Unit 10 – Site Temporary Works

Safety Management & Site

Establishment

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

Scaffolding

• 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

Framed

Common (independent) scaffolds

Tubular

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 Bird-cage

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.

Common scaffolds

Common scaffolds

Common scaffolds

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

Hoist Details

• 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 platform hoists

• 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

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)

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

Custom formwork

Custom formwork

Custom formwork

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

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

Groundworks: Groundshields

69

Trench Boxes (Shields)