Wood-FrameConstruction

Meeting theChallenges ofEarthquakes

Building Performance Series No. 5

Canadian ConseilWood canadienCouncil du bois

Building Performance Series No. 5

2

Building Performance Bulletin

The Canadian WoodCouncil is the nationalassociation representingCanadian manufacturersof wood products usedin construction.

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Wood-Frame Construction – Meeting the Challenges of Earthquakes

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Introduction

orth American single-family homes

are considered by many to be the

safest place to be in an earthquake.

This is not surprising considering that North

American housing is almost synonymous with

wood-frame construction. The lightweight and

high energy absorbing capabilities of wood

framing provides a system strong enough to

withstand the effects of powerful earthquakes.

Experience from strong earthquakes, in North

America and around the world, has shown that

well-constructed wood-frame buildings provide

safety to their occupants.

Table of Contents

Effect of Earthquakes on Wood Buildings ................... 5

What Happens in an Earthquake ........................ 5

The Response of Wood-Frame Construction ....... 6

to Earthquakes

Performance of Wood-Frame Buildings

in Past Earthquakes ................................................. 9

Lessons Learned from Past Earthquakes —

Improving Performance for the Future ..................... 13

Research ................................................................ 15

Designing Earthquake-Resistant

Wood-Frame Buildings ........................................... 16

North America ................................................ 16

World Demand for Safer Housing ..................... 17

Conclusions ............................................................ 18

References ............................................................. 19

There are over a million earthquakes annually butmost are too small to be felt. Although earthquakescan occur anywhere, there are certain locationswhere the likelihood of strong earthquakes is parti-cularly high. Around the world, earthquakes claimmany lives each year — many from damage tobuildings. There have been relatively few deathsin recent North American earthquakes. This canbe attributed to North American building practices,including the widespread use of wood framing forhousing.

The 1964 earthquake in Prince William Sound,Alaska was one of the most powerful earthquakesever recorded in North America. Considering themagnitude of the earthquake relatively few liveswere lost. Measuring 8.4 on the Richter scale, theearthquake claimed only 131 lives and 122 of theseresulted from the tidal waves caused by the earth-quake. By contrast, 15,000 people were killed in the

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1. The attachment of sheathing and finishesto the numerous wood joists and studs in atypical wood-frame house provides redun-dant load paths for the earthquake forces.There are numerous small connectionsrather than few large-capacity connections.If one connection is overloaded, its sharecan be picked up by adjacent connections.

2. Wood has a high strength to weight ratioand therefore wood buildings tend to belighter than other building types. Lightnessis an advantage in an earthquake.

3. The nailed wood connections in wood-frame systems allow the building to flexthereby absorbing and dissipating energyduring an earthquake.

4. In engineered wood-frame buildings,structural panels (plywood or OSB)acting in combination with studs andjoists, create shearwalls and diaphragms— very effective lateral-force resistingbuilding assemblies.

This Building Performance bulletin is intended toimprove the understanding of earthquakes and theireffects on wood-frame buildings. Except for a fewexceptional cases, hundreds of thousands of wood-frame buildings have provided protection for theiroccupants when exposed to the devastating effects ofsevere earthquakes. The traditional North Americanwood-frame house provides the fundamental elementsfor seismic resistance and wood-frame buildingpractices are continually evolving. New wood-basedmaterials have been introduced, building research hasprovided better details and lessons learned in pastearthquakes are being used to build better houses.

1999 earthquake in Turkey that measured 7.4 on theRichter scale.

The Geophysical Institute of the University of Alaskaexplains the relatively low losses in the 1964 AlaskaEarthquake as follows:

“The number of deaths from the earthquaketotalled 131; 115 in Alaska and 16 in Oregonand California. The death toll was extremelysmall for a quake of this magnitude due tolow population density, the time of day, thefact that it was a holiday, and the type ofmaterial used to construct many buildings(wood).”1

In California, there are over 400 million square feetof public schools and 80% of this area is wood-frameconstruction. An assessment of the damage to schoolbuildings in the 1994 Northridge Earthquake wassummarized as follows:

“Considering the sheer number of schoolsaffected by the earthquake, it is reasonableto conclude that, for the most part, thesefacilities did very well. Most of the verywidespread damage that caused schoolclosure was either nonstructural, or struc-tural but repairable and not life-threatening.This type of good performance is generallyexpected because much of the school construc-tion is of low-rise wood-frame design, whichis very resistant to damage regardless of thedate of construction.”2

In 2002, the State of California Department ofGovernment Services (DGS) completed a legislatedinventory and earthquake worthiness assessment3

of schools. School buildings that were constructedof steel, concrete, reinforced masonry or mixedsystems, designed between 1933 and July 1, 1979were required to be evaluated. Older wood-frameschools were exempted from the assessment on thebasis that, “Wood-frame buildings are known toperform well in earthquakes.”3

These endorsements of the ability of wood-frameconstruction to perform well in the face of earth-quakes are based on several researched and docu-mented wood building system characteristics.

