Observed Damage - Cables
Observed Damage - Cables |
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| The main span of the Ji Lu bridge had 68 ASTM Grade 270 High Strength 7 wire stranded stays. There were 34 on each side symmetrically placed about the central pylon. At every 6 meters along the length the were 2 cables placed 130 cm apart about the longitudinal center line of the bridge (Figure 1). |
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Figure 1 - Ji Lu Cables |
| Overall the cables faired well with the exception of cable 7th out to the south of the central pylon on the East side of the longitudinal bridge axis (Figure 2). The significant of the fracturing cable to the overall behavior of the bridge is being investigated. |  |
Figure 2 - Cable fractured and sprung loose during the earthquake |
| On the day of the earthquake, the construction was on its third phase of stressing the stays - the fine tuning. They were going back through the cables and adjusting the stress on each cable to the letter of the plans and specifications. For the stressing procedure, a large diameter steel rod was attached to the end of the cable anchorage with a large threaded sleeve (Figure 3). |  |
Figure 3 - Steel bar used in cable stressing procedure |
| At the time of the earthquake, the
rod was still attached to the anchorage of the cable with the
sleeve. The rod, as subjected to the accelerations within the deck,
under went dynamic cyclic loading. Because of the nature of the
stressing system, there were discontinuities in stress flow and a fracture
exhumed. In one direction a fracture started and penetrated roughly
30% across the diameter of the rod. As the accelerations went the
other way, the crack closed and the rod bending stiffness changed
dramatically. With this instant change in local bending stiffness,
even more dynamic load put to the other side in tension and that was
enough to fracture the anchorage completely though. The loss in anchorage
shot the cable out of its support.
At the other end of the cable, the connection to the tower was tension only. When the cable went slack, that end simply fell out of its anchorage and disconnected completely from the Ji Lu. |
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Figure 4 - Connector fracture |
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| In addition to the fractured cable, there were many that shook loose their weather protecting rubber stoppers (Figure 5). This was non structural, but shows evidence of many cycles of cable vibration. |  |
Figure 5 - Rubber stops used for weather protection |
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