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T H E STRUCTURAL DESIGN OF TALL BUILDINGS, Vol. 5, 29-44 (1996)

PROFESSIONAL STRUCTURAL ENGINEERINGEXPERIENCE RELATED TO WELDED STEELMOMENT FRAMES FOLLOWING THENORTHRIDGE EARTHQUAKE

WILLIAM E. GATESDames & Moore , Inc., 91 I Wilshire Blvd, Suite 700, Los Angeles, CA, U.S.A

ANDMANUEL MORDEN

Brandow & J ohns t on Associates, 1660 West 3r dSr, Lo s Angeles, CA, U . S . A

SUMMARYThe experiences of professional s tructu ral engineers with welded steel mo me nt frame conne ctions followingth e 1994 Northridge earthquake were gathered by personal interview. This paper ca pture s their experiencesin inspecting an d repairing these connections following the earthq uake . The engineers interviewed repr esenta wide cross section of Lo s Angeles structural engineering firms.

1 . I NTRODUCTI ONThe 17 January 1994 Northridge earthquake produced a brittle form of failure in the supposedlyductile steel moment resisting frames of numerous buildings in the San Fernando Valley, WestLo s Angeles and surrounding areas. This form of damage had not been anticipated by theengineering profession and construction industry. To capture the history and lessons learnedfrom this event, a series of interviews was conducted with key professionals involved in theinspection, repair and reconstruction of the damaged buildings. The interviews were conductedto document the significant experiences of structural engineers involved in the discovery,inspection, and evalu ation of the steel mo men t resisting frame buildings. The stru ctu ral engineersare listed in Table I. The interviews were designed for systematically gathering, synthesizingand analysing perishable data, such as impressions and unusual experiences that may havebeen encountered during the process, and for identifying key issues or concerns and lessonslearned.

2. METHODOLOGYThe structural engineers listed in Table I are representative of the professionals in the LosAngeles are a directly involved in the review and assessment of the dama ged steel mom ent fram es.Group interviews were organized with each of the engineering firms in order to capture the fullspectrum of first-hand experience in damage assessment, repair design and field observation ofC C C 1062-8002/96/010029- 160 996 by John Wiley & Sons, Ltd. Received July 1995Revised August 1995

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30 W. E. GATES A N D M . M O R DE NTable I . Interview participants

Organization Individuals(Roy Johnston-SEManny Morden-SEPeter Maranian-SErandow & Johnston, Associates

Myers Nelson Houghton

( Eugene Ungermann-SEDavid Houghton-SERichard Fallgren-SEKen Odell-CEJesse Karns-CEBarry Schindler-SESteve Hagen-CEChuck Whittaker-CEJohn A. Mart in & Associates

KPFF

Englekirk & Sabol

Joseph Stewart-SERamzi Hodall-SERoger Young-SERick Davis-CEJohn Gavan-CETom Sabol-SEBarrett Bunce-SEAlan Shiosaki-SEDani Llovet-CE

The Allen Company Jay Allen-SES. B. Barnes & Associates Clarkson Pinkham-SESE = Registered Structural Engineer.C E = Registered Civil Engineer.

construction. In addition, senior st ructural engineers in the community familiar with the historyof steel frame seismic code development were interviewed. The interviews were conducted overthe period 30 January through 24 March 1995.The interviews were organized in to four parts with specific questions developed for each part.These four parts inciuded the following.(1) Introduction to the survey with objectives defined; experience and training of participantsidentified; and the program of questioning outlined.(2) Experience and lessons from damaged welded steel moment frame (WSMF) buildings inthe Northridge earthquake. This part of the interview focused on how WSMFs weredesigned and constructed before the Northridge event and how the participants inspected,assessed damage and implemented repairs or retrofits to WSMFs following the Northridgeearthquake.(3 ) Theories and opinions on the probable causes for the unexpected brittle failures in theWSMF connections.(4 ) Specific solicited recommendations on future design practices for WSMFs and inspection,damage assessment and repair of WSMFs in future earthquakes, based on the Northridgeexperience.