“Wood-frame buildings are known to perform well in earthquakes.”3

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Effect of Earthquakes on Wood Buildings

Exterior view of a wood-frame house illustrating

damage caused by ground failure. Note cracks in

the pavement at the front of the house.

What Happens in an Earthquake

Most earthquakes originate from the regions at thejunctions of the plates that make up the earth’s crust.These plates are constantly shifting, creating stressesand distortions. Where the stress is greater than thestrength of the crust, there will be a sudden slippagethat releases energy and causes seismic waves on theearth’s surface. The place where the energy is releasedis called the focus of the earthquake, and the point onthe earth’s surface directly above the focus is calledthe epicentre. (Figure 1)

The amount of energy released at the epicentre istypically measured using the Richter Magnitude Scale(M). A magnitude M4 earthquake can be distinctlyfelt near the epicentre, M5 and M6 are moderate sizeearthquakes that may cause considerable damage,and M7 and M8 are often accompanied by wide-spread damage to buildings and other structuresand can trigger landslides and permanent grounddisplacements.

If a building is located directly over a fault, itcan be damaged by failure of the ground below thefoundation. Unstable slopes and weak soil depositscan lead to foundation failure and damage or collapseof the building. However, most damage is a resultof ground motions from the seismic waves radiatingfrom the earthquake epicentre.

When the ground motion is strong enough, it movesbuildings. The earthquake moves the foundationbut inertial forces try to keep the upper storeys of

the building in their original positions. (Figure 2)If ground displacement were to occur very gradually,the building would ride along easily, however, earth-quakes involve rapidly accelerating ground forces.The forces generated in the earthquake depend onthe weight of a building and how quickly the groundaccelerates. Since inertial forces are greater whenobjects are heavier, earthquake forces are higher inheavier building. Likewise, higher ground accelera-tions create more stress in the structure. Earthquakeswill affect buildings differently depending on thecharacteristics of the ground motion and character-istics of the building structure.

Figure 1: Earthquake Basics

Seismic waves

Epicentre

Fault

Earthís crust

Focus

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Figure 2: When the Ground Moves

Groundaccelaratesto left

Building at rest;ground at rest

Groundaccelaratesto right

Ground at rest

The type of seismic ground motion at a building siteis dependant on a number of factors:

• Distance of the building from the earth-quake’s epicentre,

• Magnitude of the earthquake,

• Depth of the earthquake’s focus, and

• Soil conditions at the building site.

The way that a building responds to an earthquakedepends on the size of the building and its stiffnesscharacteristics. Earthquakes that have high peakground accelerations pose the greatest challenge towood-frame buildings. Measured ground accelera-

tions, g, are recorded as a fraction of the accelerationdue to gravity (10 m/s2 or 32 ft/s2) and are greatest atthe epicentre. Although the Richter scale is often usedto characterize an earthquake, peak ground accelera-tion at a given location is a better indicator of thepotential damage of earthquakes on wood-framebuildings.

The Response of Wood-Frame Construction

to Earthquakes

In North America, most building codes recognize twotypes of building design and construction: 1) engineereddesign and construction and 2) design and construc-tion by conventional rules.

Engineered construction uses fully designed systems,by engineers, to resist the calculated imposed loads.Conventional construction is based on a set of pre-scriptive rules derived from traditional constructionpractice that have evolved to reflect research findings,and past performance. Conventional constructionis limited by building codes to smaller buildingswith limited occupancies. Large houses and mostnon-residential buildings are engineered.

The inertial forces generated by the ground movementof the earthquake, concentrate lateral forces in theroof and floors where the mass of the building isgreatest. The forces in the roof and floors must beresisted by walls and the entire structure must beadequately connected to the foundation. (Figure 3)The following components of wood-frame construc-tion are critical to the resistance of seismic forces:

• Anchorage to the foundation,

• Strength and ductility of the walls,

• Strength and continuity of the horizontalfloors, roof and ceilings, and

• Interconnection of all of the framingelements.

Conventional construction guidelines typically pre-scribe minimum roof, wall and floor constructionsand the connections between them. Braced walls thatmeet minimum requirements with respect to sheathinglength and type of sheathed wall must be spaced atregular intervals in the building. Anchor bolt require-ments are specified to ensure that the structure isadequately tied to the foundation.

