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 supported on cripple walls Earthquake moves the 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 This illustration represents a fully braced cripple wall. Most cripple walls are very short and do not have much overturning. Overturning will be dealt with in a future chapter. When a wall is braced like this it is known as a shear wall. 1) Earthquake pushes against the floor you walk on as illustrated by the blue arrow. 2) The floor you walk on is nailed to the floor framing. The floor framing in turn is pushed in the same direction as the floor and it tries to slide on the upper top plate. 3 This sliding movement pushes against the shear transfer ties. 4) The shear transfer ties transfer this sliding force into the upper top plate, 5) The upper top plate transfers this sliding force into the nails at the top edge of the plywood. 6) These nails transfer the sliding force down through the plywood into the row of nails at the bottom of the plywood. 7) The nails at the bottom of the plywood transfer the sliding force into the mudsill, 8). The mudsill transfers the force this sliding force into the bolts. 9) The bolts transfer the force into the foundation. 10) The foundation transfers this sliding force into the ground where it is finally dissipated. This chain of movement from the floor to the foundation through various structural components is the load path. If any component within the chain is missing or weak, the retrofit can fail. A retrofit can be defined as attaching the floor you walk on to the foundation in such a way that the floor is retrained from movement when pushed on by an earthquake. Earthquake forces pass through various structural components and their connections as they travel from the floor to the foundation

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√

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

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 Bolt cripple walls to foundation Secure cripple walls to the floor joists 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

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

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 See APA Report 154, 138, Technical Note N370C

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 Mutch Tel: 250-590-5785 M marney@ShearSeismic.com Howard Cook Tel: 510-418-1676 BayAreaRetrofit@aol.com