Wind tunnel test method to study out-of-service tower crane behaviour in storm winds

Transcription

1 tunnel test method to study out-of-service toer crane behaviour in storm inds D. Voisin a,c,*, G. Grillaud a, C. Solliec b A. Beley-Sayettat c, J- Berlaud c, A. Miton c a CSTB, Nantes, France b Ecole des Mines de Nantes, France c Potain, France Abstract An exerimental method used to study the behaviour of toer cranes in strong inds exosed to the disturbed shear flo induced by the surrounding built environment is roosed. tunnel tests on a toer crane model are described, the toer crane is considered as a rigid body slender structure equied ith a single degree of freedom art: the crane mobile jib alloing rotation ith resect to the vertical axis. Geometrical and mechanical roerties are simulated in order to reroduce realistic rotating art behavior. Instantaneous base overturning moments and rotating art ositions are measured in a boundary layer ind tunnel able to simulate the turbulent ind characteristics. Imrovements of the design of the crane in order to reduce the crane overturning moments, alleviate undesirable jib rotations and associated failure risks are investigated. Keyords: Toer crane, overturning risk, slender structures, ind loads, boundary layer ind tunnel, safety 1 INTRODUCTION In the last years, Euroe as hit by several severe indstorms, hich roduced extensive damages. Among others, several toer cranes fell don. Strong indstorm and unfavourable erection site ith surrounding buildings could result in an increase of overturning or failure risks. Those conditions usually aear on high-rise building sites during hurricanes. In revious studies related to cranes [1], the toer crane model had one degree of freedom. The rotating art as fixed in the ind direction and the only degree of freedom as the overturning angle around the tiing oint. The objective as to characterize the influence of gust duration on overturning risk. Environmental effects ere not taken into account this is to say that there ere no secial conditions of exosure or shelter. In engineering and standard ractices [2,3,4], ind loads on cranes are estimated in the hyothesis of an otimized safety osition during storms (i.e. ith the loest drag force). In a natural turbulent ind ithout any environmental effect, the rotating art of the crane moves automatically in the ind direction. This is reresentative if the crane is an isolated structure ithout any ind disturbances roduced by surrounding buildings. Hoever, due to its func- * Corresonding author: CSTB 11 rue H. Picherit BP NANTES Cedex 3 FRANCE Fax : (0) voisin@cstb.fr

2 tions, it is rarely the case. Because of the surrounding built environment, the ind flo around a toer crane may exhibit unexected and dangerous aerodynamic henomena. The aim of this research is to characterise the fluctuating loads induced on a toer crane by the disturbed aroaching ind flo, more recisely the eak overturning moments for most actual configurations. As toer cranes are lattice structures and thus are orous, aerodynamic ind induced effects are mainly drag forces [5,6,7]. The ind overturning moment induced by the drag forces acting on the hole structure is function of the ya angle beteen the ind direction and the rotating art of the crane. This ya angle is governed directly by the differential ind moment roduced by the drag forces acting on the jib and the counter jib. This is hy the heterogeneity of the ind field at rotating art level lays an essential role for crane stability. The frequency of crane's fundamental vibration mode lies in the range 0.1 Hz to 0.5 Hz (higher is the crane, loer is the frequency). The jib rotation seed observed during storms is no faster than a fe turns er minute, hich corresonds to frequencies too lo to excite the structure's natural modes. The orosity of the crane and the continuous motion of the rotating art hich alleviates critical vibratory henomenon are additional reasons to justify to neglect dynamic effects and to consider only the direct quasi-steady ind loads. Therefore the toer crane can be considered as a rigid body structure ith a single degree of freedom: the crane mobile art rotating in resect to the vertical axis. and jib deformations are neglected, as they are much smaller than the global crane motion. 2 MATHEMATICA MODE The equation of motion of the rotating art of the crane can be ritten as: I θ + C Friction θ = C (1) here θ is the jib nose angular osition, I is the mass moment of inertia of the hole rotating art about its rotating vertical axis, C is the instantaneous aerodynamic torque on acting on the rotating art (difference beteen jib and counter-jib aerodynamic loads) and C Friction the friction coefficient beteen the rotating art and the turntable toer head. The overall overturning moment is then exressed in SI units as: M Overturning Inertial RP Centrifugal ( O). y = θ m l rsinθ θ m l r cosθ (2) RP a m g [ r cosθ + a ] mrp g + lcpfx + lfx SP Gravity here M verturning ( O) Overturning moment at crane base level at tiing oint O y r O Unit vector erendicular to ind direction m Mass of the rotating art RP m Mass of the mast l ength of the mast l Height of the mast aerodynamic center CP