Wood-Frame Construction – Meeting the Challenges of Earthquakes

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Typical construction often exceeds the minimumconventional requirements in the building codes.For example, wall sheathing is often thicker thanthe specified minimum thickness and typicallythere are more walls in a building than theminimum braced wall requirements.

In engineered design, a lateral load path isestablished and each element of the load pathis designed and detailed to resist the calculatedearthquake force. Roofs and floors are designedas diaphragms and some of the walls in thebuilding will be designed as shearwalls. Thedesign of shearwalls and diaphragms includesensuring that:

• Structural wood sheathing (OSB orplywood) is thick enough to resist thecalculated forces,

• Nailing is adequate to transfer the shearforces in the sheathing to the roof, flooror wall framing,

• Blocking is specified at the edges of thestructural sheathing in the diaphragmsand shearwalls, if necessary, and

• Framing members around the perimeterof the diaphragms and shearwalls are strongenough and properly spliced to resist thecalculated tension and compression forces.

Engineered design also requires adequate connectionsbetween all of the elements in the load path. There-fore, additional nails or special framing anchors aretypically required to connect the diaphragms to theshearwalls. Special “hold-down” connections areused to hold down the corners of the shearwallsand additional anchor bolts are usually required toconnect the shearwall to the foundation. (Figure 4)

Engineered Shear Walls

Specificnail sizeand spacingrequirements

Holddownanchors

Base shearanchor bolts

Speciallydesigned studs

Woodstructuralpanelsof specificthickness

Let-in bracing Wall board WoodStructuralPanel

Figure 4: Shear Resisting Elements

Prescriptive Bracing

Figure 3: Seismic Forces on a Building

forcesof inertiaact againstroof andwalls

seismic ground motion

wallholdsroofback

forcespush roofthis way

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Wood-frame buildings have properties that naturallyenhance their performance in earthquakes, althoughto ensure that wood-frame buildings are safe inearthquakes, good construction practices must befollowed. Some of these properties are describedin the following:

Strength and Stiffness:

The lateral forces of an earthquake tend to distort thebuilding so the walls rack (become unsquare). Bracedwalls or shearwalls are critical for providing rackingresistance during an earthquake. Walls constructedwith plywood or OSB structural sheathing are veryeffective in resisting the racking forces of earthquakes.In locations where strong earthquakes are possible,the stiffness and resistance of the walls can be aug-mented by increasing the thickness of the structuralpanels and increasing the number or size of the nails.In addition, research and experience have shownthat ‘non-structural’ elements contribute to the lateralresistance of the structure. For example, interiorfinishes, partitions, and many types of exteriorcladding contribute to the lateral resistance of thestructure.

Ductility:

Compared to other materials such as masonry andconcrete that have to be carefully designed anddetailed to ensure good seismic performance, woodsystems are inherently more ductile. Ductility is theability of the structure to yield and to deform withoutcollapse. It is desirable for a building to have someflexing capability when subjected to the suddenloads of an earthquake because the flexing allowsthe building to dissipate energy. The numerous nailedjoints are very effective in providing ductility towood-frame buildings.

Weight:

Wood-frame construction is lightweight. Concretewalls used in ICF (insulated concrete form) wallconstruction are about 7 times heavier than typicalwood-frame walls. Since forces in an earthquakeare proportional to the weight of the structure,lightweight wood-frame buildings that are properlydesigned and built can be expected to perform verywell in earthquakes.

Redundancy:

Buildings that have numerous load paths are consid-ered structurally redundant and provide an extralevel of safety in earthquakes. Structures supportedby heavy frames rely on relatively few structuralmembers and connections. A design or workmanshipflaw in any one component may mean overloading ofadjacent load paths. Typical wood-frame constructionis comprised of hundreds of structural elements andthousands of nail connections. This means that thefailure of one load path can often be compensatedfor by adjacent members and joints.

Connectivity:

Wood-sheathed walls can resist the racking forces ofan earthquake, but a building must also be designedand built to resist sliding or overturning. In eithercase, the building must be adequately secured to thefoundation. The connection of the walls, floors androof framing make the house a single solid structuralunit and is an important feature for holding a housetogether during an earthquake.

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Table 1: Performance of wood-frame construction in earthquakes5

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Performance of Wood-Frame Buildings

in Past Earthquakes

In North America, the design of houses and otherbuildings for earthquakes has been primarily tominimize serious injury or death to people, andwood-frame construction has generally met thisrequirement very well. In Canada, the objectiveof earthquake-resistant design is to prevent majorfailure and loss of life. According to the StructuralCommentaries to the National Building Code ofCanada:

“Structures designed in conformance withthese provisions (of the 1995 NationalBuilding Code of Canada) should be ableto resist moderate earthquakes withoutsignificant damage and major earthquakeswithout collapse.”4

A survey of the performance of wood-frameconstruction in earthquakes shows a remarkablylow fatality level (Table 1). Most of the residentialbuildings in the survey areas were built accordingto conventional practice and were not specificallydesigned by engineers.

