EERI Nor-Cal Chapter Summer Newsletter

By Khalid M. Mosalam

This April a magnitude Mw=6.3 earthquake struck the central region of Italy near the city of L'Aquila. While the earthquake was tragic--305 people killed, 1500 injured and thousands of buildings destroyed--its aftermath provides lessons for earthquake professionals in the Bay Area.

Damaged URM building in town of Onna

In the town of Onna, a URM building is severely damaged

Within 10 km of the epicenter, the recorded horizontal peak ground acceleration exceeded 0.35g, and the ground shaking had high-frequency content with relatively short duration. The damage indicated strong effects of site conditions, and heavy damage occurred in structures founded on young sediments.

Old unreinforced masonry (URM) buildings made of mortar and multi-wythe rubble-stone or clay bricks were significantly damaged. These buildings were typically two to three stories tall, and the damage ranged from wall cracking to severe damage and collapse. Some buildings with retrofitted cross-ties to reduce out-of-plane wall deformation performed reasonably well with limited cracking and no out-of-plane collapses.

Reinforced concrete (RC) buildings ranged from two to eight stories tall. The majority of modern RC buildings were designed for horizontal acceleration of about 0.25g. In the epicentral region, little attention was paid to ductility requirements (e.g. the use smooth reinforcing bars, short lap splices, and insufficient column ties and transverse reinforcement in beam-column joints).

The designs appear to have ignored the effect of infill walls, and some construction material was of poor quality. Although these deficiencies are serious, the wide-spread damage more likely resulted from a lack of ductility and the brittleness of exterior infill walls and interior partitions. There were also isolated cases of RC frame damage due to shear failures that led to soft story mechanisms.

Beams fail due to lack of reinforcement

In the town of L'Aquila, beams fail due to inadequate steel reinforcement

The following three major lessons can be drawn from the earthquake:

1) Cross-ties restraining out-of-plane deformation of multi-wythe URM walls are effective in limiting the collapse of these types of buildings.

2) Infill walls may protect seismically-deficient RC frames and reduce their damage. But the damage in URM infill walls is hazardous, and these types of walls should be used with care and properly considered in the design.

3) Out-of-plane failure of infill walls should be accounted for in the design process of RC frames with URM infill walls. Moreover, the in-plane/out-of-plane interaction should be considered in the designs of RC frames with URM infills.

Khalid Mosalam is a Professor and Vice Chair in the Department of Civil and Environmental Engineering, University of California, Berkeley.