Structural Reconnaissance Observations

I arrived in Christchurch on March 1^st, almost a week after the earthquake.  Rescue and recovery operations were still underway so are the building assessment and critical building projects among other activities. I mostly contributed to the critical building project and gathered as much of the perishable information as possible. I have been to several of the sites within the Central Business District (CBD), where significant damage to concrete and masonry structures occurred.

Key Observations

Impact of Liquefaction

Given that significant liquefaction occurred in different parts of the CBD, the performance of   engineered structures may be summarized as follows:

• Where noticeable liquefaction was visible at the site, structural damage was found to be minimum when structure was supported fully or partially on shallow foundations, but the settlement of the structure was visible. Significant settlements will likely lead to costly repair or replacement of the structures.  (See Figures 1 and 2)
• Where noticeable liquefaction was visible at the site limited structural damage was generally found when the structure was supported on deep foundations. The ground surrounding the structure has shown significant settlement.
• When there is no significant visible liquefaction problem, damage to structures was generally visible. In some cases, well-engineered buildings exhibited noticeable damage. Damage to contents in these buildings was also high.

Extent and Cause of Structural Damage

The following observations were noted based on the inspection of the damaged and in some cases undamaged buildings:

• Buildings with asymmetric structural configuration performed poorly while uniform, regular structures appeared to have performed well.
• In well-designed, regular buildings, cracking on the beams, columns, walls appear to be consistent with expectation. The extent of cracking in the critical regions (e.g., beam hinges) appear to be limited, which is primarily due to shorter duration of the earthquake. Though expected, cracking at the base of the column in several cases was not visible, which may be due to flexibility of the foundation or extension of the column below the foundation beam.
• An unexpected failure mode was seen in some irregular buildings which were designed with structural walls at the building corners or on two sides of the buildings. Severely damaged elements were typically columns and they appeared to have failed due to excessive axial forces combined with some flexure and shear actions. The large axial loads in the columns is likely due to combination of gravity loads, outrigger effects resulting from framing action caused by rigidly connecting the floors to the columns, and possibly vertical acceleration. Given the proximity of CBD to the earthquake fault (6-8 km), the P and S waves from the fault would have arrived 1.5 to 2 seconds apart. This implies that the vertical acceleration component would have been still significant when the structures in CBD were subjected to peak excursions due to the earthquake component in the horizontal directions.

Damage to Different Structural Types

• Though not surprising, significant damage to unreinforced masonry structures (URM) was widespread largely due to out of plane motion. URM failure led to collapse of the structure, damaged to adjacent building and/or appeared to have caused significant public hazard.
• Retrofitted URM and URM with temporary shoring, which occurred following the September 2010 event, have generally minimized damage.
• 2 story frames with infill walls performed well.
• Block masonry with inadequate/poor grouting performed poorly.
• Pre1970 buildings – repairable damage was generally visible, but in some cases replacement of the structure would be required.
• Recent structures – performed well except when structural configuration was done poorly.
• Precast structures – precast components were used in several structures and they did not lead to any poor performance of structures. Two parking garages experienced collapse. Precast elements were used in both structures and the collapse of these structures appears to be due to poor connection in one case and due to punching failure of the (post-tensioned) floor slab, but both require further investigation.
• Critical buildings designed to be functional after the earthquake – performed well and they were functional during rescue and recovery operations.
• Steel frame structures – not inspected.

	Figure 1. Two leaning buildings.	Figure 2. Ground damage due to liquefaction near the buildings.