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Concrete Parking Structures and the Northridge Earthquake

Nathan C. Gould, D.Sc., S.E.

Performance and Resulting Building Code Changes

One of the most iconic images of the 1994 Northridge Earthquake is the photograph of a collapsed precast concrete parking structure at California State University, Northridge. While it was only one of many precast parking structures that suffered extensive damage as a result of the earthquake, it illustrates the juxtaposition of the incredible ductility exhibited by the perimeter columns with the collapse of the overall structure. It epitomizes one of the primary performance issues highlighted by the earthquake. The failure of numerous concrete parking structures during the earthquake, both precast and cast-in-place, led to an in-depth examination of the current design practices and ultimately led to several building code changes to improve the performance of these types of structures.

Representative Failures

California State University, Northridge

This partially collapsed garage at California State University was a relatively new, four-level precast concrete garage, approximately 18 months old at the time of the Northridge event. Given the age of the structure, it is likely that it was designed in conformance with the 1991 Uniform Building Code (UBC) requirements.

The design of the garage included a perimeter “ductile” concrete lateral force-resisting frame, with the exterior columns designed to carry all the lateral loads and the interior columns designed to carry only the vertical loads. As shown in Figure 1, the exterior columns exhibited a significant degree of ductility; however, it is likely that the interior columns, designed for vertical loads only, were unable to accommodate the loads imposed as the structure experienced significant lateral displacement. The failure of vertical-load-only columns in structures where the lateral resistance was concentrated in perimeter lateral load-resisting frames was a valuable lesson learned from the Northridge event and was the impetus for future building code changes.

Northridge Fashion Center

At the Northridge Fashion Center, two large, precast, prestressed concrete garages collapsed (Figure 2). The garages were relatively new in that the mall had just opened in 1991. The garage at the southwest corner of the shopping plaza had a vertical load-resisting system comprised of precast concrete columns supporting precast concrete beams, while the lateral load-resisting system was comprised of concrete shear walls in each of the structure’s principal directions. There was visual evidence of damage to the precast columns as well as a loss of support for the precast beams. It was interesting to note that that the concrete shear walls, which were likely intended as the primary lateral elements, suffered little if any damage. A garage in the northwest corner of the plaza, with similar construction, failed as well. There were likely several contributing factors to the collapse of the garages. The connections between the precast elements were likely insufficient to allow the elements to maintain continuity as the structure underwent significant displacement. Also, the lack of continuity reinforcement across the construction joints in the concrete slab may have limited the ability of the diaphragms to transfer the loads to the shear walls adequately.

A one-story cast-in-place concrete structure at the Center had damage to the circular concrete columns that supported the concrete drive ramp. The column’s transverse reinforcement, which was spaced at 12 inches on-center, was likely inadequate to provide the needed confinement for these columns; the columns were subjected to a high level of shear due to their increased stiffness which can be contributed to their relatively “short” length.

In addition to the practice of utilizing “independent” lateral load-resisting frames, with strength and detailing different from the standard vertical frame elements, there were several other factors that may have contributed to the significant level of damage in the concrete parking structures. These factors include the practice of designing parking structures at a minimum code compliant level, the irregularity of structural systems often found in multi-story parking structures with interior ramps, and the marginal connection of precast elements.

Changes to the Building Code

After the 1994 Northridge Earthquake, there were two code change cycles (1995 and 1996) that provided opportunities to incorporate lessons learned into the 1997 UBC, which was the premier code for seismic design at that time. Most of the lessons applicable to parking structures fell into three basic categories: Deformation Compatibility, Design of Collectors, and Design of Diaphragms.

The online version of this article contains a table that provides the 1997 UBC approved code changes in these three categories, with the reason given for the code change. The information in the table was collected from a variety of resources, including the 1997 Analysis of Revisions for the Uniform Building Code published by the International Code Council’s legacy organization, the International Conference of Building Officials.

