Revisiting Earthquake Lessons – Nonductile Concrete
Thursday, June 6, 2019
Author: Keith D. Palmer, PhD, SE
This is one in a series of short articles revisiting lessons learned in past earthquakes. This series is being presented to share past lessons with newer members of our profession who were not there to experience them first hand. Contact Dave McCormick or Kelly Cobeen with your comments and suggestions.
Structure Type: Nonductile Concrete (NDC) Structures
Earthquakes: 1971 San Fernando (Sylmar)
Lessons:The Richter magnitude 6.7 San Fernando Earthquake occurred on the morning of February 9, 1971 in the San Fernando Valley north of the city of Los Angeles. Also known as the Sylmar Earthquake, it caused 65 deaths, 2500 injuries and an estimated $550 million in damage (approximately $3.5 billion in today’s dollars1). Most of the deaths occurred at two sites: The Olive View Medical Center and the Veterans Administration Hospital in Sylmar (Figures 1 and 2). Each site experienced considerable damage, including complete collapse of concrete buildings on their campus. The damage and collapses occurring on the VA campus were not unexpected as most of those buildings were designed in the 1920s, before the concept of ductile behavior and irregularities was understood or considered. However, Olive View was a wake-up call as the damaged and collapsed buildings were relatively new and designed according to the 1965 Los Angeles County Building Code2 (similar to the 1964 Uniform Building Code (UBC3)) though in retrospect, it wasn’t surprising given that ductile detailing requirements had not yet made it into the code in 1965. Many more instances of damage occurred in numerous other concrete buildings in the surrounding area. Four hospitals were severely damaged and had to be evacuated, placing significant strain on the available medical care in the area following the earthquake. There were multiple reasons for the damage and collapses but most can be attributed to non-ductile detailing of the columns, beams and beam-column joints, and irregularities that were not prohibited or discouraged by the building codes. Even though many of the deficiencies and reasons for damage and collapse were understood at the time of the earthquake, much of this understanding had not been included in the building codes except for buildings taller than 160 feet. The San Fernando Earthquake provided motivation for adoption of this understanding and implementation into the subsequent editions of the UBC and the SEAOC Blue Book4. Additionally, it was recognized that hospitals and other essential facilities that are necessary to remain functional after the earthquake must be designed and constructed to higher standards to ensure this functionality.
Historic Background: The great 1906 earthquake prompted the City of San Francisco to include earthquake design load requirements for buildings. The publication of the first edition of the UBC in 1927 following the 1927 Santa Barbara earthquake included additional requirements for seismic design of buildings but these were relegated to an appendix and were based mainly on observations made during previous earthquakes. No significant earthquakes except the 1933 Long Beach earthquake occurred in the years following the Santa Barbara earthquake and earthquake requirements changed very little until the 1960s except for limitations on unreinforced masonry buildings and the introduction of zones in the 1937 edition. Meanwhile, construction of a large number of concrete buildings occurred during this period.
In July 1959 the SEAOC Seismology Committee published the first edition of the Blue Book, officially titled Recommended Lateral Force Requirements4. This “code” was the first to formalize the relationship between earthquake demands, building period and the ductility of the lateral-force-resisting-system. The document also included recommendations for the distribution of the shear over the building height, distribution to elements according to their stiffness and accidental eccentricity for torsion. Preference was given to moment-resisting space frames for lateral resistance relative to bearing walls through the use of a lower “K” factor which can be thought of as similar to the inverse of the “R” factor in ASCE 75. Meanwhile, researchers and practitioners were beginning to understand the advantages of ductile behavior and began testing and quantifying ways to provide this in concrete structures. In 1961, Blume, Newmark and Corning published Reinforced Concrete Buildings for Earthquake Motions6. In addition to describing building dynamics and principles of earthquake-resistant design, the book provided design methods and detailing principles for ensuring ductile behavior such as maximum allowable steel percentages, providing closely-spaced closed ties in column and beams, and providing continuous top and bottom steel for stress reversals. Unfortunately, these concepts of a ductile moment resisting frame did not find their way into codes until the 1967 UBC. However, these frames were required only for buildings greater than 160 ft. in height. These provisions required smaller tie and stirrup spacing along the lengths of moment frame columns and beams, respectively and special transverse joint reinforcement.
