A Vision To Clear Solutions.
Scaffolding is
a temporary frame used to support people and material in the construction or
building and other large structures. It is usually a modular system of metal
pieced although it can be made out of other materials. The purpose of a working
scaffold is to provide a safe place of work with safe access suitable for the
work being done. This document sets out performance requirements for working
scaffolds. These are substantially independent of the materials of which the
scaffold is made. The standard is intended to be used as the basis for enquiry
and design .The
basic materials are tubes, couplers and boards.
Basic scaffolding
The
key elements of a scaffold are standards, ledgers and transoms. The standards,
also called uprights, are the vertical tubes that transfer the entire mass of
the structure to the ground where they rest on a square base plate to spread
the load. The base plate has a shank in its centre to hold the tube and is
sometimes pinned to a sole board. Ledgers are horizontal tubes which connect
between the standards. Transoms rest upon the ledgers at right angles. Main
transoms are placed next to the standards, they hold the standards in place and
provide support for boards; intermediate transoms are those placed between the
main transoms to provide extra support for boards. In Canada this style is
referred to as "English". "American" has the transoms
attached to the standards and is used less but has certain advantages in some
situations. Since scaffolding is a physical structure, it is possible to go in
and come out of scaffolding. As well as the tubes at right angles there are crossbraces to
increase rigidity, these are placed diagonally from ledger to ledger, next to
the standards to which they are fitted. If the braces are fitted to the ledgers
they are called ledger braces. To limit sway a facade brace is fitted to the face
of the scaffold every 30 metres or so at an angle of 35°-55° running right from
the base to the top of the scaffold and fixed at every level. Of the couplers
previously mentioned, right-angle couplers join ledgers or transoms to standards, putlog or single couplers join board bearing transoms to ledgers -
Non-board bearing transoms should be fixed using a right-angle coupler. Swivel
couplers are to connect tubes at any other angle. The actual joints are
staggered to avoid occurring at the same level in neighbouring standards. Basic
scaffold dimensioning terms. No boards, bracing or couplers shown The spacing
of the basic elements in the scaffold are fairly standard. For a general
purpose scaffold the maximum bay length is 2.1 m, for heavier work the bay size
is reduced to 2 or even 1.8 m while for inspection a bay width of up to 2.7 m
is allowed. The scaffolding width is determined by the width of the boards, the
minimum width allowed is 600 mm but a more
typical four-board scaffold would be 870
mm wide from standard to standard. More heavy duty scaffolding can require 5, 6
or even up to 8 boards width. Often an inside board is added to reduce the gap
between the inner standard and the structure. The lift height, the spacing
between ledgers, is 2 m, although the base lift can be up to 2.7 m. The diagram
above also shows a kicker lift, which is just 150 mm or so above the ground.
Transom spacing is determined by the thickness of the boards supported, 38 mm
boards require a transom spacing of no more than 1.2 m while a 50 mm board can
stand a transom spacing of 2.6 m and 63 mm boards can have a maximum span of
3.25 m. The minimum overhang for all boards is 50 mm and the maximum overhang
is no more than 4x the thickness of the board. Scaffolding in Foundations Good
foundations are essential. Often scaffold frameworks will require more than
simple base plates to safely carry and spread the load. Scaffolding can be used
without base plates on concrete or similar hard surfaces, although base plates
are always recommended. For surfaces like pavements or tarmac base plates are
necessary. For softer or more doubtful surfaces sole boards must be used,
beneath a single standard a sole board should be at least 1,000 cm2
with no dimension less than 220 mm, the thickness must be at least 35 mm. For
heavier duty scaffold much more substantial baulks set in concrete can be
required. On uneven ground steps must be cut for the base plates, a minimum
step size of around 450 mm is recommended. A working platform requires certain other
elements to be safe. They must be close-boarded, have double guard rails and
toe and stop boards. Safe and secure access must also be provided. Scaffolds
are only rarely independent structures. To provide stability for a scaffolding
(at left) framework ties are generally fixed to the adjacent building / fabric
/ steelwork. General practice is to attach a tie every 4m on alternate lifts
(traditional scaffolding) prefabricated System scaffolds require structural
connections at all frames - ie.2-3m centres (tie patterns must be provided by
the System manufacturer / supplier). The ties are coupled to the scaffold as
close to the junction of standard and ledger (node point) as possible. Due to
recent regulation changes, scaffolding ties must support +/- loads (tie/butt
loads) and lateral (shear) loads. Due to the different nature of structures
there are a variety of different ties to take advantage of the opportunities.