1997 UBC Section 1633.2.4, Deformation Compatibility, was added because of the deformation-induced damage during the Northridge Earthquake to parking garage elements not part of the lateral force-resisting system. The added language required elements not part of the lateral force-resisting system, regardless of material type, to be designed and detailed to maintain support of the design dead plus live loads when subjected to the expected deformations caused by seismic forces; plus, additional considerations were stipulated. For concrete and masonry lateral force-resisting elements, the assumed flexural and shear stiffness properties were limited to a maximum of one-half the gross section properties unless a rational cracked-section analysis was performed. Also, new design and detailing requirements for concrete were added to Chapter 19 to improve deformation ductility and ensure their ability to continue to support gravity loads. These new provisions were submitted by the SEAOC Seismology Committee (Chair Bob Chittenden) for the 1995 code development cycle. The code change was “approved as revised” by the ICBO Lateral Design Code Development Committee. There were further amendments to the code change approved at ICBO’s 1995 Annual Education and Code Development Conference in Las Vegas, Nevada. The Chapter 19 deformation compatibility provisions cited in the table (online version of this article) were submitted by the Portland Cement Association (Mark Kluver). This code change was approved by the ICBO Lateral Design Code Development Committee without amendments at their meeting in Des Moines, Iowa, in February 1995.

1997 UBC Section 1633.2.6, Collector Elements, was added because collector elements failed in parking garages during the Northridge Earthquake, and lateral loads were not delivered to the shear walls as intended by design. The new provisions required that collector elements, splices, and their connections to resisting elements be designed to resist forces increased by the new overstrength factor introduced in the 1997 UBC. The overstrength factor was introduced in recognition that forces generated in the lateral force-resisting system can be two to three times the design seismic forces. Failures of collectors in the Northridge Earthquake resulting in disconnection of the building from the lateral force-resisting system and, in some cases, a loss of a portion of the vertical load-carrying system demonstrated these higher design forces were warranted. The collector element code change was submitted during the 1996 code development cycle by Forell/Elsesser Engineers Inc. (Mark Jokerst). The code change was “approved as revised” by the ICBO Lateral Design Code Development Committee with further amendments submitted by the SEAOC Seismology Committee and approved at ICBO’s 1996 Annual Education and Code Development Conference in St. Paul, Minnesota.

1997 UBC Section 1921.6.12, Diaphragms, was added because topping slabs over precast concrete members, typically intended to be used as the diaphragm to transfer the lateral loads, performed poorly during the Northridge Earthquake. Minimum thickness requirements were added as well as requirements for mechanical connectors used to transfer forces between the diaphragm and the lateral force-resisting system. The diaphragm code change (cited in the table in the online version of this article) was submitted by the California Division of the State Architect (Vilas Mujumdar) and the Portland Cement Association (Mark Kluver). It was approved by the ICBO Lateral Design Code Development Committee without amendments at their meeting in Sparks, Nevada, in February 1996.

Conclusion

The failure of numerous concrete parking garages during the Northridge earthquake highlighted several major issues with the lateral force-resisting systems of these types of structures. In “bare” structures, such as parking garages, the significant lessons learned included the importance of ductile interconnections between the different elements of the lateral force-resisting system, deformation compatibility between “vertical only” elements and the lateral force-resisting system, the importance of designing for ramp and diaphragm discontinuities, and designing non-seismic systems for the full expected seismic drift. Following the 1994 Northridge earthquake, several structural engineering and building code organizations worked together to quickly develop and adopt code modifications to address these issues in future building codes.■

The pdf version of this article includes a Table referencing several changes to the 1997 UBC resulting from the performance of parking garages in the Northridge earthquake.

References

EQE International. The January 17, 1004 Northridge, California Earthquake, An EQE Summary Report. San Francisco: March 1994.

Iverson, James K., and Hawkins, Neil M. Performance of Precast/Prestressed Concrete Building Structures During Northridge Earthquake. PCI Journal, March-April 1994: pp. 38-55.

Mitchell, Denis, DeVall, Ronald H., Saatcioglu, Murat, Simpson, Robert, Tinawi, Rene, and Trembly, Robert. Damage to Concrete Structures Due to the 1994 Northridge Earthquake. Can J. Civ. Eng., Vol. 22, 1995: pp. 361-377.