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.
Hotel
Macuto Sheraton, Caraballeda, Venezuela. Overall view of the front
facade (entrance) of the 10-story main building after the 1967 Caracas
Earthquake.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.
Detail
of structure of Slide J69 showing the damaged 1.10-m diameter columns
located at the mezzanine level (3rd story). These columns were
longitudinally reinforced with 34 to 40 25-mm smooth bars with 12-mm
circular ties spaced at about 150 mm. The ties can be seen in the
upper part of the column where the concrete was shattered. 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.
Commercial
Building Casa Micasa S.A., Managua, Nicaragua. This 2-story
reinforced concrete frame building suffered significant lateral
displacement at the second floor level during the 1972 Managua Earthquake. 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.
Olive
View Hospital, San Fernando, California. Partial View of the 5-story
Medical Treatment and Care Unit (at right and back of slide), the walkway
canopy, and the one-story Assembly Building (at left of slide)
illustrating the damage that these buildings suffered during the 1971 San
Fernando Earthquake.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).
Detailed
view of the behavior of one of the first and second story columns in the
building of Slide J72 during the 1971 San Fernando Earthquake. Note
the large permanent distortion of the first story column (because it was
part of the soft story at this level). While the well-confined
concrete of the spirally reinforced core of this column was capable of
holding the building up, the unconfined concrete cover had spalled off.
Note also the shear failure of the second story column, induced by the
shortening of this column by the wall panels that were placed at the top
and bottom of this story.
Olive View Hospital, Psychiatric Unit, San Fernando, California. 1971 San Fernando Earthquake. This unit was a 2-story reinforced concrete building. The structural system was a moment resisting frame. However, in the second story there were masonry walls that added significantly to the stiffness of this story.
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.
Three-story
apartment building, El Asnam, Algeria, damaged in the 1980 El Asnam
Earthquake. 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).
Imperial
County Services Building. Overall view of this modern 6-story
reinforced concrete building. Not the continuous shear wall at the
east end of the building which was discontinued (offset) at the second
floor level, resulting in a severe discontinuity and in a practically open
first story (soft story in the E-W direction).
Building
of Slide J76. During the 1979 Imperial Valley Earthquake,
significant inertia forces were developed simultaneously in the two main
directions (illustrated in red). As a result, the corner columns of
the building were subjected to significant bending, shear and axial forces
which led to the failure of the corner column shown in Slides J78 and J79,
as well as the first story columns at the end of the building shown in
Slide J78.
Building
of Slide J76. View of the first story columns located in the east
end of the building. Note that the explosive type of failure just
above the ground and the offset between the columns and the solid shear
wall.
Building
of Slide J76. Close-up of the failure at the bottom of the column at
the southeast corner of the building. The failure occurred in the
zone of the column where there was not adequate confinement of the
concrete and shear reinforcing steel.
Retrofitting
Buildings with Soft Stories
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.
Multistory
building in San Francisco that has been retrofitted by adding steel
diagonal braces in two of the first story bays.
Copyright
1997, The
Regents of the University of California.
Structural Engineering Slide Library, W. G. Godden, Editor
Set J: Earthquake Engineering, V. V. Bertero