Observed Damage - End Piers


Figure 1 - P13 End Pier Support

Both the north and south end piers (P11 and P13 respectively) of the Ji Lu bridge main span were heavily damaged in the 921 Taiwan Earthquake. The columns of the piers are oval shaped reinforced concrete 4.5 meters by 2 meters and heavily confined. The support beam is mostly rectangular and approximately 2 meters square. Both supports showed the largest lateral movement in the Western direction. This is contrary to the motion of the center pylon. The estimated lateral offset of the main span superstructure relative to the P13 support is 21 feet. For the P11 support, the offset is estimated at 13 feet.
	Figure 2 - P11 End Pier

The P11 and P13 damage was in the 8 inch architectural wall (Figure 1), the main span shear key, the approach span shear key, the support beam, and the pier support columns.
	Figure 1 - Pier support view showing 8 inch architectural wall

Evidence of the offsets are found from the gouging of the shear key into the architectural wall. After the key (and hence the superstructure) became dislodged from the support, the superstructure began to bounce around the curved cup formed by the architectural wall. With this pounding, the wall was being gouged out upon every collision (Figure 2). The more rigid shear key was eating away at the thin wall with each impact yielding a mess of spalled concrete and deformed steel.

Figure 2 - The shear key was dislodged and pounded the architectural wall during the Earthquake (P13 shown)

When the key landed on the P11/P13 support beam, the dynamic impact drove in diagonal shear cracking. The well reinforced support beam showed signs of local pounding effects, but given its rigidity, it was not as easily chipped away. The main damage seen in the P11/P13 support beams was diagonal shear cracking. The shear cracking started near the edge of the key restrainer and extruded outward toward the inside edges of the support columns on either side. The shear cracking is consistent with a gravity overload that is explained by the vertical component of the pounding superstructure into the support with dynamic impact (Figure 3).

Figure 3 - Diagonal Shear Cracking in the P11 support

Figure 3 - Diagonal Shear Cracking in the P11 and P13 support beam (P13 shown)

As a result of the large transverse movement of the superstructure, three out of four of the architectural closure walls were completely knocked off (Figure 4). Pounding, crushing, and spalling were also seen in the superstructure ends.
	Figure 4 - Missing architectural closure wall at the sides of the P11 and P13 supports
There was a permanent offset both longitudinally and transversely of the P13 support relative to the approach structure (Figure 5).
	Figure 5 - Relative offset between the superstructure and the approach structure
However, damage seen above the pile cap (on both P11 and P13 supports) suggests again that damage was due to gravity overload coupled with lateral forces (Figure 6). But, from the crack pattern observed, it is evident that there was no lateral mechanism formed. There was no evidence of a reversing plastic hinge at the base of the column. From this, we can conjecture the relative deformation between the superstructure and the substructure is from plastic offset of the approach spans rather than P11 or P13.
	Figure 6 - Damage locations consistent with gravity overload coupled with lateral loading. Drawn in yellow is the moment diagram form the load shown in yellow with the red gravity load. The red is the gravity load and centerline of the frame.

This relative deformation can be seen in the damage at the approach structure/main span interface at the P11 and P13 supports. Both relative longitudinal and transverse motion was observed between the main span and the approach (Figure 7). This longitudinal displacement caused the transverse restraining keys to pull out thereby making them ineffective to transmit lateral load. This, in turn, allowed freedom in the transverse direction.

Figure 7 - Relative longitudinal offsets between main span and approach structure in both P11 and P13 supports

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

Observed Damage - End Piers

Figure 1 - P13 End Pier Support

Figure 2 - P11 End Pier

Figure 1 - Pier support view showing 8 inch architectural wall

Figure 2 - The shear key was dislodged and pounded the architectural wall during the Earthquake (P13 shown)

Figure 3 - Diagonal Shear Cracking in the P11 and P13 support beam (P13 shown)

Figure 4 - Missing architectural closure wall at the sides of the P11 and P13 supports

Figure 5 - Relative offset between the superstructure and the approach structure

Figure 6 - Damage locations consistent with gravity overload coupled with lateral loading. Drawn in yellow is the moment diagram form the load shown in yellow with the red gravity load. The red is the gravity load and centerline of the frame.

Figure 7 - Relative longitudinal offsets between main span and approach structure in both P11 and P13 supports