1. 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)

2. 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

3. Northridge Earthquake 1994
Magnitude 6.7
Considered “Moderate”
$20 billion in residential damage
most residential damage deemed “preventable”

4. Northridge Residential Damage

5. Northridge Residential Damage

6. Northridge Residential Damage

7. What Happens During an Earthquake
Construction view
Forces on the house

8. 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

9. Sheathed, Bolted Cripple Wall Collapses

10. Watsonville, CA, Cripple Wall Collapse

11. Alaska Earthquake 1964

12. 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

13. 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

14. 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√

15. Interior Damage Despite Intact Appearance
Plumbing, wiring, plaster… all severely damaged after 6 foot collapse

16. 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

17. Retrofit of 2,000 sq ft two-story house using the Base Shear Formula

18. 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

19. The 3 Main Components of a Retrofit
Bolt cripple walls to foundation
Secure cripple walls to the floor joists
Install plywood shear panels

20. 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

21. Installed Mudsill Anchoring

22. Bearing Plates (Plate Washers)
Bearing plates increase strength of bolt-to-wood connection
SEAOSC Research

23. 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.

24. Mudsill Splitting Caused by over-sized holes
Over-sized holes are common because they allow builders to adjust the position of the mudsill

25. Foundations Anchors for Crawlspaces
Foundation plates replace bolts where drill-space is unavailable

26. Step 2: Secure Cripple Wall to Floor
Movement prevented by shear transfer ties

27. Shear Transfer Ties Installed

28. 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

29. 1. Directly to Flush Framing

30. 2. Nailed Blocking Method
2x4 blocks installed between 2x4 studs on 2x6 mudsill

32. 3. Flush Cut Method

33. 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

34. Plywood Installation Challenge #1
Updated plumbing, wiring and structural modifications

35. Plywood Installation Challenge #2
Old-fashioned, inconsistent and odd framing

36. Plywood Installation Challenge #3
Replacing rotted or pest-damaged wood

37. How Not to Nail Shear Panels

38. The Finished Look: Cripple Walls
Shear walls with finished-to-match panels between

39. The Finished Look: Crawlspace
Anchor plates and shear transfer ties house to foundation

40. The End
For more information please contact Marney Mutch Tel: M Howard Cook Tel: