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In order to increase the productivity of work in the construction field, one
way of improving the performances of the construction works is to produce
concrete precast elements.
It is necessary to closely and discerningly study as many variants as possible,
taking into account the fact that producing construction objects from precast
elements has an important disadvantage the necessity of an adequate
solution for the joining elements in order to re-establish the monolithic feature
of the structure.
It is therefore necessary to adopt the right structures, starting from theconception stage, structures which would fulfil the conditions of a rational
mounting with a reduced resources cost.
There will be emphasized the difficulties in mounting a ground floor
industrial hall with the main beams disposed transversely and bays of six
meters wideness.
It is also necessary to adopt joining details which are possible to be done and
will not require high precision classes, (impovarator) burden-some costs or
difficult makeups, which sometimes may be impossible to solve.
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There are not to be ignored the special characteristics of the concrete; each
type of material has its own properties, its own way of behaving, so that
choosing to use methods of metallic joining may turn up to be an aberration.
The realization of ground floor halls with tall, precast columns was expand
due to the facilities offered by the relization of the elements in a horizontal
position.
The methods of realizing the elements have to be known, especially the
deviations (abatere) allowed in the processes of precasting and mounting, the
shape of the joints, the possibilities of monolithization, etc.
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The complex process of transport, storage, handling and
mounting of the precast units
The real location of the construction site taking into account the resources,
will condition the possibility of realising different flows which, at the present,
can be summarised as follows:
in case that the main resources, respectively the precast elements, are
situated at a great distance from the site, usually more than 30 km, it is only
rational to transport them using the railway.This variant implies the existence of a storing place on the construction site,
situated near the railway, where the precast elements can be stocked until
they will be assembled.
This storing place has to be well conceived from the beginning, to be
organised and equipped with all the necessary installations; the precast
elements have to be organised in a certain order so as not to be difficult to
find them when necessary or to need further interior transportations.
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when the precast elements are realised at relatively short distances from the
construction site, usually in production plants situated near the urban areas,
it is reasonable to use the motorway for transport.
This flow can have several variants, one of them being the existence of astorage place on site situated in the operating range of the lifting device,
usually the tower crane when building the multi storey block of flats.
It is therefore reasonable to have storage place large enough to stock the
precast elements necessary to keep the mounting process running.
Although from a pessimistic point of view this variant seems kind of rigid and
difficult to realise, we still have to remember it and adopt it.
It is true that this will take more effort to organise and correlate all the
resources; not lastly it will also take an intellectual effort.
This is the variant that will lead to an increase of productivity and a reduction
of the costs.
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in case of heavy or very large elements, or small series, it is reasonable to
perform the precasting process on the construction site. We need to specify
that by precasting we mean arranging some places where the elements are
produced in the neighbourhood of the assembling devices.
These locations where the elements are produced need to be disposed such as
to allow a direct mounting, with no supplementary handling; it is also
necessary to create spaces to allow the transportation vehicles to pass.
Whenever the conditions described above can not be fulfilled it is preferable
to perform the precasting process on special construction polygons.
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START1
Handling
Placing intransportvehicles
Handling for
unloading
Storehouseplacing
Handling forloading
A
Placing intransportvehicles
Railwaytransport
Internal
transportation
Transport
Handling for
unloading
Storehouseplacing
On siteconcretecasting
Set out
Joiningmake-up
Handle formount
Temporary fixCrane release
Positionrectification
Reinforcing
Formworking
Concrete
placing
Formwork
removal
Start2n
B
C
A1
2
Auxiliary fluxes - other resources
SUPPLIER
SITE
TRANSPORTA
TION
SITE
STOREHOU
SE
TRANSPORT
WORKING PLACE ACTIVITIES
HANDLING MOUNTINGJOIN
MAKE UP
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The features of the precast elements
From the mounting process point of view we are interested in certain
characteristics of the precast elements, such as the geometrical sizes and their
framing into acceptable limits, the stress they suffer when being handledadopting the adequate holding and handling systems, the weight of the
elements, the characteristics of the joining details, the support surfaces, the
incorporated metallic plates, the exterior reinforcements, etc.
The concordance quality
The construction elements, their mounting and assembling into parts of
construction and then into a whole construction, are almost impossible to be
performed respecting their original design from the project. The making up
process tries to respect the original conception, but inaccuracies are inherent,
certain deviations from the original concept will appear during the execution
stage.
These deviations usually regard the designed geometrical characteristics of
the elements, such as dimensional deviations, geometrical ones, the reciprocal
orientation of the profiles and surfaces.
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The accuracy class of the metallic formwork
An important element for the characteristics of the precast elements is
represented by the exact shape of the pouring form. In the precast plants
there are generally used some metallic forms made in different sizes.
