Imperial+County+Services+Building+Collapse

= Imperial County Services Building Collapse (1979, El Centro) = //Christopher Coakley BAE/MAE Penn State 2013// toc

Introduction
On October 15, 1979 a 6.5 magnitude earthquake shook through the Imperial Valley area of Southern California injuring 91 people and causing an estimated $30 million in property damage. One of the most severely damaged buildings was the Imperial County Services Building, loc ated in El Centro California (USGS, 2013). The Imperial county services Building was a six story reinforced concrete frame and shear wall structure designed under the 1967 provisions of the California Uniform Building Code (Bosela, Brady, et.al., 2013). Severe damage was sustained by the columns at the east end of the building with bursting of the rebar cages and crushing of the concrete at the column base resulting in significant shortening. The failure has subsequently been attributed to the combinatory interaction of three different force actions on these columns: bi-axial bending about both principle horizontal axis' as well as increased vertical axial force resulting from a large overturning moment generated by architectural configuration (Shepherd and Plunkett, 1983). The failure is unique in that it is the failure of a building that was designed in line with a recent building code and had been previously dynamically tested, yet still failed tothe point of being judged unrepairable and thus demolished completely.

Keywords
Earthquake, Imperial County Services Building, Imperial Valley, El Centro, California, Strong-motion Instrumentation, Collapse, Shear Wall



Building Description
The Imperi l County Services was a six story, reinforced concrete building. The footprint of the building was a rectangular 136 ft 10 in by 85 ft 4 in. A regular bay system of 25 ft was set up in both principal N-S and E-W directions. The ground floor had a story height of 14 ft 6 in, where as the upper floors all had story heights of 13 ft 6 in except the top floor with a story height of 13 ft 2 in.

The gra vity system consisted of four longitudinal moment frames oriented in the E-W direction. Gravity Loads were transferred to these frames via a N-S spanning pan-joist system. All columns were typically 2 ft square except on the North and South perimeter sides where they tapered outward from 18 in to 10 in over a course of 5 ft 2 in. This was done for architectural reasons.

The lateral force resisting system consisted of a different system in each of the principle N-S and E-W directions. Lateral loads were resisted by the four longitudinal moment frames in the E-W direction. However in the N-S direction lateral resistance was provided via concrete shear walls. Shear walls were located along lines B, D, E, and F for the first story and along lines A and H for the upper stories. The shear walls at lines A and H are the east and west end walls of the structure and were cantilevered outward 5 ft 11 in above the second story. This offset creates a major discontinuity in the vertical plane of stiffness. The lateral forces developed in these upper story end shear walls needs to be transferred to the interior first story shear walls and ultimately to the foundation. The cantilevered end shear wall at line H was 30 ft 11in from the nearest first story shear wall at line F and the cantilevered end shear wall at line A was 5 ft 11 in from the nearest first story shear wall at line B. This transfer of lateral forces from the end walls to the first story walls was to occur through the floor diaphragm. Thus a thickened 5 in slab was provided at the second story as opposed to the 3" slab typical of the upper stories (Kreger and Sozen, 1989).



Damage Sustained
While damage was widespread through out the Imperial County Services Building its most severe damage can be characterized by the partial collapse just above ground level of the four reinforced concrete columns along the building's east end, column line G. Concrete at the base was spalled completely off and the vertical reinforcement buckled severely outside the column boundaries widely splaying the horizontal ties. Measurements made by the county staff indicate the columns shortening by approximately 9 in during the main shock, and approximately 3 in during the aftershock (Pardoen and Shepherd, 1983).

The settlement of these columns resulted in the formation of plastic hinges at end of the longitudinal frame bay F-G, as was evident by the yielding of the girders at these locations. This settlement also led to a sever line of cracking in all upper story floor slabs propagating in the N-S direction immediately east of the first row of interior columns. Additional less significant damage occurred with minor cracking in all columns below the second floor level, minor cracking and spalling of most columns immediately above the ground level, and diagonal tension cracks in the exterior shear walls (Shepherd and Plunkett, 1983).





