Failure Knowledge Database / 100 Selected Cases

Concrete mould timber support collapse accident in the gymnasium construction workin Maritime Self-Defense Force Atsugi air base.
[February 14th, 1992 (Fri).

The Kanagawa Prefect. Ayase City.]

Masahiko Kunishima, Takehumi Sato(note of 1)
During the concrete installation work of the beam and the slab of the second floor of
the gymnasium, installed concrete and slab’s reinforcing steel fell and 7 people died, and14 people got injured.
1EventDuring the concrete installationof the second floor, a built truss
which supported the mould for theslab fell.
After about 25 minutes, a chain
reaction collapse of the moulds andthe mould timber supports spreadfrom this falling point, and 2/3rdsof concrete and reinforcing slabs of
the second floor fell down. And theworkers who were working on theFig-1. Situation of the whole (Source: the Ministry ofLabor survey report)

inspection at the 2nd and the 1st floor were pinned under these fallen materials, and 7
died, and 14 got injured.
2CourseBy the previous day of the accident, concrete was placed in the first floor column andthe external wall under the girder mould. And during concrete installation of the slab
on the day of the accident, a beamed member of the built truss with coffer forms ( themould boards ) on its both sides with placed concrete in it fell on the shelf scaffold justbelow that ( figure 2 references ). The installation work was immediately stopped, andsome persons descended to the first floor to grasp the situation. There is a testimonythat the inflow of the concrete was resumed after a while at another section of thesecond floor.

1Failure Knowledge Database / 100 Selected Cases

Fig-2. The bend of the pillar by the built truss

Fig-3. Outline drawing of the timber support

falling (Source: the Ministry of Labor report)

collapse (Source: Asahi Shim bun article).

After about 25 minutes, a collapse of the mould timber supports began from the partof this built truss, and it spread rapidly, and it lead to the whole collapse (figure 3references).
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Cause.

・The direct cause.

Fig-4. The built slab method (Source: theMinistry of Labor report)In this construction, the built slab method was applied. A feature of this method is toomit the process by prefabricating the moulds. The members used in the mould are thebuilt trusses. The built trusses are the truss reinforcing steels of about 6m in length,
and they become finally a part of the floor, but in the concrete construction, they also
play a role as supports of the mould boards from the inside. Therefore, the traditionaltimber supports from the outside are unnecessary.
In the investigation of the part which didn’t collapse, results which show support
blocks of the built trusses were not established right under the loading point and results
which show there were the gaps between the support blocks and centers of the built

2Failure Knowledge Database / 100 Selected Cases

trusses were found, and the support blocks can break or buckle in such cases. The
similar assembly seemed to have been carried out in the accident scene. As the result,the falling of the built truss which became the origin of the accident occurred.
The mechanisms of the whole collapse are as follows. The scaffolds (the shelfscaffolds) of the workers were installed in the way that braces were fixed on the pillar,
and the stage planks were installed on them horizontally. When the built truss andconcrete fell on the shelf scaffold, the impact transmitted through the braces to thepillar, and the load in an oblique direction affected it, then the pillar reached the proof
stress, and it bent as figure 2. The structure of the mould timber support and thescaffold should have been independent from each other in order to ensure strength, buton this site, the scaffolds had been installed on the pillars through braces, and moreover,since they were fixed near the joint (the link part) which is a structural weak point of
the pillar, the strength of the pillar has been considerably lowered. Therefore, the pillar
finally lost the retention capacity force and collapsed, and this developed into a chain
reaction, and it led to the collapse of the whole mould timber support.
・The indirect cause.
The contractor carried out this construction and the plan and management of it, but
the design of the gymnasium had been done by the owner. In the design by the owner inthe beginning, the traditional mould method had been adopted. And it was changed forthe built slab method about 4 months before the accident, following an offer from thecontractor that it wanted to save the labor because of the labor shortage (figure 5references). Then, the specification which was attached to the petition for change wasadopted, and the construction became based on the specification written by thecontractor.
The built slab method was developed by Chuo Build Industry Co., Ltd, and after therecognition of Minister of Construction was received in 1979, it was adopted in about
300 cases by the time of the accident day, and it can be said that it was thecomparatively new technology. In fact, it was only the field director who had experienceof this method from all the staff at that time, and the mould timber support operation
chief was completely inexperienced. In addition, it is said that the mould timber support
operation chief didn’t even receive the changed drawing. The anxiety about the safety ofthis method spread among the subcontractors, and it seems that the whole staff wasembarrassed by the inexperienced method.
And there is a background that, the contractor was not decided even after 4 rounds of
selective competitive bidding since the target price estimated by the owner was too low,3

