nisee

National Information Service for Earthquake Engineering
University of California, Berkeley

      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’.  Inspection 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.  Slides J69-J75 illustrate such damage and therefore emphasize the need for avoiding the soft story by using an even distribution of flexibility, strength and mass.

      The structure of the 10-story main building (Slide J69) consists of a reinforced concrete frame with shear walls in the transverse (short) direction in stories 4 to 8.  The shear walls were interrupted at the fourth floor level where there were 1.10-meter diameter reinforced concrete columns (Slide J70) which were severely damaged.  The exterior walls were all constructed of tiles.  Note that there was some damage to these tile walls.

      This was the main damage to the building structure in Slide J69 and it  occurred precisely at the discontinuity in the structural system (interruption of the reinforced concrete walls).  There was also some damage to the exterior tile walls.  This main building was retrofitted by continuing the reinforced concrete walls down to the foundation.

      Note the hinging at the top and bottom of the first story columns in Slide J71.  This first story was a ‘soft story’ because, except for glass all around, it was completely open, while the second story had walls and partitions that increased significantly the lateral stiffness of this second story relative to the first.

      Note the large permanent lateral second floor level displacement of the main Treatment and Care Unit (a relative displacement with an interstory drift of 0.81 meters measured at the corner columns shown in Slide J72).  This large interstory drift, which induced significant non-structural and structural damage and which led to the demolishing of the building, was a consequence of the formation of a soft story at the first story level because of the existence of a reinforced concrete wall above the second floor level (Slide J61).  Note also the complete breakup (or crumbling) of the poorly confined concrete of the corner column.  (Compare the behavior of this first story column with the first story column of this same building shown in Slide J73).

      Lightweight concrete was used in the construction of this building.  Note that the building collapsed completely at the first (soft) story and the second floor dropped to the ground after moving laterally about 2 meters (see Slide J32).

      Although most of the buildings in this new housing development (Slide J75) remained standing after the earthquake, some of them were inclined as much as 20 degrees and dropped up to 1 meter, producing significant damage in the structural and non-structural elements of the first story.  The reason for this type of failure was the use of the ‘Vide Sanitaire,’ a crawl space about  1 meter above the ground level.  This provides space for plumbing and ventilation under the first floor slab and serves as a barrier against transmission of humidity from the ground to the first floor.  Unfortunately, the way that the vide sanitaires were constructed created a soft story with inadequate shear resistance.  Hence the stubby columns in this crawl space were sheared off by the inertia forces induced by the earthquake ground motion.

      The performance of the Imperial County Services Building in the 1979 Imperial Valley Earthquake is a good example of the need to design buildings and their structure with smooth variations in stiffness, strength and ductility, avoiding the formation of soft stories.  This building, illustrated in Slides J76 and J77, was a modern multistory reinforced building with a rectangular plan, six stories tall, with a small mechanical penthouse at the roof.  Lateral resistance was provided by moment-resisting frames in the longitudinal direction (E-W) and shear walls used in the transverse direction (N-S).  Shear walls in the upper stories were provided for the full width of the building on its east and west faces (Slides J76 and J77).  At the ground levels, the shear walls in the transverse direction were offset and considerably smaller.  Because of the use of spandrel panel walls in the stories above the first, the building response in the E-W direction was that of a soft story.  This, together with the discontinuity of the walls at their ends (offset) imposed by the desired architectural configurations, led to severe damage to the first story columns, particularly those located at the east end (Slides J78 and J79).

      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.  The use of steel diagonal braces for this purposed is illustrated in Slide J76.

The University of California, Berkeley
Copyright 1997, The Regents of the University of California.
Structural Engineering Slide Library, W. G. Godden, Editor
Set J: Earthquake Engineering, V. V. Bertero

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