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1/24A publication of the James F. Lincoln Arc Welding Foundation

Volume XIX, Number 1, 20

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2/24

Continuing the Tradition

Australia and New ZealandRaymond K. RyanPhone: 61-2-4862-3839Fax: 61-2-4862-3840

CroatiaProf. Dr. Slobodan KraljPhone: 385-1-61-68-222Fax: 385-1-61-56-940

RussiaDr.Vladimir P. YatsenkoPhone: 077-095-737-62-83Fax: 077-093-737-62-87

INTERNATIONAL SECRETARIES

It was with humility that I recently acceptedDuane Millers offer to assist in the editing ofWelding Innovation. This was true because I didnot think it would be possible for me to fill the

position left vacant by my predecessor Scott

Funderburk. He covered Welding Innovationfor several years as assistant editor, and hementored well under Duanes watchful eye. I

would expect that Scott will be sorely missedhere in the application engineering departmentand by our readership. We all wish him the very

best in his new assignment.

As is certainly the case with the roles of mostprofessionals employed in industry today, we are

required to wear many hats, and my primary

position as senior application engineer hasdovetailed nicely with my additional responsibili-ty to Welding Innovation. The irony of this is that

nearly twenty years ago, I was asked to assistthe former editor, Richard Sabo, by suggesting aname for this publication. We finally settled on

The Welding Innovation Quarterly, and thatname became synonymous with high-quality,creative design engineering concepts coupledwith fundamentally strong welding principles. If

you needed answers to tough problems, then inreturn we were prepared as a group to provide

high quality information. In principle, it is ourobjective to continue the rich tradition ofWelding Innovation, and I would like to suggestthat the articles in this issue of the magazinereflect our commitment to that objective.

Professor Henry Petroskis contribution to thisissue, The Fall of Skyscrapers, provides ananalysis of the collapse of the World Trade

Center towers resulting from the September 11terrorist attack. In addition, Professor Petroskiprovides us with ideas regarding future design

considerations, and the practicality of future

skyscraper construction. In contrast, contributing

writer Carla Rautenberg provides an article thatpresents Frank Gehrys free-form design of thePeter B. Lewis Building at the WeatherheadSchool of Management located on the Case

Western Reserve University campus. The com-pelling design of the newly constructed Gehrydesign is a clear break from the historicalconstraints of rectangular designed structures.

We are confident that this issue of Welding

Innovationwill be thought-provoking, and weencourage your innovative input for future issues.I especially urge you to consider sharing with ourreaders the value of your own experience, in theform of a submission to the Lessons Learned in

the Field column initiated by our senior designconsultant, Omer W. Blodgett. See page 18 foranother of Omers timeless and valuable engi-neering lessons.

Jeff NadzamAssistant Editor

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3/241Welding Innovation Vol. XIX, No. 1, 2002

Cover: With a shimmering stainless steelskin concealing its intricate welded structure,

the roof of the almost-completed Peter B.Lewis Building at Case Western Reserve

University definitely makes an architecturalstatement. See story on page 20.

The views and opinionsexpressed in WeldingInnovation do not neces-sarily represent those ofThe James F. Lincoln ArcWelding Foundation or TheLincoln Electric Company.

The serviceability of aproduct or structure utiliz-ing the type of informationpresented herein is, andmust be, the sole responsi-bility of the builder/user.Many variables beyond thecontrol of The James F.Lincoln Arc WeldingFoundation or The LincolnElectric Company affect theresults obtained in applyingthis type of information.These variables include,but are not limited to, weld-ing procedure, plate chem-istry and temperature,weldment design, fabrica-tion methods, and servicerequirements.

Volume XIXNumber 1, 2002

Editor

Duane K. Miller,Sc.D., P.E.

Assistant Editor

Jeff R. Nadzam

The James F. LincolnArc Welding Foundation

Omer W.Blodgett, Sc.D., P.E.Design Consultant Features

Departments

Visit us online atwww.WeldingInnovation.com

7 Design File: Designing Fillet Welds for Skewed T-Joints, Part 1

11 Opportunities: Lincoln Electric Technical Programs

17 Opportunities: Lincoln Electric Professional Programs

18 Lessons Learned in the Field: Consider the Transfer of Stress

through Members

2 Orthotropic Design Meets Cold Weather ChallengesThis overview of orthotropic steel deck bridges discusses a numberof examples that have been built in Norway, Russia, Sweden and Ukraine.

12 The Fall of SkyscrapersIn an article reprinted from American Scientist, Duke University ProfessorHenry Petroski considers some ramifications of the collapse of the World TradeCenter towers on the future skylines of the worlds cities.

20 Gleaming Waterfall Refreshes Urban CampusThe work of architect Frank Gehry inspires controversy and excitementas the new home of Case Western Reserve Universitys Weatherhead Schoolnears completion in Cleveland, Ohio.

THE JAMES F. LINCOLN ARC WELDING FOUNDATION

Dr. Donald N. Zwiep, Chairman

Orange City, IowaJohn Twyble, Trustee

Mosman, NSW, AustraliaRoy L. MorrowPresident

Duane K. Miller, Sc.D., P.E.Executive Director & Trustee
http://www.weldinginnovation.com/http://www.weldinginnovation.com/

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4/242 Welding Innovation Vol. XIX, No. 1, 2002

Orthotropic Design Meets Cold Weather ChallengesAn Overview of Orthotropic Steel Deck Bridges in Cold Regions

By Alfred R. Mangus, P.E.Transportation Engineer, CivilCalifornia Dept. of Transportation (CALTRANS)Sacramento, California

IntroductionInitially developed by German engineersfollowing World War II, orthotropicbridge design was a creative responseto material shortages during the post-

war period.Lightweight orthotropic steelbridge decks not only offered excellentstructural characteristics, but were alsoeconomical to build (Troitsky 1987).Moreover, they could be built in coldclimates at any time of the year.Engineers from around the world utilizethis practical and economic system forall types of bridges. While concretemust be at or above 5 degrees Celsiusto properly cure, it is physically possibleto encapsulate and heat the concreteconstruction process; admittedly, thisadds to construction costs (Mangus1988), (Mangus 1991).

Orthotropic steel deck bridges haveproven to be durable in cold regions.The orthotropic steel deck integratesthe driving surface as part of thesuperstructure, and has the lowesttotal mass of any practicable system.In Europe, where the advantages oforthotropic design have beenembraced with enthusiasm, there are

more than 1,000 orthotropic steel deckbridges. In all of North America, thereare fewer than 100 bridges oforthotropic design.

This article will give an overview ofimaginative steel deck bridges current-ly in operation in Norway, Russia,Sweden, and Ukraine. The examples

cover a matrix of rib types, superstruc-ture types and various bridge types.Russia has developed a mass manu-factured panelized orthotropic decksystem and has devised speciallaunching methods for cold regions.Russian engineers prefer the open ribdesign and have industrialized thissystem, while most other engineersprefer the closed rib. Researchers, aswell as the owners of orthotropic steeldeck bridges, continue to monitor theperformance of various rib types

(Figure 1).

In NorwayNordhordland Floating Bridge

The Nordhordland Bridge across theSalhus fjord is Norways second float-ing bridge and the worlds largest float-ing bridge (Meaas, Lande, and Vindoy,1994). The bridge was opened for traf-fic in 1994.The total bridge length is1615 m and consists of a high level369 m long cable stayed bridge and a1246 m long floating bridge (Figure 2).The floating bridge consists of a steelbox girder, which is supported on tenconcrete pontoons and connected toabutments with transition elements inforged steel.The main elements are ahigh-level cable stayed bridge provid-ing a ship channel and a floatingbridge between the underwater rockKlauvaskallen and the other side ofthe fjord. The cable stayed bridge pro-vides a clear ship channel. A 350 mlong ramp is required to transition from

the higher bridge deck on the cable

Figure 2. Nordhordland Floating Bridge across Salhus fjord of Norway.

Figure 1. Rib designs.

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5/24Welding Innovation Vol. XIX, No. 1, 2002 3

stayed bridge to the bridge deck 11 mabove the waterline. The steel boxgirder of the floating bridge forms acircular arch with a radius of 1,700 min the horizontal plane.The supportingpontoons are positioned with a centerdistance of 113 m and act as elastic

supports for the girder, which isdesigned without internal hinges.Thebridge follows the tidal variations byelastic deformations of the girder.

The steel box girder is the main load-carrying element of the bridge (Figure3). The octagon girder is 5.5 m highand 13 m wide. The free height belowthe girder down to the waterline is 5.5m and this allows for passage of smallboats. The plate thickness varies from14 mm to 20 mm.The plate stiffenersare in the traditional trapezoidal shapeand they span in the longitudinal direc-tion of the girder. The stiffeners aresupported by cross-frames with centerdistance of maximum 4.5 m. At thesupports on the pontoons, bulkheadsare used instead of cross-frames. Thisis done because the loads in thesesections are significantly larger than inthe cross-frames. The plate thickness

in the bulkheads varies from 8 mm to50 mm. The box girder is constructedin straight elements with lengths vary-ing from 35 m to 42 m. The elementsare welded together with a skew angleof 1.2 to 1.3 to accommodate thearch curvature in the horizontal plane.

