Chile's concrete code for buildings called into question

to the code, however. One allows the use of fewer confinement hoops, ties and hooks in walls.

Engineers are not required to confine walls because of the “satisfactory behavior of our buildings during previous quakes,” says Lagos, whose buildings did not suffer structural damage in the quake.

The more recent “Reinforced Concrete Structural Design Code (NCh 430-2008),” which is based on “ACI 318-05” with some exceptions, requires walls to be special structural walls. Frames must be special-moment-resisting frames, and the use of intermediate moment frames is allowed only if the structure has enough walls. The inconsistency between the two standards was solved in the “2010 Seismic Design Code,” approved in 2009 but still not official, according to Augusto Holmberg F., CEO of the Instituto del Cemento y del Hormigón de Chile, or the Chilean Cement and Concrete Institute. The new code adopts the criteria established in the new “RC Design Standard.”

In the years since Chile’s 1985 quake, buildings have gotten taller, while walls have become more slender — wall thickness often having dropped to 15 cm from 30 cm, says Moehle. Architectural demands, especially on narrow lots, have resulted in basement parking. There, walls have been reduced in length from the walls above to create access ramps for vehicles in a configuration called flag walls (see drawing). “This creates a stress concentration,” says Moehle. “Axial stresses are very high in some walls,” he adds.

In addition, quake ground motions were at or exceeded code levels for vibration periods between one and two seconds. “Put this together and many buildings were overloaded, with resulting compression failures,” says Moehle.

Shorter basement walls were crushed, and buildings listed somewhat. One 18-story building listed 22 cm at the roof; the confinement exception was followed in this case. “Many of the damaged buildings did not have confining steel,” he adds.

Not all Chilean engineers followed their code’s minimum requirements. Some followed the “ACI 318” provisions completely. Their buildings fared well in the quake, says Moehle.

Residents of the damaged buildings are afraid to re-inhabit them and want them demolished, says Moehle. Contractors already are shoring them. “This type of damage, in the majority of cases, is repairable and will be repaired, no matter how tall the building is,” says Lagos. “The problem is, who will pay for [the repairs],” he adds.

Moehle draws a lesson from the Chile experience for U.S. seismic engineers, saying tried-and-true code traditions should not be tampered with lightly. The engineer is concerned about the very big moment-resisting frames used more commonly in recent years by U.S. seismic engineers. A building’s seismic resistance is provided often by only one or two framing lines and a few bays. “Be careful,” he warns design engineers. “Don’t gain too much confidence in your techniques.”