Through ties are put through structure openings such as windows. A vertical
inside tube crossing the opening is attached to the scaffold by a transom and a
crossing horizontal tube on the outside called a bridle tube. The gaps between
the tubes and the structure surfaces are packed or wedged with timber sections
to ensure a solid fit. Box ties are used to attach the scaffold to suitable
pillars or comparable features. Two additional transoms are put across from the
lift on each side of the feature and are joined on both sides with shorter
tubes called tie tubes. When a complete box tie is impossible a l-shaped lip
tie can be used to hook the scaffold to the structure, to limit inward movement
an additional transom, a butt transom, is place hard against the outside face
of the structure. Sometimes it is possible to use anchor ties (also called bolt
ties), these are ties fitted into holes drilled in the structure. A common type
is a ring bolt with an expanding wedge which is then tied to a node point. The
least 'invasive' tie is a reveal tie. These use an opening in the structure but
use a tube wedged horizontally in the opening. The reveal tube is usually held
in place by a reveal screw pin (an adjustable threaded bar) and protective
packing at either end. A transom tie tube links the reveal tube to the
scaffold. Reveal ties are not well regarded, they rely solely on friction and
need regular checking so it is not recommended that more than half of all ties
be reveal ties. If it is not possible to use a safe number of ties rakers can
be used. These are single tubes attached to a ledger extending out from the
scaffold at an angle of less than 75° and securely founded. A transom at the
base then completes a triangle back to the base of the main scaffold. Putlog
scaffold As well as putlog couplers there are also putlog tubes, these have a
flattened end or have been fitted with a blade. This feature allows the end of
the tube to be within or rest upon the brickwork of the structure. They can be
called a bricklayer's scaffold and as such consist only of a single row of
standards with a single ledger, the putlogs are transoms - attached to the ledger
at one end but integrated into the bricks at the other. Spacing is as general
purpose scaffold and ties are still required.
Formwork comes in three main types:
1.Traditional timber
formwork. : -
The form work is built on site out of timber and ply
wood or moisture resistant particle board.It is easy to produce but
time-consuming for larger structures, and the plywood facing has a relatively
short lifespan. It is still used extensively where the labor costs are lower
than the costs for procuring re-usable formwork. It is also the most flexible
type of formwork, so even where other systems are in use, complicated sections
may use it.
2.Engineered
Formwork Systems:
This
formwork is built out of prefabricated modules with a metal frame (usually
steel or aluminium.) and covered on the application (concreted
side with material having the wanted
surface structure (steel, aluminium, timber, etc.). The two major advantages of
formwork systems, compared to traditional timber formwork, are speed of
construction (modular systems pin, clip, or screw together quickly) and lower
life-cycle costs (barring major force, the frame is almost indestructible,
while the covering if made of wood; may have to be replaced after a few - or a
few dozen - uses, but if the covering is made with steel or aluminum the form
can achieve up to two thousand uses depending on care and the applications).
3. Re-usable
plastic formwork:
These
interlocking and modular systems are used to build widely variable, but
relatively simple, concrete structures. The panels are lightweight and very
robust. They are especially suited for low-cost, mass housing schemes
Stay-In-Place formwork systems. This formwork is assembled on site, usually out
of prefabricated insulating formwork. The formwork stays in place (or is simply
covered with earth in case of buried structures) after the concrete has cured,
and may provide thermal and acoustic insulation, space to run utilities within,
or backing for finishes. Stay-In-Place structural
formwork systems. These are in the shape
of hollow tubes, and are usually used for columns and piers. Slab formwork (deck formwork) Some
of the earliest examples of concrete slabs were built by
Roman
engineers. Because concrete is quite strong in resisting compressive load, but
has relatively poor tensile or torsional strength, these early structures
consisted of arches,vaults anddomes.
To mold these structure, temporary
scaffolding and formwork was built in the future shape of the structure. .Timber beam slab formwork Similar
to the traditional method, but stringers and joist are replaced with engineered
wood beams and supports are replaced with metal props. This makes this method
more systematic and reusable. Traditional
slab formwork Traditional timber formwork On
the dawn of the rival of concrete in slab structures, building techniques for
the temporary structures were derived again from masonry and carpentry. The
traditional slab formwork technique consists of supports out of lumber or young
tree trunks, that support rows of stringers assembled roughly 3 to 6 feet or 1
to 2 meters apart, depending on thickness of slab. Between these stringers,
joists are positioned roughly 12 inches, 30 centimeters apart upon which boards
or plywood are placed. The stringers and joists are usually 4 by 4 inch or 4 by
6 inch lumber. The most common imperial plywood thickness is 3A
inch and the most common metric thickness is 21 millimeters. Metal Beam Slab Formwork Similar
to the traditional method, but stringers and joist are replaced with aluminium
forming systems or steel beams and supports are replaced with metal props. This
also makes this method more systematic and reusable. Hand setting modular
aluminum deck formwork. Handset modular aluminum formwork. Modular Slab Formwork These
systems consist of prefabricated timber, steel or aluminum beams and formwork
modules. Modules are often no larger than 3 to 6 feet or 1 to 2 meters in size.
