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A Case Study on Construction Safety

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A Case Study on Construction Safety
Greetings to the viewers, I am lieutenant level Arun Sekhar serving in military engineering
services as a designer. Design is part of planning and many construction safety aspects are
decided on the design table. Construction safety is closely related to constructability as the
executives at site have to be well versed with the construction sequence. Many structural
failures have happened in the past due to poor construction safety management.
I will be sharing a construction safety case study with you which will highlight the
importance of following the correct construction sequence and strict supervision at site
(Refer Slide Time: 00:41)
The structural failure led to 3 fatal casualties and the work got delayed for 3 years due to
subsequent arbitration. The construction accident happened at Srinagar in the year 2011.
(Refer Slide Time: 00:56)The proposed facility was a hanger to be used for helicopters. The picture on the screen
roughly indicates the size and shape of the proposed facility and may not be the actual one.
(Refer Slide Time: 01:09)
A hangar of size 35 meter by 50 meter by 8 meter high had to be constructed with a base
spacing of 5 meter. The steel truss of 35 meter span was to be rested on RCC column of 8
meter height. The structural drawing depicted the elevation of the truss and the support
details. After casting RCC column the steel truss was to be lifted and placed on the column.
The contractor had outsourced the fabrication work to a subcontractor.
The subcontractor was tossed to place the truss on the column by the contractor. The details
of the connections were overlooked. The protruding reinforcement of RCC column was used
for anchoring the truss by the subcontractor.(Refer Slide Time: 02:00)
After erecting 5 trusses, the rest of the work was planned for the next day. Three purlins were
fixed on the truss by tack weld. Next day at around 9 am the truss collapsed killing 3 workers
at the site the site. The client blamed contractor for the accident and the contractor blamed
design deficiencies for the mishap. The design was vetted from a premier technology institute
and the client approached the institute to find the real cause of the accident. The work got
delayed for 3 years due to the accident.
(Refer Slide Time: 02:35)
Having understood the background now we will look into the scientific reasons behind this
accident and what our lab says led to this mishap. The figure shows the typical cross section
of etrus with connection details. The support details include the foundation boards embedded
in reinforced cement concrete and the base plate at the top of RCC column.(Refer Slide Time: 02:59)
The axial forces are supposed to be transferred through the base plate to the support of the
truss and that is achieved by high strength friction grip bolts.
(Refer Slide Time: 03:10)
In our case the contractor did not purchase the j-hooks or anger boards which was supposed
to be embedded in column or he forgot to do it before casting the tip of the column. When it
was time to lift the truss using crane it was too late and the fabricator found a quick solution
by welding the base plate with the reinforcement of the column after inserting it through the
holes provided in the base plate.
(Refer Slide Time: 03:39)During high wind the truss sway and exert axial forces as well as movement on the base
plates. The very purpose of anger bolt is to resist these forces by bond in concrete and prevent
any uprooting of base plate. The weld on reinforcement neither provided enough bone
strength nor any axial force resisting capability. Srinagar is an area where the wind velocity is
considerable and in an open area the effect was even severe.
(Refer Slide Time: 04:10)
The shown picture depicts the ideal arrangement of an anger bolt and base plate. This should
have been prepared fabricated and embedded before casting the tip of the column.
(Refer Slide Time: 04:22)As construction professionals there are important takeaways for all of us. Safety in
construction is closely related to construction sequence. Erection loads are significant in steel
structures as wind plays an important role during construction. Construction sequencing has
to be designed on the design table. Close supervision during critical activities is essential and
access to supervision has to be designed during planning.
Enough lateral stability has to be ensured by purlins and wind brazing. In our case only 4
purlins were tack welded which was insufficient to resist the lateral forces in total. The cumulative lapses invited the disaster. There is no shortcut to safety, it has to be well designed. I know that the real pictures of the accident side would have helped to better understand the case, but unfortunately I cannot share them as of now due to protocol issues. Thank you so much.