The cross section dimensions of theoctagonal box girder are constant forthe length of the bridge.

The stress level varies significantlyover the length of the bridge. In theareas with the highest stresses, steelwith a yield strength of 540 MPa isused.

In the remainder of the bridge (in thecross-frames and bulkheads) normal-ized steel with yield strength of 355MPa is used.The total steel weight ofthe box girder is 12,500 tons, of whichapproximately 3,000 tons are high

strength steel. The elevated ramp isapproximately 350 m long and has agrade of 5.7 percent (Figure 4). Theelevated ramp is constructed with anorthotropic plate deck 12 mm thickand has 8 mm and 10 mm thick trape-zoidal ribs 800 mm deep. T-shapedcrossbeams support the ribs with amaximum center distance of 4.5 m.The main 1,200 mm deep box girders

are located one at each edge of theramp in order to maximize the stiffnessabout a vertical axis.The steel weightof the ramp is 1,600 tons.

Bybrua Bridge

The Bybrua cable stayed bridge has amain span of 185 m. The 15.5 m wideroadway superstructure was fabricatedin the shop in 9.0 m sections (Figure 5).There is a combined pedestrian plusbicycles area on each side of the threetraffic lanes.The cross section of themain span has a deck-plate 12 mmthick, but this increases to 16 and 20mm at the cable anchorage. The bot-tom plate varies between 8 mm and10 mm thickness, and the websbetween 12 mm and 20 mm. Atintervals of 3.0 m there are framessupporting the longitudinal stiffeningsystem. In the bridge deck this ismade up of standard trapezoidal ribs

Figure 3. Nordhordland Floating

Bridge.

Figure 4. Nordhordland Floating

Bridge.

Orthotropic bridge design

was a creative responseto material shortages

during the postwar period

Figure 5. Bybrua Bridge.

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from the German steel companyKrupp, and in the bottom flange boxsection of bulb flats open ribs were uti-lized (Aune and Holand 1981). In thelongitudinal direction the deck wasdivided into six fabricated sections,two of which were welded to the web

sections. The box bottom was fabricat-ed as three sections. The total steelweight is about 1,100 tons. All ele-ments prefabricated in the shop werewelded, as were the field splices in thedeck, whereas Huck high tensilebolts were used in all other field joints.All field joints were calculated as fric-tion connections.The whole steelstructure is metallized with zinc andpainted according to the specificationsof the Norwegian Public RoadsAdministration. The superstructure

received the maximum live loadstresses during the erection of thebridge. The wearing surface of thebridge deck is the same as that devel-oped by the Danish State RoadLaboratories for the Lillebelt Bridge ofDenmark.

Storda and Bomla BridgesThe Triangle Link project connectsthree islands off the Norwegian coastsouth of Belgen to the mainland withthree bridges (Larson and Valen

2000). The entire project was complet-ed in April 2001. The two orthotropicsteel deck suspension bridges are

known as the Storda Bridge and theBomla Bridge.The Storda Bridge is1,076 m long, has a main span of 677m, with towers 97 m high and a verti-cal clearance of 18 m (Figure 6). TheBomla Bridge is 990 m long with amain span of 577 m and the towerheight is 105 m. The roadway of bothbridges is 9.7 m wide. Scanbridge AS

of Norway fabricated the BomlaBridges steel approach superstruc-ture, which was launched out over thetops of the columns from the shore.The steel components for the mainspan superstructure of the StordaBridge were prefabricated in the

Netherlands (Figure 6) and the mainspan superstructure of the BomlaBridge was prefabricated in Italy. Theorthotropic ribs for the Storda Bridgewere prefabricated in France. Theorthotropic sections were transportedto the site by barge, and were liftedinto position by a crane.

In RussiaRussian engineers have standardizedtheir orthotropic deck plates usingopen or flat plate ribs as shown inFigure 1. They have several launchingsolutions or standardized methods forpushing the superstructure across ariver or gorge. There are a limitednumber of bridge case histories docu-mented in English, but they provide anoverall view of Russian techniques(Blank, Popov, and Seliverstov 1999).In the city of Arkhangel, Russia, a ver-tical lift record span bridge of 120.45m was completed in 1990 (Stepanov

1991). The Berezhkovsky twin parallelbridges are multi-cell box girderbridges consisting of three spans of110 m + 144.5 m +110 m. Each bridgehas four traffic lanes 3.75 m wide.These bridges were the first to belaunched with inclined webs(Surovtsev, Pimenov, Seliverstov, andIourkine 2000).

Oka Bridge

The four-lane orthotropic twin boxgirder bridge crossing the Oka Riveron the bypass freeway around the cityof Gorki, Russia, was opened to trafficin 1991 (Figure 7).The 966 m longsuperstructure consists of 2 spans x

84 m + 5 spans x 126 m + 2 spans x84 m (Design Institute Giprotransmost1991). This bridge is a single continu-ous superstructure with a fixed bearing420 m away from one of the abut-ments. The total bridge width (29.5 mincluding steel traffic barriers) providestwo sidewalks 1.5 m wide each, fourtraffic lanes, four safety shoulders anda center median.The total weight ofsteel for the superstructure is 10,635tons, or 373 kg/m2.The orthotropicsteel superstructure comprises five

basic elements (Figure 7). There aretwo main box girders assembled fromtwo L-shaped sections for the bottomface and sides.The intermediateorthotropic plate sections were usedfor the top flange of the two box gird-ers, as well as the majority of thedeck.The end sections of theorthotropic plate were panelized withtapered ends, because only sidewalkloading is required.The transversediaphragms are steel trusses betweenthe box girders. The diaphragmsrequired extra steel beams at the bot-tom flange of the box girder above thebearings. The main box girder wasshop fabricated in L-shaped sectionsthat are 21 m long and 3.6 m deep.The intermediate orthotropic weldedsteel deck plate was shop fabricated inpanelized sections 2.5 m wide and11.5 m long.

The longitudinal ribs of the orthotropicdeck and steel box girders are flat rib

plates spaced at 0.35 m, and thespacing of transverse ribs is 3.0 m forboth components.The stiffening ribs ofthe main girder are located on bothsides of every web. The vertical split-Tribs of the box girders were alignedwith the transverse ribs of the ribbedplate, thus creating the integral inter-nal diaphragms. The longitudinal stiff-ening ribs are at a constant spacing

Researchers continueto monitor the performance

of various rib types

Figure 6. Storda Bridge.

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along the bridge. Depending on theweb thickness, additional vertical stiff-ening ribs were required between thediaphragms. The superstructure waserected using continuous launchingfrom one bank of the Oka River. Theshop-fabricated elements were addedpiece by piece to form a continuous

structure at the erection slip area onthis riverbank.The joints of the hori-zontal sections of the orthotropic deckand ribbed plates, as well as the jointsof the web of the main girder, wereautomatically welded.The joints of thelongitudinal ribs of the ribbed platewere manually welded. All the remain-ing joints used high strength bolts.

In SwedenHigh Coast Suspension Bridge

The High Coast Suspension Bridge ofSweden is almost the same size asSan Franciscos Golden Gate Bridge(Merging with Nature 1998). The mainspan is 1,210 m long with suspended

side spans of 310 m and 280 m. Thewidth of the roadway is 17.8 m, allow-ing for a possible future extension to 4lanes (Figure 8). The distancebetween the main cables is 20.8 mand there are 20 m between the hang-ers. The girder is continuous throughthe towers extending 1,800 m from

abutment to abutment where expan-sion joints and hydraulic buffers arelocated. The 48 box girder sectionswere fabricated at a shipyard inFinland (Pedersen 1997).The stan-dard section is 20 m long with two

sets of hangers each and weighs 320tons (Figure 9).The 20 m long panelsfor the deck, sides and bottom werefabricated with a maximum width of 10m. They were fabricated from steelplates, typically 9 to 14 mm thick,10 mlong and 3 to 3.3 m wide. The ribswere 20 m long trapezoidal ribs with aplate thickness of 6 to 8 mm.

The plates were placed on a planeand welded in the transverse and lon-gitudinal direction and the trough stiff-eners were fitted and welded

longitudinally. Plates connecting thepanels and the diaphragms were weld-ed between ribs.The 20 m long edgesections and units for the transversediaphragm, or bulkhead, were prefabri-cated. The bottom and inclined sideswere placed first. Each 4 m deep

transverse diaphragm or bulkhead wasfitted. The edge sections were installedand finally the two deck panels wereplaced on top, completing a 20 m longsubsection. The 31 bridge girder sec-tions for the main span were transport-ed from the fabrication yard in Finlandon the three barges in the same wayas the sections for the side spans, anderected with a floating sheerleg crane,130 m boom, starting from mid-spanand proceeding towards the towers(Edwards and Westergren 1999).

In UkraineSouth Bridge over the Dnipro River

The 1992 signature span of the SouthBridge over the Dnipro [Dnepr] Riverin Kyiv [Kiev], Ukraine is an unsym-metrical cable stayed bridge with amain span of 271 m (Korniyiv and.Fuks 1994). The main span side of theH-tower is a continuous three-spansteel box girder with orthotropic steeldeck. The back-span superstructure onthe opposite side of the H-tower is asegmental prestressed concrete boxsection. Concrete construction for theshorter back span of 60 m was usedas a counter-weight mass equal to thelonger orthotropic main span.Thebridge carries a six-lane roadway, tworail tracks and four large-diameterwater pipes (Figure 10). The total live

Figure 7. Twin box girder bridge crossing the Oka River, Russia (splitsection).