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WELDED STEEL MOMENT FRAMES 31The interviews were conducted by Bill Gates and Manny Morden with Allan Porush as analternative and often third participant. They were conducted in the offices of the engineeringfirms. Most of the interviews were conducted at lunchtime to minimize lost productive hourson the part of the participants. The typical interview lasted 1.5 to 2 hours. The entire interviewwas recorded with a cassette tape recorder as well as by hand-written notes. The tapedinterview was transcribed to hard copy for interpretation, analysis and summary in thisreport.To prioritize the opinions and observations of the engineers interviewed, a formalquestionnaire was mailed to each individual. The questionnaire was prepared along the lines ofquestioning in the interviews and was based on input from the engineers. The survey formprovided an opportunity for independent expression of opinions and impressions on thecausative factors for the WSMF damage.Th e major structu ral engineering firms and individuals (see Tabie I ) selected for th e interviewswere the original designers for many of the steel moment resisting frames in the Los Angelesregion. Th e stru ctu ral engineering staff in these firms has han ds-on experience in the review andrepair of approximately 50% of the damaged W SM Fs as a result of the Northridge earthquake.The majority of the engineers interviewed have either participated in or are currently involvedin developing seismic design s tanda rds used in W SM F construction.Before the Northridge event, none of the engineers interviewed anticipated that WSMFconstruction would fail in brittle fracture at the welded connection in an ear thq uak e. All expectedsusbtantially ductile behaviour to occur with local plastic deformation of the connection similarto that witnessed in the university test programs that were used to qualify the weldedbeam -to-colum n joint for mod ern steel frame design. In hindsight, a few of the engineers adm ittedthat they were somewhat skeptical of the ability of the welded joints to develop fully plastichinges before some form of failure might occur in the highly stressed weld zone. One of theengineers was even suspicious that w elded join ts might behave in a brittle ma nn er when subjectedto very rapid strain rates under intense seismic loading. However, none of the engineers had abasis on which to reject the building code premise tha t ductile yielding in this type of constructioncould be achieved in an earthquake.Most of the engineers interviewed referred to the components of the moment connection asfollows:(a) Joint-the welded atta chm ent of a flange of a beam to a colum n.(b) Connection--the full interface joining a beam to a colum n, including the m eans offastening.

3. C H R O N O L O G Y O F D IS CO V ER YIt was the general impression of the stru ctu ral engineering commu nity, immediately followingthe Northridge earthquake, that WSMFs survived the event with no significant damage. Noneof the buildings showed signs of plastic deformation that would normally have been anticipatedas an indication of structural distress and none of the steel frames had collapsed. The firstindication that steel frame structures had experienced damage was reported from newconstruction in which the steel frames were still exposed. Within days, it became apparent thatseveral of the steel frame structures in the cities of San ta Clarita, Valencia an d W oodland Hills(Can oga Par k) were out of plumb a nd could no t be maintained in alignment d uring the seriesof aftershocks. As the interior surfaces and fireproofing were removed from the steel frames ofthese buildings, it became evident that significant brittle fractures had developed in the welded

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32 W. E. GATES AND M . MORDENbeam-to-column joints. The damage consisted of cracks through the beam-to-column welds, orthrough the base metal of the beam or column flanges. These cracks resulted in a loss ofseismic mom ent resistance in the dam aged connections; however, the connections still transferredgravity loads which, in the opinion of the engineers interviewed, may explain why there wereno collapses.By the end of two months, more than 17 to 20 W SM Fs h ad been identified as having damageto the welded beam-column connections. In the succeeding mo nth s up to the present, the num berof damaged WSMF structures has continuously increased. As of August 1995, i t is estimatedthat over 150 steel frame buildings, o r roughly 75% of the building struc tures investigated, havesome form of brittle cracking in one or mo re of the beam-column connections.

4. DAMAGE S URVEY AND TES TI NG4 .1 . Methods of damage assessment

Initially, the steel frame structures were surveyed for damage using standard pre-Northridgemethodologies. This consisted of visual inspection of the fireproofed steel members to verify thatthe flanges in the beams at the column joint of the WSMF had not buckled. Shear tabs in thejoints were also checked. Unfo rtunately, the cracks in the welds and flanges of the join t elementswere not visible through the fireproofing. However, as soon as the engineering communitylearned that steel moment resisting frames had suffered a brittle form of damage, resulting incracked welds and flanges, the engineers turned to new methods for surveying and testing tolocate and identity the damage.When trying to estimate whether a WSMF building had been damaged in the earthquake,all of the engineers interviewed indicated that they were generally misled by the state of damageor lack thereof observed in the building cladding surrounding the steel frame. There are a fewinstances where the non-structural elements were heavily damaged and it appeared that thebuildings frame must also be damaged. However, when the moment connections were fullyexposed by removing the fireproofing, the joints were found to be essentially undamaged. Inother cases, the non-structural elements of the building had virtually no signs of damage, yetwhen the steel frame connections were exposed and examined, in some cases they were foundto have cracks extending through the columns, leaving the beams of the frame free to separatefrom the columns at a floor level. Thus, the normal damage survey conducted by the City ofLos Angeles and adjacent communities in which buildings were red, yellow, and green taggedbased on the visually observed degree of damage and risk to life, turned out to be unreliablefor identification of damaged WSMFs. Instead, as engineers discovered damaged WSMFs in acluster of similar structures, they would extend the survey process to the nearby buildings simplyto verify that these structures were structurally safe, even if the buildings appeared totallyundamaged.The engineers resorted to a series of post-Northridge survey techniques that had rarelybeen used in the past for assessing earthquake damage to WSMFs. These included thefollowing:

(a) plumb bob and transit surveys for potential vertical misalignment;(b) visual inspection for ma jor cracks thr oug h welds, flanges and shea r tabs-with fireproofing(c) ultrasonic tests (UT) for cracking in welds and parent materials, including root cracks(d) magnetic particle and dye penetrant tests.

