JNIOSH

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.
    From the results of the investigation,it was found both in Japan and overseas that collapse accidents during construction accounted for the majority of construction accidents and about 10% of the serious collapse accidents were due to wind. In particular, collapse of scaffolds due to wind was a major problem. On the other hand, as for the number of fatalities due to wind, falls made up the majority. In order to examine countermeasures to accidents during construction work in strong wind, prevention of scaffolds collapse and prevention of falls need to be studied.
    For these reasons, the research 'Development of an assessment method on wind resistance in temporary structures' covered the following three themes.
  1. Development of a practical equation for estimating wind loads on temporary structures under construction.
    Field measurement and wind tunnel experiments were conducted to measure wind loads acting on scaffolds. Based on the results, an estimation method of wind loads acting on scaffolds will be proposed.
  2. Study on the risk of the collapse of temporary structures due to the construction errors in reinforcing.
    The stiffness of scaffolds and stresses in the reinforcement members, i.e. the ties connecting the scaffolds to the building wall, were experimentally studied. Based on these parameters, a numerical simulation was performed. From the results, a strengthening method for scaffolds in strong wind will be proposed.
  3. Development of a risk assessment method for assembling and dismantling temporary structures under strong wind.
    The experiments were carried out on the risk of the assembling and dismantling work of scaffolds in strong wind. Based on the results, the methods for evaluating wind resistance in scaffolds were examined in consideration of the wind environment of the workers.

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.
    The latest design code was instituted more than five years ago to prevent these fatal accidents, but despite the introduction of the new code several accidents have still occurred. Therefore, in this study, a field measurement of wind loads acting on the actual scaffolds that were 7 story - 6 bay frame structure was carrie dout. The scaffolds were set alongside the building in the National Institute of Industrial Safety. The surface of the scaffolds was covered with plastic sheets of 0 % porosity, and ties to the building wall were installed every 2 story - 2 bay of the scaffolds.
    In the measurement, the wind loads acting on the sheets of the scaffolds and the ties were measured when wind speed around the scaffolds exceeded 10 m/s. The sampling frequency and the measurement duration time for measuring wind loads were 50 Hz and 10 minuets, respectively. The measurement was carried out from November 20th, 2003 to August 31st, 2004. The results were compared with the estimated values using the existing design method.
    The results of this study are as follows:
  1. The loads acting on the ties due to strong wind varied and concentrated on only some specific ties, especially the ties at the upper part of the scaffolds in this experiments.
  2. From the results of the comparison between the measured and design values on the load acting on the ties, the measured values were higher than the design values on some ties.
  3. Especially on ties that were set at the upper part of the scaffolds, the measured value was approximately 1.4 times higher than the design values.
  4. Therefore, it can be concluded that the design load acting on the ties should be increased or the allowable strength of the ties should be decreased in the design stage for the scaffolds to the wind loads.

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.
    By the way, this kind of experiments need reliability which can express the full scale scaffolds characteristics. Therefore, it is necessary to figure out in advance the reliability of wind tunnel test results.
    The purpose of this study is to investigate reliability of wind tunnel tests, the relation between full scale scaffolds characteristics of wind pressure acting on natural wind and miniature model characteristics of wind pressure acting on artificial wind by wind tunnel test.
    The results of this study are as follows:
  1) It is possible to estimate qualitatively the wind pressure characteristics of the full scale scaffolds by the data of wind tunnel tests.
  2) Wind tunnel test results are relatively close to the full scale experiment and extremely precise if the mean wind speed is not very low or the influences of local properties such as timbers do not occur.
  3) It is necessary to be equal to the mean wind speed of full scale tests as much as possible if the wind pressure characteristics of full scale scaffolds are estimated by wind tunnel tests.