Figure 8. High Coast, Sweden, section.

Figure 9. High Coast, Swedencomponents.

Figure 10. South Bridge (cable stayed) of Ukraine (splitsection).

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load is about 246 kN/m. The three-span (80.5 m + 90 m + 271 m) contin-uous steel box girders are made oflow-alloy steel with a minimum yieldstrength of 390 MPa.

The bridge was divided into segments

that were shop-welded. Field spliceswere either welded or joined by high-strength bolts. Bolting was used whereautomatic welding was impracticalbecause of the short length of theweld or difficult access. The cross sec-tion of the twin two-cell box girderbridge consists of six vertical webs,the upper deck plate and the lowerbox flanges.The narrow cell is for thecable stayed bridge anchorage. In thecentral portion of the cross sectionthat carries the two hot water supply

pipes, the lower flange was omitted topreclude the undesirable effects ofunequal heating inside a closed box.The bearings at the piers permit lateraldisplacement of the superstructurebecause of the 41.5 m bridge width.The orthotropic decks, bottom flangesof the boxes and the webs have openflat-bar stiffening ribs, a common fea-ture in Ukrainian and Russian bridges.Longitudinal flat or open ribs wereplaced on the top face of theorthotropic deck plate under the railtracks, thus avoiding intersections oflongitudinal and transverse stiffeners.

This facilitated fabrication, at the sametime precluding stress concentrationsat crossing welds that would be sus-ceptible to fatigue under dynamic trainloading. Longitudinal ribs under thetrain tracks have a depth in excess ofthe design requirements, which per-mitted longitudinal profile adjustmentsof the tracks after erection of the

bridge superstructure. The steel gird-ers were pre-assembled on the bankof the south and erected by launching.The twin two-cell box girders wereequipped with a launching nose andstiffened with a temporary strut system(Rosignoli1999). Single erection rollerswere used at the tops of supports andhad a friction factor of less than 0.015.The erection of the 271 m main span

was accomplished with two false worksupports providing three equal spansof about 90 m. At the H-tower of thecable stayed bridge, hinged conditionsare provided by supports with limitedrotational capability in the vertical andthe horizontal planes. The torsional

rigidity of the bridge is supplied by thetwo planes of cable stays, plus thestiffness of the closed box sections.Under one-sided loading of the bridge(three traffic lanes), the deck crossslope of 0.3% was measured in fieldtests, less than the calculated value of0.35%. Eccentric hinged connectionsbetween the bottom flanges of thestiffening girders and the H-tower wereconstructed, considerably reducing thebending moments in the girders.

ConclusionThe foregoing examples illustrate arange of creative responses to thechallenge of designing and construct-ing orthotropic steel deck bridges incold weather regions. The versatility,economy and structural integrity ofwelded orthotropic design undoubtedlywill continue to inspire bridge design-ers and structural engineers in the21st century.

Figure CreditsFigure 1 from Ballio, G., Mazzolani, F. M. 1983,Theory and Design of Steel Design Structures,Chapman & Hall Ltd. courtesy of Dr. Mazzolani;Figures 2, 3, & 4 courtesy of Dr. Ing. A-AasJakobsen, AS Structural EngineeringConsultants, Oslo, Norway; Figure 5 adapted fromAune, Petter, and Holand, Ivar (1981); Figure 6courtesy of Mr. L. Adelaide of Profilafroid, France;Figures 8 & 9 courtesy of Claus Pedersen ofMondberg & Thorsen AS, Copenhagen, Denmark;Figures 7 & 10 courtesy of IABSE InternationalAssociation of Bridge Structural Engineers.

ReferencesAune, P., Holand, I. (1981) Norwegian Bridge

Building A Volume Honoring Arne Selberg,Tapir, Norway

Blank, S. A., Popov, O. A., Seliverstov, V. A., (1999)Chapter 66 Design Practice in Russia, BridgeEngineering Handbook, 1st ed., Chen, W.F.,and Duan, L. Editors, CRC Press, Boca RatonFlorida

Design Institute Giprotransmost, (1991) TheBridge-Crossing over the River Oka onthe Bypass Road near Gorki StructuralEngineering International, IABSE, Zurich,Switzerland, Vol. 1, Number 1, 14-15

Edwards,Y. & Westergren P., (1999), Polymermodified waterproofing and pavement systemfor the High Coast Sweden NordicRoad &Transport Research No. 2, 28

Korniyiv, M. H. and Fuks, G. B (1994) The SouthBridge, Kyiv, Ukraine Structural EngineeringInternational, IABSE, Zurich, Switzerland, Vol.4, Number 4, 223-225

Larsen J., and Valen, A., (2000) Comparison ofAerial Spinning versus Locked-Coil Cablesfor Two Suspension Bridges (Norway),Structural Engineering International, IABSE,10(3), 128-131

Mangus, A., (1988) Air Structure Protection ofCold Weather Concrete, ConcreteInternational, American Concrete Institute,Detroit, MI, October 22-27

Mangus, A., (1991) Construction Activities Insideof Air Structures Protected From the ArcticEnvironment, International Arctic Technology

Conference, Society of Petroleum Engineers,Anchorage, AK, May 29-31

Mangus, A, and Shawn, S., (1999) Chapter 14Orthotropic Deck Bridges, Bridge EngineeringHandbook, 1st ed., Chen, W.F., and Duan, L.Editors, CRC Press, Boca Raton Florida

Mangus, A., (2000) Existing Movable BridgesUtilizing Orthotropic Bridge Decks, 8th HeavyMovable Bridge Symposium Lake Buena VistaFlorida, Heavy Movable Structures, P. O. Box398, Middletown, NJ

Merging with Nature Hoga Kusten [High Coast](1998), Bridge Design & Engineering, 10, (1),5056

Meaas, P., Lande, E., Vindoy, V., (1994) Design ofthe Salhus Floating Bridge (Nordhordland

Norway), Strait Crossings 94, Balkema,Rotterdam ISBN 90-5410 388-4 3(3), 729-734

Pedersen, C., (1997) The Hoga Kusten (HighCoast) Bridge-suspension bridge with 1210 mmain span construction of the superstructure,Mondberg & Thorsen A/S Copenhagen,Denmark

Rosignoli, M.,(1999) Launched Bridges Prestressed Concrete Bridges built on theground and launched into their final position,ASCE Press, Reston, VA

Stepanov, G. M., (1991), Design of MovableBridges, Structural Engineering International,IABSE, Zurich, Switzerland, Volume 1,Number 1, 9-1

Surovtsev, V., Pimenov, S., Seliverstov, V., &

Iourkine, S, (2000) Development of StructuralForms and Analysis of Steel Box Girders withinclined webs for operation and erection condi-tions Giprotransmost J.S.Co, PavlaKortchagina str. 2, 129278 Moscow, RussianFederation

Troitsky, M. S., (1987) Orthotropic Bridges - Theoryand Design, 2nd ed., The James F. Lincoln ArcWelding Foundation, Cleveland, OH

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Designing Fillet Welds for Skewed T-jointsPart 1Practical Ideas for the Design Professional by Duane K. Miller, Sc.D., P.E.

Design File

Introduction

Detailing fillet welds for 90-degree T-joints is a fairlystraightforward activity.Take the 90-degree T-joint and skewitthat is, rotate the upright member so as to create anacute and obtuse orientation, and the resultant geometryof the fillet welds becomes more complicated (see Figure1). The greater the degree of rotation, the greater the differ-ence as compared to the 90-degree counterpart.

A series of equations can be used to determine weld sizesfor various angular orientations and required throat dimen-sions. Since the weld sizes on either side of the joint arenot necessarily required to be of the same size, there are avariety of combinations that can be used to transfer theloads across the joint. While there are theoretical savingsto be seen by optimizing the combinations of weld sizes,rarely do such efforts result in a change in fillet weld size ofeven one standard size.

Codes prescribe different methods of indicating therequired weld size. These are summarized herein.

When acute angles become smaller, the difficulty ofachieving a quality weld in the root increases. The AWSD1.1 Structural Welding Codedeals with this issue byrequiring the consideration of a Z-loss factor.

This edition of Design File addresses the situation wherethe end of the upright member in the skewed T-joint is par-allel to the surface of the other member. A future DesignFile column will consider the situation in which the uprightmember has a square cut on the end, resulting in a gap onthe obtuse side. Also to be addressed in the future areweld options other than fillet welds in skewed T-joints.

The GeometryFigure 1 provides a visual representation of the issue. Forthe 90-degree orientation, the weld throat is 70.7% of theweld leg dimension. This relationship does not hold true forfillet welds in skewed joints. On the obtuse side, the weldthroat is smaller than what would be expected for a filletweld of a similar leg size in a 90-degree joint, and theopposite is the case for the acute side.These factors mustbe considered when the fillet weld leg size is determinedand specified.

Careful examination of the fillet welds on the skewed jointraises this question: What is the size of the fillet weld in askewed joint?