The vast majority of wood-frame buildings survivedthe strong shaking almost unscathed or with variousdegrees of superficial and structural damage. A fewwood-frame buildings with recognized structural

San Fernando, CA, 1971 6.7 0.6+ 63 4 100,000

Edgecumbe NZ, 1987 6.3 0.32 0 0 7,000

Saguenay QC, 1988 5.7 0.15 0 0 10,000

Loma Prieta CA, 1989 7.1 0.5 66 0 50,000

Northridge CA, 1994 6.7 1.0 60 16 + 4* 200,000

Hyogo-ken Nambu, 6.8 0.8 6300 0** 8,000**

Kobe Japan, 1995

* 16 deaths occurred in the collapse of one apartment building. Four deaths were from foundation failures that caused collapse of buildings on hillsides.

** Pertains to modern North American style wood-frame houses in the affected area.

*** Numbers taken from Rainer and Karacabeyli document.6

Earthquake Force Approximate No. of No. of Platform FramePersons Killed Wood Buildings Strongly

Shaken (estimated)Richter Maximum Total In FailuresMagnitude Peak Ground of Platform

M Acceleration Frame WoodMeasured Buildings

g***

deficiencies collapsed. A summary of the deficienciesis given in the next section.

Three recent California earthquakes provide insightinto the performance of North American style wood-frame construction.

San Fernando Earthquake, 1971

The earthquake occurred in a northerly suburbanarea of Los Angeles and consequently affected a largenumber of single-family homes as well as hospitalsand commercial structures. Hospitals and officebuildings built of unreinforced masonry and rein-forced concrete collapsed, or were severely damagedto the point where they had to be demolished.

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Loma Prieta Earthquake, 1989

The epicentre of the earthquake was located 100 kmsouth of San Francisco but its effects were felt on theNorth Shore of San Francisco Bay. The earthquakecaused the collapse of a number of engineered struc-tures including the double-deck freeway in Oaklandthat resulted in the death of 49 motorists. At theepicentre, housing was subjected to peak groundaccelerations as large as 0.5 g and possibly larger.Newer housing at the epicentre generally performedwell unless they were situated on ground fissuresor had large openings in lower storey walls.6

Some older housing at the epicentre experiencedextensive damage due to inadequate braced walls,cripple wall failures and inadequate connectionsto the foundations. In the Marina District of SanFrancisco some older four-storey buildings wereparticularly susceptible due to ground storey park-ing that resulted in non-reinforced soft storeys.7

Northridge, 1994

The earthquake struck the densely populated SanFernando Valley, in northern Los Angeles on January17th, 1994. Although moderate in size (M6.7) thepeak ground accelerations were amongst the highestever recorded and significantly higher than thosespecified in the building codes at the time.8 Therewere numerous building collapses, many in largestructures. Amongst the reasons given for the limiteddeath and injuries from the earthquake was the timeof the earthquake:

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Aerial view of the damage to the San Fernando Veterans Administration Hospital and complex. The collapsed

masonry structure was built in 1926, before earthquake building codes were in effect. Forty-seven deaths

occurred as a result of the collapse of the structure.

The majority of the wood-frame houses performedwell, especially from the standpoint of life safety.Damage in older wood-frame houses in the SanFernando area ranged from superficial to partialcollapse. Serious damage included houses slidingfrom foundations, collapse of “cripple walls” incrawl spaces, collapse of add-ons such as porchesand collapse of masonry chimneys. Newer two-storeyapartment buildings with large ground-level openingswere also severely affected. Typically, these apart-ments were constructed to allow parking at theground level. Because one wall of the garage storeywas open, these floors were “soft storeys.”6

The importance of providing strong walls and founda-tion connections was recognized in this earthquakeand building codes were updated accordingly.

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The second level of the concrete parking structure of the Northridge Fashion Center

collapsed onto the lower level.

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The 1994 Northridge Earthquake has been exten-sively studied. A very high percentage of wood-framehouses performed well in the earthquake. Most of thedamage to such buildings was non-structural in natureand easily repairable.2

There were several typical modes of failure that hadbeen experienced in previous earthquakes. Althoughbuilding codes had addressed these issues in newconstruction, the failures reflected the need to upgradeexisting buildings to remove obvious deficiencies. Inthe Northridge Earthquake, a vulnerable wood-framebuilding type was low-rise, multi-storey, apartmentstructures with a soft first storey. Such buildings, withlarge, often continuous openings for parking, did nothave enough wall area and strength to withstand theearthquake forces and resulted in several collapsesof the ground floor. The most tragic collapse of theearthquake occurred when 16 people died in onesuch apartment building. Although the building wasengineer-designed, the peak ground acceleration at

“The earthquake occurred at 4:31 a.m. when

the majority of people were sleeping in their

wood frame single family dwellings, gener-

ally considered to be the safest type of

building in an earthquake. If the earthquake

had occurred during the day, say at 11:00

a.m., several hundred people would have

been killed at the retail store and parking

garage of the Northridge Fashion Mall alone,

where actually only one person was killed.

