Formwork Supports, Scaffolds and Failure.pdf

Sanjivani Rural Education Society's
Sanjivani College of Engineering, Kopargaon 423603.
-Department of Strucutral Engineering-
By
Mr. Sumit S. Kolapkar (Assistant Professor)
Mail Id- kolapkarsumitst@sanjivani.org.in

Ø Introduction- Formwork supports
1.Vertical support-
• Single leg type (Shore or prop)- Are tubular and telescopic type
• Multi leg type- Frame or tripod or trestle
2.Horizontal support- Light weight trusses
Horizontal Support- Truss

Single Leg Type-Shore

Multi Leg Type- Frame, Tripod and Trestle

Ø Introduction- Shore/prop and dropheads
1.Shore/prop-
• Timber shores/props are commonly used in low rise construction where floor
to floor height may be between 3-4 m.
• The l/d ratio is an important design parameter for timber shores and thus
while using timber shores, sufficient thickness of timber should be ensured
to safeguard them against buckling.
• Bracing of shores should also be ensured. A typical arrangement for timber
shores/props is given in Fig.
• The arrangement of jointing of shores/props to achieve additional heights is
also shown in fig.
• The shores/props made up of steel are very common these days.
• Modern props are made with high tensile steel and tubes.
• They are of light weight yet offer high strength.
• The props are usually used for supporting the formwork in low clearance
construction, up to heights of about 5 m.
• For height more than 5 m it need to be properly tied and braced so as to
form a rigid structure.

1.Shore/prop-
• Like timber shores, bracings are also important in steel shores.
• Bracings are provided by means of tubes and clamps.
• These props come in different designs and have different load carrying
capacities.

1.Shore/prop-

2.Dropheads-
• Devices which are fitted on the top of props or supports to support the slab
while the remaining form for the decking could be struck for reuse.
• It remain in contact with the underside of the slab at all times for the full
curing period, while the majority of the formwork materials, including
plywood, can be removed as early as 3-4 days after pouring of the concrete.
• This results in great economy as far as the cost of the formwork is
concerned.
• The dropheads are suited primarily for flat slabs and multi-story construction.

2.Dropheads-

Ø Multilegged shoring towers
Commonly used multi-legged shoring systems for high clearance
construction (floor to floor heights are considerable) are the
1. frame based system,
2. tube and coupler system,
3. trestle system
1. Frame based system,-
• Those are pre-fabricated tubular frames which come in a variety of shapes
and modular sizes which can be assembled one over the other to get the
required heights.
• The frames are usually braced together by means of ledgers and cross
braces to form a rigid structure.
• In order to achieve varying heights, i.e. for adjustments in height,
accessories such as screw jacks (tower spindle) are provided either at the
top or the bottom or both.

1. Frame based system-

2. Tube and coupler system-

3. Trestle system- Only suitable for light work like painting, cleaning etc.

Ø Classification based on Hurd and Shapira
and Raj

Ø Classification based on Hurd and Shapira
and Raj
1. Hurd (1963) Classification- Classification is based on the safe working
load of each leg of the tower
Standard: Safe working load of each leg is 27 kN
Heavy duty: Safe working load of each leg is 45 kN
Extra heavy duty: Safe working load of each leg is 180 kN
Note-
1. With the introduction of aluminum towers and towers with
ultrahigh-load carrying capacity, the safe working load of each leg
has increased up to 450 kN.
2. The service lifetime of the shoring tower material has increased
to 15-20 years now.

Ø Classification based on Hurd and Shapira and Raj
2. Shapira and Raj (2005) Classification-
Type ‘A’:
• It can be assembled in square and rectangular shapes.
• Each tier is made up of two parallel frames which are
connected by two pairs of cross braces.
• The cross braces are used to interconnect two separate towers
or to make a larger tower.
• Depending on the manufacturer, the height of each may vary
from 910 mm to 2,100 mm.
• The working load per leg of the tower may vary from 45 to 80
kN. Such towers are available in aluminum and painted steel.
• The tower width varies from 610 mm to 1,830 mm.
• It cannot be assembled in the triangular tower form.

