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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

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