safety management & site establishment · e.g. as falsework supporting a formwork system (to be...
TRANSCRIPT
How access to heights is gained and how structures are temporarily
supported in the construction process
Different styles of formwork for moulding concrete
Excavation and trenching and other groundwork techniques
Learning Outcomes
Temporary i.e. non-permanent works
Temporary Works are installed on site to help fulfil the
execution of the actual (permanent) contract works
Temporary Works will be dismantled/removed from site upon
fulfilment of their respective purposes.
This Unit looks at four types of temporary works: o Access
o Support
o Protection
o Groundworks (including excavations, ground supports and
hydro-geological controls)
Introduction
Frequently, temporary works play multiple roles but their selection
is based on the primary project requirements
This Unit details typical examples of the intertwined functions of
temporary works to be expected on modern sites
It also describes the indispensable roles that they typically
function in every construction project
These are roles that usually account for a significant percentage
of the project’s running costs
Introduction
An example showcasing the extensive and combined utilities of temporary support and access structures.
Ladders should be a means
of access, not a working
platform
Main selection considerations
will reside on suitability of
work, safety/durability, cost
and portability.
Easily available, ladders are essential as both
the main and back-up tools of access
Ladders
Must be positioned so as not to tip the scaffold
Hook-on and attachable ladders must be
specifically designed for use with the type of
scaffold on which they are used
Have rest platforms provided at a max. of 10m
vertical intervals
Portable, hook-on, and attachable
ladders:
When positioned their bottom step is not
more than 24 inches above the scaffold
supporting lever.
Have rest platforms at maximum vertical
intervals of 12 feet
Have a minimum step width of 16
inches, except for mobile scaffold
stairway-type ladders, which shall have
a minimum step width of 11½ inches.
Have slip-resistant treads on all steps
and landings.
Steps and rungs of ladders and
stairway-type ladders shall line up
vertically with each other between rest
platforms.
Stairway-type ladders:
• Conventional step-ladders have rectangular stiles and flat
treads that are arranged to be horizontal when in use, and are
restrained in position by means of stays, chains or cords.
• Some variants use flat-topped rungs, while others could be of
tubular construction.
Step ladders
• Despite being temporary structures, the design of scaffolds
follows the principles laid down for permanent structures
• There must be no deviation from sound structural principles
• There is generally enough variety in standard scaffold equipment
to erect a platform to suit most work requirements
• Scaffolds are sometimes used for purposes other than access
e.g. as falsework supporting a formwork system (to be discussed
later) during concreting processes
Common (independent) scaffolds
Common scaffolds
Putlog
A putlog scaffold
consists of a single row
of standards, parallel to
the face of the building
and set as far away from
it as is necessary to
accommodate a platform
of four or five boards
wide, with the inner
edge of the platform as
close to the wall as is
practicable
Common scaffolds Birdcage
• Birdcage scaffolds are commonly used for access to soffit or ceiling, as well as to provide heavy-duty and sturdy falsework support for horizontal slab casting.
• Due to its modular assembly and adjustable members, the entire mass of support components can be easily shaped to provide horizontal support to massive areas.
• It also assists in providing a more uniformly distributed loading pattern from the structure to the ground
17
Tower scaffolding
•More efficient than a ladder,
•All towers have to be endorsed by a professional engineer,
•Most commercially produced towers can be assembled without the use of tools
•Easy to transport within short distances due to the casters.
18
Truss-out scaffold
•It is useful in high-rise construction,
•It is useful in avoiding busy sidewalk pedestrian traffic,
•Requires highly skilled personal to erect and dismantle.
•There is an extra risk on H&S due to vibrations and heights etc.