Figure 1 illustrates the fillet weld leg size for a skewed T-joint, and is designated by . This, however, is inconsis-tent with AWS Terms and Definitions(AWS A3.0-94) whichdefines a fillet weld leg as The distance from the jointroot to the toe of the fillet weld. According to this definition,and as shown in Figure 1, the fillet weld leg is dimensionb. The dimension that is labeled is the distance from amember to a parallel line extended from the bottom weld

f1

f2LEGSIZE LEG

SIZE

DIHEDRAL ANGLE135 MAX

0

DIHEDRAANGLE60 MIN

0

b1b2

b2 b1

t1t2

1

2

1

2

2

1

Figure 1. Equal throat sizes (t1 = t2).

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toe. While not technically correct according to AWS A3.0, itis the dimension and terminology used when fillet welds inskewed joints are discussed in the AWS D1.1 StructuralWelding Code, as well as other AWS publications (i.e., TheWelding Handbook, ninth edition, volume 1). Such termi-nology will be used here.

This raises an additional question: What would a weldinspector actually measure when dealing with a fillet weldin a skewed T-joint? Conventional fillet weld gauges couldbe used to measure the obtuse sides fillet weld leg dimen-sion as shown in Figure 1. Dimension b would be diffi-cult to measure directly since the location of the weld rootcannot be easily determined. Welds on the acute side areimpossible to measure using conventional fillet weldgauges. The face dimension f, however, offers an easyalternative: when this dimension is known for the weld sizeand the dihedral angle, the welder and inspector can easilydetermine what the actual size is by using a pair ofdividers. Alternately, a series of simple gauges of various

widths could be made to directly compare the requirementsto the actual weld size. Thus, dimension f may be impor-tant for controlling weld sizes in skewed T-joints.

When sizing a fillet weld for 90-degree T-joints or skewed T-joints, the starting point is to determine the required throatsize needed to resist the applied loads. From the throatdimension, the fillet weld leg size can be determined. Threeoptions will be considered:

Where the throat size is the same on either side of the

joint (see Figure 1)To determine the required fillet weld size for a given throat,the following relationship can be used:

The width of the face of the weld (f) can be found fromthis equation:

Dimension b, that is, the true fillet leg size, can be foundfrom this relationship:

Finally, the cross-sectional area of the weld metal can bedetermined from the following:

Where the leg size is the same on both sides

(see Figure 2)If the designer decides to make both welds with the sameleg size (as is illustrated in figure 2), the first required stepis to determine the composite total dimension of the two

throat sizes. This dimension tT is then inserted into the fol-lowing equations to determine the two throats t1 and t2.

Equations 1 4 can be used to find the corresponding filletweld leg size, face dimension, b dimension, and cross-sectional area. These calculations will be made using theapplicable throat dimension t determined from equations5 and 6, not the total throat dimension t

T

used in equa-tions 5 and 6.

Where a minimum quantity of weld metal is used(see Figure 3)Even a casual review of Figure 1 shows that, when filletweld leg sizes are specified to be of the same size oneither side of the skewed T-joint, the use of weld metal isas efficient as could be. Minimum weld metal can beobtained by taking advantage of the more favorable condi-tion that results on the acute side where a greater weldthroat can be obtained for the same quantity of metal thatwould be placed on the obtuse side.

To minimize the volume of weld metal used in the combina-tion of the two welds, the following equations may be usedonce the total throat dimension tT is known:

= 2 t sin( )

2

f = 2 t tan ( )

2

cos ( )b =

2

t

A = t2 tan ( )

2

bb

b b

t1t2

2 1

Figure 2. Equal fillet weld leg sizes (1 =2).

t1 = tT

cos ( )

2

1

cos ( )21

+ cos ( )22

t2 = tT

cos ( )

2

2

cos ( )

2

1+ cos ( )

2

2

t1 =

1 + tan2( )

2

1

tT

t2 =

1 + tan2( )

2

2

tT

(1)

(5)

(6)

(7)

(8)

(2)

(3)

(4)

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Although the preceding calculations are not particularly dif-ficult, Table 1 has been provided to simplify the process.Columns A and B are used to determine fillet weld legsizes and face widths for various dihedral angles. To obtainthe required fillet weld size, the calculated throat is multi-plied by the factor in Column A. Face widths can be found

following the same procedure.

If the same leg size is desired on either side of the joint,columns C-E are used. In this case the total weld throat tTis used, as opposed to what was done with columnsA and B.

For the minimum weld volume, columns FH can be used.Again, the total weld throat tT is used.

As will be discussed below, for dihedral angles of 3060degrees, the D1.1 Code requries the application of a Z-lossfactor.Thus, the values in Table 1 that apply to dihedral angles

where this applies are shown in blue numbers to remind theuser to incorporate this factor into the weld throat sizes.

Influence of Dihedral AngleAWS D1.1 Structural Welding CodeSteelprovides for fivegroupings of skewed T-joints, depending on the range ofsizes of the dihedral angle: a) Obtuse angles greater than100 degrees, b) angles of 80100 degrees, c) acute anglesof 6080 degrees, d) acute angles of 3060 degrees, and

e) acute angles of less than 30 degrees. Each is dealt within a slightly different manner.

A B C D E F G H

phi1 deg leg size face width throat leg size face width throat leg size face width30 0.517 0.536 0.788 0.408 0.422 0.933 0.483 0.536

35 0.601 0.630 0.760 0.457 0.479 0.910 0.547 0.630

40 0.684 0.728 0.733 0.501 0.533 0.883 0.604 0.728

45 0.765 0.828 0.707 0.541 0.585 0.854 0.653 0.828

50 0.845 0.932 0.682 0.576 0.635 0.822 0.694 0.932

55 0.923 1.04 0.657 0.607 0.684 0.787 0.726 1.04

60 1.00 1.15 0.634 0.634 0.731 0.750 0.750 1.15

65 1.07 1.27 0.611 0.656 0.778 0.712 0.764 1.27

70 1.15 1.40 0.588 0.674 0.823 0.671 0.770 1.40

75 1.22 1.53 0.566 0.689 0.868 0.630 0.766 1.53

80 1.29 1.68 0.544 0.699 0.912 0.587 0.755 1.68

85 1.35 1.83 0.522 0.705 0.956 0.544 0.735 1.83

90 1.41 2.00 0.500 0.707 0.999 0.500 0.707 2.00

95 1.47 2.18 0.478 0.705 1.043 0.457 0.673 2.18

100 1.53 2.38 0.456 0.699 1.087 0.414 0.633 2.38

105 1.59 2.60 0.434 0.689 1.131 0.371 0.589 2.60110 1.64 2.85 0.412 0.675 1.175 0.329 0.540 2.85

115 1.69 3.14 0.389 0.656 1.221 0.289 0.488 3.14

120 1.73 3.46 0.366 0.634 1.267 0.250 0.434 3.46

125 1.77 3.84 0.343 0.608 1.314 0.214 0.379 3.84

130 1.81 4.28 0.318 0.577 1.363 0.179 0.324 4.28

135 1.85 4.82 0.293 0.542 1.413 0.147 0.271 4.82

140 1.88 5.48 0.267 0.502 1.465 0.117 0.221 5.48

145 1.91 6.33 0.240 0.458 1.519 0.091 0.173 6.33

150 1.93 7.44 0.212 0.409 1.576 0.067 0.130 7.44

b1

b2

b2 b1

t1

t2

Figure 3. Minimum weld volume.

Leg & Face DimensionsMultipy by t

Same Leg SizeMultipy by tT

Minimum Weld VolumeMultipy by tTDihedral Angle

Table 1.

Blue numbers indicate that Z-loss factors must be considered.

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Obtuse angles greater than 100 degreesFor this category, contract drawings should show therequired effective throat. Shop drawings are to show therequired leg dimension, calculated with equation 1, or byusing columns C or D of Table 1 (AWS D1.1-2002, para2.2.5.2, 2.3.3.2).

Angles of 80100 degreesFor this group, shop drawings are required to show thefillet leg size (AWS D1.1-2002, para 2.2.5.2). While notspecifically stated in the code, the assumption is thatcontract drawings also show this dimension.

Angles of 6080 degrees

For this category, contract drawings should show therequired effective throat. Shop drawings are to show therequired leg dimension (AWS D1.1-2002, para 2.2.5.2,2.3.3.2)

Angles of 3060 degreesContract drawings are to show the effective throat. Shopdrawings are required to show the required leg dimensionsto satisfy the required effective throat, increased by the Z-loss allowance ... (AWS D1.1-2002, para 2.2.5.2, 2.3.3.3).The Z-loss factor is used to account for the likely incidenceof poor quality welding in the root of a joint with a smallincluded angle. The amount of poor quality weld in the rootof the joint is a function of the dihedral angle, the weldingprocess, and the position of welding. Table 2.2 of D1.1summarizes this data as contained below:

Once the Z-loss dimension has been determined, it is addedto the required throat dimension. Even though part of theweld in the root is considered to be of such poor quality asto be incapable of transferring stress, the resultant weld willcontain sufficient quality weld metal to permit the transfer ofimposed loads. Figure 4 illustrates this concept.

The data in Table 1 that applies to dihedral angles of3060 degrees has been shown in blue numbers becausethese values must be modified to account for the Z-lossfactors. Such a modification has not been done for the datain the Table.

10 Welding Innovation Vol. XIX, No. 1, 2002

Acute angles less than 30 degrees

The D1.1 code says that welds in joints with dihedralangles of less than 30 degrees shall not be considered aseffective in transmitting applied forces and then goes

on to discuss a single exception related to tubular struc-tures. In that exception, with qualification of the weldingprocedure specification, such welds may be used for trans-ferring applied stresses. For plate (e.g., non-tubular) appli-cations, such an option is not presented in the code.

The practical application of this principle is that when weldsare placed on the acute side, no capacity is assigned tothe weld. Rather, the full load is assumed to be transferredwith the weld on the obtuse side.

Practical ConsiderationsThe most straightforward, and easiest, approach to deter-mining the required weld size is to assume two welds withequal throat dimensions will be used, calculate therequired weld throat dimension, and then calculate therequired fillet weld leg size, using either equation 1 or Table1, columns A and B. Simple? Yes. Best? Lets see.

The optimizing method that uses equations 6 and 7 willresult in reduced weld metal volumes. But, reduced howmuch? The significance increases with greater rotationsfrom the 90-degree T-joint orientation. For angles of 80, 70,and 60 degrees, the differences in weld volume are

approximately 3, 12 and 25%. However, note that these dif-ferences are functions of the leg size squared. Accordingly,the change in leg size is approximately 1, 6, and 13%. Inpractical terms, for dihedral angles between 60 and 120degrees, there will not be a standard fillet weld leg sizeuntil the welds become quite large. In the case of a 70-degree dihedral angle, for example, and assuming a 1/8 in.increment for standard sizing of fillet welds over 1 in. legsize, the leg size would need to be 2 in. before the opti-mized weld size would result in a smaller weld. For a 2 mmstandard size, this would equal a 34 mm fillet.

Z t

1

Figure 4. Z-loss.

Table 2. Z-loss dimension.

Dihedral Angle

60o > > 45o

Position of Welding Position of WeldingV or OH H or F

Process Z (in.) Z (mm) Process Z (in.) Z (mm)

SMAW 1/8 3 SMAW 1/8 3FCAW-S 1/8 3 FCAW-S 0 0

FCAW-G 1/8 3 FCAW-G 0 0

GMAW N/A N/A GMAW 0 0

SMAW 1/4 6 SMAW 1/4 6FCAW-S 1/4 6 FCAW-S 1/8 3

FCAW-G 3/8 10 FCAW-G 1/4 6

GMAW N/A N/A GMAW 1/4 6

45o > > 30o

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For angles less than 60 degrees, there can be significantdifferences in weld volume. These are situations where theZ-loss factor must be considered as well. Thus, for anglesof 3060 degrees, optimization of weld size makes sense,and the Z-loss factors can be considered at the same time.

It must be recognized that other code provisions may fur-

ther affect these results. For example, when optimized forminimum weld volume, welds on the obtuse side may besmaller than minimum fillet weld sizes as contained in Table5.8 of D1.1. The calculated sizes, if less than these mini-mums, must be increased to comply with this requirement.

There does not appear to be any intrinsic value in havingwelds on opposite sides of the skewed T-joint be of thesame size. If this approach is used, the resultant weld vol-umes will fall somewhere between the results for the samesized throat and the optimized sizes.

After the welds are detailed, the joint must be welded.

Practical considerations apply here too. It must be recog-nized that the ratio of the face width f to the throat dimen-sion t constitutes the equivalent of a width-to-depth ratiofor the root pass. On the obtuse side, this ratio is large,exceeding 1:6 for dihedral angle of 106 degrees or more. Itis very difficult to get a single weld bead to wash out this

wide without electrode manipulation (weaving). On theacute side, the ratio is less than 1:2 for angles of 62degrees. This can lead to width-to-depth ratio cracking.

RecommendationsWhen determining fillet weld details for skewed T-joints with

dihedral angles from 60120 degrees, it rarely matterswhich method of proportioning of weld sizes is used. Usingequal throat dimensions is a straightforward method, similarto what is typically done for fillet welds on either side of a90-degree T-joint. Unless the weld size is large, optimizing itwill probably not result in a smaller specified weld size.

For fillet welds on skewed T-joints with dihedral angles from3060 degrees, the Z-loss factor must be considered.Based on the specific dihedral angle, the welding process,and the position of welding, the Z-loss factor can be deter-mined, and this dimension added to the required weldthroat dimension.

It is important to consider how these dimensions should becommunicated between the designer, fabricator, welder andinspector.The face dimension is a practical means of verifyingthat the proper weld size has been achieved.

Lincoln Electric Technical Programs

Opportunities

Welding Technology Workshop

June 10-14, 2002July 29 August 2, 2002The purpose of this program is tointroduce or enhance knowledge ofcurrent thinking in arc welding safety,theory, processes, and practices. Thecourse is designed primarily for weld-ing instructors, supervisors and pro-fessional welders. Fee: $395.

Welding of Aluminum Alloys,

Theory and Practice

October 15-18, 2002Designed for engineers, technologists,technicians and welders who arealready familiar with basic weldingprocesses, this technical trainingprogram provides equal amounts ofclassroom time and hands-on welding.

Fee: $595.

Space is limited, so register early to avoid disappointment.For full details, see

www.lincolnelectric.com/knowledge/training/seminars/

Or call 216/383-2240, or write to Registrar, Professional Programs, The Lincoln ElectricCompany, 22801 St. Clair Avenue, Cleveland, OH 44117-1199.

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the World Trade Center towers shouldhave been viewed as a poor invest-ment and so might not have beenundertaken as a strictly private enter-

prise. In fact, it was the Port of NewYork Authority, the bi-state governmen-tal entity now known as the PortAuthority of New York and New Jersey,that in the 1960s undertook to buildthe towers. With its ability to issuebonds, the Port Authority could affordto undertake a financially risky projectthat few corporations would dare.

Sometimes private enterprise doesengage in similarly questionableinvestments, balancing the tangible

financial risk with the intangible gain inpublicity, with the hope that it willtranslate ultimately into profit. This wasthe case with the Empire StateBuilding, completed in 1931 and nowthe seventh tallest building in theworld. Although it was not heavily

occupied at first, the cachet of theworlds tallest building made it a presti-gious address and added to its real-estate value. The Sears Tower standsan impressive 110 stories tall, thesame count that the World TradeCenter towers once claimed.This sky-scraper gained for its owner the pres-tige of having its corporate name

associated with the tallest building inthe world. The Sears Tower, completedin 1974, one year after the secondWorld Trade Center tower was fin-ished, held that title for more than 20years-until the twin Petronas Towerswere completed in Kuala Lumpur,Malaysia, in 1998, emphasizing therise of the Far East as the location ofnew megastructures.

Building InnovationIt is not only the innovative use of ele-vators, marketing and political will thathas enabled super-tall buildings to bebuilt. A great deal of the cost of such a

structure is in the amount of materialsit contains, so lightening the structurelowers its cost. Innovative uses ofbuilding materials can also give a sky-scraper more desirable office space.Now more than 70 years old, the steelframe of the Empire State Building hasclosely spaced columns, which breakup the floor space and limit office lay-outs. In contrast, the World TradeCenter employed a tubular-construc-tion principle, in which closely spacedsteel columns were located around the

periphery of the building. Sixty-foot-long steel trusses spanned betweenthese columns and the inner structureof the towers, where further columnswere located, along with the elevatorshafts, stairwells and other non-exclu-sive office space. Between the coreand tube proper, the broad column-less space enabled open, imaginativeand attractive office layouts.

The tubular concept was not totallynew with the World Trade Center, ithaving been used in the diagonallybraced and tapered John HancockCenter, completed in Chicago in 1969.The Sears Tower is also a tubularstructure, but it consists of nine 75-foot(23-meter)-square tubes bundledtogether at the lower stories.The vary-ing heights of the tubes give the SearsTower an ever-changing look, as it pre-sents a different profile when viewedfrom the different directions fromwhich one approaches it when driving

the citys expressways.When new tothe Manhattan skyline, the unrelieved209-foot(64-meter)-square plans andunbroken 1,360-foot(415-meter)-highprofiles of the twin World Trade Centerbuildings came in for considerablearchitectural criticism for their lack ofcharacter. Like the Sears Tower, how-ever, when viewed from differentangles, the buildings, especially as

they played off against each other,enjoyed a great aesthetic synergy. Theview of the towers from the walkway ofthe Brooklyn Bridge was especially

striking, with the stark twin monolithsechoing the twin Gothic arches of thebridges towers.

Although the World Trade Center tow-ers did look like little more than tallprisms from afar, the play of the ever-changing sunlight on their aluminum-clad columns made them newbuildings by the minute. From a closerperspective, the multiplicity of unbro-ken columns corseting each buildingalso gave it an architectural texture.