3 r a g Rotating art barycenter to crane vertical axis length Cross shaed-base sacing Gravity acceleration Fx Drag comonent of ind force on Fx Drag comonent of ind force on rotating art SP r r m RP g Force on Sleing Part Inertia & Centrifugal Forces Force on Counter-jib I θ θ TOP VIEW y x Force on C Friction θ l r m g θ Force on Jib l CP a O y Overturning Moment Fig. 1. Moments and forces schemes x The overturning moment at crane base level is the difference beteen restoring moment and overturning moment. It can be searated into four major comonents (Eq. 2): inertial and centrifugal rotating art moment, gravity moment and fluctuating ind moment. Gravity moment (resulting of the mass of the counter-eights on the counter jib used to balance the carried loads during service conditions) module is constant but its action is function of the angle θ. Overturning moment is mainly governed by the gravity moment and the fluctuating ind moment hen the rotating art is steady or hen the angular seed and acceleration of the rotating art of the crane are small. 3 DIMENSIONA ANAYSIS AND SIMIARITY CONDITIONS In order to evaluate direct ind effects as ell as the environmental effects on toer cranes a study on a reduced scale model in a boundary layer ind tunnel as initiated. The scale of the model is a comromise beteen similarity scales and the size of the ind tunnel test section. On one hand, to take into account a large built area surrounding the toer crane in the ind tunnel, the model scale needs to be small enough. But on the other hand, the model crane itself has to match geometrical and kinematic similarity conditions more easily satisfied if the geometrical scale of the model is large. The model geometrical scale factor as then chosen as λ =1/80 hich reresents an aroriate comromise beteen these to oosite conditions using a 9.2 m² cross section area ind tunnel.

4 3.1 Kinematic similarity The second ste of the similarity study as then to satisfy the kinematics roerties. To model the toer crane rotating art that moves into the air, e must ensure that the model rotation seed is similar to that of the rototye. This is achieved if the ratio of the acceleration of the structure to the acceleration of a fluid article is the same for both model and rototye, Fay [8]. Thus, to model the behaviour of the toer crane rotating art hich describes a disc of radius ith an angular velocity θ ith a seed ind V e must check that the ratio of the centrifugal acceleration of the rotating art 2 V. V V / 2 θ is the same for both model and rototye. to a tyical acceleration of a fluid element In fact, kinematic similitude is ensured by the equality of the Strouhal number for both model and rototye (Eq. 3) θ V m = θ V or ( V ( V / θ ) m / θ ) = λ In the resent case, the crane model angular seed observed in ind tunnel (in strong environment effect conditions) for a 4.7 m/s ind seed can be around 2 rad/s (20 rm), it corresonds to a crane rototye angular seed equals to 0.23 rad/s (2.2 rm) in a 41.7 m/s ind seed (150 km/h). 3.2 Dynamic similarity Dynamic similarity exists beteen geometrically and kinematically similar systems if the ratios of all forces beteen the model and the rototye are the same. Therefore it is necessary to reroduce realistic rotating art behaviour to ensure that governing differential equation of motion (Eq. 1) and overturning moment equation at bases (Eq. 2) of the model and rototye are similar. The gravity term in Eq. 2 imlies to reroduce a Froude number similarity. Consequently the square of the Froude number is relates to the ratio of inertial to gravity forces, it is used to define the ind seed similarity scale. Mass distribution, rotating art inertia and friction coefficient ere reroduced at model scale in resect to similarity las (Eq. 4, 5, 6). (3) M m = λ M (4) 3 I m = λ I (5) 5 C Friction m = λ C (6) λ 4 Friction Force m = λ Force (7) 3 Moment m = λ Moment (8) 4 This leads to very light model arts. For examle 8000 kg of the model jib matched by 16 gr at model scale.