removed;hidden by the back-up bars;

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W E L D E D S T E E L M O M E N T F RAMES 33The plumb bob and transit surveys for vertical misalignment were found to be relevantin only a few instances. The magnetic particle and dye penetrant tests are only applicableat the surface of the welds and are normally used for the fillet welds on shear tabs. Thistesting procedure was used more extensively in the repair of cracked welds. The visualinspection for major cracks as well as the ultrasonic tests for cracking in the welds andparent materials turned out to be the dominant mode of damage inspection used onW S MF s .The ultrasonic test proved to be the most effective method in locating cracks within the weldand base materials. These cracks were generally found to start at the root weld in the bottomand top flange joints of the beam and to propagate throu gh the weld zone into the base metalsof the beam or colum n flange. Th e presence of the back-up bar at the r oot weld pass often resultedin erroneous readings, indicating the presence of a minor crack (e.g. AWS-D1.1 Type W1) inthe root of the weld. When the back-up bar was removed and the weld retested ultrasonically,

i t was reported that generally 50% of the welds turned ou t to have no cracks. Thus, the engineershad t o be rathe r cautious a bou t calling a building damaged until all the minor ( W l) cracksidentified ultrasonically were confirmed through direct visual inspection or ultrasonic retestingwith the back-up bar removed.Metallurgical tests were conducted on some of the first steel frame buildings found to haveweld failures. This samp ling and testing process was no t normally carried o ut on other b uildingsin the investigative phase; however, the metallurgical test pro cedure is sometimes used in testingto establish the repair procedure.4.2 . Selection of candidate connections

Inspection of connections in the WSMF buildings turned out to be a costly process thatinvolved removal of architectural finishes and surfaces, scraping away fireproofing (some of itincluding asbestos), wire brushing the steel surfaces, then condu cting the non-destructive visualand ultrasonic tests and finally replacing fireproofing and non-structural surfaces and finishesafter the inspection process was completed. Since the damage did not occur in every momentconnection, and tended to follow a random pattern, the engineers attempted to minimize thecostly inspection process by identifying the probable locations or hot spots where seismicallyinduced failures might have occurred.Each engineering firm interviewed had its own method ology for identifying the most p robablehot spots in the structure for inspection. Most of the engineers reviewed the design drawings toidentify the critically stressed connections and large member sizes that represented connectionswith a high poten tial for damage. They often relied on p rior design and analysis experience withsimilar structures and on Northridge experience gained from other damaged buildings. ThisNorth ridge experience included directionality in the earthqu ake damage patterns as reported tothem by other engineers. For WSMFs located in the San Fernando Valley, the north-southoriented frames were generally more heavily damag ed, while in the San ta M onica a nd W est LosAngeles areas, the frames that were oriented in the east-west direction parallel to S ant a Mon icaBoulevard tended to be more heavily damaged.The engineers also took advantage of information from other engineers on the distributionof dama ge in low-rise and high-rise structures. For low rise (e.g. two to six stories), there was ahigher percentage of connection failure in the first two floors above ground than in the higherstories of the building. The damage was also more random in these structures. For high rise(e.g. ten stories and up), the damage tended to be concentrated in the upper two-thirds of thebuilding.

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34 W. E. GATES AND M . MORDENOther engineers attempted to locate the hot spots in the WSMFs using computer modelsto simulate the dynamic earthquake response. These analyses were generally performed withthe ETABS computer program using the most readily available design earthquake such as theUniform Building Code design response spectra; or the analyses were performed using recordedground motion time-histories of historic earthquakes. Generally, earthquake records from theNorthridge event near the particular building site were not available for the initial screeningstudy. T he success ratio in identifying potentially dam aged join ts by either analysis or engineeringjudgment was significantly lower than the engineers had anticipated, particularly in low-risebuildings.

4.3. Minim um inspectionSince location of the dam aged mom ent resisting connections was not readily predictable withany degree of certainty by e ither linear elastic analysis o r engineering judgm ent, a preliminaryguideline for inspection practice was developed by the City of Los Angeles Department ofBuilding and Safetys Task G ro up on Steel Buildings. This Guideline specified that the m inimuminspection should include 5 to 10% of the welded connections in high-rise buildings over sixstories and approximately 15% of the connections in low-rise buildings of six stories and less.

These percentages would vary depending on the total number of connections per level and inthe entire building. For buildings with heavy external damage or those in which there wasobvious misalignment as evidenced by vertical alignment surveys, the engineers normally hadlittle difficulty in convincing the ow ner to let them expose the connection s an d inspect. However,where the building appeared undamaged, the engineers often found it difficult to convince thebuilding owner of the need to expose 10 to 15% of the connections, or at least the percentagenecessary to satisfy the engineer that the structure had little risk of hazardous performance infuture earthquakes d ue to connection damage.Typical inspection costs ranged from $800 to $1200 per beam-to-column connection in acommercial building without asbestos removal. If asbestos removal was necessary, the costscould increase by an a dded $1000 to $2000 per connection. For high class residential buildingswith gypsum or plaster w alls an d ceiling enclosures, the cost to inspect a typical frame connectioncould range from $2000 to $5000. For institutional structures such as hospitals, the soft cost(i.e. dislocation of operations) often equalled the hard costs, where the hard costs generallyinclude: ceiling and wall access and restoration; fireproofing removal and restoration; clean-up;testing services of an inspection laboratory and engineering fees.