The close spacing of the columns wasdictated by the desire to make thestructure as nearly a perfect tube aspossible. A true tube, like a straw,would be unpunctured by peripheralopenings, but since skyscrapers areinhabited by people, windows are con-sidered a psychological must. At thesame time, too-large windows in verytall buildings can give some occupantsan uneasy feeling. The compromisestruck in the World Trade Center wasto use tall but narrow windowsbetween the steel columns. In fact, thewidth of the window openings wassaid to be less than the width of a per-sons shoulders, which was intended

by the designers to provide a measureof reassurance to the occupants.Since the terrorist attack, however,one of the most haunting images ofthose windows is of so many peoplestanding sideways in the openings,clinging to the columns and, ultimately,falling, jumping or being carried totheir death.

The tubular conceptwas not totally new

with the WTC

The Port Authority couldundertake a financially riskyproject that few corporations

would dare

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Failure AnalysesTerrorists first attempted to bring theWorld Trade Center towers down in1993, when a truck bomb exploded inthe lower-level public garage, at thebase of the north tower. Power waslost in the tower and smoke rose

through it. It was speculated that theterrorists were attempting to topple thenorth tower into the south one, buteven though several floors of thegarage were blown out, the structurestood. There was some concernamong engineers then that the base-ment columns, no longer braced bythe garage floors, would buckle, andso they were fitted with steel bracingbefore the recovery work proceeded.After that attack, access to the under-ground garage was severely restricted,

and security in the towers was consid-erably increased. No doubt the 1993bombing was on the minds of manypeople when the airplanes struck thetowers last September.

As they had in the earlier bombing, theWorld Trade Center towers clearly sur-vived the impact of the Boeing 767 air-liners. Given the proven robustness ofthe structures to the earlier bombingassault, the thought that the buildingsmight actually collapse was probablyfar from the minds of many of those

who were working in them on September

11. It certainly appears not to havebeen feared by the police and firefighters who rushed in to save peopleand extinguish the fires. Indeed, thesurvival of the World Trade Centerafter the 1993 bombing seems to havegiven an unwarranted sense of securi-ty that the buildings could withstandeven the inferno created by the esti-mated 20,000 gallons (76,000 liters) of

jet fuel that each plane carried. (That

amount of fuel has been estimated tohave an energy content equivalent toabout 2.4 million sticks of dynamite.)

Steel buildings are expected to be fire-proofed, and so the World Trade

Center towers were. However, fire-proofing is a misnomer, for it only insu-lates the steel from the heat of the firefor a limited period, which is supposedto be enough time to allow for the fireto be brought under control, if notextinguished entirely. Unfortunately, jetfuel burns at a much higher tempera-ture than would a fire fed by normalconstruction materials and the custom-ary furniture and contents found in anoffice building. Furthermore, conven-tional fire-fighting means, such as

water, have little effect on burning jetfuel. The World Trade Center fire, esti-mated to have produced temperaturesas high as of the order of the meltingpoint of steel, continued unabated. Ithas been speculated that some of thesteel beams and columns of the struc-ture that were not destroyed by theimpact eventually may have beenheated close to if not beyond theirmelting point, but this appears to havebeen unlikely.

Even if it did not melt, the prolongedelevated temperatures caused thesteel to expand, soften, sag, bend andcreep. The intense heat also causedthe concrete floor, no longer adequate-ly supported by the steel beams andcolumns in place before the impact ofthe airplane, to crack, spall and breakup, compromising the synergisticaction of the parts of the structure.Without the stabilizing effect of the stifffloors, the steel columns still intact

became less and less able to sustainthe load of the building above them.When the weight of the portion of thebuilding above became too much forthe locally damaged and softenedstructure to withstand, it collapsedonto the floors below. The impact ofthe falling top of the building on thelower floors, whose steel columnswere also softened by heat transferalong them, caused them to collapse

in turn, creating an unstoppable chainreaction. The tower that was strucksecond failed first in part because theplane hit lower, leaving a greaterweight to be supported above thedamaged area. (The collapse of the

lower floors of the towers under thefalling weight of the upper floorsoccurred for the same reason that abook easily supported on a glass tablecan break that same table if droppedon it from a sufficient height.)

Within days of the collapse of the tow-ers, failure analyses appeared on theInternet and in engineering class-rooms. Perhaps the most widely circu-lated were the mechanics-basedanalysis of Zdenek Bazant of

Northwestern University and the ener-gy approach of Thomas Mackin of theUniversity of Illinois at Urbana-Champaign. Each of these estimatedthat the falling upper structure of aWorld Trade Center tower exerted onthe lower structure a force some 30times what it had once supported.Charles Clifton, a New Zealand struc-tural engineer, argues that the fire wasnot the principal cause of the collapse.He thinks that it was the damagedcore rather than the exterior tubecolumns that succumbed first to theenormous load from above. Once thecore support was lost on the impactedfloors, there was no stopping the pro-gressive collapse, which was largelychanneled by the structural tube tooccur in a vertical direction. In thewake of the World Trade Center disas-ter, the immediate concerns were, ofcourse, to rescue as many people asmight have survived. Unfortunately,even to recover most of the bodies

proved an ultimately futile effort. Thetwin towers were gigantic structures.Each floor of each building encom-passed an acre, and the towersenclosed 60 million cubic feet each.Together, they contained 200,000 tonsof steel and 425,000 cubic yards (325cubic meters) or about 25,000 tons ofconcrete. The pile of debris in someplaces reached as high as a ten-storybuilding. A month after the terrorist

The survival of the WTCafter the 1993 bombing

seems to have givenan unwarranted sense

of security

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attack, it was estimated that only 15percent of the debris had beenremoved, and it was estimated that itwould take a year to clear the site.

Forensic EngineeringAmong the concerns engineers hadabout the clean-up operation was howthe removal of debris might affect thestability of the ground around the site.Because the land on which the WorldTrade Center was built had been partof the Hudson River, an innovativebarrier had to be developed at thetime of construction to prevent riverwater from flowing into the basementof the structures. This was done withthe construction of a slurry wall, in

which the water was held back by adeep trench filled with a mudlike mix-ture until a hardened concrete barrierwas in place. The completed structureprovided a watertight enclosure, whichcame to be known as the bathtubwithin which the World Trade Centerwas built.The basement floors of thetwin towers acted to stabilize the bath-tub, but these were crushed when thetowers broke up and collapsed into theenclosure. Early indications were thatthe bathtub remained intact, but inorder to be sure its walls do not col-lapse when the last of the debris andthus all the internal support isremoved, vulnerable sections of theconcrete wall were being tied back tothe bedrock under the site even as thedebris removal was proceeding.

Atop the pile of debris, the steelbeams and columns were the largestand most recognizable parts in thewreckage.The concrete, sheetrock

and fireproofing that were in the build-ing were largely pulverized by the col-lapsing structure, as evidenced by theubiquitous dust present in the after-math. (A significant amount ofasbestos was apparently used only inthe lower floors of one of the towers,bad publicity about the material havingaccelerated during the construction ofthe World Trade Center. Nevertheless,in the days after the collapse, the

once-intolerant EnvironmentalProtection Agency declared the airsafe.) The grille-like remains of thebuildings facades, towering precari-ously over what came to be known asGround Zero, became a most eerie

image. Though many argued for leav-ing these cathedral wall-like skeletonsstanding as memorials to the dead,they posed a hazard to rescue work-ers and were in time torn down andcarted away for possible future reusein a reconstructed memorial. As isoften the case following such atragedy, there was also some dis-agreement about how to treat thewreckage generally. Early on, therewas clearly a need to remove as muchof it from the site as quickly as possi-

ble so that what survivors there mightbe could be uncovered.This necessi-tated cutting up steel columns intosections that could fit on large flatbedtrucks. Even the disposal of the wreck-age presented a problem. Much of thesteel was marked for immediate recy-cling, but forensic engineers worriedthat valuable clues to exactly how thestructures collapsed would be lost.

All of the speculations of engineersabout the mechanism of the collapseare in fact hypotheses, theories of

what might have happened. Althoughcomputer models will no doubt be con-structed to test those hypotheses andtheories, actual pieces of the wreck-age may provide the most convincingconfirmation that the collapse of thestructures did in fact progress ashypothesized.Though the wreckagemay appear to be hopelessly jumbledand crushed, telltale clues can surviveamong the debris. Pieces of partially

melted steel, for example, can providethe means for establishing how hot thefire burned and where the collapsemight have initiated. Badly bentcolumns can give evidence of bucklingbefore and during collapse. Even the

scratches and scars on large pieces ofsteel can be useful in determining thesequence of collapse. This will be thetask of teams of experts announcedshortly after the tragedy by theAmerican Society of Civil Engineersand the Federal EmergencyManagement Agency. Also in theimmediate wake of the collapse, theNational Science Foundation awardedeight grants to engineering and socialscience researchers to assess thedebris as it is being removed and to

study the behavior of emergencyresponse and management teams.

Analyzing the failure of the towers is aHerculean task, but it is important thatengineers understand in detail whathappened so that they incorporate thelessons learned into future designpractices. It was the careful failureanalysis of the bombed FederalBuilding in Oklahoma City that ledengineers to delineate guidelines fordesigning more terrorist-resistantbuildings. The Pentagon was actuallyundergoing retrofitting to make it betterable to withstand an explosion when itwas hit by a third hijacked plane onSeptember 11. Part of the section ofthe building that was struck had in fact

just been strengthened, and it sufferedmuch less damage than the old sec-tion beside it, thus demonstrating theeffectiveness of the work.