Also, due to timing of the earthquake, people

were not present on sidewalks to be injured

from falling debris, particularly from

unreinforced masonry and tilt-up buildings

or falling facades from other buildings.” 9

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that location was substantially greater than the 0.4 gdesign value used in the building codes at the time.6

Other wood-frame construction performed exceed-ingly well. In a statistical-based study of the seismicperformance of residential construction in theNorthridge earthquake, the authors concluded:

“SFD (Single family dwelling) homes suf-

fered minimal structural damage to elements

that are critical to the safety of occupants.

Structural damage was most common in the

foundation system. The small percentage of

surveyed homes (approximately two percent)

that experienced significant foundation

damage were located in areas that endured

localized ground effects or problems

associated with hillside sites.”10

Following an earthquake, it is necessary to ensuredisplaced people can be provided with emergencyaccommodation. If too many houses are destroyed,emergency shelters will be overwhelmed. In the

days after the Northridge Earthquake, many buildingsin the Los Angeles area were inspected to determineif they were safe for occupancy. When there wasconcern about the safety of a building, an initialinspection was carried out and the building wastagged either “Red” for hazardous, “Yellow” forbuildings that posed a threat to life, but not so muchthat an occupant could not re-enter to remove posses-sions and “Green” for buildings that did not posea life-safety hazard to the occupants. After 4 days,almost 50,000 residential buildings had been in-spected and over 85% of these were “Green Tagged”(Figure 5).

Most of the buildings were inspected because resi-dents and owners were concerned the condition oftheir homes might be unsafe. Therefore the buildingsinspected were generally more damaged than thevast majority of wood-frame buildngs for which noinspection was requested. Furthermore, many ofthe red and yellow-tagged buildings were subse-quently revised to green.10

Survey results have shown that in a number ofearthquakes wood-frame construction has withstoodpeak ground accelerations of 0.6 g or greater withoutserious distress and often without any signs of struc-tural damage. Most of the wood-frame houses thatexperienced the strong earthquake forces in Californiawere built using the prescriptive conventionalconstruction requirements.

Figure 5: L.A. Department of Building and Safety

Building Inspection Results

100%

80%

60%

40%

20%

0%

Single-Family Single-Family Low-Rise

Detached Attached Multifamily

Green

Yellow

Red

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Weak First Storey: Experience in past earthquakes hasshown that weak and soft first storeys will makebuildings susceptible to the effects of earthquakes.Providing adequately braced or sheathed walls isessential to ensure conventionally built wood-framebuildings can resist earthquakes. Large openings inthe walls such as windows, doors and garage doorscan leave too little wall area to provide adequateresistance to the lateral forces imposed by earth-quakes. Unless these walls are adequately reinforced,lack of wall capacity leads to large distortions andeventual collapse. California experience has shownthat multi-family apartments built with first storeygarages are particularly vulnerable to earthquakesand building codes have recognized that buildingswith soft storeys have to be carefully designed anddetailed to resist earthquakes. Engineering solutionscan be provided for new or existing structures thatrequire additional wall reinforcement.

Connections to Foundations: Anchor bolts are used toprevent a structure from moving off its foundation.Some old houses were built without using anchorbolts to connect the wood structure to the foundation.Experience has shown that these structures may slideoff their foundations during an earthquake. Thesestructures can be retrofitted by adding connectionsbetween the wood framing and foundation.

Shored building in the Marina District of San Francisco

following the 1989 earthquake. Note the garage area

on the first floor beneath the apartments that created

a weak first storey.

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This turn-of-the-century wooden residence sustained

major damage when it moved off its foundation in the

1989 Loma Prieta Earthquake.

Lessons Learned from Past Earthquakes

— Improving Performance for the Future

The evaluation of building performance has identifiedproblems with some wood-frame buildings eventhough, overall, wood-frame buildings have beenfound to be safe in earthquakes. These deficienciesare described in brief to provide an understandingof failure modes and how they can be avoided.

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These overturned bookcases are typical of the damage

that can occur during a strong earthquake.

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Chimney damage from the Northridge Earthquake.

Failure of cripple wall in the Loma Prieta Earthquake.

