to a fourth aspect of the invention, there is provided a
method of making a building structural element comprising the steps of:
constructing a pair of beams, each beam in said pair of beams
comprising a first flange portion, a second flange portion and a web portion
extending between said first flange portion and said second flange portion;
forming and assembling a plate member such that the plate member
engages one of either said first flange portion or said second flange portion
of
each beam so as to create an interior space defined by said pair of beams and
plate member; and
pouring a cementitious material into said interior space to form a
non-unitary building structural element such that on curing of said
cementitious
material and application of a post-tensioned pre-stressing force, said
building
structural element has substantially no deflection under dead load.
The pouring step may be done separately or as part of pouring the
adjacent floor spans which the element supports. The element may have its ends
initially supported on temporary support structures adjacent to the permanent
end
supports of the beam with possible additional supports) along the span.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the invention will hereinafter be
described, by way of example only, with reference to the drawings wherein:
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Figure 1 is a plan view of part of a building floor system extending
between a core wall of a building and an edge of the building;
Figure 2, separated into Figures 2A and 2B, is a side sectional view
taken on the line A-A of Figure 1;
Figure 3 is a sectional view of a structural element in use according
to one embodiment of the invention taken on the line B-B of Figure 2;
Figure 4 is a sectional view of the structural element in use taken on
the line C-C of Figure 2;
Figure 5 is a sectional view of a structural element in use according
to a further embodiment and similar to Figure 3;
Figures 5(a) and 5(d) are side views of the structural element in
Figure 5 showing separate conditions of the structural element;
Figure 6 is a sectional view of the further embodiment of the
structural element in use similar to Figure 4;
Figure 7 is a side sectional view showing the structural element
applied to a tunnel cover;
Figure 8 is a side view of a prior art pre-cast structural element; and
Figure 9 shows a plan view and a side view of structural elements
across a floor span with secondary supporting beams.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Shown in Figure 1 is a plan view of a pair of structural elements 1
that each extend from a perimeter column 2 on an outer edge of a building and
the
core wall 3 in the central part of the building. The span of each structural
element
may extend up to 18 metres in length and beyond and the separation between
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each element 1 is dependent on the separation between each perimeter column
which as previously discussed may be equivalent to fit three spaces for cars
in
parking levels underneath the office floors which typically may be anywhere
between 6 to 9 metres. Between the perimeter columns 2 extends an edge beam
16 and the floor 4 extends between the adjacent elements 1. The edge beam 16
can incorporate an inner steel beam that stops just short of the sides of the
perimeter columns 2, so as not to require any physical connection to the
column 2
and is supported on the same external support frame 19 that supports the main
structural element 1. This steel component of the edge beam 16 is precambered
to take the dead loads, allowing minimal deflections along the edge of the
building
that may affect any facade glazing. It is to be noted that in Figure 1
although only
two beams are shown any number of beams needed to support a designated floor
area may be used.
With reference to Figures 3 and 4 the element 1 is essentially
constructed of an outer shell, typically made from steel which comprises first
and
second (side shell) beams 5 generally in the shape of I-beams each having a
web
portion 30 and at either end of the web portion 30 exists first and second
flange
portions 31 and 32. Extending at the lower portion of the element 1 is a plate
member or (form tray deck soffit)13 generally made from metal and more
particularly steel whereby this extends between each bottom or second flange
portion 32 of the side shell beams 5 and is affixed or otherwise engaged with
the
flange portions 32. Shear studs 12 extending from the web portion 30 of the
side
shell beams 5 are used to obtain integral action with the cementitious
material that
is poured into an interior space 57 of the shell defined by each beam 5 and
the
plate member 13. Ligatures 23 generally in the form of a U-shape extend from
within the floor 4 downwardly into the interior space 57 and through raised
ribs of
the plate member 13. This is also used for extra support for the cementitious
material that is poured into the interior of the shell. Furthermore
reinforcement
bars 24 are also provided within the element 1. A closer or spacer 15 also is
positioned within the floor 4 to tie the cage structure to the side shell
beams 5.
Pre-stressed tendon elements 10 are several in number and shall be described
with reference to Figure 2 hereinafter.
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It is to be noted that the pair of beams 5, and various reinforcing for
the beam can be pre-fabricated off site in a controlled production line
environment
and be transported to the site with its reinforcement bars 24 and ligatures
23, pre-
stressed tendon elements 6, 8 and 10 and bracing 15 to keep the reinforcing
elements in place. Tendon element 6 is a dead end anchor for one of the
tendons
and tendon element 8 is a pressed metal form incorporating a live end anchor
recess. The two steel side shell beams 5, coupled with bracing 25 for handling
and
transport, enable the complete assembly to be delivered to a site and in one
simple lift, generally of around 4.5 tonnes for an 18 metre beam, be in place
on
10 pre-set support frames at its two ends, the support frames being designated
by 18
and 19 and at its mid span with support frame 20.
