Observations on RC construction from EERI Team

The large majority of reinforced concrete buildings in the epicentral area fared well if we consider that they were subjected to a ground motion equal or larger than those they were designed for. In the last 25 years L’Aquila and its province were considered to be in the Class 2 seismic area of the building code and these buildings were designed for a horizontal acceleration of 0.25g. In the epicentral area maximum accelerations up to 0.67g were measured although a recording station went off scale at 1g (see discussion below).

Among this construction class the modern RC frame buildings (i.e., those designed after 2003 when the last building code was issued) performed much better than those of the 60s to 80s vintage. The modern RC frame buildings usually suffered no exterior damage or minor/moderate damage to the clay partitions both internal and external (see Fig 1 for a typical example). We only found one modern building on the verge of collapse whose many columns were completely severed in shear possibly due to a faulty design that did not properly considered torsional effects (i.e., buildings rotation along the vertical axis during the ground shaking due to stiffness and plan irregularities) (see figure 2 ).  This building is unlikely to be recovered and may be demolished.

In the nearby town of Pettino, the EERI survey team found several instances of soft story effects. Two out of four almost identical buildings collapsed in this same failure mode and two survived. In the particular case shown (in figure 3) a three story structure collapsed when the columns of the first story,used for parking space, failed. A survivor of the earthquakes told us that the mainshock of April 6 generated a significant damage that lowered the first floor by about a foot but the first story collapsed completely in an aftershock.

The first story column-beam connections displayed insufficient stirrup reinforcement (large spacing and small diameter) and short anchorage lengths around the beam longitudinal rebar in the column. The second collapsed building in this cluster of four failed instead during the mainshock. The other two remained standing although one showed significant separation of the infill walls in the first story, indicating an incipient similar failure mechanism that was not fully developed while the fourth one experience minor exterior damage (figure 4).

The green shed in figure 5. houses the strong ground motion station at the “Moro” location that registered a peak ground acceleration of over 1g, saturating shortly thereafter. Investigations are still taking place to make sure that no malfunctioning of the instrument occurred during the shaking. The station is surrounded by several identical six-story reinforced concrete structures (figure 6) built in the late eighties to early 90s that showed no apparent structural damage but only moderate cracking to the infill walls. Note the green 3-story RC frames recently remodeled (figure 5) across the street from the station that was completely unscathed. This case study is sure to stem thorough investigations in the engineering community to understand how these buildings could survive relatively untouched to such a large ground motion