Abstract of Special Research Report (SRR-91)

National Institute of Occupational Safety and Health, Japan

Study on Method of Dynamic Evaluation For Temporary Structures



: In recent years, as major civil engineering works such as highway and bridge construction have increased drastically in Japan, the collapse accidents of such temporary structures as concrete shorings for bridge construction works have occured very often as mentioned in this report. Since these accidents may result in many casualties, the establishment of the preventive measures has become a great demand.
    The causes of those accidents have been attributed to insufficient evaluation on stability of framework of temporary structures at planning and design stage, to unsafe execution of costruc tion works from structural viewpoints. Besides, those asccidents are also caused by inadequate safety management such as poor checking and inspection system for the temporary structures.
    Traditionally, the government in Japan has made the laws and regulations on construction works so as to prevent the accidents other than technical standards for further safety man agement. In addition to the government, private associations and suppliers also have prepared design and planning technical standards or manuals for construction works, which have also been helpful as the preventive measures for the accidents in construction works.
    However, it may be necessary to clarify the items, such as design load of supports, etc, in which definitive technical standards have not yet been established, in the light of the fact that the accidents in the major construction sites were mostly due the insufficient standards.
    In this study the following items were conducted for the improvement of safety design and planning, and applications of those results to preparation of safety standards of temporary struc ture for the establishment of preventive measures against collapse accidents in construction works.
  1) Determination of horizonal and working load in bridge construction works.
  2) Evaluation of stability for horizonal load of temporary structure.
  3) Evaluation of strength of members of temporary structure.
   (1) Strength of bracket and anchorbolt for beam-type supports.
   (2) Strength of strut with combined members using jack, ajustable pieces and steel H.

Actual Measurement of Load Acting on Frame Type Shoring

Katsunori OGAWA, Yoshimasa KAWAJIRI and Kinichi KINOSHITA

: On concrete works of bridges, many types of shoring have been widely used, such as the frame tpye, built-up post type and steel H type. On designing of these shorings, it is necessary to make clear the substitute forces and horizontal substituting forces. But the standards has not been clearly established present, and hence there are many cases in which the substitute forces aren't fully considered at design stage.
    Strength of temporary structures like shorings are usually calculated by simple models, be cause of the complexity of the structures.
    Comparing them with permanent structures, the construction precision and the joints of temporary structures aren't perfect, and there were some cases of collapse accidents in which the causes had been thought to be derived to these factors.
    Therefore, actual measurement on the frame type shoring was carried out to clarify the calculating method of forces liberated to each member, and to determine the values of substitute forces due to site work activities. This measurement was conducted at the costruction site of Tokyo Bay side high way in Ogishima, Kanagawa prefecture.
    This actual measurement was carried out to investigate axial forces and deformations of some parts of the shoring a week after concrete placing. After the measurement, the measured values were compared with the theoretically calculated values, examining from the stand point of safety design and construction works.
    The results from this actual measurement and calculation are summarized below;
  (1) Axial forces acting on supports vary in a wide range, and it was found that they had acted on some supports locally.
  (2) There were some supports of which measured values were 2.5 times as large as to the calculated values or exceeded the permissible compressive strength.
  (3) Substitute forces due to site work activities also concentrated locally, and it was found that they should be estimated as to 350kgf/m2 .
  (4) Axial forces acting on joints and braces, vertical and horizontal displacement of shorings were measured small, and the influence of temperature change on axial forces were also estimated to be small.

Actual Measurement of Load Acting on Steel H Tyepe Shoring

Yoshimasa KAWAJIRI, Katsutoshi OHDO, Katsunori OGAWA and Kinichi KINOSHITA

: On bridge constructions, frame type shorings and steel H type shorings have widely used for concrete placing. It is easy to calculate stresses of each member of the steel H type shoring because of the simplicity of the structure compared with the frame type shoring.
    In general, the differences between calculated stresses and actual stresses of steel H type shoring are not so large as compared with that one of the frame type shoring. For this reason, in designing the members of shorings, they are usually calculated by simple models. However, temporary structures are not rigid, because they are connected by half rigid joints with a few bolts, and angles braces are not strong either.
    Therefore there is a need to confirm safety the substitute forces due to site work activi ties, horizontal substituting force and deformation of the whole frame. For this purpose, actual measurements were carried out on the steel H type shoring at the construction site of Tokyo Metropolitan Bay area high way.
    This actual measurements was carried out to measure the axial forces and deformations of some parts of the shoring a week after concrete placing. After this actual measurement, measured values were compared with theoretically calculated values, examing from the stand point of safety design and construction of steel H type shorings.
    The results of this experiment are summarized as below;
  (1) Axial forces acting on supports, stringers and joints were assumed to be large by calcu lation, and these estimation was confirmed by actual measurements.
  (2) There were some members which exceeded the calculated axial forces, but it was con firmed that calculated values were similar to measured ones in compliance with the calculating model.
  (3) On substitute forces due to site work activities, it is necessary to estimate them at 350kgf/m2 in case of the frame type shoring, and 190 - 253 kgf/m2 for steel H type shorings.
  (4) The influence of temperature change on axial forces was estimated to be small.