The technical specifications mention the accuracy classes, according to the
method of realising the form, table 1. These accuracy classes are imposed by
the processes taking place in the metallic confections and mechanical
processing fields.
Due to the processes that take place during the fluxes, the dimensional
variations of the metallic components, the temperature, the contraction of the
concrete, etc, the precast elements result to be classified into an inferior
accuracy class than that stipulated by the metallic form.
The designer is the one to establish the accuracy classes, according to the
special conditions of fitting, support, joining, the complexity of the mounting
method, taking into account the main dimensions of the precast element,
parameters which condition the mounting possibilities.
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Table 1 Accuracy (precision) classes
No.
Formwork feature
Dimensional Tolerances (T)Basic Dimension (mm)
900 901 3000 3000 9000 >9000
1 Fix forms, metal cutting &pouring PC5 PC5 PC5 PC6
2 Forms with mobile elements,metal cutting & pouring
PC5 PC5 PC6 PC63 Fix forms, welded metallic
profiles, plates & strips, metal cuttingPC6 PC6 PC6 PC7
4 Forms with mobile elements,welded metallic profiles, plates &strips, metal cutting PC6 PC6 PC7 PC7
5Fix forms, welded metallic
profiles, plates & strips, without metalcutting PC6 PC6 PC7 PC7
6 Forms with mobile elements,welded metallic profiles, plates &strips, without metal cutting PC7 PC7 PC7 PC7
7 Metallic profiles, plates & stripsassembled by clinch (nituire) PC7 PC8 PC8 PC8
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Precast elements attaching systems for handling and mounting
devices
These devices are hung or settled to the precast units or they are attached
using other procedures in accordance with the precast elements features and
the specific of transportation, storage and mounting.
In order to realize the transportation and the mounting there are needed somehandling devices with specific parameters corresponding to the precast
elements features.
The mounting systems have to be conceived so as to function perfectly, to be
safe during all phases, to require a minimum material consumption which
could be reused.
After use there should be no complicated dismantle procedures.
One of the simplest mounting systems is made of steel, it is loop shaped and
it has the extremities incorporated in the element during the forming phase; it
is currently in use although it doesnt have the best performances.
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Mounting system with hanging shackle
This system called hanging shackle (ureche de agare) has several
disadvantages as shown in the figure.
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Thus, in the case there is used a handling device with inclined cables and the
shackles are located in the longitudinal plane of the element, figure a, during
the moments of hanging and loading the arms of the system place themselves
following the direction of the cable; the interior arm tends to get curved
without being loaded.In this situation the entire load is on the arm from exterior which can
collapse.
It is therefore necessary that, when using devices with inclined cables the
shackles should be disposed in a perpendicular plane on the device plane; in
this case the arms will bent equally.
It is also necessary to use the same type of device during all the phases -
transportation, storage, and mounting in order to maintain the same angle
between the cables, otherwise the repeated bending of the shackles arms
could lead to possible breaking.
Other disadvantages of this system are: a quite large consumption of steel; the
usage of high supports at storage in order to create spaces between the
elements bigger than the height of the shackles; the need to cut or bend the
shackles after the mounting process is completed.
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Next we are going to present the devices fulfilling the same functions, but
having clear advantages. Thus the mounting system is made of two elements.
One of the elements formed of a metal pipe screwed at the interior or of a wire
made coil, has welded around it steel curls (bucla) and it is incorporated in
the concrete. The other element is made of a bolt having a screwed hole on its
superior part. After it is stored or placed at the mounting place the bolt is
retrieved.
Mounting system with bolt screwed in the element14
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The following figure presents how the mounting
system made of a cable is introduced into a hole in
the precast element through a plastic tube.
Mounting system made of a cable introduced into the
holes of the element
The mounting system presented in figure implies
realising some holes into the element when it is
produced, holes which will allow, during the
handling stage, to introduce metal rods screwed attheir ends in order to hang the handling devices.
Mounting system made of a bolt introduced into the
element through a hole15
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Holding systems by laying are presented in figure. Thus, the system presented
includes a yoke with its sizes corresponding to the section of the element; this
yoke has on its inferior part several holes.
The use of the device requires placing it on the superiorpart of the precast element and introducing several pipes
into the holes from the inferior part.
In order to safeguard the operation it is necessary to put
some spacers (distantier) between the yoke and the
element.
Mounting systems by laying
The system on the left side of the figure is fork shaped
and is frequently used when handling the precast strips.16
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Handling and mounting devices
The handling devices have to be conceived so as to satisfy certain
requirements such as: a good functionality, a reduced consumption of labour,
a certain degree of universality in order to use them on a wide range of
elements.
The precast elements have to be held by lifting devices in order to allow
handling procedures during the last manufacture stages, to allow loading and
unloading in the intermediate storages and also to allow mounting.
