Abstract：Earthquake resistance of temporary structures is inferior to that of permanent structuresA tower crane supported by a pile foundation，which is one of the temporary structures，is often used when constructing high-rise buildingsThe tower crane installed in a high position may be vibrated largely by strong seismic ground motion because of its slender structureDynamic response characteristics of the structure with a pile foundation are greatly influenced by the surrounding ground and the foundation type as wellIn current Japanese design mode，however，the seismic load is only to be defined as 20% of the weight of a crane，neither dynamic unstability of the structure nor influence of the surrounding ground type is sufficiently consideredIn this paper，a series of centrifugal shaking table tests was performed to investigate the seismic characteristics of the tower crane supported by a pile foundation in soft clayTwo different weight conditions were set for the crane jib to accurately simulate practical situationsFrom the test results，the dynamic behaviours of the tower crane and the influenced factors were discussedIn addition，it was concluded that the weight conditions for the crane jib give significant influence on the level of pile damage and might lead to different failure patterns of the tower crane
　　Keywords：seismic interaction，centrifuge modelling，tower crane，pile foundation，shaking table test
　　CLC Number：TU473Document Code：AArticle ID：1000-0666（2017）01-0035-10
　　Introduction
　　A tower crane supported by a pile foundation is installed on the outside of a structure during construction when it cannot be installed in the insideThe pile foundations have been designed based on the standard of the building constructor because there are no design guidelines on pile foundations of the tower craneHowever，even a temporary structure such as a tower crane can cause large damage if it over-turns due to an earthquakeTherefore，it is necessary to clarify the earthquake resistance of the tower crane and determine the design standards of a pile foundationThe dynamic response characteristics of tall structures are influenced by the vibration characteristics of the related factors such as structure，a pile foundation and a surface groundFurthermore，the rigidity of the ground surrounding the pile foundation may deteriorate during the seismic ground motion and eventually the structure may become unstableTherefore，when determining the structural stabilities，the interaction between the ground and the structural system has to be taken account，especially in case of soft clay ground which is weak against vibrationGenerally，the earthquake-resistance of a tower frame structure used during construction works is smaller than that of the permanent structure because it is only temporarily used and its structure is likely to be designed without considering seismic vibration to minimize construction costsHowever if the temporary tower crane is struck by an earthquake，there is a strong possibility that the resulting damage may affect not only the construction site but also the surrounding houses and passersbyIn fact，there was a serious accident during the Chi-Chi earthquake in Taiwan where a climbing jib crane fell to the surface ground from a height of 260 m and five people diedIn this case，the climbing jib crane，installed on the top of a skyscraper under construction，was thought to be damaged by the unexpected strong vibration due to the large earthquakeAccording to the design code of the crane，an earthquake load is assumed to be 20% of vertical loadHowever，in the case of a strong earthquake，the assumed maximum earthquake load on a tower crane may be exceeded because of its unstable slender structureTherefore，it is essential to analyze the dynamic characteristics of the tower cranes to prevent any damageThe effect of loading conditions on the response characteristics have been analyzed through the previous centrifuge testing of tower craneIn this paper，to develop an understanding of its dynamic behaviors，a model tower crane supported by a pile foundation in soft clay was investigated by using dynamic centrifugal shaking table tests 　　地震研究40卷第1期Kazuya ITOH， et al：Seismic Response of Tower Crane Supported by Pile Foundation in Soft Clay1Centrifuge Model Tests
　　11Centrifuge Facility and Model Container
　　All the tests described here were conducted on the JNIOSH Mark II Centrifuge（Horii et al，2006）owned by the National Institute of Occupational Safety and Health，Japan（JNIOSH），as shown in Figure1Technical specifications of JNIOSH Mark II Centrifuge are listed in Table 1Unlike in other centrifuges，its arms（forms）are asymmetric，which is one of its special featuresOne side of the arm is provided with a bridge plate where a swinging platform is fixed to its inner end plate with the help of a pair of hydraulic suspension jacks when the platform is lifted up（Dynamic platform）.In order to balance this portion（weight of the end plate）of the dynamic side arm while swinging，two counter weights are overhung on the two sides of opposite arm which is used for non-shaking or static tests（Static platform）.Figure 2 shows the relationship between capacity of beam centrifuge in terms of platform size and effective radius of rotationAs seen in the figure，in the type of middle size centrifuge used here，longer radius and larger platform could be obtained，equivalent to those of large size centrifugesIn this research，dynamic platform was used for the model testFiguer 1JNIOSH Mark II Centrifuge
