Seismic Retrofit of Historic Buildings: Keeping Preservation in the Forefront

16 Pages·1997·1.9 MB·English

by  Look, David

29.84/4:41
^RESERVATION
41
BRIEFS
The Seismic Retrofit of Historic Buildings:
Keeping Preservation in the Forefront
David W. Look, AIA, Terry Wong, PE,
and Sylvia Rose Augustus
P
deposTtoryitem
U.S. DepartmentoftheInterior
NationalPark Service JAN 15 1998
Cultural Resources
Heritage Preservation Services CLEMSON
LIBRARY
Violent, swift, and unpredictable, earthquakes result from This Preservation Briefprovidesessential informationon
sudden movements ofthe geological plates that form the howearthquakesaffect historicbuildings, howa historic
earth's crust, generally along cracks orfractures known as preservation ethiccan guide responsible decisions, and how
"faults." Ifa building has notbeen designed and various methods ofseismic retrofitcan protect human lives
constructed toabsorb these swaying ground motions, then andhistoricstructures. Because many ofthe termsused in
major structural damage, oroutrightcollapse, can result, this Briefare technical, a glossary is provided on page 7.
with grave risk to human life. Historicbuildings are The Brieffocuses on unreinforced masonrybuildings
especially vulnerablein this regard. As a result, moreand because theseare themostvulnerableofourolderresources,
morecommunities arebeginning to adopt stringent but the guidanceis appropriatefor all historicbuildings.
requirements forseismic retrofitofexistingbuildings. And Damage to non-structural elements such as furnishingsand
despitepopularmisconceptions, therisksofearthquakes are collections isbeyond the scopeofthis Brief,butconsider-
notlimited to the WestCoast, as theSeismicZone Map on ation should begiven to securing and protecting these
page 14illustrates. cultural resources aswell.
Although historicand otherolderbuildings canbe retro- Planning theretrofitofhistoricbuildingsbeforean earth-
fitted to surviveearthquakes, many retrofit practices damage quake strikes isa process thatrequires teamworkon the part
ordestroy the very features that makesuch buildings ofengineers, architects, codeofficials, and agency admini-
significant. Life-safetyissuesare foremostand, fortunately, strators. Accordingly, this Briefalso presents guidanceon
therearevarious approaches which can savehistoric assembling a professional team and ensuring its successful
buildingsboth from the devastationcaused by earthquakes interaction. Project personnelworking togethercan ensure
and from the damageinflicted bywell-intentioned but that thearchitectural, engineering, financial, cultural, and
insensitiveretrofit procedures. Building owners, managers, social values ofhistoricbuildingsarepreserved, while
consultants, and communities need tobe actively involved rendering themsafe forcontinued use.
in preparingdocumentsand readying irreplaceablehistoric
resources from these damages (seeillus.l).
a.
i
QO
loo
1. Earthquakedamagetohistoricbuildingscan berepaired ina maimersensitiveto theirhistoriccharacterasseen in thisca. 1928fivestoryapartment
building. Theowners usedacombinationoffederalrehabilitation taxcredits,communitydevelopmentblockgrants,andpostearthquakegrants tofunda CIO
portionoftherehabilitationandseismic upgradecosts. Photos:HistoricResources Group,LosAngeles.
Balancing Seismic Retrofit and Preservation Earthquake Damage
Reinforcinga historic building to meetnew construction to Historic Buildings:
requirements, as prescribedby manybuilding codes, can Assessing Principal
destroy much ofa historicbuilding's appearanceand Risk Factors
integrity. This isbecause themostexpedientways to
reinforceabuilding according to suchcodes areto impose Typical earthquake damage
structural members and to fill irregularities or large to most olderand historic
openings, regardless oftheplacement ofarchitectural buildings—results from p—oor
detail. The results canbequite intrusive (see illus. 2). ductility or flexibility of
However, structural reinforcementcanbeintroduced thebuilding and,
sensitively. In such cases, its design, placement, patterning, specifically, poorstructural
and detailingrespect thehistoric characterofthebuilding, connectionsbetween walls,
evenwhen thereinforcement itselfis visible. floors, and foundations
combined with thevery
Three importantpreservation principles shouldbekept in heavyweightand massof
mind when undertakingseismic retrofitprojects: historic materials that are
moved by seismic forces
• Historic materialsshouldbepreservedand retainedto the and mustberesisted. In
greatestextentpossibleand not replacedwholesalein the buildings that have not
processofseismicstrengthening; been seismically upgraded,
• Newseismic retrofitsystems,whetherhidden orexposed, particularly unreinforced
shoidd respect thecharacterandintegrityofthehistoric masonrybuildings,
buildingandbevisuallycompatiblewith itin design; parapets, chimneys, and
and, gableends maydislodge
• Seismicworkshouldbe "reversible" to thegreatestextent and fall to the ground
possible toallowremovalforfutureuseofimproved duringa moderate to severe
systemsand traditional repairofremaininghistoric earthquake (see illus. 3).
materials. Walls, floors, roofs,
skylights, porches, and
Itis strongly advised thatall owners ofhistorically stairs which rely on tied
significantbuildings contemplatingseismic retrofitbecome connections may simply
familiarwithTheSecretaryoftheInterior's Standardsfor the
fail. Interiorstructural
TreatmentofHistoricProperties, whichare published by the supports maypartially or
National ParkServiceand cited in thebibliographyofthis
totally collapse.
publication. Thesestandards identify approaches for
Unreinforced masonry
workingwith historicbuildings, including preservation,
wallsbetween openings
rehabilitation, and restoration. Code-required work to
often exhibit shear (or
makebuildings functionaland safe is an integral diagonal) cracking. Upper
componentofeach approach identified in the Standards. stories maycollapseonto
While someseismic upgradingworkis more permanent under-reinforced lower
than reversible, caremustbe taken topreservehistoric
floors with large perimeter
materials to the greatest extentpossible and fornew work
openings oratriums.
to havea minimalvisual impacton the historicappearance
Unbraced infill material
ofthebuilding.
between structural orrigid
frame supports may
dislodge. Adjacent
buildings with separate
foundations maymove -r—T
differently in an earthquake
creatingdamagebetween
them. Poorly anchored
wood framebuildings tend ezx
T~E i i i rr
to slideofftheir
foundations. Ruptured gas
and waterlines often cause XLZL
^^m
fire and waterdamage.
