NORTHRIDGE EARTHQUAKE: STATISTICAL ANALYSIS OF RESIDENTIAL WOODFRAME DAMAGE. G. G. Schierle 1

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2 NORTHRIDGE EARTHQUAKE: STATISTICAL ANALYSIS OF RESIDENTIAL WOODFRAME DAMAGE G. G. Schierle 1 Abstract Key findings of Statistical Analysis of Field Investigations of the CUREE- Caltech Woodframe Project of the Northridge Earthquake are presented. The study of residential wood structures is based on electronic files of the City of Los Angeles of damaged and existing buildings. The findings are mostly relative, correlating damage to the existing building stock, presented in graphs, maps, and tables; graphs and maps for correlations and tables for numeric backup. Due to space limitations, this essay includes only graphs and maps. Recommendations are also included. Introduction The statistical study of residential wood structures damaged in the Northridge Earthquake is based on electronic files of the Department of Building and Safety of the City of Los Angeles. The research included six studies: General Study (all red and yellow-tagged damaged residential woodframe buildings) Sample Study (random sample of 1230 projects for detailed study) Focus Study: Apartments with Tuck-under Parking Focus Study: Expensive Repair (projects estimated at 60% or more damage) Focus Study: Demolished Buildings NAHB Study Follow-up All studies correlated 18 categories: 2 Types (single family [SFD] and multifamily [MFD] dwellings) 3 Vintages (built 0-40, 41-76, 77-93) 3 Intensities (< 30, 30-60, >60 percent PGA [Peak Ground Acceleration]) Methodology The Field Investigations are for residential woodframe buildings in the City of Los Angeles. The limitation to LA is because the Los Angeles Department of Building and Safety has the most extensive electronic file of buildings damaged in the Northridge Earthquake, as determined by studying available data in cities affected by the Earthquake (Table 1). For convenience, the electronic file of damaged buildings will be referred to as damage file in this essay. The city of Los Angeles also made available an electronic file of existing buildings, referred to as existing file. The existing file was used to determine damage as percent of existing, considered more relevant to measure performance than numbers damaged. Correlating the damage and existing files required some adjustment since condominiums are listed as MFD in the damage file but SFD in the existing file. This essay considers condominiums as MFD. 1 Professor of Architecture, University of Southern California Los Angeles, CA

3 Table 1. City Data Files of Northridge Earthquake Damage Column index A B F H I J K L M N O P Q R S T U V W X Y LA County E E E E E E E E Los Angeles E E E E E E E E E E E E E E E E E E E E E Beverly Hills E E E E P P E Burbank M M M M M M M M M M M M M M Glendale n o d a t a a v a i l a B l e San Fernando i n c l u d e d i n L A C o u n t y Santa Clarita E E E E E E E Santa Monica E E E E E E Ventura County P P P Calabasas E E E E E E E Fillmore P P P P P P P Simi Valley P P P P P P P P P E = electronic file; M = microfiche; P = paper file Column index: O Percent damage A Record number P Repair cost B Street number Q vacate building (P = partial, T = total) F Street name R Permit required H Number of units S Posted (green, yellow, red) I Council district T Number of stories J Damage type (EQ, fir, flood, etc.) U Type of Construction (I, II, III, IV, V, URM) K Use (R = residential) V Approximate building dimensions L Occupancy W Zip Code M Number of dwelling units X Year built N Number of units vacated Y Description of repair job Key Findings 1. MFD built after 1976 had a greater percentage damaged than older ones; but SFD built after 1976 had a smaller percentage damaged than older ones (Fig. 1). 2. Apartments with tuck-under parking confirm the above, with repair costs of newer buildings about double the cost of older ones (Fig. 3). 3. MFD built after 1976 have the highest repair cost (Fig. 5). 4. MFD built have the highest percentage vacated. 5. Apartments with tuck-under parking built have the most vacated units 6. As percent of existing, most demolished SFD and MFD were built (Fig. 4) 7. The above findings, and similar findings by others (EQE, 1995a, 6-10, 6-12), are contrary to what should be expected, considering that 1976 UBC changes require more conservative seismic designs. The poor performance of new MFD may be due to less construction observations by design professionals since the increase of lawsuits in the mid 1970s resulted in reduced quality control.

4 8. 9. As percent of existing, most demolished SFD had two stories (Fig. 6). Their resonant periods of about 0.2 seconds correlate with the Northridge Earthquake dominant ground period of about 0.2 seconds and may be a major factor for their failure. 10. Damage patterns correlate better with intensity of PGA (Peak Ground Acceleration) than PGV (Peak Ground Velocity). While this differs from other studies (Boatwright et al, 1994), it is compelling in Figures 7 and 8. The respective map of existing SFD has uniform distribution. Hence, damage patterns are defined by seismic intensity, rather than existing SFD density. This may be due to the period and stiffness of low-rise woodframe buildings. 11. There are large discrepancies between rapid screening estimates of repair costs and damage ranges vs. cost recorded of building permits. Maybe some building owners under-reported repair costs to avoid property tax increases. 12. Apartment buildings with tuck-under parking, supported by pin columns cost over five times more to repair than those constructed with moment frame and over 7 times more compared to moment frames built at the same time period of Of 122 demolished SFD, 72 (59%) are on hillsides, likely rock sites; while only 16% of existing SFD are on rock sites (EQE, 1995a) Of 122 demolished SFD, 107 (88%) were red-tagged (Table 6-1) Of 45 demolished MFD, 14 (31%) are on hillsides, likely rock sites; while only 8% of existing MFD are on rock sites (EQE, 1995a) Of 45 demolished MFD, 38 (84%) were red-tagged (Table 6-2) Of 60 vacated SFD, 43 (72%) were red tagged (table 5-1). 18. Of 34 vacated MFD, 30 (88%) were red-tagged (Table 5.2). 5. Footings are the most costly items to repair of SFD (Graph 3-4b, 5-4b) Chimneys are most frequently damaged items in SFD built before Non-structural items are most frequently damaged in expensive repair SFD and in the NAHB Study Follow-up Shear walls are the most frequently damaged and the most costly item to repair at MFD 2. 1 While the EQE data includes other jurisdictions, it is deemed similar to Los Angeles City. 2 Building replacement and earthquake general repair are not considered in these conclusions.