5. T Y PE S O F D A M A G E5.1. Introduction-moment resisting connec tions

The various types of failure patterns to the WSMF connections are illustrated in Figures 1an d 2. In general, the do m inan t mode of observed failure in the mom ent resisting connectionswas a brittle fracture initiating at the bottom beam flange, generally starting in the root of thewelded joint. T he fracture followed various paths from the roo t weld into the base m aterials orheat affected zone and resulted in a wide variety of fracture patterns, as illustrated in Figure 2.Once the bottom flange failed, the bolted and welded gravity shear tab would often unzip,starting from the bottom and working up the connection (see Figure 1, S3). The most strikingand potentially hazardous mod e of failure was the brittle fracture propagating from the root ofthe weld through the column flange and web (see Figure 1, P5).

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WELDED STEEL MOMENT FRAMES

Relerence Detail: MR F Joint Damage TypesNote: See Relerence Schedule lor Description

Figure 1. Types of connection damage

BYTESmG

Relerence Detail.MRF Joint Damage TypesNote: See Relerence Schedule or Description

Figure 2. Tvpes of weld damage

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36 W. E. GATES AND M . MORDEN5.2. Degree of damage

The degree of observed cracking in the welded moment connections varied from building tobuilding an d from engin eering office to enginee ring office interviewed. F or p urposes of discussion,three degrees of damage were defined during the interview based on the severity of cracking.These were a s follows.(1 ) Minor cracks. Non-visual root weld cracks identified through ultrasonic tests (UT) and(2) Signijcant cracks. Non-visual, UT cracks of structural significance (greater than in(3) Severe cracks. Visually identified and extending through the weld or base metals of the

defined as AWS D1.l, Class W1 (a depth or less).depth).beam or column flange and of major structural significance.

Table I I illustrates the variation in the degree of observed weld damage in the momentconnections. David H ough ton, of Myers Nelson H ough ton, noted th at when you can visuallyfind weld connection damage, it is pretty much going to be widespread. And its goingto be severe, as opposed to a little minor damage. In contrast, Brandow & JohnstonAssociates engineers found 60 to 75 % of the cracks to be minor in the buildings theysurveyed.One of the major issues brought up by all engineering firms interviewed was the significanceof minor cracks in the root weld. These are the non-visible cracks identified through ultrasonictesting and defined under ASW D1 .l, Class W1, as acceptable flaws (non-rejectable under norm alconstruction).The engineers were nearly unanimous about accepting minor root weld cracks as insignificantrelative to structu ral safety. However, they were split on their interp retation of the cause of thecracks. Some thought they were a pre-existing condition while others believed they representedminor crack damage caused by the Northridge earthquake.When the extent of cracking reached AWS D1.l, Class W2 or W3, the cracks were consideredstructurally significant in terms of safe performance in future earthquakes. Most of theengineering firms interviewed had found that, by removing the back-up bars when Class W 2cracks were found in the U T inspection an d retesting, the cracks were downgraded to

Class W 1-which was considered acceptable. The bac k-u p bar introduc ed a n additionaldegree of crack dep th in the U T inspection process that m ade the connection a ppear t o bedamaged to an unacceptable degree. Thus, much of the significant damage in the form ofroot cracks was downgraded to acceptable flaws by simply removing back-up bars andrepeating the ultrasonic tests. The engineer would then replace the back-up bar with a filletweld.

Table 11. Degree of observed weld damage (%)Engineering firm

Degree of da m a ge M N H B&JA JAMA KPFF E& S TA CMinor cracks 10-20 60-75 20 10-15 50-60 15-30Significant cracks) 50-60 20-30 60 25-30 20-30 60-70Severe crackd3) 30-40 5-10 20 10 10-20 10-15N u m b e r of buildings represented 20 30+ 30+ 4 20+ 10

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WELDED STEEL MOMENT F RAMES 37Table 111. Ratio of beam top flange to bottom flange weld cracks and weld or base metal fracture-engineers impressions based o n buildings surveyed

Engineering firmItem M N H B&JA J A M A K P F F E&S TA C

1~ ~ ~ -.-1 1 I I 1~10 20 10 5 10 10_- _Weld cracks identified by UT or visual inspection

Weld or base metal fracture 1~ __I 1 110 40 100 10 20 40-~_N u m b e r of buildings surveyed 20 30+ 30+ 4 20+ 10

