A note on dynamic fracture of the bridge bearing due to the great Hanshin–Awaji earthquake
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International Journal of Impact Engineering 27 (2002) 153160
A note on dynamic fracture of the bridge bearing due to thegreat HanshinAwaji earthquake
Shinji Tanimuraa,*, Takashi Satob, Tsutomu Umedaa, Koji Mimuraa,Osamu Yoshikawac
aDivision of Mechanical Systems Engineering, Graduate School of Engineering, Osaka Prefecture University,1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan
bGraduate School of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, JapancOsaka Institute of Technology, 5-16-1, Omiya, Asahi-ku, Osaka, Osaka 535-8585, Japan
Received 14 December 2000; received in revised form 15 June 2001; accepted 6 July 2001
The great HanshinAwaji earthquake, which occurred in Japan in 1995, caused fracture and destructionof a large number of structures. One of the examples, whose mechanism is not clear, is the fracture of abridge bearing part of the Nielsen bridge type that does not occur under ordinary static or dynamic loading.The fracture probably resulted from a very high stress due to an unexpected dynamic mechanism or animpact. In this paper, the three dimensional dynamic behaviour of a bridge of the Nielsen bridge type wasanalysed, for a collision/impact between the upper and the lower bridge bearings, which might haveoccurred in the great HanshinAwaji earthquake. The numerical results show that an impact due to arelative velocity of 56m/s between the upper and the lower bridge bearings generates a stress sucient tolead to the fracture in the upper bridge bearing. The location and the direction of the maximum principalstress determined numerically in the upper bridge bearing coincide fairly well with the observed features ofthe actual fracture plane. r 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Dynamic fracture of structure; Collision; Earthquake; Numerical simulation; Bridge bearing
One of the examples of unusual fractures of structures which occurred due to the greatHanshinAwaji earthquake , is the fracture of a bridge bearing part of the Nielsen bridgetype, as shown in Fig. 1, that might not occur under ordinary static or dynamic loadings. One
*Corresponding author. Tel.: +81-722-54-9209; fax: +81-722-54-9904.
E-mail address: firstname.lastname@example.org (S. Tanimura).
0734-743X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
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Fig. 1. Fracture of bridge bearing of Nielsen bridge type: (a) Nielsen bridge. (b) Fracture of the upper bridge bearing.
(c) An expanded view of the fracture surface near the root of boss. (d) Detail of the fracture surface near the root ofboss.
S. Tanimura et al. / International Journal of Impact Engineering 27 (2002) 153160154
explanation of the cause of that fracture was presented by assuming a high stress concentration atthe root of the boss as shown in Fig. 1(c). Through careful observation of the fracture surfacenear the root of the boss, however, one may nd that the fracture surface does not start at the rootof the boss but somewhat inside of the root of boss as shown in Fig. 1(c) and (d). The assumptionis also not substantiated suciently to explain the cause of the high enough tensile force whichacts to the upper part of the bearing and leads to the tensile fracture. Multi-fracture like a multi-spallation, which is sometimes caused by a plate impact, can be observed on part of the fracturesurface, as shown in Fig. 1(c).We then hypothesized a new mechanism by which such an unusual fracture can be
explained reasonably. That is, we assumed that the upper part of bridge bearing was onceseparated from the lower part of that during a severe vibration of the Nielsen bridge (Fig. 1(a))due to the strong near-source earthquake and then collided with the lower part of the bearing.This might cause a suciently high tensile stress to lead to the unusual fracture of the upper partof the bearing.To substantiate this hypothesis, we analysed the three dimensional dynamic behaviour of the
entire Nielsen bridge together with the ground by using DYNA3D code . It was found that,when the upper and lower parts collide with each other with the relative velocity of about 6m/s,the high tensile stress, which might cause the tensile fracture, occurs on the plane in the upperbearing which corresponds well to the actual fracture surface, and also the principal axis ofmaximum principal stress is almost perpendicular to the actual fracture plane in the upperbearing.
2. Numerical methods and results
2.1. Numerical methods
Fig. 1(b) is a view of the fractured upper part of the bridge bearing which was locatedat position as shown in Fig. 2. Cracks were observed also in the upper part of the bearingof . These bridge bearings consisted of an upper part with a concave surface of thespherical bearing and a lower part with a convex surface (Fig. 3). The nite element model of abridge of the Nielsen bridge type together with the ground was made on the basis of the designdrawing of them and the measured values of the elastic properties of the ground around thebridge, as partly shown in Fig. 2. The whole model shown in Fig. 2 was separated into two parts.That is, the upper part of the model consisted of the upper beam, the lower beam and the upperpart of bearing, and the lower part of the model consisted of the lower part of bearing, the pierand the ground.The material of the upper part of bearing, where fracture occurred, was SCMn1A of JIS
standard (yield strength is X 275MPa, tensile strength is X 540MPa, elongation is X17%). Thematerials of the upper beam, lower beam and pier were of steel. The material properties of theroad section, ground and others were chosen as the values given in the design drawing and themeasured values of ground properties around the bridge. In the modeling, all of the materials wereassumed to remain elastic to evaluate simply the values of the dynamic tensile stresses which occurduring the three dimensional dynamic behaviour of the models after the collision.
S. Tanimura et al. / International Journal of Impact Engineering 27 (2002) 153160 155
The boundary conditions on the dynamic contact surface between the concave and convexsurfaces of the spherical bearing allowed separation from each other and rotation. Friction on thecontact surface was taken into account. The acceleration due to gravity 9:8 m=s2 was also takeninto account in the direction of the z-axis.
Fig. 2. View of whole FE model in y direction (m).
Fig. 3. Oblique view of FE model of bridge bearing (mm).
S. Tanimura et al. / International Journal of Impact Engineering 27 (2002) 153160156
To sustantiate the hypothesis, the three dimensional dynamic behaviour of whole model wasanalysed for various relative impact velocities between the upper and lower models by changingthe initial velocity of the upper model in the vertical direction.The accuracy of the analysis was checked to conrm the acceptability of the obtained numerical
results by changing the total number of nite elements from 4000 to 5000 and choosing smallerelements for the parts of the bearing near to the dynamic concave and convex contact surfaces ofthe bearing.
It was observed that the maximum value of the principal stress s1 occurred insideof the upper part of bearing at the elapsed time after collision tC0:3 s; and also theprincipal axis of the maximum stress was almost perpendicular to the actual fractureplane in the upper part of the bearing. Fig. 4 shows an example of distributions of principalstress on the plane which corresponds fairly well to the actual fracture surface of the upper part ofbearing.The maximum values of the principal stresses occurred on the plane corresponding to the actual
fracture surface of bearing shown in Fig. 4. The relative impact velocity between the upperand lower parts of the bearing was changed from 1 to 6m/s, as shown in Fig. 5, where h denotesthe depth of the ground. On the bottom of the lower model, that is on the bottom surface of theground, the displacement of the bottom surface in the vertical direction is restricted in theDYNA3D code. To evaluate the eect of this restriction, which might cause reection of somepart of the stress waves, the depth of ground h was assumed as h 140m when the eect might berelatively small. We can observe in Fig. 5 that the maximum values when the depth was assumedas h 140m are about 80MPa smaller than the h 35m case.When the value of mechanical impedance rc only was changed to the values of 50% and
10% of that value based on the properties measured at the ground around the bridge, andthe other conditions were not changed, the values of the maximum principal stresses became91% and 84% of those values shown in Fig. 5, respectively. These results imply that the valuesof the maximum principal stresses in the upper part of bearing due to the collision betweenthe upper and lower parts of bearing, are also aected by the mechanical properties of theground.The dynamic yield strength of the material, SCMn1A, of the upper part of the bearing at strain
rates 102100 s1 can be estimated as about 400MPa, and the strain hardening rate of the materialis almost zero. If high enough tensile stresses over about 4002500MPa occurred in the upper partof the bearing due to the collision, the initiation of the tensile failure might be caused at thosepoints in the bearing. A fracture like the actual bearing fracture shown in Fig. 1(b) might thereforebe caused when the upper and lower parts of bearing collide with each other with a relativevelocity of about 526 m=s: It might be noted that a relative velocity of about 526 m=s is not theground motion but is the relative velocity between the upper and