Case Ditails

Scenario
Case Name The workers got injured because the cantilever beam fell down due to the break off of the suspension chain.
Pictograph
Location A construction site of a steel factory
Overview In a construction work of a steel factory, while the workers were erecting the structural steel, the cantilever beam which had just been mounted fell down. Two steeplejacks fell down to the ground about 6 meters below, and were nearly killed.
Incident The workers fell down along with the falling steel, and they were injured.
Sequence * During the structural steel erection work, the victim A and the victim B were installing the structural steel erection of overhanging slab of the second floor, using the transportable crane.
* The cantilever beam was installed by the temporary bolt.
* At first, they had adopted the method using the tightening bolt first. But they changed it to the other method, because they would not be in time with using tightening bolts. After this, they changed the method involving using the temporary bolt first, and started to tighten the bolt second.
* They had installed 4 bolts before it was hung. However, there was only 1 trace on the top and 2 traces below the gusset plate. One bolt may have dropped or loosened. They had used the chain with hooks at both tips and lever block at the medium part. They reinforced the beam by the temporary suspension in the way to put the tip book on that already installed one edge at the trap part of the steel column, and install the other edge at clamp part with the cantilever beam tip side.
* The beam was hung, and it was installed with the temporary bolt.
* The clamp for the gitch was removed from the hung beam.
* First, the suspension chain came off. About ten seconds later, the beam suddenly fell down because of the breakage of the temporary bolt in the cantilever of the column side.
* Then, steeplejacks A and B on the beam hang their life line on the main rope stretched by using the stanchion. But the main rope came off with the falling of beam. They fell to the earth floor about 6 meters below.
* The victim A fell straight down. He broke his thoracic vertebrane and rib closure and rested from work for 60 days. There were small impacts on the victim B, because he clung to the falling main rope. So, he had a bruise of the heel of hos foot, and had rested from work for 3days.
* The suspension chain had effectively worked at the beginning. But it broke because of excessive tension operated on the bolts in root of this cantilever beam.
Cause * In the structural steel erection plan, they didn't make a manual about the operational precedure of the cantilever beam.
* The calculating of the shear strength of the bolt, the examination about type, the number and mounting position of temporary bolts had not been made.
* There may have been hold a little and the suspension chain have slipped by swinging, etc. because the selection, and the way of mounting the tip hooks of the suspension chain, and the clamp were not appropriate.
* It was loaded before the strength of the root of this cantilever beam was ensured.
* At first, there was a question about the quality of the temporary bolts used in this case. However, it was confirmed that there was no problem with the quality after the test of the public institution.
* They worked without the appropriate reinforcement plan. Because they had little recognition that the welding of the cantilever beam and the column was weak under the external force in an emergency, and it had been the unstable structure until the top and the bottom of the cantilever beam flange and the column was finished.
Countermeasures * They must decide and make a manual about the operational procedure of the cantilever beam, clearly.
* They have to use `semi-finished bolt' as a temporary bolt, and put over the half of a bolt into the beam and use more than two bolts in the case of the compound junction. The diameter of `semi-finished bolt' must be equi-diametral with the tightening bolt.
* They must hang the tip of the cantilever beam from the upper part.
* They make sure of the prevention against the coming off in the way to weld the suspension piece with the tip of the beam or to install the suitable clamp for the shape of hook when they hang the cantilever beam from the upper part.
Knowledge Comment * They must make and abide by the manual about the operational procedure.
* They make a sufficient plan to cope with the external force in an emergency.
Background The background is as followis, which shows that work planning was not done sufficiently.
* The drastic change occurred to the plan of the pile and main frame construction work from the viewpoint of VE. And they spent the considerable time to deal with the methane gas which arose in the site. They couldn't make the sufficient system for the plan of structural steel construction.
* Many members interchanged (for expample director, technician), so, a settled environment was not ensured.
* The director took up the post in the middle of the construction, and didn't attend the meeting for construction planning. So, he took over them with the minutes of the proceeding. But, it didn't mentioned specifically about the construction plan after the structural steel weld was adopted.
* In the basic main frame construction work, the term of construction was delayed by the shortage of reinforced-bar placer and mould carpenter. So, the main frame construction work had to be done by many additional staffs during the summer vacation.
* The structural steel work began under such a situation, and the construction plan was made. But it had become a general plan without making a monual about the concrete operational procedure about the change to the welding in the site.
Reason for Adding to DB This case is a typical accident caused by the organization problems.
Scenario
Primary Scenario Insufficient Analysis or Research, Insufficient Prior Research, Insufficient Analysis or Research, Insufficient Prior Research, Poor Concept, Poor Organization, Organizational Problems, Poor Management, Organizational Problems, Inflexible Management Structure, Planning and Design, Poor Planning, Planning and Design, Poor Planning, Bodily Harm, Harm to Physical Well-being, Bodily Harm, Injury
Sources Japan Civil Engineering Constructors Assoc. Inc.
Number of Injuries 2
Field Civil Engineering
Author HOJO, Tetsuo (Institute of Technologists)
Scenario