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Porter 2006 Woodframe nontechnical

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Information about Porter 2006 Woodframe nontechnical
Education

Published on January 22, 2008

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The Value of Seismically Strengthening a House, a Nontechnical Overview :  The Value of Seismically Strengthening a House, a Nontechnical Overview Pasadena Professionals in Real Estate Women’s City Club, Pasadena, CA 10 November 2006 © Keith A. Porter, PE, PhD GW Housner Senior Researcher California Institute of Technology Outline:  Outline Common problems of woodframe buildings Strengthening schemes Estimating the benefit of retrofit Conclusion: is retrofit worthwhile? Warning: there will be one equation… Common seismic problems of woodframe buildings:  Common seismic problems of woodframe buildings Things that have caused problems in past earthquakes Lack of foundation bolts:  Lack of foundation bolts 5/2/83 M6.5 Coalinga 1948 house 1964 Alaska earthquake (tsunami damage in Crescent City, CA) 5/2/83 M6.5 Coalinga 5/2/83 M6.5 Coalinga Cripple wall collapses:  Cripple wall collapses 1989 M7.1Loma Prieta 1983 M6.5 Coalinga 1992 Mendocino 1984 Morgan Hill 1979 Imperial Valley Soft story; tuckunder parking:  Soft story; tuckunder parking 10/17/89 M7.1 Loma Prieta 1/17/94 M6.7 Northridge Northridge Meadows Still an issue: buildings like these often have these conditions:  Still an issue: buildings like these often have these conditions Pasadena Arcadia Altadena Altadena How to reduce damage through structural modifications:  How to reduce damage through structural modifications Strengthening schemes:  Strengthening schemes New steel frames at tuckunder New bolts, bracing, etc. Some other strengthening schemes:  Some other strengthening schemes Add plywood sheathing to soft story Manufactured strongwalls Mechanical energy-dissipation devices Lots of measures for household contents Strap water heater to frame Latches on cabinets Strap tall furniture to frame Shelf lips What do they cost?:  What do they cost? Bolts only: ~$1-2k Add bolts sheath cripple wall : ~$2-6k Add plywood to soft story walls: ~$10-20k Steel braces at garage openings: ~$20-50k That’s the easy question. The hard one is… What are they worth?:  What are they worth? Qualitatively… Reduced future earthquake repair costs Reduced chance of being red-tagged or otherwise uninhabitable after an earthquake Reduced chance of death and injury Peace of mind … but how much? I’ll talk about item 1 today (watch for a hint about the equation) One way to measure whether strengthening is cost-effective:  One way to measure whether strengthening is cost-effective Benefit-cost ratio:  Benefit-cost ratio Benefit: present value of reduced future loss, considering uncertain earthquakes, time value of money, and inflation Cost = up-front cost of construction Benefit-cost ratio (BCR) = Benefit  Cost If BCR > 1, then measure is cost-effective If BCR < 1, then not Federal government has used BCR in regulatory decision-making since 1993; so will we to answer the question, Is seismic retrofit worthwhile?:  Is seismic retrofit worthwhile? Under what conditions is BCR > 1? CUREE-Caltech Woodframe Project:  CUREE-Caltech Woodframe Project CA earthquake costs ~$5B/yr, up to $100B/event Moderate events can be very costly: Northridge: 33 deaths, $20B ppty loss, 48,000 homes Woodframe contribute substantially to risk: 99% of homes, 96% of LA buildings are woodframe CUREE-Caltech Woodframe Project grant $5.2M FEMA grant for testing, fieldwork, code development, economic modeling, education Economic modeling component (small part of the $5.2M) We created mathematical models of 19 particular houses The models use new lab test data, new engineering techniques We calculated & mapped benefits of retrofit & redesign 4 houses, 19 alternatives examined:  4 houses, 19 alternatives examined Small house: 1200 sf, 2 bdrm, 1 ba Large house: 2,400 sf, 3 bdrm, 2½ba Townhouse: 2,000 sf, 3+2 Apartment building : 10 850-sf units Our imaginary study houses look much like these real ones:  Our imaginary study houses look much like these real ones What are the 19 alternatives?:  What are the 19 alternatives? 3 quality levels each house (low/med/high) Stronger stucco, better nailing (not sure what cost) 7 retrofit or redesign options ($300-$30k cost) Small house: brace cripple walls ($1,400*) Large house: different nailing ($300*), more plywood ($7,500k*), different design assumption ($300*) Townhouse: thicker plywood, more nails ($1,700*) Apartment building: steel frames ($29,000*), new shearwalls ($11,000*) * For the study houses in 2002 in LA, not for every house everywhere So how do we estimate benefit?:  So how do we estimate benefit? From past earthquakes (“empirical approach”): See how similar buildings performed in past earthquakes with & without retrofit or design feature Won’t work; not enough data—not even from insurers Expert opinion approaches: A.k.a. old guys sitting around a room and guessing Works, but not very convincing From mathematical models (“analytical approach”): Really hard to do this, but like the saying goes, How do you eat an elephant? One bite at a time. Here are the bites… It’s called “performance-based earthquake engineering”:  It’s called “performance-based earthquake engineering” It means “how we estimate the future seismic performance of buildings and other facilities in terms of repair costs, health impacts, and repair durations (dollars, deaths and downtime)” Step 1: define the building to be studied:  Step 1: define the building to be studied Site, soil Building Step 2: how strongly does the ground shake?:  Step 2: how strongly does the ground shake? Site, soil Building Step 3: how much does the building deform?:  Step 3: how much does the building deform? Site, soil Building Step 4: what gets broken?:  Step 4: what gets broken? Site, soil Building be patient, this is cool … Step 5: what does it cost to fix?:  Step 5: what does it cost to fix? Site, soil Building Step 6: Repeat many times with different assumptions, to account for uncertainty:  Step 6: Repeat many times with different assumptions, to