With reference to Figure 2 at the core wall 3 of the building the
element 1 may be housed adjacent to a rebate 22 in the wall 3 and the
individual
reinforcement bars 14 and 17 screwed into ferrules 21 that have been precast
into
the jump form core wall 3 for both the top and bottom reinforcing connections
to
respectively go to reinforcement bars 14 and 17. At the other end of the beam,
connection to the external column 2 all that is required is that the
connecting
reinforcing bars 14 and 17 extend the required length past the face of the
column
2. These reinforcing bars 14 and 17 together with the beam ligatures 11 at the
end sections of the element 1 are the only reinforcing steel for the element 1
that
needs to be fixed on site.
Whilst small (up to 150mm diameter) penetrations for fire sprinkler
pipes, sewer pipes etc can be accommodated through the web of the element 1,
each end of each beam can be stepped up to accommodate any major service
duct reticulation without impinging on the ceiling height by simply stepping
up the
bottom flange portion 32 of each steel side shell beam 5. This is more clearly
shown in Figure 2 where the step 7 is shown such that the space between the
step and either the core wall 3 or column 2 designated as 9 provides a space
for
such a service duct reticulation if required. This step also allows the
housing of
the live or jacking end for one or more pre-stressed tendons 10. The tendons
10
as mentioned are post-tensioned on site and may include any number as desired
to be located in the interior space between each steel side shell beam 5.
Shown
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in Figure 2 a pre-stressed tendon extends between the steps 7 in the bottom
flange portion 32 and further pre-stressing tendons 10 are shown extending the
full length of the beams and having a drape, in other words stressed in a
concave
manner and this is done to provide an uplift force or component that cancels
out
5 anywhere between 50% to 100% of the total dead load, although about 50-60%
is
usually sufficient for these beams, with the balance of the dead load and
transient
load capacity being provided by the steel shells of the prefabricated beam and
the
cementitious material. The dead load is deemed that weight comprised of the
floor itself, the beams and permanent superimposed loads such as ceiling
10 surfaces and floor finishes. The two steel side shell beams 5 are generally
precambered so that once the temporary props are removed all of the deflection
due to the dead load has been accommodated. In reality, the steel side shell
beams 5, the pre-stressing tendons and the concrete core filling the interior
space
of the pair of beams 5 interact together to share the total load.
Nevertheless, the
beams 5 and the pre-stressing tendons substantially prevent the reinforced
component from taking any significant proportion of the load and certainly
will not
allow the concrete component to creep substantially as the pre-stressing
tendons
plus the precambered steel shell beams 5 between them are capable of taking
the
total dead load before the beam has deflected back to the horizontal.
In use, as mentioned, before the construction of the side shell beams
5, shear studs and prestress tendons 10 and most of the beams reinforcement
are
done offsite and the reinforcing members 14, 17 and ligatures 11 to the end
section are done on site. The element 1 is then placed on its temporary
supports
just short of the rebate 22 in the core wall and the column at the other end.
The
reinforcing bars 14 and 17 are then placed into the respective ferrules and
together with the ligatures 11 placed on site and thereafter the floor is
poured such
that the cementitious material, or generally concrete, is poured into the
floor
structure and allowed to fill a substantial volume of the interior space 57
within the
side shell beams 5. The combination of the two steel side shell beams 5 and
the
cementitious (concrete) material act integrally firstly by the concrete being
poured
between and over the steel beams and also via the shear studs 12 which are
shop
welded to the insides of each of the webs 30 of the beams 5.
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Thus the two side steel beams 5 significantly increase the axial steel
content of the composite beam which acts as a compression member under the
action of the pre-stressing load, such that loss of any prestress force due to
axial
creep shortening of the beam is minimal. This hybrid beam construction also
then
can have no overall deflection from dead load or it can be provided with a
slight
residual upwards camber before the transient loads. The remaining robust
concrete beam as an uncracked section is available to assist the other two
components with the incremental deflections due to the live load and any other
transient loads.
As the external columns do not initially support any of the floor 4 and
element 1, the floor 4 and element 1 and columns 2 could be poured the same
day. The columns 2 may be poured whilst the floor formwork and reinforcing to
the floor panels 4 between the hybrid elements 1 is being installed. It is
envisaged
that with a well organised work force, even for large floor areas, three day
floor
cycles or even less could regularly be achieved by pouring half the floor on
the
first day, pouring the other half on the second day and preparing columns and
lift
perimeter shutters etc on the third day.