Scaffold Class Activity:
Scaffold Types;
•Tower scaffolds
•Common scaffolds (independent scaffold)
•Putlog scaffolds
•Birdcage scaffold
•Truss-out scaffold
Working as a group;
1. Produce a sketch drawing of the each type, highlight main elements,
2. Find 3 examples works/cases/scenarios for each, when these scaffold types would be more useful than others,
3. Identify safety and practicality concerns for each type,
4. Identify safety and practicality benefits for each type,
• Hoists are used to transport personnel and
materials to different working levels
• Modern hoists operate on the rack and pinion
system
• The hoist unit have its drive motor fitted on top
of the car, along with brake and gear
• This technology enables the car to climb up
and down the mast at a controlled speed
Hoists
• Mobile elevating platforms are used as an alternative to scaffolds and
suspended cradles (described later in the Unit)
• They are particularly suitable for short duration tasks requiring high
mobility of the access structures, especially so for retrofitting projects
where clients are seeking short durations
• The market offers a wide variety of these platforms, and guidance
on specific applications should be sought from the manufacturers
• There are generally two categories of mobile elevating platforms:
Self-propelled and Vehicle-mounted
• http://www.youtube.com/watch?v=_sqTICuXsMg
Mobile elevating platforms
Self-propelled Platform Hoists
Articulated-telescopic boom Self-propelled Scissor platform
•These can easily be manoeuvred
into position by onboard controls.
•They are mainly available with the
following boom types, namely:
scissor, telescopic and articulated
Vehicle-mounted platform Hoists
• These platforms come in various sizes
and capacities, from small trailer-
mounted platforms to large truck-
mounted types
• Smaller platforms are commonly used
in highway and other road-related
maintenance (e.g. pruning trees,
servicing street-lamps etc.)
• Larger versions are used where
access by other methods are deemed
either too expensive or time-
consuming
• Mast climbing platforms allow access to a localised
area of a project
• The rack and pinion drive gives an adjustable
working platform that can be positioned exactly
to suit the task in hand
• Tools and materials can be carried up to the
work site, together with the operatives
• Mast climbing platforms are for work and access
purposes only and must not be used for transporting
men and materials between levels.
• The three main components are; Mast(s) or tower(s);
a platform capable of supporting persons &
equipment and a chassis supporting the tower/mast
Mast climbing platform hoists
• Two-point adjustable suspension scaffolds, also known as
‘swing-stage scaffolds’ or ‘gondolas’, are perhaps the most
common type of suspended scaffold.
• Hung by ropes or cables connected to stirrups at each end of
the platform, they are commonly seen to be used by window
cleaners on skyscrapers, and play a prominent role in high-
rise construction
Suspended cradle (gondola)
Suspended cradle (gondola)
Suspended cradle (gondola)
• Abseiling (or industrial roped
access) can provide a safe and
cost-effective method of access
for light work commonly of
maintenance or inspection nature.
• Modern roped access equipment
and techniques allow fully trained
specialist operatives to reach
highly inaccessible locations,
some of extreme conditions (e.g.
from narrow mineshafts to
overhanging external details
under apexes of skyscrapers).
Abseiling
It is a highly feasible method to consider if
• Ground conditions are either unknown or
unsuitable to support any vertical access (e.g.
crane, scaffold & the like).
• In situations when the heights of work locations
cannot be reached from the ground (e.g. due to
safety or physical constraints) and no strong
anchorage points are available for gondolas.
• Time and cost efficient for light tasks (e.g.
maintenance of the main glass pyramid at Louvre
Museum , France)
Abseiling
BS 6100, Section 6.5,[10] defines formwork as ‘A
structure, usually temporary, but in some cases wholly
or partly permanent, used to contain poured concrete
to mould it to the required dimensions and support it
until it is able to support itself. It consists, primarily, of
the face contact material and the bearers that directly
support the face material.’
Kindly explain the difference between the formwork
and the falsework?
Formwork
Formwork vs Falsework
The term ‘formwork’ is commonly confused and
associated with another, namely ‘falsework’, the latter
being a term used to described temporary support systems
such as those scaffold supports as previously covered.
BS 5975[12] defines
falsework as ‘Any
temporary structure
used to support a
permanent structure
during its erection and
until it becomes self
supporting.’
• Selection of the formwork system is a key factor that governs the
success of a project in terms of time, cost, quality and safety
• For high-rise buildings, the most effective plan is for the works to
achieve a very short floor cycle
• The key to achieving this is to exploit an efficient and
appropriately designed formwork system.