Unrestrained Furnishings, Components and Appliances:

Tall objects such as bookcases can topple, andsliding or rolling objects can become projectiles.Heavy, unbraced interior partition walls can fallover, and improperly attached canopies and cur-tain walls (for example, brick cladding) can fallon passers-by. Domestic gas water heaters, unlessproperly braced and secured, can easily tip overor displace, causing a possible leakage of gas andthe risk of explosion. Bracing furnishings, compo-nents and appliances can be a cost effective wayof reducing earthquake damage and costs.

Building codes are continuously evolving to addressstructural issues related to earthquake safety but theydo not typically deal with furnishing and appliancesnor do they address existing buildings. Increasinglyhomeowners in areas prone to earthquakes are as-sessing their houses or having their houses assessedby professionals. Homeowners can easily undertakemany of the upgrades themselves. Governmentagencies provide upgrade information and muchof it is available electronically. The Associationof Bay Area Governments at www.abag.ca.govand the California Seismic Safety Commission atwww.seismic.ca.gov both have useful informationfor homeowners and building professionals. InCanada, the Canada Mortgage and Housing Cor-poration has prepared a Residential Guide toEarthquake resistance.11

Cripple Walls: Cripple walls, also known as knee wallsor pony walls, are the short stud walls between thefloors and foundations in some houses. Many olderbuildings that had unbraced cripple walls sustainedstructural damage during California earthquakes.Strengthening the walls using wood structural sheath-ing is an effective method of bracing cripple walls.

Chimneys: Unreinforced masonry chimneys are par-ticularly susceptible to earthquake damage. Failureof the chimneys can cause damage to the roof andwalls of the structure below. Also, damaged chimneysmay be dangerous in aftershocks following the initialearthquake. Bracing chimneys or using chimneys fromlighter materials can help prevent damage in futureearthquakes.

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building codes and standards, economic aspects,and education and research.

The project included full-scale shake tests thatinvolved the construction of a house on a specialplatform capable of simulating earthquake loadsof different magnitudes. Three different shake tableprojects were conducted: tests of a simplified full-scale two-storey single family house (Universityof California San Diego), tests of a full-scale multi-storey apartment building (University of California

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Full-scale shake tests involved the construction of

a house on a special platform capable of simulating

earthquake loads of different magnitudes.

Research

Wood-frame construction has benefited from manyresearch projects ranging from tests on the smallestnail to full-scale earthquake simulations on wholehouses. The observed seismic performance of tradi-tional wood-frame construction is difficult to modelmathematically due to the many load paths andcontributions of “non-structural” elements. By gaininga better understanding of the properties of materialsand assemblies, researchers are better able to predictthe performance of whole houses. Full-scale assem-blies are being used to develop sophisticated modelsof how a whole house works together to resist earth-quake loads. This research helps to explain theperformance of conventional wood constructionand can be used to provide more effective designsolutions.

Forintek Canada Corporation is Canada’s foremostwood research institute and has an ongoing multi-phase seismic research program focusing on compo-nent testing and modeling. Results from their researchwere used to augment the seismic provisions in theCanadian wood design standard. Forintek and itsresearch partners have also worked on computermodels used to evaluate the performance of wood-frame buildings in earthquakes and this work showspromise as a tool that can help to efficiently designearthquake-resistant wood-frame buildings.12

Tests on full-scale wood-frame buildings were under-taken as early as 1965.12 Since then, there have beenother tests at research institutions around the world.The more recent tests involve using a “shake table”to closely simulate the effects of a real earthquake.Of particular interest is the CUREE project in Cali-fornia. A parallel research project was carried outat the University of British Columbia (UBC).

The Consortium of Universities for Research inEarthquake Engineering (CUREE) (www.curee.org.)is a non-profit organization, established in 1988, toadvance earthquake engineering research, educationand implementation. The project is funded by theUS Federal Emergency Management Agency (FEMA)through a grant administered by the CaliforniaGovernors Office of Emergency Services. TheCUREE-Caltech Woodframe Project has five com-ponents; testing and analysis, field investigations,

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Designing Earthquake-Resistant

Wood-Frame Buildings

Construction practices are constantly evolving.For example, longer member spans leading to moreopen concept structures, larger garages and the desirefor more openings in exterior walls have providedchallenges for designing earthquake-resistant build-ings. However, the widespread use of wood structuralpanels allows us to build stronger floors and wallsand our understanding of earthquake-resistant designis continuously evolving.

North America

In North America, building codes provide prescriptiveframing requirements for wood houses. The 1995and earlier editions of the National Building Codeof Canada do not explicitly include prescriptiveprovisions for earthquake design. Framing guidelinesfor earthquake-prone regions of Canada can be foundin the Canadian Wood Council’s (CWC) EngineeringGuide for Wood Frame Construction13 that includesa set of simple prescriptive requirements based onconventional construction guidelines that have beenused in California. The CWC guide can also be usedto engineer small wood-frame buildings.

