Abstract of Special Research Report (SRR-No.31)
National Institute of Occupational Safety and Health, Japan
Development of an Assessment Method on Wind Resisting in Temporary Structures
Introduction
SRR-No.31-1 |
Katsutoshi OHDO |
: The weather has a major influence on work performed on construction sites, because construction work usually takes place in the open air. Due to bad weather, e.g. strong wind, construction accidents sometimes occur. We examined the yearbooks of the Japan Construction Safety and Health Association to investigate wind-induced accidents during construction. These data were compared with data of overseas wind-induced accidents collected from the Engineering News-Record.
Keywords; Scaffolds, Strong wind, Collapse, Falling down, Risk |
Measurement of Wind Load Acting on the Scaffolds
SRR-No.31-2 |
Katsutoshi OHDO, Seiji TAKANASHI, Yasumichi HINO and Koichi SAITO |
: In Japan, temporary scaffolds are typically covered with plastic sheets to prevent construction equipment from falling. However, wind loads acting on scaffolds are increased significantly due to these sheets. This seriously impacts safety because wind loads have a major effect on the stability of the scaffolds due to the scaffolds' inherent instability under horizontal loads. In fact, the scaffolds often collapse under strong wind during construction and many people have been injured and killed. To prevent these fatal accidents, a design code for scaffolds under wind loads has been established and continuously developed.
Keywords; Field measurement, Scaffolds, Wind loads, Tie, Construction, Design code |
The Predictability of Wind Pressure Acting on the Actual Scaffolds by the Wind Tunnel Test
SRR-No.31-3 |
Yasumichi HINO, Noboru SATO, Songpol PHONGKUMSING, Katsutoshi OHDO and Seiji TAKANASHI |
: There are various shapes, heights of under construction buildings and temporary scaffolds. Locations of construction sites also are various. In construction sites, collapse accidents of the scaffolds sometimes occur due to strong wind. It is necessary to understand reliable wind pressure characteristics acting on the scaffolds to prevent these collapse accidents. Hence, the investigations considered various situations for the quantitative evaluation of wind pressure acting on the scaffolds are needed. In such cases, wind tunnel tests have been usually carried out by using miniature models, for making it possible to do the parametric studies.
Keywords; Wind tunnel experiment, Wind Pressure, Scaffolds |
Study on the Estimation Method of Wind Pressure Acting on the Temporary Scaffolds
SRR-No.31-4 |
Yasumichi HINO and Songpol PHONGKUMSING |
: Temporary scaffolds are usually attached to buildings under construction. Also, construction companies take into account safety and sound insulation by using scaffolds with mesh sheets or sound proof panels. However, collapse accidents of scaffolds due to strong wind happen every year. These accidents involve not only the construction workers and also pedestrians. There accidents also have a deleterious effect on public transportation, e.g. closing streets and business activities. Therefore, it is necessary that strict countermeasure to prevent scaffold collapse should be imposed.
Keywords; Wind-resistant design, Wind tunnel experiment, Wind Pressure, Scaffolds |
Study on a New Reinforcement Method of Temporary Scaffolds
SRR-No.31-5 |
Yasumichi HINO and Katsutoshi OHDO |
: Temporary scaffolds used at the construction sites are designed and erected with safety considerations to prevent scaffold collapse due to the strong wind. Some kinds of countermeasures are carried out to prevent it. For example, a lot of ties and the mesh-sheet are usually used. Ties are tools for connecting scaffolds to buildings under construction and to resist the wind pressure acting on the scaffolds. Also, the mesh-sheet is a tool for prevention of some kind of construction materials falling. It also can reduce the wind pressure because its surface area is smaller than that of normal sheets. However, there are some cases some regions with the strong wind in every year that required very large numbers of ties, which are calculated based on the wind-resistant design. Therefore, it is necessary for such a construction site to create a new concept design or reinforcement methods for the scaffolds that is based on detailed investigations to reduce the necessary amount of the ties.
