Federal Emergency Management Agency FEMA 354/November 2000

A Policy Guide toSteel Moment-Frame

Construction

DISCLAIMER

This document provides information on the seismic performance of steel moment-frame structures and theresults and recommendations of an intensive research and development program that culminated in a seriesof engineering and construction criteria documents. It updates and replaces an earlier publication with thesame title and is primarily intended to provide building owners, regulators, and policy makers with summarylevel information on the earthquake risk associated with steel moment-frame buildings, and measures thatare available to address this risk. No warranty is offered with regard to the recommendationscontained herein, either by the Federal Emergency Management Agency, the SAC Joint Venture,the individual Joint Venture partners, or their directors, members or employees or consultants.These organizations and their employees do not assume any legal liability or responsibility for theaccuracy, completeness, or usefulness of any of the information, products or processes included inthis publication. The reader is cautioned to review carefully the material presented herein andexercise independent judgment as to its suitability for specific applications. This publication hasbeen prepared by the SAC Joint Venture with funding provided by the Federal Emergency ManagementAgency, under contract number EMW-95-C-4770.

Cover Art. The background photograph on the cover of this guide for Policy Makers is a cityscape of aportion of the financial district of the City of San Francisco. Each of the tall buildings visible in thiscityscape is a steel moment-frame building. Similar populations of these buildings exist in most otherAmerican cities and many thousands of smaller steel moment-frame buildings are present around the UnitedStates as well. Until the 1994 Northridge earthquake, many engineers regarded these buildings as highlyresistant to earthquake damage. The discovery of unanticipated fracturing of the steel framing following the1994 Northridge earthquake shattered this belief and called to question the safety of these structures.

A Policy Guide toSteel Moment-frame Construction

SAC Joint Venturea partnership of:

Structural Engineers Association of California (SEAOC)Applied Technology Council (ATC)

California Universities for Research in Earthquake Engineering (CUREe)

Prepared for SAC Joint Venture byRonald O. Hamburger

Project Oversight Committee

William J. Hall, Chair

Shirin AderJohn M. Barsom

Roger FerchTheodore V. Galambos

John GrossJames R. HarrisRichard Holguin

Nestor IwankiwRoy G. Johnston

Len JosephDuane K. Miller

John Theiss John H. Wiggins

SAC Project Management CommitteeSEAOC: William T. Holmes

ATC: Christoper RojahnCUREe: Robin Shepherd

Program Manager: Stephen A. MahinProject Director for Topical Investigations: James O. MalleyProject Director for Product Development: Ronald O. Hamburger

SAC Joint Venture

Structural Engineers Association of Californiawww.seaoc.org

Applied Technology Councilwww.atcouncil.org

California Universities for Research in Earthquake Engineeringwww.curee.edu

November, 2000

THE SAC JOINT VENTURE

SAC is a joint venture of the Structural Engineers Association of California (SEAOC), the AppliedTechnology Council (ATC), and California Universities for Research in Earthquake Engineering (CUREe),formed specifically to address both immediate and long-term needs related to solving performance problemswith welded, steel moment-frame connections discovered following the 1994 Northridge earthquake.SEAOC is a professional organization composed of more than 3,000 practicing structural engineers inCalifornia. The volunteer efforts of SEAOC’s members on various technical committees have beeninstrumental in the development of the earthquake design provisions contained in the Uniform Building Codeand the 1997 National Earthquake Hazards Reduction Program (NEHRP) Recommended Provisions forSeismic Regulations for New Buildings and Other Structures. ATC is a nonprofit corporation founded todevelop structural engineering resources and applications to mitigate the effects of natural and other hazardson the built environment. Since its inception in the early 1970s, ATC has developed the technical basis forthe current model national seismic design codes for buildings; the de-facto national standard for postearthquake safety evaluation of buildings; nationally applicable guidelines and procedures for theidentification, evaluation, and rehabilitation of seismically hazardous buildings; and other widely usedprocedures and data to improve structural engineering practice. CUREe is a nonprofit organization formed topromote and conduct research and educational activities related to earthquake hazard mitigation. CUREe’seight institutional members are the California Institute of Technology, Stanford University, the University ofCalifornia at Berkeley, the University of California at Davis, the University of California at Irvine, the Universityof California at Los Angeles, the University of California at San Diego, and the University of SouthernCalifornia. These university earthquake research laboratory, library, computer and faculty resources areamong the most extensive in the United States. The SAC Joint Venture allows these three organizations tocombine their extensive and unique resources, augmented by consultants and subcontractor universitiesand organizations from across the nation, into an integrated team of practitioners and researchers, uniquelyqualified to solve problems related to the seismic performance of steel moment-frame structures.

ACKNOWLEDGEMENTS

Funding for Phases I and II of the SAC Steel Program to Reduce the Earthquake Hazards of Steel Moment-Frame Structures was principally provided by the Federal Emergency Management Agency, with tenpercent of the Phase I program funded by the State of California, Office of Emergency Services. Substantialadditional support, in the form of donated materials, services, and data has been provided by a number ofindividual consulting engineers, inspectors, researchers, fabricators, materials suppliers and industrygroups. Special efforts have been made to maintain a liaison with the engineering profession, researchers,the steel industry, fabricators, code-writing organizations and model code groups, building officials,insurance and risk-management groups, and federal and state agencies active in earthquake hazardmitigation efforts. SAC wishes to acknowledge the support and participation of each of the above groups,organizations and individuals. In particular, we wish to acknowledge the contributions provided by theAmerican Institute of Steel Construction, the Lincoln Electric Company, the National Institute of Standardsand Technology, the National Science Foundation, and the Structural Shape Producers Council. SAC alsotakes this opportunity to acknowledge the efforts of the project participants – the managers, investigators,writers, and editorial and production staff – whose work has contributed to the development of thesedocuments. Finally, SAC extends special acknowledgement to Mr. Michael Mahoney, FEMA ProjectOfficer, and Dr. Robert Hanson, FEMA Technical Advisor, for their continued support and contribution to thesuccess of this effort.

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INTRODUCTION

The Northridge earthquake of January 17, 1994,caused widespread building damage throughoutsome of the most heavily populated communitiesof Southern California including the San FernandoValley, Santa Monica and West Los Angeles,resulting in estimated economic lossesexceeding $30 billion. Much of the damagesustained was quite predictable, occurring intypes of buildings that engineers had previouslyidentified as having low seismic resistance andsignificant risk of damage in earthquakes. Thisincluded older masonry and concrete buildings,but not steel framed buildings. Surprisingly,however, a number of modern, welded, steel,moment-frame buildings also sustained significantdamage. This damage consisted of a brittlefracturing of the steel frames at the welded jointsbetween the beams (horizontal framing members)and columns (vertical framing members). A few ofthe most severely damaged buildings couldreadily be observed to be out-of-plumb (leaning toone side). However, many of the damagedbuildings exhibited no outward signs of thesefractures, making damage detection both difficultand costly. Then, exactly one year later, onJanuary 17, 1995, the city of Kobe, Japan alsoexperienced a large earthquake, causing similarunanticipated damage to steel moment-framebuildings.

Following discovery of hidden damage in LosAngeles area buildings, the potential forsimilar, undiscovered damage in SanFrancisco and other communities affected bypast earthquakes was raised.

Ventura Boulevard in the San FernandoValley. Many of these buildings had hiddendamage.

Prior to the 1994 Northridge and 1995 Kobeearthquakes, engineers believed that steelmoment-frames would behave in a ductilemanner, bending under earthquake loading, butnot breaking. As a result, this became one of themost common types of construction used formajor buildings in areas subject to severeearthquakes. The discovery of the potential forfracturing in these frames called to question theadequacy of the building code provisions dealingwith this type of construction and created a crisisof confidence around the world. Engineers did nothave clear guidance on how to detect damage,repair the damage they found, assess the safetyof existing buildings, upgrade buildings found tobe deficient or design new steel moment-framestructures to perform adequately in earthquakes.The observed damage also raised questions as towhether buildings in cities affected by other pastearthquakes had sustained similar undetecteddamage and were now weakened and potentiallyhazardous. In fact, some structures in the SanFrancisco Bay area have been discovered to havesimilar fracture damage most probably dating tothe 1989 Loma Prieta earthquake.

