Engineering and Construction

A soft story, also known as a weak story, is defined as a story in a building that has substantially less resistance, or stiffness, than the stories above or below it. In essence, a soft story has inadequate shear resistance or inadequate ductility (energy absorption capacity) to resist the earthquake-induced building stresses. Although not always the case, the usual location of the soft story is at the ground floor of the building. This is because many buildings are designed to have an open first-floor area that is easily accessible to the public. Thus the first floor may contain large open areas between columns, without adequate shear resistance. The earthquake-induced building movement also causes the first floor to be subjected to the greatest stress, which compounds the problem of a soft story on the ground floor.

Concerning soft stories, the National Information Service for Earthquake Engineering (2000) states:

In shaking a building, an earthquake ground motion will search for every structural weakness. These
weaknesses are usually created by sharp changes in stiffness, strength and/or ductility, and the effects of these weaknesses are accentuated by poor distribution of reactive masses. Severe structural damage suffered by several modern buildings during recent earthquakes illustrates the importance of avoiding sudden changes in lateral stiffness and strength. A typical example of the detrimental effects that these discontinuities can induce is in the case of buildings with a ‘soft story.’ Inspections of earthquake damage as well as the results of analytical studies have shown that structural systems with a soft story can lead to serious problems during severe earthquake ground shaking. [Numerous examples] illustrate such damage and therefore emphasize the need for avoiding the soft story by using an even distribution of flexibility, strength, and mass.

Three examples of buildings having a soft story on the ground floor are as follows:

1. Chi-chi earthquake in Taiwan on September 21, 1999. In Taiwan, it is common practice to have an open first floor area by using columns to support the upper floors. In some cases, the spaces between the columns are filled-in with plate-glass windows in order to create ground-floor shops.

Figure 14.1 shows an example of this type of construction and the resulting damage caused by the Chi-chi earthquake.




2. Northridge earthquake in California on January 17, 1994. Many apartment buildings in southern California contain a parking garage on the ground floor. In order to provide an open area for the ground floor parking area, isolated columns are used to support the upper floors. These isolated columns often do not have adequate shear resistance and are susceptible to collapse during an earthquake. For example, Figs. 14.2 and 14.3 show the collapse of an apartment building during the Northridge earthquake caused by the weak shear resistance of the first floor garage area.






3. Kocaeli earthquake in Turkey on August 17, 1999. In terms of building conditions during this earthquake, it has been stated (Bruneau, 1999):

A typical reinforced concrete frame building in Turkey consists of a regular, symmetric floor plan, with square or rectangular columns and connecting beams. The exterior enclosures as well as interior partitioning are of non-bearing unreinforced brick masonry infill walls. These walls contributed significantly to the lateral stiffness of buildings during the earthquake and, in many instances, controlled the lateral drift and resisted seismic forces elastically. This was especially true in low-rise buildings, older buildings where the ratio of wall to floor area was very high, and buildings located on firm soil. Once the brick infills failed, the lateral strength and stiffness had to be provided by the frames alone, which then experienced significant inelasticity in the critical regions. At this stage, the ability of reinforced concrete columns, beams, and beam-column joints to sustain deformation demands depended on how well the seismic design and detailing requirements were followed both in design and in construction.

A large number of residential and commercial buildings were built with soft stories at the first-floor level. First stories are often used as stores and commercial areas, especially in the central part of cities.
 
These areas are enclosed with glass windows, and sometimes with a single masonry infill at the back.

Heavy masonry infills start immediately above the commercial floor. During the earthquake, the presence of a soft story increased deformation demands very significantly, and put the burden of energy dissipation on the first-story columns. Many failures and collapses can be attributed to the increased deformation demands caused by soft stories, coupled with lack of deformability of poorly designed columns. This was particularly evident on a commercial street where nearly all buildings collapsed towards the street.

Examples of this soft story condition are shown in Figs. 14.4 and 14.5.





Concerning the retrofitting of a structure that has a soft story, the National Information Service for Earthquake Engineering (2000) states:

There are many existing buildings in regions of high seismic risk that, because of their structural systems and/or of the interaction with non-structural components, have soft stories with either inadequate shear resistance or inadequate ductility (energy absorption capacity) in the event of being subjected to severe earthquake ground shaking. Hence they need to be retrofitted. Usually the most economical way of retrofitting such a building is by adding proper shear walls or bracing to the soft stories.