The 1971 San Fernando Earthquake laid bare the deficiencies of the current building codes and highlighted the need for much faster inclusion of new knowledge into the codes. The collapse and partial-collapse of buildings on the Olive View campus could possibly have been prevented were this not the case. The Medical Treatment and Care Unit Building and the Psychiatric Unit were found to be compliant with the code at the time of design (1965 Los Angeles County Building Code2). The irreparable damage to the Treatment and Care Unit occurred because the first and second floor lateral system comprised a moment-resisting frame while the lateral system of the four stories above comprised reinforced concrete shear walls. This resulted in a soft and weak-story resulting in a residual 10 to 15% story drift of the first story above grade. Additionally, many of the columns used ties at large spacings which failed in shear and caused complete failure of the core concrete which was not confined (Figure 3). These columns lost all gravity load carrying ability. Thankfully, several columns were detailed with closely-spaced spirals and provided enough strength after failure of the tied columns to prevent collapse (Figure 4). The concrete shells outside the confined cores of the spirally-reinforced columns spalled off, but the cores remained intact. Additionally, confinement and shear reinforcement were not provided through all beam-column joints and this caused significant distress in numerous locations. Figure 5 compares reinforcement detailing of non-ductile and ductile columns. Prior to the 1967 UBC, there were no stirrup requirements and columns were oftentimes detailed according to Figure 5a. The 1967 UBC limited the stirrup and supplementary ties to 4 in. and the 1973 UBC added the requirement that the spacing of laterally supported longitudinal bars not exceed 14 in (Figure 5b). Additionally, 135- or 180-degeree hooks were required to prevent “unraveling.” Figure 5c shows current ACI 3189 requirements which are very similar to the 1973 UBC requirements except that the maximum spacing of 4 in. has been relaxed to 6 in. if hx is less than or equal to 8 in.
The damage described above was not limited to Olive View but occurred in numerous buildings in the Valley and bordering areas and included many construction types other than nonductile concrete. Immediately following the earthquake, SEAOC in conjunction with EERI performed large-scale reconnaissance and studies and memorialized the results in numerous publications7,8.
Recommendations Following the Earthquake: The reconnaissance and studies by the structural engineering community resulted in several recommendations. Some of these recommendations included information already understood by engineers but were slow to be included in the codes3,4.
- Increase specified code values of ground motion parameters. Recorded horizontal and vertical ground motions during the San Fernando earthquake were much higher than the current codes had anticipated.
- Provide adequate ductility in all concrete members, not only those in the lateral-force-resisting system. All members should be designed to sustain actual displacements expected during an earthquake.
- Design all members for shear based on the ultimate moment capacities at the ends of the members. Moment capacities should be calculated based not on nominal values but on expected values.
- Design all members for reversal of stress that occur due to the cyclic nature of buildings during an earthquake. Continuous bottom and top reinforcement should be provided and anchored or developed to develop the strength of the reinforcement.
- Provide adequate ties and spacing to provide confinement of the core.
- Require ductile frames and walls in all buildings, not just those greater than 160 feet in height.
- Critical facilities should be designed to remain functional following an earthquake. These facilities include hospitals, emergency services, utilities, communications and transportation networks.
- A change is story stiffness or strength should require a certain specified increase in strength and ductility.
Many of the above recommendations found their way into the 1973 and 1976 UBC. 1973 saw the addition of the requirement of designing framing elements not part of the lateral system for moment due to four times the distortion resulting from code-required lateral forces. There was a significant increase in base shear and the addition of a fourth Zone in the 1976 UBC. Additionally, an occupancy importance factor was added in the 1976 UBC that increased the demands on essential facilities and buildings with large occupancies (greater than 300 people in one room). The 1976 UBC is considered to be the first code to provide seismic resistance similar to the current code for reinforced concrete buildings. Given the lag in construction year relative to design year, the benchmark that most engineers use for determining if a concrete building is nonductile is 1980. However, it should not be assumed that this is always the case. It is possible that some buildings built after 1980 may have been designed per the 1973 UBC.
