JNIOSH

Abstract of Special Research Report (SRR-No.20)

National Institute of Occupational Safety and Health, Japan

Study on Safety Countermeasure of Cranes Against Earthquake

Introduction

SRR-No.20-1
Yutaka MAEDA and Yoshio KITSUNAI

: The Southern Hyogo-prefecture earthquake hit around the Kobe City in the early morning of 17th January 1995, and caused a great deal of damage to many cranes. Through on-the-spot investigation and questionnaire, main feature of damage to cranes by this earthquake was found that (1) tower cranes and jib cranes suffered more damage compared with overhead travelling cranes and bridge cranes including container cranes. (2) main damage of overhead travelling cranes was falling down or derailment from the runway girder due to large deformation of building, and container cranes and unloaders were damaged due to movement of foundation associated with soil liquefaction, and (3) tower cranes fixed to buildings with stays were more severely damaged than ones standing by themselves, and the stays seemed to pay the important part of damping or amplification of seismic vibration.
    In Japan two kinds of standards of crane for proof against earthquake exist; one is the construction code of crane, and another is the Japan Crane Association standard. The construction code of crane has the legal force and all the cranes are to have the uniform value 0.2 of the lateral seismic factor. The Japan Crane Association standard decides the corrected seismic coefficient to overhead travelling crane and bridge crane, but not to jib crane nor tower crane. Considering these circumstances, the vibrating characteristics of tower crane with or without stays were selected for the main subject of this study.
    The chapter 2 presents the survey on the damage of cranes in the Southern Hyogo-prefecture Earthquake. And the results of examination to obtain the dynamic characteristics about the tower crane are dealt in the chapter 3. The chapter 4 and 5 deal the vibration test on model of a tower crane and the buckling strength of stays respectively. Concluding remarks are dealt in the chapter 6.

Survey of Damage of Cranes in the Southern Hyogo-prefecture Earthquake

SRR-No.20-2
Yoshio KITSUNAI and Yutaka MAEDA

: On the early morning of 17th January 1995, a very severe earthquake hit around the southern portion of the Hyogo prefecture, particularly the city of Kobe. The earthquake caused catastrophic disaster, with more than 6,400 people dead, about 300,000 injured due to collapse of more than 100,000 buildings resulting from severe shaking and fire. All types of cranes such as container cranes in Kobe harbor, unloaders, jib-cranes in shipbuilding yards, tower cranes in building lots, overhead travelling cranes in manufacturing works, etc. were broken or failed significantly.
    The epicenter was near Awaji island and located at 34° 36.4' North latitude, 135° 2.6' East longitude. The Richter Magnitude of the earthquake was estimated to be 7.2. The focal depth was approximately 14.3 km. Peak ground accelerations at Kobe Ocean Meteorological Observatory were 818 gal in the north-south direction (NS), 617 gal in the east-west direction (EW) and 332 gal in the vertical direction (VD), and duration of strong shaking was less than 10 sec. The area which suffered acceleration above 600 gal, extended from Takatori through Nishinomiya, and was a narrow band of approximately 20 km long and 1 km wide.
    National Institute of Industrial Safety (NIIS) and Japan Crane Association (JCA) dispatched a reconnaissance team to Hanshin area to assess the damage of cranes. In addition, questionnaires were sent to heavy industry works, manufacturing factories, civil engineering works, and port facilities etc. in Kansai area. 117 replies for the questionnaires were received. The number of damaged cranes identified by the questionnaire is corresponding to 56.6% of a total number of damage cranes of 207 cases.
    The main results are follows;
  (1) The cranes with high height such as a tower crane and a jib erase suffered more damage compared with the crane with low height such as an overhead traveling crane.
  (2) Main damage cause of overhead traveling cranes was falling down of main girder and derailment from rail due to large plastic deformation of runway girder.
  (3) Damage of portal jib cranes was particularly concentrated around roller path. About a half of the portal jib cranes damaged due to buckling of the columns.
  (4) Plastic deformation and collapse of jib accounted for approximately 50% of whole damage causes of tower and climbing cranes.
  (5) The tower crane fixed with stay was severely damaged, while the tower crane without stay suffered relatively minor damage. The tower crane mounted on the rooftop of 17 stories building overturned and fell down due to brittle failure from bottom of the tower.
  (6) Container cranes and unloader installed along waterfront of reclaimed lands were severely damaged due to movement of foundation associated with soil liquefaction.