Many ofthese
vulnerabilities canbe
mitigated by understanding
how the forcesunleashed 3.Forces from moderatetoserious
in an earthquakeaffect the earthquakescauseda) theuntiedgable
2. Standardapproaches toseismicretrofit,asseen with thediagonally building, then planning tofail, b) thefirstfloortocollapsec)
binrtarcuesdivfer.amSeolcurtoisosnisn,gsiuncfhroanstuosfinthgehhiidsdteornicmowimnednotwsfr,aamreesvairsuoaulnldythe and implementing acdrajcakcsenftrboumiltdhienpgso,unadnidndg)eaffnedctdoifagonal
perimeterofthewindow,willmeet thegoalsofhistoricpreservationand appropriate remedial cracks in exteriormasonrybetween
seismicretrofit. Photo: SteadeCraigo. treatments. windows toform. Photos: DavidLook.
Six principal factors influence howand why historic Type ofBuildingand Construction
beuairltdhiqnugaskearaenddasmuabgseedquiennatnsteraerntghtqhuaokfee:ar(t1h)qdueapktehwoafvtehse A historicbuilding's construction and materials determine
reaching the surface; (2) duration ofthe earthquake, witosobdeheanviforramdeursitnrgucatunreesa,rtahrqeuqaukiet.edSuocmtielebuainldd,intghsu,s,saubclheatso
includingafter-shock tremors; (3) proximity ofthebuilding absorb substantial movements. Others, such as unreinforc-
to theearthquake epicenter, although distance is not
ed brickoradobebuildings comprised ofheavy individual
necessarily a directrelationship; (4) geological and soil load-bearing units, aremoresusceptible todamage from
conditions; (5) building construction details, including
materials, structural systems, and plan configuration; and shaking. Ifan earthquake is strong, or continues fora long
time, building elements thatare poorly attached orunrein-
(6) existingbuilding condition, including maintenance forced may collapse. Most historicbuildings still standing
level. inearthquakezones have survived some shaking, but may
—
The first th—ree factors thedepth, duration, and proximity be structurally weakened.
etoartthheqfuaaukltes haarveebesyhoonwdnhtuhemafonurctohntfraoclt.or,Regceeonltogical soil Buildings ofmore rigid construction techniques may also
have seismic deficiencies. Masonry infill-wall buildings are
conditions, tobe as importantas anyofthe otherfactors
generallybuilt ofsteel orconcrete structural frames with
because loose, softsoils tend toamplify ground motion,
unreinforced masonry sections or panels setwithin the
thereby increasing damage. Further, there is the tendency frame. While the structural frames maysurvivean earth-
ofsoft,unstablesoils to "liquefy" as the ground vibrates,
causing thebuilding foundations to sinkunevenly. This quake, the masonry infill can crackand, insome cases,
dislodge. The reaction ofconcretebuildings and concrete
fourth factor, geological and soil conditions, is difficult to
ainddnreewsscionnsatrruectrtoifoint.siTthuaetiloans,t talwtohfoaucgthorist—catnhebebupilladninnge'dsfor cfornafmiegusrtartuicotunroefsiirsolnarogrelstyedelepreeinndfeonrtceumpenotn.thEearelxytent and
— buildings constructed ofconcreteare often inadequately
construction typeand its existing physical condition are
the two factorsoverwhichbuildingownersand managers reinforced, inadequately tied, orboth, and are thus
have control and canultimatelyaffecthowthehistoric susceptible to damage during earthquakes.
property performs in an earthquake (see illus. 4). Recognition ofthe configuration ofthe historic structure
and inherent areas ofweakness are essential to addressing
appropriate alternatives forseismic retrofit. Forexample,
the plan and elevation maybeas importantasbuilding
materials and structural systems in determining a historic
building's survival in anearthquake. Small round, square,
iiAt
orrectangularbuildings generally survive an earthquake
if
because theirgeometry allows forequal resistanceoflateral
forces in all directions. The more complex and irregularthe
plan, however, the more likely thebuildingwillbe
damaged during an earthquakebecauseofits uneven
strength and stiffness in different directions. Structures
havingan "L," "T," "H," "U," or "E" shape have unequal
resistance, with the stress concentrated atcorners and
intersections. This is ofparticular concern ifthebuildings
have flexible structural systems and/oran irregular layout
ofshear walls which may cause portions ofthebuilding to
4. Thecompactsizeandgoodconditionofthemasonrybuildingon the pull apart.
leftwithstoodtheearthquakeexceptforthelossofthe unsupported
chimneyat theroofline. Thebrickbuildingon therightappearstohave Similarly, the more complex and irregularabuilding
sustainedmoredamage. Photo:SteadeCraigo. elevation, the more susceptible itis to damage, especiallyin
tall structures. Large or multiple openings around the
Although historicbuildings present problems, theway they building on the ground level, such as storefronts orgarage
were constructed often has intrinsicbenefits that should not openings, or floors with columns and walls running in only
beoverlooked. Diagonal subflooringunder tongue-and- onedirection are commonly known as "soft stories" and are
groove nailed flooring can provide a diaphragm, or prone to structural damage.
horizontal membrane, that ties thebuilding together.
