Observations on Concrete Buildings from the EERI Team

Failures of concrete structures by far the biggest threat to life-safety

• Even though there was a far greater number of unreinforced masonry building failures as compared to concrete structures, the collapse of midrise concrete structures led to a disproportionate loss of life.

Non-ductile concrete structural damage was generally as expected, but many of these structures performed well. Typical failures / damage include:

• Lack of beam-column joint reinforcing led to joint damage, sway mechanism formation, and likely one the main factors leading to catastrophic collapse.
• Inadequate lap splices in beams initiating splitting of beams and shear failures
• Inadequate column ties / confinement leading to shear failure, compression failure and column bar buckling
• Lack of collector reinforcement and weak diaphragms inhibiting the transfer of seismic inertial forces to shear walls and separation of diaphragms from walls.
• Lack of consideration of potential drift demands on elements led damage of those elements and likely a major contributor to the failure of stairways.

Irregular structures

• Improper layout of lateral force resisting elements led to torsional response problems, increased drift and deformation demands on elements and concentration of damage in those elements.
• Vertical stiffness and mass irregularities leading to concentration of drifts in certain levels
• Discontinuous shear walls caused major damage to their supporting framing

Improper proportioning of elements

• Strong beams / weak columns leading to concentration of damage in columns.
• Slab damage / partial floor collapse due to column punching shear.
• Short column shear damage
• Thin shear walls, in combination with the lack of proper boundary elements and confinement led to compression failure / buckling of walls (similar to what was seen in Chile)
• Large variations in shear wall stiffness within buildings led to heavy concentration of damage is some walls, while other walls had little damage.

Future questions regarding performance of concrete structures

• What are the true implications of the large vertical accelerations? How big a factor were these accelerations to the compression failures of elements and damage to column transfer girders, slab punching shear failures? Should design standards be updated to account for higher vertical acceleration component?
• Why did many non-ductile concrete structures have satisfactory performance, while others suffered major damage / collapse?
• Is this due to the short duration of the strong motion? The short duration of the event meant that concrete structures were not subject to many stress reversals, and therefore showed little cyclic degradation in strength and stiffness. If the structures would have been subjected to longer duration of strong motion there likely would have been much more damage and likely more major failures.
• How big of a factor is use of precast elements versus cast-in-place (in-situ) construction to the overall damage levels / types of failures.
• How do we better protect foundations from liquefaction induced permanent residual drifts? Some structures that showed little-to-no damage will need major work, or potentially complete deconstruction. Can these structures be saved? How? Even if the foundations are addressed, can they be protected from future liquefaction induced damage?

Learning from Earthquakes: First person reports