California State University, Northridge Parking Structure C CollapseLuke T. Donovan, MS, Penn State, 2009

Introduction


At 4:30 a.m. on 17 January 1994, Martin Luther King Jr. Day, in the San Fernando Valley of Los Angeles, California an earthquake of magnitude 6.7 struck. The earthquake resulted in approximately 57 fatalities, 9,000 injuries, and $25 billion in direct losses. This report focuses on the resulting structural collapse of California State University Northridge’s (CSUN) Parking Structure C which was located approximately 30 km from the epicenter.
(ASCE, pp. 3-6)


Key Words


Parking Garage, Concrete, Pre-Cast, Post-Tensioned, Earthquake, Northridge, Ductile.

Design and Construction


A. T. Curd Builders constructed the five-story, 2,500-space, $11.3 million, reinforced concrete parking garage located at the corner of Zelzah Avenue and Plummer Street under a design-build contract in 1990-91.
(LA Times) The main structural makeup of the garage, designed using the 1988 UBC, was a mix of cast-in-place columns, pre-cast girders, and post-tensioned flooring elements. The exterior frame was finished with a brick facing and had rough plan dimensions of 433 feet by 366 feet. Four full floors and a partial roof area resulted in approximately 681,000 square feet of floor space. Each of the four corners was “trimmed” at a 45 degree angle and utilized for a stairwell. (Mitchell, pp.361-377)

Pre-cast concrete systems are commonly utilized in parking structure construction due to the facilities functional long span and large open area demands. The open nature of parking structures commonly results in less redundant structures with fewer shear walls. Parking Structure C's main interior girders, which spanned in a north-south direction, were precast-post tensioned elements supported on corbels on the exterior and interior site-columns.
Donovan_-_AE_537_CSUN_Garage_AutoCAD_Layout.JPG
Figure 1: CSUN Parking Structure C Damage Plan
These pre-cast beams supported a 5-inch thick cast-in place floor slab (and ramps) spanning in the east-west direction.
(Dames & Moore) The slab post tensioning was anchored in the perimeter frames. The slab was assumed to act as the lateral load resisting system. Vertical stirrups provided horizontal shear connections between the precast beams and the cast-in place post-tensioned slab. Dowels were placed horizontally through the precast columns and extended into the slab and provided partial moment connections in the beam column joints. The perimeter ductile moment-resisting frame was designed to resist the lateral loads while the non-ductile interior columns were designed and detailed for gravity loads only. The exterior frame was constructed of four story precast “trees.” (Mitchell, pp. 361-377)

Collapse


The earthquake epicenter was located at 34deg 12’N, 118deg 32’W and had an approximate
Donovan_-_AE_537_CSUN_Garage_Picture_#1.JPG
Figure 2: CSUN Parking Structure C Exterior Frame Curvature (Courtesy: NGDC and USGS)
focal depth of 15-20 km. Strong shaking lasted approximately 15 seconds in the epicenter area and the USGS assigned a seismic moment magnitude (Mw) of 6.7. The earthquake produced a strong amount of ground motion and the peak horizontal accelerations recorded were large for a quake of its magnitude. The largest aftershock (M5.9) occurred one minute after the main shock and there were over 3000 aftershocks of M>1.5 recorded in the first three weeks.
(ASCE, pp. 3-6)

Approximately 40% of the structure collapsed, occurring at the east and west ends of the structure, the remainder of the building was damaged but standing. Observable failures modes at the east and west ends included brittle failure of the interior gravity columns, beams pulling off the supporting corbels and sloping ramp failures. The partial collapse degraded the lateral and gravity live loading systems of the entire garage. Prior to multiple aftershocks and final demolition, the garage was well pictorially documented due to the extreme curvature in the exterior frame. (Dames & Moore)

Causes of Failure


CSUN hired Dames & Moore Inc. to complete an analysis of the collapse. The report is the most extensive single document on this specific collapse but only one of many technical articles that led multiple concrete code revisions following the Northridge earthquake. (LA Times, SEAOC)

Donovan_-_AE_537_CSUN_Garage_Picture_#3.1.jpg
Figure 3: Stair Collapse (Photo By: Ken Fowler, Coutesy: Earthquake Images)
Dames & Moore conducted eight different computer models for three dimensional linear analyses. Each model had seven different loading types, ranging from static dead load to vertical motion. The computational values used for horizontal and vertical acceleration were obtained from a motion instrument located at the corner of Saticoy Street and White Oak. The instrument was maintained by USC and was located within 5 km of the parking garage. Significant items of note from the Saticoy-White Oak instrument are the high values of vertical acceleration, and that the horizontal acceleration values were not significantly higher than those assumed in building codes. (Dames & Moore)

Results concluded the collapse may have occurred due to column or beam/corbel connection failures. Interior column collapse, due to flexural-shear failure, effectively eliminated girder and floor support. Girder collapse subsequently allowed the post-tensioned slab to pull inward from the exterior framework. The floor post-tensioning was connected to the exterior frame which was then pulled inward creating extreme curvature in the exterior frame. This failure mechanism created doubts of the effectiveness of mixing ductile and non-ductile systems. Also, the ramp cast-in place ramps may have failed from excessive shear or bending or in turn from the previous mentioned failures.

