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1. ACCIDENT
STUDYING AN
REPORT
INVESTIGATION

2. On July 20, 2013, 3 workers employed by Sub-contractor,
expired after an under-construction roof slab collapsed. The
building the roof slab casting was almost nearing completion,
when the entire slab along with the beams collapsed.
STUDYING AN ACCIDENT INVESTIGATION REPORT 2

3. Events before the incident
 The entire formwork was checked by the Client’s Site
civil engineer & Main Contractor prior to concrete
pouring. The clearance for concrete pouring was given.
 Concrete pouring started from 10 am on 20th July’13.
The concrete was poured manually using two tower
cranes. The use of Tower cranes in this remote area
helped in significantly reducing the amount of manpower
at the roof.
 The beams and slabs were cast simultaneously as per
instructions from the client.
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4. Events before the incident
 At the time of the incident, 12 workmen were at the roof
top, secured by a life line and 3 carpenters were at the
bottom. The carpenters were inspecting the condition of
the formwork for leakages, buckling, bulging, prop
support etc. which is a general practice in India by all
civil construction companies.
 10 minutes prior to the incident there was a fourth
carpenter, who had gone under the roof slab to inspect
and found no abnormalities. This carpenter had come out
before the incident.
 Moments before the incident, a movement on the
concrete top surface, near the west end, was observed.
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5. The description of the incident is based on inputs taken from the Main
contractor’s Site Manager, the Contractor’s Site Manager and Contractor’s
workmen, all of whom were at the spot at the time of the incident.
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The incident occurred at around 3 PM on 20th July’13. The entire roof
collapsed in a matter of seconds starting from the west end.
At the time of the incident, the concrete pouring was nearing completion
and finishing work was in progress.
7 workers out of the 12 on the roof top were injured. The injured were
rushed to the nearest hospital. It is worth mentioning that the life line with
full body harness and fall arresters saved these 12 lives. This amply
demonstrates the importance of adhering to safety guidelines.
Incident
Description

6. General Findings
Design of Building
 The structural design of the GIS sub-station and control
room building was done in-house by Civil design team
and approved by Client’s civil design department.
 The contract does not specify the roof architecture.
Initially design & drawings were approved for Flat roof
construction. However considering the snow-fall region
customer insisted for pitched (sloping) roof design.
Therefore a revised design was done for the pitched roof
and approved by Client.
 Basic Roof design – Sloping roof, pitched design. Slab
thickness 150mm. Concrete , grade M20 (1:1.5:3). Roof
(eave) height – 9 meters. Ridge Height – 12 meters
STUDYING AN ACCIDENT INVESTIGATION REPORT 6

7. General Findings - Shuttering Support
Shuttering Support Design
 The General practice in India is for shuttering support
to be done at site.
 The shuttering support design was done by the Site
Manager in discussion with the contractor’s Site
Manager. The design is done on the basis of load per
unit area.
Shuttering Material
 The shuttering material used is Steel scaffolding with
cup lock system.
Installation
 Pictures from incident site show that at certain
locations the fixation of props is not proper.
 The base made for the props was on virgin soil
with Wooden battens or steel base. There was
evidence at site of some props not having a
proper base and put directly on ground.
 The roof to be cast, including the scaffolding,
was checked & cleared by Main contractor and
site engineers . The clearing document was
verified and found in order.
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8. General Findings contd. . .
Supports & Surface
 Surface, Virgin soil.
Location was cut during
land development by 1.5 to
2 mtrs
 Base for props at some
places were made of No
evidence of any settlement
was found wooden battens  Joints at some places were not
proper
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9. General findings –
Concrete pouring sequence
 Concrete pouring sequence followed
is as per Standard Industry practice
which enables beams to set and take
load
 Separate pouring of concrete for
Beams and Slab was ruled out as
Client wanted beams and slabs to be
cast as monolithic structure
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B4
B3
A3
A4
A2
B1
B2 A1
Tower Cranes
A
B
Circular movement of concrete seen on this spot just before collapse
All 3 bodies recovered below this spot at GL.
West end
Note : Slab casting in same sequence as beam sequence A5, A6 & B5, B6
9m eave
height
Slab
Ridge line-12m height
East end
Beams
(wt 8 Ridge line
– 12m height
Slab 9m eave
height
B3 A4
– tons each)
B6 B5 A6 A5

10. General Findings
Safety Measures
 2 EHS officials were present full time during the
concreting
 Lifeline , Fall support was in place
 Workers at top having body harness and this was also
anchored. This prevented them from falling on the
collapsed roof structure thus avoiding more fatalities.
 Concreting using tower cranes enabled to reduce the
manpower requirement at top.
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11. General findings contd.. . .
 Collapsed roof view from North West side  East end
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12. Analysis –
Review of Shuttering support design
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Design of shuttering support structure
 The shuttering support structure was verified by an independent and
qualified structural expert.
 The shuttering support had vertical props and lateral bracings.
 No cross bracings were observed on the shuttering supports. Cross
bracings are required for countering the lateral forces arising out of
sloping roof structures.
 The Staad model of the shuttering support, indicates that even with a
horizontal loading of 10% of vertical load, the supports failed However
a simulated model with cross bracings withstood the lateral forces.
Cross Bracings

13. Root Causes
Absence of cross bracings
in the shuttering supports
to counter the lateral forces
due to sloping RCC roof.
Hence, improper shuttering
support design to suit the
sloping roof structure is
probably the main cause
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Absence of further design
review of shuttering
supports, as a process, for
complex and heavy
structures
Improper installation of
shuttering supports in
some places, particularly
base support pp and joints
probably was a contributing
factor but not a primary
cause

14. Recommendations &
measures to prevent
recurrence
 Standardized Shuttering support design basis should be
made available to site engineers.
 Design of shuttering support for complex civil structures
like pitched roof etc should be done by experienced civil
engineers and must be approved by design department.
 This requirement should be integrated in the overall
Design Review process
 For complex scaffolding structures, installation should be
verified and approved by Experienced scaffolding expert.
This requirement should be included in the FQP
STUDYING AN ACCIDENT INVESTIGATION REPORT 14

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