SAC Publications

Report No. SAC 95-06

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Surveys and Assessment of Damage to Buildings Affected by the Northridge Earthquake of January 17, 1994, by D. Bonowitz, M. Durkin, W. Gates, M. Morden, and N. Youssef, December, 1995.

Executive Summary

This volume includes the technical reports documenting data collection, assessment and interpretation of damage to steel moment resisting frame buildings in the January 17, 1994 Northridge Earthquake. These studies comprise Task 2.1, 2.2, and 2.3 of Phase I of the SAC Steel Project. The goals of these tasks were to gather quantitative information related to both damaged and undamaged steel buildings in the area of strong shaking and to assess this information to provide insight and identify trends that would be of use in the development of the SAC Interim Guidelines and standards of practice. These tasks included the following projects:
  1. A regional survey of steel building performance to identify the number and distribution of damaged and undamaged buildings and general information related to the size, age, level of damage, status of any repairs, etc.
  2. A detailed survey of a smaller sample of buildings that focused on the type and distribution of damage within the subject structures
  3. A series of interviews to gather and synthesize experience related to inspection, evaluation, and repair of damaged buildings by various participants in the process
  4. Assessment and interpretation of the data collected. A series of regional maps were produced by the California Office of Emergency Services GIS Office from the data collected in these tasks.
Between late January and early March of 1995, Durkin and Associates performed a regional survey to estimate the prevalence of steel frame building structural inspection, damage and repair in regions where the peak ground acceleration exceeded 0.20 g. A total of 163 buildings were selected at random from a list of 1284 candidate buildings. A standardized questionnaire was used in the collection of data via telephone interviews with the building owner, manager and/or engineer, The data collected was then analyzed and evaluated. Locational information was provided for use in regional mapping efforts. The survey indicated that eighty-three per cent of the buildings had visual inspections performed, but only thirty-eight per cent had intrusive inspections, and only eighteen per cent included ultrasonic testing as part of the inspection. While only thirteen per cent of the buildings reported having steel connection damage, such damage was found in sixty-six per cent of the buildings that had ultrasonic inspection of a number of connections. Steel frame damage was concentrated in buildings between 200,000 and 300,000 square feet in area, less than five stories or between eleven and twenty stories in height, and constructed in the 1980's. The majority of the damaged buildings in the survey were located ley or Santa Monica. A relatively high proportion of the damaged in the San Fernando Val buildings were exposed to peak ground accelerations estimated to between 0.25 g and 0.35 g and p@ velocities of 20 to 35 cm/sec. Repair work had been completed in over half of the damaged buildings at the time of the survey, and work was in progress in the remaining structures. While uncertainties over proposed inspection ordinances delayed in-depth inspections, other external forces such as lenders, insurers and out-of-state owners sometimes required such investigations.

Nabih Youssef and Associates collected detailed survey and inspection damage of eighty-nine steel frame buildings. This total includes the results from a previous damage survey that was initiated and funded by the National Institute of Standards and Technology. Detailed connection damage data was included for eight buildings that were the subject of detailed analytical investigation in Task 3.1 of the SAC Steel Project. Analysis of the data on a statistical basis was performed in an attempt to identify trends and potential damage and performance predictors. About twenty per cent of the buildings surveyed reported no damage, and thirty per cent reported only weld damage. In the eight case study buildings, over seventy per cent of the connections were undamaged, and seventeen per cent had weld damage only, half of which was limited to root cracking that was only detectable through ultrasonic inspection. Weld only cracking was noted three times more frequently dian fractures that included base metal. Bottom flange weld cracking was reported in three times as many cases as top flange fractures, but it should be noted that inspection of the top flange was much less frequent than at the bottom flange. Also note that top flange cracking was not found more frequently in buildings with wood floor diaphragms, indicating that composite beam behavior does not completely explain the prevalence of bottom flange fractures. Frame redundancy, as evidenced by the floor area per moment connection, appears to be a damage predictor. While connection damage appears to be related to ground motion parameters such as peak ground acceleration or velocity, a lack of data at the various levels makes this conclusion unable to confirm statistically. The survey indicated that in low-rise buildings, damage is likely to be concentrated in the lower floors, while in buildings over eighteen stories tall, very little damage was found in the lower third of the building stories. A strong directionality effect of the damaged buildings was noted for buildings in the San Fernando Valley. Two patterns of connection damage were found in this survey, one that appeared to be related to the quality of construction, and another that is related to structural demand. Root weld damage appears to be unrelated to structural demand parameters, while other forms of weld and base metal cracking showed limited, but definite correlation to demand. This observation was borne out by evaluation of various analytical predictors for the group of case study buildings, although each individual building did not follow this pattern.