Keywords; Scaffolds, Strong wind, Collapse, Falling down, Risk |
Influence of Construction Error on the Mechanical Properties of Tie
SRR-No.31-6 |
Seiji TAKANASHI and Katsutoshi OHDO |
: Accidents due to scaffolding collapse following exposure to strong winds do occur. In general, the scaffolding is a large-scale structure, tens of meters high and wide. When such scaffolding collapses, many workers will be exposed to the disaster at the same time. Generally speaking, the scaffolding also lacks horizontal stability, and its stability is retained through ties connecting it with the building. The performance demanded of the tie is decided by the structural standard while its strength requirement is 8.82kN or more. Because the safety rate is twofold or more in the design studied, the allowable strength is 4.41kN. However, there are numerous poorly-constructed ties throughout the construction site. Tests used to examine the performance of the tie with construction error were performed, with four main parameters. 1) Angle of the tie. 2) Distance between fulcrums. 3) Depth of the anchor bolt for fixation. 4) Torque of clamping. The results were as follows. Although the effects of distance between the fulcrums of the tie on the compression strength were small, the influence which the angle of tie exerted on the compression strength was considerable and compression strength was found to decrease in proportion to the angle of the tie within five degrees. When five degrees was exceeded, compression strength remained a constant value, regardless of the tie angle. As for the tension strength, the influence caused by the distance between fulcrums was comparatively large. The compression strength was smaller than the tension strength, regardless of the distance between the fulcrums and the angle of the tie. The rigidity of the tie was also found to be greatly influenced by its angle. When ties of very different rigidity co-exist in a scaffold, there is a high possibility of the load concentrating on a specific tie, and if this phenomenon occurs continuously, there is considerable potential for a form of progressive collapse. The ties of very different angles must be avoided; a construction error which is easy to spot. Accordingly, such ties must be reset appropriately or otherwise increased. Strength and stiffness decrease remarkably when construction errors in the anchor bolt are found, meaning the tie becomes unable to demonstrate effective performance. It is difficult to pinpoint this construction error following construction of the tie, meaning the worker must be instructed as to the correct fixation method. Although the influence which the clamp torque exerted on the strength of tie was small, there remained the possibility of the bolt coming off during usual work or daily wind if the torque was too weak. Conversely, the life of the tie used repeatedly shortens if the torque is too strong. |
Influence of Construction Error on the Mechanical Characteristics of Scaffold
SRR-No.31-7 |
Seiji TAKANASHI and Katsutoshi OHDO |
: The number of work accidents involving the construction industry as a proportion of all work accidents is relatively high, with many such accidents related to the temporary structures, in particular those occurring annually due to the collapse of scaffolding. In general, scaffolds are used for relatively short periods of time. Therefore, it is a structure which can be dismantled very easily. Moreover, the scaffold cannot resist horizontal loads like wind forces to which it is exposed, meaning the ties have to resist them. When such scaffolds are exposed to strong wind, the stress of the ties was measured using an experiment. Scaffold under various conditions was used for this experiment, with the purpose of the research to compare the test and the calculation results respectively. The loading tests using the actuator, meanwhile, were performed indoors. Moreover, when the scaffold constructed outdoors was attacked by strong winds, the wind force and the behavior of the scaffold were observed. During the indoor experiment, the influence exerted by the specific arrangement of the scaffold board on the axial force of the tie was examined. The load was found to be concentrated on other ties when scaffold boards adjacent to the tie were removed, with a consequent increase in the risk of damage to such specific ties increases. In addition, the influence of any construction errors which were present and exerting force on nearby ties was examined. As a result, the axial forces of ties near that with a construction error were found to be remarkably increased. The experimental and calculation results of the axial force of the tie were compared, using general methods of calculation. The experimental results when a construction error was discovered in a specific tie were 1.5 times the calculation result, representing a construction error within the tolerable range. However, a safe design cannot be established based on these calculation results. Therefore, when the construction error is confirmed, correct construction must be repeated or measures such as increasing ties become necessary. The results by the outdoor test were similar to the results of the indoor test. Even if the scaffold is not exposed to strong winds, in the event of substandard construction, considerable initial axial forces may be generated in the tie. An axial force of 57% of the allowable strength of tie was generated by the construction test. Moreover, the axial force exceeded the allowable strength of tie during the construction test. The worker should perform corrective work. |
Experimental Study on Risk of Assembling and Dismantling Works of the Scaffolds under Strong Winds
SRR-No.31-8 |
Katsutoshi OHDO, Yasumichi HINO, Seiji TAKANASHI and Noboru SATO |
: The weather has a major influence on construction work, and construction accidents sometimes occur due to bad weather, e.g. strong wind. Therefore, wind-induced accidents during construction were investigated. The investigation showed that serious accidents, in which more than three workers were killed or injured, occurred mostly due to the collapses of the scaffolds.
Keywords; Scaffolds, Strong wind, Falling down, Risk |