5.3. Distribution of damage within the connectionTh e greater percentage of the observed dam age in the welded mom ent con nections was locatedin the bottom beam flange welded joint and its interconnected components (e.g. beam flange,

shear tab, column flange and web). A small percentage of the top flange joints were found tobe damaged. Table 111 summarizes the engineers impressions of the ratio between top flangeand bottom flange weld cracks, as well as the ratio between weld and base metal fracture. Theengineers impressions varied, in some cases significantly, as a result of their method ofinvestigation and their lasting impressions of what they actually found. In general, thetop- o-bottom flange ratio of weld cracks identified by ultrasonic testing and visual inspectionrange from 1 :5 to 1 :20. The ratio for fracture through the weld or base metal ranged from1 :10 to 1 : 100.Most of the engineers interviewed concluded that the composite action of the concrete floorslab in conjunction with the top beam flange minimized the potential for failure in this beamlocation. However, some of the engineers theorized that since the root weld in the top flangewas not located in the extreme fiber of the beam, as i t is in the bottom flange, there was a lowerstress riser induced in the top flange by the root weld pass, thus reducing the probability ofbrittle failure in the top flange.

5.4. Distribution of damage within the buildingT he dam age within the building was found to be distributed in a pattern with height depend ingon whether the building was a low rise or a high rise. In the low-rise structures, there was ahigher percentage of connection failure in the first two floors above ground than in the higherstories of the building. In the high-rise buildings, the distribution of damage tended to beconcentrated in the upper two-thirds or upper half of the building. Several of the engineersobserved a higher percentage of damage in the moment connections at the corner columns ofthe WSMFs than at the interior connections.Local directionality of the dominant earthquake motion played a significant role in the degreeand distribution of damage in the WSMFs. For structures located in the San Fernando Valley,the north-south oriented frames were generally mo re heavily damaged, while in the Sant aMonica and W est Los Angeles areas, the frames oriented in the east-west direction parallel toSanta Monica Boulevard, tended to be more heavily damaged.

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38 W. E. GATES AND M . MORDEN5.5. Damaged gravity connections

The gravity connections in the non-seismic frames of the WSMF buildings often became thesecond line of lateral load resistance when the m omen t connections of the W SM Fs failed. Manyof the engineers interviewed stated that when they had observed heavy damage in the form ofcracking through beam or column flanges or heavy nugget tear-out of beam flanges from columnflanges, there was also associated damage to the gravity connections in the non-seismic frames.Th e estimated ratio between W SM F connection damage (heavy cracking a nd fracture) to gavityconnection damage, ranged from 3 :2 all the way down t o 20 : 1.The damage to the gravity connections typically included sheared bolts in the shear tabconnection, or tearing of the shear tab . In on e instance, an engineer reported that eight of theeight bolts at both ends of the beam had failed in the gravity connection, permitting the beamto hang by its shear studs from the concrete floor slab. In at least two other instances, it wasreported by engineers that many of the gravity connections in the building surveyed hadexperienced damag e as a result of a construction err or in which A307 machine bolts, rather thanthe specified A325 bolts, had been used in the gravity shear tab connections. In one building,over 80 sheared bolts were found lying in the ceiling by contractor when he opened i t up fordamage inspection.These examples indicate the degree of paritial structural collapse at gravity connections thatmay be associated with the brittle failures of WSMF connections. The engineers interviewedrecommended that gravity connections in heavily damaged buildings be exposed and visuallyinspected as part of the routine WSMF earthquake damage survey.

5 .6 . Deterioration of W S M F connectionsOne case of progressive crack propagation was reported in an eleven-story high-rise buildingthat had been designed with an im por tan ce factor of 1.5 in accordance w ith the Uniform BuildingCode Zone 4 requirements. Over a period of time, startin g with July 1994 thro ugh December1994, the building was ultrasonically tested three times a nd the size of cracks in the welds andbase metals identified. In the second and third ultrasonic test programs, conducted in Octoberand December of 1994, the cracks were found to have propagated from the welds into the basemetal of the column or beam flanges. The engineers involved postulated that the crackpropagation may have been due to continuing relaxation of the original residual stressesand readjustment of strains in the frames induced by the Northridge earthquake. Thisbuilding showed no signs of frame damage in the form of non-structural or architecturaldamage.

6. FACTORS P E R C E I V E D T O C O N T R I B U T E TO D A M A G E6.1 . Introduction

A series of factors may have contribu ted to the brittle m ode of failure observed in the W SM Fsfollowing the Northridge earthq uake . At the time of the interview, there was no clear evidenceto identify which factors were most significant in contributing to the failures. Thus, it was notsurprising to find that the engineers had differing opinions and theories as to the actual causeof the damage observed. The opinions and theories expressed were based on personalobservation, information gained from reading and talking with others, training and backgroundin the design an d construction of WS M Fs an d personal research.

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WELDED STEEL M O M E N T F RAMES 39The engineers opinions on the factors contributing to the brittle failure of welded WSMFsin the Northridge earthquake have been broken down into six key topic areas as outlinedbelow.( 1 ) Education, training, and qualifications.(2) Design practice.(3) Connection qualification tests.(4) C onstru ction materials.(5) Welding and inspection practices.(6 ) Earth quak e characteristics.A summary of the interliew findings is also presented in Table IV .