Understanding how the World Trade

Center towers collapsed will enableengineers to build more attack-resis-tant skyscrapers. Even before adetailed failure analysis is completed,however, it is evident that one way tominimize the damage to tall structuresis to prevent airplanes and their fuelfrom being able to penetrate deeplyinto the buildings in the first place.This is not an impossible task. When aB-25 bomber struck the Empire State

All of the speculationsabout the mechanismof collapse are in fact

hypotheses

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Building in 1945, its body stuck outfrom the 78th and 79th floors like along car in a short garage. The build-ing suffered an 18-by-20-foot (5.5-by-6-meter) hole in its face, but there wasno conflagration, and there certainlywas no collapse. The greatest damage

was done by the engines comingloose and flying like missiles throughthe building. The wreckage of theplane was removed, the local damagerepaired and the building restored toits original state. Among the differ-ences between the Empire StateBuilding and World Trade Center inci-dents was that in the former case, rel-atively speaking, a lighter plane strucka heavier structure. Furthermore, thepropeller-driven bomber was on ashort-range flight from Bedford,

Massachusetts to Newark airport and

so did not have on board the amountof fuel necessary to complete atranscontinental flight or to bring downa skyscraper.

Modern tall buildings can be strength-ened to be more resistant to full pene-tration by even the heaviest of aircraft.This can be done by placing more andheavier columns around the peripheryof the structure, making the tubedenser and thicker, as it were. The ulti-mate defense would be to make thefacade a solid wall of steel or concrete,or both. This would eliminate windows

entirely, of course, which would defeatsome of the purpose of a skyscraper,which is in part to provide a dramaticview from a prestigious office or boardroom.The elimination of that attraction,in conjunction with the increased massof the structure itself, would providespace that would command a signifi-cantly lower rent and yet cost a greatdeal more to build. Indeed, no onewould likely even consider building or

renting space in such a building.Hence, the solution would be a Pyrrhicvictory over terrorists.

The World Trade Center towers mighthave stood after the terrorist attack ifthe fires had been extinguished quick-ly. But even if the conventional sprin-kler systems had not been damaged,water would not have been effectiveagainst the burning jet fuel. Perhapsskyscrapers could be fitted with arobust fire-fighting system employingthe kind of foam that is laid down onairport runways during emergencylandings, or fitted with some other oxy-gen-depriving scheme, if there couldalso be a way for fleeing people tobreathe in such an environment. Suchschemes would need robustness andredundancy to survive tremendousimpact forces, so any such systemmight be unattractively bulky and pro-hibitively expensive to install. Otherapproaches might include more effec-tive fireproofing, such as employingceramic-based materials, thus at leastgiving the occupants of a burningbuilding more time to evacuate.

The evacuation of tall buildings will nodoubt now be given much more atten-tion by architects and engineers alike.Each World Trade Center tower hadmultiple stairways, but all were in thesingle central core of the building. Incontrast, stairways in Germany, forexample, are required to be in differentcorners of the building. In that configu-ration, it is much more likely that one

stairway will remain open even if aplane crashes into another corner. Butlocating stairwells in the corners of abuilding means, of course, that primeoffice space cannot be located there.In other words, most measures tomake buildings safer also make them

more expensive to build and diminishthe appeal of their office space.Thisdilemma is at the heart of the reasonwhy the future of the skyscraper isthreatened.

It is likely that, in the wake of theWorld Trade Center collapses, anysuper-tall building currently in thedevelopment stage will be put on holdand reconsidered. Real-estateinvestors will want to know how theproposed building will stand up to the

crash of a fully fueled jumbo jet, howhot the ensuing fire will burn, how longit will take to be extinguished and howlong the building will stand so that theoccupants can evacuate. The investorswill also want to know who will rent thespace if it is built.

Potential tenants will have the samequestions about terrorist attacks.Companies will also wonder if theiremployees will be willing to work onthe upper stories of a tall building.Managers will wonder if those employ-ees who do agree to work in the build-ing will be constantly distracted,watching out the window for approach-ing airplanes. Corporations will wonderif clients will be reluctant to come to aplace of business perceived to be vul-nerable to attack. The very need tohave workers grouped together onadjacent floors in tall buildings is alsobeing called into question.

After the events of September 11, theincentive to build a signature structure,a distinctive super-tall building thatsticks out in the skyline, is greatlydiminished. In the immediate future, asleases come up for renewal in existingskyscrapers, real-estate investors willbe watching closely for trends. It isunlikely that our most familiar skylineswill be greatly changed in the foresee-able future. Indeed, if companies begin

Figure 2. Structural design of the

World Trade Center towers was a tubewith a central core.

The towers might have stoodafter the attack if the fires had

been extinguished quickly

Courtesy

ofA

merica

nScientist

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to move their operations wholesale outof the most distinctive and iconic ofsuper-tall buildings and into more non-descript structures of moderate height,it is not unimaginable that cities likeNew York and Chicago will in time seethe reversal of a long-standing trend.

We might expect no longer to seedevelopers buying up land, demolish-ing the low-rise buildings on it, andputting up a new skyscraper. Instead,owners might be more likely to demol-ish a vacant skyscraper and erect inits place a building that is not signifi-cantly smaller or taller than its neigh-bors. Skylines that were onceimmediately recognizable even in sil-houette for their peaks and valleysmay someday be as flat as a mesa.

There is no imperative to such aninterplay between technology andsociety. What really happens in thecoming years will depend largely onhow businesses, governments and

individuals react to terrorism and thethreat of terrorism. Unfortunately, theimage of the World Trade Center tow-ers collapsing will remain in our collec-tive consciousness for a few

generations, at least. Thus, it is no idlespeculation to think that it will be atleast a generation before skyscrapersreturn to ascendancy, if they ever do.Developments in micro-miniaturization,telecommunications, information tech-nology, business practice, manage-ment science, economics, psychology

and politics will likely play a much larg-er role than architecture and engineer-ing in determining the immediatefuture of macro-structures, at least inthe West.

BibliographyBazant, Zdenek P., and Youg Zhou. 2001. Whydid the World Trade Center collapse?-Simpleanalysis. Journal of Engineering Mechanics,vol. 128 (2002) pp 2-6. www3.tam.uiuc.edu/news/200109 c/

Clifton, G. Charles. 2001. Collapse of the World

Trade Center towers. www.hera.org.nz/pdffiles/worldtrade centre.pdf

Mackin, Thomas J. 2001. Engineering analysisof tragedy at WTC. Presentation slides for ME346, Department of Mechanical Engineering,University of Illinois at Urbana-Champaign.

Skylines once recognizable insilhouette may someday be asflat as a mesa

Lincoln Electric Professional Programs

Opportunities

Blodgetts Design of WeldedStructures

September 24-26, 2002

Blodgetts Design of Steel Structures isan intensive 3-day program whichaddresses methods of reducing costs,improving appearance and function,and conserving material through theefficient use of welded steel in a broadrange of structural applications.Seminar leaders: Omer W. Blodgett andDuane K. Miller. 2.0 CEUs. Fee: $595.

Blodgetts Design of WeldmentsJune 4-6, 2002

October 29-31, 2002

Blodgetts Design of Steel Weldmentsis an intensive 3-day program for thoseconcerned with manufacturing machinetools, construction, transportation,material handling, and agriculturalequipment, as well as manufacturedmetal products of all types. Seminarleaders: Omer W. Blodgett and DuaneK. Miller. 2.0 CEUs. Fee: $595.

Fracture & Fatigue Controlin Structures:Applications of Fracture Mechanics

October 15-17, 2002

Fracture mechanics has become theprimary approach to analyzing andcontrolling brittle fractures and fatiguefailures in structures. This course willfocus on engineering applicationsusing actual case studies. Guest sem-inar leaders: Dr. John Barsom and Dr.Stan Rolfe. 2.0 CEUs. Fee: $595.

Space is limited, so register early toavoid disappointment.For full details, see

http://www.lincolnelectric.com/knowledge/training/seminars/

Or call 216/383-2240, or write to

Registrar, Professional Programs, TheLincoln Electric Company, 22801 St.ClairAvenue, Cleveland, OH 44117-1199.

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IntroductionHere I am, back again. In the secondissue of 2001 (Vol. XVIII, No. 2), theeditors of Welding Innovationweredelighted to publish an excellent piecein this space: Persistence Pays Off byRob Lawrence of Butler Manufacturing.

Now, where are the submissions fromthe rest of you out there?

I started this column a couple of yearsago with the idea of providing a forumin which our readers could share theimportant principles gleaned from theeveryday challenges of working in thefield. It seems to me that often theevident solution to a problem turnsout to be a dead end. I call these myah-ha! moments. Surely theyve hap-pened to many of you.Think aboutwhat you actually learned from theseexperiences, that you were able toapply again in other situations.Thensend an email describing your columnidea to Assistant Editor Jeff Nadzamat [email protected] worry about preparing a fin-ished, illustrated article. Our writers,editors and artists can help with that.Were just looking for a description ofthe real-life circumstances, and astatement of what you learned.