The analyses summary concluded multiple possible failure mechanisms may have initiated the collapse, but it is more likely that all of them contributed at one or more locations. Each failure mechanism can be explained by analyses results which only include horizontal motions. Vertical motions, although high, were not needed to explain the failure of the parking garage. The contribution of vertical motions did contribute to the collapse, but were “not a primary cause of the failures.”
(Dames & Moore) Field observations did not note any material specifications or workmanship deviations from the Construction drawings.

Dames & Moore concluded four key points regarding the collapse:

  • Horizontal floor displacements were far in excess of what non-ductile gravity columns could have withstood.
  • Vertical motions may have made a bad situation worse, but the structure would have failed even if there had been no vertical motions.
  • From interpretation of the Bluebook Commentary, the design did not meet the “intent” of the code. The design was consistent with the accepted practice of the time. It did meet the specific code provisions, but did not meet the general code.
  • The design did not conform to the code itself, due to inadequate ductility and continuity in the gravity system. (Dames & Moore)

Professional and Procedural Aspects


The Dames and Moore report criticized the design-build contract method which essentially allows the one company to control both project components and their checking processes. During the design process the Esgil Corporation reviewed the plans on behalf of CSUN and claimed Curd’s plans used a higher for the structure’s ability to flex with earthquake shock waves than the design warranted. CSUN sided with the builder over the plan check. Litigation over the collapse was not pursued and may have been “embarrassing to university officials because…they previously sided with the builder over their own plan-check consultants.” (LA Times)

Technical


The 1994 Northridge earthquake incited numerous changed to structural element codes. As proven through analysis and study of the collapse of the CSUN garage, large lateral drift can cause the unseating and collapse of beams and interior and exterior columns may not possess sufficient strength or ductility to safely undergo the same seismic movements. This case was a significant contributing factor to the American Concrete Institutes’ Building Code Requirements for Structural Concrete (ACI 318) revised requirements for the following elements: diaphragms and collectors, pre-stress tendons, strength factors, beam to column connections and transverse reinforcement of frame members. (SEAOC)

Donovan_-_AE_537_CSUN_Garage_Picture_#2.jpg
Figure 4: CSUN Parking Structure C Collapse (Courtesy: NGDC and USGS)
Conclusion


Thankfully the timing of the 1994 Northridge earthquake limited the amount of collateral damage and did not cause any human injuries at the CSUN parking garage. It did result in the total destruction of a three year old facility and prompted necessary changes to the parking garage industry. The lateral system of the garage was not adequately stiff to minimize and protect the structures interior (gravity system) against excessive lateral displacements and/or the interior (gravity) system was not ductile enough to withstand the lateral displacements. Also, connections between the slab and column and the ramp and the ramp segments were inadequate in strength.
(Dames & Moore)

Bibliography

  • Frammolino, R. (1994, February 11). CSUN Garage Not Up to Code, Review Warned. Los Angeles Times. Retrieved from: http://articles.latimes.com/1994
  • DeSamtis, J. (1995, March 14). Northridge - Demolition of CSUN Parking Structure Starts. Los Angeles Times. Retrieved from: http://articles.latimes.com/1995
  • Chandler J. & Smith D. (1995, March 21). Collapsed Parking Structure Violated Code, Report Says. Los Angeles Times. Retrieved from: http://articles.latimes.com/1995
  • Bolin, R., & Lois, S. (1998). The Northridge Earthquake: Vulnerability and Disaster. London & New York: Routledge
  • Chen W.F. & Lui E.M. (2006) Earthquake Engineering for Structural Design. Boca Raton, London, & New York: Taylor & Francis Group
  • American Society of Civil Engineers (1995). Northridge Earthquake: Lifeline Performance and Post-Earthquake Response. Technical Council on Lifeline Earthquake Engineering. Monograph Number 8. New York: Author, pp.3-6
  • Mitchell, D., DeVall R., Saatcioglu M., Simpson R., Tinawi R., & Tremblay R. (1995). Damage to Concrete Structures Due to the 1994 Northridge Eartrhquake. Can. J. Civ. Eng. Vol. 22, 361-377. Retrieved from: http://article.pubs.nrc-cnrc.gc.ca/RPAS/rpv?hm=HInit&afpf=l95-047.pdf&journal=cjce&volume=22
  • Dames & Moore. (1990). The Northridge Earthquake: A Special Report by Dames & Moore. Author.
  • SEAOC Seismology Committee (2009). “Concrete Parking Structures,” June, 2009, The SEAOC Blue Book: seismic design recommendations, Structural Engineers Association of California, Sacramento, CA. Retrieved from: http://www.seaoc.org/bluebook/index.html
  • Earthquake Recovery Bulletin, Vol. 1, No. 1, Fall 1994. California State University, Northridge. Office of Public Relations. Retrieved from: http://digital-library.csun.edu/u?/UniversityArchives,254

Additional Sources: