Abstract of Special Research Report (SRR-No.37)

National Institute of Occupational Safety and Health, Japan

Study on Instability of Bridge and Development of Safety Construction Technique during Bridge Erection


Katsutoshi OHDO

: In this project study, to prevent collapse accidents and falling accidents during bridge erection, the instability of jack and supporting stand, the safety of bridge girder erection work methods, wire grip management technique used in cable erection, and instability factors on suspended scaffolds are examined experimentally and analytically. From the results of these studies, the risk evaluation methods during the bridge erection are examined for preventing the collapse and falling accidents.
    The sub-themes of this project study are as follows;
    1. Study on instability of jack and supporting stand during bridge girder erection
    2. Study on evaluation of safety of bridge girder erection method
    3. Establishment of wire grip management technique used in cable erection, etc.
    4. Study on instability factors on the installation or demolition work of suspended scaffolds

Study on Stability of Bridge Girder in Launching Method

Seiji TAKANASHI and Katsutoshi OHDO

: The launching method is one of the construction method used to erect a bridge girder. In this method, a bridge girder is placed on a launching apparatus, and it is moved into a prescribed position by the launching apparatus or other equipment. The advantages of this method are that a large-sized crane is not required, and a bridge girder can be installed in a short time. The disadvantages of this method are that a working area is required for the adjacent to the construction site, and also that a large patch load occur onto the bridge girder during the launching operation. Since this load disappears when the bridge is completed, it may not be taken into consideration at the design stage. If a bridge girder's web plate is damaged due to this load, the stability of the bridge girder may be lost, and it may collapse. Therefore, in this study a reproduction test was carried out using a launching apparatus that is commonly used in construction sites and a full-scale bridge girder model.
    The test was carried out for the following purposes:
1. To clarify the stress state in the web plate of the bridge girder during the launching operation.
2. To clarify the influence caused by the eccentricity occurring between the bridge girder and the launching apparatus.
3. To confirm the validity of the methods of reinforcing the web plate against the construction loads.
The test results were evaluated based on the magnitude of the stress generated in the web plate. The stress was calculated from the strain that was measured by a three-axis rosette gauge attached to the web plate of the bridge girder.
    The findings obtained from the test were as follows:
1. The effect of eccentricity occurring between a bridge girder and a launching apparatus cannot be eliminated in actual construction sites. As the test result showed, the stress of the web plate increased with the occurrence of eccentricity. In particular, when the launching apparatus was misused, the stress increased noticeably, and the risk of damage to the bridge girder became extremely high.
2. Several reinforcement methods for the bridge girder's web plate against a construction load have been proposed. In this test, the validity of three typical reinforcement methods was investigated. The strength of the bridge girder increased by 15% when the simplest reinforcement method was employed. However, it increased by only 25% even when the most complicated method was employed.

Experimental Study on Horizontal Stability of Saddle

Katsutoshi OHDO, Seiji TAKANASHI and Hiroki TAKAHASHI

: When constructing or reconstructing bridge girders, a temporary structure called a saddle is often used as a support. The saddle is composed of many stacked steel H-beams, each with a width and height of 150 mm, in a double cross. The load on the saddle comprises the vertical load due to the weight of the bridge girders, and the horizontal load due to the launching erection. Although the strength against vertical loads is considered when designing the saddles, the horizontal stability against the horizontal load has largely been judged based on the experience of the construction workers.
    In recent years, 2-edge girders have been widely used in steel bridges to reduce the cost, but fewer girders means not only a greater girder height but also higher saddles to support them. When saddles are higher than 5 m and conventional construction methods that rely on workers' experience are used, the horizontal stability of the saddles could constitute a risk as skilled workers are decreasing in number. However, few studies have focused on the horizontal stability of saddles through experiments and analyses. Therefore, in this study we conducted experiments which involved applying vertical and horizontal loads to actual saddles stacked in a basic arrangement to obtain fundamental data on the horizontal stability of the saddles. In the experiments, vertical and horizontal loads were applied to saddles measuring in height from 1 m to 4 m for a single saddle, and 3 m and 5 m for the twin saddles. The twin saddles were connected to each other by steel angles and braces. All of these conditions were decided based on the opinions of on-site engineers and considered to be similar to the conditions at actual construction sites. The vertical loads increased in increments of 500 kN from 500 kN to 3000 kN and the horizontal load, which was 5%, 10%, and 20% of each vertical load, was applied to the top of the saddle to examine the maximum vertical load that could be applied to the corresponding horizontal load.
    The results are summarized as follows:
    1) The 2-m single saddles could bear the horizontal load of 10% until the vertical load reached 3000 kN. However, the 3-m single saddles could not bear the horizontal load of 10% until the vertical load reached even 2000 kN, and the 4-m single saddles could not bear the horizontal load of 10% even before the vertical load reached 500 kN.
    2) When considering seismic load, the 2-m saddle and 1000 kN vertical load appears to be the limit.
    3) On the other hand, the twin saddles connected to each other were stable at the height of even 5 m against seismic load.
    4) Therefore, it is concluded that single saddles should be connected to each other when the saddle height exceeds 2 m in consideration of the limits of the combination loads.