The Vibration Characteristics of the Tower Crane Used in the Construction Site

SRR-No.20-3
Seiji TAKANASHI, Akira TSUBOTA and Kimio KIKUCHI

: As for the tower crane, earthquake resistance has been taken into consideration and design has been done in the same way as for other structures. However, the dynamic behavior has been hardly examined so far. Therefore, in this study, a free vibration test was carried out to examine the dynamic characteristics such as natural frequencies, damping factors and the mode shapes of the tower crane. The experiment was conducted on a tower crane used in the construction site. The experiment was done two cases. The first was for the tower crane without stays standing by itself with a tower mast of 24m length. And the second was for the tower crane with a tower mast of 75m length, with stays installed in 2 steps in the height direction. For the latter case, the test was repeated for different plays of stays. A jack was set in a stay, as the mechanism to adjust length. If the initial stress was added to the jack, there was no play in the stay. If not so play was left in the stay.
    The way of shaking the tower crane was shown in the following.
  1) A revolving frame is turned first, and is stopped suddenly. Bending moment and torsionnal stress occur in the mast by this method. And, the shearing force and bending moment occur in the side direction of jib.
  2) Another way is to drop the weight. Big shearing force and bending moment occur in the mast by this method.
    The followings are the experiment results.
  1) The results of the free vibration test on the without stays type tower crane showed that the natural frequency in the lateral direction of the jib was 0.27 Hz, and the torsionnal frequency of the mast was 1.4 Hz. The natural frequency in the horizontal direction of the tower crane was 0.44 Hz in the 1st mode, and 1.1 Hz in the 2nd mode.
  2) In case of the with stays type tower crane, frequency in the horizontal direction of the tower crane was 0.48 Hz in the 1st mode, and 1.17 Hz in the 2nd mode.
  3) The 1st mode damping factors were 1.2% in both cases with and without stays, and the damping factor was calculated by the logarithmic decrement method.
  4) However, it was found to be 3.4% when there was play in stays in case of the with stays type tower crane. When a balance of the arrangement of the stays was bad, the tower crane vibrated in the direction where the power wasn't added to.

Vibration Test on Model of Construction Tower Crane

SRR-No.20-4
Katsutoshi OHDO and Takanobu SUZUKI

: The Hanshin Awaji earthquake caused much damage to construction tower cranes, and some cranes collapsed or fell to the ground. Fortunately, the earthquake occurred in the early morning, so fatal accidents by the collapsed cranes had not been happened. However, if the earthquake had occurred in the daytime, it was assumed that not only the construction workers but also many pedestrians or car drivers around the sites would had been killed or injured.
    The earthquake resistant design for overhead traveling cranes have been proposed, but the earthquake resistance of the construction tower cranes were not so much considered. Therefore, the horizontal and vertical motion of the construction tower cranes was investigated experimentally and analytically to obtain the fundamental data for a new earthquake resistant designs. Vibration tests to a tower crane model were performed by a shaking table and free vibration analyses were performed by using a rigid-body pendulum model, which was proposed to calculate the complex motion of the tower cranes simply. Both results were compared and the earthquake resisting of the tower cranes was examined.
    The results of the experiments and analyses were summarized as follows.
  (1) Natural frequencies of the tower crane were similar in both results of the experiments and analyses. Therefore, it was found that natural frequencies of it were calculated simply by using the proposed rigid-body pendulum model.
  (2) The rotational angles of the vibration modes of the tower crane in the experiments were bigger than those in the analyses due to the vibration of the jib. Therefore, the vibration of the jib needs to be considered in the analyses.
  (3) It was assumed that the tower crane which was the object of this study had a similar natural frequency to the dominant frequency in the Hanshin Awaji earthquake by the experiments and analyses.