In the United States, building codes contain conven-tional construction requirements for houses thatinclude provisions for earthquakes. Earthquake design

Berkeley) with tuck-under parking garages, and testsof a simplified box-type wood-frame building model(UBC).

The two-storey house tests included specimens sim-ulating a conventionally framed house and engineeredhouses. Some specimens were built without finishingmaterials (bare), and others were finished withstucco. All specimens were subject to at least a 0.5gpeak ground acceleration and some of the engineeredhouses were subject to 0.89g ground accelerations.A review of the test results by Canadian researchers12

led to the following general observations:

• In all projects, none of the specimens builtwith finishes (i.e. stucco) reached a “nearcollapse” state.

• None of the two-storey “bare” specimenstested in the CUREE project reached a“near-collapse” state.

• One three-storey “bare, tuck under garage”specimens tested in the CUREE projectreached a “near collapse” state.

• Test results are in substantial agreementwith previous surveys of the performanceof wood-frame construction.

• The amount of deflection increases with thedegree of openings in the walls.

• Deflection in the conventionally constructedhouse was higher than deflections in thecomparable engineered house.

• Adding a stucco exterior finish and gypsumwallboard interior finishes significantlyreduces the lateral deflection, reducing thedamage to the finished structure substan-tially.

The CUREE tests at the University of California SanDiego and Berkeley, and tests performed at UBC andForintek have produced many pertinent results thatcan be applied to the seismic design of North Ameri-can style wood-frame houses.

The amount and quality of research devoted towood-frame construction around the world meansan increasing ability to understand how earthquakesaffect wood structures and how to design them toperform even better.

Wood-Frame Construction – Meeting the Challenges of Earthquakes

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provisions can also be found in the American Forestand Paper Association’s Wood-frame ConstructionManual for One and Two Family Dwellings14.

When framing systems are not based on prescriptiverequirements, earthquake load paths must be engi-neered. Loads are determined using the buildingcode requirements and the load-resisting elementsare designed and detailed based on the wood designstandards. In both Canada and the United States,recent editions of the national wood design standardshave been updated to include enhanced provisionsfor earthquake design. The Canadian StandardAssociation’s latest edition of CSA O86–0115 “Engi-neering Design in Wood,” includes provisions thatmake it easier to design wood-frame buildings toresist earthquake loads. Likewise, the most recentedition of the National Design Specification® forWood Construction16 in the United States includesall the information required to design wood-framemembers to resist earthquake loads.

WoodWorks® Shearwalls design software has beenspecifically developed to assist engineers to designearthquake-resistant wood-frame buildings. In addi-tion to providing wood design solutions, the softwarewill also generate seismic loads on the building. Thesoftware is available for use in both Canada and theUnited States. Information on the software can befound at www.woodworks-software.com.

In North America, building codes and wood designstandards provide the information required to designearthquake-resistant wood structures. The codes andstandards are updated on an ongoing basis to reflectfield studies and research on the effects of earthquakeson wood buildings.

World Demand for Safer Housing

There are many reasons why wood-frame construc-tion, which has been so successfully employed inNorth America over the past century, is attractinginterest in other parts of the world. In some cases, theincreasing affluence of emerging economies is creatinga demand for increased comfort. In other cases, thefailure of buildings and numerous deaths from recentearthquakes is fueling the demand for safer housing.17

The entire island of Taiwan is exposed to a very highearthquake risk. A 1999 earthquake killed more than2,200 people and left over 100,000 homeless.As a result, the government has indicated strongsupport for introducing wood construction and isdeveloping Taiwanese codes based on North Ameri-can and Japanese Models.

Japan is another country situated almost entirelyin a region of high earthquake risk. First introduced20 years ago, North American style wood-frameconstruction occupies a fast-growing niche of theJapanese home market, particularly since its exem-plary performance during the 1995 Kobe Earthquake.

Increasing economic development is driving thedemand for improved housing in China. The govern-ment has recently adopted modified versions of NorthAmerican building codes for wood-frame constructionthat will facilitate construction of high-quality,durable wood-frame housing in China. Like manyparts of Asia, areas of China face high exposure tonatural hazards like earthquakes and typhoons.

Concrete and masonry are the primary buildingmaterials in Turkey. The 1999 earthquake in Turkeycaused 15,000 deaths, mostly from building collapse,and left 600,000 people homeless. Turkish officialshave recognized the need to improve building stand-ards and introduce new construction technologies,including wood-frame construction for single familyand low-rise apartment buildings.

