Observed Damage - Main Span
blueline.JPG (929 bytes)
The main spans of the Ji Lu Cable Stayed Bridge consist of two 120 meter segments symmetric about the main Pylon (Figure 1).  The majority of the damage to the main span was concentrated near the center pylon with minor deck damage at the span ends.

Post Earthquake, the main span was shored with temporary supports.

mainSpan.JPG (23865 bytes)
Figure 1 - Longitudinal view of the main span
The spine of the main span was cast in place concrete; then, prestressed longitudinally, transversely, and in a reverse-v configuration about the center web (Figure 2).  Then, precast, prestressed wings were added and post tensioned into place. xSection.jpg (24904 bytes)
Figure 2 - The cast in place spine showing location of prestressing
The Side panels (wings) were precast and transversely field post tensioned into place.  The closure pour on the main span deck was meant to tie the longitudinal segments together.  However, at the time of the earthquake, the closure pour had not been done.  Without a positive longitudinal connection, the wings simply opened up under tension rather than transmitting any longitudinal forces (Figure 3). wings.JPG (15662 bytes)
Figure 3 - Post tensioned wings opened up during the earthquake making them ineffective to transmit any tension forces
Another important piece to the forensic puzzle is the missing precast wings (Figure 4).  For construction reasons, these panels were left out until the very end.  They were not in place at the time of the earthquake and that had an interesting effect on behavior.  

The precast panels were incapable of transferring tension; however, they have been assumed able to transfer compression.  The stiffness under transverse bending is thereby direction dependent because of the missing panels. 

 missingPiece.JPG (23773 bytes)
Figure 4 - Three precast wings had not yet been placed at the time of the earthquake
The bending stiffness in the direction where the missing precast wings would have been in compression is a factor of 3 (Figure 5) less than the bending stiffness going the other way.  Because of the large influence of the deck on the first transverse mode, the structural period of the bridge is quite a bit different depending on the direction of transverse motion.   This, in turn, has a large impact on the overall behavior - both for displacements and for forces.  This effect of asymmetric stiffness a direction depend period of vibration is being investigated further. Stiffness.jpg (13790 bytes)
Figure 5 - Deck properties and assumptions have a great effect on the strong bending stiffness of the Ji Lu Deck
Inconsistent with simple lateral or vertical bending failure is a large crack running along the spine of the bridge (Figure 6) just to the side of the center web in the box girder.  The reason for this crack and an investigation into this failure mode is currently being researched.  
Figure 6 - Picture showing the under side of the deck on the south side of the Pylon (P13 side) 
Because of this large drop in stiffness near the critical location of the bridge, there was a concentration of damage (Figure 7).  All bending forces that were accumulating down the spine of the member were taken by the uncracked- with- wing stiffness until nearing the joint.  Then, at that juncture, the moment capacity as well as the stiffness, dropped.  This discontinuity in stiffness and yielding force resistance on the lateral load path resulted in a concentration of inelasticity.  The section showed most of its damage on the right (East) side.
Figure 7 - Damage Map of the Ji Lu spine cross section on the South side of the Pylon.  Click in damaged area to see photograph


[Overview] [Foundation] [End Piers] [Main Span] [Pylon] [Cable Damage]