Buckling Strength of Stay of Construction Tower Crane

SRR-No.20-5
Katsutoshi OHDO and Etsuji YOSHIHISA

: The Hanshin Awaji earthquake caused much damage to construction tower cranes, and many stays that connected the tower cranes to buildings under construction were broken. As a result, some cranes collapsed or fell on the ground. In spite of this situation, the buckling strength of the stays has not yet been made clear, and even experiments on the buckling strength of the stays has hardly been performed. To prevent the collapse accidents of tower cranes due to the earthquake, the design method of the stay needs to be established.
    Therefore, in this study, compression tests to the actual sized stays, which were composed of steel H-beams and jacks, were performed to obtain the fundamental data for establishing the design method of the stay. In the experiments, the lengths of the steel H-beams were changed, and maximum compression loads were measured. The results of the compression tests were compared with the Euler's buckling loads and the relationship between the lengths and the buckling strength of the stays were examined.
    The results of this study were summarized as follows.
  (1) When slenderness ratios of the steel H-beams were approximately less than 100, the jacks of the stays were broken. Therefore, it is assumed that the buckling strength of the stays is equal to the compression strength of the jacks under 100 at slenderness ratios of the steel H-beams.
  (2) When slenderness ratios of the steel H-beams were approximately more than 100, the steel H-beams of the stays were buckled. Therefore, it is assumed that the buckling strength of the stays is equal to the Euler's buckling loads of the steel H-beams over 100 at slenderness ratios of the steel H-beams.

Concluding Remarks

SRR-No.20-6
Yutaka MAEDA

: This specific research started with the occasion of the Southern Hyogo-prefecture earthquake in 1995, which caused a great deal of damage to many cranes. The dynamic effect of earthquake on jib cranes and tower cranes is not prescribed in the standard for proof against earthquake of crane.
    Considering the damage of cranes by this earthquake related in the chapter 2, the tower crane used in the construction site was selected as the main object of this specific research. Results of the studies are summarized as follows;
  (1) Survey on damage of cranes due to the Southern Hyogo-prefecture Earthquake
    Jib cranes and tower cranes suffered more damage compared with bridge cranes (container cranes) and overhead travelling cranes. Damage of portal jib cranes was particularly concentrated around roller path, and about a half of this damage was due to buckling of the columns. The tower cranes fixed with stays were severely damaged, while the tower cranes without stay suffered relatively minor damage. Main damage cause of overhead travelling cranes was falling down and derailment due to large plastic deformation of runway girders. Container cranes and unloaders installed along waterfront of reclaimed lands were severely damaged due to movement of the foundation associated with soil liquefaction.
  (2) The vibration characteristics of a tower crane used in the construction site
    To obtain the dynamic characteristics, an actual tower crane was excited by sudden release of the lifted load or by stopping the rotation. The natural frequencies of the tower crane standing by itself were 0.27 Hz in the lateral direction of jib axis and 1.4 Hz for the tortional vibration of the mast, and the first mode damping factor was 1.2%. The natural frequencies of tower crane standing with stays were 0.48 Hz in case the stays were connected without play, and 0.34 Hz with play, and the damping factors were 1.2% and 3.4%, respectively. This showed that the damping of a crane was greatly influenced by the play between stay and crane.
  (3) Vibration test by the model of a construction tower crane
    Vibration tests using tower crane model were performed using a shaking table and free vibration analyses were performed by using a rigid-body pendulum model. Natural frequencies of the tower crane were similar in both the results of experiments and analyses. It was assumed that the tower crane had a similar natural frequency to the dominant one of the earthquake from the experiments and analyses.
  (4) Buckling strength of stays for construction tower cranes
    Compression tests were performed to the actual sized stays composed of steel H-beams and jacks.
The experiment showed that jacks of the stays were broken when the slenderness ratios of beams were less than 100, and the steel H-beams were buckled when these ratios were more than 100.
    Therefore, it was assumed that the buckling strength of the stays was equal to the compression strength of the jacks when the slenderness ratio was less than 100, and was equal to the Euler's buckling load when this ratio was over 100.

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