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?

“…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

Mmarney@ShearSeismic.com

Howard CookTel: 510-418-1676

BayAreaRetrofit@aol.com