The Seismic Retrofit of Historic Buildings
David W. Look, AIA, Terry Wong, PE, and Sylvia Rose Augustus
If a building has not been designed and constructed to absorb these swaying groundmotions, then major structural damage, or outright collapse, can result, with grave riskto human life. Historic buildings are especially vulnerable in this regard. As a result,more and more communities are beginning to adopt stringent requirements for seismicretrofit of existing buildings. And despite popular misconceptions, the risks ofearthquakes are not limited to the West Coast.
Although historic and other older buildings can be retrofitted to survive earthquakes,many retrofit practices damage or destroy the very features that make such buildingssignificant. Life-safety issues are foremost and, fortunately, there are variousapproaches which can save historic buildings both from the devastation caused byearthquakes and from the damage inflicted by well-intentioned but insensitive retrofitprocedures. Building owners, managers, consultants, and communities need to be activelyinvolved in preparing documents and readying irreplaceable historic resources from thesedamages
This Preservation Brief provides essential information on how earthquakes affecthistoric buildings, how a historic preservation ethic can guide responsible decisions, andhow various methods of seismic retrofit can protect human lives and historicstructures. Because many of the terms used in this Brief are technical, a glossary isprovided at the end. The Brief focuses on unreinforced masonry buildings because these arethe most vulnerable of our older resources, but the guidance is appropriate for allhistoric buildings. Damage to non-structural elements such as furnishings and collectionsis beyond the scope of this Brief, but consideration should be given to securing andprotecting these cultural resources as well.
Planning the retrofit of historic buildings before an earthquake strikes is aprocess that requires teamwork on the part of engineers, architects, code officials, andagency administrators. Accordingly, this Brief also presents guidance on assembling aprofessional team and ensuring its successful interaction. Project personnel workingtogether can ensure that the architectural, engineering, financial, cultural, and socialvalues of historic buildings are preserved, while rendering them safe for continued use.
Reinforcing a historic building to meet new construction requirements, as prescribed bymany building codes, can destroy much of a historic building's appearance andintegrity. This is because the most expedient ways to reinforce a building according tosuch codes are to impose structural members and to fill irregularities or large openings,regardless of the placement of architectural detail. The results can be quite intrusive.However, structural reinforcement can be introduced sensitively. In such cases, itsdesign, placement, patterning, and detailing respect the historic character of thebuilding, even when the reinforcement itself is visible.
Both exteriors and interiors can be severely damaged in an earthquake. This Craftsman Style bungalow was successfully restored and seismically upgraded after the Northridge earthquake in California. Photographs courtesy Historic Preservation Partners in Earthquake Response.
Three important preservation principles should be kept in mind when undertaking seismicretrofit projects:
- Historic materials should be preserved and retained to the greatest extent possible and not replaced wholesale in the process of seismic strengthening;
- New seismic retrofit systems, whether hidden or exposed, should respect the character and integrity of the historic building and be visually compatible with it in design; and,
- Seismic work should be "reversible" to the greatest extent possible to allow removal for future use of improved systems and traditional repair of remaining historic materials.
It is strongly advised that all owners of historically significant buildingscontemplating seismic retrofit become familiar with The Secretary of theInterior's Standards for the Treatment of Historic Properties, which arepublished by the National Park Service and cited in the bibliography of this publication.These standards identify approaches for working with historic buildings, includingpreservation, rehabilitation, and restoration. Code-required work to make buildingsfunctional and safe is an integral component of each approach identified in the Standards.While some seismic upgrading work is more permanent than reversible, care must be taken topreserve historic materials to the greatest extent possible and for new work to have aminimal visual impact on the historic appearance of the building.
Typical earthquake damage to most older and historic buildings results from poorductility--or flexibility--of the building and, specifically, poor structural connectionsbetween walls, floors, and foundations combined with the very heavy weight and mass ofhistoric materials that are moved by seismic forces and must be resisted. In buildingsthat have not been seismically upgraded, particularly unreinforced masonry buildings,parapets, chimneys, and gable ends may dislodge and fall to the ground during a moderateto severe earthquake. Walls, floors, roofs, skylights, porches, and stairs which rely ontied connections may simply fail. Interior structural supports may partially or totallycollapse. Unreinforced masonry walls between openings often exhibit shear (or diagonal)cracking. Upper stories may collapse onto under-reinforced lower floors with largeperimeter openings or atriums. Unbraced infill material between structural or rigid framesupports may dislodge. Adjacent buildings with separate foundations may move differentlyin an earthquake creating damage between them. Poorly anchored wood frame buildings tendto slide off their foundations. Ruptured gas and water lines often cause fire and waterdamage. Many of these vulnerabilities can be mitigated by understanding how the forcesunleashed in an earthquake affect the building, then planning and implementing appropriateremedial treatments.
Six principal factors influence how and why historic buildings are damaged in anearthquake: (1) depth of the earthquake and subsequent strength of earthquake wavesreaching the surface; (2) duration of the earthquake, including after-shock tremors; (3)proximity of the building to the earthquake epicenter, although distance is notnecessarily a direct relationship; (4) geological and soil conditions; (5) buildingconstruction details, including materials, structural systems, and plan configuration; and(6) existing building condition, including maintenance level.
The first three factors--the depth, duration, and proximity to the fault--are beyondhuman control. Recent earthquakes have shown the fourth factor, geological soilconditions, to be as important as any of the other factors because loose, soft soils tendto amplify ground motion, thereby increasing damage. Further, there is the tendency ofsoft, unstable soils to "liquefy" as the ground vibrates, causing the buildingfoundations to sink unevenly. This fourth factor, geological and soil conditions, isdifficult to address in a retrofit situation, although it can be planned for in newconstruction. The last two factors--the building's construction type and its existingphysical condition--are the two factors over which building owners and managers havecontrol and can ultimately affect how the historic property performs in an earthquake.
Although historic buildings present problems, the way they were constructed often hasintrinsic benefits that should not be overlooked. Diagonal subflooring undertongue-and-groove nailed flooring can provide a diaphragm, or horizontal membrane, thatties the building together. Interior masonry walls employing wire lath with plaster alsoadd strength that binds materials together. The typical construction of older buildingswith partition walls that extend from floor to ceiling ( instead of just to the undersideof a dropped ceiling) also provides additional support and load transfer during anearthquake that keeps shifting floors from collapsing. Moreover, buildings constructed ofunreinforced masonry with a wall thickness to height ratio that does not exceed coderequirements can often survive shaking without serious damage. The stability ofunreinforced masonry walls should not be underestimated; while the masonry may crack, itoften does not shift out of plumb enough to collapse.
Type of Building and Construction
A historic building's construction and materials determine its behavior during anearthquake. Some buildings, such as wooden frame structures, are quite ductile and, thus,able to absorb substantial movements. Others, such as unreinforced brick or adobebuildings comprised of heavy individual load-bearing units, are more susceptible to damagefrom shaking. If an earthquake is strong, or continues for a long time, building elementsthat are poorly attached or unreinforced may collapse. Most historic buildings stillstanding in earthquake zones have survived some shaking, but may be structurally weakened.
Buildings of more rigid construction techniques may also have seismic deficiencies.Masonry infill-wall buildings are generally built of steel or concrete structural frameswith unreinforced masonry sections or panels set within the frame. While the structuralframes may survive an earthquake, the masonry infill can crack and, in some cases,dislodge. The reaction of concrete buildings and concrete frame structures is largelydependent upon the extent and configuration of iron or steel reinforcement. Earlybuildings constructed of concrete are often inadequately reinforced, inadequately tied, orboth, and are thus susceptible to damage during earthquakes.
Recognition of the configuration of the historic structure and inherent areas ofweakness are essential to addressing appropriate alternatives for seismic retrofit. Forexample, the plan and elevation may be as important as building materials and structuralsystems in determining a historic building's survival in an earthquake. Small round,square, or rectangular buildings generally survive an earthquake because their geometryallows for equal resistance of lateral forces in all directions. The more complex andirregular the plan, however, the more likely the building will be damaged during anearthquake because of its uneven strength and stiffness in different directions.Structures having an "L," "T," "H," "U," or"E" shape have unequal resistance, with the stress concentrated at corners andintersections. This is of particular concern if the buildings have flexible structuralsystems and/or an irregular layout of shear walls which may cause portions of the buildingto pull apart.
Similarly, the more complex and irregular a building elevation, the more susceptible itis to damage, especially in tall structures. Large or multiple openings around thebuilding on the ground level, such as storefronts or garage openings, or floors withcolumns and walls running in only one direction are commonly known as "softstories" and are prone to structural damage.
Much of the damage that occurs during an earthquake is directly related to thebuilding's existing condition and maintenance history. Well maintained buildings,even without added reinforcement, survive better than buildings weakened by lack ofmaintenance. The capacity of the structural system to resist earthquakes may be severelyreduced if previous alterations or earthquakes have weakened structural connections or ifmaterials have deteriorated from moisture, termite, or other damage. Furthermore, inunreinforced historic masonry buildings, deteriorated mortar joints can weaken entirewalls. Cyclical maintenance, which reduces moisture penetration and erosion of materials,is therefore essential. Because damage can be cumulative, it is important to analyze thestructural capacity of the building.
Over time, structural members can become loose and pose a major liability. Unreinforcedhistoric masonry buildings typically have a friction-fit connection between horizontal andvertical structural members, and the shaking caused by an earthquake pulls them apart.With insufficient bearing surface for beams, joists, and rafters against the load bearingwalls or support columns, they fail. The resulting structural inadequacy may cause apartial or complete building collapse, depending on the severity of the earthquake and theinternal wall configuration. Tying the building together by making a positive anchored orbraced connection between walls, columns, and framing members, is key to the seismicretrofit of historic buildings.
The two goals of the seismic retrofit in historic buildings are life safety and theprotection of older and historic buildings during and after an earthquake. Becauserehabilitation should be sensitive to historic materials and the building's historiccharacter, it is important to put together a team experienced in both seismic requirementsand historic preservation. Team members should be selected for their experience withsimilar projects, and may include architects, engineers, code specialists, contractors,and preservation consultants. Because the typical seismic codes are written for newconstruction, it is important that both the architect and structural engineer beknowledgeable about historic buildings and about meeting building code equivalencies andusing alternative solutions. Local and state building officials can identify regulatoryrequirements, alternative approaches to meeting these requirements, and if thejurisdiction uses a historic preservation or building conservation code. Even on smallprojects that cannot support a full professional team, consultants should be familiar withhistoric preservation goals. The State Historic Preservation Office and the local historicpreservation office or commission may be able to identify consultants who have beensuccessful in preserving historic buildings during seismic retrofit work. Once the teamhas been assembled, their tasks include:
Compiling documentation. The team should review all availabledocumentation on the historic building, including any previous documentation assembled tonominate the structure to the National Register of Historic Places, and any previousHistoric Structures Reports. Original plans and specifications as well as those showingalterations through time often detail structural connections. Early real estate orinsurance plans, such as the Sanborn Maps, note changes over time. Historicphotographs of the building under construction or before and after previous earthquakesare invaluable. Base maps for geological or seismic studies and utility maps showing thelocation of water, gas, and electric lines should be also identified. The municipal orstate office of emergency preparedness can provide data on earthquake hazard plans for thecommunity.
Evaluating significant features and spaces. The team must also identifyareas of a historic building and its site that exhibit design integrity or historicalsignificance which must be preserved. It is critical, and a great challenge, to protectthese major features, such as domes, atriums, and vaulted spaces or highly decorativeelements, such as mosaics, murals, and frescoes. In some cases, secondary areas of thebuilding can provide spaces for additional reinforcement behind these major features, thussaving them from damage during seismic retrofit work. Both primary and secondary spaces,features, and finishes should, thus, be identified.
Assessing the condition of the building and the risk hazards. The teamthen assesses the general physical condition of the building's interior and exterior,and identifies areas vulnerable to seismic damage. This often requires a structuralengineer or testing firm to determine the strength and durability of materials andconnections. A sliding scale of potential damage is established, based on the probabilityof hazard by locale and building use. This helps the owner distinguish between areas inwhich repairable damage, such as cracking, may occur and those in which life-threateningproblems may arise. These findings help guide cost-benefit decisions, especially whenbudgets are limited.
Evaluating local and state codes and requirements. Few codes considerhistoric buildings, but the California State Historical Code and the Uniform Code forBuilding Conservation provide excellent models for jurisdictions to adopt. Code officialsshould always be asked where alternative approaches can be taken to provide life safety ifthe specified requirements of a code would destroy significant historic materials andfeatures. Some jurisdictions require the removal of parapets, chimneys, or projectingdecoration from unreinforced masonry buildings which is not a preservation approach.Professionals on the team should be prepared with alternatives that allow for mitigatingpotential damage to such features while retaining them through reattachment orstrengthening.
Developing a retrofit plan. The final task of the project team is todevelop a retrofit plan. The plan may require multiple treatments, each more comprehensivethan the last. Treating life-safety issues as well as providing a safe route of exitshould be evaluated for all buildings. Developing more comprehensive plans, often combinedwith future rehabilitation, is reasonable. Long-term restoration solutions phased in overtime as funding is available should also be considered. In every case, owners and theirplanning teams should consider options that keep preservation goals in mind.
There are significant advantages of completing a seismic survey and analysis even ifresources for implementing a retrofit are not yet available. Once the retrofit plan isfinished, the project team will have a document by which to assess future damage andproceed with emergency repairs. If construction is phased, its impact to the wholebuilding should be understood. Some partially completed retrofit measures have leftbuildings more rigid in one area than in others, thereby contributing to more extensivedamage during an ensuing earthquake.
The integrity and significance of the historic building, paired with the cost andbenefit of seismic upgrading, need to be weighed by the owner and the consulting team.Buildings in less active seismic areas may need little or no further bracing or tying.Buildings in more active seismic zones, however, may need more extensive intervention.Options for the level of seismic retrofit generally fall into four classifications,depending on the expected seismic activity and the desired level of performance.Realistically, for historic buildings, only the first three categories apply.
1) Basic Life Safety. This addresses the most serious life-safety concerns by correcting those deficiencies that could lead to serious human injury or total building collapse. Upgrades may include bracing and tying the most vulnerable elements of the building, such as parapets, chimneys, and projecting ornamentation or reinforcing routes of exit. It is expected that if an earthquake were to occur, the building would not collapse but would be seriously damaged requiring major repairs.
2) Enhanced Life Safety. In this approach, the building is upgraded using a flexible approach to the building codes for moderate earthquakes. Inherent deficiencies found in older buildings, such as poor floor to wall framing connections and unbraced masonry walls would be corrected. After a design level earthquake, some structural damage is anticipated, such as masonry cracking, and the building would be temporarily unusable.
3) Enhanced Damage Control. Historic buildings are substantially rehabilitated to meet, to the extent possible, the proscribed building code provision. Some minor repairable damage would be expected after a major earthquake.
4) Immediate Occupancy. This approach is intended for designated hospitals and emergency preparedness centers remaining open and operational after a major earthquake. Even most modern buildings do not meet this level of construction, and so for a historic building to meet this requirement, it would have to be almost totally reconstructed of new materials which, philosophically, does not reflect preservation criteria.
Devising the most appropriate approach for a particular historic building will dependon a variety of factors, including the building's use, whether it remains occupiedduring construction, applicable codes, budgetary constraints, and projected risk ofdamage. From a design perspective, the vast majority of historic buildings can tolerate awell-planned hidden system of reinforcement. Utilitarian structures, such as warehouses,may be able to receive fairly visible reinforcement systems without undue damage to theirhistoric character. Other more architecturally detailed buildings or those with morefinished interior surfaces, however, will benefit from more hidden systems; installationof such systems may even require the temporary removal of significant features to assuretheir protection. Most buildings, particularly commercial rehabilitations, can incorporateseismic strengthening during other construction work in a way that ensures a high degreeof retention of historic materials in place.
Cost plays a critical role in selecting the most appropriate retrofit measure. It isalways best to undertake retrofit measures before an earthquake occurs, when options areavailable for strengthening existing members. Once damage is done, the cost will besubstantially higher and finding engineers, architects, and contractors available to dothe work on a constricted schedule will be more difficult.
Planned seismic retrofit work may add between $10 and $100 per square foot to the costof rehabilitation work depending on the level of intervention, the condition of thebuilding, and whether work will be undertaken while the building is occupied. Costs canexceed several hundred dollars a square foot for combined restoration and seismic upgradecosts in major public buildings, in order to provide a level of structural reinforcementthat would require only minor repairs after a major earthquake. But maintenance andincremental improvements to eliminate life-safety risks are within the cost realm ofresponsible upkeep.
Each property owner has to weigh the costs and benefits of undertaking seismic retrofitin a timely manner. Owners may find that an extended engineering study evaluating a widerange of options is worthwhile. Not only can such a study consider the most sensitivehistoric preservation solution, but the most cost-effective one as well. In many cases,actual retrofit expenses have been lower than anticipated because a careful analysis ofthe existing building was made that took the durability and performance of existinghistoric materials into consideration. Most seismic retrofit is done incrementally orincorporated into other rehabilitation work. In large public buildings, seeminglyexpensive "high-tech" solution such as installing foundation base isolators canturn out to be justified because significant historic materials do not have to be removed,replaced, or replicated. The cost for a fully retrofitted building can offset thepotential loss of income, relocation, and rebuilding after an earthquake. Without carefulstudy, these solutions often are not evaluated.
Some municipalities and states provide low-interest loans, tax relief, municipal bonds,or funding grants targeted to seismic retrofit. Federal tax incentives for therehabilitation of income-producing historic buildings include seismic strengthening as anallowable expense. Information on these incentives is available from the State HistoricPreservation Office. It is also in the best interest of business communities to supportthe retrofit of buildings in seismically active areas to reduce the loss of sales andproperty taxes, should an earthquake occur.
Seismic strength within buildings is achieved through the reinforcement of structuralelements. Such reinforcement can include anchored ties, reinforced mortar joints, bracedframes, bond beams, moment-resisting frames, shear walls, and horizontal diaphragms. Mosthistoric buildings can use these standard, traditional methods of strengtheningsuccessfully, if properly designed to conform to the historic character of the building.In addition, there are new technologies and better designs for traditional connectiondevices as well as a greater acceptance of alternative approaches to meeting seismicrequirements. While some technologies may still be new for retrofit, the key preservationprinciples explained above should be applied, to ensure that historic buildings will notbe damaged by them.
There are varying levels of intervention for seismically retrofitting historicbuildings based on the owner's program, the recommendations of the team, applicablecodes, and the availability of funds. The approaches to strengthening buildings beginningon page 10 are to show a range of treatments and are not intended to cover all methods.Each building should be evaluated by qualified professionals prior to initiating any work.
Adequate maintenance ensures that existing historic materials remain in good conditionand are not weakened by rot, rust, decay or other moisture problems. Without exception,historic buildings should be well maintained and an evacuation plan developed. Expectationthat an earthquake will occur sometime in the future should prepare the owner to haveemergency information and supplies on hand.
- Check roofs, gutters, and foundations for moisture problems, and for corrosion of metal ties for parapets and chimneys. Make repairs and keep metal painted and in good condition.
- Inspect and keep termite and wood boring insects away from wooden structural members. Check exit steps and porches to ensure that they are tightly connected and will not collapse during an emergency exit.
- Check masonry for deteriorating mortar, and never defer repairs. Repoint, matching the historic mortar in composition and detailing.
- Contact utility companies for information on flexible connectors for gas and water lines, and earthquake activated gas shut-off valves. Strap oil tanks down and anchor water heaters to wall framing.
- Collect local emergency material for reference and implement simple household or office mitigation measures, such as installing latches to keep cabinets from flying open or braces to attach tall bookcases to walls. Keep drinking water, tarpaulins, and other emergency supplies on hand.
This is not an exhaustive list, but illustrates that most measures to reducelife-safety risks rely on using mechanical fasteners to tie a building together.Incorporating these measures can be done incrementally without waiting for extensiverehabilitation. An architectural or engineering survey should identify what is needed.Care should be taken to integrate these changes with the visual appearance of thebuilding.