Pulling-out Strength of Anchorbolts for Bracket

Katsunori OGAWA, Yoshimasa KAWAJIRI and Katsutoshi OHDO

: The bracket is used for the supporting point of beam-type shoring in RC high bridge formation works. The anchor for this bracket-mounting application is embedded into bridge pier. This anchor plays an important role to support the brackets.
    For this reason, we have performed a series of experiments on pulling-out strength, shear strength, etc. to clarify the dynamic properties of this bracket-mounting application.
    In the pulling-out test, we have performed the experiments to pull out one or two anchors embedded into bridge pier at a certain interval simultaneously. For experiments two types of deformed-bar-type anchors having a bore diameter of 35 mm, whose embedding depth is 30 cm and 40 cm, respectively were prepared, so as to examine strength properties for.the pulling-out resistance.
    The result we have found are followings;
  1) If the design strength of concrete is 300 kgf/cm2, the limitation embedding depth which is free from pulling out or repture of anchorbolts is 30 cm.
  2) If the design strength of concrete is 240 kgf/cm2, the minimum limitation embedding depth free from pulling out or repture of achorbolts is 40 cm.
  3) If two or more anchors are applied, it is recommended to make their embedding interval should be larger than 20 cm or more in order to prevent intensive drop of the anchors.
  4) The loads that generate crack were 12 tf and 17 tf when embedding depths were 30 cm and 40 cm, respectively.
  5) It is effective way to mount small-piece steel bar to the anchor for pulling-out-proof measures.
  6) In designing the allowable loads, it is recommend to assume the safety factor as of 1.5 for the crack load, 3.0 for pulling-out strength, or for rupture of anchor or bolt, 1.5 for the yield point of the material, and 2.0 for pulling-out strength, respectively.
  7) The following expression can be used for calculating the pulling-out strength ( P max ) of anchor;
    P max = n ·φ ·L ·α ·F c
        n : The numbers of anchors
        φ: Peripheral length of anchor (cm)
        L : Embedding depth (cm)
        α: Proportinal constant on design strenth of concrete
        F c : Design strength of concrete ( kgf/cm2 )

Shear Strength of Anchors for Braket

Yoshimasa KAWAJIRI, Katsutoshi OHDO and Katsunori OGAWA

: Brackets are widely used as support of beam-type shorings for concrete placing in bridge construction and are the most important members of the support system to prevent their collapse accident.
    Usually these brackets are installed using bolts in internal thread anchors embedded in con crete of bridge piers. The purpose of this study is to clear the shear strength of these anchors and bolts, with the emphasis on the followings.
  (1) Effect of bolt fastening on the shear strength.
  (2) Effect of clearance between bolts and their holes on the shear strength.
    In experiments, plates installed to anchors embedded in concrete block model were pushed out by loading machine.
    Result of tests are summarized as follows;
  (1) Crack load and ultimate load increase in proportion to fastening torque in bolts.
  (2) In case of plural bolts having clearances between bolts and their holes, shear strength decreases compared with that one with no clearance.
  (3) Prom the results in this time, crack load, P nc (tf), and ultimate load, P nu (tf), are given by equations below;
    P nc = n · [0.64 T 0 + β · (5.76 - 3.76)], P nu = α ·n · (18.3 + 0.33 T 0 )
    where n : number of bolt, α: 0.85 - 1, β: 1/n - 1, T 0 : bolt axial force (tf)

Strength of Brackets for Temporary Beam

Yoshimasa KAWAJIRI, Katsutoshi OHDO and Katsunori OGAWA

: Brackets are used as supports for beam-type shoring in concrete placing of bridge con struction, as shown schematically in Fig.1.
    It is study one type of installed bracket using bolts in internal thread anchors embedded in concrete of bridge piers will be examined to clarify the strength of this type bracket.
    The strength of anchors was already examined in the former studies.
    This study aims at investigating the strength of brakets systematically.
    In the experiments, push-out tests were performed against installed brackets using bolts to internal thread anchors embedded in concrete block model.
    Result of tests are summarised as follows;
  (1) Ultimate strength of brakets depend on the strength of concrete blocks are installed in and in case of non-reinforced concrete 104 - 131 tf for concrete design strength 240 kgf/cm2, 123 - 140 tf for the same 300 kgf/cm2 and in case of reinforced concrete 156 - 177 tf.
  (2) Allowable load for brackets should be 40 - 60 tf according to the test results in this time.
  (3) Measured axial forces in the top bolts are larger than the calculated values by conven tional method in some cases.
    Then another calculating method is proposed and verified good agreement with measured values.

On the Strength of Steel H Supported by Jack

Katsutoshi OHDO and Yoshimasa KAWAJIRI

: On bridge constructions, mechanical jacks are used in steel H type shorings. Sometimes accidents due to collapse have happened in these shorings caused by buckling of steel H supported by jacks. Each strength of the jack and the steel H is cleared, but the strength of their compound structure is not cleared.
    For the purpose of preventing these accidents, it is necessary to make clear the compound strength. So, three types of experiments were performed to clarify the compound strength of the steel H supported by the jack. The first type was on the lower end fixed jack, the second type was on the lower end semi-fixed jack. These experiments were performed to analyze the effect of the length of the steel H and the eccentric distance between the steel H, the jack and the jack base-beam. The third one was on the jack base-beam reinforced by the stiffener.
    As a result of these experiments, it was concluded that the steel H supported by the jack should be designed considering the strength of the steel H as well as the jack and reinforcement of the jack base-beam by the stiffener under the jack.

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