The Problem

By itself, concrete is brittle and easily cracked during an earthquake. But with the addition of reinforcing steel, concrete buildings can be made strong enough to withstand earthquake forces.

However, the concrete walls or columns of structures erected before 1975 often lack enough reinforcing steel to keep them from collapsing or being damaged beyond repair.

Larger imageLack of Reinforcing steel - If the confining reinforcing steel in a column is too widely spaced (left), it will not be able to keep the vertical reinforcing bars and the concrete in place when it is shaken by an earthquake (right).

Source: California Seismic Safety Commission

These buildings can pose the greatest threat to life in major earthquakes because, though total collapse of these buildings is rare, just one collapse could cause hundreds of deaths. In 1971, three concrete hospital buildings in the San Fernando Valley collapsed, killing 52 people; 43 people were killed in the collapse of a concrete freeway viaduct during the Loma Prieta earthquake of 1989. These structures lacked sufficient steel to confine the concrete and allow it to bend but not fall apart (See drawing right).

How to Identify

Hire an engineer or architect to help you review your building’s plans and determine whether it has this weakness, particularly if the building has many large windows, an irregular shape, or a tall, open first floor.

Larger imageShort columns in this parking structure failed in the 1994 Northridge earthquake. Modern building codes now require much more reinforcing steel for such short columns.

Source: California Seismic Safety Commission

Larger imageThis medical office building’s exterior cladding affected the way its frame responded to shaking, and the columns failed during the 1994 Northridge earthquake. The building was demolished and all medical records inside were destroyed.

Source: A. Johnson