In addition to these examples of countries movingtoward residential wood-frame construction forearthquake safety reasons, there are many otherexamples of countries adopting wood-frame construc-tion because it is researched, proven, economical,flexible, and capable of meeting code requirements.Europe and the emerging countries of the formerSoviet Union are also showing interest in NorthAmerican construction technology.

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Conclusions

The lightweight and high energy absorbing capabili-ties of wood-framing are inherent characteristics thatmake it a preferred building system in earthquakeregions. Surveys have shown that wood-frame build-ings meeting the basic requirements for wall bracing,connectivity and anchorage, provide safety to theiroccupants during earthquakes. The prescriptiverequirements of building codes for smaller wood-frame buildings provide a basic resistance to lateralearthquake loads. Larger wood-frame buildings areeffectively engineered to resist earthquake forces.

Research has been done on North American wood-frame construction over the past twenty years tobetter understand and enhance its performance inearthquakes. Wood-frame construction has alsobenefited from observed performance in NorthAmerican earthquakes. Test results from research

projects are in substantial agreement with the findingsof the previous surveys of the performance of wood-frame construction. Building codes and design stand-ards for wood-frame construction are continuouslyevolving to incorporate earthquake field observationsand research.

Wood-frame construction is a proven buildingmethod that has provided safety to people in devastat-ing earthquakes. In the Alaska earthquake of 1964,the low death toll is attributed to the fact that mostpeople were at home — in wood-frame buildings —when the earthquakes struck.1 The same observationwas made following the Northridge Earthquake thirtyyears later.9 Wood framing also offers the prospectof safety for those people living in areas of the worldthat are at high risk to the devastating effects ofearthquakes.

In the Alaska earthquake of 1964, the low death toll is attributed to the fact that most people were at

home — in wood-frame buildings — when the earthquakes struck.1 The same observation was made

following the Northridge Earthquake thirty years later.9

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References

1. Geophysical Institute of the University of Alaskawww.aeic.alaska.edu/Seis/html_docs/Alaska_1964_earthquake.html

2. The January 17, 1994 Northridge, CA Earthquake,EQE Summary Report, March 1994. www.eqe.com/publications/northridge/commerci.htm

3. Seismic Safety Inventory of California Public Schools,Department of General Services, 2002. www.dgs.ca.gov

4. User’s Guide – NBC 1995 Structural Commentaries.Canadian Commission on Building and Fire Codes.National Research Council of Canada, 1996.

5. Insuring Good Seismic Performance with Platform-Frame Wood Housing, Construction TechnologyUpdate 45, Institute for Research in Construction,National Research Council, 2000.

6. Wood-Frame Building Construction in Earthquakes,J. Hans Rainer, Erol Karacabeyli, Forintek CanadaCorp., Special Publication No. SP-40, 1999.

7. Earthquake Damage in the San Francisco Area andProjection to Greater Vancouver, Canada Mortgageand Housing Corporation, 1990.

8. The Northridge Earthquake, American WoodCouncil, American Forest and Paper Association,1994.

9. An overview of the effectiveness of design and con-struction practices and building codes in theNorthridge Earthquake of January 17, 1994.Adelman, A.A., 1994. Lessons Learned from theNorthridge Earthquake. Hearing Before the UnitedStates House of Representatives Committee onScience, Space and Technology, March 2, 1994.

10. Assessment of Damage to Residential BuildingsCaused by the Northridge Earthquake, U.S. Depart-ment of Housing and Urban Development, 1994.

11. Residential Guide to Earthquake Resistance CanadaMortgage and Housing Corporation, 1998.

12. Data Base for Seismic Design of Part 9 Structures.Unpublished report prepared for the Canadian WoodCouncil. Forintek Canada Corporation, Hans Rainerand Erol Karacabeyli. Vancouver, BC. 2002.

13. Engineering Guide for Wood-frame Construction,Canadian Wood Council, Ottawa, 2001.

14. Wood-Frame Construction Manual for One and TwoFamily Dwellings. American Forest and PaperAssociation, 2001.

15. CSA O86-01, Engineering Design in Wood, CanadianStandards Association, 2001.

16. National Design Specification® for Wood Construc-tion, American Forest and Paper Association, 2001.

17. A Report to the Governor and the Legislature onLessons Learned from Recent Earthquakes In Turkey,Greece, and Taiwan. Seismic Safety Commission, 2000.

Canadian Wood Council/Conseil canadien du bois99 rue Bank Street, Suite/bureau 400Ottawa, Ontario K1P 6B9Tel/Tél : 1-800-463-5091Fax/Téléc : (613) 747-6264

www.cwc.ca

© 2003 CANADIAN WOOD COUNCIL

Other titles in the Canadian Wood CouncilBuilding Performance Series