Download - Seismic Retrofitting Of Older Homes
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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)
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Wood-frame Construction: What’s The Risk?
“…after the last large earthquake
in California there were few
reports of structural damage to
single-family homes.”
NOT TRUE!
Times Colonist July 19
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Northridge Earthquake
1994
• Magnitude 6.7
• Considered “Moderate”
• $20 billion in residential damage
• most residential damage deemed “preventable”
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Northridge Residential Damage
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Northridge Residential Damage
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Northridge Residential Damage
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What Happens During an Earthquake
Construction view
Forces on the house
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What Happens to Cripple Walls
House is supportedon cripple walls
Earthquake movesthe foundation
House rocks on the cripple walls House falls when
cripple wall collapses
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Sheathed, Bolted Cripple Wall Collapses
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Watsonville, CA, Cripple Wall Collapse
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Alaska Earthquake 1964
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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
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Load Path of Earthquake Forces
Earthquake forces pass through various structural components and their connections as they travel from the floor to the foundation
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Ç√
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
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Interior Damage Despite Intact Appearance
Plumbing, wiring, plaster… all severely damaged after 6 foot collapse
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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
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The Base Shear Formula
Plywood bracing 14 each side 8,400# $3,6401/2" bolts w/MSP's 7 each side 9,345# 1,820L90, H10R, LTP4or homemade STT's 14 each side 8,400# 1,400
TOTAL COST $6,860
Retrofit of 2,000 sq ft two-story house using
the Base Shear Formula
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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
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The 3 Main Components of a Retrofit
1. Bolt cripple walls to foundation
2. Secure cripple walls to the floor joists
3. Install plywood shear panels
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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
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Installed Mudsill Anchoring
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Bearing Plates (Plate Washers)
Bearing plates increase strength of bolt-to-wood
connection SEAOSC Research
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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.
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Mudsill Splitting
• Caused by over-sized holes
• Over-sized holes are common because they allow builders to adjust the position of the mudsill
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Foundations Anchors for Crawlspaces
Foundation plates replace bolts where drill-space is unavailable
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Step 2: Secure Cripple Wall to Floor
Movement prevented by shear transfer ties
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Shear Transfer Ties Installed
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There are three ways to attach a plywood panel to the mudsill:
• Directly to Flush Framing
• The Nailed Blocking Method
• The Flush Cut Method
Step 3: Install Plywood Shear Panels
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1. Directly to Flush Framing
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2. Nailed Blocking Method
2x4 blocks installed between 2x4 studs on 2x6 mudsill
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3. Flush Cut Method
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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
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Plywood Installation Challenge #1
Updated plumbing, wiring and structural modifications
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Plywood Installation Challenge #2
Old-fashioned, inconsistent and odd framing
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Plywood Installation Challenge #3
Replacing rotted or pest-damaged wood
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How Not to Nail Shear Panels
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The Finished Look: Cripple Walls
Shear walls with finished-to-match panels between
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The Finished Look: Crawlspace
Anchor plates and shear transfer ties house to foundation
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The End
For more information please contact
Marney MutchTel: 250-590-5785
Howard CookTel: 510-418-1676