They also should not determine higher loads on the elements handled than
those appearing during exploitation, while other types of loads should be as
small as possible.
The handling devices should not be very high, nor heavy in order to use the
working parameters of the lifting devices as rational as possible, to beperfectly safe during exploitation.
Furthermore they should be properly conceived and dimensioned so as to
avoid their breaking or some of their elements falling apart provoking work
accidents.
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The picture presents the angles that can be realised by the cables with the
horizontal.
The most simple and frequently used devices with two holding points are
made of inclined cables.
The loads that appear due to cables
tilting
Devices for linear elements with two holding points
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Thus, decomposing the action of the gravitational load G/2, in the holding
point, following the direction of the cable (N) and the axes of the element (H)
the result will be:
For different inclinations of the cables, respectively values of the angle , it
results the dimension of the loads N and H.
Therefore, for = 10 it results the value of load from the cable three times
higher than the weight of the element, also similar values for the compressionstrength H.
For = 30 the load from the cable drops down to the value of the elements
weight, for = 45 it is 0.7 G; it reaches 0.5 G in the case of the value = 90.
It is important to remember that for small angles ( = 10
) the load N in thecables is six time higher than in the case of their vertical placement.
The load H which compresses the element offers concordant values as well.
The angle is limited to a maximum 45 in order not to introduce unfavourable
loads into the element and there wouldnt be needed thick cables.
tan2
G
H;
sin2
G
N
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It is not allowed the free passage of a cable over the hook of the lifting device
since this can slide and lead to shocks, respectively to cable breakings and
accidents during exploitation.
At their inferior part the cables are provided with hooks. The hook is a piece
made by forging, having a specific form.
During forging the fibres are disposed following the direction of the hooks
shape so as there would be deformation before cracking while under loads.
Each hook is marked with nominal load from manufacture.
The devices from this group have
cables held at their superior part with
a closed or opened ring
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In order to reduce the inclination of the cables, respectively the loads existing
within these and the element, there can be conceived systems with short
horizontal elements (sleepers traversa), which have the advantage of
reduced weights and relatively short heights .
The systems with cables positioned vertically, case in which there areminimum loads, need to be provided with long sleepers, accordingly
dimensioned.
The result will be relatively heavy devices having reduced heights.
Device with long sleeper
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The following figure presents a more functional device with a higher degree of
universality.
The holding points are realised so as to be functional in any position on
horizontal through some carriages moving on the sleeper.
Device which automatically places the cables
vertically
The movement of
proximity or that of
remoteness is
synchronized through a
continuous cable passedover two pulleys placed in
a horizontal plane at the
ends of the sleeper. One
carriage is held by a cable
on one of the sleepersside, the other one on the
other side.
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When the element is held in any position, the cables are coming automatically
on vertical under the action of the horizontal components moving the
carriages. The movement is realised during the beginning of the lifting phase,
while the device starts to load itself and there are only reduced forces to
induce important horizontal loads in the element.
In case of the particular system of forces, respectively the action produced by
the gravitational load, a linear element needs two simple connections only.
In order to produce devices with more than two connections it is necessary
that each supplementary connection should be compensated with a proper
system such as a cantilever with equal arms or a fixed pulley.
Devices for linear elements with more than two attaching points
The following figure presents a device with three holding points used whenhandling large panels.
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The panel with the same configuration as that in the figure can not be handled
by catching in two points because there would be some loads that can damage
it; therefore there are needed three catching points properly placed.
The device realises the compensation for the extra connection, respectively it
creates equal loads in the catching points through an equal armed lever.
Device with three attaching points used
when handling large panels
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When handling a truss there are conceived proper devices with two pulleys in
order to realise loads similar to those from the exploitation phase, respectively
compression at the superior end and stretching at the inferior one.
The truss can be held in four points while the compensation of the twoconnections is realised by a continuous passing of two cables over the pulleys.
Device used for handling the trusses
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Long linear elements with reduced transversal section need a large number of
holding points.
The compensation of the extra connections, meaning realising equal loads in
the holding points, is realised with the device presented in figure.
Device used for handling linear
elements with n attaching points
This device is made up of a sleeper
on the extremities of which it is held
a continuous cable passed over a
system of fixed and mobile pulleys.
Since in the continuous cable the
stress is the same, the load in the
holding points is represented by the
relation: N = G/n.
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In order to conceive rational devices both from the point of view of the static
equilibrium as well as from that of the compatibility with the features of the
elements being handled it is necessary to analyse these aspects and to realise a
correlation between them. It is presented a floor element held in a three connections device.
Device incompatible with the features of the
elements
Handling devices for surface elements
Although the device is correctly conceived fromthe point of view of static equilibrium under the
action of the gravitational load it is not compatible
with the features of the element.
