powerpoint sw poon
TRANSCRIPT
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Control ofFalsework Construction
Dr. S.W. Poon
Associate Professor
Department of
Real Estate & Construction
The University of Hong Kong
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Introduction
Falsework is any temporary structure to support apermanent structure while the latter is not self-supporting
during construction
Braggs Committee identified technical reasons and
procedural inadequacies were the main causes
Study of bridge collapses revealed that over 50% were
falsework related failures 50%
Causes were inadequate review of falsework design,
inadequate control during construction and improper
procedures in falsework removal
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Analysisof
Falsework Failures
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Elliot (1973) described seven collapses
occurred within two years in California
and he recommended that the contractoris required to have a licensed engineers
check on the design of the falsework. (1973)
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Smith (1976) studied 143 bridge
failures since 1877. (1976)
1877 143
Twenty three of them happened during
bridge construction and about 40% ofthese were due to failure of temporary
supports. 23
40%
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Hadipriono and Wang analyzed 126 falsework
failures in concrete structures. 126
42% of them were related to bridge construction. 42%
Falsework collapses during construction stage weresummarised and about half of these 85 cases
occurred during concrete pouring.
85
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According to Hadipriono, three types of causes, i.e.
the enabling, triggering and procedural causes, were
classified.
The enabling causes are events that contribute to
the deficiencies in the design and construction of
falsework.
The triggering causes are usually external events
that could initiate a falsework collapse.
Procedural causes are hidden events that produce
the enabling and, quite often, the triggering event
as well.
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Most of the enabling and
triggering causes stemmed frominadequate procedural methods.
Evaluation of these factors is
generally only available in moredetailed investigation reports.
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The most noticeable causes:
lack of review of falsework design or
construction
unqualified persons to monitor the
erection procedures
lack of supervision in monitoring
changes during construction
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Hadipriono (1985) further identified
external events and deficiencies in both
the design and construction were the
principal sources of 150 major
structural failures. (1985)
150
H l d th t th bli t
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He revealed that the enabling events were
caused by inadequacy in the institutional and
procedural methods such as confusions thatoccurred at interfaces between contractors,
subcontractors, construction managers,
design engineers, architects and the clients
representatives. Consequently, they resulted
in inadequate design review and improperconstruction monitoring.
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Many failures were stemmed from
inadequate design review procedures
design calculations subcontracted to aprofessional were not thoroughly checked
detailing of important components, or the
design of a complex falsework, was
performed without fully verified.
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Another trend spotted was the lack of
monitoring during construction phases.Frequently, inspection was performed in
superficial ways and proper erectionprocedures were not adhered to.
Also lack of expertise and facilities in
performing unconventional constructionprocesses were very common.
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Hadipriono suggested that:
A need to analyze potential problemsoccurred in the past.
To avoid confusions among parties
involved, improve the procedures duringdesign and construction processes.
Adequate risk analysis for structures in
services and during construction.
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Me c h a n i s m s
i n c o n t r o l l i n g
f a l s e w o r k a c t i v i t i e s
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1. U.K. Falsework Coordinator
-- The contractor employs a falsework
coordinator who is responsible for thechecking of the design and
construction of falsework, andcoordination with other parties
involved in falsework construction.
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2. Hong Kong Independent
Checking Engineer --
A professional engineer, employed bythe contractor, checks the design and
construction of falsework. His permitwould be required at critical stages of
falsework construction.
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3. Developing countries
Contractor designs the falsework
subject to the Engineer or R.E.sapproval but without responsibility.
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Failure PredictionMethods
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In 1973, Pugsley outlined an
approach to the problem ofassessing the proneness to
structural accidents. 1973
He suggested the following parameters of
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He suggested the following parameters ofsignificance in accident history:
new or unusual materials
new or unusual methods of construction
new or unusual types of structure
experience and organisation of design andconstruction team
research and development background
industrial climate
financial climate and
political climate.
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Blockley (1975) outlined the fuzzy set approach to
the problem of predicting the likelihood of a
structure failure. (1975)
A failure occurs because there is a major error and/or
several smaller errors combined to eliminate thefactor of safety.
/
These factors are difficult to quantify but may be
measured using fuzzy linguistic variables.
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Six main categories (parameters) used
include: ( )
materials
type of structure
design experience time
construction
externals
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Each parameter is assigned the
gravity and consequence rating.
The overall effect is then related
to a safety index.
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Blockley(1977) classified 24 parameters
and assessed for 23 major structural
accidents and one existing structure. (1977) 24 23
The accidents are ranked in their order of
inevitability.
Problems such as poor site control, errors
of judgement, time and financial pressurewere highlighted.
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Human errors were predominant in
causing the failures and can beprevented by good communications
between all parties concerned and bywell-defined responsibilities and
procedures under the contract.
However, the parameters are
assessed in giving an overall score
only.