• Modern buildings are generally complex in terms of scale and
size so the design and use of the right formwork system, will
contribute substantially to the overall success of a project.
Formwork
Kindly discuss the common formwork materials and types available in
construction. Produce a spider diagram to include the following;
• Common formwork materials
• Common types of formwork,
• Implications and uses of each category,
• Disadvantages of each type.
Formwork
Categories of Formwork
• Size
• Location of use
• Material of construction
• Nature of operation
• Proprietary system
• Materials used for formwork are
traditionally limited due to the dilemma
between cost and performance.
• Timber in general is still the most
popular formwork material, due to its low
initial cost and adaptability.
Categories of Formwork
• In recent years, full aluminium formwork system has been used but
the performance is being questioned by many, particularly with
regards to additional costs and the need for specialised workmen - http://www.youtube.com/watch?feature=endscreen&v=Yqu21vPBylY&NR=1
Categories of Formwork
• Steel in either hot-rolled or cold-formed sections and in combination
with other sheeting materials, is another popular choice of formwork
material.
Categories of Formwork
Most timber and aluminium forms can be assembled manually, due to
their weight, design and construction.
• It is labour intensive, and used in simpler jobs; or occasionally
used in very large or complex buildings to attain the benefit in
flexibility
• Some systems are equipped with a degree of mobility to ease the
erection and striking processes
• These formworks are generally categorised as either the crane-
lifted types or the mechanised slip-form systems.
• In the crane-lifted category large panels are fabricated either in
steel sections and sheeting, or using plywood sheeting and
stiffened by metal studs and soldiers.
• These large panels can be positioned either on a solid slab or
fixed onto brackets (e.g. should they be used for external walls or
shafts).
Formwork
• Slip-form formwork systems use hydraulic or screw-jack systems (either
automated or manual), and these systems allow for continuous casting till the
end of a typical section is reached.
• Slip form formwork is raised vertically in a continuous process. It is a method
of vertically extruding a reinforced concrete section and is suitable for
construction of core walls in high-rise structures – lift shafts, stair shafts,
towers, etc.
• Slip-form system derived its name from the fact that the formwork itself
actually ‘slip off’ a previously cast structure
• It moves when the structure has taken physical shape with both its
cementitious properties and composite bond with the reinforcements being set
(i.e. harden) to a safe and acceptable level for the absence of the physical
form support.
• The process is continuous and encourages a sense of urgency in the steel-
fixers and casting crew to adhere to appropriately timed and scheduled
activities in order to compliment the continuum.
Slip-form formwork
Slip-form formwork
http://www.youtube.com/watch?v=LKoj-N0-YyA
http://www.youtube.com/watch?v=zrUNHQeDkXw
Climbing-form formwork
http://www.youtube.com/watch?v=xIPpBvY
Sx8Q
Repetitive sequence of work:
• High-rise block structures usually create highly repetitive cycles of
work and may be suitable for certain kinds of formwork.
• However, for horizontally spanned buildings, the level of
repetitiveness will be limited
Physical site constraints:
• Sites with numerous physical and contractual restrictions (e.g.
sloped grounds, minimal site access or manoeuvre space, close
proximity to sensitive structures), will increase difficulties from the
mobilisation stage (i.e. getting the formwork onto site and storing
them) to subsequent erection.
Formwork: Construction-related factors
Speed of work:
• Work on low rise construction sites can be accelerated by
the introduction of additional sets of formwork to create more
independent work sites.
• This increases costs and should be considered only when
time is of the essence e.g. when the risk of imposed delay
penalties exceeds the costs of having additional systems
• For high-rise buildings, the mere increase of formwork input
cannot often fulfil the need for speed in construction, as the
critical path depends on individual floor-cycles times.
Therefore the selected formwork design needs to support
minimal floor-cycle times.