In response to the many concerns raised bythese damage discoveries, the FederalEmergency Management Agency (FEMA)sponsored a program of directed investigation anddevelopment to identify the cause of the damage,quantify the risk inherent in steel structures and

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develop practical and effective engineering criteriafor mitigation of this risk. FEMA contracted withthe SAC Joint Venture, a partnership of theStructural Engineers Association of California(SEAOC), a professional association with morethan 3,000 members; the Applied TechnologyCouncil (ATC), a non-profit foundation dedicatedto the translation of structural engineeringresearch into state-of-art practice guidelines; andthe California Universities for Research inEarthquake Engineering (CUREe), a consortiumof eight California universities with comprehensiveearthquake engineering research facilities andpersonnel. The resulting FEMA/SAC project wasconducted over a period of 6 years at a cost of$12 million and included the participation ofhundreds of leading practicing engineers,university researchers, industry associations,contractors, materials suppliers, inspectors andbuilding officials from around the United States.These efforts were coordinated with parallel effortsconducted by other agencies, including theNational Science Foundation and NationalInstitute of Standards and Technology (NIST), andwith concurrent efforts in other nations, includinga large program in Japan. In all, hundreds oftests of material specimens and large-scalestructural assemblies were conducted, as well asthousands of computerized analyticalinvestigations.

As the project progressed, interim guidancedocuments were published to provide practicingengineers and the construction industry withimportant information on the lessons learned, aswell as recommendations for investigation, repair,upgrade, and design of steel moment-framebuildings. Many of these recommendations havealready been incorporated into recent buildingcodes. This project culminated with thepublication of four engineering practice guidelinedocuments. These four volumes include state-of-the-art recommendations that should be includedin future building codes, as well as guidelines thatmay be applied voluntarily to assess and reducethe earthquake risk in our communities.

This policy guide has been prepared to provide anontechnical summary of the valuable informationcontained in the FEMA/SAC publications, anunderstanding of the risk associated with steelmoment-frame buildings, and the practicalmeasures that can be taken to reduce this risk.It is anticipated that this guide will be of interestto building owners and tenants, members of thefinancial and insurance industries, and togovernment planners and the building regulationcommunity.

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FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 353 / July, 2000

Recommended Seismic Design Criteria ForNew Steel Moment-Frame Buildings

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FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 352 /July, 2000

Recommended Seismic Design Criteria ForNew Steel Moment-Frame Buildings

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FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 351 /July, 2000

Recommended Seismic Design Criteria ForNew Steel Moment-Frame Buildings

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FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 350 / July, 2000

Recommended Seismic Design Criteria ForNew Steel Moment-Frame Buildings

FEMA 350 Recommended Seismic DesignCriteria for New Steel Moment-FrameBuildings

FEMA 351 Recommended Seismic Evaluationand Upgrade Criteria for Existing WeldedSteel Moment-Frame Buildings

FEMA 352 Recommended Post-earthquakeEvaluation and Repair Criteria for WeldedSteel Moment-Frame Buildings

FEMA 353 Recommended Specifications andQuality Assurance Guidelines for SteelMoment-Frame Construction for SeismicApplications

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AN OVERVIEW OF THE PROBLEM

What is a steel moment-frame building?

All steel-framed buildings derive basic structuralsupport for the building weight from a skeleton (orframe) composed of horizontal steel beams andvertical steel columns. In addition to being ableto support vertical loads, including the weight ofthe building itself and the contents, structuresmust also be able to resist lateral (horizontal)forces produced by wind and earthquakes. Insome steel frame structures, this lateralresistance is derived from the presence ofdiagonal braces or masonry or concrete walls. Insteel moment-frame buildings, the ends of thebeams are rigidly joined to the columns so thatthe buildings can resist lateral wind andearthquake forces without the assistance ofadditional braces or walls. This style ofconstruction is very popular for many buildingoccupancies, because the absence of diagonalbraces and structural walls allows completefreedom for interior space layout and aestheticexterior expression.

Columns

Beams

Beam/ColumnConnection

VerticalForce

Lateral Force

A steel moment-frame is an assembly ofbeams and columns, rigidly joined together toresist both vertical and lateral forces.

Construction of a modern steel frame buildingin which the ends of beams are rigidly joinedto columns by welded connections.

Are all steel moment-frame buildingsvulnerable to the type of damage thatoccurred in the Northridge earthquake?

The steel moment-frame buildings damaged in the1994 Northridge earthquake are a special type,known as welded steel moment-frames (WSMF).This is because the beams and columns in thesestructures are connected with welded joints.WSMF construction first became popular in the1960s. In earlier buildings, the connectionsbetween the beams and columns were eitherbolted or riveted. While these older buildings alsomay be vulnerable to earthquake damage, theydid not experience the type of connectionfractures discovered following the Northridgeearthquake. Generally, welded steel moment-frame buildings constructed in the period 1964-1994 should be considered vulnerable to thisdamage. Buildings constructed after 1994 andincorporating connection design and fabricationpractices recommended by the FEMA/SACprogram are anticipated to have significantly lessvulnerability.

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What does the damage consist of?

The damage discovered in WSMF buildingsconsists of a fracturing, or cracking, of the weldedconnections between the beams and columnsthat form the frame, or skeleton, of the structure.This damage occurs most commonly at thewelded joint between a column and the bottomflange of a beam. Once a crack has started, itcan continue in any of several different patternsand in some cases has been found to completelysever beams or columns.

Steel backing

Co l umn f lange

Beam bottom flangeWeld

Fracture

Column flange

Beam bottom flange

Damage consists of fractures or cracks thatinitiate in the welded joints of the beams tocolumns.

Damage ranges from small cracks that aredifficult to see, to much larger cracks. Here, acrack began at the weld and progressed intothe column flange, withdrawing a divot ofmaterial.

What does the damage look like?

There are several common types of damage, eachof which looks somewhat different. The mostcommon cracks initiate in the weld itself or justnext to the weld. These cracks often are very thinand difficult to see. In a few cases, crackscannot be seen at all. In some cases, crackscause large scoop-like pieces of the columnflange, called divots, to be pulled out. In stillother cases, the cracks run across the entirecolumn, practically dividing it into twounconnected pieces.

What is the effect of the damage?

WSMF buildings rely on the connections betweentheir beams and columns to resist wind andearthquake loads. When the welded joints thatform these connections break, the building losessome of the strength and stiffness it needs toresist these loads. The magnitude andsignificance of this capacity loss depends on theunique design and construction attributes of eachbuilding, as well as the extent and type ofdamage sustained. Few buildings were damagedso severely in the Northridge earthquake that theyrepresented imminent collapse hazards. However,significant weakening of some buildings didoccur. Once the welded joints fracture, othertypes of damage can also occur includingdamage to bolted joints. Damage that results inthe complete severing of beams or columns ortheir connections poses a serious problem andcould result in the potential for localized collapse.

Fracturing of welded connections can lead todamage to the bolted connections that holdthe beams onto the columns, creatingpotential for localized collapse.

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Why did this damage occur?

We now understand that the vulnerability ofWSMF structures is a result of a number of inter-related factors. Early research, conducted in the1960s and 70s suggested that a particular styleof connection could perform adequately.Designers then routinely began to specify thisconnection in their designs. However, theparticular style of connection tends toconcentrate high stresses at some of theweakest points in the assembly, and in fact,some of the early research showed somepotential vulnerability. As the cost of constructionlabor increased, relative to the price ofconstruction materials, engineers adopteddesigns that minimized the number ofconnections in each building, resulting in largermembers and increased loads on theconnections. At the same time, the industryadopted a type of welding that could be used tomake these connections more quickly, butsometimes resulted in welds that were moresusceptible to cracking. Although building codesrequired that inspectors ensure the quality of thiswelding, the inspection techniques andprocedures used were often not adequate. Finally,the steel industry found new ways toeconomically produce structural steel with higherstrength. Although the steel became stronger,designers were unaware of this and continued tospecify the same connections. Often, theseconnections did not have adequate strength tomatch the newer steel material and weretherefore, even more vulnerable. In the end, thetypical connections used in WSMF buildingswere just not adequate to withstand the severedemands produced by an earthquake.

Fractures commonly initiate at the weldedjoint of the beam bottom flange to column.

Beam

Co l umn

Welds

The typical connection used prior to 1994.Severe stress concentrations inherent in itsconfiguration were not considered in thedesign.

How widespread was this damage?