Type ‘A’:

Type ‘B’:
• Each tower section is made up of four telescopic props.
• The props are connected by sets of four ledger frames.
• The props may also be used separately as single post shores.
The ledger frames are used to interconnect two separate
towers or to make a larger tower.
• Depending on the manufacturer, the height of each tier may
vary from as low as 800 mm to as high as 6,250 mm.
kN.
• The tower width varies from 550 mm to 3,800 mm.
• The towers cannot be assembled in the triangular tower form.

Type ‘B’:

Type ‘C’:
• These towers come in square configuration in the plan.
• As in the case of Type ‘A’ towers, each tier is made up of two
parallel frames. In Type ‘C’ towers, the tiers are turned 90° in
relation to each other.
• The ledger frames are used to interconnect two separate
towers or to make a larger tower.
vary from a minimum of 500 mm to a maximum of 1,800 mm.
• The working load per leg of the tower may vary from 50
• to 60 kN.
• The tower width varies from 1,000 mm to 1,520 mm.
• The towers of type ‘C’ usually cannot be assembled in
triangular tower form; however some manufacturers can have it.

Type ‘C’:

Type ‘D’:
• Such towers can be assembled in triangular, square, and
rectangular shapes.
• Each tier is made up of four frames connected to each other. As
in the other types of towers, these can also be interconnected
to make a larger tower.
vary from a minimum of 500 mm to a maximum of 1,500 mm.
kN.
• The tower width varies from 1,000 mm to 2,000 mm.

Type ‘D’:

Ø Trestle (crib) shoring-
• It is used to act as a shoring tower for heavy construction such
as the bridge girders, slabs and culverts.
• It is usually made up of angle sections (ISA) and is braced
appropriately.
• The angle sections are arranged in different patterns to suit the
different requirements of carrying various loads.
• Some of the arrangements of angle sections to get the box
shape are shown in Fig.
• The boxes are of varying dimensions and they use varying
number of angle sections.
• The load carrying capacity of each of the box sections varies.
• The angle sections are laced with each other to form the box
section.
• The independent trestles (box sections) are also braced with
each other as per the requirement.

Ø Design of vertical supports for formwork-
• Shoring tower design is concerned with finding the appropriate
distances between the joists and stringers, and distances
between towers the in two directions.
• The stability aspect of the tower needs careful consideration.
• The distances between the joists are governed by the
maximum allowable span of the sheathing elements.
• For safety reasons, the joist’s spacing is determined in such a
way that the sheathing element is supported with no cantilevers
on its two sides.
• Distances between the tower rows are similarly governed by
the maximum allowable span of the joists.
• For practical considerations, a tower placed next to a wall
should be no closer than a minimum distance from the wall
(around 200 mm, measured from the tower leg’s axis) so as to
leave room for the tower footings and to allow manipulation of
the tower’s screw jacks (tower spindle).

Ø Forces acting on a shore-
• The total load on the intermediate shores/props = Total load on
the formworks (w) x spacing of the primary beams x span of the
secondary beams
1. Total load on the formworks (w)-
a. Self–weight of concrete slab = thickness of slab x unit weight of
freshly placed wet concrete including reinforcement (= 26 kN/m3
as per Indian Standard)
b. Imposed load-It comprises the loads from
(i) lateral pressure of concrete (IS :14687-1999), (ii) loads from
construction personnel, plant and equipment, vibration and impact
of machine delivered concrete (IS :875 part-2-1987), (iii)
unsymmetrical placement of concrete (IS :875 part-2-1987), and
(iv) concentrated load and storage of construction materials
(IS :875 part-2-1987).

secondary beams
1. Total load on the formworks (w)-
c. Load of formwork-
Note- 1.The code recommends the self-weight of formwork be
determined on the basis of the actual measurement in accordance
with IS: 875 (Part 1)–1987.
2. In the absence of actual measurement, it may be assumed as
500 N/m2 for the purpose of initial calculations.
Therefore,
Total load on formwork = sum of a + b + c

secondary beams
2. spacing of the primary beams-
3. span of the secondary beam-
Note-
Assume the required prop height, depnding on height the
permissible load carrying capacity of the props from the
manufacturer’s data and then provide prop centre to centre.