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Melchers (1978) commented on the
contents of various failure reportsranging from the formal government
inquiry reports to professionalmagazines. (1978)
Non-technical problems such as human
errors were not always included in failurereports.
e o ow ng are pro ems en e romf b id f il
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four bridge failures :
1. Failure to appoint an experiencedbridge engineer
2. Negligence in checking the falseworkdesign and failing to submit thefalsework drawings.
3. The consulting engineers failed to
require the contractor to submit details
of falsework for approval. 4. Routine design work is commonly done
by inexperienced engineers.
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Melchers (1978) suggested a
complementary approach on the in-
depth study of failed projects, i.e. a
pathological approach so that projects
would be studied from an organisationas well as a technical viewpoint.
(1978)
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Yao (1981) explored the gap between the
calculated probability of failure (10-6) andBrowns perceived failure rate (10-3) for a certain
type of structure. (1981)
(10-6) (10-3)
In his example, two subjective factors i.e. the
design and construction factors were assessed fortheir gravity and consequence. The failure
probability is found to be of the order of 10-4,
which is closer to Browns perception.
10-4
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Melchers et al (1983) summarised the experience
gained from the study of structural failures andsatisfactory construction, and commented on the
accuracy and completeness of reporting. (1983)
Most failures can be shown to occur because of
gross human errors - the need for a means ofassessing the effectiveness in controlling the
changes in both the design and construction process
on the occurrence of gross human errors. --
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Hadipriono (1985) pointed out human based
uncertainties are abundant but are seldomincluded in the assessment of falsework
performance - A method based on fuzzy set
concept has been developed. (1985)
--
Reduction of enabling and triggering events
can achieve a desired level of overall falsework
performance. However, procedural errors are
not included.
Ellingwood (1987) concluded 10% off il bl h i
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failures were traceable to stochastic
variability in loads and capacities. Theremaining 90% were due to other causes,including design and construction errors,
modeling and analysis uncertainties.Independent control stops should beinstituted at key decision points in the
project, especially where responsibility forproject phases changes hands. (1987) 10%
90%
Pidgeon et al (1990) described that Event
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Pidgeon et al (1990) described that Event
Sequence Diagram (ESD) provides apowerful means of representing and
accessing information about the sequences
of events preceding a failure or near-missincident. (1990) (ESD)
The ESD shows the temporal order and
relationship of events leading up to aparticular outcome. ESD
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Lucas (1990) stated that we must learn
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Lucas (1990) stated that we must learn
from experience to prevent future
crises from occurring. (1990)
The fundamental concept is to find the
cause, to derive effective remedies
and to prevent future accidents.
Turner (1992) identified that large
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Turner (1992) identified that large
scale accidents have many causesand that their preconditions build up
over a period of time. (1992)
Crises and disasters developed in a
covert and unnoticed fashion during
an incubation period.
Development sequence of systems failures
Development sequence of systems failures
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1.Situation 'notionally normal'
2.Incubation period
3.Trigger event
4.Onset
5.Rescue and salvage
6.Full cultural readjustment
1.Situation 'notionally normal'2.Incubation period
3.Trigger event
4.Onset5.Rescue and salvage
6.Full cultural readjustment
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Air
pressure
( pronenessto failure )
Blockley : Balloon Model
Dias and Blockley (1995) argues that
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Dias and Blockley (1995) argues that
engineering students and practisingengineers could upgrade their
knowledge by learning from casehistories of design and construction,
and of failure. (1995)
Reflection on failures will result inimproved design and construction.
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Development of aModel for Falsework
FailureAnalysis/Prediction /
The shortcomings of models mentioned
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previously are:
1. Procedural inadequacy has not been
considered and assessed.
2. Most models can only be used to
assess the likelihood of an eventualfailure without evaluating the (safety)
condition at various stages of the
falsework construction. ( )
arac er s cs o e mo e o e eve ope :
1 Th k d i i l i i i f f l k d d
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1. The key and critical activities of falsework are grouped under tfive essential stages and presented by event sequence diagram.
2. Different sub-models are derived, e.g. the conventional,independent checking engineer and falsework coordinator
system.
3. Controls regarding the following common critical activities areincluded
(a) Construction method of the permanent works and risk offalsework collapse.
(b) Changes in falsework design and construction method.
4. The activity or procedure performance will include the effect opersonnels characteristics.
5. Communications between parties are shown in the flow diagra
include recommendations by researchers on failure analysis.
er s o e mo e :1. To show clearly the activities flow for the
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yparties; their roles, responsibilities and
involvement.
2. Inadequacy in design is taken into consideration
for assessing falsework safety.
3. The Balloon model is adopted. At each stage, theactivities and procedures are assessed and effects
leading to failure are aggregated to indicate therisk.
4. For checking any missing activities orinadequacy in procedures.
5. A simple tool to assess the falsework condition.