5 Figure 1. Damaged Projects as Percent of Existing (General Study) Figure 2. Expensive repair projects as percent of existing

6 Figure 3. Repair cost of MFD with tuck-under parking by year built Figure 4. Demolished projects as percent of existing

7 Figure 5. Average repair cost by PGA and year built (NAHB study follow-up) Figure 6. Demolished SFD by number of stories and year built

8 Figure 7. Damaged SFD built on PGA contours Figure 8.Damaged SFD built on PGV contours

9 Recommendations Given the focus of this study on Los Angels City, with the most comprehensive electronic data file of buildings damaged in the 1994 Northridge Earthquake, the following recommendations refer to the LA experience but are recommended for other jurisdictions as well. For some issues, design guidelines are recommended instead of code provisions. Design guidelines are expected to encourage good design practice, while code provisions may be perceived to restrict creative options. The objective is to engage the entire building community in seismic design issues. 1. Rapid Screening should record structural material. Type of construction defined by UBC is not sufficient since type V, for example, may be any material. This would not only benefit future statistical investigations of specific technologies, such as this Woodframe Project; but also identify buildings at risk, such as steel frame failures of the Northridge Earthquake. 2. Rapid Screening should record structural material at different levels, such as wood structures over concrete parking. 3. Repair permits should, if possible, replace the category Earthquake General Repair by itemized repair and associated costs, to help refining design guidelines and code provisions. 4. Research should study the effect of resonant building period matching the site period. 5. Research should study the correlation of PGA and PGV with damage of diverse structures. 6. Research should study effects of decoupling finishes by seismic joints to reduce damage. 7. Research should study SFD footing design options to reduce earthquake repair costs. 8. Research should study chimney design to reduce frequent earthquake damage. 9. Building codes should upgrade shear wall design, the most costly item to repair in MFD. 10. Building codes should upgrade footing design, the most costly item to repair in SFD. 11. Building codes should upgrade design of frequently damaged nonstructural items. 12. Building codes should require designers to observe construction to improve quality control. 13. Design guidelines should recommend seismic joints at intersections H, L, T, U and similar configurations, to prevent diaphragm failure like those of the Northridge Meadows building (Arnold and Reitherman, 1982). 14. Design guidelines should recommend attaching beams to posts with post caps and to pilasters and walls with metal anchors to prevent slippage under seismic forces. 15. Design guidelines should recommend replacing pin-joint columns with moment resisting frames for buildings with tuck-under parking. 16. Design guidelines should recommend adapting hillside buildings to the site to minimize cutand-fill that causes earthquake damage due to differential grading settlement as demonstrated by the high percentage of damaged hillside structures and findings of Prof. John Stuart s study of 600 homes damaged during the Northridge Earthquake. This design policy provides better earthquake performance. Several hundred such units designed by the author with vertically aligned shear walls, survived major earthquakes unharmed (Fig. 9).

10 To avoid expensive earthquake settlement repairs adapt building to site instead of adapting site to building Figure 9. Adapt building to site instead of adapting site to building

11 Acknowledgements The research of this essay was part of the CUREE-Caltech Woodframe project, Earthquake Hazard Mitigation of Woodframe Construction; funded by the Federal Emergency Management Agency (FEMA) of a Hazard Mitigation Grant Program award - administered by the California Governor s Office of Emergency Services (OES). The author likes to thank: Professor John Hall, Caltech, Project Manager, Robert Reitherman, Executive Director, CUREE, Project Director, the Woodframe Project Advisory Committee; Andrew Adelman, Manager and Nick Deli Quadri, Manager of Engineering, Department of Building and Safety, City of Los Angeles. Disclaimer This research is a public service by the California Institute of Technology and the Consortium of Universities for Research in Earthquake Engineering. No liability for the accuracy of this study is assumed by them, by the University of Southern California, the research team, the Federal Emergency Management Agency, and the California Governor s Office of Emergency Services Bibliography Arnold, C, Reitherman, R, 1982, Building Configuration and Seismic Design, John Wiley Boatwright, J, Thywissen, K, Seekins, L, 1994, The Correlation of Ground Motion and Intensity for the Jan.17, 1994, Northridge, CA Earthquake, USGS Schierle, G G, 2001, Northridge Earthquake Field Investigations: Statistical Analysis of Woodframe Damage, Consortium of Universities in Earthquake Engineering, Report W-02 Schierle, G G, 2001, Northridge Earthquake Field Investigations: Case Studies of Woodframe Damage, Consortium of Universities in Earthquake Engineering, Report W-04 EQE, 1995, Part A, The Northridge Earthquake of January 17, 1994: Report of Data Collection and Analysis Part A: Damage and Inventory Data, California OES