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.
    Most scaffolds that are erected, are based on some kind of design method for temporary scaffolds. But most of the design methods do not take into consideration the fact that buildings under construction have some holes on the exterior walls. According to the disaster researches, some collapse accidents happened due to the strong wind blows into the holes. Therefore, it is necessary to investigate the wind effects.
    In this study, the fundamental characteristics of wind pressure acting on temporary scaffolds erected alongside of the building under construction, were investigated. And also, the design method considered the holes in the exterior walls and the difference of the shape of temporary scaffolds was proposed.
    From the result of this study, fundamental characteristics of wind pressure acting on scaffolds were shown clearly. And the design method for temporary scaffolds was proposed based on the results of this study. The based data for the proposed design method are shown as follows.
  1) Wind pressure acting on the outside of the scaffolds, C(+) can be estimated by considering the depth of buildings under construction, and it can be estimated by existing wind-resistant design method usually used for exterior walls.
  2) Wind pressure acting on the inside of the scaffolds, C(-) should be divided into 2 types by wind direction facade (from 270 to 90 degree) and backside (from 90 to 270 degree).
  3) In case, that the wind direction is facade, the maximum value of C(-) occurs at the upper area, and it is affected by the existence of the buildings under construction, but most of the value of C(-) is not affected by the holes in the exterior walls.
  4) In case that the wind direction is backside, the value of C(-) is affected by buildings, values of C(-) increase pro rata in relation to the hole size of exterior walls, and finally C(-) becomes nearly equal to the value and distribution of the only scaffolds model if the hole size nearly equal to the wall size.

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.
    The wind pressure acting on the scaffolds is estimated as the sum of its front side and the rear side. And also,the wind pressure acting on the scaffolds attached to buildings under construction is larger than the only scaffold model. The reason is mainly the increase of wind pressure acting on the rear side of scaffolds. This knowledge is based on past investigations. Accordingly, this study tried to investigate the possibility of a new concept for scaffold. This method is to control the wind pressure acting on the rear side of scaffolds, to cover the gaps between the scaffolds and buildings under construction by using some sheets without mesh, not to add many ties, not to change from normal sheets to the mesh-sheets. In other words, it is predictable that the wind pressure acting on the rear side of the scaffolds would not affect if all the gaps are covered by some sheets without mesh.
    In this study, wind tunnel tests by artificial wind and full scale tests by natural wind were carried out to investigate the possibility of this reinforcement method based on such a concept.
    The major findings obtained in this study can be summarized as follows.
  1) Wind pressure acting on the rear side of scaffolds mostly reduced by the reinforcement method.
  2) The total of the wind pressure is also reduced by covering all the gaps between the scaffolds and buildings under construction.
  3) This method can also be used for the scaffolds attached to buildings under construction which has some holes in exterior walls, if the holes area is less than 25% of the exterior walls.
  4) This result was verified by not only the wind tunnel tests, but also by full scale tests by using actual scaffolds.

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.
    In this specific research, to prevent these collapse accidents, a field measurement and wind tunnel experiments for evaluating the wind loads acting on the scaffolds were carried out. The strength of the ties that connect the scaffolds to the building wall was also evaluated experimentally, and the wind resistant properties of the scaffolds were examined.
    However, as for the number of fatalities due to wind, the number of falls was the highest. Therefore, to examine the total safety countermeasures against dangers in construction work on scaffolds in strong wind, the prevention of falls needs to be studied.
    For these reasons, experiments were carried out on the risk in the assembling and dismantling work of scaffolds in strong wind. Ten construction workers participated in the experiments, and the assembling and dismantling work of the scaffolds were carried out in the wind tunnel under a uniform flow and gusts.
    The results of this study are as follows:
  1. From the results of the experiments under the uniform flow, the minimum value that was felt to be a high risk by the subjects was 8 m/s at the mean wind speed.
  2. From the results of the experiments under gusts, the minimum value that was felt to be a high risk by the subjects was 14 m/s at the maximum instantaneous wind speed.
  3. There was high risk in the assembling and dismantling work of the scaffolds even under 10 m/s at the mean wind speed, which is the standard for work to be stopped according to the regulations of Labor Safety and Health.
  4. A limited wind speed for performing the assembly and disassembly of scaffolds was proposed.

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