The concrete part of the element 1 is proportioned to take the
reduced fire design load condition being 1.1 times the dead load plus 0.4
times the
live load in Australia and similar figures used in other countries. Thus there
is no
need for either fire protection or a complicated fire engineering analysis if
this can
in fact provide a solution. The combined capacity of the steel and pre-
stressed
concrete beams is more than adequate to carry the in-service load which is
generally 1.25 times the dead load plus 1.5 the live load in Australia and
similar
figures are used in other countries.
Most codes set deflection limits for incremental deflection, due to any
creep component of the dead load plus the deflection due to live load and plus
the
weight of the partitions, at span/500. Thus for an 18 metre span this limit is
36
mm. For any building with large internal spans parallel to and adjacent to the
side
of the building, this magnitude of differential deflection between the floor
next to a
column at the side of the building, where there is zero deflection, and the
mid span
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deflection at the adjacent internal beam, which may be as close as 2.5 metres
for
a steel beam floor system, is obviously too much. More stringent deflection
criteria than the code requires may be necessary. The present invention
through
its hybrid steel and concrete beam can achieve maximum incremental deflections
of the order of under 20 mm for such 18 meter spans.
With reference to Figures 5 and 6 there is shown a further
embodiment of the present invention wherein the structural element 1 is made
from a single unitary or joined construction. Specifically, instead of having
a pair of
opposed beams linked by a plate member as in the first embodiment, the
embodiment in these Figures has a generally U-shape channel formed of a pair
of
side walls 33 and 34 and a bottom or further portion 35 linking each of the
side
walls 33 and 34. Preferably, the construction is made out of steel. Upper
portions
of the side walls 33 and 34 respectively have flange elements 36 and 37 for
supporting part of the floor 4.
With reference to Figures 5(a), (b), (c) and (d) the structural element
can be made from either a single piece of steel plate for example utilising
four
folds indicated in Figure 5(a) at 38, 39, 40 and 41 (with no welding).
Alternatively,
the construction could be welded at point 42 and retain the four folds 38
through to
41 as shown in Figure 5(b) giving a two plate construction. In Figure 5(c)
there is
shown an alternative arrangement for the structural element retaining folds 38
and
41 but welded at points 43 and 44, providing a three plate construction.
Finally in
Figure 5(d) five pieces of steel plate could be used with no folds and four
welds as
indicated at points 45, 46, 47 and 48.
The invention can also be used for shorter spans using shallower
side shell steel beams with or without the use of pre-stressing tendons and
with or
without notches in the bottom of the ends of the beam to accommodate major
service duct reticulation.
Where pre-stressing tendons are used for such shallow beams that
do not have service duct step-ups that double as stressing anchor points then
the
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anchors can be stressed from rebate pockets in the top of the floor that are
filled in
later after stressing and grouting of the tendons.
With reference to Figure 7, and as mentioned previously, the
invention is suitable for use in road tunnels using a "cut and cover" method.
This
involves lifting into position each of the structural elements, which already
have
their reinforcement and prefabricated in place, using relatively low load
capacity
cranes. This compares to extremely heavy precast pre-stressed concrete beams
49 used in prior art systems and shown in Figure 8. The heavy precast pre-
stressed concrete beams 49 have required extremely large cranes to be used for
such lifting applications. With reference to Figure 8 the beams 49 have
relatively
thin flanges 50 and a topping slab 51 normally used for such applications.
There is
usually the necessity for tanking or using a waterproof membrane 52 over the
full
extent of the deck, which in turn requires a protective wearing slab 53. All
of these
can be dispensed with by using a watertight integral pour that is possible
with the
use of the present invention. With reference back to Figure 7 this involves
the use
of fluid barrier means or more particularly water stops 58 and local tanking
(or
using waterproof membranes) 54 fitted to ensure that the roof of the tunnel is
watertight. The floor deck 55 of the span between the structural elements 1
can be
sufficiently thick and poured in sections of suitable size and, if necessary,
pre-
stressed between control joints 56. The water stops 53 sand the local tanking
or
membranes 54 are fitted to each of the control joints 56.
With reference to Figure 9 and as mentioned previously, the
invention is also suitable for large multi-bay floor spaces, particularly with
high
floor to floor dimension and/or long spans in one or both directions and/or
high
floor loadings such as floor decks for retail, recreational or other use. The
building
element (1 ) can be used to span between columns (59), to which secondary
beams (60) are bolted (61 ), supporting the floor slab (4) between the
structural
elements (1 ).
The structural elements can be tailored to span the self-weight of the
floor structure, plus construction live load during construction as simply
supported
between columns.
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The structural element then forms a reinforcement concrete or pre-
stressed concrete element that is continuous over several spans for the loads
that
need to be supported by the floor deck.
