Buildings and Earthquakes

John Nurre17may2013

Author’s noteThis 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

the Evolution of Buildings

Life is good

the Evolution of Buildings

Life is good but sometimes the weather isn’t

the Evolution of Buildings

A solid roof

the Evolution of Buildings

A solid roof needs solid walls

the Evolution of Buildings

A good building

the Evolution of Buildings

A good building needs a good foundation

the Evolution of Buildings

A good home

the Evolution of Buildings

A good home is good under most situations

the Evolution of Buildings

Earthquakes are not like “most situations”

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

Two Types of Earthquakes

Strike-Slip: Horizontal Motion Dip-Slip: Vertical Motion

Loma Prieta (1989)• Dip-Slip (vertical motion)

• 14mm max lateral displacement

• 7.1 Richter Scale

• “Lift-and-drop” damageLos Gatos CA

San Francisco CA

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

http://quake.wr.usgs.gov/info/1906/got_seismogram_lp.html

Local (Richter) Intensity ≠ Total Radiated Power

Earthquake IntensitiesModified 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”

Effects on Buildings

Vertical MotionCompressive failure

Horizontal MotionBuilding slips off foundation

Horizontal MotionFoundation secure, building

leans

(“shear failure”)

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

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

Structure Foundation in Horizontal Shake

• Structure must be tied to foundation

• If structure slips from foundation, it will settle and cannot be restored

Foundation Upgrade

Floor Joist

Sill Plate

FoundationUpgrade:

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

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

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 failsPanel can move

as shown

Beam joint failsBeam can move

as shown

Joint bears XX pounds without shearing

Joint bears XX pounds without shearing

Bad and Good Panel Joints

• Staple joints not allowed by CA building code 18.08

• http://www.codepublishing.com/CA/SantaClarita/html/SantaClarita18/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

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

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