　　（Horii et al，2006）Table 1Major specifications of JNIOSH Mark II centrifuge（Horii et al，2006）
　　（a）Main centrifuge machine（b）Ancillary equipmentMaximum accelerationDynamic50 gStatic100 gMaximum number of rotationsDynamic143 rpmStatic194 rpmMaximum payloadDynamic1000 kgStatic500 kgPayload capacityDynamic50 G – tonsStaticEffective radiusDynamic220 mStatic238 mPayload sizeDynamic11 m × 095 mStatic11 m × 15 mMain motor440V90 kW（DC）Rotary jointsOilMaximum pressure210 MPaNumber of ports2AirMaximum pressure07 MPaNumber of ports1WaterMaximum pressure05 MPaNumber of ports2Electricity slip ringCapacityAC100V，30 ANumber of channels2Slip ringCapacityAC100V，1 ANumber of channels44Figure 2Relationship between capacity of beam
　　centrifuges in terms of platform size and
　　effective radius（Itoh et al，2009）The aluminum model container is 450 mm in width，200 mm in breadth，and 347 mm in heightThe inner side of the container was anodized to reduce wall face frictionThe front face of the container is a transparent window to observe the deformation of the model groundTwo sponges（260 mm in height，200 mm in breadth and 10 mm in thick）were placed near both sides of the container to absorb the stress due to the side boundariesEffectiveness of the sponges is unknown，nevertheless，comparing the stiffness of the sponges to that of the aluminum wall，the stiffness of the sponges is far similar to that of the soilTherefore，installing the sponges may absorb the influence of the boundary conditions 　　12Modelling of Tower Crane and Pile FoundationIn this study，a hammerhead type of tower crane，which has a simple figure relatively easy to grasp the seismic behaviors，was modeled as the superstructure of the model tower crane as shown in Figure 3Sizes and materials of the model tower crane were determined by considering the similarity rules for centrifuge modellingProperties of the model tower crane are listed in Table 2Previous studies by Itoh et al（2004）and Arai et al（2006a，b）were carried out under different weight conditions of the crane jibs to simulate the actual phenomenon accuratelyIn this study，similarly to their previous studies，there were two cases of the test set up with different weight conditions for crane jib to accurately simulate practical situationsOne was the case of “Balance” which corresponded to the crane having no load and the other was “Unbalance” suspending the loadsIn both cases，the total weights of the crane jibs were arranged to be the sameIn order to deepen the understanding of properties of the tower crane，different waves，model piles and model ground were set up for the tests
　　The natural frequencies of the model tower crane were investigated by a resonant search test in 1 g fieldThe dynamic acceleration of the model tower crane was horizontally recorded by an accelerometer located at upper part of the tower craneFigure 4 shows the relationship between the amplifications and the frequency in the resonant search testThe first and the second natural frequencies were 05 Hz and 16 Hz in the case of the balanced condition and 05 Hz and 12 Hz in the unbalanced condition，respectively at prototype scale according to the peak values of amplificationFigure 3Model of tower craneTable 2Properties of tower crane model
　　popertyPrototypeModelReal scaleCentrifuge modelHeight of tower crane30 m30 m600 mmLength of jib30 m30 m600 mmSoil layer13 m13 m260 mmYoung modulus206000 N/mm270270 N/mm270270 N/mm2Bending rigidity154×106 kN·m2190×106 kN·m2304×108 N·mm2Primary natural frequency05 Hz05 Hz25 HzDiameter of pile075 m075 m15 mLength of pile135 m135 m270 m
　　Figure 4Relationship between amplification and
　　frequency at upper part of tower craneThe model substructure of the tower crane has four model piles fixed in the aluminum footingThe pile spacing to diameter ratio was s/d=333 in the excitation direction and s/d=400 in the transverse direction of shaking axisPile 1 was installed in the main-jib side and Pile 2 was installed in counter weight side，as shown in Figure 5The model pile was approximately equivalent to bending rigidity of a solid concrete pile with a diameter of 800 mm at prototype scale Considering the similarity rule for centrifuge modelling，the aluminum tubes with an outside diameter of 15 mm，thickness of 1mm，and length of 309 mm were used for the model piles 　　13Experimental Procedures
　　Fujinomori clay was used for the model clay layer in this test because it is well-characterized in the research by Japan Geotechnical Society（1993），as shown in Table 3Deaired slurry of the clay，which was prepared at 70 % water content，was first consolidated in a strong box on the laboratory floor under a pressure of 50 kPaThen the model piles were installedFigure 5Experimental setupin the model ground by using a template in order to insure the location and verticalityAll instrumentations were set and centrifugal acceleration was applied up to 50 g allowed for selfweight settlement of the soilThen，after the excess pore pressure in the ground disappeared，a shaking table test was carried outTwo cases of different weight conditions for the crane jib were investigated in this test as mentioned aboveTable 3Material properties of Fujinomori clay