Ø Introduction to Scaffolds-
• It is a temporary structure for gaining access to the higher
levels of the permanent structure during construction.
• Scaffolds are often used because they are convenient, versatile,
and economical.
• They are needed in all the stages of construction.
• It is one of the essential formwork parts to provide temporary
platforms at various levels for carrying out all those works which
cannot be conveniently and easily carried out either from the
ground level or any other floor of the building or by the use of a
ladder.
• Besides providing access, the scaffoldings are also used for
(i) centering for the formwork, and (ii) for supporting heavy loads
at great heights.

Ø Parts of Scaffolds-
• It consists of standards, putlogs, ledgers which are generally
made up of bamboo, timber or metal to provide a working
platforms for workmen and materials in the course of
construction, maintenance, repairs and demolition, and also to
support or allow hoisting and lowering of workmen, their tools
and materials.
Standard or Upright: A vertical member used in the construction of
scaffold for transmitting the load to the foundation.
Ledger: A horizontal member which ties the standard at right
angles and which may support putlogs and transoms.
Putlog / Bearer: A scaffolding member spanning from ledger to
ledger or from ledger/ standard to a building and upon which the
platform rests.
Transom: A member spanning across ledgers/ standards to tie a
scaffold transversely and which may also support a working
platform.

Ø Parts of Scaffolds-
Brace: A member fixed diagonally across two or more members in
a scaffolding to afford stability.
Bracing: Bracing is a system of braces or ties that prevent
distortion of a scaffold.
Guard Rail: A horizontal rail secured to uprights and erected along
the exposed edges of scaffolds to prevent workmen from falling.
Toe-Board: A barrier placed along the edge of the scaffold platform
and secured there to guard against the falling of material and
equipment.
Base Plate: Base plate is used so that the standard/ poles do not
get inserted into the ground due to the heavy load on the top of
the scaffold boards due to the masons. These base plates are
generally made up of hard metal.

Ø Classification of Scaffolds-
1. Based on normal use
2. Based on type of construction

Ø Classification of Scaffolds-
1. Based on normal use-
Timber Metal

Ø Classification of Scaffolds- Timber Scaffold
1. Single pole scaffold-
The single row of the upright poles is fixed close to the building or
wall and connected horizontally by ledgers along with the length
connected to the buildings or walls by means of cross timbers
known as ‘Putlogs’.

2. Double pole scaffold (Independant scaffolds)-
• There are two rows of uprights kept about 1 to 1.5 m apart
across the building and at suitable intervals along the length.
• The two rows of uprights are connected by cross timbers viz.
putlogs or transoms and longitudinally by ledgers.
• Suitable diagonal bracings are also provided for the same. In
case of bamboo or bally scaffolds built with sawn ropes or coir
of hemp, and in case of scaffolds built with sawn timber
sections, the joints are usually made with bolted/ nailed
connections.

2. Double pole scaffold (Independant scaffolds)-

Ø Classification of Scaffolds- Metal Scaffold

Ø Classification of Scaffolds- Single pole or putlog
• Is physically tied into the brickwork using putlogs or tubes with
putlog adapters
• A slot is left between bricks to accommodate them and once in
place the wall itself becomes the inside support.
• The outside support is formed by a series of standards (upright
tubes) and ledgers (horizontal tubes) which are connected
using double couplers, the putlogs then sit directly onto the
ledgers and are secured using single (putlog) couplers.
• Once all the putlogs are in place boards are laid across them to
form the working platform and a further two rows of tube are
fitted horizontally to form a guardrail (safety barrier).

Individual component type-
• It consist of single row of uprights connected together by
ledgers.
• Putlogs are fixed to the ledgers and built into the wall of the
building as the construction progresses.
• The scaffold system essentially consists of a base plate, ledger,
uprights, double coupler for coupling the ledger to the uprights,
putlog, putlog coupler, horizontal tie member, longitudinal
diagonal brace, toe board, and guard rail.
• Unit Frame Type- The system consists of base plate, unit
vertical, unit Putlog cross bar, unit type longitudinal diagonal
brace, horizontal tie coupled with double couplers, scaffold
boards, toe board, and guard rail.