5 inear ind surface distribution as resected all along the rotating art, this distribution generates an aroriate ind torque on the rotating art. In the ind tunnel velocity range of 0 to 10 m/s, the aerodynamic coefficients of the model roved to be constant, no Reynolds number effect ere observed. A carbon tube used to reroduce the mast, it has similar drag characteristic than the full scale element. It has been verified that the mast ind surface S.Cx hich is equal to 22.9 m 2 at full scale (40 m/s) is reroduced by a m 2 at model scale (in ind tunnel velocity range of 0 to 10 m/s). 4 EXPERIMENTA FACIITIES AND PROCESS 4.1 tunnel facility descrition The tests ere conducted in one of the CSTB boundary layer return tye ind tunnel hose cross-section is 2.3 m high 4 m ide. The airflo velocity is adjustable in a range 0 to 10 m/s. is simulated using calibrated roughness on the tunnel floor for scales ranging from 1/50 to 1/ Instrumentation The fluctuating ind loads acting on the rotating art of the crane, its angular osition as ell as the overall overturning moment at the base of the crane are the target arameters necessary to evaluate the crane behaviour. The Froude similarity criteria is associated to the matching of mass and inertia similarities of the rotating art and also of the mast of the toer crane model. This Froude condition imlies also lo ind velocity ranges (belo 5 m/s) in the ind tunnel. Therefore the loads acting on the toer crane model are very small, estimates of the ind moment ere 0.01 N.m for an along ind jib osition and 0.1 N.m for a jib erendicular to the ind. Thus the 15 m high (full scale) mast element of the model suort of the rotating art as designed as a strain gauge balance. This balance is built using to aluminium stages fitted ith gauges and connected to each other by the light 10 mm carbon tube (Fig. 2). The square cross sections of the uer and loer stages are resectively mm 2 and mm 2. Both mass and stiffness ere roerly tuned and the natural vibration frequency is high enough to not affect measurements. The strain gauges of the mast are ired into a Wheatstone bridge circuit, hich gives a voltage outut roortional to the overturning moment after reliminary calibration. 45 m full scale 0.56 m model 15 m full scale 0.19 m model Strain gauge balance Angular encoder Fig. 2. 1/80 scale model of a Potain MD238 crane in CSTB boundary layer ind tunnel

6 The lo friction torque at the taer roller bearing imlies the use of a non-intrusive system of measurement of the angular osition, hich doesn't introduce additional friction. An incrementing angular encoder integrated into the toer crane model, consisting in three otoelectronic sensors and a reflecting surface, is used to measure the angular osition. 4.3 Methodology Exerimental results issued of the ind tunnel tests are the angular rotating art osition, function of time, and the instantaneous overturning moment at the base of the crane. Angular seed and angular acceleration are obtained by time derivation of the angular rotating art osition. Therefore inertial, centrifugal and gravity moments at the crane base can be easily calculated. Finally instantaneous ind moment at base crane level is deduced from the subtraction of instantaneous inertial, centrifugal and gravity moments (Eq. 2). 5 RESUTS AND DISCUSSION 5.1 Crane ithout surrounding built environment Preliminary to analyse the toer crane behaviour in a built surrounding environment, some reliminary exeriments ere carried out to simulate the model crane behaviour ithout any built environment. One of the former tests as to observe the rotating art motion initially erendicular to the ind. The time delay T 90 0 necessary for the rotating art initially in a erendicular ind osition to reach the along ind osition can be estimated using a simlified ind torque hyothesis (i.e. the ind torque is constant in 90 to 45 angular sector and equals to its 90 value, and it is null beteen 45 to 0 angular sector) by integration of equation 1. So, the T 90 0 estimation for the Potain MD238 real scale rotating art exosed to a 45m/s ind seed is close to 10.6s hich corresonds to 1.19s at model scale. In the ind tunnel the measured corresonding time delay T 90 0 is 1.42s (Fig. 4a). The 19% difference beteen estimates and ind tunnel tests is due to the integration hyotheses and the friction coefficient estimate. The rotating art motion describes only to oscillations before being steady. The maximum ya angle observed during the counterflo eriod is less than 30, and the total time delay for the crane to be in the alongind safety osition (in the ind direction) is 4.5 s. The friction art of the torque can be neglected (Fig. 4b), inertial torque contribution is redominant. Figure 4c illustrates overturning moment (y value) and its comonent moment at base crane level. The gravity comonent is null hen the rotating art is erendicular to the ind, inertia and centrifugal comonents are negligible. Comared to ind moment, gravity moment has an oosite sign. Oosed to ind effects, negative moments contribute to crane stability. Polar diagram (Fig. 4d) shos the resultant overturning moment decrease function to angular osition.

7 Fig. 4. Crane ithout surrounding environment (a) Rotating art model angular osition, seed and acceleration (b) Torques on rotating art around a vertical axis (c) Moments at base level (d) Polar reresentation of the overturning moment function to angular osition. (Results at model scale) 5.2 Crane in turbulent flo and in a built environment Surrounding built environment can generate heterogeneous and unbalanced ind flo around the crane. This kind of flo may be found in the ake and in the shear layer of a building sensibly higher and/or larger than the crane (Fig. 5). But, it is interesting to notice that if the crane is a third higher than the building the henomenon does not occur any more as the sensitive art of the crane is out of the erturbed flo. To reroduce and estimate the effects of such built environment effects, a 0.87 m high, 0.16 m ide and 0.48 m long model of building as set uind to the crane, 0.6 m front of the rotation axis crane. velocity and crane characteristics ere the same than the former uniform flo test. Heterogeneous Field o Seed Area Acceleration Area Fig. 5. Examle of environment ind effect: Heterogeneous ind field