- Bolt sill plates to foundations and add plywood stiffeners to cripple wall framing around wood frame buildings. Keep reinforcement behind decorative crawlspace lattice or other historic features.
- Reinforce floor and roof framing connections to walls using joist hangers, metal straps, threaded bolts, or other means of mechanical fasteners. Tie columns to beams; reinforce porch and stair connections as well.
- Repair weakened wooden structural systems by adding, pairing, or bracing existing members. Consider adding non-ferrous metal straps in alternating mortar joints if extensive repointing is done in masonry walls.
- Reinforce projecting parapets and tie parapets, chimneys, balconies, and unsecured decorative elements to structural framing. Make the connections as unobtrusive as possible. In some cases, concrete bond beams can be added to reinforce the top of unreinforced masonry or adobe walls.
- Properly install and anchor new diaphragms, such as roof sheathing or subflooring, to the walls of a structure prior to installing finish materials.
- Avoid awkwardly placed exposed metal plates or rosettes when using threaded bolts through masonry walls. When exposed plates will interfere with the decorative elements of the facade, use less visible grouted bolts or plates that can be set underneath exposed finished materials.
- Use sensitively designed metal bracing along building exteriors to tie the unsupported face of long exterior walls to the floor framing. This is often seen along side or party walls in commercial or industrial buildings.
When buildings are being rehabilitated, it is generally the most cost effective time tomake major upgrades that affect the structural performance of the building. New elements,such as concrete shear walls or fiber reinforcing systems can be added while the structureis exposed for other rehabilitation or code compliance work.
- Inspect and improve all lateral tie connections and diaphragms.
- Reinforce walls and large openings to improve shear strength in locations of doors, windows, and storefront openings. Carefully locate "X" and "K" bracing to avoid visual intrusion, or use moment frames, which are a hidden perimeter bracing in large openings. From a preservation perspective, the use of a more hidden system in finished spaces is generally preferable.
- Strengthen masonry walls or columns with new concrete reinforcement or fiber wrap systems. Avoid the use of heavy spray concrete or projecting reinforced walls that seriously alter the historic relationship of the wall to windows, trim, and other architectural moldings or details.
- Selectively locate new shear walls constructed to assist the continuous transfer of loads from the foundation to the roof. If these walls cannot be set behind historic finishes, they should be located in secondary spaces in conjunction with other types of reinforcement of the primary spaces or features.
- Consider the internal grouting of rubble masonry walls using an injected grout mixture that is compatible in composition with existing mortar. Ensure that exposed areas are repaired and that the mortar matches all visual qualities of the historic mortar joints in tooling, width, color and texture.
- Evaluate odd-shaped buildings and consider the reinforcement of corners and connections instead of infilling openings with new construction. Altering the basic configuration and appearance of primary facades of buildings is damaging to those qualities that make the building architecturally significant.
New technologies, being developed all the time, may have applicability to historicpreservation projects. These specialized technologies include: vertical and center coredrilling systems for unreinforced masonry buildings, base isolation at the foundations,superstructure damping systems, bonded resin coatings, and reproducing lost elements inlighter materials.
However, many new technologies may also be non-reversible treatments resulting indifficulties of repair after an earthquake. The reinforcement of historic materials withspecial resins, or the use of core drilling to provide a reinforced vertical connectionfrom foundation to roof may not be as repairable after an earthquake as would moretraditional means of wall reinforcement. New technologies should be carefully evaluated bythe design team for both their benefits as well as their shortcomings.
Using computer modeling of how historic buildings may act in an earthquake suggestsoptions for seismic upgrade using a combination of traditional methods and newtechnologies. While most projects involving base isolation and other complex dampingsystems constitute only a small percentage of the projects nationwide that are seismicallyreinforced, they may be appropriate for buildings with significant interior spaces thatshould not be disturbed or removed during the retrofit. Each building will needs its ownsurvey and evaluation to determine the most appropriate seismic reinforcement.
"New structural steel and restoration of the historic stucco and decorative tile work and a repaired tile roof reinstated this earthquake damaged building as a major element of the historic district. Northridge, CA. Photo courtesy of Historic Preservation Partners for Earthquake Response.
Should a historic building suffer damage during an earthquake, it is the owner whohas a plan in place who will be able to play a critical role in determining itsultimate fate. If the owner has previously assembled a team for the purpose of seismicupgrading, there is a greater chance for the building to be evaluated in a timely fashionand for independent emergency stabilization to occur. In most municipalities, a survey,often by trained volunteers, will be conducted as soon as possible after an earthquake,and buildings will be tagged on the front with a posted notice according to their abilityto be entered. Typically red, yellow, and green tags are used to indicate varying levelsof damage--no entry, limited entry, and useable--to warn citizens of their relativesafety. Heavily damaged areas are often secured off-limits and many red tagged, butrepairable, buildings have been torn down unnecessarily because owners were unable toevaluate and present a stabilization plan in time. Owners or members of the preservationcommunity may engage their own engineers with specialized knowledge to challenge ademolition order. Because seismic retrofit is complex and many jurisdictions are involved,the coordination between various regulatory bodies needs to be accomplished beforean earthquake.
