Seismic Retrofitting Of Older Homes

Presentation created by: Howard Cook Bay Area Retrofit Oakland, California Marney Mutch Shear Seismic Inc. Victoria, BC

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.” Times Colonist July 19

NOT TRUE!

• Magnitude 6.7

• Considered “Moderate” • $20 billion in residential damage • most residential damage deemed “

preventable

”

Forces on the house Construction view

House is supported on cripple walls Earthquake moves the foundation House rocks on the cripple walls House falls when cripple wall collapses

• 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

Earthquake forces pass through various structural components and their connections as they travel from the floor to the foundation

Ç√ • 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

Plumbing, wiring, plaster… all severely damaged after 6 foot collapse

• Establishes how much earthquake force the base of a house will be subjected to • Properly retrofit house must be able to resist this force

Retrofit of 2,000 sq ft two-story house using the Base Shear Formula Plywood bracing 14 each side 8,400# $3,640 1/2" bolts w/MSP's 7 each side 9,345# 1,820 L90, H10R, LTP4 or homemade STT's 14 each side 8,400# 1,400

TOTAL COST $6,860

• • • 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

1.

Bolt cripple walls to foundation 2.

Secure cripple walls to the floor joists 3.

Install plywood shear panels

• 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

Bearing plates increase strength of bolt-to-wood connection SEAOSC Research

• 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.

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

Foundation plates replace bolts where drill-space is unavailable

Movement prevented by shear transfer ties

There are three ways to attach a plywood panel to the mudsill: • Directly to Flush Framing • The Nailed Blocking Method • The Flush Cut Method

2x4 blocks installed between 2x4 studs on 2x6 mudsill

• 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

Updated plumbing, wiring and structural modifications

Old-fashioned, inconsistent and odd framing

Replacing rotted or pest-damaged wood

Shear walls with finished-to-match panels between

Anchor plates and shear transfer ties house to foundation

For more information please contact Marney Mutch Tel: 250-590-5785 [email protected]

Howard Cook Tel: 510-418-1676 [email protected]