The beams and formwork are typically set by hand and pinned, clipped, or
screwed together. The advantages of a modular system are: does not require a
crane to place the formwork, speed of construction with unskilled labor,
formwork modules can be removed after concrete sets leaving only beams in place
prior to achieving design strength. Table
or flying form systems These systems consist
of slab formwork "tables" that are reused on multiple stories of a
building without being dismantled. The assembled sections are either lifted per
elevator or "flown" by crane from one story to the next. Once in
position the gaps between the tables or table and wall are filled with
"fillers". They vary in shape and size as well as their building
material. The use of these systems can greatly reduce the time and manual labor
involved in setting and striking the formwork. Their advantages are best
utilized by large area and simple structures. It is also common for architects
and engineers to design building around one of these systems.Structure A
table is built much the same way as a beam formwork but the single parts
of this system are connected together in a way that makes them transportable.
The most common sheathing is plywood, but steel and fiberglass are also in use.
The joists are either made from timber, wood I-beams, aluminium or steel. The
Stringers are sometimes made of wood I-beams but usually from steel channels.
These are fastened together (screwed, weld or bolted) to become a
"deck". These decks are usually rectangular but can also be other
shapes.Support All
support systems have to be height adjustable to allow the formwork to be placed
at the correct height and to be removed after the concrete is cured. Normally
adjustable metal props similar to (or the same as) those used by beam slab formwork
are used to support these systems. Some systems combine stringers and supports
into steel or aluminium trusses. Yet other systems use metal frame shoring
towers, which the decks are attached to. Another common method is to attach the
formwork decks to previously cast walls or columns, thus eradicating the use of
vertical props altogether. In this method, adjustable support shoes are bolted
through holes (sometimes tie holes) or attached to cast anchors. Size The size
of these tables can vary from 70 sqft. to 1500 sqft. or 8 m2 to 150
m2. There are two general approaches in this system.
·
Crane
handled: this approach consists of
assembling or producing the tables with a large formwork area that can only be
moved up a level by crane. Typical widths can be 15, 18 or 20ft. or 5 to 7
meters but their width can be limited, so that it is possible to transport them
assembled, without having to pay for an oversize load. The length vary and can
be up to 100ft. (or more) depending on the crane capacity. After the concrete
is cured, the decks are lowered and moved with rollers or trolleys to the edge
of the building. From then on the protruding side of the table is lifted by
crane whiles the rest of the table is rolled out of the building. After the
center of gravity is outside of the building the table reattached to another
crane and flown to the next level or position.
This technique
is fairly common in the United States and east Asian countries. The advantages
of this approach are the further reduction of manual labor time and cost per
area of slab and a simple and systematic building technique. The disadvantages
of this approach are the necessary high lifting capacity of building site
cranes, additional expensive crane time, higher material costs and little flexibility.
·
Crane fork or elevator
handled:
By
this approach the tables are limited in size and weight. Typical widths are
between 6 to 10 ft or 2 to 3 meters, typical lengths are between 12 and 20ft or
4 to 7 meters, though table sizes may vary in size and form. The major
distinction of this approach is that the tables are lifted either with a crane
transport fork or by material platform elevators attached to the side of the
building. They are usually transported horizontally to the elevator or crane
lifting platform single handedly with shifting trolleys depending on their size
and construction. Final positioning adjustments can be made by trolley. This
technique enjoys popularity in the US, Europe and generally in high labor cost
countries. The advantages of this approach in comparison to beam formwork or
modular formwork is a further reduction of labor time and cost. Smaller tables
are generally easier to customize around geometrically complicated buildings, (round
or non rectangular) or to form around columns in comparison to their large
counterparts. The disadvantages of this approach are the higher material costs
and increased crane time (if lifted with crane fork).
Usage
For
removable forms, once the concrete has been poured into formwork and has set
(or cured), the formwork is struck or stripped (removed) to expose the finished
concrete. The time between pouring and formwork stripping depends on the job
specifications, the cure required, and whether the form is supporting any
weight, but is usually at least 24 hours after the pour is completed. For
example, the California Department of Transportation
requires the forms to be in place for
1-7 days after pouring, while the Washington State Department of Transportation
requires the forms to stay in place for 3 days with a damp blanket on the
outside.
Spectacular
accidents have occurred when the forms were either removed too soon or had been
under-designed to carry the load imposed by the weight of the uncured concrete.
Less critical and much more common (though no less embarrassing and often
costly) are those cases in which underdesigned formwork bends or breaks during
the filling process (especially if filled with a high-pressure concrete pump).
This then results in fresh
concrete escaping out of the formwork in a
form blowout, often in large quantities.
Concrete exerts
less pressure against the forms as it hardens, so forms are usually designed to
withstand a number of feet per hour of pour rate to give the concrete at the
bottom time to firm up.
The
hardening is an asymptotic process, meaning that most of the final strength
will be achieved after a r short time, though some further hardening can occur
depending on the cement type and admixtures.
Wet
concrete also applies hydrostatic pressure to formwork. The pressure at the
bottom of the form is therefore greater than at the top. In the illustration of
the column formwork to the right, the 'column clamps' are closer together at
the bottom. Note that the column is braced with steel adjustable 'formwork
props' and uses 20 mm 'through bolts' to further support the long side of the
column.
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