Building Condition
Interior masonry walls employing wire lath with plaster
also add strength thatbinds materials together. The typical Much ofthe damage thatoccurs duringan earthquake is
construction ofolderbuildings with partition walls that directly related to thebuilding's existingcondition and
extend from floor to ceiling (instead ofjust to theunderside maintenance history. Well maintained buildings, even
ofa dropped ceiling) also provides additional supportand withoutadded reinforcement, survivebetter than buildings
load transferduring an earthquake that keeps shifting weakened by lackofmaintenance. Thecapacity ofthe
floors from collapsing. Moreover, buildings constructed of structural system to resist earthquakes maybe severely
unreinforced masonrywith a wall thickness to height ratio reduced ifprevious alterations or earthquakes have
thatdoes not exceed code requirements can often survive weakened structural connections or ifmaterials have
shaking without serious damage. Thestability of deteriorated from moisture, termite, orotherdamage.
unreinforced masonrywalls should notbeunderestimated; Furthermore, in unreinforced historic masonrybuildings,
while the masonry may crack, itoften does not shiftoutof deteriorated mortarjoints can weaken entirewalls. Cyclical
plumb enough to collapse. maintenance, which reduces moisture penetration and
erosion ofmaterials, is thereforeessential. Becausedamage these major features, such as domes, atriums, and vaulted
canbecumulative, it is important to analyze the structural spaces or highly decorative elements, such as mosaics,
capacity ofthebuilding. murals, and frescoes. In somecases, secondary areas ofthe
building can provide spaces foradditional reinforcement
Overtime, structural members canbecome loose and pose
behind these major features, thus saving them from damage
a major liability. Unreinforced historic masonrybuildings
during seismic retrofit work. Both primary and secondary
typically havea friction-fit connection between horizontal
spaces, features, and finishes should, thus, be identified.
and vertical structural members, and theshakingcausedby
anearthquake pulls them apart. With insufficientbearing Assessing thecondition ofthe buildingand the riskhazards. The
surfaceforbeams, joists, and rafters against theload team then assesses the general physical condition ofthe
bearing walls orsupportcolumns, they fail. Theresulting building'sinteriorand exterior, and identifiesareas
structural inadequacy maycause a partial orcomplete vulnerableto seismic damage. This often requires a
building collapse, depending on the severity ofthe structuralengineerortesting firm to determine the strength
earthquakeand the internal wall configuration. Tying the and durability ofmaterials and connections (see illus. 5). A
building togetherby making a positiveanchored orbraced sliding scale ofpotential damage is established,based on
connectionbetween walls, columns, and framing members, theprobability ofhazard by localeand building use. This
is key to the seismic retrofitofhistoricbuildings. helps the ownerdistinguish between areas in which
repairable damage, such as cracking, mayoccurand those
Putting a Team Together inwhich life-threatening problems may arise. These
findings help guide cost-benefitdecisions, especiallywhen
The two goals ofthe seismic retrofitin historicbuildings budgets are limited.
are life safetyand the protection ofolder and historic 5.Acarefulprogramofin-place
buildings duringand afteran earthquake. Because testingisessentialtoevaluate
rehabilitation should be sensitive to historic materials and theexistingseismiccapacityof
thebuilding's historiccharacter, itisimportant to put a building. This masonrypush-
togethera team experienced inboth seismic requirements testuseshydraulicjacksto
and historic preservation. Team members should be estimatetheshearcapacityof
thewall. Testlocationsshould
selected fortheir experience with similar projects, and may beinareasthatdo notdestroy
include architects, engineers, code specialists, contractors, significantfeaturesandrepairs
and preservationconsultants. Because the typical seismic shouldbecarriedoutcarefully.
codes arewritten fornewconstruction, it is important that PGhrootuop:,ASrachnitFreacntcuirsacloR.esources
both the architectand structural engineerbe knowledge-
able abouthistoricbuildings and about meetingbuilding
code equivalencies and using alternative solutions. Local
and statebuilding officials can identifyregulatory
requirements, alternative approaches to meeting these Evaluatinglocalandstate
codesand requirements. Few
requirements, and ifthejurisdiction uses a historic
codesconsiderhistoric
preservation orbuilding conservation code. Even on small
buildings,buttheCalifornia
projects thatcannot supporta full professional team,
StateHistoricalCodeand
consultants should befamiliarwith historic preservation
theUniformCodefor
goals. TheState Historic PreservationOffice and the local
historic preservation office orcommission maybe able to BuildingConservation
identify consultants who havebeen successful in provideexcellentmodelsforjurisdictions toadopt. Code
officialsshould alwaysbeasked wherealternative
preserving historicbuildings during seismic retrofitwork.
Once the team hasbeen assembled, their tasks include: approachescanbe taken toprovidelifesafetyifthespecified
requirementsofa codewould destroysignificanthistoric
Compilingdocumentation. The team should reviewall materialsand features. Somejurisdictions requirethe
available documentation on the historicbuilding, including removalofparapets, chimneys,orprojectingdecorationfrom
any previous documentation assembled to nominate the unreinforced masonrybuildingswhich isnota preservation
structure to the National RegisterofHistoric Places, and approach. Professionalson the teamshould beprepared
any previous HistoricStructures Reports. Original plans withalternatives thatallowformitigatingpotentialdamage
and specifications as well as those showing alterations tosuch featureswhileretaining them through reattachment
through time often detail structural connections. Early real orstrengthening.
estate or insurance plans, such as the Sanborn Maps, note
changes over time. Historic photographs ofthebuilding Developinga retrofitplan. Thefinal taskoftheprojectteamis
todevelopa retrofit plan. Theplan mayrequiremultiple
underconstructionorbeforeand afterprevious
earthquakes are invaluable. Base maps forgeological or treatments,each morecomprehensivethan thelast. Treating
seismic studies and utility maps showing thelocationof life-safetyissuesaswellas providingasaferouteofexit
water, gas, and electric lines should bealso identified. The shouldbeevaluated forallbuildings. Developing more
comprehensiveplans,oftencombined with futurerehabili-
municipal or stateofficeofemergency preparedness can
providedata on earthquake hazard plans for the tation, isreasonable. Long-term restoration solutionsphased
community. inovertimeas fundingisavailableshould alsobeconsid-
ered. In everycase,ownersand theirplanning teams should
Evaluatingsignificantfeaturesandspaces. The team mustalso consideroptions thatkeeppreservationgoalsin mind.
identify areas ofa historicbuilding and its site that exhibit
Therearesignificant advantages ofcompleting a seismic
design integrity or historical significance which mustbe
preserved. Itis critical, and a great challenge, to protect survey and analysis even ifresourcesforimplementinga
retrofitare notyetavailable. Once the retrofit plan is
finished, the project team will havea documentby which to
assess future damage and proceed with emergency repairs.
Ifconstruction is phased, its impact to thewholebuilding
should be understood. Some partially completed retrofit
measures haveleft buildings more rigid in onearea than in
others, thereby contributing to more extensivedamage
during an ensuingearthquake.
Planning for Seismic Retrofit: How Much
and Where?
The integrity and significanceofthe historic building,
paired with thecostand benefitofseismicupgrading, need
tobeweighed by the ownerand the consulting team.
Buildings in lessactive seismicareas may need little or no
furtherbracingor tying. Buildings in moreactive seismic
zones, however, may need moreextensiveintervention.
Options for thelevel ofseismic retrofit generally fall into 7.Moreextensiveseismicissuescan beaddressed throughstructural
fourclassifications, depending on the expected seismic reinforcement, themostcommon methods usinganchortiesandbraces.
activity and the desired level ofperformance. Realistically, Shownhereisan interiordiagonalframe,tobecovered,which will
dampenand transferseismicloads inadesignedpathfromfoundation to
forhistoricbuildings, only the first three categories apply. roof. Photo:DavidLook.
1) Basic Life Safety. This addresses the most serious life-
safety concerns bycorrecting those deficiencies that
could lead to serious human injuryortotal building
collapse. Upgrades may includebracing and tying the
mostvulnerable elements ofthebuilding, such as
parapets, chimneys, and projecting ornamentation or
reinforcing routes ofexit, (see illus. 6). It is expected that
ifanearthquake wereto occur, thebuilding would not
collapsebutwould be seriously damaged requiring
major repairs.
2) Enhanced Life Safety. In this approach, thebuildingis
upgraded using a flexibleapproach to thebuilding codes
for moderate earthquakes. Inherentdeficiencies found in
olderbuildings, such as poorfloor to wall framing
connections and unbraced masonrywalls would be
corrected (see illus. 7). Aftera design level earthquake,
somestructural damage is anticipated, suchas masonry
cracking, and thebuilding would be temporarily
unusable.
SheathingCrippleWalls
WallDetail
Framingclips
plaratoten16na"encsdnpnadgcmitnjoogpist e8.arFtuhlqluaskeeiswmiitchraesmtiruncitmuruimngoftodeanmsaugreemthaaytbiunivlodlivnegesxsteunrsviivveeamajor
reinforcement. Uponcompletion, thechanges to thisca. 1932 Gothic
Revivalbuildingtoaddbaseisolationat thefoundation werenotvisually
apparent. Photo:©Jonathan Farrer,courtesy UniversityofCaliforniaat
LosAngeles(UCLA),CapitalPrograms.
3) Enha)iced DamageControl. Historicbuildings are
substantially rehabilitated to meet, to the extent possible,
'ReinbfaorrscingNpHVoa*ltvdlecedmsnoosddlwtewiohnafagilntetohac8p'acatnheedl Naatlpilnygwehoooo1lcdWehbsjeostiotinoStvdosae«aevnen/ndtsitbluoadttsitoonm aSthomemapjeroemrsicenraoirrbtehrdqeupbaaukiierl.adbilnegdcaomdaegperowvoisuilodnb(eSeeexiplelcust.e8d).after
MATERIALSNEEDED
SSa1iid/2mm"cNpposs8SoomTnnnmaRoiAHUln3DsCS2onTruAdUlaelRrqhnuAoiilLvndagdtIeoconlwrilnpCso-wrDieiXqhupilvayjwconold JCFA1iirn-rg1acc/smhu2aio"lwnraghrobslosleqaluwssaarwe PlywCohoadlkbllaidnee 4dc)eensItimegmrnesadtireaedtmeahioOnscipcinutgpaalonspceaynn.daTnehdimseorapgeperpanrtcoiyaocnpharleispafaitrneetrdeannedmesadsjofror
earthquake. Even mostmodern buildings do notmeet
6. Often simpleapproaches,suchas nailingplywoodstiffenersbetween this level ofconstruction, and so fora historic buildingto
cfseroiauswnmdliasctpiadocanemsasgtcueadn.smIalalnkudsetoranattdioroafnml:aoRotreicpjordiosidtffuaecbreoednvceweiaitnnhdpprbeoortlmteiicsntsgiinsoginlalfpbrluoadtmedsHinotogmferom rmeeceotnstthrisucrteeqduiorfenmeenwt,maittewroiuallsdwhhaivceh,tophbieloaslompohsitcatloltya,lly
EarthquakePreparednessGuide. EQEIncorporated,San Francisco,CA. does not reflect preservation criteria.
—
e
Devising the mostappropriateapproach for a
particularhistoricbuilding will depend on a
variety offactors, including thebuilding's use,
whetheritremainsoccupied during
construction, applicablecodes, budgetary i
constraints, and projected riskofdamage. 9
From a design perspective, thevastmajorityof
historicbuildings can toleratea well-planned 7\:
hidden systemofreinforcement. Utilitarian ^#f: 1 fiyitiv.
structures, such aswarehouses, maybeable to
Pr!r
receive fairly visible reinforcement systems
without unduedamage to their historic
character. Othermore architecturally detailed
buildings or those with more finished interior
surfaces, however, willbenefit from more
hidden systems; installation ofsuch systems
mayeven require the temporary removal of
significant features to assure theirprotection. 9.Thesestudiesforapublicbuildingcompared, in theshadedareas, theamountofhistoric
Mostbuildings, particularly commercial material thatwouldbeaffectedby(a) the Uniform BuildingCoderequirements,(b)engineering
rehabilitations, can incorporate seismic alternatives thatprotectedsignificanthistoricmaterials,and(c) theuseofbaseisolationsystems.
strengthening during otherconstruction work Thecostforimplementingthe3proposalswassimilar,andwhile proposal(c)wasselected there
in a way that ensures a high degree of wEherreenmkraannytzp,osEictkisvteuta,sp&ectKsuthonboAtrhch(ibt)ecatnsd.(c). Photos: GeorgeSiekkinen,withpermissionfrom
retention ofhistoric materials in place.
Assessing the Cost of Seismic Retrofit Some municipalities and states provide low-interestloans,
taxrelief, municipal bonds, or funding grants targeted to
Costplays a criticalrolein selecting the mostappropriate seismic retrofit. Federal tax incentives for the rehabilitation
retrofit measure. Itisalwaysbestto undertake retrofit ofincome-producing historicbuildings include seismic
measuresbefore an earthquakeoccurs, whenoptions are strengthening as an allowable expense. Information on
available forstrengtheningexisting members. Once these incentives isavailable from the State Historic Preser-
damage is done, thecostwill besubstantially higherand vationOffice. It is also in thebest interestofbusiness
finding engineers, architects, and contractors available to do communities to support the retrofit ofbuildings in
theworkona constricted schedulewillbemoredifficult. seismically activeareas to reduce thelossofsales and
Planned seismic retrofitwork mayadd between $10 and property taxes, should an earthquakeoccur.
$100 persquarefootto thecostofrehabilitation work
depending on thelevel ofintervention, the condition ofthe Seismic Strengthening Approaches
building, and whetherworkwillbeundertaken while the
building is occupied. Costs can exceed several hundred Seismic strength within buildings is achieved through the
dollars a square foot forcombined restoration and seismic reinforcement ofstructural elements. Such reinforcement
upgrade costs in majorpublic buildings, in order to provide canincludeanchored ties, reinforced mortarjoints,braced
a level ofstructural reinforcement thatwould require only frames,bond beams, moment-resisting frames, shearwalls,
minorrepairsaftera majorearthquake. But maintenance and horizontal diaphragms. Most historicbuildings can use
and incremental improvements to eliminate life-safety risks thesestandard, traditional methods ofstrengthening
arewithin the cost realmofresponsibleupkeep. successfully, ifproperly designed to conform to the historic
characterofthebuilding. In addition, there are new
Each propertyownerhas to weigh the costs andbenefits of technologies and betterdesigns for traditional connection
undertaking seismicretrofitin a timely manner. Owners devices as wellasa greateracceptance ofalternative
may find thatan extended engineering study evaluating a approaches to meeting seismic requirements. Whilesome
wide range ofoptions is worthwhile. Not only can such a technologies may stillbenew forretrofit, the key preserva-
study consider the mostsensitive historicpreservation tion principles on page2 should be applied, to ensure that
solution, but the mostcost-effective one as well. In many historicbuildings will notbe damaged by them. Foran
cases, actual retrofitexpenses havebeen lowerthan illustrated design guideline forusingsome ofthe more
anticipated becausea careful analysis ofthe existing traditional methods on theexteriors of historic unrein-
building was made that took the durabilityand forced masonrybuildings, see illustration 10 on pages 8-9.
performance ofexisting historic materials into
consideration. Mostseismic retrofit is done incrementally Therearevarying levels ofintervention forseismically
or incorporated into other rehabilitation work. In large retrofitting historicbuildingsbased on the owner's
publicbuildings, seemingly expensive "high-tech" solution program, therecommendations ofthe team, applicable
such as installing foundation base isolators can turn out to codes, and the availability offunds. Theapproaches to
bejustified becausesignificant historic materialsdo not strengtheningbuildingsbeginning on page 10 are to show a
have tobe removed, replaced, orreplicated (see illus. 9). rangeoftreatments and are not intended tocoverall
Thecost fora fully retrofitted building can offset the poten- methods. Each building should be evaluated byqualified
tial loss ofincome, relocation, and rebuildingafteran earth- professionals prior to initiating any work.
quake. Without careful study, thesesolutions often are not
evaluated.
Maintenance/Preparedness
GLOSSARY:
Adequatemaintenanceensures thatexisting historic
materialsremain ingood conditionand arenotweakened by
AnchorTies orbolts: Generally threaded rods orbolt
rot, rust, decayorothermoistureproblems. Without which connectwalls to floorand roofframing. Washers,
exception, historicbuildings should bewell maintained and plates, orrosettesanchor thebolt in place.
anevacuation plan developed. Expectation thatan earth-
quakewill occursometimein thefutureshould preparethe Base isolation: theability to isolate the structures from
ownertohaveemergencyinformationand supplieson hand. the damaging effects ofearthquakesby providinga
flexible layerbetween the foundations and vertical
• Checkroofs, gutters, and foundations formoisture supports.
problems, and forcorrosion ofmetal ties for
parapets and chimneys. Make repairs and keep Diagonal Braces: the use ofdiagonal, chevron orother
type ofbracing (X orK) to provide lateral resistance to
metal painted and in good condition.
adjacentwalls.
• Inspect and keep termite and wood boring insects
away from wooden structural members. Check Core drilling: a type ofvertical reinforcementof
masonry walls that relies on drillinga continuous
exitsteps and porches to ensure that they are vertical core that is filled with steel reinforcing rods and
tightly connected and will notcollapse during an grouting to resistin-planeshearand out-of-plane
emergencyexit. bending.
• Check masonry fordeteriorating mortar, and Cripple wall: A frame wall between a building's first
neverdeferrepairs. Repoint, matching the historic floorand foundation.
mortarin composition and detailing.
Diaphragm: A floor, roof, orcontinuous membrane that
• Contactutility companies for information on flex- provides for the transferofearthquake loading to the
ible connectors forgas and water lines, and earth- exterior orinterior shearwalls ofthe structure.
quakeactivated gas shut-offvalves. Strap oil tanks
down and anchorwaterheaters to wall framing. Fiberwrap reinforcement: A synthetic compound of
filaments thatincrease the shearcapacity ofstructural
• Collectlocal emergency material for referenceand members.
implementsimple household oroffice mitigation
measures, such as installing latches to keep Grouted bolts: anchorbolts set, generally on an angle, in
cabinets from flying open orbraces toattach tall a concrete grout mixture, avoid the problem ofusingan
bookcases to walls. Keep drinking water, exposed washer. Requires a greaterdiameter hole than
an anchorboltwith washer.
tarpaulins, and other emergency supplies on hand.
Lateral forces: Generally the horizontal forces
Basic/TraditionalMeasures transferred to thebuilding from the dynamic effects of
This is not anexhaustivelist,but illustrates thatmost wind or seismic forces.
measures toreduce life-safety risks rely onusing Life-safety: providing a levelofassurance thatrisk of
mechanical fasteners to tieabuilding together. loss oflife is keptto minimal levels. Forbuildings, this
Incorporating thesemeasures canbe doneincrementally includes strengthening to reduce 1Structural collapse,2)
without waiting forextensiverehabilitation (see illus.ll- falling debris, 3)blocking exits oremergency routes, and
12). An architectural orengineering survey should identify 4) preventionofconsequential fire.
what is needed. Care should be taken to integratethese Moment-resisting frame: A steel frame designed to
changes with the visual appearanceofthebuilding. provide in-plane resistance to lateral loads particularly
by reinforcing thejointconnectionbetween column and
beamswithout addinga diagonalbrace. Often used as a
perimeterframearound storefronts orlargedoor and
window openings.
Seismic retrofit: All measures that improvethe
earthquake performanceofa building especially those
thataffect structural stability and reduce the potential
forheavy structural damage orcollapse.
Shearstress: A concept in physics where forces actona
body in oppositedirections,butnot in the same line.
Horizontal forces applied to a wall that is insufficientto
movewith theseforces will crack, often in a diagonalor
X pattern. Connections atbeams and walls will also
crackfromshearstress.
Shearwall: A wall deliberately designed to transferthe
building's loads from the roofand floors to the
foundation thereby preventinga building from collapse
from wind orearthquake forces.
11. Limitedinterventionshouldcorrectobviousstructuraldeficiencies, Unreinforced Masonry (URM): This designation refers
suchas tyingvulnerableelements togetherand repointingmasonry.
Seen hereis 1)anchoredbolt,2) metaljoist strap,and3) repointingand to traditional brick, block, and adobe construction that
reinforcing masonryjoints. Upon replasteringandpainting these relieson theweight ofthemasonry and thebonding
reinforcements will notbevisible. Photo:HistoricPreservation Partners capacity ofmortar to provide structural stability.
forEarthquakeResponse.
KEEPING PRESERVAT1
Anchor Bolts:
Typically 'A" bolts with flat metal
washers (sometimes called plates or
rosettes) are probably the mostcommon
retrofitprocedure. The tie the exterior
wall to the floorsand roofcausing the
building to moveas a single unit.
The washers are the mostnoticeable part
ofthesystem. Anchorbolt locations are
determined by thestructural engineer.
Decorative washers, such as cast iron
stars, carefully placed, can enhance the
building. Poorly placed orcarelessly
aligned washers areverynoticeable.
Itisimportant to control rustby painting
ferrous metal washers. Newwasherscan
be specified as stainless orgalvanized
steel. In circumstanceswherewashers Recommended NotRecommended
arevisibly intrusive, thepreferable ••UAlsiegndewcaorsahteirvsetwoacsrheeartseoirndaerrelaysawpipteharhaingche.visibility. •Tahheapahnaczhaorrdboflatsshioonn.thMiosrbeuicladriengshwoeurledpoleacteadkeinn
solution would be to recess thembelow •Usestainlessorgalvanizedsteelandpaintwhen toaligntheanchorboltwashers.Also,painting
the face material. This is particularly •Aaptptreompprtiattoe,cotnocperalevtehnetbroulsttssatnrdeakwsa.shersbelowthe tthhaetwraesshuletrsfrcoamnwreeadtuhceeritnhge.unsightlyruststreaks
applicable to stuccobuildings. exteriorfinish,whenappropriate. •Donotplaceanchorboltsatlocationswithhigh
reliefornamentation.
Infill Windows:
From an architectural standpoint, infill of
openings is nota desirableremedyand
should beused only as a last resort. Itis
often possible touse abraced frames
instead ofinfilling openings,butitmay
bemoreexpensive.
Thepurposeoffillingtheopeningsisto
increasetheshearcapacityand reducethe
stresseson theunreinforcedmasonrywall.
Itisnotadequatetojustinfillwith the
sameunreinforcedmasonry,butgenerally
areinforced concrete, reinforcedblockor
reinforced brick isspecified. Ifinfilling
theopeningsappearstobetheonlyreal-
isticmethod, thedesignsolution should be
sensitive,and ifpossible,limitedto
secondaryelevations. Theopeningshould
Recommended NotRecommended
besetbackand thefacingmaterial should •Infillofwindowsshouldbeavoidedinallcases. •Infilltechniquessuchasthisarenotencouraged.
becompatiblewiththesurrounding Whereabsolutelyrequired,however,the Suggestionofaformerwindowopeningshould
appearanceofawindowopeningshouldberetained havebeenemphasizedbyslightlyrecessingthe
material. tosuggesttheoriginalvisualrhythmofthefacade. formeropening.
KEY QUES1
Questions to AskWhen Planning Seismic Retrofit: Arethevisible featuresofthereinforcement, suchasanchor
Thesequestionsshouldbeasked with theassistanceofthe teamto washersorexteriorbuttressesadequately designed toblend
determineacceptablealternatives. Sincethereisnevera single withthehistoricbuilding?
rightanswer, thedesign teamand codeofficialsshould work Canhiddenorgroutedboltsbesetonanangletotiefloors
together todeterminetheappropriatelevel ofseismicretrofitwith and wallstogether, instead ofusing traditionalboltsand
thelowestvisual impactonthesignificantspaces, features, and exposed washersorrosettesonornamentalexteriors?
finishesofboth theinteriorand exteriorofhistoricbuildings. Arediagonal frames, suchasX, K,orstrutslocated tohavea
Aswith the illustrationsabove, thisguideis not intended to minimal impacton theprimaryfacade?
proscribehowseismicretrofitshould bedone,butrather, to Aretheysetbackand painted a recedingcolorifvisible
illustrate thateveryphysicalchange toabuildingwillhavesome throughwindowsorstorefronts?
consequence. Byaskinghowimpactscanbereduced, theowner Can moment framesorreinforcedbracingbeadded around
will haveseveral options fromwhich tochoose. historicstorefronts inordertoavoid unsightlyexposed
Canbracingbeinstalled withoutdamagingdecorative reinforcement,suchasXbraces,within theimmediate
detailsorappearanceofparapets, chimneys,orbalconies? viewingrangeofthepublic?
*
IN THE FOREFRONT
Sf
Securing Exterior Ornamentation:
Ornament is one ofthecharacter-defining
features ofa building. Careful forethought
***Z "^^tM'wwS and analysis should always precede
]f]|TW! alteration ofa building's ornament.
Generally methods to secure ornament-
— mvm m ationby repairand reinforcing
f??*^ i 1 :^r connections should be undertaken.
Repairs orreinforcement should blend
lV«Afc-p-Matu« with theappearance oftheornamentation
and should be designed toprevent future
R•eAlcloomrmigeinndalebduildingornamentationenhancesthe N•oItfitRiescdoemtemremnidneeddthatornamentationmustbe failures such as cracking due to thermal
architecturalvalueandshouldberetainedand securedorremoved,effortshouldbemadetosecure and seismic stress orunsightlydifferential
maintained. it.Theparapetofthisbuildingshowsa"scar"where weathering.
ornamentationwasremoved.
Ifornamental elements mustbe removed
during the repairprocess, they should be
reinstalled orreplaced in-kind. The useof
substitutematerials maybe acceptable if
no otheroptions exist.
Exterior Buttresses:
Exteriorbuttresses, an integral part of
Gothicarchitecture, arenottraditionally
Hilt part ofourarchitecture. In retrofitting an
existingbuilding, itis usuallybetter touse
m- m W1 ! an in-wall orinteriorbracing system rather
- r than a visibleexteriorsystem. When used
as anexteriorbracing system, care mustbe
taken toavoid damage to existing decora-
tive elements. Even ifsaved, exteriorbut-
tresses can obscure decorative elements.
Recommended NotRecommended Another problem requiring careful study
•Ethxetebruiiolrdbirnga'csinngatourrbaultltirneess.siTnhgeshexotuelrdioirnsctoereplorate •Taphpeeaexrtaenrcieo.rbCraarceinsghoounldthbiesbtuaiklednintogddeosmiignnatexetseriitsor is the integration ofthebuttresses with the
bracingappearstobeanoriginalbuildingelement bracingtoblendwithorenhancethebuilding's existing structural system. Their attach-
becauseitrunsparalleltothecorniceline. naturallines. ment penetrates thebuilding skin making
thebuilding more vulnerable to moisture
damage. In a few cases where the interior
building fabric is highly significant, exter-
Adaptedfrom "Architectural Design Guidefor Exterior Treatme'ntsofUnreinforced iorbuttresses maybe preferred. Care
Masonry Buildings During SeismicRetrofit." Usedwith permissionfrom the San should be taken to avoid damage or
Francisco Chapter, TheAmerican InstituteofArchitects. Drawings©Cassandra obscuring existing architectural details.