Influence of Residual Deformation on Stability of Saddle in Bridge Construction

Hiroki TAKAHASHI, Katsutoshi OHDO, Seiji TAKANASHI

: In the construction of a bridge girder, a temporary "saddle" structure often used. The saddle is composed of multiple stacked steel H-beams, each having a width and height of 150mm. The vertical load acting on the saddle is due to the weight of the bridge girders. The saddle member might become deformed by this load. After removing the load, the deformation might remain. This deformation is known as residual deformation. The slightly deformed saddle members are used repeatedly as well. However, there is no management standards for the saddle. In this study, the influence of residual deformation on the stability of the saddle is examined and a management standard for the saddle is proposed.
    To confirm the strength of the saddle, the distribution of load in the saddle was examined by experimentation using saddle members at construction sites. It is difficult to obtain quantitative data on the stability of the saddle by experimentation, as residual deformation exist in real saddle members due to welding of the flat steel bar for strengthening of the H-beam. Therefore, the influence of residual deformation on the stability of the saddle was examined by numerical analysis. In addition to that the experimental results were compared with the analytical results. A numerical analysis was also carried out in consideration of the residual deformation of the saddle member. The finite element method was used for this analysis.
    The results of this study are summarized as follows:
    1. Even if residual deformation affects at least one member of the saddle, its influence of residual deformation on saddle stability is minimal. The results of this study suggest that the probability of saddle failure is low when a bridge girder is supported on a saddle.
    2. However, if the flat surface at the flange of the saddle member is less than three times the thickness of the flat steel bar, the saddle member should not be used.
    3. If the saddle member is managed appropriately such that an extremely deformed saddle member is removed, the stability of the saddle would be guaranteed. Moreover, the saddle is assumed to be used originally within the elastic deformation even if there is the bridge on saddle. When the saddle is used, the residual deformation should not remain in the saddle. It is should use the saddle of the enough number in consideration of the weight of the bridge girder.

Investigation of Wire Rope Grip Usage Standard

Tetsuya SASAKI, Takashi HONDA, Kenta YAMAGIWA

: The cable erection method is used in the construction of steel bridge. Even though this method is very versatile and used only as a last resort to build an arch-type bridge over a deep valley,the nature of the method requires the use of many wire ropes and each one must be terminated using wire rope grips. One of the major problems with this method is the risk of wire rope slipping through the wire rope grips in the case of improper fitting. All grips are installed according to the wire rope grip usage standard; nevertheless, accidents due to wire rope slipping through the grips frequently occurs at bridge construction sites employing the cable erection method. This fact suggests that the existing usage standard for wire rope grips may be insufficient.
    To clarify this problem, an intensive study was carried out to reveal the adequacy of the current usage standard for U-bolted wire rope grips. Since the efficiency of termination using U-bolted wire rope grips depends on many factors including the type and diameter of the wire rope, number of grips, tightening torque, etc., the wire rope types mainly used for the cable erection method were firstly determined through a survey of bridge construction companies. According to the survey, fiber core (FC) 6×24 and 6×37 type wire ropes are mainly used for small-diameter wire ropes less than 22.4mm, and IWRC 6×Fi(25) and FC 6×37 types for large-diameter wire ropes over 22.4mm. Based on this result, loading tests were conducted to reveal the effect of several factors on the efficiency of wire grip termination. The main conclusions from this study are as follows:
    (1) For large-diameter wire ropes over 16mm, the current standard for U-bolted wire grips cannot provide sufficient efficiency of termination.
    (2) Repeatedly used wire grips may lead to decreased efficiency of termination.
    (3) M-type grips fitted under the manufacturer's standard exhibit sufficient efficiency of termination.

Strength of Roughly Assembled Suspended Scaffolds

Yasumichi HINO

: Suspended scaffolds supported by many chains are generally used at bridge construction sites for work such as painting, assembling of form panels, etc., which involve only lightweight materials. However, after the Hanshin-Awaji earthquake in 1996, numerous bridge reinforcement projects were implemented, involving the handling of 10 kN of heavy-load materials on suspended scaffolds.
    As the safety design method for suspended scaffolds has not been confirmed, there is an urgent need to develop countermeasures for the heavy-load materials. In fact, an accident has already occurred in which material fell from the suspended scaffolds onto the express highway and crashed into several cars. The accident was caused by the fracture of one of the scaffold chains due to the heavy load.
    The objective of this study was to investigate the strength of suspended scaffolds and the suitable use method of their components by experimental studies. Especially, the relationship between the difference in the relative number of chains and the increment load was investigated. The major findings obtained from the results of this study are summarized as follows:
1) The increment load of the suspended scaffolds roughly depends on the additional number of chains .However, the maximum load of the suspended scaffolds with a small length of chain could not be estimated in proportion to the number of chains.
2) The loads acting on multiple chains have especially great disparity at the elastic area. This disparity depends on the difference in relative chain length.
3) Suspended scaffolds must always be used in the elastic area. Hence, the design load values cannot be estimated in proportion to the number of chains.
4) The chain length must be equalized for ensuring the safety of suspended scaffolds for supporting the heavy-load parts.
5) Maximum lateral strength of the clamp was very small even under appropriate installation. When the torque of the clamp was insufficient or its installation angle was large, the maximum value was very small.
6) The lateral load of suspended scaffolds can be reduced through effective planning, design and execution management.

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