During exploitation the element is being supported
on four sides and it is therefore accordingly
dimensioned. In the situation presented in the
figure on one of the sides there has been introduced
a support in the middle realising some cantilevers
which are not reinforced properly; these will lead
to cracks and will destroy the element.27
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It is thus compulsory the compatibility between the device and the features of
the element. Presently, on sites, there is frequently used a device with four
holding points, with no compensation.
Device with four holding points and nocompensation
As it can be seen in the figure, the lack of
compensation of the extra connections
leads to the deformation and cracking ofthe element until the longest cable start
working.
Although the crack closes after the element
is placed on the supports it still allows the
access of noxious, corrosive agents,especially during the mounting of the floor
panels in the area of the toilets at the
apartmentbuildings.
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The handling device presented in the following picture is correctly designed
because it ensures both of the imposed condition (equilibrium against the
gravitational load and compatibility with the features of the element). It is
used a fix pulley to compensate the forth connection.
Device with four cables provided with
pulley to compensate the extra connections
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In the case of some surface elements having one of the sides longer it is
necessary to place six holding points in order to realise the compatibility with
the exploitation situation.
Device with six cables and three
compensation pulleys
The starting point is the ring hanged in the
cranes hook with three connections, while
the six cable are compensated by three fixed
pulleys.
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In order to manipulate
relatively tall colums but with
reduced transversal sections it is
presented a device that has six
attaching points.
Device for handling columns
The column has three holes
where there are introduced rods
so as to realise six holding
points. Although the column is a
linear element it behaves as asurface one because of the
particular way of attachment to
the device.
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To perform the handling process on horizontal, the shipping and unshippingprocesses, as well as that of the superimposed precasting, the column is held in
all the six connections.
Two of the connections are compensated by two pulleys attached on one lever,
while the third one, at the base of the pillar is compensated by an equal
armed cantilever.
In order to place the column vertically the connections from the bottom are
untied, the column turns around it while the pulleys from the sleeper realise
the compensation of the lengths. In this phase the column supports itself onfive points two of which are compensated by the pulleys of the sleeper.
After reaching the vertical position the pillar is lifted from the ground while it
is supported on four points compensated by the pulleys of the sleeper.
The following figures present a handling device used for shipping, unshippingand mounting. It is also used for transporting the floors vertically.
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Device for handling plates during
transportation in horizontal position
Device for handling plates during transportation
in vertical position
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The conditions imposed by the norms are:
it is strictly forbidden to improvise handling devices on the construction site;
in case of devices with extra non-compensated connections some of the cables
do not work, the other cables overheat so that the safety during exploitation is
compromised;
the devices have to be certified by ISCIR (Inspectia de Stat pentru Controlul
Cazanelor, Recipientelor sub Presiune si Instalatiilor de Ridicat)and manufactured in specialised laboratories only;
when the working period has started the devices are checked on a daily basis,
and they are periodically loaded with loads higher than the nominal one,
according to the indications from the norms.
When performing the handling manoeuvres during the mounting phase one
of the pulleys from the superior part is blocked by a bolt. Pulling out the bolt
will turn the system into a mechanism and will allow tipping.
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Transportation and storage of the precast elements
In accordance with the features of the precast elements their transportation
and storage need specific proceduresmethods and means.
No matter the variant of transportation and storage chosen, there need to beobserved some specific rules in order to prevent the damage of the elements
and to avoid work accidents. These rules are:
the position of the holding points in the handling device and that of the
support areas during storage should be chosen so as not to change the support
diagram from exploitation or, if there are any other schemes adopted, there
should be no need for further reinforcements. To check the bearing capacity
as a consequence of the handling, transportation and mounting shocks, the
gravitational load is amplified with a dynamic coefficient of 1.5, according to
the norms;
during all the transportation, handling and mounting stages the holding
devices and the supports should be conceived so as the precast elements will
not loose stability;
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it is necessary for the support systems of the transport means to guarantee
stability for the elements when starting, sudden braking or sharp turnings;
according to the features of the precast elements the supports will be
manufactured so as not to damage the edges or the finished surfaces;
when the precast elements are stacked, it is necessary that the supports of
each precast element should be placed on the same vertical so as not to
damage the elements located at the base.
The supports between the precast elements should have a proper height so as
not to damage the holding system. When handling the elements with forkdevices the heights of the supports should be correlated with the features of
the device.
In order to cope with the supplementary loads appearing during handling and
transportation, the slender elements should be strengthen with pre
compressed coupling bars disposed following a rational course.
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There are conceived fixed racks (stativ) to place the elements in warehouses
which have some degree of permanency.
Fixed racks
Inclined elements
The elements are inclined and
support each other; there will be
needed supplementary manoeuvres
to choose a panel from the central