　　PropertyParticle density，ρs/（g·cm-3）2719Maximum grain size/mm025Mean particle diameter，D50/mm0027Liquid Limit，wL/（%）627Plastic Limit，wp/（%）278Plasticity Index，Ip349Velocity of the S wave，VS /（m·s-1）89
　　14Input Waves
　　Ground motions based on the recordings during the 1940 El Centro earthquake were used for the random input wave in this experimentFigure 6 is the input acceleration wavesThe maximum acceleration was normalized to about 100 gal（called “Small shake 1”），about 600 gal（called “Large shake”）and about 150 gal（called “Small shake 2”）and used for the experimentThe influence of the ground deterioration was examined by the continuous dynamic loading using these three kinds of wavesThis paper mainly discusses the behaviors of the models under Large shakeFigure 6Input acceleration waves and
　　their Fourier spectrumsAll the results of the centrifuge tests are presented in terms of prototype scale，hereafter
　　2Experimental Results and Discussions21Response Acceleration Waves
　　Figure 7 shows the time histories of horizontal acceleration in both cases at all measurement points when inputting large shakeMeasurement points were at the top of the tower crane，the footing and the model groundThese waves might be divided into 7～20 s（1st interval）and 20～40 s（2nd interval）.The 1st interval contained a principal motion and the 2nd interval was a motion after the principal motionThe response acceleration at the top of the tower crane showed a similar tendency to that at the footing，although its amplification factor was lowerAs for the response acceleration of the ground，the amplitude of acceleration decreased with the decreasing depthThe tendency was distinguished in the surface ground（GL-34 m）at the 1st interval because of the clay softening under cyclic loadingIt should also be added that there were spiky waves observed in the surface ground（GL-34 m）in both casesThis may be explained by the collision between the pile and the ground 　　22Comparisons of Dynamic Characteristics between Various Weight Conditions of the Crane JibThe time histories of the horizontal displacement of the footing in both cases at Large shake are plotted in Figure 8As for the vertical axis in this figure，the positive side means the deformation in the direction of the main jibNote that the point where the footing displacement was measured 10 mm above the ground levelAs shown clearly in the figure，in the case of the balance weight condition，the horizontal displacement of the footing was nearly zeroOn the other hand，in the case of the unbalance weight condition，inertial deformation took place at the first intervalAfter the first interval which contained a principal motion，horizontal displacement of the footing could not be observed because measurement range was overAt the end of shaking，the observed permanent horizontal displacement of footing was about 150 mm in the direction of the main jib which was located at the heavy weight sideThen Figure 8b makes additional double integration of acceleration records on the footing（blue line）.For the results of horizontal displacement of the footing，it appears that not only kinematic forces resulting from cyclic ground displacement but also inertial forces from the superstructure may come to play important roles（a）Balanced condition（b）Unbalanced conditionFigure 7Response acceleration at each measurement point（a）Balanced condition（b）Unbalanced conditionFigure 8Horizontal displacement response measured and calculated on footingFigure 9 shows comparisons of the axial forces observed at piles in both casesIn the vertical axis，the positive side means the compression of pileIn addition，Pile-1 is on the side of the main jib and Pile-2 is on the side of the counterweight jib，respectivelyIn both cases，the axial forces of Pile-1 changed in the opposite phase to the axial forces of Pile-2It means that rocking motion occurred in the motion of the superstructureAdditionally，the axial force of Pile-2 was much larger than that of Pile-1 at upper part of pileIn the unbalance weight condition，the residual axial forces recorded with Pile-1 and Pile-2 took placeIn comparison with the result of the balance weight condition，it is obvious that the residual axial force at the pile in the unbalance case is larger than that in the balance case because of the overturning moment caused by unbalance weight condition for superstructure（a）Balanced condition（b）Unbalanced conditionFigure 9Time histories of axial forces of pile and load of pile tipFigure 10 shows the time histories of bending moments in both cases at all measurement points，together with the locations of upper and lower part of the towerIn the unbalance case，the results show that the bending moment of the lower part of tower crane corresponds to the amplitude of the upper part of tower craneOn the other hand，the results in the balance case show that the bending moment of the upper part of tower crane corresponds to the amplitude of the lower part of tower crane 　　In the case of the balance condition，the magnitude of the bending moment of Pile-1 was larger than that of Pile-2 at the middle part under the ground（GL-875 m）.In both cases，it can be pointed out from the distributions that inertial forces from the superstructure mainly dominateIn such a case which the pile at the position of the middle part of under the ground is heavily damaged，there may be great risk in tilting failure of tower craneThe results also provided that it is important to consider the dynamic interaction between the piles and the ground