Mettling-Davis.
4S TO ASK:
Canshortersectionsofreinforcementbe "stitched" into Should baseisolation, wall dampingsystems, orcore
theexistingbuildingtoavoid removaloflargesections of drillingbeconsidered? Aretheyprotectingsignificant
historicmaterials? Thisisparticularly trueforthe materialsbyreducingtheamountofintervention?
insertionofroofframingsupports. Aretheseismic treatmentsbeingconsidered "reversible"
Canshearwallsbelocated in utilitarianinteriorspaces to ina waythatallowsthemostamountofhistoricmaterials
reducethe impactonfinishes in the primaryareas? toberetained and allowsfuturerepairand restoration?
Aretheresituationswherethinnerapplied fiberreinforced
coatingwould adequatelystrengthenwallsorsupports
withouttheneed forheavierreinforced concrete?
Can diaphragmsbeadded tonon-significantfloorsin 10. Keepingpreservation in theforefront is a criticalaspectof
order toprotecthighlydecorated ceilingsbelow,orthe seismic retrofit ofhistoricbuildings. Thesekeyquestions will
reverseifthefloorismoreornamental thantheceiling? help keeppreservation in mindas decisionsare madeabouthow
Are thereadequatefunds toretain,repair, orreinstall best to improve thestructuralperformanceofhistoric buildings.
ornamental finishes oncestructural reinforcementshave
been installed?
V
Ifthrough-bolts are used,
considerexteriorappearance
tr
in location and detailingof rffrrrrrrrrrrrrrfrrrrr
bolt plates SSBrfffSffiTW
w w u
Ifexteriorappearance is sen- \\
[rrrrcrffrrrrrrrrrrrrrrFf
sitive, considergrouted bolts
»
n&g rc&qjg
pf
New steel angle brace at- fffrrrr
tached toexisting roof mm
Masonry wall
12. Bracingparapets,as illustratedhere, andsupportingchimneys
usingmetalstrutsorties,aresimplemethods toprotecttheseheavy
elementsfromfalling. Drawing:ArchitecturalResources Group.
13.Installingdiagonalframes, underwayin this rehabilitation,area
• sBtoilftfesinlelrpsltaotecsritpopfleouwnadlaltifornasmianngdaardodunpdlywwoooodd Xwtar,laldK,i,tdioeornsadiliganmgeeodtnhatolodcbrrooafscsesseb,iesthmhiiencydrssehoiloniufdlodrwcabelemlesonnats.tmheTuocinhrseiaddsuecpoeofststhihebelipem,epraaincmdettoefrthe
framebuildings. Keep reinforcementbehind paintedarecedingcolorwherevisible. Photo:DavidLook.
decorativecrawlspacelattice orother historic
features.
• Reinforce floorand roofframing connections to
wallsusingjoist hangers, metal straps, threaded
bolts, orothermeansofmechanical fasteners. Tie
columns tobeams; reinforceporch and stair
connectionsaswell. steelmomentframe
• Repair weakened wooden structural systemsby
adding, pairing, orbracingexisting members.
Consideradding non-ferrous metal straps in
alternating mortarjoints ifextensive repointingis
done in masonrywalls.
• Reinforce projecting parapets and tieparapets,
chimneys, balconies, and unsecured decorative
elements to structural framing. Makethe
connections as unobtrusive as possible. In some
cases, concretebond beams canbeadded to
reinforce the top ofunreinforced masonryoradobe
• wParlolpse.rly install and anchornewdiaphragms, such d14u.riTnhgeaurseehaobfilaisttaeteilonmoelmiemnintaftredamtehetoneseudpptoorptlatcheedlairaggeonoaplenbrsatcoerseforront
as roofsheathing or subflooring, to the walls ofa otherintrusivesupports inahighlyvisibleareaofahistoricbuilding.
structure priorto installing finish materials. Photo: DavidLook.
• Avoid awkwardly placed exposed metal plates or
rosettes whenusing threaded bolts through
masonrywalls. When exposed plates will interfere s*
with the decorative elements ofthe facade, useless
visible grouted bolts orplates thatcanbe set
underneath exposed finished materials.
• Usesensitively designed metalbracingalong
buildingexteriors to tie theunsupported face of
long exteriorwalls to the floor framing. This is L... ' i
often seen along sideor party walls in commercial
orindustrial buildings.
V
Rehabilitation ««?"
Whenbuildings arebeing rehabilitated, it is generally the gjSflk" '
mostcosteffective time to make major upgrades thataffect
thestructural performance ofthebuilding (seeillus. 13-17).
Newelements, suchas concrete shearwalls orfiber <*" Mi
reinforcing systems canbe added while the structure is
exposed forotherrehabilitation orcode compliancework.
15. Theuseoffibercompositematerialscan enhancetheshearcapacityof
• Inspectand improve all lateral tie connections and existingstructuralcomponents -beams,columns,andsurfaceelements,
diaphragms. suchaswallsandfloors. In this roofingapplication, theexistingroof
diaphragm isbeingstrengthenedand thereisadditionalbenefitto the
shearreinforcementoftheparapetwall. Photo: TheCrosbyGroup.
10