Formwork: Construction-related
factors cont’d
Formwork: Construction-related factors cont’d
Recycling of formwork:
• The number of times timber formwork can be reused is usually
limited to its durability after every striking process (i.e. the removal
of falsework, struts and wedges, followed by plywood sheetings)
• Oil-based coatings are applied to contact surfaces of the plywood
sheetings and left to dry, prior to the erection process
• Timber form may usually be used for up to ten casts, thus making it
economically viable as the main option for formwork
• Though reusability of metal form is greatly superior, its high initial
and maintenance costs will often discourage its choice of use
• Careful balance between cost, speed, performance and quality of
outputs should be properly maintained when making the selection
Construction planning and management:
• Planning i.e. phasing or sectioning arrangements, integration of
the structures, site-layout and setting up arrangements, and the
hoisting and concrete placing facilities, etc., are influential factors
in the selection and use of formwork.
Area or volume of cast per pour:
• The optimum volume of cast per pour will be different and in
accordance to the types of formwork used, elements of structure
to be placed and specific scale of work
• Usually volume of concrete ranging from 50m3 (non-continuous
pour from approx. 10 safely-laden ready-mixed concrete trucks) to
200m3 (continuous pour involving (e.g.) elephant concrete pumps
from approx. 40 trucks of the same) per pour can be comfortably
planned for most site environments.
Formwork: Construction-related
factors cont’d
Formwork: Construction-related factors cont’d
Continuity of structures and construction joints:
• Introducing a large number of construction joints in a large
structure subdivides the works into effective and workable sizes,
• Being the weak physical links of any structure, construction-joints
are inevitable in all forms of building.
• Design engineers conceptualise form systems and
site staff exercise common sense, in conjunction
with strict adherence to design specifications, to
ensure the rigidity of a structure.
• In order to ensure rigidity of the overall system
during the casting process, form-ties are
incorporated into the formwork design
• These accessories once (partially or wholly)
removed after casting, have their locations
patched with high strength grout and should not
affect the overall structural integrity of their
structures.
Involvement of other construction techniques:
• The applications of tensioning (http://www.youtube.com/watch?v=7JsuNg5r4Is) and
prefabrication techniques are often involved in the construction of
modern high-rise buildings, especially so in the Far East.
• This may impede the casting schedules and dictate the selection
and use of formwork, especially where pre-cast elements are to
be incorporated during the casting process.
• Additional provisions of temporary supports, slot spaces and
boxed-out positions in the formwork for the pre-cast elements, or
additional working spaces for the placing of stressing tendons
and the onward jacking process, should be allowed in such
cases.
Formwork: Construction-related factors
cont’d
Excavations and trenching
• An excavation is any man-made cut, cavity, trench, or depression
in an earth surface that is formed by earth removal.
• A Trench is a narrow excavation (in relation to its length) made
below the surface of the ground.
• In general, the depth of a trench is greater than its width, and the
width (measured at the bottom) is not greater than 5m.
• If a form or other structure installed or constructed in an
excavation reduces the distance between the form and the side of
the excavation to 5m or less (measured at the bottom of the
excavation), the excavation is also considered to be a trench.
Groundworks
Safety introduction:
• Excavating is recognized as one of the most hazardous
construction operations and this Unit will highlight various
trenching methods, hazards and their preventions.
Groundworks
Soil mechanics: An overview
• A number of stresses and deformations can occur in an open cut site or trench. For example, increases or decreases in moisture content can adversely affect the stability of a trench or excavation.
• The following diagrams show some of the more frequently identified causes of trench failure.
Groundworks
Groundworks: Soil mechanics
TENSION CRACKS. Tension cracks usually
form at a horizontal distance of 0.5 to 0.75
times the depth of the trench, measured from
the top of the vertical face of the trench.
Figure 9.7.1: Tension Cracks
SLIDING or sluffing may occur as a result of
tension cracks.
Figure 9.7.2: Sliding
TOPPLING. In addition to sliding, tension
cracks can cause toppling. Toppling occurs
when the trench's vertical face shears along the
tension crack line and topples into the
excavation.