During times of emergencies, many communities, banks, and insurance agencies will notbe in a position to evaluate alternative approaches to dealing with damaged historicbuildings, and so they often require full compliance with codes for new construction forthe major rehabilitation work required. Because seismic after-shocks often create moredamage to a weakened building, the inability to act quickly--even to shore up thestructure on a temporary basis--can result in the building's demolition. Penetrating rain,uneven settlement, vandalism, and continuing after-shocks can easily undermine abuilding's remaining structural integrity. Moreover, the longer a building isunoccupied and non-income-producing, the sooner it will be torn down in a negotiatedsettlement with the insurance company. All of these factors work against saving buildingsdamaged in earthquakes, and make having an action plan essential.
Having an emergency plan in place, complete with access to plywood, tarpaulins, bracingtimbers, and equipment, will allow quick action to save a building following anearthquake. Knowing how the community evaluates buildings and the steps taken to secure anarea will give the owner the ability to be a helpful resource to the community in a timeof need.
If the federal government is asked to intervene after a natural disaster, technicalassistance programs are available. Often after a disaster, grant funds or low-cost loansfrom federal, state, and congressional special appropriations are targeted to qualifiedproperties, which can help underwrite the high cost of rehabilitation (see informationabout FEMA)
Recent earthquakes have shown that historic buildings retrofitted to withstandearthquakes survive better than those that have not been upgraded. Even simple efforts,such as bracing parapets, tying buildings to foundations, and anchoring brick walls at thehighest, or roof level, have been extremely effective. It has also been proven that wellmaintained buildings have faired better than those in poor condition during and after anearthquake. Thus, maintenance and seismic retrofit are two critical components for theprotection of historic buildings in areas of seismic activity. It makes no sense toretrofit a building, then leave the improvements, such as braced parapets or metal boltswith plates, to deteriorate due to lack of maintenance.
Damage to historic buildings after an earthquake can be as great as the initialdamage from the earthquake itself. The ability to act quickly to shore up and stabilize abuilding and to begin its sensitive rehabilitation is imperative. Communities withoutearthquake hazard reduction plans in place put their historic buildings--as well as thesafety and economic well-being of their residents -- at risk.
Having the right team in place is important. Seismic strengthening of existing historicbuildings and knowledge of community planning for earthquake response makes theprofessional opinions of the team members that much more important when obtaining permitsto do the work. Local code enforcement officials can only implement the provisions of themodel or historic preservation codes if the data and calculations work to ensure publicsafety. Buildings do not need to be over-retrofitted. A cost-effective balance betweenprotecting the public and the building recognizes that planned for repairable damage canbe addressed after an earthquake. Engineers and architects, who specialize in historicbuildings and who have a working knowledge of alternative options and expectedperformance for historic structures, are critical to the process.
It is clear that historic and older buildings can be seismically upgraded in acost-effective manner while retaining or restoring important historic character-definingqualities. Seismic upgrading measures exist that preserve the historic character andmaterials of a buildings. However, it takes a multi-disciplined team to plan and toexecute sensitive seismic retrofit. It also takes commitment on the part of city, state,and federal leaders to ensure that historic districts are protected from needlessdemolition after an earthquake so that historic buildings and their communities arepreserved for the future.
Most local jurisdictions measure seismic risk based on seismic zones established bycode, such as the Uniform Building Code with its 4 risk zones [1=low to 4=high]. There arealso maps, such as this one, which identify the Effective Peak Acceleration (EPA) whichfurther reflect the light, moderate, and severe shaking risks as a percentage of theacceleration of gravity that can be expected in an area.
In the United States, the greatest activity areas are the western states, Alaska, andsome volcanic island areas. However, noted historical earthquakes occurred inMassachusetts (1755), Missouri (1811), South Carolina (1886), and Alaska (1964). TheCaribbean Islands and Puerto Rico have been sites of severe earthquakes. The history ofearthquakes in the United States has been recorded for over 200 years and new areas ofconcern include moderate risk areas in southern and mid-western states.
The Richter Magnitude Scale, first published in 1935, records the size of an earthquakeat its source, as measured on a seismograph. Magnitudes are expressed in whole numbers anddecimals between 1 and 9. An earthquake of a magnitude of 6 or more will cause moderatedamage, while one of over 7 will be considered a major earthquake. It is important toremember that an increase of one whole number on the Richter Scale is a tenfold increasein the size of the earthquake.
The Federal Emergency Management Agency -- FEMA -- is an independent agency of thefederal government, reporting to the President. Since its founding in 1979, FEMA's missionhas been to reduce loss of life and property and protect our nation's criticalinfrastructure from all types of hazards through a comprehensive, risk-based, emergencymanagement program. FEMA works with the state and local governments and the private sectorto stimulate increased participation in emergency preparedness, mitigation, response andrecovery programs related to natural disasters. To minimize damage-repair-damage cycles,FEMA carries out and encourages preventive activities referred to as hazard mitigation.
The FEMA Hazard Mitigation Program, established in 1988 with the passage of the RobertT. Stafford Disaster Relief and Emergency Assistance Act, offers a framework forprotecting historic structures from natural disasters. In the event of a federallydeclared disaster, state and local governments as well as eligible non-profit applicantsmay receive financial and technical assistance to identify and carry out cost-effectivehazard mitigation activities.
FEMA encourages hazard mitigation projects, including the restoration of buildings, byproviding technical assistance and funding through the Hazard Mitigation Grant Program(HMPG), which can underwrite up to 50% of the cost of the project.
FEMA's public-assistance program provides financial and other assistance to rebuilddisaster-damaged facilities that serve a public purpose, such as schools, hospitals,government buildings and public utilities.
In terms of technical assistance, FEMA, under a cooperative agreement with the BuildingSeismic Safety Council has produced two volumes of comprehensive material dealing with theseismic retrofit of existing buildings (see Further Reading). Inaddition an ongoing project ATC-43 involves earthquake analysis procedures forUnreinforced Masonry Buildings and Reinforced Concrete Buildings. These documents containnationally applicable technical criteria intended to ensure that buildings will withstandearthquakes better than before. There is a great deal of information that is applicable tohistoric buildings, although historic buildings are not necessarily identified as acategory. Write for FEMA publications at:
FEMA, PO Box 70274, Washington, DC 20024
For current information about emergency activities, federally declared disaster areas,or how to contact regional offices see the FEMA website: http://www.fema.gov/
These questions should be asked with the assistance of the team to determine acceptablealternatives. Since there is never a single right answer, the design team and codeofficials should work together to determine the appropriate level of seismic retrofit withthe lowest visual impact on the significant spaces, features, and finishes of both theinterior and exterior of historic buildings.
As with the illustrations above, this guide is not intended to proscribe how seismicretrofit should be done, but rather, to illustrate that every physical change to abuilding will have some consequence. By asking how impacts can be reduced, the owner willhave several options from which to choose.
Can bracing be installed without damaging decorative details or appearance of parapets, chimneys, or balconies?
Are the visible features of the reinforcement, such as anchor washers or exterior buttresses adequately designed to blend with the historic building?
Can hidden or grouted bolts be set on an angle to tie floors and walls together, instead of using traditional bolts and exposed washers or rosettes on ornamental exteriors?
Are diagonal frames, such as X, K, or struts located to have a minimal impact on the primary facade? Are they set back and painted a receding color if visible through windows or storefronts?
Can moment frames or reinforced bracing be added around historic storefronts in order to avoid unsightly exposed reinforcement, such as X braces, within the immediate viewing range of the public?
Can shorter sections of reinforcement be "stitched" into the existing building to avoid removal of large sections of historic materials? This is particularly true for the insertion of roof framing supports.
Can shear walls be located in utilitarian interior spaces to reduce the impact on finishes in the primary areas?
Are there situations where thinner applied fiber reinforced coating would adequately strenghten walls or supports without the need for heavier reinforced concrete?
Can diaphragms be added to non-significant floors in order to protect highly decorated ceilings below, or the reverse if the floor is more ornamental than the ceiling?
Are there adequate funds to retain, repair, or reinstall ornamental finishes once structural reinforcements have been installed?
Should base isolation, wall damping systems, or core drilling be considered? Are they protecting significant materials by reducing the amount of intervention?
Are the seismic treatments being considered "reversible" in a way that allows the most amount of historic materials to be retained and allows future repair and restoration?
Buildings at Risk: Seismic Design Basics for Practicing Architects. Washington,DC. AIA/ACSA Council on Architectural Research. February, 1992
Controlling Disaster: Earthquake-Hazard Reduction for Historic Buildings.Washington, DC. National Trust for Historic Preservation. 1992.
Earthquake-Damaged Historic Chimneys: A Guide to the Rehabilitation and Reconstructionof Chimneys. Oakland, CA. Historic Preservation Partners for Earthquake Response. July,1995.
Eichenfield, Jeffrey. 20 Tools That Protect Historic Resources After an Earthquake;Lessons learned from the Northridge Earthquake. Oakland, CA. California PreservationFoundation.1996.
History at Risk, Loma Prieta: Seismic Safety & Historic Buildings. Oakland,CA. California Preservation Foundation. 1990.
Kariotis, John C., Roselund, Nels; and Krakower, Michael. Loma Prieta, AnEngineer's Viewpoint. Oakland, CA; California Preservation Foundation, 1990.
Langenbach, Randolph. "Bricks, Mortar, and Earthquakes; Historic Preservation vs.Earthquake Safety." Apt Bulletin, Vol.21, Nos.3/4 (1989), pp.30-43.
Langenbach, Randolph. "Earthquakes: A New Look at Cracked Masonry." CivilEngineering. November, 1992. pp. 56-58.
NEHRP commentary on the Guidelines for the Seismic Rehabilitation of Buildings (second ballot version). Washington, DC. Building Seismic Safety Council (Prepared forFederal Emergency Management Agency) Draft, April, 1997. FEMA 274.
NEHRP Handbook of Techniques for the Seismic Rehabilitation of Existing Buildings. Washington,DC. Building Seismic Safety Council (Prepared for Federal Emergency Management Agency)1992. FEMA 273.
The Secretary of the Interior's Standards for Rehabilitation with IllustratedGuidelines for Rehabilitating Historic Buildings. Washington, DC. Government PrintingOffice, 1992.