8 Fig. 6. Crane in heterogeneous ind field (a) Rotating art model angular osition, seed and acceleration (b) Torques on rotating art (c) Moments at crane base level (d) Polar reresentation of the overturning moment function to angular osition. (Results at model scale) Starting the test, the ind tunnel is off, the initial rotating art osition is arallel to the ind tunnel longitudinal axis. Then, starting the roeller, ind velocity increases from 0 to 5 m/s in aroximately 4 s before being steady at 5 m/s. An auto-rotating henomena arises, the rotating art revolved six times during 30 s. For each turn the angular seed is in a range of 70 deg/s to 180 deg/s and the maximum angular acceleration value is 270 deg/s 2 (Fig. 6a). As reviously mentioned, inertial torque contribution is redominant comared to friction torque in the estimation of the ind torque (Fig. 6b). At the beginning of the test, the overturning moment is equal to the gravity moment N.m, the toer crane stay in safety osition. 5 to 10 s later, ind moment increases as ind seed increases but the rotating art is not moving significantly before starting to rotate. During the first half turn, overturning moment increases to a maximum of 0.08 N.m. Then, hen rotating, the overturning moment is in a range from N.m to 0.08 N.m. Figure 6d shos that overturning moment maximum values corresond to -240 to 120 angular sectors and minimum values to 60 to 300. Inertial and centrifugal moment module values are less than 0.01 N. In comarison ith the crane ithout any built environment, the rotating art behaviour is tremendously different: it does not sto to turnaround. Consequences on moments at the crane base level are their large amlitude variation function of the angle of attack and angular seed and acceleration (Fig.6c). Most imortant variations are due to the gravity moment ith resect to the osition of rotating art's barycenter and the aerodynamic ind moment deending of the ind angle of attack. Thus, the overturning moment of the crane in this heterogeneous ind field is strongly fluctuating and its maximum value is more than tice higher than in the case ithout any built environment.

9 6 CONCUSION Measurements of overturning moments at crane base level and angular rotating art osition ere carried out on a 1/80 scaled toer crane model. To environment conditions (ith and ithout an uind surrounding building) ere tested in a boundary layer ind tunnel to analyse the ind field influence on the toer crane behaviour. Overturning moments at base crane level ere identified and evaluated. It as observed that inertial and centrifugal moments are much smaller in comarison to gravity and ind moments. Concerning the role of the friction coefficient on rotating art motion in storm ind conditions, it aears that its contribution to ind torque on rotating art is not redominant comared to the inertial torque contribution. This model crane is an efficient tool to study any unsusected surrounding built environment effect on toer cranes submitted to strong inds. Also, it ill be essential to test imrovements of the toer cranes designs in order to reduce environmental effect on overturning crane moment and alleviate undesirable rotating art auto-rotating henomenon. Tests on reduced drag cranes and various differential jib/counter-jib aerodynamics eightings are in rogress. 7 ACKNOWEGMENTS Potain S.A. acts as sonsor on this study. The assistance and co-oeration of the sonsor are gratefully acknoledged. In articular the authors ould like to extend thanks to Guy Galand ho is folloing this research. REFERENCES [1] Eden J.F., the late Iny A. and Butler A.J. (1981) Cranes in storm inds. Eng. Struct.,1981, Vol. 3, july [2] Eden J.F., Butler A.J. and Patient J.(1983) tunnel tests on model crane structures. Eng. Struct.,1983, Vol. 5, October Building Reseach Establishment, Garston, Watforf WD2 7JR, UK [3] Fetizon F., Jouannet J.C., Watremetz M. (1979) Toer crane in turbulent ind. Pratical exeriences ith flo induced vibrations Symosium, Karlsruhe 3-6/setember 1979 [4] Vreugdenhll J. (1995) Out-of-service -oading of cranes gusts. Bulk Solids Handling/Volume15/No.1/January-March 1995 [5] Eden J.F., Butler A.J. and Patient J. (1985) A ne aroach to the calculation of ind forces on latticed structures. Building Research Institut The Structural Engineer/ Volume 63 A/No. 6/June 1985 [6] ESDU (1988) attice structures Part 1 : mean fluid forces on single and multile lane frames. Engineering Sciences Data Unit - Engineering Sub-Series, Data Item 81027, ondon, [7] ESDU (1988) attice structures Part 2 : mean fluid forces on toer-like sace frames. Engineering Sciences Data Unit - Engineering Sub-Series, Data Item 81028, ondon, [8] James A. Fay, N. Sonalkar A fluid mechanics hyercourse Massachusetts Institute of Technology 1991 htt://.mas.ncl.ac.uk/~sbrooks/book/nish.mit.edu/2006/textbook/nodes/cha10/node1.html