All right, then, here are some morelessons I learned, not in school, butworking in the field.

Provide a Path forTransfer of StressA common design oversight is the failureto provide a path so that a transverseforce can enter that part of the member(section) that lies parallel to the force.

Given what is needed for the propertransfer of force (as shown in Figure 1),lets consider some examples.

The top of Figure 2 shows a lug thathas been welded to a flanged beam inthe simplest and most efficient man-nerso the force goes into the web,the part parallel to it. In the centersketch of Figure 2, the lug is placedacross the bottom flange, necessitat-ing the use of either rectangular or tri-angular stiffeners to transfer the loadto the web. If, for some reason, the cir-

cumstances require the lug to beplaced in this manner, the stiffeners(with the attendant increase in weldingand material usage they entail) aremandatory. Merely welding the lugacross the more flexible flange couldresult in an uneven load on the weld.

Note that the stiffeners are not weldedto the top flange. There would be noreason to weld them there, since theflange will not take the force. At thebottom of Figure 2, the member is in adifferent position, and the lug is cor-

rectly welded to the flanges that willtake the load. It is not welded to theweb, since that would serve little pur-

pose in transferring the force.

Figure 3 illustrates how a lug might bewelded to a box section so as to trans-fer force to the parts parallel to it. Thesketch at the top, of course, is notapplicable to the rolled section shown,since there would be no way of gettingthe diaphragm inside the box. But if itwere a fabricated box section, thediaphragm could be welded in beforewelding the top plate on. The center

Lessons Learned in the FieldBy Omer W. Blodgett, Sc.D., P.E.

Consider the Transfer of Stress through Members

Figure 1.

Figure 2.

Figure 3.

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and bottom drawings in Figure 3 showadditional ways to attach a lug to abox section. In the center, the lug isshaped as a sling and directly welded

to the flange. At the bottom, the lug isdesigned so it will transfer the forceinto the two webs. This is a very effi-cient way to transfer the force on thelug into the webs.

When the Member Is CircularFigure 4 illustrates two methods ofapplying a transverse force to a circu-lar member. The rationale for thesemethods of attachment is shown in

Figure 5. At the top of Figure 5, thebeam is welded to a support. In stan-dard practice, it is assumed that theflanges transfer the bending momentsand the web transfers vertical shear. Inthe case of the circular member at the

bottom of Figure 5, however, it is diffi-cult to decide which part of the mem-ber is flange, and which part is web.Mathematical analysis has shown that

if a tube is divided into four quadrants,the top and bottom quadrants willtransfer 82% of the bending moment,and the side quadrants, 82% of thevertical shear.The methods of attach-ing the lug shown in Figure 4, there-fore, are methods that transfer forcetending to cause vertical shear into theareas of the circular section mostclosely parallel to the force.

More Complicated ExamplesFigure 6 provides a more complicatedexample of force transfer. A tank tohaul water on a truck is made up of1/4 in. (6.4 mm) thick plate, with thesides overlapping the ends so as toprovide fillet welds. Considering theforces from the water pressure on thetank ends, the only place for them to

go is through the welds and into thesidesthe parts parallel to their direc-tion. The forces get there by bendingthe end plate. In service, the welds

cracked.Three remedies were triedsuccessively, as shown in Figure 6,using longitudinal and corner stiffen-ers, and finally both longitudinal andend stiffeners with corner stiffeners.

Figure 7 shows the center sill of apiggyback railroad car to which abracket is welded to carry a 500 lb.(227 kg) air compressor unit. There areno interior diaphragms. The vertical

force from the weight of the unit istransferred as moment into the bracket,creating bending at the web. The two

horizontal bending forces must eventu-ally transfer to the parallel flanges, butwith an open box section there are noready pathways. As a result, the webflexes and fatigue cracks appear in theweb.The sketches at the bottom ofFigure 7 illustrate two possible meansfor correcting the faulty design. In one,a stiffener is added before the webopposite the bracket side is weldedinto the assembly. The stiffener is weld-ed to both flanges and to one web.There are now paths for the bendingforces to get to the flanges.The sec-ond way to correct the design is toshape the bracket so it can be weldeddirectly to the sill flanges in new fabri-cations, or to add pieces to the bracketon existing cars to accomplish thesame purpose.

ConclusionThe foregoing are just a few examplesintended to illustrate the importance of

considering the transfer of forcethrough members. Sometimes weengineers act a little like horses withblinders on: we concentrate so single-mindedly on the problem at hand, thatwe cant see what is going on aroundus. The ideas discussed in this columnshould demonstrate how critical it isfor us as engineers to take our blind-ers off, expand our limited views, andtest our assumptions.

Figure 4.

Figure 6.

Figure 7.

Figure 5.

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Controversy and intense interest havesurrounded The Peter B. LewisBuilding of the Weatherhead School ofManagement at Case Western

Reserve University ever since architectFrank Gehry unveiled his design. Butmany Cleveland area residents whoshuddered when they first saw pho-tographs of the model in the cityspaper, The Plain Dealer, have beenwon over as they watched the shiningsculptural curves of the roof take solidshape and form.

According to Plain Dealerarchitecturecritic Steven Litt, the building depictsGehrys vision of a gleaming waterfallsplashing over boulders in a mountainstream. Sure enough, the stainlesssteel skin that slinks over and aroundthe sensuous curves of the steel struc-

ture glistens in the sun like so muchrushing water. The sight, etched againsta blue sky, can be breathtaking.What no longer shows is the meticu-lous planning, shop fabricating andfield welding work that went into creat-

ing the structural steel supports forthat elegant silvery gown of shinglesthe $61.7 million building now wears.When the projects lead contractor,Hunt Construction Group ofIndianapolis, called for bids to fabri-cate and erect the structural steel, theresponse was apparently less thanoverwhelming. However, Mariani MetalFabricators, Ltd., based across LakeErie from Cleveland in Toronto,Ontario, answered the call.

Software Jumps IndustriesGreg Kern, vice president of the 16-year-old firm, readily admits that theproject was a challenge, not only tobuild, but to price. This was one of thefirst uses of CATIA software in the

steel construction industry, he pointsout. CATIA, which was developed forautomotive design and drafting applica-tions, is employed by architect FrankGehry. Therefore, the geometry wasthere for us, because [Gehry] hadworked out the models, says Kern.After winning the $6 million structuralsteel contract, Mariani Metal hired adrafting and software training companywith automotive industry expertise to

produce project models and about1,200 drawings using CATIA. ParallelCATIA software stations were estab-lished in the Mariani fabrication shop

and at the construction site to providedesign and fabrication adjustments inreal-time, a step which prevented manypotential disruptions in production.

Devising a Practical ApproachWhen considered in the light of tradi-tional steel construction concepts, cre-ating the three-dimensional negativeand positive curves that comprise theroof structure posed practical prob-lems both structurally and in terms ofcost. After analyzing the complexgeometry from a real-world erectionstandpoint, Mariani Metal proposedfabricating the structural framework ina series of ladders, infills, truss panelsand support members, which would beshop-fabricated (Figure 1) and thenassembled and field-welded on site. Itwas basically a modular approach,notes Kern.

Gleaming Waterfall Refreshes Urban CampusBy Carla Rautenberg

Welding InnovationContributing WriterThe James F. Lincoln Arc Welding FoundationCleveland, Ohio

Figure 2.

Figure 1.

Figure 3.

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Another challenge was devising amethod of bending the hundreds of

pipes which form the lines of ruling(Figures 2 and 3). Almost every pipe

utilized in the roof structure had aunique curvature and length. With the

help of the CATIA software, MarianiMetal developed a system which per-mitted unique members to be created

within a production line. This allowedstockpiling of pipe sections which

could then efficiently feed the shopfabrication process.

Standard AWS D1.1 connection

details were employed to weld the 700tons of Grade 50 pipe and structuralsteel used to create the framework for

the roof. Mariani Metals, which has afull-time workforce of thirty, employed

eight welders on the job in the shop;field welding was done by a crew thataveraged between eight and sixteen

welders. The process most used wasshielded metal arc, with semi-automatic

flux cored arc welding in selectedapplications. We kept the field welding

operation as straight-forward as possi-ble, doing the most complex welding inthe shop, says Kern. He proudly

states that the skin lies directly on ourpipes, with the pipes themselves cre-

ating the geometry of the surface, andadds that the whole process required

the precision of building a Swisswatch in full scale. That kind of preci-sion is apparently something the folks

at Mariani thrive on; Kern maintainsthat they would hasten to work on

additional Gehry projects.

Across the street from the constructionsite in Clevelands University Circle,the stone caryatids of Case Western

Reserves gothic style MatherMemorial Building have silently

watched a monument to 21st centuryarchitecture take dramatic form. When

the 149,000 ft.2 (13,843 m2) Peter B.Lewis Building is dedicated later this

year, the city of Cleveland will havea new landmark.

This soaring structural steel framework is now hidden by the outer skin and

inner walls of the building.

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24/24

NON-PROFIT ORG.

U.S. POSTAGE

PAID

JAMES F. LINCOLN

ARC WELDING FND.

Story on page 20.

P.O. Box 17188Cleveland, OH 44117-1199

TheJames F. LincolnArc WeldingFoundation

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