Ø Classification of Scaffolds- Double pole or
Independant
• It consists of two rows of uprights connected together
longitudinally by ledgers and transversely by putlogs or
transoms.
• The system consists of base plate, ledger, uprights, double
coupler for coupling the ledger to the uprights, transom, putlog,
putlog couplers to couple the transom to the putlog, longitudinal
diagonal and cross braces, swivel coupler, toe board, and guard
rail.

Ø Classification of Scaffolds- Double pole or
Independant

Ø Classification of Scaffolds-Outrigger (Cantilever)
scaffolding
• This is usually an independent type of scaffolding which does
not rest on the ground but is cantilevered from the face of the
buildings or structures.

Ø Classification of Scaffolds-Outrigger (Cantilever)
scaffolding

Ø Classification of Scaffolds-Platform scaffolds
• This consists of two or more rows of uprights connected
together by ledgers and transoms and usually a working
platform is placed on top of the scaffold.
• This type is used normally for supporting heavy loads at the top
level and for providing an access platform at one level.

Ø Classification of Scaffolds-Tower scaffolds
• This consists of uprights connected together by ledgers and
transoms. This may be made mobile by mounting it on the
castors.

Ø Classification of Scaffolds-Suspended scaffolds
(cradles)
• This is an independent scaffold which is hung from a building or
structure and not supported on the ground.

Ø Codes of practice for metal scaffolds-

Ø Safety provisions in building the scaffolds-
(i) Every scaffold should be braced by means of longitudinal and transverse bracing
systems so as to form a rigid and stable structure. Also every scaffold should be
effectively tied to a building to prevent the movement of the scaffold either away or
towards the building.
(ii) Where heavy winds or gale forces are expected, it is necessary to take special
precautions and install additional ties to the scaffold to prevent overturning and collapse.
(iii) Guide rails and toe boards must be provided for all the working platforms to ensure
the safety of the workmen.
(iv) All working platforms should be fully covered to prevent materials falling and causing
injury to the workers or passersby.
(v) Safety nets or other screens should be provided to catch any falling materials.
(vi) The use of barrels, boxes, loose earth pads or other unsuitable objects as supports
for uprights and working platform, should not be permitted.
(vii) Care should be taken to see that no uninsulated wire exists within 3 m of the
working platforms, gang ways, runs, etc. of the scaffolds.
(viii) Scaffolds on thoroughfares should be provided with warning light, if general light is
not sufficient, to make it clearly visible.
(ix) Men should not be allowed on scaffolds during storms or high winds.
(x) Grease, mud, paint, gravel or plaster or any such material should be removed from
the scaffold platforms immediately.
(xi) Either sand or saw dust or any other suitable material should be spread on the
platforms to prevent slipping.

Ø Safety provisions in building the scaffolds-
(xii) All projecting nails from the platforms or other members should be removed.
(xiii) During dismantling of scaffolding, necessary precautions should be taken to prevent
injury to the persons due to the falling of loose materials. The bracing and other
members of the scaffolds should not be removed prematurely while dismantling the
entire scaffold so as to avoid the danger of collapse.
(xiv) When scaffolds are to be used to a great extent and for a long period of time, they
should be inspected from time to time to ensure their soundness.
(xv) Boards and planks used for platforms, gangways should be of sound quality and
proper thickness, closely laid and securely fastened and placed.

Ø Design issues in building the scaffolds-
1. Scaffold support systems should use jack bases, even on a concrete foundation. This
will allow for:
i) first, an increase in system stiffness at the base;
ii) secondly, adjustments to different heights off the ground (such as when stairs are
being built); and
iii) finally, adjustment of height due to improper or uneven installation of steel scaffolds.
If a system of steel scaffold is twisted because of improper installation, some of
the scaffolds may not be in contact with the ground, which may lead to instability
problems.
2. The wooden planks beneath the wooden shores should be fastened directly to the
dried reinforced concrete columns and walls.
3. Nails should be used at both the top and the bottom of the wooden shores in the
scaffold support system.
4. The bamboo and the steel scaffolds should be fastened together with wires in order to
prevent buckling of the bamboo. In addition, it is recommended that the bamboo braces
on the out of-plane surface of the steel scaffolds be replaced with steel braces since
bamboo’s capacity to resist the bending moment may be inadequate for this application.