6.2. EducationltraininglexperienceAll the engineers interviewed, without exception, stated that they had not anticipated thatsteel moment frames would crack in a brittle fashion rather than deforming plastically, asdemonstrated in laboratory tests on this form of connection. Without exception, the engineersfelt that their formal education and background in material science and metallurgy did notprepare them adequately to cope with the design repair problems that they have faced onW SM Fs since the North ridge earthq uake . Most of the engineers feel that they have come up avery steep learning curve in the past year in welding technology and metallurgy.Many of the engineers felt that their formal education in welding was limited to the physicalstrength characteristics of the various types of welds with little training on the metallurgyassociated with the weld materials and the base metals. Few of the engineers had ever beentrained in non-destructive test procedures for welded connections and virtually none of theengineers had h eard of or fully understood what a W elding Procedu re Specification was supposedto accomplish in the construction of a W S M F .A few of the engineers stated that they had questioned whether the high state of stresscalculated in the welded beam-column connectio n could be sustained witho ut prema ture failure.However, they had no bias on which to refute the test reports from the universities or the factthat the design concept for the special welded moment connection was accepted as a buildingcode standard.

6.3. Design practice6.3.1. Redundancy. The engineers interviewed had strong opinions on the influence ofstructural redundancy on the observed earthquake damage to the WSMFs. Some of the firmsstated that i t was their stand ard design practice to use multi-bay W SM Fs to resist earthquak esin all pre-Northridge designs, while others indicated that for economic reasons, they generallyused single or two-bay frames. Table IV illustrates the wide spread of engineering opinion onthe significance of redund ancy in the damage observed.To validate the hypothesis that redundancy reduces the degree of damage, John A. Martin& Associates engineers presented tw o case studies from the Northridge earthquake. The firstwas a four-story building with four single-bay WSMFs, two in each principal direction. Thestructure had 4 inches of permanent lateral deflection at the roof after the earthquake. Of th e

40 moment connections, 34 were damaged. Many of these connections had cracks through thewelds or base metal with separations of $ t o 4".A s a consequence of the heavy damage to themoment connections, 80% of the gravity connections in the structure were also damaged.

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40 W. E. GATES AND M. MORDENTable IV. Factors perceived to contribute to WSMF connection damage in the Northridge earthquake

Relative importanceGeneral/specific factor

~

H M L N/A

Designpractice

r 1 . Lack of redundancy in bays2. Design force levels3. Welde d joint concept-is the prescriptive connec tion aflawed design?4. High triaxiality (restrain in 3 directions)5. Back-up bar (weld stress riser)6. Stress concentration factors (column web against beam7. Added flexural stress in beam flange caused by colum n8. Lack of continuity plates9. Low cycle fatigue

flange)panel zone shear deformation

10. Poor understanding of materia l sciences by the designer< 11 . Other-poor unde rstan ding of welding procedu res

1. Limited test program for prescriptive connection (staticloads, small scale members, ideal welding conditions inthe lab)2. Over optimism reflected test results by engineeri . O t h e r 4 i d no t remember the poor test resul tsTest program

1. Toughness of welds2. Lowe r fracture resistance with higher strain rates3. Low temperature fracture resistance4. High strength steel (lower toughness and ductility)5. Base metal acceptance standards. (Beam strength greaterthan anticipated in design relative to weld and columnstrength.)I 6. Residual stress from mill production (thick colum n flange)ConstructionmaterialsWeldingpractice

1. Non-conformance to AWS S tandard s and Practices2. Preh eat an d cool do wn (residual stresses)3. High deposition weld process (lack of fusion)4. AWS acceptan ce sta nd ard is to o lax-permits inclusions5. Welder qualification/certfication6. Inspector qualification/certification7. Inspection practices

that are stress risers

1. Larger ground motion t han previously expected by2. Larg e vertical com pone nt of ground m otion from thrust3. Very rapid energy release (high strain ra te in loading)

engineersfaultingEar thquake

28 39 28 55 44 56 050 39 11 028 56 17 050 33 17 050 33 1 1 511 22 67 0

6 50 39 511 28 50 1117 50 22 1111

67 22 0 I 161 28 5 6544 50 6 050 33 11 60 28 50 2211 56 33 028 56 17 0

28 56 17 033 50 17 039 56 5 061 39 0 011 56 33 05 78 17 0

5 67 22 017 56 22 522 33 44 0

5 33 56 572 22 6 0

Perceived factors contributing to the brittle welded connection failures have been ranked in relative importance usingqualita tive terms: high-H; medium-M; low-L; an d no t applicable-N/A. Th e relative impo rtance has beenquantified in the table using the percentage of the 18 engineers interviewed that selected the particular qualitativefactor. The numerical values indicate the degree of consensus between engineers on each of the perceived contributingfactors to th e WSMF connection damage.