Failure Knowledge Database / 100 Selected Cases

and the price which the contractor received by optional contract was about 60 million
yen lower than the bidding price. So the design change for cost saving might have been
done without considering the risks.
4Immediate Action
In the first disaster in which a built truss fell, the concrete inflow work was stopped,

and the field director and some workers went to confirm the situation, consider thecountermeasures and conduct treatment the fallen concrete on the first floor. And thesecondary disaster occurred afterwards.
After the accident, Kondo Minister of Labor inspected the field, and directed
investigation. The remedial work began in April, and the Ministry of Labor establishedthe special technology survey team and undertook the investigation of the disaster
cause.
5Countermeasure
In the accident investigation report by the Ministry of Labor, the cautionary note in

the construction, and the necessity of ensuring safety of mould timber supports by
information collection during the innovative technique introduction, etc. are described.
From April of the same year, the construction cost integration standard in public
works was revised for the improvement in the technical capacities of field workers, andsafe training for the workers in the field more than once a month for half a day wasimposed, and the cost for that was newly appropriated.
In "the secondary structural improvement promotion program" of the constructionindustry decided in the same year, improvement of work conditions and security andfosterage of the personnel were stated as the plan to be carried out.
6Generalization
This construction gives the strong impression that the contractor hurried up theconstruction very much. One of the causes was the serious manpower shortage. The
manpower became insufficient, while the construction technology advanced, and thepossibility of the major accident by the expansion of divergence between technology andproficiency was indicated, and it raised the social problem of labor shortage in theconstruction industry.
7Knowledge
The safety is first. … Stop at the red signal even in a hurry. Escape when the floor

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Failure Knowledge Database / 100 Selected Cases

falls even if you want to execute construction.
Do ordinary work properly …It is unexpectedly difficult to adequately do work whichis easy even for amateurs.
Direct your eye to what supports us. …Never treat the pillar carelessly which
supports all.
When facing something new, have both of anxiety and expectation. …Be careful
because there is a risk, even if the construction period can be shortened.
8SequelThe field director had the fault of neglecting the duty to evacuate the workers, butcharges were dropped because of his death. The other construction parties’ indictments
were suspended the indictments for the reason of concluding the private settlementwith the bereaved families.
9On the Side
1992 was the year of the bubble economy collapse. The people were still obsessed

about the bubble economy. At that time, the manpower of Japan consisted mainly ofwhite-collars, and the construction industry was shunned as "3K" (dirty, severe anddanger) and suffered a serious labor shortage. Especially, the shortage of the skilledworkers such as reinforcing-bar placers and mould engineers, etc. peaked in 1990 and91, and this accident occurred just at that period. Afterwards, the construction labor
shortage rate has descended as well as stock prices and land prices, and returned tozero.
<Reference>
The investigation result report of the concrete mould timber support collapse disaster inthe gymnasium construction work in Maritime Self-Defense Force Atsugi air base(Ministry of Labor 1993)
The construction accident (Nikkei construction 2000)
The House of Representatives conference minutes, the 123rd Diet, Labor Relations
Commission, No.1.
(http://kokkai.ndl.go.jp/SENTAKU/syugiin/123/0340/12302260340001c.html)
The House of Representatives conference minutes, the 123rd Diet, Labor Relations
Commission, No.4.
(http://kokkai.ndl.go.jp/SENTAKU/syugiin/123/0340/12304150340004c.html)
Chuo Build Industry Co., Ltd (http://www.chuo-build.co.jp/sub4461.html)

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Failure Knowledge Database / 100 Selected Cases

Construction labor supply and demand survey result(http://www.mlit.go.jp/toukeijouhou/chojou/rodo.htm)
Second structure improvement promotion program.
(http://www.mlit.go.jp/sogoseisaku/const/kengyo/kouzoukaizen1992.pdf)(Note of 1) Department of civil engineering, the Univ. of Tokyo.

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