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Seismic Retrofitting Of Older Homes

Seismic Retrofitting Of Older Homes. Adapted from a presentation created by: Howard Cook, Bay Area Retrofit, Oakland CA Thor Matteson, Structural Engineer, Berkeley, CA Danielle Hutchings, Federal Emergency Management Agency (FEMA). Wood-frame Construction: What’s The Risk?.

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Seismic Retrofitting Of Older Homes

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  1. Seismic Retrofitting Of Older Homes Adapted from a presentation created by: Howard Cook, Bay Area Retrofit, Oakland CA Thor Matteson, Structural Engineer, Berkeley, CA Danielle Hutchings, Federal Emergency Management Agency (FEMA)

  2. Wood-frame Construction: What’s The Risk? Times Colonist July 19 “…after the last large earthquake in California there were few reports of structural damage to single-family homes.” NOT TRUE!

  3. Northridge Earthquake1994 • Magnitude 6.7 • Considered “Moderate” • $20 billion in residential damage • most residential damage deemed “preventable”

  4. Northridge Residential Damage

  5. Northridge Residential Damage

  6. Northridge Residential Damage

  7. What Happens During an Earthquake Construction view Forces on the house

  8. What Happens to Cripple Walls House rocks on the cripple walls House is supported on cripple walls Earthquake moves the foundation House falls when cripple wall collapses

  9. Sheathed, Bolted Cripple Wall Collapses

  10. Watsonville, CA, Cripple Wall Collapse

  11. Alaska Earthquake 1964

  12. Every Older House is Unique…Retrofits Are Too • Infinite variety of cripple wall configurations in homes built before building code standards introduced • Design and retrofit installation is unique to each • Retrofit engineering principles make it possible to retrofit a house no matter how it’s built

  13. Load Path of Earthquake Forces Earthquake forces pass through various structural components and their connections as they travel from the floor to the foundation

  14. Failure in Load Path • House fell 6 feet because somewhere the load path from floor to foundation failed • Interior walls, plumbing and electrical systems suffered catastrophic damage • An effective load path would have resulted in house remaining fully habitable Ç√

  15. Interior Damage Despite Intact Appearance Plumbing, wiring, plaster… all severely damaged after 6 foot collapse

  16. The Base Shear Formula • Establishes how much earthquake force the base of a house will be subjected to • Properly retrofit house must be able to resist this force

  17. The Base Shear Formula Retrofit of 2,000 sqft two-story house using the Base Shear Formula

  18. Calculating Plywood and Hardware Quantities • The amount of plywood and hardware used in previous example is calculated by dividing it’s shear value into 8,000 • Shear value of H10 shear transfer tie = 585 • 8,000 / 585 = 14 Simpson Strong Tie Catalogue

  19. The 3 Main Components of a Retrofit • Bolt cripple walls to foundation • Secure cripple walls to the floor joists • Install plywood shear panels

  20. Step 1: Bolting Mudsill to Foundation • Bolting is only of value where walls have been braced with plywood • The number of bolts is calculated to have the same resistance capacity as the plywood

  21. Installed Mudsill Anchoring

  22. Bearing Plates (Plate Washers) Bearing plates increase strength of bolt-to-wood connection SEAOSC Research

  23. Problems With Existing Bolts • Oversized bolt holes are found in most existing bolt installations. • More than 1/16 of an inch larger than diameter of bolt is considered oversized.

  24. Mudsill Splitting • Caused by over-sized holes • Over-sized holes are common because they allow builders to adjust the position of the mudsill

  25. Foundations Anchors for Crawlspaces Foundation plates replace bolts where drill-space is unavailable

  26. Step 2: Secure Cripple Wall to Floor Movement prevented by shear transfer ties

  27. Shear Transfer Ties Installed

  28. Step 3: Install Plywood Shear Panels There are three ways to attach a plywood panel to the mudsill: • Directly to Flush Framing • The Nailed Blocking Method • The Flush Cut Method

  29. 1. Directly to Flush Framing

  30. 2. Nailed Blocking Method 2x4 blocks installed between 2x4 studs on 2x6 mudsill

  31. 3. Flush Cut Method

  32. Nailed Blocking > Wood Splitting • Improper nailing often causes wood splitting • Pneumatically driven staples can be driven closer and reduce splitting at time of driving and when loaded in shear

  33. Plywood Installation Challenge #1 Updated plumbing, wiring and structural modifications

  34. Plywood Installation Challenge #2 Old-fashioned, inconsistent and odd framing

  35. Plywood Installation Challenge #3 Replacing rotted or pest-damaged wood

  36. How Not to Nail Shear Panels

  37. The Finished Look: Cripple Walls Shear walls with finished-to-match panels between

  38. The Finished Look: Crawlspace Anchor plates and shear transfer ties house to foundation

  39. The End For more informationplease contact Marney MutchTel: 250-590-5785Mmarney@ShearSeismic.com Howard CookTel: 510-418-1676BayAreaRetrofit@aol.com

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