Although no comprehensive survey of all of thesteel buildings affected by the Northridgeearthquake has been conducted, the City of LosAngeles did enact an ordinance that requiredmandatory inspection of nearly 200 buildings inareas that experienced the most intense groundshaking. Initial reports from this mandatoryinspection program erroneously indicated thatnearly every one of these buildings hadexperienced damage and in some cases, thatthis damage was extensive. It was projected thatperhaps thousands of buildings had beendamaged. It is now known that damage wasmuch less widespread than originally thought andthat many of the conditions that were originallyidentified as damage actually were imperfectionsin the original construction work. Of the nearly200 buildings that were inspected under the Cityof Los Angeles ordinance, it now appears thatonly about 1/3 had any actual earthquakedamage and that more than 90% of the totaldamage discovered occurred within a small groupof approximately 30 buildings. Therefore,although this damage was significant, and doeswarrant a change in the design and constructionpractices prevalent prior to 1994, it appears thatthe risk of severe damage to buildings is relativelyslight, except under very intense ground shaking.

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Engineers expect steel to be ductile, capableof extensive bending and deformation withoutfracturing, as shown in this test specimen.The development of cracks in the steel atrelatively low levels of loading wasunexpected.

Why was this damage a surprise?

In its basic form, steel is a very ductile material,able to undergo extensive deformation anddistortion before breaking. This is the type ofbehavior desired for earthquake resistance, soengineers believed WSMF buildings would bequite earthquake resistant. Following the 1971San Fernando earthquake, the 1987 WhittierNarrows earthquake, and the 1989 Loma Prietaearthquake, there were few reports of significantdamage to steel buildings, especially ascompared to other types of construction,confirming this general belief. However, relativelyfew WSMF buildings were subjected to severeground motion in these events. As the industry’sconfidence in the ability of WSMF buildings toresist earthquake damage grew, design practice,material production processes and constructiontechniques changed, adding unexpectedvulnerability. The 1994 Northridge earthquakewas the first event in which a large number ofrecently constructed WSMF buildings weresubjected to strong ground motion.

Have other earthquakes caused similardamage?

When damaged WSMF buildings were firstdiscovered following the Northridge earthquake,there was speculation that this was a result ofsome peculiar characteristic of the earthquakeitself or of local design and construction practicesin the Los Angeles region. It has now beenconfirmed that similar damage occurred to somebuildings affected by the 1989 Loma Prietaearthquake and also the 1992 Landers and BigBear earthquakes. In 1995, the Kobe earthquakeresulted in damage to several hundred steelbuildings, and the collapse of 50 older steelbuildings. Japanese researchers have confirmedproblems similar to those experienced in theNorthridge earthquake. Engineering researchersaround the world have recognized this behavior asa common problem for welded steel moment-frame buildings designed and constructed usingpractices prevalent prior to the Northridgeearthquake and have been working together tofind solutions.

Many steel moment-frame buildings weredamaged in the 1995 Kobe earthquake. Thebuilding shown here, lying on its side, is oneof more than 50 older steel buildings thatcollapsed.

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How many WSMF buildings are there?

Thousands of WSMF buildings have beenconstructed in all regions of the United States.One of the factors contributing to the seriousnessof this problem is that WSMF construction isoften used in many of the nation’s most importantfacilities, including hospital buildings, emergencycommand centers and other federal, state andlocal government office buildings. It is commonlyused for commercial office structures. Most high-rise buildings constructed in the United States inthe last 30 years incorporate this type ofconstruction. WSMF construction has alsofrequently been used for mid-rise and to a lesserextent, low-rise commercial and institutionalconstruction, auditoriums and other assemblyoccupancies, and has seen limited application inindustrial facilities. It is relatively uncommon inresidential construction, although some high-risecondominium type buildings are of thisconstruction type.

Many of the tallest buildings in our majorcities are of WSMF construction.

Are existing WSMF structures safe?

No structure is absolutely safe and, if subjectedto sufficiently large loads, a structure made of anymaterial will collapse. Building codes in theUnited States permit structures to be damagedby strong earthquakes, but attempt to preventcollapse for the most severe levels of groundmotion which can be expected at the site. NoWSMF structure in the United States has evercollapsed as a result of earthquake loading.However, research conducted as part of theFEMA/SAC project confirms that WSMFstructures with the style of beam-columnconnection typically used prior to the Northridgeearthquake have a higher risk of earthquake-induced collapse than desired for new buildings.

How does the risk of WSMF structurescompare to other types of buildings?

Many other types of buildings present muchgreater risks of collapse than do WSMFbuildings. Based on observations from pastearthquakes, buildings that present greater risksinclude buildings of unreinforced masonryconstruction, buildings of concrete and masonryconstruction that pre-date the mid-1970s,buildings of precast concrete construction andeven some types of wood frame construction.However, many WSMF buildings are large andhouse many occupants. The earthquake-inducedcollapse of one such structure could result in avery large and unacceptable life loss.

Older concrete frame, tilt-up and masonry buildings are generally more prone to earthquake-inducedcollapse than WSMF buildings.

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AFTER THE NEXT EARTHQUAKE

Can one tell if a WSMF building isdamaged?

Some damaged WSMF buildings permanentlylean to one side after an earthquake and havesevere damage to finishes, providing a clearindication that the building has sustainedstructural damage. However, many WSMFbuildings do not exhibit any obvious signs ofdamage. To determine if these buildings aredamaged, it is necessary to conduct anengineering inspection of the framing andconnections. To conduct these inspections, it isnecessary first to remove building finishes andfireproofing. These inspections can be disruptiveof occupancy and very expensive, ranging from afew hundred dollars to more than one thousanddollars per connection. Large buildings may haveseveral thousand connections. The presence ofasbestos, sometimes used in fireproofing inbuildings constructed prior to 1979, substantiallyincreases the inspection costs.

The most severely damaged buildings willoften lean to one side. However, there may beno obvious indications of the damage in somebuildings.

Is it safe to occupy a damaged building?

If a building is severely damaged, there is a riskthat aftershocks or subsequent strong winds orearthquakes may cause partial or total collapse ofthe building. The more severe the damage, thehigher this risk. Most buildings damaged by theNorthridge earthquake were judged to be safe forcontinued occupancy while repairs were made.FEMA 352 provides engineering procedures thatcan be used by building officials and engineers toquantify the risk associated with continuedoccupancy of damaged buildings. FEMA 352 alsoprovides recommendations as to when a buildingshould be deemed unsafe for further occupancy.

How does an owner know if a building issafe?

Following an earthquake, building departmentswill typically perform rapid inspections of affectedbuildings to identify buildings that may be unsafe.Once such an inspection has been performed, thebuilding department will typically post the buildingwith a placard that indicates whether the buildingis safe for occupancy. FEMA 352 providesprocedures that building officials can use toconduct these rapid inspections. If the buildingdepartment doesn’t provide this service, buildingowners can retain private engineers or inspectionfirms for this purpose.

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Is the building department inspectionenough?

Rapid inspections conducted by buildingdepartments after an earthquake are intended toidentify those buildings at greatest risk ofendangering the public safety. They are notadequate to detect all damage a building hassustained. If a building has experienced strongground motion, the building owner should retainan engineer to conduct more thoroughinspections in accordance with FEMA 352, even ifthe building department posts the building assafe.

After an earthquake, is it necessary toinspect every WSMF building?

The amount of damage sustained by a building isclosely related to the severity of ground motion atthe building site. Unless a building experiencesstrong ground motion, it is unlikely that it willsustain significant damage. FEMA 352recommends that all buildings thought to haveexperienced ground motion in excess of certainlevels be subjected to detailed inspections todetermine if they sustained significant damage.Because detailed inspections can be timeconsuming and costly, FEMA 352 alsorecommends that an initial, rapid investigation beperformed to look for obvious signs of severedamage that could pose an immediate threat tolife safety.

Is it necessary to inspect everyconnection in a building?

The only way to ensure that all damage in astructure is found is to inspect all of theconnections. However, as previously discussed,connection inspections are costly and disruptive.Therefore most owners would prefer not to inspectevery connection, if possible. Because damagetends to be distributed throughout a structure, itis possible to inspect a sample of the totalnumber of connections in a building and makejudgments as to how widespread damage is likelyto be throughout the entire building. FEMA 352provides recommendations for selecting anappropriate sample and drawing conclusions onthe condition of the building, based on thedamage found in the sample.

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FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 352 / July, 2000

Recommended Post-earthquakeEvaluation and Repair Criteriafor Welded Steel Moment-FrameBuildings

FEMA 352 contains recommended proceduresfor identifying earthquake damage in WSMFbuildings and the risk of continuedoccupancy.

Is it possible to repair any damagedbuilding?

It is technically possible to repair almost anydamaged building. However, if a building isseverely damaged, the cost of repair may exceedthe replacement cost. Repair may be particularlydifficult and costly if a building has experiencedlarge, permanent sideways displacement,sometimes called drift. In such cases, thestructure may even be unsafe for occupancy evenfor repair. In these cases, it may make moresense to demolish a building and replace it ratherthan to repair it. Following the 1994 Northridgeearthquake, the owner of one building elected todo this.

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How is the fracture damage repaired?

Each type of fracture requires a somewhatdifferent repair procedure. FEMA 352 providesrecommendations for many types of repair.Generally, repairs consist of local removal of thedamaged steel using cutting torches or electricarcs, and welding new, undamaged material backin its place. To do this cutting and weldingsafely, it is necessary first to remove anycombustible finishes from the work area and alsoto provide ventilation to remove potentially harmfulfumes. In some cases, it may be necessary toprovide temporary shoring of the damagedelement while the repair work is done. Theseoperations are disruptive to normal occupancy ofthe immediate work area and also quite costly.Typical connection repair costs can range from$10,000 to $20,000. Additional costs areassociated with the temporary loss of use of floorspace during repair operations.

Flange removal and repalcementper Figure 6-6, if required

Weld access holes as requiredfor weld terminations

FEMA 352 provides detailed repair proceduresfor different types of damage. One repairprocedure is shown here as an example.

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DEALING WITH THE RISK OF EXISTING BUILDINGS

Is it possible to upgrade an existingWSMF building?

It is technically feasible to upgrade an existingsteel moment-frame building and improve itsprobable performance in future earthquakes. Themost common methods are upgrades of theindividual connections, addition of steel braces,concrete or masonry walls, or addition of energydissipation systems. Each of these approachesmay offer advantages in particular buildings.

Buildings can be upgraded by modifyingconnections, adding braces and othermethods.

Connections can be upgraded by welding orbolting new plates onto the beams andcolumns to change the connection shape andreduce stress concentrations. In some cases, itmay also be appropriate to replace defectivewelds and welds with low toughness withwelds with improved toughness andworkmanship.

What is a connection upgrade?

Connection upgrades are the most directapproach to improving the seismic performance ofan existing steel moment-frame structure. Aspreviously discussed, existing connections canfracture because their shape results in thedevelopment of large stress concentrations; somewelds have large defects that reduce theirstrength and the weld metal itself may be of lowtoughness and unable to resist the large stressesimposed on it. Connection upgrades addressthese problems directly by modifying the shape ofthe connection to reduce the stressconcentrations. In addition, depending on theapproach used, the upgrade may includereplacement of defective welds and welds with lowtoughness with new welds that have improvedtoughness and better workmanship. FEMA 351provides information and design criteria for anumber of alternative methods of upgradingconnections.

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How does the addition of braces or wallsupgrade a building?

Connection fractures occur when the forcedelivered by the earthquake exceeds theconnection strength or when connection elementsexperience low-cycle fatigue. This is similar towhat happens to a paper clip, when it is bentback and forth repeatedly. As the metal is bentback and forth, it dissipates energy, but alsobecomes damaged. Eventually, so muchdamage occurs that the metal breaks. If aconnection is bent through a large angle ofdeformation, or is relatively weak or brittle, it mayonly be able to withstand one or two cycles, thatis, be bent back and forth once or twice.However, if the bending deformation is relativelysmall, or if the connection material has hightoughness, the connection may be able towithstand a number of such cycles. Earthquakesproduce many cycles of motion.

When steel braces or masonry or concrete wallsare added to a building, they stiffen it and reducethe amount that the building will sway in anearthquake. This reduces the amount of bendingon the connections. Guidance on the use ofthese techniques is already available in suchpublications as FEMA-273, NEHRP Guidelinesfor Seismic Rehabilitation of Buildings and,therefore, is not repeated in FEMA 351.

Energy dissipation systems are typicallyinstalled in buildings as part of a verticalbracing system that extends between thebuilding’s floors.

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FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 351 / July, 2000

Recommended Seismic Evaluation and UpgradeCriteria for Existing WeldedSteel Moment-Frame Buildings

FEMA 351 provides engineering proceduresfor evaluating the probable performance ofbuildings in future earthquakes. Theseprocedures address the safety as well asfinancial aspects of earthquake performance.

What is an energy dissipation system?

Energy dissipation systems reduce the amountthat buildings sway in an earthquake byconverting the earthquake’s energy into heat.Several types of energy dissipation devices areavailable. One type is a hydraulic cylinder,similar to the shock absorbers in an automobile.Other types dissipate energy through friction. Inorder for these systems to be effective, one end ofthe device must move relative to the other end.The most common way to do this is to install thedissipation devices as part of a bracing systembetween the building floors. As the buildingsways in an earthquake and one floor movesrelative to another, this drives the device anddissipates the energy as heat. Detailed guidanceon the design of energy dissipation systems isalready available in such documents as FEMA273 NEHRP Guidelines for Seismic Rehabilitationof Buildings and, therefore, is not covered in detailin the steel moment-frame criteria documents.

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What type of upgrade is best?

No one type of upgrade is best for all buildings.Basic factors that affect selection of an optimalalternative include the individual building’scharacteristics, the severity of motion anticipatedat the building site, desired building performance,cost of the upgrade, the feasibility of performingupgrade work while the building remainsoccupied, and the effect of the upgrade onbuilding appearance and space utilization.Addition of diagonal bracing may be the leastcostly alternative; however, its effect on buildingappearance and functionality may be viewed bysome owners as unacceptable. Addition ofenergy dissipation devices will result in betterperformance of the building, but may cost more.Modification of individual connections would havethe least effect on building appearance, but wouldinconvenience the existing tenants more thansome other approaches and may cost more toimplement.

How does an owner pick an appropriateupgrade approach?

To determine the best method of upgrading abuilding, an owner should retain an engineer toevaluate the building’s probable earthquakeperformance. An upgrade should be considered ifthe probable performance is unacceptable. In theabsence of ordinances that require upgrade ofbuildings, it will be necessary for each owner todecide what constitutes acceptable performance.The engineer can assist the owner inunderstanding the various options. Once aperformance objective is selected, the engineercan prepare preliminary designs for variousupgrade approaches, together with estimates ofprobable construction cost. The owner can thenselect the most appropriate design consideringthe cost, structural performance improvements,aesthetics and other impacts of each upgradeapproach on the building.

Should all existing WSMF buildings beupgraded?

A building should be upgraded only if the risk oflosses associated with its probable performancein future earthquakes is deemed unacceptable.Individual owners may make this judgement, or insome cases, the local community may decidethat risk is unacceptable. The severity ofearthquake risk associated with a building’sperformance in future earthquakes, as well as theacceptability of this risk, must be determined ona building-specific basis. The earthquakeperformance of some WSMF buildings housingcritical occupancies and located in zones likely toexperience frequent intense ground motion maybe unacceptable, whereas many other WSMFbuildings will be capable of performing adequatelyin the levels of ground motion they are likely toexperience. To determine if a building should beupgraded, an owner should retain the services ofan engineer to evaluate the building’s probableperformance in future earthquakes, permitting theowner to balance the assessed risks against thecost of mitigation.

What is the cost of upgrading a WSMF?

Upgrade costs are dependent on a number offactors including the upgrade approach selected,the structural configuration of the building, thespecific seismic deficiencies present, the severityof earthquake motion used as a basis for design,the intended performance of the upgradedbuilding, the building’s occupancy, and the natureof tenant improvements and furnishings. Costscan range from about $10 per square foot ofbuilding floor area to as much as $30 or more.This can be compared to the typical costsassociated with new building construction.Exclusive of the costs of land acquisition,planning and design, the construction cost of anew steel frame building shell will range fromabout $70 to $125 per square foot. The cost oftenant improvements, including interior partitions,ceilings, lighting, and furnishings can equal thebuilding construction costs.

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When is the best time to upgrade?

Much of the cost of performing a seismic upgraderelates to the need to demolish and restorearchitectural finishes and to displace tenantstemporarily from construction work areas. Inmany commercial buildings these costs areincurred on a regular, periodic basis as leasesexpire and tenants in a building change.Therefore, the most economical time to performan upgrade is during a change of tenantoccupancy. Most buildings have many tenantswith staggered lease periods. If upgrade work isto be done when tenant space is being changedfrom one leaseholder to the next, it will usually benecessary to phase the work over a period ofyears. If such an approach is taken, the probableperformance of the building in the event that anearthquake occurs while it is only partiallyupgraded should be evaluated for each of thepotential stages of completion, to ensure that anunsafe condition is not created inadvertently.Another beneficial time to perform upgrade workis at the time of property transfer. If upgrade workis done as part of a building ownership transfer,financing can include additional funding to performthe upgrade work.

Is upgrading economically feasible?

The feasibility of an upgrade depends on theindividual economic circumstances of eachbuilding, its tenants and owners. Earthquakesare infrequent events. Even in areas of highseismic risk, like California, most buildings willexperience at most only one or perhaps twodamaging earthquakes over their lives. Seismicupgrades can greatly reduce the financial and lifelosses that occur as a result of suchearthquakes. However, because the probability ofoccurrence of a damaging earthquake is usuallysmall, the present value of these avoided lossesrarely exceeds the initial cost of the upgradeunless there are large life loss or occupancyinterruption costs associated with the potentialdamage. If upgrades are performed concurrentlywith other building renovation work, such asremodeling or asbestos removal, the upgradework will cost less and produce a more attractivereturn on the investment.

How can an owner evaluate a building’sprobable future performance?

To determine the probable performance of abuilding in future earthquakes, an owner shouldretain an engineer. FEMA 351 provides structuralengineers with several methods for evaluating theprobable performance of buildings in futureearthquakes. These methods include SimplifiedLoss Estimation, Detailed Loss Estimation andDetailed Performance Evaluation, a procedurethat is compatible with modern performance-based design approaches.

How can an owner tell if a building willbe safe in future earthquakes?

FEMA 351 presents engineering procedures thatcan be used to estimate the probability that abuilding will experience life-threatening damage infuture earthquakes. The ability of a building toresist earthquakes without endangering life isdependent on two primary factors, the capacity ofthe building and the intensity of future earthquakeground motion. It must be remembered that anybuilding has the potential to experience life-threatening damage, if it experiences sufficientlyintense ground motion. In typical regions withsignificant earthquake risk, earthquakes thatproduce low levels of ground motion may be feltrelatively frequently, perhaps one time every tento twenty years. Such ground motion rarelycauses damage. Intense ground motion, capableof causing severe damage to modern buildings,will occur much less frequently, perhaps only onetime in a few hundred to a few thousand years.The procedures of FEMA 351 allow determinationof the probability that a building will experienceeither partial or total collapse, if earthquakeground motion of a specified intensity occurs. Touse this method, the owner and engineer mustagree upon an appropriate return period for theground motion to be used as a basis for theevaluation. The return period is the averagenumber of years, for example, 100, 500, etc.,between damage-causing earthquakes.

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What is the Simplified Loss EstimationMethodology?

The Simplified Loss Estimation Methodologycontained in FEMA 351 consists of a series ofgraphs that indicate the probability that WSMFbuildings will experience various levels of damageif they are subjected to certain levels of groundmotion. These graphs were compiled from dataobtained for buildings in the Los Angeles areafollowing the 1994 Northridge earthquake. To usethese graphs, an engineer must estimate anintensity of ground motion to which a building willbe subjected. Using the graph, an engineer canobtain an estimate of the percentage of thebuilding’s connections that will be damaged andthe probable cost of connection repairs, if thebuilding experiences an earthquake of a certainintensity. These graphs do not take into accountthe individual characteristics of a building, exceptin an approximate manner and, therefore, do notprovide precise estimates. Rather, they providean indication of the potential range of damage andrepair costs and the probability that damage andrepair costs will exceed certain amounts, basedon the behavior of the buildings that were affectedby the Northridge earthquake.

The Simplified Loss Estimation Methodologyuses a series of graphs to relate probableearthquake losses to ground motion intensity.

What is the Detailed Loss EstimationMethodology?

In addition to the Simplified Loss EstimationMethodology, FEMA 351 also presents aprocedure for developing building-specific,damage and loss estimates for WSMF buildings.This method can be implemented in HAZUS,FEMA’s nationally applicable loss estimationmodel, to explore the potential benefits to acommunity of requiring upgrade of steel buildings.The method can also be used to develop lossestimates for individual buildings to assist ownersin making upgrade decisions. Loss estimatesgenerated using this technique take intoconsideration the specific characteristics ofindividual buildings and, therefore, provide a moreaccurate basis for cost-benefit studies for seismicupgrades. However, the method is somewhatcomplex and requires specialized expertise andtraining to implement.

How safe should an existing building be?

There is no single commonly accepted minimumlevel of safety for existing buildings. However, itis always useful to compare the safety of anexisting building with that intended for newbuildings as well as with that of other existingbuildings. New buildings are designed to providea high level of confidence, on the order of 90%,that they will survive the most severe groundmotion likely to be experienced, every 1,000 to2,500 years, without collapse. Therefore, it isunlikely that most new buildings would everexperience earthquake-induced collapse. Mostexisting buildings will not be capable of providingthe same performance as a new building. Anexisting building may be acceptably safe if it canprovide high confidence that it will resist collapseunder ground motion intensities likely to occurevery 500 years or so. The selection of anacceptable level of safety depends on a number offactors including the number of occupants in thebuilding, and its use. FEMA 351 presentsevaluation procedures that can be used toestimate the probability that a building willcollapse in future earthquake ground motion andto associate a confidence level with this estimate.

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Will it be possible to reuse a buildingafter an earthquake?

In addition to safety concerns, many owners andtenants in buildings are concerned with theirability to return to a building after an earthquake,and continue to live or work in it. FEMA 351provides an evaluation procedure that may beused to determine the probability that a buildingwill experience so much damage that it shouldnot be reoccupied following an earthquake. Touse this procedure, it is necessary for the ownerand the engineer to select an appropriate returnperiod for the ground motion upon which thisevaluation will be based and the performance thatwill be acceptable if this ground motion occurs.

Is it cost effective to upgrade?

While earthquakes can cause catastrophicdamage, in most cases the probability that abuilding will be affected by such an event is low.Even in the most seismically active regions ofCalifornia, ground motions likely to causesubstantial damage to WSMF buildings arecurrently projected to affect a building site onlyone time every hundred years or so. Given thatmany investors retain ownership in a buildingasset for a relatively small number of years, thelikelihood that a loss will actually occur during anownership period is low. Therefore, when ownersbalance the cost of a seismic upgrade programagainst the probable economic benefit to begained during their ownership period, a goodreturn on investment is often found to exist only ifpost-earthquake occupancy of a building isperceived to be critical for economic or otherreasons, or if life loss is anticipated and theeconomic value of such loss is included in theevaluation.

Why is building performance expressedin probabilities?

The amount of damage that a building will sustainin an earthquake depends on a number of factorsincluding the configuration and strength of thebuilding, the quality of its construction, and thespecific characteristics of the ground motionproduced by the earthquake. Although engineerscan develop estimates of each of these factors, itis not possible to predict any of these thingsprecisely. Therefore, engineers must expressfuture building performance in probabilistic terms.

Should communities adopt mandatoryupgrade programs for steel framebuildings?

Given the low benefit/cost ratio for seismicupgrade of steel frame buildings, it is unlikely thatmany owners will perform such upgradesvoluntarily. Mandatory upgrade ordinances canbe an effective measure to assure that thesebuildings are upgraded. For example, a numberof California cities, including Los Angeles andSan Francisco, have successfully implementedordinances requiring upgrade of unreinforcedmasonry buildings, a type of building that cancollapse in even moderate earthquakes. However,similar ordinances requiring upgrade of WSMFbuildings may not be an effective application ofthe limited resources available to a community. Itis likely that other financial and health risks facedby the community are more significant than thehazards posed by the earthquake performance ofsteel frame buildings. Indeed, many communitieshave large inventories of buildings that are farmore hazardous than typical steel framebuildings, including unreinforced masonrybuildings and older concrete buildings. Beforeadopting a mandatory upgrade ordinance for steelframe buildings, communities should carefullyconsider the costs and benefits on a community-wide basis and weigh these against otherpotential applications of the limited resourcesavailable. FEMA’s HAZUS loss estimation modelis available to communities and can be used toprovide guidance in deciding on the advisability ofadopting a mandatory upgrade ordinance.

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CRITERIA FOR NEW BUILDING DESIGN

How should new buildings be designed?

Buildings must be designed to meet minimumcriteria specified in building codes. Modelbuilding codes are developed on a national basisby professional organizations representingengineers, architects, building officials and firemarshals, and are adopted, sometimes withmodification, by individual cities, counties andstates. Prior to the 1994 Northridge earthquake,the building codes contained prescriptiverequirements for the design and construction ofsteel moment-frame buildings. Theserequirements included specification of minimumpermissible strength and stiffness for resistingearthquake loading, and specific requirements forconnections between beams and columns. TheNorthridge earthquake demonstrated that some ofthese prescriptive requirements were notadequate. Following that discovery, theprescriptive requirements were removed frombuilding codes used in regions of high earthquakerisk and replaced with a requirement that designsinclude test data to show that reliableperformance could be achieved for each newbuilding. These new requirements were bothcostly and difficult to enforce and resulted in adecrease in the number of steel moment-framebuildings constructed.

Studies conducted under the FEMA/SACprogram to reduce earthquake hazards in weldedmoment-resisting steel frames confirm that someof the design requirements contained in thebuilding codes prior to the Northridge earthquakewere inadequate. In the time since, some ofthese inadequate requirements have beenimproved based on interim findings andrecommendations of the FEMA/SAC program.Now that the program is complete, FEMA 350presents a series of additional criteria for thedesign and construction of steel moment-framebuildings that should make performance of thesestructures in future earthquakes much morereliable. It is recommended that the buildingcodes adopt these new recommendations andthat, until this occurs, engineers and ownersvoluntarily adopt these criteria when developingnew buildings.

What types of changes to design andconstruction practice does FEMA 350recommend?

The FEMA 350 recommendations affect nearly allphases of the design and construction process.The recommendations address the types of steelused, the way in which columns and beams areconnected, the way the steel is fabricated, thetype of welding that is performed, and thetechniques that are used to assure that theconstruction work is performed properly.

In addition, FEMA 350 provides engineers with aseries of performance-based design criteria thatcan be used to design and construct buildings toresist earthquakes reliably while sustaining lessdamage than anticipated by building codes.

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FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 350 / July, 2000

Recommended Seismic Design Criteria ForNew Steel Moment-Frame Buildings

FEMA 350 provides recommendations fordesign and construction of new steel moment-frame buildings. These recommendationsaffect the materials of construction, designand construction procedures, and methods ofquality assurance.

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What are the recommended changes tothe types of structural steel that shouldbe used?

In the last few years, the steel industry in theUnited States has undergone rapid change withold rolling mills phased out and new mills, usingmore modern technologies, coming on line.Although the chemical composition and physicalproperties of steel have changed rapidly duringthis period, industry standard specifications anddesign practice largely neglected this. Workingwith the FEMA/SAC project, the AmericanInstitute of Steel Construction (AISC) developednew industry standard specifications for structuralsteel material. These new specifications providebetter control of the critical properties that areimportant to earthquake performance. FEMA 350recommends the use of these more modernsteels and also recommends design proceduresthat properly account for the high strength ofmodern steel materials.

Methods of steel production have undergonedramatic change in recent years and industrystandard design and material productionspecifications did not adequately reflect thematerial currently produced. FEMA 350updates the code requirements to assurespecification of proper materials and propertreatment in design.

If inadequately constructed, welded joints inmoment-frames can be weak links that failprematurely. FEMA 350 recommends carefulcontrol of the materials, workmanship andprocedures used to make these joints andverify their adequacy.

Are there any changes to weldingrequirements?

FEMA 350 and its companion document, FEMA353, present extensive new recommendations forwelding of moment-resisting connections inframes designed for seismic applications. Thenew recommendations address the types ofmaterials that may be used for this work, thewelding processes and procedures that should befollowed, the environmental conditions underwhich welding should be performed, and the levelof training and workmanship required of weldersand inspectors engaged in this work. In addition,FEMA 353 provides detailed recommendations forquality control, quality assurance and inspectionprocedures that should be put into place toassure that this critical work is properlyperformed. These new recommendations affect allsegments of the design and construction processand require designers, producers, fabricators,welding electrode manufacturers, welders andinspectors, among others, to adopt newpractices. These recommendations have beensubmitted to the American Welding Society forincorporation into the structural welding code.

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What is different about the new designprocedures?

Prior to the Northridge earthquake, most steelmoment-frame designs used a standardconnection that was prescribed by the buildingcode. We know now that the configuration ofthese connections was problematic and resultedin unreliable connection performance. Followingthis discovery, the building codes were changedto require testing of each new connection design.This was very costly and difficult to implement.FEMA 350 presents a series of new connectionconfigurations that are recommended asprequalified for use in moment-frames conformingto certain limitations. The use of theseprequalified connections will greatly simplify thedesign process and make the construction of newmoment-frame buildings more economical andmore reliable.

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General:Applicable systems OMF, SMF

Hinge location distance sh dc/2Critical Beam Parameters:

Depth Up to W36 (OMF)Up to W30 (SMF)

Minimum Span OMF: 15 ft.SMF: 20 ft.

bf/2tf of flange 52√F y ; 35√Fy recommended minimumFlange thickness Up to 1-1/4” (OMF)

Up to ¾” (SMF)Permissible Material Specifications A36, A572 Grade 50, A992

Critical Column Parameters:Depth Not Limited

Permissible Material Specifications A572, Grade 50; A913 Grade 50Minor Axis Connection Pre-

qualifiedOMF: YesSMF: No

Beam/Column Relations:PZ strength Section 3.3.3.2 for SMF; Cpr=1.2; Rv/Ru < 1.2 (recommended)

Column/beam bending strength Section 2.8.1; Cpr=1.2Connection Details

Web connection Section 3.5.3.2. Welding QC Level 2.Continuity plate thickness Section 3.3.3.1

Flange welds Section 3.5.3.1: Welding QC Level 1.Weld electrodes CVN 20 ft-lbs at -20 oF and 40 ft-lbs at 70 oF

Weld access holes Not Applicable

FEMA 350 provides prescriptive criteria forprequalified beam-to-column connectionscapable of reliable performance.

The Reduced Beam Section, or “dog bone”connection is one of 12 different types ofprequalified connections that engineers canspecify without project-specific testing.

How many different types of connectionsare prequalified?

FEMA 350 contains design criteria for twelvedifferent types of prequalified, welded and boltedbeam-to-column connections. Eachprequalification includes specification of thelimiting conditions under which the connectiondesign is valid, the materials that may be used,and the specific design and fabricationrequirements.

What is the basis for the newprequalified connections?

The new prequalified connections contained inFEMA 350 are based on extensive laboratory andanalytical investigations including more than 120full-scale tests of beam-to-column connectionassemblies and numerous analytical studies ofthe behavior of different connection types.Connections were prequalified only after theirbehavior was understood, analytical models weredeveloped that could predict this behavior andlaboratory testing demonstrated that theconnections would behave reliably.

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What is the best type of connection touse?

Each of the prequalified connections offers certainadvantages with regard to design considerations,fabrication and erection complexity, constructioncost, and structural performance capability. Noone connection type will be most appropriate forall applications. It is likely that individualengineers and fabricators will develop preferencesfor the use of specific types of connections, andthat certain connections will see widespread usein some parts of the country but not others.

Do the prequalified connections cover allpossible design cases?

The prequalifed connections should be applicableto many common design conditions. However,they are not universally applicable to allstructures and, in particular, are limited inapplicability with regard to the size, types andorientation of framing members with which theycan be used. If a design requires the use oftypes or sizes of framing that are not included inthe ranges for a prequalified connection, FEMA350 recommends that supplemental testing of theconnection be performed to demonstrate that itwill be capable of performing adequately.

Is it possible to use types of connectionsother than those that are prequalified?

FEMA 350 contains information on several typesof proprietary connections that are not prequalifiedand nothing in FEMA 350 prevents engineers fromdeveloping or using other types of connectiondesigns. However, FEMA 350 does state that,when other types of connections are used, atesting program should be conducted todemonstrate that these connections can performadequately. FEMA 350 also presents informationon the types of testing that should be conductedand how to determine if connection performanceis acceptable.

Will designs employing these newrecommendations cost more?

The cost of a steel frame building is generallydependent on the amount of labor required tofabricate and erect the steel and the total numberof tons of steel in the building frame. The newdesign recommendations both formalize andsimplify design procedures that generally havebeen in use by engineers since the 1994Northridge earthquake. They do not result in anincrease in steel tonnage or significantly greaterlabor for fabrication and erection. However, moreextensive construction quality assurancemeasures are recommended and these will resultin some additional construction cost. It isanticipated that this additional cost will amount toless than 1% of the total construction cost fortypical buildings.

Will buildings designed to the newrecommendations be earthquake proof?

It is theoretically possible to design and constructbuildings that are strong enough to resist severeearthquakes without damage, but it would not beeconomical to do so and we could not afford toconstruct many such buildings. The objective ofmost building codes is to design buildings suchthat they might be damaged by severeearthquakes but not collapse and endangeroccupants in any earthquake they are likely toexperience. The FEMA 350 recommendationsadopt this same design philosophy. It isanticipated that there would be a very low risk oflife threatening damage in buildings designed andconstructed in accordance with the FEMA 350recommendations. However, they mayexperience damage that will require repair,including permanent bending and yielding of theframing elements and possible permanent lateraldrift of the structure. Although it is possible thatsome connections designed using the newprocedure may fracture, it is anticipated that thiswill not be widespread.

