Observed Damage - Pylon

The central pylon of the Ji Lu rises 76 meters above the pile cap and 58 meters above the main span deck.  The pylon has 78 cables extending outward in the bridge longitudinal direction as consistent with a cable stayed harp design.

Figure 1 - View upward on the North face of the center pylon P12 (on the side of P11)

The East side of P12 saw the most damage.  The damage was, however, was only superficial.  The concrete outer most fiber strain demand was not large and the cause for the spalling was a simple weak plane (Figure 2).

The cross section had a transverse reinforcing ratio of 3%.  When that is considered against the immense cross sectional area of concrete, the result is a near solid wall of transverse reinforcing steel at the cover/core interface.  

This heavily confined section was well designed for very large ductilities.  However, for serviceability consideration, at moderate cover strains, a weak plane developed and the concrete cover just split off.

Figure 2 - East side view showing large missing sections of the concrete cover

From the south side of the pylon, the splitting crack at the cover/core interface can be seen clearly.  

From the damage pattern, we can see that though there was no plastic hinging, there was substantial moment demand in the transverse direction.  Furthermore, we can see that the moment was higher when bending positively about the Northern direction with the East side in compression.  This direction of bending means, for a cantilever, that the top of the tower moves in the Eastern direction.  

Upon observation of the P13 and P11end piers, we can see that this is opposite in direction to the largest lateral motion of the keyway (21 feet on P13 and 13 feet on P11).  At the P13 support and P11 supports, the largest motion was in the Western direction.  This information is consistent with the model formulation and the structural mode of vibration.

Figure 3 - South side view showing splitting crack

On the west side of the P12 pylon above the deck level, there was only minor damage.  This is the same side as the direction of largest motion noted on the P11 and P13 support.  This side is opposite to the side of the missing wings and opposite to the side of the most main span damage. 

Figure 4 - West side view.  Only minor damage