account for uncertainty Site, soil Building Now comes the integral calculus:  Now comes the integral calculus Okay, let’s skip the integral calculus. Briefly, How frequently does the ground shake a little? How much do we save in a little shaking? Repair cost without strengthening minus repair cost with strengthening, given a little shaking How frequently does the ground shake a lot? Etc. Add up frequency x savings, get “annual savings” Pretend annual savings are like an annuity, with a given interest rate and fixed life. The price of that annuity is the “benefit” of the strengthening Divide benefit by strengthening cost, that’s BCR Nice. Did we make all this up ourselves?:  Nice. Did we make all this up ourselves? USGS created the hazard data (how strongly does the ground shake and how often) California Geological Survey compiled the soil data (soil affects shaking locally) Others in CUREE-Caltech Woodframe Project Made up the index buildings Created the mathematical structural models Did the laboratory tests We acquired the necessary data, created & carried out the methodology Here are the hazard & soil data:  Here are the hazard & soil data USGS data used for seismic hazard on reference site coil conditions (Frankel & Leyendecker 2001) California Geological Survey data used to account for soil conditions (Wills et al. 2000) That’s how we ate the elephant. Now how did it taste?:  That’s how we ate the elephant. Now how did it taste? … in other words, where is it likely to be cost-effective to seismically strengthen a house? Brace cripple walls in the small house* Cost: $1400; Benefit: up to $11,000; BCR: up to 8:  Brace cripple walls in the small house* Cost: $1400; Benefit: up to $11,000; BCR: up to 8 * i.e., our small house, not every small house Wood shearwalls on apartment building* Cost: $11,000; Benefit: up to $74,000; BCR: up to 7:  Wood shearwalls on apartment building* Cost: $11,000; Benefit: up to $74,000; BCR: up to 7 * i.e., our apartment building, not every one High-quality construction of apartment building* Cost: unknown; benefit: up to $120,000:  High-quality construction of apartment building* Cost: unknown; benefit: up to $120,000 * i.e., our apartment building, not every one That’s nice, but are the mathematical models any good?:  That’s nice, but are the mathematical models any good? Or, how is this not just old guys sitting around guessing, but using fancy computer models to do it? Validation:  Validation Our model What we know from past earthquakes Shaking intensity,not magnitude. MMI 7 is strong, 9 is really really strong “Mean damage factor” means average repair cost ÷ replacement cost Yes, I said there weren’t enough past earthquake data:  Yes, I said there weren’t enough past earthquake data Our models distinguish houses with and without the strengthening schemes The historical data don’t distinguish these or other features Good mathematical models would all generally pass through the horizontal bars of the past earthquake data We would just expect the strengthened ones to pass through the bars at a different place, suggesting lower losses at the same intensity So what does the validation tell us?:  So what does the validation tell us? The models generally agree with past earthquake experience Not definitive, but tends to support the model If the model is right, then there are many places where houses like the study ones can be cost-effectively strengthened BCRs for other strengthening measures:  BCRs for other strengthening measures Steel frames on apartment building* BCR up to 3, i.e., benefit as high as 3 x cost Cost effective in 120 ZIP Codes (out of 1,650) Large house design alternatives* None are cost effective House was pretty good to begin with Townhouse design alternative* BCR up to 5 Cost-effective in 300 ZIP Codes * Our apartment building, etc., not every one everywhere Is that all there is in terms of benefit?:  Is that all there is in terms of benefit? The foregoing was just about building damage and repair, but there are other earthquake losses Other benefits: reduced red-tag probability:  Other benefits: reduced red-tag probability This chart shows how construction quality or retrofit reduce the chance that the small house, at a particular location, will get red-tagged, either (a) assuming a particular earthquake occurs (“scenario”), or (b) accounting for the chances of various size earthquakes, during a particular number of years (“planning period”). Point is, one can calculate & map this, we just didn’t do it. How earthquakes affect contents:  How earthquakes affect contents Conclusions:  Conclusions State-of-the-art economic analyses of the seismic performance of wood homes New lab data, rigorous uncertainty propagation, validation, peer review Official hazard and soil data Mitigation can be cost-effective Saving up to 8x initial construction cost BCR > 1 tends to be near faults, on softer soil Greater savings considering contents, loss of use, and improved safety For technical details see www.sparisk.com/publications.htm Acknowledgments (who is “we”?):  Acknowledgments (who is “we”?) The research was performed under the supervision of Professors Charles Scawthorn (Kyoto University) and Jim Beck (Caltech), with funding by the Federal Emergency Management Agency, the California Governor’s Office of Emergency Services, and Caltech’s George W. Housner Fund. Tom Boyd, Kelly Cobeen, Robert Reitherman, James Russell, and Hope Seligson provided valuable advice and assistance throughout the project. The research also greatly benefited from the help of Vanessa Camello, Ken Campbell, Ken Compton, André Filiatrault, Bryan Folz, Bill Graf, Hiroshi Isoda, David Johnson, David McCormick, Goetz Schierle, Ed Sylvis, Tom Tobin, and Ray Young. John Hall was the overall Principal Investigator of the CUREE-Caltech Woodframe Project. Many of the photos were provided courtesy of the National Information Service for Earthquake Engineering, University of California, Berkeley. The contributions of all these individuals and organizations are gratefully acknowledged. Thanks for your attention:  Thanks for your attention keith@cohen-porter.net

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