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WELDED STEEL MOMENT FRAMES 41In the second case, a building had single-bay, double-b ay a nd triple-bay WS MF s that were alldamaged. However, the degree of damage to the single-bay frames was found to be much moresevere and the distribution of damage more extensive than in the multi-bay frames of the samebuilding. Barry Schindler of Joh n A. Martin & Associates summarized the issue on redundancyby simply stating On e thing we learned from the eart hq uak e was that single-bay frames probablyare not a good idea.6.3.2. Design force levels. Were the force levels used in the lateral earthquake design of theWSMF buildings too low? It was the opinion of the 18 engineers who completed the surveyquestionnaire that the design force levels used under the Los Angeles City Building Code werenot a major factor contributing to the damage. Table IV indicates that in their opinion i t witsranked as a low-to-medium factor.6.3.3. Welded joint concept. In the opinion of the engineers, the majority felt that the weldedjoint concept used in the standard prescriptive welded moment connection was flawed. It didnot perform as they had anticipated. This factor was assigned a high-to-medium importance.6.3.4. High triuxiality. Triaxiality is a condition in which deformation in the welded joint isrestrained in all three directions. High triaxiality requires large stresses for plastic flow andreduces the fracture energy, while the fracture ultimate stress is relatively unaffected. This makesthe fracture an easier energy dissipating mechanism than yielding. The engineers assigned to amedium-to-high importan ce t o the high computed triaxial state of stress in the welded interfacebetween beam and column flanges.6 . 3 . 5 . Back-up bar. The stress riser produced by the notch between the ba ck-up ba r and thecolumn flange at the root weld pass was considered to high-to-medium contributing factor tothe connection dam age observed.6.3.6. Stress concentration factors. Stress concentration factors or stress risers in thecolumn-web/beam-flange interface were studied by a team of Jay Allen (The Allen Company),Ralph Richard an d James Partrid ge (Smith-Emery Com pany ). Their analytical and experi-mental investigations identified not only the high stress riser in the weld interface between thecolumn web and beam flange, but also the significance of strain rate effect due to the stressconcen tration. M ost o f the engineers interviewed were aware of this analytical and experimentaltest pro gra m and were of the opinion th at stress concen tration had a high-to-medium importan cein contributing to earthquake damage.6.3.7. Beam Pany e jlexure caused b y panel zone shear deformation. Th e added flexural stresseswere produced in the beam flange by shear deform ations in the colum n panel zone. The flexuralstresses and induced deformation patterns were studied analytically by the team of JayAllen/Ralph Richard/Jim Partridge as well as by K P F F . The engineers who conducted theanalytical investigations were of the opinion that the added flexural stress represented asignificant contributing factor to the connection damage. However, the rest of the engineersinterviewed felt that the added flange flexural stresses contributed little to the connectiondamage.6.3.8. Lack of continuity plates. There are various opinions on the significance of continuityplates in the transfer of tension and compression forces through the column flange and webs

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42 W. E. GATES AND M . MORDENzone under beam bending. In general, the engineers felt that the lack of continuity platesrepresented a medium-to-low contributing factor to the damage.

6.3.9. Low cycle fatique. In general, the engineers concluded that the contribution to failurefrom low cycle fatigue was probably low to possibly moderate.6.3.10. Understanding of material sciences. As noted in Section 6.2, most of the engineers feltthat their education in the material sciences and welding procedures was not adequate to coverthe problems that they faced following Northridge. Poor understanding on the part of theengineers, fabricators and welders was considered to be a contributing factor to the damage. Inthe opinion of the engineers interviewed, the understanding of material sciences was rankedmedium-to-low.

6.4 . Connection qualification testsA series of university test programs was performed during the 1960s and early 1970s thatserved as the basis for development of the prescriptive connection used in welded momentresisting frames and specified in the Uniform Building Code. The connection design was basedon a static cyclic test program with small scale members, constructed under ideal weldingconditions in the laboratory. Even under these ideal conditions, some of the conclusions thatwere tested failed prematurely. It was the opinion of the engineers interviewed that the testprogram itself did not accurately reflect the behaviour of the larger size members andwelding procedures being employed in the field, nor did they reflect the dynamic impulsiveloading imposed by the earthquake. In the opinion of the engineers, these pre-Northridgetest results were viewed by the profession in an overly optimistic manner. In fact, the poor testresults were either not remembered or overlooked. By far the majority of the engineersinterviewed identified the pre-Northridge prescriptive connection test programs as a highfactor contributing to the use of the welded moment connection design in modern WSMFbuildings.

6.5. Construction materialsThe following physical characteristics of the steels used in the fabrication of the welded WSMFconnections were assessed in terms of their contribution to the connection failure.(1) Toughness of the welds-medium-to-high contributing factor.(2) Low fracture resistance at high strain rates-high-to-medium contributing factor.( 3 ) Low temperature fracture resistance-low-to-not a contributing factor.(4)High strength steels with their lower toughn ess an d ductility-medium-to-low contribu tingfactor.(5) Base metal acceptance standards-beam steel strengths provided by the mills are oftengreater than specified or anticipated by the engineer. As a consequence, the beam steelyields at a higher stress level than the column and weld material-medium-to-highcontributing factor.(6) Residual stresses from mill production of thick flanged columns lead to weakthrough-thickness characteristics. Th e nugget type failures witnessed with column flangepullout may be related to this weakness-medium-to-high contributing factor.