　　23p-y Curve
　　In order to discuss soil stiffness transition during an earthquake，this study focuses on p-y behaviors by back-calculating the recorded bending moment distribution M（z）along a pile based on a simple beam theory according to the equationsIn this study，four numerical differentiation methods were used to determine soil stiffness transition：weighted residual derivatives，central difference formulas，differentiation
　　（a）Balance condition（b）Unbalance conditionFigure 10Time histories of bending moment of tower and pileof cubic spline interpolation functions，and differentiation of polynomial interpolation functionsThen，integrating to solve for p and y requires specification of two boundary conditionsThe measured footing displacement relative to the container base was used for one of the boundary conditions and the pile tip displacement and moment was assumed to be zero for the other
　　Figure 11 shows p-y curve at GL-03 m in both casesThese p-y behaviors were divided into 1st interval（7-20 s）and 2nd interval（20-40 s），respectively；The observed p-y behaviors in the case of the balanced condition show a typical linear shapeWhereas the observed p-y curve in the case of the unbalanced condition follows a softening spring type shape to the stress strain curve during a cyclic triaxial test of soft clayFurthermore，in the case of the unbalanced condition，it may not at the same point in horizontal space as when it detached，resulting in the shift in the axes of oscillation of displacement in Figure 11 b.（a）Balance condition（b）Unbalance condition（a）Balance condition（b）Unbalance conditionFigure 11Calculated p-y curve3Conclusions
　　A series of centrifugal model tests verified the influence of weight condition of superstructure，which is the crane jib，on the dynamic characteristics of tower crane supported by a pile foundation in soft clay groundThe main knowledge which is clarified by the centrifuge experiment is as follows； 　　（1）As for the response acceleration of the ground，there were spiky waves observed in the surface ground in both cases
　　（2）For the results of horizontal displacement of the footing，it appears that not only kinematic forces resulting from cyclic ground displacement but also inertial forces from superstructure may come to play important roles
　　（3）As for the response axial force observed at piles，it means that rocking motion occurred in the motion of the superstructureFurthermore，it is obvious that the residual axial force at the pile in the unbalance case is larger than that in the balance case because of the overturning moment caused by unbalance weight condition for superstructure
　　Consequently，in the case of seismic stability of tower crane，it is necessary to consider the interaction of the surface ground and the pile foundation
　　Acknowledgment
　　Former Tokyo City University formerly known as Musashi Institute of Technology graduate students MrMasao Jingu and MrNaoyuki Itoh provided some of centrifuge tests，and supported some data processingAuthors would like to thank them
　　References：
　　ARAI F，ITOH K，SUEMASA N，Katada T，et al2006bDynamic response of tower crane with pile foundation[C]//International Conference on Physical Modelling in Geotechnics – ICPMG 06，Hong KongLondon：Taylor & Francis Group，971-974
　　ARAI F，KATADA T，SUEMASA N，et al2006aSeismic stability of Tower crane with pile foundation in loam ground[C]//First European Conference on Earthquake Engineering and Seismology 2006（1st ECEES）：Joint Event of the 13th European Conference on Earthquake Engineering and the 30th General Assembly of the European Seismological CommissionSwiss Society for Earthquake Engineering and Structural Dynamics，768
　　HORII N，ITOH K，TOYOSAWA Y，et al2006Development of the NIIS Mark-II geotechnical centrifuge[C]//International Conference on Physical Modelling in Geotechnics – ICPMG 06，Hong KongLondon：Taylor & Francis Group，141-146
　　ITOH K，SUEMASA N，TAMATE S，et al2004Dynamic loading test for pile supported tower crane in soft clay[C]//Proceedings of the 13th World Conference on Earthquake Engineering，Vancouver B C，Canada
　　ITOH K，TOYOSAWA Y，Kusakabe O2009Centrifugal modelling of rockfall events[J].International Journal of Physical Modelling in Geotechnics，9（2）：1-22
　　The Japanese Geotechnical Society1995Research committee on dynamic characteristics of clay[J].Cooperative study of strength of clay on the cyclic load，Tsuchi to Kiso，The Japanese Geotechnical Society，43，（5）：79-82