Figure 9.7.3: Toppling
SUBSIDENCE AND BULGING. An
unsupported excavation can create an
unbalanced stress in the soil, which, in turn,
causes subsidence at the surface and bulging of
the vertical face of the trench. If uncorrected,
this condition can cause face failure and
entrapment of workers in the trench.
Figure 9.7.4: Subsidence and Bulging
HEAVING OR SQUEEZING. Bottom
heaving or squeezing is caused by the
downward pressure created by the weight of
adjoining soil. This pressure causes a bulge in
the bottom of the cut, as illustrated in the
drawing above. Heaving and squeezing can
occur even when shoring or shielding has been
properly installed.
Figure 9.7.5: Heaving or Squeezing
BOILING is evidenced by an upward water
flow into the bottom of the cut. A high water
table is one of the causes of boiling. Boiling
produces a "quick" condition in the bottom of
the cut, and can occur even when shoring or
trench boxes are used.
Figure 9.7.6: Boiling
TENSION CRACKS. Tension cracks usually
form at a horizontal distance of 0.5 to 0.75
times the depth of the trench, measured from
the top of the vertical face of the trench.
Figure 9.7.1: Tension Cracks
SLIDING or sluffing may occur as a result of
tension cracks.
Figure 9.7.2: Sliding
TOPPLING. In addition to sliding, tension
cracks can cause toppling. Toppling occurs
when the trench's vertical face shears along the
tension crack line and topples into the
excavation.
Figure 9.7.3: Toppling
SUBSIDENCE AND BULGING. An
unsupported excavation can create an
unbalanced stress in the soil, which, in turn,
causes subsidence at the surface and bulging of
the vertical face of the trench. If uncorrected,
this condition can cause face failure and
entrapment of workers in the trench.
Figure 9.7.4: Subsidence and Bulging
HEAVING OR SQUEEZING. Bottom
heaving or squeezing is caused by the
downward pressure created by the weight of
adjoining soil. This pressure causes a bulge in
the bottom of the cut, as illustrated in the
drawing above. Heaving and squeezing can
occur even when shoring or shielding has been
properly installed.
Figure 9.7.5: Heaving or Squeezing
BOILING is evidenced by an upward water
flow into the bottom of the cut. A high water
table is one of the causes of boiling. Boiling
produces a "quick" condition in the bottom of
the cut, and can occur even when shoring or
trench boxes are used.
Figure 9.7.6: Boiling
Groundworks: Soil mechanics
Ground shoring is the provision of a support system for trench walls,
used to prevent movement of soil, underground utilities, roadways,
and foundations. Shoring or shielding is used when the location or
depth of the cut makes sloping back to the maximum allowable
slope impractical. There are generally two types of shoring systems,
each with its own sub-categories:
• Ground shores support soil structures usually beneath ground
level and are commonly used in conjunction with trenching and
sheet-piling systems to prevent the inward collapse of the
surrounding earth.
• Structural shores typically support either existing building
structures that are deemed too structurally dilapidated to be in
self-support, or as a falsework in support of the erection process
of new structures.
Groundworks: Groundshores
Strut shoring
• Strut shoring is the most basic
form of ground shoring support.
The system consists of posts,
wales, struts, and sheeting
• The supporting struts are the main
components that resist the push
factor from the surrounding earth
• The preferred materials for struts
are timber and aluminium, the
former being cheap and readily
available, with the latter being
strong and light
Groundworks: Groundshores
Hydraulic shoring
• The modern trend is towards
the use of hydraulic shoring, a
prefabricated strut and/or wale
system manufactured of
aluminium or steel.
• Hydraulic shoring provides a
critical safety advantage over
traditional strut shoring as
workers do not have to enter
the trench to install or remove
the shoring components
Groundworks: Groundshores
• Ground shields or trench boxes are
different from shoring.
• Instead of shoring upwards or otherwise
supporting the trench face, they are
intended primarily to protect workers
from cave-ins and similar incidents.
• The excavated area between the
outside of the trench box and the face of
the trench should be as small as
possible.
• The space between the trench boxes
and the excavation side are backfilled to
prevent lateral movement of the box.
Groundworks: Groundshields