seismic retrofitting of older homes
DESCRIPTION
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?. - PowerPoint PPT PresentationTRANSCRIPT
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?
“…after the last large earthquake
in California there were few
reports of structural damage to
single-family homes.”
NOT TRUE!
Times Colonist July 19
Northridge Earthquake
1994
• Magnitude 6.7
• Considered “Moderate”
• $20 billion in residential damage
• most residential damage deemed “preventable”
Northridge Residential Damage
Northridge Residential Damage
Northridge Residential Damage
What Happens During an Earthquake
Construction view
Forces on the house
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
Sheathed, Bolted Cripple Wall Collapses
Watsonville, CA, Cripple Wall Collapse
Alaska Earthquake 1964
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
Load Path of Earthquake Forces
Earthquake forces pass through various structural components and their connections as they travel from the floor to the foundation
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
Interior Damage Despite Intact Appearance
Plumbing, wiring, plaster… all severely damaged after 6 foot collapse
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
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
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
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
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
Installed Mudsill Anchoring
Bearing Plates (Plate Washers)
Bearing plates increase strength of bolt-to-wood
connection SEAOSC Research
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.
Mudsill Splitting
• Caused by over-sized holes
• Over-sized holes are common because they allow builders to adjust the position of the mudsill
Foundations Anchors for Crawlspaces
Foundation plates replace bolts where drill-space is unavailable
Step 2: Secure Cripple Wall to Floor
Movement prevented by shear transfer ties
Shear Transfer Ties Installed
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
1. Directly to Flush Framing
2. Nailed Blocking Method
2x4 blocks installed between 2x4 studs on 2x6 mudsill
3. Flush Cut Method
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
Plywood Installation Challenge #1
Updated plumbing, wiring and structural modifications
Plywood Installation Challenge #2
Old-fashioned, inconsistent and odd framing
Plywood Installation Challenge #3
Replacing rotted or pest-damaged wood
How Not to Nail Shear Panels
The Finished Look: Cripple Walls
Shear walls with finished-to-match panels between
The Finished Look: Crawlspace
Anchor plates and shear transfer ties house to foundation
The End
For more information please contact
Marney MutchTel: 250-590-5785
Howard CookTel: 510-418-1676