Ø Design issues in building the scaffolds-
5. If possible, the wooden shores should be replaced with tubular steel adjustable
shores. The end of these tubular steel adjustable shores can be connected the same
way as the joints between steel scaffolds (i.e., in place of nail joints). This can greatly
increase the critical load of the scaffold support system. If this is not possible, tubular
steel adjustable shores should at least be used temporarily in the interior of the scaffold
support system where the largest force is exerted.
6. Whenever possible, simple steel scaffolds with one joint should be used. In addition,
the connection pins at the joints should be lengthened in order to increase the stiffness
of the joints.

Ø Causes of the collapse of scaffolds-
1. Construction loads may exceed the critical load of the scaffold support
system-
• According to Peng et al. (1996), the failure of a scaffold support system in high-
clearance structures is usually a problem of structural stability.
• The collapse of the scaffold support system is often the result of the actual
construction load exceeding the critical load of the system.
Note- At present, there are no guidelines which can be used to predict the critical load
(i.e., the buckling load) of the entire scaffold support system.
2. Horizontal instability of the wooden shores-
• As shown in Fig., it is difficult to accommodate the inner clearance of high-clearance
structures by using a stacked arrangement of steel scaffolds alone and thus there is a
need to use wooden shores at the top in combination with steel scaffolds.
• The wooden shores are used primarily for filling the gap between the formwork and
steel scaffolds.
• Peng et al. (1996) reported that it is almost impossible for wooden shores to buckle
under general construction loads. However, the connections at their ends are unable
to carry moment and the bottom of the wooden shores may move horizontally
together with the wooden planks on the top of the steel scaffolds after construction
loads are applied on the formwork.

3. Partial loading of the fresh concrete may reduce the critical load for the
scaffold support system-
• The concrete pour duration for high-clearance structures is usually one day except in
the case of a very large slab pouring area.
• Peng et al. (1996) that, the concrete load over the complete slab area may be
considered to be a sequence of different partial load cases.
4. A specific (possibly asymmetric) placement pattern of fresh concrete may
decrease the critical load of the system-
• The fresh concrete is usually placed according to some specific pattern. However, a
uniform load is usually assumed in structural analysis.
• According to Peng et al. (1996), the placement pattern may cause the critical load of
the temporary support system to be smaller than that under the uniform load
assumed in the design.
• Thus, the design strength of the temporary support may be inadequate for the actual
external load.

Ø Causes of failure of formwork-
1. Improper Stripping and Shore Removal-
• Premature stripping of forms, premature removal of shores, and careless practices in
reshoring can produce catastrophic results.
2. Inadequate Lateral Bracing—Wind, Construction Loads-
• The more frequent causes of formwork failure, are other effects that induce lateral
force components or induce displacement of the supporting members.
• Inadequate cross bracing and horizontal bracing of shores is one of the factors most
frequently involved in formwork accidents.
• Note- Investigations prove that many accidents causing damage worth thousands of
rupees could have been prevented only if a few hundred rupees had been spent on
diagonal bracing for formwork support.
• High shoring with heavy load at the top is vulnerable to eccentric or lateral loading.
• Diagonal bracing improves the stability of such a structure, as do guys or struts to
solid ground or completed structures.
3. Vibration Due to Concrete Placing Equipment
• Forms sometimes collapse when their supporting shores or jacks are displaced by
the vibration caused by passing traffic, the movement of the workers and the
equipment on the formwork, and the effect of vibrating concrete to consolidate it.
• Diagonal bracing can help prevent failure due to vibration.

Ø Causes of failure of formwork-
4. Unstable Soils under Mudsills, Shoring not Plumb-
• Unstable soils under the mudsills can also cause the formwork to fail. The mudsills
act as a base for a shore or post in formwork.
• The mudsills could be a timber plank, a frame, a small footing or pedestals.
• Formwork should be safe if it is adequately braced and constructed; so all loads are
carried to the solid ground through vertical members.
• Shores must be set plumb and the ground must be able to carry the load without
settling.
• Shores and mudsills must not rest on the frozen ground; moisture and heat from the
concreting operations, or changing air temperatures, may thaw the soil and allow
settlement that overloads or shifts the formwork.
• Site drainage must be adequate to prevent a washout of soil supporting the mudsills.

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