2. Author’s note
This package was created for a planned unit community
(condo complex) in Mountain View California, during a
retrofitting project.
The package addresses a question asked on
22nov12, regarding need for shear nailing. The
package was originally published on 14dec2012.
The package is pertinent to the retrofitting project at one
condo complex. It has been reviewed and approved by
the construction manager and the professional
engineer. It can be used as an example for similar
projects, but each project should rely on the advice of
a professional engineer for appropriate application.
This version is a first-revision, and seeks to make the
original explanations clearer for the layperson.
John Nurre, 17 May 2013
10. the Evolution of Buildings
A good home is good under most situations
11. the Evolution of Buildings
Earthquakes are not like “most situations”
12. Earthquakes and Buildings
• Types of earthquakes
– Shake direction– vertical vs horizontal
– Shake intensity– epicenter and radiated
• Effects on buildings
– Varies with shake direction
• Reinforcing against earthquakes
– Foundation retrofit: good attachment to pad
– Shear nailing: strengthen walls above foundation
13. Two Types of Earthquakes
Strike-Slip: Horizontal Motion Dip-Slip: Vertical Motion
14. Loma Prieta (1989)
• Dip-Slip (vertical motion)
• 14mm max lateral
displacement
• 7.1 Richter Scale
• “Lift-and-drop” damage
Los Gatos CA
San Francisco CA
15. San Francisco (1906)
• Strike-Slip (horizontal)
• 8 ft shift in Woodside
• 7.9 Richter Scale
• 6.3 x more energy
• Lateral-shake damageFolger Ranch (Woodside CA)
Palo Alto CA
17. Earthquake Intensities
Modified Mercalli Intensity Scale, USGS-preferred measure for earthquake intensity
1906 “Great San Francisco”
Boatwright and Bundock 2005
-122.103
+37.402
-122.103
+37.402
1989 “Loma Prieta”
18. Effects on Buildings
Vertical Motion
Compressive failure
Horizontal Motion
Building slips off foundation
Horizontal Motion
Foundation secure, building
leans
(“shear failure”)
19. Wood Structure in Vertical Shake
W
One panel removed
Example of column buckling
• Can withstand vertical
tension
• Joints are steel reinforced
• Can withstand vertical
compression
• Wood is “tough”
• Can withstand shock
• Safe against buckling
• Little (almost zero) load
• Panels give stability
20. Reinforcing against Earthquakes
• Vertical shake (“dip slip”) less important
– Comparatively less damaging to wood structures
– Witness little damage from Loma Prieta
• 7.1 Richter
• Horizontal shake (“strike slip”) is important
– Known weakness for wood structures
– Witness wood structure damage in 1906
– San Andreas fault (Portola Valley) usually produces
horizontal shaking
21. Structure Foundation in Horizontal Shake
• Structure must be
tied to foundation
• If structure slips from
foundation, it will
settle and cannot be
restored
22. Foundation Upgrade
Floor Joist
Sill Plate
Foundation
Upgrade:
Simpson-Strongtie UFP10
• May 2011
• Issue: weak joint
between sill plate
and foundation
• Fix: Retrofit with
dedicated shear
ties
• Engineered by Bill
Dailey, Hohbach-
Lewin
23. Upper Structure in Horizontal Shake
• Under lateral jolt:
• Vertical supports tend to
lean
• Beam joints can’t restrain
against leaning
• Shear panels:
• Can stop “lean” action
• If joints are not strong:
• Joints (staples) pop
• System leans as shown
W
W
24. Bad and Good Panel Joints
• Earthquake is similar to
gravity “turned sideways”
• Similar to figure shown
• If all joints together cannot
bear load, then all joints fail
• Structure leans
• If sum of strengths ( XX lbf)
can bear load, then force is
supported
• Structure remains square
and true
W
W
Panel joint fails
Panel can move
as shown
Beam joint fails
Beam can move
as shown
Joint bears XX
pounds without
shearing
Joint bears XX
pounds without
shearing
25. Bad and Good Panel Joints
• Staple joints not allowed by
CA building code 18.08
• http://www.codepublishing.com/CA/SantaClarita/html/SantaClarita
18/SantaClarita1808.html
• Upgrade:
• Old: 0.060 dia staples, 5
inch spacing
• New: 0.130 dia nails, 4
inch spacing
• Benefit
• To-code earthquake “best
practice”
• 6x strength increase
26. Earthquake Retrofit Project
• Cancel poor-value earthquake insurance, use
savings to strengthen structures
• Insurance averaged $35k per year (2007, 2008, 2009)
• First phase: bring foundation up to code
• 2011
• Aprx $54k (initial firm price, cannot find final)
• Second Phase: bring panel-attachment to code
• Initial plan: accumulate savings from cancelled
insurance, schedule when funds are sufficient
• Initiated 12 September 2012 (Executive Session)
• Draeger quote $18k for 10 buildings
27. Summary
• Preparing for earthquakes is appropriate
• Prime risk is a lateral-shift earthquake
• Two phases of preparation:
• Secure buildings to foundations
• Strengthen building walls with additional
nails
• Initial phase (foundations) complete
• Second phase (nailing) has started