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Will the new recommendations beincluded in the building codes?

The seismic provisions of current building codesare largely based on the NEHRP RecommendedProvisions for Seismic Regulations of Buildingsand Other Structures, supplemented by standardspecifications developed by industryassociations, including the American Institute ofSteel Construction (AISC) and the AmericanWelding Society (AWS). Many of the designrecommendations contained in FEMA 350 havealready been incorporated into the standarddesign specifications developed by AISC and aredirectly referenced by the 2000 InternationalBuilding Code. The remaining recommendationsare being proposed for incorporation into latereditions of the AISC and AWS specifications.This material is also being included in the NEHRPRecommended Provisions for SeismicRegulations of New Buildings and OtherStructures. Some of the earlier material hasalready been included in the 1997 edition (FEMA302/303) and the remainder of the material will beincorporated into the new 2000 edition (FEMA368/369), due to be released in spring 2001. It isanticipated that many jurisdictions around theUnited States will adopt building codes based onthese criteria as the basis for building regulationin their communities.

Is it possible to design to the newrecommendations before the newbuilding codes are adopted?

Many of the new design recommendations arecompatible with existing building code regulationsthat are already in force around the United States.It should be possible for engineers to utilize thenew recommendations, on a voluntary basis, as asupplement to the existing building coderequirements. However, the local building officialis the ultimate authority as to the acceptability ofthis practice. Engineers should verify that thebuilding official will accept the new documentsprior to using them to design buildings.

Is it possible to design for betterperformance?

As an option, FEMA 350 includes a methodologywhich engineers can use to design for superiorperformance relative to that anticipated by thebuilding code. This approach is similar to theprocedures contained in FEMA 351 for theevaluation and upgrade of existing buildings. Inthese procedures, a decision must be made as towhat level of performance is desired for a specificlevel of earthquake motion. As an example, oneof the available performance levels, termedImmediate Occupancy, results in such slightdamage that the building should be available foroccupancy immediately following the earthquake.The performance-based procedures of FEMA 350can be used to design a building such that thereis a high probability the building will provideImmediate Occupancy performance for any levelof earthquake intensity that is desired. As aminimum, however, buildings should be designedto satisfy the prescriptive criteria of the applicablebuilding code, which are intended to protect lifesafety.

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CONSTRUCTION QUALITY CONTROL AND ASSURANCE

What is construction quality control?

Construction quality control includes that set ofactions taken by the contractor to ensure thatconstruction conforms to the specified materialand workmanship standards, the requirements ofthe design drawings and the applicable buildingcode. Construction quality control includes hiringworkers and subcontractors that have thenecessary training and experience to perform theconstruction properly; making sure that theseworkers understand the project requirements andtheir responsibility to execute them; andperforming routine inspections and tests toconfirm that the work is properly performed.Some contractors may retain independentinspection and testing agencies to assist themwith their quality control functions.

What is construction quality assurance?

Construction quality assurance is that set ofactions, including inspections, observations andtests, that are performed on the owner’s behalf toensure that the contractor is conforming to thedesign and building code requirements. Inessence, construction quality assurancerepresents a second line of defense andsupplements the contractor’s own quality controlprogram. Building codes specify minimum levelsof quality assurance for different types ofconstruction. If the contractor on a project doesnot perform adequate quality control it may beappropriate to provide more quality assurancethan required by the applicable building code.Quality assurance tasks are typically performedby design professionals and special inspectionand testing agencies specifically retained by theowner for this purpose. It is recommended thatthe engineer assist the owner in determining thelevel of quality assurance appropriate to a specificproject, and also that the engineer be retained bythe owner to actively monitor and participate inthe quality assurance process.

Why is construction quality control andquality assurance important?

Investigations performed after every earthquakeindicate that much of the damage that occurs is aresult of structures not being constructed inaccordance with the applicable building code orthe designer’s intent. This problem has affectednearly every type of building construction,including wood frame, masonry, concrete andsteel buildings. Construction errors andconstruction work that is improperly performedcreate weak links where damage in structurescan initiate. Earthquakes can place extremeloading on structures and, if structures containimproper construction, damage is likely to occurwhere that poor construction occurs.

Was inadequate construction quality asignificant factor in the damagesustained by steel buildings in theNorthridge earthquake?

One of the contributors to the damage sustainedby steel moment-frame buildings in the Northridgeearthquake was construction that did not conformto the applicable standards. In particular,investigations conducted after the earthquakerevealed that critical welds of beams to columnswere not made in accordance with the buildingcode requirements. A number of defects werecommonly found in the construction including:welds that had large slag inclusions and lack offusion (bonding) with the steel columns, weldsthat were placed too quickly and with too muchheat input into the joint; and welding aidsincluding backing and weld tabs that wereimproperly installed. These construction defectsresulted in welded joints that had lower strengthand toughness than should have been provided,resulting in a propensity for damage.

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Who is responsible for constructionquality control and assurance?

Each of the participants in building construction,including the design engineer, the contractor, thebuilding official, and special inspectors, plays acritical role in assuring that construction meetsthe appropriate standards. The engineer mustspecify the applicable standards and the qualityassurance measures that are to be used. Thecontractor must perform the work in the requiredmanner and perform inspections to verify that thisoccurs. Building codes require that the ownerretain a special inspection agency to performdetailed inspections and assure that thecontractor is executing the work properly. Theresponsible building official typically monitors theentire process and makes sure that each of theparties fulfills their individual responsibilities.

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FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 353 / July, 2000

Recommended Specificationsand Quality Assurance Guidelines for Steel Moment-Frame Constructionfor Seismic Applications

FEMA 353 provides recommendedspecifications for fabrication, erection, andquality assurance of steel moment-framestructures designed for seismic applications.

Where can one find recommendationson appropriate quality control andquality assurance procedures?

FEMA 353 provides detailed recommendations forprocedures that should be followed to assure thatsteel moment-frame construction complies withthe applicable standards. It includes informationthat is useful to engineers, building officials,contractors and inspectors. This information ispresented in the form of specifications that can beincluded in the project specifications developedby engineers for specific projects. It alsoincludes commentary that explains the basis forthe recommendations and methods that can beused to implement them.

Are the new quality recommendationssignificantly different than thoserequired in the past?

Many of the recommendations contained inFEMA 353 are a restatement and clarification ofrequirements already contained in building codesand in the standard AISC and AWSspecifications. There are also some importantnew recommendations. These include newrequirements intended to assure that weld fillermetals and welding procedures are capable ofproviding welded joints of adequate strength andtoughness. In addition, comprehensiverecommendations are provided as to the extent oftesting and inspection that should be performedon different welded joints. In part, these newrequirements are based on findings that theinspection methods traditionally used in the pastto assure weld quality are incapable of reliablydetecting nonconforming construction. It isanticipated that many of these new requirementswill be incorporated into the AWS standards inthe future.

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PREPARING FOR THE NEXT EARTHQUAKE

What should communities do before thenext disaster occurs?

Before the next disaster strikes, communitiesshould conduct a thorough examination of theirvulnerability to natural hazards and the risks theymay present to their citizens. In addition to apotential earthquake threat, communities may bevulnerable to other significant hazards, such asflooding, high winds, hurricanes, tornadoes,tsunami, etc. While this publication addressesthe potential earthquake risk of steel moment-frame buildings, communities may have moresignificant risks from other more hazardous typesof buildings, such as unreinforced masonry, non-ductile concrete frame or tilt-up concretebuildings. Communities should carefully considerall of these potential risks and how they mayaffect the different types of construction present.

Regardless of the risk posed by existingconstruction, before the next disaster occurscommunities should adopt reliable building codesand building regulation practices that will ensurethat new buildings are adequately designed andconstructed to resist the future earthquakes andother disasters that will inevitably occur. Buildingcodes that incorporate the latest edition of theNEHRP Recommended Provisions for SeismicRegulation for Buildings and Other Structures arestrongly recommended.