Current Status. The ASCE 75 and ACI 3189 standards contain the requirements for concrete structures designed for seismic resistance. The information provided therein represents a vast body of knowledge gained through observations after earthquake and theoretical and experimental research performed at universities. These standards have been tested in recent earthquakes (e.g. 1989 Loma Prieta and 1994 Northridge) and have demonstrated adequate performance of modern reinforced concrete buildings. There were some exceptions including: flat slab buildings that exhibited two-way flexure/shear damage at the columns (but the ductile shear walls and moment frames in these buildings performed as intended), failure of gravity columns attributed to large drifts and captive columns, precast concrete failures at connections and chord failure due to thin topping slabs. There are still thousands of pre-1980 nonductile concrete buildings in high seismic regions in the U.S. and abroad. The California Seismic Safety Commission estimates that there are 40,000 in California10. The SEAOC Existing Buildings Committee, in cooperation with EpiCenter recently completed an inventory based on all Sanborn maps for the City of San Francisco that resulted in an estimate of 3,400 pre-1980 concrete buildings. This study verified the estimate calculated by the Concrete Coalition11. The risk of these buildings has not been quantified and is difficult to do given the variability of building configurations, system types and oftentimes lack of drawings. Methodologies to determine the risk of these buildings include ASCE 4112 Tiers 1, 2 and 3 and the recently-developed ATC 7813 methodology for ranking buildings in an inventory. Several Southern California cities have recently adopted ordinances that require owners to assess the collapse potential of their older concrete buildings and retrofit these if the assessment deems this necessary. These cities include the City of Los Angeles, West Hollywood and Santa Monica. San Francisco is currently deciding what to do about the nonductile concrete building stock in the city. We currently have the knowledge to design concrete buildings safely. Hopefully we will be able to retrofit our current stock of nonductile concrete buildings in an economical way before the next big one hits.
1. Bureau of Labor Statistics: https://data.bls.gov/cgi-bin/cpicalc.pl
2. County of Los Angeles. Dept. of County Engineer. Building and Safety Division. County of Los Angeles: Official Compilation. Los Angeles: Building News, 1965.
3. Uniform Building Code, various years, International Conference of Building Officials. Whittier, CA.
4. SEAOC, Recommended Lateral Force Requirements, various years, Structural Engineers Association of California, San Francisco, CA.
5. ASCE, Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7, American Society of Civil Engineers, Reston, VA.
6. Blume, J.A., Newmark, N.M., Corning, L.H. Reinforced Concrete Buildings for Earthquake Motions, 1961. Portland Cement Association, Chicago, IL.
7. NOAA, San Fernando, California, Earthquake of February 9, 1971. National Oceanic and Atmospheric Administration, U.S. Department of Commerce, Washington, D.C, 1973.
8. BRD, Engineering Aspects of the 1971 San Fernando Earthquake, Building Research Division, Institute for Applied Technology, National Bureau of Standards, Department of Commerce, Washington, D.C. 1971.
9. ACI, Building Code Requirements for Structural Concrete, ACI 318-14. American Concrete Institute, Farmington Hills, MI.
10. OES, State of California Multi-Hazard Mitigation Plan. 2018. California Governor’s Office of Emergency Services. Mather, CA.
11. Kadysiewski, S. Inventory of Concrete Buildings in San Francisco. Concrete Coalition 2010.
12. ASCE, Seismic Evaluation and Retrofit of Existing Buildings, ASCE/SEI 41-17, American Society of Civil Engineers, Reston, VA.
13. ATC 78, Seismic Evaluation of Older Concrete Frame, Frame-Wall, and Bearing Wall Buildings for Collapse Potential. In Press. Applied Technology Council, Redwood City, CA.
Figure 1. Olive View Hospital (Source: NISEE-PEER, University of California, Berkeley)
Figure 2. Veteran’s Administration Hospital (Source: NISEE-PEER, University of California, Berkeley)
Figure 3. Olive View Hospital – failed tied column. (Source: NISEE-PEER, University of California, Berkeley)
Figure 4. Olive View Hospital – spalled spiral column supporting gravity load due to intact confined core. (Source: NISEE-PEER, University of California, Berkeley)
Figure 5. Concrete column hoop and supplementary ties requirements: (a) no requirements prior to 1967 UBC; (b) 1967 and 1973 UBC requirements; (c) Modern ACI 318 requirements