Response of the Structure
The Imperial County Services building has the un usual advantage for study due to the installment of of strong-motion instrumentation prior to its failure. Strong-motion instrumentation was installed and maintained by the California Division of Mines and Geology consisting of a 13-channel remote-accelerometer system positioned to enable translational, torsional and in-plane floor bending responses to be identified. Also a triaxial accelerograph was installed at ground level approximately 100 meters to the east of the building (Rojahn and Mork,1983).

Study of the strong motion data over the time history of the earthquake helps to further reveal the cause of the damage previously described. Two main features of the accelerogram can be distinguished as being particularly important. First the abrupt long period motion in the E-W direction occurring at the roof, fourth, and second floors at a time of 6.8s. The second important feature being a .5s cluster of high amplitude, high frequency motion at 11s in the N-S direction which the recordings indicate occurring directly above the failed exterior columns. Records in between these two times of 6.8s and 11s are characterized by clusters of low amplitude, high frequency behavior (Shepherd and Plunkett, 1983). This first feature, the long period motion at 6.8s is believed to correspond to the initial yielding of the longitudinal moment frames in the E-W direction. This is followed by the clusters of low amplitude, high frequency motions associated with the continuation of yielding and plastic hinging of these E-W frames. Finally followed with the high-amplitude, high-frequency behavior thought to signify the full settlement of the exterior eastern face columns (Pauschke, Oliveira, et.al., 1981).

Furthermore the strong motion data shows that beginning at this 11s mark and lasting untill the 11.5s mark, the frequency content at opposite ends of the roof acting in the N-S direction changed abruptly where previously they were in phase. This is taken to imply that the stiffness characteristics in the N-S direction changed substantially between 11s and 11.5s thus indicating start of the exterior eastern column collapse at 11s (Rojahn and Mork,1983).

Analysis
By analyzing the appropriate records of this strong-motion data a confirmation of the behavior of the Imperial County Services Building and its failure mechanisam can be developed. The following section will explain in brief the common and agreed upon results of these analysis. These analyses provide a confirmed general understanding of the building's macroscopic elastic response to dynamic loading as well as indication of what members should have been expected to fail and experience unacceptable inelastic behavior under high seismic loading (Pardoen and Shepherd, 1983).

Various analytical computer models have been constructed over the years by many different people, to which the recorded strong motion acceleration data has been applied in an attempt to verify the failure mechanism of the Imperial County Services Building. The main conclusions of these models supports the observed failure damage at the building. The large overturning moment of the east end shear wall was transferred via the thickened second story slab to the first line of interior columns. Here this overturning moment created high axial loads particularly in the northernmost and southernmost columns. A corresponding reduction in column moment capacity then occurred as will happen when both bending and axial load are interacting in a single member. This reduction in moment capacity of the eastern exterior columns then allowed the large longitudinal (E-W) moments to overpower and yield the member. The remaining columns' behavior, both interior and exterior, were characterized by yielding and/or damage resulting from E-W longitudinal moments (Pardoen, Moss, et.al., 1983).

Conclusions
It is evident by extensive computational analysis of actual recorded strong-motion data and by real observations of the failure that the ultimate cause of the partial collapse and resulting demolition of the Imperial County Services Building can be attributed to an irregular and improper architectural configuration of structural shear walls. Had the ground level shear wall at column line F been located at column line G the generation of axial load in the eastern exterior columns would have been much less significant. Also of course had the end shear walls been continuous to the ground the structure would have been much less prone to damaging movements. This resulting arrangement led to unforeseen interaction of principal forces in the exterior columns ultimately causing them to fail as well other significant damage throughout the building.

Additional Resources & References
Brandow, Gregg and Leeds, David (1980), “Imperial County, California, Earthquake, October 15, 1979: Reconnaissance Report”, Earthquake Engineering Research Institute.
 * **Investigation Report**

Zeris, Christos, (1984), “Investigation of the Response of the Imperial County Services Building to the 1979 Imperial Valley Earthquake and Implications to Earthquake Resistant Design”, SESM Division, University of California Berkeley.
 * ** Investigation Report **

Pardoen, Gerard, (December 1983), “Ambient Vibration Test Results fo the Imperial County Services Building”, Bulletin of the Seismological Society of America, Vol. 73, pp. 1895-1902.


 * **Journal Article**: This article presents the results of ambient vibration test results conducted on the Imperial County Services Building prior to its failure.