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WE L DE D ST E E L MOME NT FRAME S 436.6. Welding and inspection practices

Table IV summarizes the key factors in welding practice that the engineers identified assignificant. These included the following.(1 ) Non-conformance of the welding to AWS Standards and Practices-medium-to-high

(2) The proper application of preheat and cool down to minimize residual stresses due to( 3 ) High deposition welding with the resulting lack of ade qua te fusion-high contribution.(4) Th e relatively lax AWS acceptance sta nda rds for inclusions an d flaws in the weld, resulting( 5 ) Welder qualification and certification-medium contribution.(6) Inspector qualifications and certification-medium contribution .(7) Inspection practices-medium contribution.

contribution.welding-medium-to-high contribution.

in stress risers-medium-to-low contributing factor.

6.7. Earthquake characteristicsI t is interesting that the engineers interviewed did not consider the Northridge earthquake to

be unusual in terms of the amplitude of ground motion or the large vertical component fromthe near field blind thrust fault. These factors were considered low-to-medium in terms ofinducing the brittle fractures. However, the very rapid energy release and associated high strainrates of loading was identified by three-quarters of the engineers interviewed as a highcontributing factor to the brittle form of damage.

7. C IT Y IN S P E C T IO N O R D IN A N C EOn 22 Febr uary 1995, a little more than a year and a m onth after the Northridg e earthqua ke,the Los Angeles City Council passed Ordinance 170406, addressing the inspection and repairof earthquake damaged welded steel moment frame buildings. The passage of this ordinanceamended the Los Angeles Municipal Code by adding Section 91.8908, which deals withcommerical buildings constructed with WSMFs and located in high damage areas associatedwith the Northridge event. The ordinance specifically requires that the building owner submita report to the Departm ent of Building and Safety (B&S) within 180 days of notification. Thisreport must indicate the number, location and description of damaged welded connectionslocated in the building along with proposed repair procedures. The report is to be preparedunder the supervision of an d signed by a registered stru ctu ral engineer. Within 90 days ofsubmittal of the report to the Department of Building and Safety, the repairs mustcommence and all repairs must be completed within two years of the date of the permit forrepair.The ordinance applies to a limited class of WSMFs (commercial buildings), to a limitedgeographic are a (the San Fer nan do Valley an d West Los Angeles), and to a limited number ofconnections within the structure.

A C K N O W L E D G E M E N T SFu ndi ng for this research is provided by the Federal Emergency M anagem ent Agency (F EM A)through the SAC Joint Venture. SAC is a partnership of the Structural Engineers Association

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44 W. E. GATES AN D M . MORDENof California, the Applied Techno logy C ouncil an d the C alifornia Universities for Research andEarthquake Engineering.

A number of engineering firms graciously donated the time of key individuals to participatein the interviews. Thanks are due to each of these organizations and the individuals listed inTable I of this report, who contributed time and effort to the interviews. Thanks are also dueto the following, who contributed valuable suggestions, guidance and materials to the surveyeffort: SAC Task 2 Technical Advisory Panel members Professor Vitelmo Bertero, ProfessorGa ry Hart, David Ho ughto n and Ray Tide; SAC Project Managers Steve Ma hin and Jim Malley;and alternative interviewer Allan Porush.

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AP P ENDI X: DAMAGE CLAS S AND TYP E REF ERENCE S CHEDULE F ORF I G U R E S 1 A N D 2Girder damageG I buck led f langeG2 y ie lded f langeG3 flange tearout near we ldG4 f lange crack outs ide HA ZColumn f lange damagec 1c 2c 3c 4c 5 lamel lar f lange tear ing

incipient f lange crack (detected by UT)complete f lange tearout or divotful l or partial cross-flang e crack in H AZful l or partial cross-flang e crack outside H A Z

Flange wel d damageW 1w 2w 3 fracture at g i rder in ter facew 4 fracture at co lumn in ter face

incipient crack, especial ly at wel d ro ot (detected by UT)crack through weld metal, ful l or partial width of f lange

Shear conne ction damages 1s 2s3s 4s 5

column t o we b or co lum n to shear tab w eld crackwe b t o shear tab supplementa l we ld crackw e b or shear tab crack, especial ly th roug h bo lt holesweb or shear tab deformation, especially at holesloose, damaged or m issing bolts

Panel zone damageP1P2P3P 4

fracture, buckle or yield of continu ity platecrack i n cont inu i ty p late weldsbuckle, yield or ducti le deformation of doubler plate or column webcrack in doubler plate we lds

Co lumn we b damageP5P6 partial dep th crack in colum n we b or doubler plate (extension of C 3 or C 4)fu l l or near fu l l depth crack i n co lumn w eb or doubler p la te