Gates and Morden conducted a series of interviews with key professionals involved in the inspection, evaluation and repair of damaged steel buildings. Structural engineers, testing and inspection agencies, contractors, and building officials from the City of Los Angeles were selected as participants. Eleven interviews were conducted, with a total of thirty-four participants. The interviews were intended to extract significant experiences of the various participants that would be useful in the development of the Interim Guidelines, and for other professionals that may face similar challenges after future earthquakes. Individual theories and opinions as the causes of the connection damage were solicited, as were specific recommendations on improvements to future practice in the design, inspection, construction, and repair of this class of structures. It should be noted that many of the conclusions from these interviews were consistent with those drawn from evaluation of the data collected by Nabih Youssef and Associates. Additional results from the interviews included the following:

  1. The brittle mode of failure was not anticipated by any of the engineers and the fractures have eroded their confidence in the performance of the SMRF system relative to other structural systems
  2. The knowledge of practicing structural engineers related to metallurgy and welding issues is seriously lacking
  3. Many possible causes were cited for the connection fractures, such as triaxial stress states, stress risers due to backing bars, stress concentration factors at the center of weld, lack of correlation between previous laboratory testing and field applications of these connections, impulsive and/or dynamic loading effects, and/or construction quality
  4. Welding procedures and standards need to be improved in critical applications
  5. The range of cost for connection inspection was from $800 to $1200, unless asbestos fireproofing was present
  6. Gravity frame connection damage was also prevalent in buildings with damaged moment connections
  7. Individual engineering judgment guided repair and retrofit processes, with varying solutions resulted
  8. Repair work does not include any modification of the connection in the vast majority of cases
  9. Costs for repair range from $3,000 to $20,000 per connection, with most cases in the $5,000 to $8,000 range
  10. Other costs, such as business interruption and lost rental income, often greatly exceed the repair costs.
The final report in this volume is a brief interpretation of the data collected in the other projects in Task 2. This report, by Gates and Morden, also identifies areas of uncertainty in the data, and makes recommendations for future data collection and analytical investigations. Interpretation of the regional maps developed for this project indicate that the north-central portion of the San Fernando Valley experienced the most severe shaking during the Northridge Earthquake. All of the steel moment frame buildings in this area that were included in the surveys experienced significant damage. In the surveyed buildings, no damage was found where the peak ground acceleration was less than 0.24g and the peak ground velocity was below 15 cm/sec. A number of buildings in the range of ground motions just above these lower bounds experienced only root weld cracking. Approximately two-thirds of the buildings that experienced moment frame connection damage also experienced damage to the gravity frame connections. Increased damage was also noted with increased member sizes. The following areas for future possible future work were identified:
  1. Expansion of the existing data base to include all of the buildings to be included in the City of Los Angeles Ordinance
  2. Collection of data on a connection basis for all buildings
  3. Examination of the influence of foundation flexibility on frame performance
  4. A more rigorous statistical evaluation of the existing data
  5. Examination of the effects of soil stiffness
  6. Evaluation of the relative merits of various ground motion parameters as response predictors.
As a quality assurance measure, all SAC Steel Project Investigations are overseen by a Technical Advisory Panel (TAP). The panel for the Task 3 investigations included practicing engineers experienced in the inspection, evaluation and repair of damaged steel buildings and researchers with expertise in steel building performance in earthquakes and statistical data interpretation.

Note regarding fold-out maps produced by the California Office of Emergency Services.
The six fold-out maps included in this report contain plots of building locations, indications of the extent of damage, and contours of peak ground acceleration, velocity and velocity response. The smoothed contours are for average alluvial site conditions and do not consider site-specific effects. The contours are generated from a grid of values, whose basic estimates come from an empirical attenuation relation fitting Northridge data. The basic estimates are modified by an interpretation of any nearby strong-motion recordings, permitting soil or rock sites to be taken into account. The extent of the modification depends on the proximity of the recording and the relative confidence held in the recording compared to the attenuation relation. Contouring uses a standard algorithm, including smoothing, which itself depends further on the grid size and the smoothing distance. Given that the generation of the contours involves these individual judgments, there is substantial uncertainty in the ground motion estimate at a specific site. Furthermore, it is likely that contours developed by others would be different from these. However, sound scientific and engineering judgment could be relied on to produce similar and satisfactory contours.

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