Communities may wish to encourage buildingowners to upgrade their existing hazardousbuildings to minimize potential damage,economic and life loss. FEMA 351 can be usedas a technical basis for both voluntary andmandatory upgrade programs. In addition,communities should put emergency responsesystems into place. Building departments shouldtrain their personnel to conduct buildinginspections. In large cities, building departmentswill quickly become overwhelmed by thisresponsibility. Before the next earthquake, thebuilding department should make arrangementsfor assistance from outside the affected area.Some states have agencies that will facilitatethis. Communities should also consider adoptingordinances to govern the post-earthquake buildinginspection and repair process. Experience has

shown that some building owners will not actresponsibly to inspect and repair buildings,unless required by law to do so. FEMA 352 canbe used as a basis for developing post-earthquake inspection and repair ordinances.

Is help available?

FEMA has developed a number of tools thatcommunities can use to identify their exposure tonatural disasters, including both earthquake andflood hazard maps. FEMA has also developedstandardized, GIS-based computer software thatcan help a community to assess the risk thatthese hazards present. This software package,called HazardsUS, or HAZUS, presentlyaddresses earthquake risk, however modules arecurrently under development that will also addressthe risk from floods and high winds. FEMA hasalso published a series of guidance documentsfor both technical and non-technical audiences toassist in addressing specific hazard-relatedissues. All of these aids are available, withoutcharge, from FEMA.

Once the hazard and risk are known, FEMA hasa community-based initiative called ProjectImpact to help encourage the implementation ofpre-disaster prevention, or mitigation, activities.This initiative, which may include a one-time seedgrant, encourages the formation of community-based partnerships between public and privatestakeholders. For more information on this andother programs, visit FEMA’s website atwww.fema.gov.

Project Impact assists communities to formpublic and private partnerships to reduce thepotential for natural disaster losses.

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THE FEMA/SAC PROJECT AND DISSEMINATING ITS RESULTS

Why did FEMA elect to undertake suchan effort?

FEMA, as part of its role in the NationalEarthquake Hazards Reduction Program(NEHRP), is charged with reducing the ever-increasing cost of damage in earthquakes.Preventing losses before they happen, ormitigation, is the only truly effective way ofreducing this cost. In this light, the role of thisnation's building codes and standards inmitigating earthquake losses is critical, andFEMA is committed to working with this nation'sseismic codes and standards to keep themamong the best in the world. Because thedamage that resulted from the Northridgeearthquake called into question the designassumptions and building code requirementsassociated with steel moment-frame constructionand because of the unknown life safety hazard ofthe damaged buildings, it was crucial thatadequate repair and retrofitting procedures bequickly identified. This was particularly critical forFEMA, because many of the damaged steelbuildings were publicly owned and therefore thecost of damage repair was eligible for fundingunder the public assistance provisions of theRobert T. Stafford Disaster Relief and EmergencyAssistance Act. To fully respond to this need,development of reliable and cost-effectivemethods for use in the various building codes andstandards that address the design of newconstruction and the repair and/or upgrading ofexisting steel moment-frame buildings wasnecessary.

The solution of this problem required acoordinated, problem-focused program ofresearch, investigation and professionaldevelopment with the goal of developing andvalidating reliable and cost-effective seismic-resistant design procedures for steel moment-frame structures. This work involvedconsideration of many complex technical,professional and economic issues includingmetallurgy, welding, fracture mechanics,connection behavior, system performance, andpractices related to design, fabrication, erectionand inspection.

How were related policy issuesaddressed by the FEMA/SAC project?

As part of the project, an expert panel withrepresentation from the financial, legal,commercial real estate, construction, and buildingregulation communities was formed to reviewsocial, economic, legal and political issuesrelated to the technical recommendations. Thispanel was charged with evaluating the potentialimpact of the recommended design andconstruction criteria on various stakeholdergroups, identifying potential barriers to effectiveimplementation of the recommendations andadvising the project team on potential ways ofmaking the guidelines more useful and effective.The panel held a workshop in October 1997 thatbrought together representatives of theseconstituencies, and their recommendations wereconsidered during the development of technicalrecommendations and in the preparation of thispublication.

Why did it take so long to develop thenecessary information?

The damage experienced by steel moment-framestructures in the Northridge earthquake called toquestion the entire portions of building codes thataddressed this type of construction, and whichhad been developed over a period of more than 20years. This project basically had to start fromscratch and either revalidate all of the existingcriteria, or when this criteria was found to beinadequate, develop new design criteria andconstruction standards that could be relied upon.This involved a complete re-evaluation of theproperties of structural steels and weldingmaterials, the assumptions used in designing andevaluating different types of moment-connections,and the methods used in inspecting andevaluating these connections and their behavior.In the process, all of this material had to bedeveloped and presented in a rigorous andscientifically defensible manner so that it wouldbe recognized as reliable and used by thebuilding design, construction and regulatorycommunities. This project accomplished in sixyears what originally evolved over 20 years.

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How will this information be put untopractice?

The new FEMA guidance documents are nowbeing widely disseminated to the structuralengineering and building regulation communities.Three of these publications provide recommendedcriteria for design of more reliable newconstruction (FEMA 350), upgrading existingstructures to minimize the potential for futuredamage (FEMA 351), and evaluating and repairingbuildings after an earthquake (FEMA 352). Thefourth provides technical specifications andquality assurance guidelines (FEMA 353). Muchof the information contained in these publications,especially for new construction, is beingincorporated into both the NEHRP RecommendedProvisions for Seismic Regulation of Buildingsand Other Structures and the latest standarddesign specification published by AISC.Together, these publications serve as theconsensus design standard for steel structuresnationwide. This standard is routinely adopted byreference into the model building codes where itis in turn used as part of state and/or localbuilding codes. AISC has also published adesign guide, developed jointly with NIST, forupgrade of existing structures, and whichcompliments the recommendations contained inFEMA 351.

How can one obtain additionalinformation?

The four FEMA publications described above canbe ordered free of charge by calling 1-800-480-2520. In addition, a series of state-of-the-artreports have been developed which summarizethe current state of knowledge that forms thetechnical basis for the recommended criteriapublications. These reports are available fromFEMA in CD-ROM format (FEMA 355). Individualreports on the technical investigations performedin support of the FEMA/SAC project are availableas well. These technical reports and FEMA 355may be purchased in hard copy format from theSAC Joint Venture. The SAC Joint Venture willalso continue to maintain a publicly accessiblesite on the World Wide Web atwww.sacsteel.org. Future information andtraining opportunities will be available from theAmerican Institute of Steel Construction (AISC)and other organizations. Consult AISC’s WorldWide Web site at www.aisc.org for additionalinformation.

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INDEX

American Institute of Steel Construction (AISC), 2, 18,21, 23, 26

American Welding Society (AWS), 21, 23building code, 1, 15, 17, 19, 20, 21, 22, 23, 25collapse, 4, 7, 8, 14, 20connection, 3, 5, 8, 9, 10, 11, 12, 15, 19, 20, 25construction, 20cost, 2, 5, 9, 13, 14, 15, 16, 19, 20, 25crack, cracks, cracking 4, 5, 6damage, i, 3, 4, 5, 6, 8, 9, 10, 12, 14, 15, 16, 17, 20, 21, 22,

25, 26design, 1, 2, 4, 5, 6, 11, 12, 13, 14, 17, 18, 19, 20, 21, 23,

25, 26FEMA 350 Recommended Seismic Design Criteria

for New Moment Resisting Steel Frame Buildings,2, 17, 18, 19, 20, 21, 26

FEMA 351 Recommended Seismic Evaluation andUpgrade Criteria for Existing Welded Steel FrameBuildings, 2, 11, 12, 14, 15, 16, 21, 26

FEMA 352 Recommended Post EarthquakeEvaluation and Repair Criteria for Welded SteelMoment Frame Buildings, 2, 8, 9, 10, 24, 26

FEMA 353 Recommended Specifications and QualityAssurance Guidelines for Steel Moment FrameConstruction for Seismic Applications, 2, 18, 23, 26

fracture, 1, 4, 10, 20, 25inspection, 5, 8, 9, 23, 24, 25Kobe, 1, 6Loma Prieta Earthquake, 1, 6materials, 19National Institute of Standards and Technology

(NIST), 2, 26National Science Foundation (NSF), 2NEHRP Recommended Provisions for Seismic

Regulation of Buildings and Other Structures(FEMA 302/303), 12, 21, 25

Northridge earthquake, i, 3, 4, 6, 8, 9, 15, 17, 19, 20,22, 25

quality assurance, 17, 20, 22, 23, 26quality control, 22, 23repair, 1, 2, 9, 10, 15, 20, 24, 25safety, 7, 8, 9, 14, 15upgrade, 2, 11, 12, 13, 14, 15, 16, 21welding, 4, 11, 18, 22, 23welded joint, 1, 4