Seismic Retrofit Alternatives for San Francisco's Unreinforced MasonryBuildings: Estimates of Construction Cost & Seismic Damage. San Francisco, CA.City and County of San Francisco Department of City Planning (prepared by Rutherford &Chekene, Consulting Engineers). 1990
The Seismic Retrofit of Historic Buildings Conference Workbook. San Francisco,CA. Association for Preservation Technology, Western Chapter. 1991. [contains an excellentbibliography of additional sources].
Schuller, M.P. Atkinson, R.H. and Noland, J.L. "Structural Evaluation of HistoricMasonry Buildings."APT Bulletin, Vol 26, No. 2/3,pp. 51-61.
State Historical Building Code. Sacramento, CA: State Historical Building CodeBoard.
Uniform Code for Building Conservation. Whittier, CA: International Conferenceof Building Officials, 1991.
Anchor Ties or bolts: Generally threaded rods or bolt which connect walls tofloor and roof framing. Washers, plates, or rosettes anchor the bolt in place.
Base isolation: the ability to isolate the structures from the damaging effectsof earthquakes by providing a flexible layer between the foundations and verticalsupports.
Diagonal Braces: the use of diagonal, chevron or other type of bracing (X or K)to provide lateral resistance to adjacent walls.
Core drilling: a type of vertical reinforcement of masonry walls that relies ondrilling a continuous vertical core that is filled with steel reinforcing rods andgrouting to resist in-plane shear and out-of-plane bending.
Cripple wall: A frame wall between a building's first floor and foundation.
Diaphragm: A floor, roof, or continuous membrane that provides for the transferof earthquake loading to the exterior or interior shear walls of the structure.
Fiber wrap reinforcement: A synthetic compound of filaments that increase theshear capacity of structural members.
Grouted bolts: anchor bolts set, generally on an angle, in a concrete groutmixture, avoid the problem of using an exposed washer. Requires a greater diameter holethan an anchor bolt with washer.
Lateral forces: Generally the horizontal forces transferred to the building fromthe dynamic effects of wind or seismic forces.
Life-safety: providing a level of assurance that risk of loss of life is kept tominimal levels. For buildings, this includes strengthening to reduce 1)structuralcollapse, 2) falling debris, 3)blocking exits or emergency routes, and 4) prevention ofconsequential fire.
Moment-resisting frame: A steel frame designed to provide in-plane resistance tolateral loads particularly by reinforcing the joint connection between column and beamswithout adding a diagonal brace. Often used as a perimeter frame around storefronts orlarge door and window openings.
Seismic retrofit: All measures that improve the earthquake performance of abuilding especially those that affect structural stability and reduce the potential forheavy structural damage or collapse.
Shear stress: A concept in physics where forces act on a body in oppositedirections, but not in the same line. Horizontal forces applied to a wall that isinsufficient to move with these forces will crack, often in a diagonal or X pattern.Connections at beams and walls will also crack from shear stress.
Shear wall: A wall deliberately designed to transfer the building's loadsfrom the roof and floors to the foundation thereby preventing a building from collapsefrom wind or earthquake forces.
Unreinforced Masonry (URM): This designation refers to traditional brick,block,and adobe construction that relies on the weight of the masonry and the bondingcapacity of mortar to provide structural stability.
NOTE: The printed version of this preservation brief features 25illustrations. This electronic version includes some of the original illustrations, butthey are not keyed into the text as they are in the printed version; however many of thereferencing illustration numbers remain in the text.
Front Cover. Historic buildings damaged by earthquakes can be rehabilitated and seismically retrofitted. The posted tag in the window warns that this building, temporarily, cannot be entered Photo: David Look.
David W. Look, AIA, is the Chief, Cultural Resource Team, Pacific Great Basin Support Office, National Park Service. Terry Wong, PE, is the Chief, Structural Engineering, Denver Service Center, National Park Service. Sylvia Rose Augustus, is the Historical Architect, Yosemite National Park
The authors wish to thank their collaborator, Sharon C. Park, FAIA, Senior Historical Architect, Heritage Preservation Services, NPS, who undertook the technical editing of the publication and took the authors' original manuscript and developed it into the Preservation Brief complete with compiling information from other sources and selecting the photographs. Kay D. Weeks and Michael J. Auer, Heritage Preservation Services, NPS, contributed substantially to the published manuscript by revising the draft with an eye to articulation of policy, organizational structure, and cohesiveness of language.
The authors also wish to thank the following for providing information for the publication and/or reviewing the final draft: Steade R. Craigo, AIA, Senior Restoration Architect, State of California; Randolph Langenbach, Architect, FEMA; Bruce Judd, FAIA, Architectural Resources Group; Melvyn Green & Associates; Cassandra Mettling-Davis and Carey & Co. Inc. Architecture; Curt Ginther, Architect, the University of California, Los Angeles (UCLA) Capital Program; The Crosby Group; American Institute of Architects, San Francisco Chapter; Jeffrey L. Eichenfield, California Preservation Foundation; Michael Jackson, Illinois Historic Preservation Agency; George Siekkinen, the National Trust for Historic Preservation; and colleagues at Heritage Preservation Services, NPS, including:
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