| Case Name |
In a tension work, a back of a fixed part was broken. |
| Pictograph |
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| Date |
November 7, 1994 |
| Place |
Muko City, Kyoto Prefecture. |
| Location |
The PC girder, in the construction site to widen an expressway. |
| Overview |
In the tension work of the precast PC (prestressed concrete) girder, the back of the part which the main direction PC steels were fixed to deviated and caused the shear fracture, and the anchorage sank into the main girder. |
| Incident |
The style of structure was the ear girder (a girder in the most outlying place) of the post-tensioning-system precast girder bridge. The skew angle was 70° . The form of the cross section was approximately a rectangle. The height of the girder (1.15) was limited lower than the standard of the height, for the length of the girder (37.0m) (Ratio of height and span of the girder h/L=1.150/37.0m=1/32.2). In the tension work, the back of the part fixed to the main direction PC steels was broken and the anchorage sank into the main girder. |
| Sequence |
The process of this accident from the outbreak to the procedure is as follows.
1.The four of the five main girders (except the broken one) in 1 span were placed temporarily until the building started. There was no abnormality, especially.
2.The broken girder had 4 main direction PC steels (12S12.7-strand) which were going to be fixed to the edge. The two of the four steels (C1, C2) had finished being tensioned.
3.In the tension work of the PC steel, C3 which was the outside of the girder, the back of the concrete, the anchorage part, deviated and caused a shear fracture, and the anchorage caved in.
4.The edge of the main girder (fixed part of the PC steels) had broken, so the tension work was stopped.
5.The tension worker was placed in the safe zone which was on the side of the girder, and no one was injured in the accident. |
| Cause |
The causes of the accident were as follows.
1.Box-out for the transverse prestressing anchorage was close to the back of the main direction PC steels (12S12.7).
2.Because the main girder had 70° as an angle of skew, the geometry of the box-out for the transverse prestressing anchorage was big and deep.
3.The anchorage arrangement interval and the distance of both edges were satisfied with the regulation of the PC method standard, but the box-out form in the main girder production was become unity. So, a large sectional damage was made in the back.
4.The plan was to check when it was designed and when the main girder was made. But they didn't recognize the structure of the back of the anchorage sufficiently. |
| Response |
1.The concerned discussed and examined the materials to determine the measure of the broken girder (`repair and reuse' or `rejection and reproduction').
2.As a measure, they should perfectly pull down the damage part , and partially joint them . And they should change the anchorage and the PC steels. They must confirm the strength and the capacity of the bearing loads of the main girder by loading tests, etc. (The repair utilization plan and) the broken girder should be completely destroyed. They should improve the structure partly and make a new girder. (The plans of destruction and re-producing) were examined but in this case, it was impossible to change the time of the building. If they examined during only the loading test, they didn't confirm the capacity of the bearing loads of the main girder which was repaired, and the durability. So, they took the measure of throwing it away and reproducing the main girder. |
| Countermeasures |
The countermeasures of recuurence prevention in this accident are as follows.
1.Because the prestressed force converges at the place near the PC steels anchorage of prestressed concrete materials, the concrete needs the determined compressive strength and the sections. They need to take countermeasures to place reinforced steels such as stirrups, lattice-like reinforcements, spiral reinforcements, etc. when they design. Similarly, the concrete around the anchorage of the PC steels has to be reinforced by placing reinforcement steels, to bear the tensile stress which occurs in the right-angled direction of the PC steels.
2.In the case of placing many anchorages on the same plane, they have to determine the section size of the concrete materials, considering the number of the anchorage, the strength of the tensile force, the necessary least interval of each anchorage, etc.
Like the above mentioned, the anchorage of the prestressed concrete materials is an important part. When the prestress is introduced, the same failure and accident like this may happen. The materials aren't satisfied with the demand and performance as materials. It may occur not only the materials, but also a serious accident in a tension work. Therefore, when the plan was checked in designing and building, structure details of the anchorage should be examined sufficienly in advance, and improve them if there is a problem (for example, the interlacing of the main direction PC steels anchorage and the lateral direction anchorage, the appropriate arrangement of reinforcing bars, the existence of the partial defect of cross section, the condition of the concentrated arrangement of the grout hose, etc.).
There is a similar case of the accident in the post-tension tension work. If they concetrate grout hose on the back of the anchorage, and pour the concrete into the form flame, there may be defects of the section and air space in the back of the anchorage, as a result. So, the anchorage may sink into the concrete materials in the tension work. It becomes impossible for this PC steels to be tensioned. They have to pull down the important part of the concrete materials, and produce the reinforced one again. |
| Knowledge Comment |
1.The items to check (check sheet etc.).
2.The transmission of information to the party, etc. |
| Scenario |
| Primary Scenario
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Insufficient Analysis or Research, Insufficient Prior Research, Carelessness, Insufficient Understanding, Planning and Design, Poor Planning, Failure, Fracture/Damage
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| Field |
Civil Engineering
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| Author |
TANAKA, Miyo (Japan Prestressed Comcrete Constructors Association)
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